Thu, Dec 14, 2017
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Arizona Solar Center Blog

Commentary from Arizona Solar Center Board Members and invited contributors.

While blog entries are initiated by the Solar Center, we welcome dialogue around the posted topics. Your expertise and perspective are highly valued -- so if you haven't logged in and contributed, please do so!

Passive Solar Energy - The Starting Point

PASSIVE SOLAR ENERGY - IMAGE 01 The sun’s energy is an incredible bounty. The energy contained in solar rays make their way through our filtering atmosphere and is critical to life on this planet...

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and is fundamental to human survival. It can also provide for our comfort. 

The use of the sun’s power in solar energy design is usually identified in 2 contexts - Passive Solar - that which uses natural processes without mechanical equipment and additional electrical or gas energy to operate, and Active Solar - that which uses nature’s resources with the inclusion of mechanical equipment and hardware driven by electricity and gas.


All solar design starts from a simple base - Passive Solar First.

What can be achieved by using all of the natural resources available to meet specific needs? This is the basic question and tenet of Passive Solar applications whether it be applied to heating and cooling a building, lighting, heating water, cooking, etc. Passive solar applies both to buildings and equipment.

Sound fundamentals of good passive applications and integration can beneficial and are directly related to active solar equipment use and implementation:

  1. by meeting needs with no mechanical equipment dependent on external energy incorporation,

  2. in improving conditions which reduce the amount and size of equipment required to meet needs,

  3. by improving the conditions for active solar equipment applications, and

  4. in minimizing the commensurate costs that accompany the purchase and use of any equipment, solar or non-solar.

In short , Passive solar design and applications is the base which sets the conditions for effective active solar incorporation and use.

Passive and Active solar applications should be considered as elements of the same palette - sort of the one-two punch of living with the sun,

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one-two punch

and the mutuality is undeniable. Both rely on the same design considerations of orientation, access to the sun, behavior of materials, and appropriate use of site resources, and vary only in the inclusion external energy of electricity and gas. Most active system guidelines even point out starting with passive considerations first.

Besides providing for direct meeting of needs, Passive Solar design is a primary basis for enhancing the quality of active solar systems. Passive solar actions can result in the reduction of quantity of equipment needed to meet a particular task. For example, daylight is an available resource to meet illumination needs. Good day lighting design of buildings uses that resource effectively, and reduces the need and cost of daytime artificial lighting and equipment.

The beginning point consideration is at the end use side of things whether using traditional equipment or using solar equipment. Considerable savings can be gained in applying natural energy actions to reduce the cost of both supplying equipment as well as running and maintaining it. Quite simply, the less work that needs to be accomplished by equipment, the less amount of equipment is needed, and the less it needs to run when used - this all translates to less cost for purchase of the equipment, and less on-going cost for running and maintaining. Passive solar applications mitigate the quantity of active solar equipment needed, and resulting the tandem of both is optimal. Information about the sun and how to use it effectively is common in both applications.


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Traditionally, the term Passive Solar has been identified with heating and cooling of buildings, but it has a broader context and application. There is, of course passive solar heating and cooling of buildings. There is also passive solar water heating, solar cooking, natural lighting, passive solar heating of pools, and even passive solar devices which move things - equipment, air, etc. Even the process of direct conversion of sunlight to electricity can be considered a “passive” action since it occurs through the appropriate use and placement of materials and capitalizes on the behavior of the combinations created, without infusing man-made energy sources and machines to make it work.

Knowledge and understanding of natural processes is the heart of Passive Solar. Knowledge about the composition, attributes and behavior of sunlight and heat; the behavior of heat flow; the behavior and capacities of materials, both in nature and man-made; the sun’s annual, seasonal, and daily movement; diurnal and seasonal temperatures and conditions; human sensory response and comfort ; the patterns of nature and of people; and the physiology and psychology of the interaction between people and Nature, all are applied to effective solar application and utilization.


PASSIVE SOLAR

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NATURE’S CONTRIBUTION - a gift that also keeps us on our toes

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intense heat, cold) The conditions that nature provides, in the form of climate, is variable. Cold in the winter, hot in the summer, nice other times of the year. Arizona climate covers the entire spectrum with extremes at the desert and mountain locations. Simultaneously, nature also provides the tools for mitigation of the extreme conditions. Sunlight and materials for a warming system; breezes, water, earth, gravity, and materials for a cooling system. It is the application of these resources into a system that addresses conditions that makes passive, and active, solar so effective.

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The sun’s available energy varies in amount and impact through the year. The amount and intensity of energy from the sun that impacts the earth is affected by the composition of the earth’s atmosphere, and the angle of the solar radiation waves. The more dense the atmosphere, whether by clouds or smog, the less solar energy reaches the ground. Additionally the more directly perpendicular the sun is to the earth’s surface, the more concentrated the energy is in a given area and the more intense its impact. The highest capitalization of solar radiation for heat is when surfaces are perpendicular to the sun, allowing the most density of radiation at a given point.

PASSIVE SOLAR ENERGY - IMAGE 14We know the sun’s position every day of the year and the amount of radiation that position provides, both to the earth’s surface, as well as to various positions of building walls and/or equipment. A south facing wall, or piece of equipment gets more energy from the sun than any other position. An angle directly perpendicular to the sun gets more energy per square foot than one that is at an angle. The sun is less available in the winter (shorter days) than in the summer. There is less solar energy availability in the winter than the summer due to the sun’s position at an angle to the earth and therefore more atmosphere to penetrate. 

We also know that cool air settles and warm air rises, and that this action occurs with fluids like water. We know about heat flow and capabilities of materials in their capacity to absorb, hold, and give up heat. We know how to let sunlight in, how to capture and create air movement for cooling, and prevent unwanted heat.

THE BUILT ENVIRONMENT IN TUNE WITH NATURE

PASSIVE SOLAR ENERGY - IMAGE 15Passive solar buildings are environmentally responsive and use nature’s elements in providing shelter and comfort to people in a manner that is healthy and minimally destructive of the environment; are non-depleting of natural resources; and use the building itself in the comfort creating process. They are characterized throughout the recent years with terms as “sustainable”, “renewability”, and “green”. Quite simply, these terms refer to the same thing - a nature incorporating , comfort generating, security providing environment in which the building composition itself is the “machinery” that creates protection, health and comfort, and incorporates appropriate solar equipment to attain higher degrees of performance.

HISTORY

PASSIVE SOLAR ENERGY - IMAGE 16Arizona history is replete with examples of people living with the sun - both in using it as a resource as well as dealing with it’s negatives. Passive solar was integrated into Arizona architecture and buildings, both in private and public buildings. While incorrectly called Arizona’s first solar building, (there is no indication that this was a conscious effort since a number of cliff dwellings built in the same period by the same people do not show the same kind of solar application) the construction of Montezuma’s Castle does embody some solar principles of orientation, thermal mass, “overhangs” for summertime shading, and south facing winter courts

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PASSIVE SOLAR ENERGY - IMAGE 19Desert buildings used proper orientation, thick masonry walls, natural cross ventilation, indoor and outdoor living spaces, and natural and man-made shade for summer cooling, and south facing courts, and windows with tile floors which, when coupled with the thick masonry walls, provided for capture and storage of warmth during winter conditions. Higher elevations of Arizona utilized the similar principles with differing amounts of wall mass and windows for heating, and porches and cross ventilation for summer evening relaxation and sleeping.

PASSIVE SOLAR ENERGY - IMAGE 20Arizona desert buildings, both private and public, used passive means of shading to provide respite from the intense sun. Passive solar equipment, in the form of water heaters, were prevalent in Arizona as well as Southern California. the historic Ellis-Shackleford House in Phoenix and the historic Tempe Bakery had direct gain solar hot water heaters.

PASSIVE SOLAR ENERGY - PRELUDE TO SOLAR EQUIPMENT CONSIDERATION
There are a number of passive energy fundamentals which can be considered in reducing the amount of equipment and/or its’ operation. 

ORIENTATION - It’s a necessary thing...


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Like all direct solar applications, capturing the sun as a resource is as simple as providing for its clear path to where it can do its work - be it heating water, cooking food, or warming a space. Orientation is a fundamental concept of solar use for passive, and active, systems -

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orientation of a solar device or orientation of a building or a solar cooker

PASSIVE SOLAR ENERGY - IMAGE 26 Orientation and a direct relationship with the sun is the first rule of solar energy use when trying to capitalize on its heat providing attributes.  The sun’s traverses the sky every day -

In the winter it is a low and short path and in the summer a long and high path - and even though the sun’s location is constantly changing, it is a predictable path that can be used in incorporating the sun’s energy to meet needs, and to exclude when we want to minimize the same. Applied knowledge of both the sun’s movement, position at any given time, and time of the year, as well as impact in the form of radiation (solar incidence) , enables us to take advantage of these attributes to meet needs, and to make use of our buildings and our equipment more effective and efficient.

Proper orientation is critical to optimizing the solar resource. A properly oriented building can optimize solar gain for human comfort heating, and with proper shape and overhangs can minimize summertime overheating. Likewise, properly oriented solar equipment, be it a solar water heating panel or a photovoltaic electricity generating panel, will have optimum production and minimal negative impact. Using natural winter heating and minimizing summer heat impacts reduces the size of heating and air conditioning equipment as well as the energy, traditional or photovoltaic, needed to provide energy to these systems. Additionally, proper orientation allows for full benefit of the operation of electric solar panels and maximizing solar water heating operations.


PASSIVE SOLAR ENERGY - IMAGE 27 Proper building orientation also eases the integration of active solar equipment into the building form and shape, and mitigates the conflicts of solar installations contested in numerous subdivisions regulations.

Proper orientation with direct exposure to the south, is best for passive solar heating of building spaces as well as the operation of solar equipment (water heaters, pv panels, cookers, pool panels, etc.). slight adjustments to the east or west of south allow for earlier or later use of the sun's energy, or for mitigating it.


FORM - It’s a right thing

PASSIVE SOLAR ENERGY - IMAGE 28Solar buildings employ a form and shape that is responsive to the elements of nature that impact upon it, as well as the solar equipment that is part of the passive/active solar approach. Elongated along the east west axis, an Arizona building optimizes the southern exposure for good wintertime direct heating, while minimizing the east and west exposures which are severely impacted negatively in the summer, especially in the Arizona desert areas. Good building form is also beneficial when it comes to integration of solar equipment. Instead of unsightly racks, collector panels can be blended into the building architecture, and seem as seamless as a skylight or clerestory window.

For this reason roof design is important re: slopes and orientations to the sun’s path. (slide 28) Equally important is the integration of passive solar strategies to building additions such as thermal chimneys to accelerate cross ventilation, cooling towers, and north facing clerestories which incorporate hot water and pv panels on their back sides.


LOCATION - It’s the effective thing

PASSIVE SOLAR ENERGY - IMAGE 29Location of a building and the placement of the spaces within are a critical passive element in optimizing the use of natural resources for comfort, and proper placement also optimizes integrated solar equipment by minimizing piping runs and complex plumbing and electrical transfers. 90% of the winter sun’s energy is received at the earth’s surface between 9 a.m. and 3 p.m. so open and continued exposure is important for natural heating. Habitable spaces that benefit from solar heating are best located on the south side of a building with support spaces (garage, storerooms, etc.) located to the perimeter and to the north side. In this way the sun can directly, or indirectly, provide it’s energy to warm the spaces which means less heating equipment.

Additionally, ancillary spaces located to the perimeter east and west sides provide a thermal barrier zone to the habitable spaces, thereby reducing the heating and cooling loads to an easily manageable level. Proper orientation and spatial design allows for optimum use of the sun for providing thermal comfort in both winter and summer, and reduces the amount of heating and cooling equipment, as well as the energy required to run it. Additionally, proper location can reduce the amount of solar equipment needed in a radiant floor system and/or for pv systems which provide power to air conditioning and heating machinery.

PASSIVE SOLAR ENERGY - IMAGE 30Good location planning extends to the integration of solar equipment as a building component by reducing piping runs and the commensurate “line losses” , thereby allowing more of the solar heat captured in a water heating system to get to the storage and/or use point.

MATERIALS - It’s the smart thing:


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All solar heating and cooling systems are based on the ability to gather and store solar energy within a material for a period of time. This is accomplished by using a material which will hold heat until it is needed for heating, or capturing heat that will be dispelled at a later time. Solar water heaters use water. Solar buildings use their own structure - floors, walls, even roofs. Of course, some materials are better for this purpose than others. Glass, wood, and insulation are not good holders of heat. More dense materials like earthen materials (adobe, stone, brick, etc) and man-made materials like concrete are very good. This attribute is called thermal mass. 

Heating application of thermal mass is to select material(s) that will absorb heat from solar exposure during the day, hold that heat for a time during non-solar periods, then give it up as conditions warrant. The same action can be incorporated for building cooling. As an area heats up, heat can be absorbed into the thermal mass material in the walls, floor or ceiling, like a thermal sponge, then held until evening time where effective cooling practices using cross ventilation, night sky radiation and even whole house mechanical ventilation (remember - nighttime electricity rates are lower than daytime). This action is based upon fundamental principles of thermal transfer.

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Nature is always seeking to even out things so if there is a “difference”, there is a natural action which moves to make every thing the same or to even out. In the case of heat transfer, heat migrates to cold, so in a building, hot walls will radiate into cold spaces, and conversely, hot, summertime spaces will have their heat migrate to cooler walls. An analogy that has been used is one of 2 buckets of water - 1 full and one empty. If they are placed at an equal height, and there is a connection between the 2, below the water line of the first, water will flow to the second until there is an equal amount in the second where all action will stop because a balance has 
been achieved. 

Passive solar buildings utilize the very fabric of the building as part of the comfort system for heating and cooling, and the addition of active solar system for running naturally heated or cooled water to through a thermal mass wall, floor, or roof structure enhances the performance of the system by the additional thermal mass capacity and heat capture/transfer attributes of the 
water.


WINDOWS - It’s the clear thing.

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One of the major design considerations affecting a building’s energy consumption is the location and size of windows. Windows are the weakest point of the building envelope and usually the leakiest when it comes to energy, both in terms of losing heat in the winter, and gaining heat in the summer. A square foot of glass will lose 12 time more energy than a wood wall with insulation. As a rule, windows should be located primarily on the south side where they can be used as part of the heating system, as well as provide for natural lighting. That is the side where the sun is!. East and west sides of desert buildings should have minimal or no windows since these are the 2 worst exposures for early morning and late afternoon summer sun. East windows allow for early summer morning heat up and west windows allow for late afternoon negative impacts.

Solar windows should be sized in accordance with the heating and cooling performance of the building. Typical oversizing to have the “feel of the great outdoors” is not an optimal situation when it comes to solar design.

Clerestorey windows are a design tool for getting sunlight benefits (heating and lighting) to areas not able to be located at the south face. Clerestories also provide a mechanism for diffusing the direct impact of sunlight and moderating glare. Additionally, operable clerestorey windows are a good device for house ventilation cooling in the summer. 

Reverse clerestories, those opening to the north can be a benefit in desert conditions . Facing north, they provide even natural light to interiors and their angled backs can be a perfect mounting structure and angle for solar equipment like photovoltaic and solar water heating panels.


THERMAL DECOMPRESSION - It’s the healthy thing
A building trying to maintain a comfortable internal temperature will always be in conflict with the temperatures adjacent to the exterior. Heat always moves to cold - in the winter , interior warmth is moving toward the exterior cold. In the summer, the external heat is trying to move to the interior coolness. In both situations, the greater the difference in temperature between inside and outside conditions, the faster the movement of heat and the greater the amount of heat moved, and the more equipment is required to mitigate conditions.

In temperate times when inside and outside are at or near the same temperatures, there is minimal movement and therefore minimal need for equipment. Add to this the fact that sudden and abrupt changes in temperature are not positive to the human body which has to react rapidly to changed conditions, and good passive site planning of thermal decompression is important for not only comfort, but for health. 

Thermal decompression simply means that there is layering of vegetation, landscape features, and built elements that gradually temper the environment to a point where the temperatures adjacent to the building are much closer to its internal temperature. This decompression approach establishes a condition where the difference between the internal temperature and the temperature on the building skin are much closer, so less heat is gained (or lost) and less mitigating equipment, and commensurate energy, is required.


PASSIVE SOLAR APPLICATIONS - It’s first thing
Natural Lighting -

PASSIVE SOLAR ENERGY - IMAGE 39The sunlight received by a building will provide more than sufficient illumination to meet daily needs. Use of day lighting is a passive solar application. The sun’s capacity to provide light, when integrated correctly in a building, means no need to use artificial lighting during the day, which means no energy used for those lights, which means no utility cost, except at night when the sun doesn’t shine. Solar building design incorporates day lighting strategies of letting light into all spaces either directly with proper window placement, clerestories and even skylights, or indirectly with light reflecting color choices, light shelves, and transparent and translucent walls. PASSIVE SOLAR ENERGY - IMAGE 40 This glazing has dual benefit - while providing for illumination, it can also provide for wintertime heating. Good passive design then incorporates both attributes of sunlight - illumination and heating, and the building construction and finishes are used to capitalize on both. Light colored surfaces and transparent/translucent interior panels for “bouncing” or directing sunlight for illumination, and dark, thermal mass surfaces for absorbing the sun’s rays for heating. Multi-faceted and multi-applicable, day lighting design is an effective passive solar approach which has a direct impact on the building’s

However, addition of a solar electricity generation system (photovoltaics) allows for the capture of daytime sunlight and its transformation into electricity, which can then be stored and used in the night. Add to this the use of efficient fixtures and systems, and costs in both resources and dollars are further reduced.


Water Heating -
Batch or Integrated Collector/Storage (ICS) System

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Simply water in a tank within a container and exposed directly to the sun. This is the basis of batch/ breadbox, systems which combine collection, heating, and storage of water into a single component. Direct heating of the storage tank or tanks, makes this system compact, simple, and effective. These units are called a passive systems because they do not rely on equipment to make them function. When hot water is removed, it is replaced by an equal amount of "new" water.

The "batch" approach has been used for quite some time and improvements in design have enhanced their effectiveness in increasing water heating capabilities. Newer systems use a number of small-diameter connected storage tanks connected to expose more of the water surface to the sunlight, heating the water at a faster rate. In some cases reflectors are integrated, bouncing more of the sun's rays onto the water tank, and when the sun falls, the reflectors, made of highly insulating material, fold over the glazing to insulate the tank. Some systems use evacuated glass tubes (like a thermos bottle) around the collector to keep heat loss to a minimum. Thermosyphon Systems.

PASSIVE SOLAR ENERGY - IMAGE 42Hot water rises and cold water settles. This is because hot water is less dense than cold water due to its molecular "excitement" in being heated. In a typical water heater, colder water is at the bottom of a tank. When it is heated by the heating element or burner, it becomes less dense and rises to the top of the tank, while being replaced by cooler, settling water, which is, in turn heated, rises, etc.. This cycle is called a convective action. A thermosyphon solar water heating system incorporates natural convection to move fluid heated by the collector to a storage tank. In order to do this naturaly, the collector is located at some point below the storage storage tank. As the fluid at the bottom of the storage tank cools (more dense) it flows to the bottom of the collector where it is reheated making it rise back to the top of the storage tank. This process is continuous. As a result, thermosyphon systems do not need pumps and for that reason they are considered a passive system - that system that does not rely on equipment to make it function.

HEATING/COOLING

PASSIVE SOLAR ENERGY - IMAGE 43Passive solar applications for heating and cooling a building mitigate expensive heating and cooling with conventional equipment driven by electricity and gas, and good passive design reduces the energy consumed and the allied cost of utility resources to maintain comfort.

There are basic elements of passive energy buildings which use the form and materials to provide comfort. Some of these are applicable to solar equipment design and use, even to the point where there is solar equipment which are passive in their operation - i.e. thermal energy flows in the system naturally. Solar water heating is one type of equipment that can be a passive solar piece of equipment. A “batch” water heater and a thermosiphon water heater can be considered passive solar equipment - since they do not rely on out side energy source to make them function. Of course, when talking about passive and active solar, optimum conditions and control occur best when these two are coupled.

Basics of passive applications are rooted in dealing with the sun (exposure to and capture of the sun’s energy when we want heat; protection from the sun when we want cooling), the materials used (for effective capture, storage, and use), and natural processes of physics for both). Every passive system for solar heating, whether it is heating water, or heating a building, requires exposure to the sunlight and trapping it - this is done by glazing - windows for a building and glass covers for solar panels. Every passive system is dependent upon materials which will absorb the sun’s heat, store a good quantity of it and easily distribute it. In a building, the effective material can be the structure itself, in the form of thermal mass. 

Thermal mass is characterized by those dense materials like concrete and earthen materials, and also by an extremely good material - water. These materials can readily absorb solar radiation, hold its warmth, and easily and evenly give it up to adjacent spaces.

PASSIVE SOLAR ENERGY - IMAGE 44Heat capture, storage and distribution follow a natural and predictable behavior. Sunlight heats the surfaces it strikes. The amount of heat held within the material depends on the material composition - straw is a terrible holder, concrete is a better holder. When sunlight is no longer available the material gives its’ captured heat to adjacent cooler conditions.

Generally there are 3 passive heating building concepts - Direct Gain, Indirect Gain and Isolated Gain These concepts have inherent within them cooling strategies and applications as well.

Direct Gain
Simply stated, sunlight comes directly through windows into the space to be heated.


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The building materials struck by the sunlight are thermal mass materials - concrete/tile floor, masonry walls, or even strategically placed containers of water. 

Building windows act in exactly the same way as solar panel glazing - they let the sunlight (short wave radiation) in and inhibit heat (long wave radiation) from escape. Direct gain design system is always working, letting in not only direct sunlight but also the diffuse light of cloudy days, and the intense light of summer.

Like any system, optimization is the goal - so the building eaves and overhangs become a designed-in optimizing element - summertime conditions, when heating is not required, are mitigated by keeping the sunlight off of the windows via the overhang, while in the winter, the sun is much lower in the sky and can easily skirt under the building’s brow.


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Heating is quite simple in this approach - sunlight, absorbed by the thermal mass materials, solid and/or liquid, is stored as heat. When the space cools in the evening, the heat migrates to the cooling spaces directly (radiation) or by air movement across the surface of the material (convection). For this approach, a careful consideration of the site, solar energy availability, and seasonal conditions, are all necessary to determine the appropriate amount of windows and thermal mass. Too many windows in an Arizona desert setting will result in a human cooker; too few windows in a Rim setting will result in not enough capture. 

This system has worked effectively in Arizona designs, as well as that sunniest of place of Liverpool, England.

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For effective cooling, such as the desert setting, Direct Gain Avoidance is the rule, BUT the thermal mass of the building can still be used in the cooling cycle. The materials, by nature of their thermal mass attribute, remain cool (or can be cooled during nighttime conditions - naturally by cross ventilation, or mechanical y by lowest cost energy driven fans). This coolness allows their absorption of unwanted heat in the building - acting as a sort of thermal sponge, moving heat away from people and holding to the evening, where cross ventilation or even whole house fans can dispose of the captured heat. Control of Direct Gain systems is done with the addition of movable insulation, either on the exterior or with interior blinds, and cross ventilation planning with placement of low wall vents on the cool side of the building, and high wall vents on the warm side of the building.


Indirect Gain -
Indirect gain is an “next step” of a Direct Gain system. Sunlight penetrates south facing windows, then strikes thermal mass located behind the window and between the sun and living space. There are basically three types of indirect gain systems, each defined by where the thermal mass is located. The three strategies are:

  • Thermal Wall and Plenum

  • Sunspace

  • Thermal Roof


Thermal Wall and Plenum -


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South facing windows front a thermal mass wall of masonry, and/or water, placed directly behind to create a vertical plenum or chase. The sun side of the thermal wall is typically dark to capture more of the solar spectrum. This mass absorbs, stores and distributes heat while acting as a buffer to the interior spaces, and moderates temperature changes and provides for extended use of thermal gain well into the evening. Sunlight passes through the glass and converts to heat energy as it impacts the thermal mass and is absorbed, slowly saturating or moving through the mass until it radiates into the living space - the wall is a delayed action radiator. 

At the same time the air trapped between the windows and the mass heats up, and the addition of vents at the top and bottom of the wall allow for direct passive heating. Warmed between the glass and the wall spills into the living space through the opened upper vent and since Nature abhors a vacuum, cooler room area enters the plenum through the bottom vent, and is heated by the sun warmed wall, rises, spills into the room and is replaced by cooler air again, and this natural convection process continues as long as there is sunlight. There are a number of examples of this application - the Trombe wall which uses masonry, earthen materials like, and water like Steve Baer’s water barrel. Variations in thermal mass wall materials vary from commercial water tubes to incorporation of stone.

The Baer barrel wall installation provides for optimizing the heating capabilities as well as cooling of the adjacent spaces with the addition of movable insulating panels. During Winter conditions, Insulating panels are moved to allow for solar access to heat the water barrels, then at night the insulating panels are raised to cover the glazing and the barrels radiate their warmth to the space. In the summer heat, the insulation is raised and the barrels, with their cool water, act as absorbers, pulling unwanted heat from the spaces. At nightfall, the insulation is lowered, and the barrels give up their stored heat to the exterior by radiation and convection. the water, now cooled, is ready to act as a cooling absorber the next day.


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Sunspaces are a combination of Direct Gain and Thermal Wall systems, utilizing both approaches in tandem with a dedicated Direct Gain area (Sunspace) adjacent (fronting) the living space, with a Thermal Wall placed between the two. The Sunspace, has extensive south glazing and large daily fluctuations, while the adjacent living space is protected from these fluctuations by the Thermal Wall separating the spaces. Vents or operable doors and windows in the Thermal Wall allow warmed Sunspace heat to circulate to adjacent living spaces by natural convective actions during the day, and radiate the absorbed Sunspace heat to the living spaces in the evening. An additional usable area, Sunspaces are often used as solar greenhouses. Temperature control is best achieved with operable venting windows and cross ventilation.


Sunspaces - green houses:

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Thermal Roof -

The thermal Roof approach places thermal mass on the roof rather than at a wall, and is very effective as both a passive heating and cooling strategy. The system is both a radiator and an absorber and replaces standard heating and cooling mechanical systems and the inherent ductwork distribution system. Using water as thermal mass, roof ponds are constructed directly on top of heat conducting ceilings of metal pans or metal decking so there is direct thermal transfer. Movable insulation is placed above the ponds to facilitate better retention of heat in the winter and to prevent absorption of external heat in the summer.


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The operation is quite simple. During wintertime conditions, insulating panels are rolled or pivoted back, exposing water contained in UV inhibiting water beds to the sun. The ponds gather the sun’s warmth and at nightfall, the insulating panels are replaced to contain the gained heat and prevent loss to the cold night air. The heat stored in the bags, warms the supporting metal decking and the entire ceiling is a radiant ceiling throughout the cold winter night. The next morning, the insulating panels are removed when the sun appears and the cycle begins again.

Summer cooling is a reverse process. Ponds, covered during the daytime heat, remain cool and act in concert with the supporting metal ceilings as a thermal sponge absorbing interior heat generated from people, equipment, and infiltration from the outside. At night, panels are removed and the ponds throw off their gathered heat to the night sky by means of radiation, convections, and if wet down, by evaporation. 

Roof pond heating and cooling is optimized when all living spaces except the bathrooms and high water use areas are covered by the system. In areas that generate humidity, like a shower, the metal ceilings will tend to “rain” due to the temperature difference of steam vapors and cool metal.

Roof ponds, like Harold Hay’s Skytherm system, have been designed and used in the hot climate of Arizona and New Mexico to the moderate temperatures of the California coast and even planned for the twin cities of Minneapolis/St. Paul.


Isolated Gain -

PASSIVE SOLAR ENERGY - IMAGE 75This is basically an indirect system where solar collection for heating are isolated from the living spaces, and while the system functions independently, heating can be called for by simply opening some floor vents and letting the natural behavior of hot air rise through the spaces. The most common application of this approach is the convective loop. Much like a thermosiphon water heater, heat transfer material of air or water, is moved across a collector panel system facing the sun, and circulated into a tank surrounded by rock (water transfer system) or a rock bin (air transfer system) in a continuous operating loop. Natural thermosiphoning occurs when the collector is lower than the heat storage area which is usually located under the building.

A hybrid of this system can include moving heated water or air through a radiant floor system where the masonry floor itself acts as the thermal mass storage. This variation can also use cool water to create a “cool” floor by running house supply water, or water from an adjacent pool, through a floor system.


Cool Towers -

PASSIVE SOLAR ENERGY - IMAGE 76 Evaporative cooling systems which utilize gravity effect on dense, cooled air to drop and spill into living spaces. The system is comprised of wet cooler pads mounted high in an area which provides no obstructions to air movement, which comes into contact with the pads. The warm dry air contacting the wet pads, cool and becomes more dense and heavier and falls down the tower, usually positioned over or adjacent to a major living space. The falling cool air, spills into the living space, pushing warmer out at strategic venting areas. As the process continues, the cooler air ponds in the area, providing a cool environment in Az. desert conditions.

A variation to this system is the addition of a south facing thermal chimney to pull cool tower air through the house. Located at an opposite location from the cool tower, the thermal chimney provides an escape vent for interior warm air , which moves more quickly as it get heated and is driven out. This rapid venting has a drawing effect on the cool tower air and it is distributed more extensively through the building. The solar chimney can be set up to become a recirculating air heater during winter conditions.


Natural Cooling -

There are three sources of undesirable heat - direct summer sun solar gains through windows and glazing; heat transmission through the building envelope; and internal heat produced by people, their activities, and their equipment. Direct solar heat gain at windows and glazing can be easily controlled by shading the house - preventing the sun from reaching it (except for good day lighting and operation of solar equipment) and with external shading devices and vegetation as well as thermal insulating shutters. Heat transmission conditions can be nullified by setting up layers of thermal decompression with vegetation, built structure like porches, and water features. While there is not much that can be done to reduce natural heat production by people, equipment heat generation can be impacted by careful selection of energy efficient equipment and by good timing - do the laundry in the evening.


SOLAR COOKING


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Use of the sun for food preparation is fun, energy saving, and saves money, both in the cooking operation, and in the cooling costs saved when the heat is taken out of the kitchen during the summer. A variety of cooking tools from box cookers to slat faced ovens are available - whether they be commercial products or hand built by the inspiring solar chef.


Passive Solar Energy has many faces and applications and an effective Passive solar building incorporates many of these elements. Natural processes and incorporation of building and site elements to provide for comfort as well as mitigation of untoward conditions are the hallmark of good passive design, and results in establishing a basis for reduction of equipment (solar and otherwise) for achieving comfort, and reduction of equipment purchase and operations costs. Passive solar energy is Direct,

  • natural,

  • effective,

  • cost effective,

  • and inherent to the building form, structure, materials and use.

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This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Arizona Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources.


Financial support for this presentation has been provided by the Arizona Department of Commerce (Energy Office) and the U.S. Department of Energy through (DOE) Grant No. DE-FG51-01R021250. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the Energy Office or U.S. DOE. The State of Arizona and U.S. DOE assume no liability for damages arising from errors, omissions or representations contained in this presentation.

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Passive Solar Energy - The Starting Point

PASSIVE SOLAR ENERGY - IMAGE 01 The sun’s energy is an incredible bounty. The energy contained in solar rays make their way through our filtering atmosphere and is critical to life on this planet...

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and is fundamental to human survival. It can also provide for our comfort. 

The use of the sun’s power in solar energy design is usually identified in 2 contexts - Passive Solar - that which uses natural processes without mechanical equipment and additional electrical or gas energy to operate, and Active Solar - that which uses nature’s resources with the inclusion of mechanical equipment and hardware driven by electricity and gas.


All solar design starts from a simple base - Passive Solar First.

What can be achieved by using all of the natural resources available to meet specific needs? This is the basic question and tenet of Passive Solar applications whether it be applied to heating and cooling a building, lighting, heating water, cooking, etc. Passive solar applies both to buildings and equipment.

Sound fundamentals of good passive applications and integration can beneficial and are directly related to active solar equipment use and implementation:

  1. by meeting needs with no mechanical equipment dependent on external energy incorporation,

  2. in improving conditions which reduce the amount and size of equipment required to meet needs,

  3. by improving the conditions for active solar equipment applications, and

  4. in minimizing the commensurate costs that accompany the purchase and use of any equipment, solar or non-solar.

In short , Passive solar design and applications is the base which sets the conditions for effective active solar incorporation and use.

Passive and Active solar applications should be considered as elements of the same palette - sort of the one-two punch of living with the sun,

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one-two punch

and the mutuality is undeniable. Both rely on the same design considerations of orientation, access to the sun, behavior of materials, and appropriate use of site resources, and vary only in the inclusion external energy of electricity and gas. Most active system guidelines even point out starting with passive considerations first.

Besides providing for direct meeting of needs, Passive Solar design is a primary basis for enhancing the quality of active solar systems. Passive solar actions can result in the reduction of quantity of equipment needed to meet a particular task. For example, daylight is an available resource to meet illumination needs. Good day lighting design of buildings uses that resource effectively, and reduces the need and cost of daytime artificial lighting and equipment.

The beginning point consideration is at the end use side of things whether using traditional equipment or using solar equipment. Considerable savings can be gained in applying natural energy actions to reduce the cost of both supplying equipment as well as running and maintaining it. Quite simply, the less work that needs to be accomplished by equipment, the less amount of equipment is needed, and the less it needs to run when used - this all translates to less cost for purchase of the equipment, and less on-going cost for running and maintaining. Passive solar applications mitigate the quantity of active solar equipment needed, and resulting the tandem of both is optimal. Information about the sun and how to use it effectively is common in both applications.


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Traditionally, the term Passive Solar has been identified with heating and cooling of buildings, but it has a broader context and application. There is, of course passive solar heating and cooling of buildings. There is also passive solar water heating, solar cooking, natural lighting, passive solar heating of pools, and even passive solar devices which move things - equipment, air, etc. Even the process of direct conversion of sunlight to electricity can be considered a “passive” action since it occurs through the appropriate use and placement of materials and capitalizes on the behavior of the combinations created, without infusing man-made energy sources and machines to make it work.

Knowledge and understanding of natural processes is the heart of Passive Solar. Knowledge about the composition, attributes and behavior of sunlight and heat; the behavior of heat flow; the behavior and capacities of materials, both in nature and man-made; the sun’s annual, seasonal, and daily movement; diurnal and seasonal temperatures and conditions; human sensory response and comfort ; the patterns of nature and of people; and the physiology and psychology of the interaction between people and Nature, all are applied to effective solar application and utilization.


PASSIVE SOLAR

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NATURE’S CONTRIBUTION - a gift that also keeps us on our toes

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intense heat, cold) The conditions that nature provides, in the form of climate, is variable. Cold in the winter, hot in the summer, nice other times of the year. Arizona climate covers the entire spectrum with extremes at the desert and mountain locations. Simultaneously, nature also provides the tools for mitigation of the extreme conditions. Sunlight and materials for a warming system; breezes, water, earth, gravity, and materials for a cooling system. It is the application of these resources into a system that addresses conditions that makes passive, and active, solar so effective.

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The sun’s available energy varies in amount and impact through the year. The amount and intensity of energy from the sun that impacts the earth is affected by the composition of the earth’s atmosphere, and the angle of the solar radiation waves. The more dense the atmosphere, whether by clouds or smog, the less solar energy reaches the ground. Additionally the more directly perpendicular the sun is to the earth’s surface, the more concentrated the energy is in a given area and the more intense its impact. The highest capitalization of solar radiation for heat is when surfaces are perpendicular to the sun, allowing the most density of radiation at a given point.

PASSIVE SOLAR ENERGY - IMAGE 14We know the sun’s position every day of the year and the amount of radiation that position provides, both to the earth’s surface, as well as to various positions of building walls and/or equipment. A south facing wall, or piece of equipment gets more energy from the sun than any other position. An angle directly perpendicular to the sun gets more energy per square foot than one that is at an angle. The sun is less available in the winter (shorter days) than in the summer. There is less solar energy availability in the winter than the summer due to the sun’s position at an angle to the earth and therefore more atmosphere to penetrate. 

We also know that cool air settles and warm air rises, and that this action occurs with fluids like water. We know about heat flow and capabilities of materials in their capacity to absorb, hold, and give up heat. We know how to let sunlight in, how to capture and create air movement for cooling, and prevent unwanted heat.

THE BUILT ENVIRONMENT IN TUNE WITH NATURE

PASSIVE SOLAR ENERGY - IMAGE 15Passive solar buildings are environmentally responsive and use nature’s elements in providing shelter and comfort to people in a manner that is healthy and minimally destructive of the environment; are non-depleting of natural resources; and use the building itself in the comfort creating process. They are characterized throughout the recent years with terms as “sustainable”, “renewability”, and “green”. Quite simply, these terms refer to the same thing - a nature incorporating , comfort generating, security providing environment in which the building composition itself is the “machinery” that creates protection, health and comfort, and incorporates appropriate solar equipment to attain higher degrees of performance.

HISTORY

PASSIVE SOLAR ENERGY - IMAGE 16Arizona history is replete with examples of people living with the sun - both in using it as a resource as well as dealing with it’s negatives. Passive solar was integrated into Arizona architecture and buildings, both in private and public buildings. While incorrectly called Arizona’s first solar building, (there is no indication that this was a conscious effort since a number of cliff dwellings built in the same period by the same people do not show the same kind of solar application) the construction of Montezuma’s Castle does embody some solar principles of orientation, thermal mass, “overhangs” for summertime shading, and south facing winter courts

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PASSIVE SOLAR ENERGY - IMAGE 19Desert buildings used proper orientation, thick masonry walls, natural cross ventilation, indoor and outdoor living spaces, and natural and man-made shade for summer cooling, and south facing courts, and windows with tile floors which, when coupled with the thick masonry walls, provided for capture and storage of warmth during winter conditions. Higher elevations of Arizona utilized the similar principles with differing amounts of wall mass and windows for heating, and porches and cross ventilation for summer evening relaxation and sleeping.

PASSIVE SOLAR ENERGY - IMAGE 20Arizona desert buildings, both private and public, used passive means of shading to provide respite from the intense sun. Passive solar equipment, in the form of water heaters, were prevalent in Arizona as well as Southern California. the historic Ellis-Shackleford House in Phoenix and the historic Tempe Bakery had direct gain solar hot water heaters.

PASSIVE SOLAR ENERGY - PRELUDE TO SOLAR EQUIPMENT CONSIDERATION
There are a number of passive energy fundamentals which can be considered in reducing the amount of equipment and/or its’ operation. 

ORIENTATION - It’s a necessary thing...


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Like all direct solar applications, capturing the sun as a resource is as simple as providing for its clear path to where it can do its work - be it heating water, cooking food, or warming a space. Orientation is a fundamental concept of solar use for passive, and active, systems -

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orientation of a solar device or orientation of a building or a solar cooker

PASSIVE SOLAR ENERGY - IMAGE 26 Orientation and a direct relationship with the sun is the first rule of solar energy use when trying to capitalize on its heat providing attributes.  The sun’s traverses the sky every day -

In the winter it is a low and short path and in the summer a long and high path - and even though the sun’s location is constantly changing, it is a predictable path that can be used in incorporating the sun’s energy to meet needs, and to exclude when we want to minimize the same. Applied knowledge of both the sun’s movement, position at any given time, and time of the year, as well as impact in the form of radiation (solar incidence) , enables us to take advantage of these attributes to meet needs, and to make use of our buildings and our equipment more effective and efficient.

Proper orientation is critical to optimizing the solar resource. A properly oriented building can optimize solar gain for human comfort heating, and with proper shape and overhangs can minimize summertime overheating. Likewise, properly oriented solar equipment, be it a solar water heating panel or a photovoltaic electricity generating panel, will have optimum production and minimal negative impact. Using natural winter heating and minimizing summer heat impacts reduces the size of heating and air conditioning equipment as well as the energy, traditional or photovoltaic, needed to provide energy to these systems. Additionally, proper orientation allows for full benefit of the operation of electric solar panels and maximizing solar water heating operations.


PASSIVE SOLAR ENERGY - IMAGE 27 Proper building orientation also eases the integration of active solar equipment into the building form and shape, and mitigates the conflicts of solar installations contested in numerous subdivisions regulations.

Proper orientation with direct exposure to the south, is best for passive solar heating of building spaces as well as the operation of solar equipment (water heaters, pv panels, cookers, pool panels, etc.). slight adjustments to the east or west of south allow for earlier or later use of the sun's energy, or for mitigating it.


FORM - It’s a right thing

PASSIVE SOLAR ENERGY - IMAGE 28Solar buildings employ a form and shape that is responsive to the elements of nature that impact upon it, as well as the solar equipment that is part of the passive/active solar approach. Elongated along the east west axis, an Arizona building optimizes the southern exposure for good wintertime direct heating, while minimizing the east and west exposures which are severely impacted negatively in the summer, especially in the Arizona desert areas. Good building form is also beneficial when it comes to integration of solar equipment. Instead of unsightly racks, collector panels can be blended into the building architecture, and seem as seamless as a skylight or clerestory window.

For this reason roof design is important re: slopes and orientations to the sun’s path. (slide 28) Equally important is the integration of passive solar strategies to building additions such as thermal chimneys to accelerate cross ventilation, cooling towers, and north facing clerestories which incorporate hot water and pv panels on their back sides.


LOCATION - It’s the effective thing

PASSIVE SOLAR ENERGY - IMAGE 29Location of a building and the placement of the spaces within are a critical passive element in optimizing the use of natural resources for comfort, and proper placement also optimizes integrated solar equipment by minimizing piping runs and complex plumbing and electrical transfers. 90% of the winter sun’s energy is received at the earth’s surface between 9 a.m. and 3 p.m. so open and continued exposure is important for natural heating. Habitable spaces that benefit from solar heating are best located on the south side of a building with support spaces (garage, storerooms, etc.) located to the perimeter and to the north side. In this way the sun can directly, or indirectly, provide it’s energy to warm the spaces which means less heating equipment.

Additionally, ancillary spaces located to the perimeter east and west sides provide a thermal barrier zone to the habitable spaces, thereby reducing the heating and cooling loads to an easily manageable level. Proper orientation and spatial design allows for optimum use of the sun for providing thermal comfort in both winter and summer, and reduces the amount of heating and cooling equipment, as well as the energy required to run it. Additionally, proper location can reduce the amount of solar equipment needed in a radiant floor system and/or for pv systems which provide power to air conditioning and heating machinery.

PASSIVE SOLAR ENERGY - IMAGE 30Good location planning extends to the integration of solar equipment as a building component by reducing piping runs and the commensurate “line losses” , thereby allowing more of the solar heat captured in a water heating system to get to the storage and/or use point.

MATERIALS - It’s the smart thing:


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All solar heating and cooling systems are based on the ability to gather and store solar energy within a material for a period of time. This is accomplished by using a material which will hold heat until it is needed for heating, or capturing heat that will be dispelled at a later time. Solar water heaters use water. Solar buildings use their own structure - floors, walls, even roofs. Of course, some materials are better for this purpose than others. Glass, wood, and insulation are not good holders of heat. More dense materials like earthen materials (adobe, stone, brick, etc) and man-made materials like concrete are very good. This attribute is called thermal mass. 

Heating application of thermal mass is to select material(s) that will absorb heat from solar exposure during the day, hold that heat for a time during non-solar periods, then give it up as conditions warrant. The same action can be incorporated for building cooling. As an area heats up, heat can be absorbed into the thermal mass material in the walls, floor or ceiling, like a thermal sponge, then held until evening time where effective cooling practices using cross ventilation, night sky radiation and even whole house mechanical ventilation (remember - nighttime electricity rates are lower than daytime). This action is based upon fundamental principles of thermal transfer.

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Nature is always seeking to even out things so if there is a “difference”, there is a natural action which moves to make every thing the same or to even out. In the case of heat transfer, heat migrates to cold, so in a building, hot walls will radiate into cold spaces, and conversely, hot, summertime spaces will have their heat migrate to cooler walls. An analogy that has been used is one of 2 buckets of water - 1 full and one empty. If they are placed at an equal height, and there is a connection between the 2, below the water line of the first, water will flow to the second until there is an equal amount in the second where all action will stop because a balance has 
been achieved. 

Passive solar buildings utilize the very fabric of the building as part of the comfort system for heating and cooling, and the addition of active solar system for running naturally heated or cooled water to through a thermal mass wall, floor, or roof structure enhances the performance of the system by the additional thermal mass capacity and heat capture/transfer attributes of the 
water.


WINDOWS - It’s the clear thing.

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One of the major design considerations affecting a building’s energy consumption is the location and size of windows. Windows are the weakest point of the building envelope and usually the leakiest when it comes to energy, both in terms of losing heat in the winter, and gaining heat in the summer. A square foot of glass will lose 12 time more energy than a wood wall with insulation. As a rule, windows should be located primarily on the south side where they can be used as part of the heating system, as well as provide for natural lighting. That is the side where the sun is!. East and west sides of desert buildings should have minimal or no windows since these are the 2 worst exposures for early morning and late afternoon summer sun. East windows allow for early summer morning heat up and west windows allow for late afternoon negative impacts.

Solar windows should be sized in accordance with the heating and cooling performance of the building. Typical oversizing to have the “feel of the great outdoors” is not an optimal situation when it comes to solar design.

Clerestorey windows are a design tool for getting sunlight benefits (heating and lighting) to areas not able to be located at the south face. Clerestories also provide a mechanism for diffusing the direct impact of sunlight and moderating glare. Additionally, operable clerestorey windows are a good device for house ventilation cooling in the summer. 

Reverse clerestories, those opening to the north can be a benefit in desert conditions . Facing north, they provide even natural light to interiors and their angled backs can be a perfect mounting structure and angle for solar equipment like photovoltaic and solar water heating panels.


THERMAL DECOMPRESSION - It’s the healthy thing
A building trying to maintain a comfortable internal temperature will always be in conflict with the temperatures adjacent to the exterior. Heat always moves to cold - in the winter , interior warmth is moving toward the exterior cold. In the summer, the external heat is trying to move to the interior coolness. In both situations, the greater the difference in temperature between inside and outside conditions, the faster the movement of heat and the greater the amount of heat moved, and the more equipment is required to mitigate conditions.

In temperate times when inside and outside are at or near the same temperatures, there is minimal movement and therefore minimal need for equipment. Add to this the fact that sudden and abrupt changes in temperature are not positive to the human body which has to react rapidly to changed conditions, and good passive site planning of thermal decompression is important for not only comfort, but for health. 

Thermal decompression simply means that there is layering of vegetation, landscape features, and built elements that gradually temper the environment to a point where the temperatures adjacent to the building are much closer to its internal temperature. This decompression approach establishes a condition where the difference between the internal temperature and the temperature on the building skin are much closer, so less heat is gained (or lost) and less mitigating equipment, and commensurate energy, is required.


PASSIVE SOLAR APPLICATIONS - It’s first thing
Natural Lighting -

PASSIVE SOLAR ENERGY - IMAGE 39The sunlight received by a building will provide more than sufficient illumination to meet daily needs. Use of day lighting is a passive solar application. The sun’s capacity to provide light, when integrated correctly in a building, means no need to use artificial lighting during the day, which means no energy used for those lights, which means no utility cost, except at night when the sun doesn’t shine. Solar building design incorporates day lighting strategies of letting light into all spaces either directly with proper window placement, clerestories and even skylights, or indirectly with light reflecting color choices, light shelves, and transparent and translucent walls. PASSIVE SOLAR ENERGY - IMAGE 40 This glazing has dual benefit - while providing for illumination, it can also provide for wintertime heating. Good passive design then incorporates both attributes of sunlight - illumination and heating, and the building construction and finishes are used to capitalize on both. Light colored surfaces and transparent/translucent interior panels for “bouncing” or directing sunlight for illumination, and dark, thermal mass surfaces for absorbing the sun’s rays for heating. Multi-faceted and multi-applicable, day lighting design is an effective passive solar approach which has a direct impact on the building’s

However, addition of a solar electricity generation system (photovoltaics) allows for the capture of daytime sunlight and its transformation into electricity, which can then be stored and used in the night. Add to this the use of efficient fixtures and systems, and costs in both resources and dollars are further reduced.


Water Heating -
Batch or Integrated Collector/Storage (ICS) System

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Simply water in a tank within a container and exposed directly to the sun. This is the basis of batch/ breadbox, systems which combine collection, heating, and storage of water into a single component. Direct heating of the storage tank or tanks, makes this system compact, simple, and effective. These units are called a passive systems because they do not rely on equipment to make them function. When hot water is removed, it is replaced by an equal amount of "new" water.

The "batch" approach has been used for quite some time and improvements in design have enhanced their effectiveness in increasing water heating capabilities. Newer systems use a number of small-diameter connected storage tanks connected to expose more of the water surface to the sunlight, heating the water at a faster rate. In some cases reflectors are integrated, bouncing more of the sun's rays onto the water tank, and when the sun falls, the reflectors, made of highly insulating material, fold over the glazing to insulate the tank. Some systems use evacuated glass tubes (like a thermos bottle) around the collector to keep heat loss to a minimum. Thermosyphon Systems.

PASSIVE SOLAR ENERGY - IMAGE 42Hot water rises and cold water settles. This is because hot water is less dense than cold water due to its molecular "excitement" in being heated. In a typical water heater, colder water is at the bottom of a tank. When it is heated by the heating element or burner, it becomes less dense and rises to the top of the tank, while being replaced by cooler, settling water, which is, in turn heated, rises, etc.. This cycle is called a convective action. A thermosyphon solar water heating system incorporates natural convection to move fluid heated by the collector to a storage tank. In order to do this naturaly, the collector is located at some point below the storage storage tank. As the fluid at the bottom of the storage tank cools (more dense) it flows to the bottom of the collector where it is reheated making it rise back to the top of the storage tank. This process is continuous. As a result, thermosyphon systems do not need pumps and for that reason they are considered a passive system - that system that does not rely on equipment to make it function.

HEATING/COOLING

PASSIVE SOLAR ENERGY - IMAGE 43Passive solar applications for heating and cooling a building mitigate expensive heating and cooling with conventional equipment driven by electricity and gas, and good passive design reduces the energy consumed and the allied cost of utility resources to maintain comfort.

There are basic elements of passive energy buildings which use the form and materials to provide comfort. Some of these are applicable to solar equipment design and use, even to the point where there is solar equipment which are passive in their operation - i.e. thermal energy flows in the system naturally. Solar water heating is one type of equipment that can be a passive solar piece of equipment. A “batch” water heater and a thermosiphon water heater can be considered passive solar equipment - since they do not rely on out side energy source to make them function. Of course, when talking about passive and active solar, optimum conditions and control occur best when these two are coupled.

Basics of passive applications are rooted in dealing with the sun (exposure to and capture of the sun’s energy when we want heat; protection from the sun when we want cooling), the materials used (for effective capture, storage, and use), and natural processes of physics for both). Every passive system for solar heating, whether it is heating water, or heating a building, requires exposure to the sunlight and trapping it - this is done by glazing - windows for a building and glass covers for solar panels. Every passive system is dependent upon materials which will absorb the sun’s heat, store a good quantity of it and easily distribute it. In a building, the effective material can be the structure itself, in the form of thermal mass. 

Thermal mass is characterized by those dense materials like concrete and earthen materials, and also by an extremely good material - water. These materials can readily absorb solar radiation, hold its warmth, and easily and evenly give it up to adjacent spaces.

PASSIVE SOLAR ENERGY - IMAGE 44Heat capture, storage and distribution follow a natural and predictable behavior. Sunlight heats the surfaces it strikes. The amount of heat held within the material depends on the material composition - straw is a terrible holder, concrete is a better holder. When sunlight is no longer available the material gives its’ captured heat to adjacent cooler conditions.

Generally there are 3 passive heating building concepts - Direct Gain, Indirect Gain and Isolated Gain These concepts have inherent within them cooling strategies and applications as well.

Direct Gain
Simply stated, sunlight comes directly through windows into the space to be heated.


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The building materials struck by the sunlight are thermal mass materials - concrete/tile floor, masonry walls, or even strategically placed containers of water. 

Building windows act in exactly the same way as solar panel glazing - they let the sunlight (short wave radiation) in and inhibit heat (long wave radiation) from escape. Direct gain design system is always working, letting in not only direct sunlight but also the diffuse light of cloudy days, and the intense light of summer.

Like any system, optimization is the goal - so the building eaves and overhangs become a designed-in optimizing element - summertime conditions, when heating is not required, are mitigated by keeping the sunlight off of the windows via the overhang, while in the winter, the sun is much lower in the sky and can easily skirt under the building’s brow.


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Heating is quite simple in this approach - sunlight, absorbed by the thermal mass materials, solid and/or liquid, is stored as heat. When the space cools in the evening, the heat migrates to the cooling spaces directly (radiation) or by air movement across the surface of the material (convection). For this approach, a careful consideration of the site, solar energy availability, and seasonal conditions, are all necessary to determine the appropriate amount of windows and thermal mass. Too many windows in an Arizona desert setting will result in a human cooker; too few windows in a Rim setting will result in not enough capture. 

This system has worked effectively in Arizona designs, as well as that sunniest of place of Liverpool, England.

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For effective cooling, such as the desert setting, Direct Gain Avoidance is the rule, BUT the thermal mass of the building can still be used in the cooling cycle. The materials, by nature of their thermal mass attribute, remain cool (or can be cooled during nighttime conditions - naturally by cross ventilation, or mechanical y by lowest cost energy driven fans). This coolness allows their absorption of unwanted heat in the building - acting as a sort of thermal sponge, moving heat away from people and holding to the evening, where cross ventilation or even whole house fans can dispose of the captured heat. Control of Direct Gain systems is done with the addition of movable insulation, either on the exterior or with interior blinds, and cross ventilation planning with placement of low wall vents on the cool side of the building, and high wall vents on the warm side of the building.


Indirect Gain -
Indirect gain is an “next step” of a Direct Gain system. Sunlight penetrates south facing windows, then strikes thermal mass located behind the window and between the sun and living space. There are basically three types of indirect gain systems, each defined by where the thermal mass is located. The three strategies are:

  • Thermal Wall and Plenum

  • Sunspace

  • Thermal Roof


Thermal Wall and Plenum -


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South facing windows front a thermal mass wall of masonry, and/or water, placed directly behind to create a vertical plenum or chase. The sun side of the thermal wall is typically dark to capture more of the solar spectrum. This mass absorbs, stores and distributes heat while acting as a buffer to the interior spaces, and moderates temperature changes and provides for extended use of thermal gain well into the evening. Sunlight passes through the glass and converts to heat energy as it impacts the thermal mass and is absorbed, slowly saturating or moving through the mass until it radiates into the living space - the wall is a delayed action radiator. 

At the same time the air trapped between the windows and the mass heats up, and the addition of vents at the top and bottom of the wall allow for direct passive heating. Warmed between the glass and the wall spills into the living space through the opened upper vent and since Nature abhors a vacuum, cooler room area enters the plenum through the bottom vent, and is heated by the sun warmed wall, rises, spills into the room and is replaced by cooler air again, and this natural convection process continues as long as there is sunlight. There are a number of examples of this application - the Trombe wall which uses masonry, earthen materials like, and water like Steve Baer’s water barrel. Variations in thermal mass wall materials vary from commercial water tubes to incorporation of stone.

The Baer barrel wall installation provides for optimizing the heating capabilities as well as cooling of the adjacent spaces with the addition of movable insulating panels. During Winter conditions, Insulating panels are moved to allow for solar access to heat the water barrels, then at night the insulating panels are raised to cover the glazing and the barrels radiate their warmth to the space. In the summer heat, the insulation is raised and the barrels, with their cool water, act as absorbers, pulling unwanted heat from the spaces. At nightfall, the insulation is lowered, and the barrels give up their stored heat to the exterior by radiation and convection. the water, now cooled, is ready to act as a cooling absorber the next day.


PASSIVE SOLAR ENERGY - IMAGE 52 PASSIVE SOLAR ENERGY - IMAGE 53

Sunspaces are a combination of Direct Gain and Thermal Wall systems, utilizing both approaches in tandem with a dedicated Direct Gain area (Sunspace) adjacent (fronting) the living space, with a Thermal Wall placed between the two. The Sunspace, has extensive south glazing and large daily fluctuations, while the adjacent living space is protected from these fluctuations by the Thermal Wall separating the spaces. Vents or operable doors and windows in the Thermal Wall allow warmed Sunspace heat to circulate to adjacent living spaces by natural convective actions during the day, and radiate the absorbed Sunspace heat to the living spaces in the evening. An additional usable area, Sunspaces are often used as solar greenhouses. Temperature control is best achieved with operable venting windows and cross ventilation.


Sunspaces - green houses:

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PASSIVE SOLAR ENERGY - IMAGE 59

PASSIVE SOLAR ENERGY - IMAGE 60


Thermal Roof -

The thermal Roof approach places thermal mass on the roof rather than at a wall, and is very effective as both a passive heating and cooling strategy. The system is both a radiator and an absorber and replaces standard heating and cooling mechanical systems and the inherent ductwork distribution system. Using water as thermal mass, roof ponds are constructed directly on top of heat conducting ceilings of metal pans or metal decking so there is direct thermal transfer. Movable insulation is placed above the ponds to facilitate better retention of heat in the winter and to prevent absorption of external heat in the summer.


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PASSIVE SOLAR ENERGY - IMAGE 65 PASSIVE SOLAR ENERGY - IMAGE 66 PASSIVE SOLAR ENERGY - IMAGE 67 PASSIVE SOLAR ENERGY - IMAGE 68
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The operation is quite simple. During wintertime conditions, insulating panels are rolled or pivoted back, exposing water contained in UV inhibiting water beds to the sun. The ponds gather the sun’s warmth and at nightfall, the insulating panels are replaced to contain the gained heat and prevent loss to the cold night air. The heat stored in the bags, warms the supporting metal decking and the entire ceiling is a radiant ceiling throughout the cold winter night. The next morning, the insulating panels are removed when the sun appears and the cycle begins again.

Summer cooling is a reverse process. Ponds, covered during the daytime heat, remain cool and act in concert with the supporting metal ceilings as a thermal sponge absorbing interior heat generated from people, equipment, and infiltration from the outside. At night, panels are removed and the ponds throw off their gathered heat to the night sky by means of radiation, convections, and if wet down, by evaporation. 

Roof pond heating and cooling is optimized when all living spaces except the bathrooms and high water use areas are covered by the system. In areas that generate humidity, like a shower, the metal ceilings will tend to “rain” due to the temperature difference of steam vapors and cool metal.

Roof ponds, like Harold Hay’s Skytherm system, have been designed and used in the hot climate of Arizona and New Mexico to the moderate temperatures of the California coast and even planned for the twin cities of Minneapolis/St. Paul.


Isolated Gain -

PASSIVE SOLAR ENERGY - IMAGE 75This is basically an indirect system where solar collection for heating are isolated from the living spaces, and while the system functions independently, heating can be called for by simply opening some floor vents and letting the natural behavior of hot air rise through the spaces. The most common application of this approach is the convective loop. Much like a thermosiphon water heater, heat transfer material of air or water, is moved across a collector panel system facing the sun, and circulated into a tank surrounded by rock (water transfer system) or a rock bin (air transfer system) in a continuous operating loop. Natural thermosiphoning occurs when the collector is lower than the heat storage area which is usually located under the building.

A hybrid of this system can include moving heated water or air through a radiant floor system where the masonry floor itself acts as the thermal mass storage. This variation can also use cool water to create a “cool” floor by running house supply water, or water from an adjacent pool, through a floor system.


Cool Towers -

PASSIVE SOLAR ENERGY - IMAGE 76 Evaporative cooling systems which utilize gravity effect on dense, cooled air to drop and spill into living spaces. The system is comprised of wet cooler pads mounted high in an area which provides no obstructions to air movement, which comes into contact with the pads. The warm dry air contacting the wet pads, cool and becomes more dense and heavier and falls down the tower, usually positioned over or adjacent to a major living space. The falling cool air, spills into the living space, pushing warmer out at strategic venting areas. As the process continues, the cooler air ponds in the area, providing a cool environment in Az. desert conditions.

A variation to this system is the addition of a south facing thermal chimney to pull cool tower air through the house. Located at an opposite location from the cool tower, the thermal chimney provides an escape vent for interior warm air , which moves more quickly as it get heated and is driven out. This rapid venting has a drawing effect on the cool tower air and it is distributed more extensively through the building. The solar chimney can be set up to become a recirculating air heater during winter conditions.


Natural Cooling -

There are three sources of undesirable heat - direct summer sun solar gains through windows and glazing; heat transmission through the building envelope; and internal heat produced by people, their activities, and their equipment. Direct solar heat gain at windows and glazing can be easily controlled by shading the house - preventing the sun from reaching it (except for good day lighting and operation of solar equipment) and with external shading devices and vegetation as well as thermal insulating shutters. Heat transmission conditions can be nullified by setting up layers of thermal decompression with vegetation, built structure like porches, and water features. While there is not much that can be done to reduce natural heat production by people, equipment heat generation can be impacted by careful selection of energy efficient equipment and by good timing - do the laundry in the evening.


SOLAR COOKING


PASSIVE SOLAR ENERGY - IMAGE 77 PASSIVE SOLAR ENERGY - IMAGE 78

Use of the sun for food preparation is fun, energy saving, and saves money, both in the cooking operation, and in the cooling costs saved when the heat is taken out of the kitchen during the summer. A variety of cooking tools from box cookers to slat faced ovens are available - whether they be commercial products or hand built by the inspiring solar chef.


Passive Solar Energy has many faces and applications and an effective Passive solar building incorporates many of these elements. Natural processes and incorporation of building and site elements to provide for comfort as well as mitigation of untoward conditions are the hallmark of good passive design, and results in establishing a basis for reduction of equipment (solar and otherwise) for achieving comfort, and reduction of equipment purchase and operations costs. Passive solar energy is Direct,

  • natural,

  • effective,

  • cost effective,

  • and inherent to the building form, structure, materials and use.

PASSIVE SOLAR ENERGY - IMAGE 79


This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Arizona Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources.


Financial support for this presentation has been provided by the Arizona Department of Commerce (Energy Office) and the U.S. Department of Energy through (DOE) Grant No. DE-FG51-01R021250. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the Energy Office or U.S. DOE. The State of Arizona and U.S. DOE assume no liability for damages arising from errors, omissions or representations contained in this presentation.

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Residential PV Stand Alone Systems Presentation

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Residential PV Stand Alone Systems Presentation

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Residential PV Grid-Tie Presentation

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Residential PV Grid-Tie Presentation

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Dutch entrepreneurs warm to solar energy

Hermann Scheer inspires Dutch participants at major conference

Press release; 5/5/2009

http://www.solarplaza.com/article/dutch-entrepreneurs-warm-to-solar-energy

When it comes to solar energy, Dutch entrepreneurs are successful all over the world. This was evident at The Solar Future in Rotterdam, the biggest solar energy conference ever held in the Netherlands. Here, with over 250 participants, it also emerged that a large group of new entrepreneurs are ready to invest in solar energy. Conspicuous by its absence from the conference, organized by Rotterdam-based company Solarplaza, was the Dutch government. One of the presentations demonstrated that the stop-go policy of different subsidy schemes has set the Netherlands back a long way within Europe. Flanders alone installed over 10 times more new PV capacity in 2008 – around 50 MegaWatt. Comparisons with Germany are even more painful. Germany instals the same amount in a day as the Netherlands does in a year.

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AZ Dept. of Commerce Report - Birth of a Technology

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The Birth of a Technology

French scientist Edmund Becquerel in 1839 discovered the photovoltaic effect – that light falling on certain materials can produce electricity.  Later physicist, including Albert Einstein, found that tiny photons, or particles of light, could interact with electrons surrounding the nucleus of an atom.  That interaction causes a free stream of electrons - the basis of electricity.

Scientists in the 1930s developed a theory for the electrical properties of silicon and other crystalline materials, substances that came to be known as “semi-conductors.”  Primitive photovoltaic cells were developed using selenium, but they were very expensive, only one percent efficient, and little more than a scientific curiosity.

Early in 1954, a small team of scientists at Bell Laboratories tried to find a practical way to generate electricity for telephone systems in rural areas not connected to power lines.  Using crystalline silicon, they fashioned an enormous solar cell capable of turning six percent of the sunlight that struck it into electricity.  Soon the efficiency was raised to eleven percent and scientists realized that the new devices could have practical applications. They had another reason for optimism; the material they were using, silicon, is the world’s second most abundant element, comprising 28 percent of the earth’s crust.

These achievements were greeted with much fanfare amid the technological developments of the 1950s.  Solar cells seemed to promise an unlimited supply of electricity and roused considerable excitement.  A 1957 article in “Business Week” envisioned an automatically controlled solar car in which “all riders could sit comfortably in the back seat and perhaps watch solar-powered TV.”

It was an unfavorable time, however, to develop a new energy technology.  Oil was priced at less than $2 per barrel, and large fossil-fuel power plants were built at a record pace.  Moreover, in 1954 construction began on the world’s first commercial nuclear reactor.  Nuclear power was envisioned as a source of electricity “too cheap to meter,” and most government energy funds were devoted to that technology.

Photovoltaic researchers also faced an unsettling economic reality. Silicon cells developed in the 1950s were extremely expensive, with costs as high as $600 per watt (compared to $5 or $6 today).  Funding for research to reduce the cost was not available in an era with falling electricity prices and minimal concerns about the environment.

The space program rescued photovoltaics from the technological scrap heap.  American scientists in the late 1950s went searching for a lightweight, long-lasting power source for satellites.  Photovoltaic cells, which could take advantage of the continuous sunlight of space, were their choice. In 1958 just four years after the Bell laboratory breakthrough, silicon solar cells were boosted into orbit aboard Vangard I, the second U.S. Satellite.

With the help of large contracts from the National Aeronautics (NASA) four U.S. commercial companies enter the photovoltaics business and by the late 1960’s were producing hundreds of thousands of solar cells a year.  Amid the heady competition of the post-sputnik space race, the Soviet Union began equipping its satellites with photovoltaics as well.  Today, solar cells power virtually all satellites.

Achievements in photovoltaic cell research during the peak years of the space program included a major increase in efficiency and reduction in cost of more than 400 percent.  However, the space-related PV market leveled-off and photovoltaic cell production declined.

In 1973, America was jolted by its first oil crisis – gasoline prices soared and interest in alternative fuels was awakened.  Beginning in 1975, the U.S. government funded a steadily growing research and development program aimed at making photovoltaics economical for terrestrial use.  Japan and European countries followed suit.  Perhaps a dozen private companies entered the solar cell research or production business.

Since the 1970s, the overall market for photovoltaics has increased more than ten-fold.  In the same period, the cost of PV modules has dropped from about $50 per watt to $5 or $6 per watt.  Production has increased to more than 250 times the highest level during the peak years of the space program, and can barely keep up with demand.

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AZ Dept. of Commerce Report - Fundamentals of Photovoltaics

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The Fundamentals of Photovoltaic Systems

The basic element of a photovoltaic system is the solar cell. Modern solar cells are approximately 4 inches square and are most often made from silicon, a semiconductor.

The photovoltaic effect occurs when sunlight shines on the silicon, freeing electrons and generating an electric current. The electricity is collected and transported by metal contacts on the top and bottom of the cell. The current flows through a wire to provide electricity.

Groups of cells are mounted on a rigid, rectangular plate and wired together to form a module often called a” panel” or “flat-plate collector.”  The module is sealed with plastic or glass for protection. Two or more modules connected together form an “array.”

Single cells produce little power and are not often used individually.  They can be found, however, in some items such as small yard lights. Individual panels, which usually produce 40 to 60 watts of electricity, power larger devices such as downtown Phoenix bus stop lights.  Arrays, depending on the number of panels used, can provide all the electricity for a home or even create a huge generating station.

Modules or arrays are sometimes mounted on tracking systems, which follow the sun across the sky.  These devices help maximize electricity production because sunlight shines directly on the PV modules throughout the day.  Single-axis trackers move as the sun changes position from the east to west.  Two-axis trackers not only follow the suns east to west movement, but also allow for its apparent change in attitude with different seasons. Trackers can increase the energy production of a photovoltaic system by nearly 40 percent.

Electricity storage is a critical component of many PV systems. If power is needed at night or on cloudy days, solar-generated electricity can be stored in batteries.  In large PV-powered homes, for example, it is not uncommon to find banks of 50 or more batteries.  These battery banks usually store sufficient electricity to power the home through one or two cloudy days and nights.

All photovoltaic, cells produce direct current (dc) electricity.  That electricity can be used immediately if the PV cell is connected to a device designed for dc power – many refrigerators in recreational vehicles, for example. However most homes and appliances are designed to operate on alternating current (ac) electricity provided by utility companies.  For applications, an inverter, which changes dc electricity to ac must be added to the PV system.

Charge controllers are also important components of many PV systems.  These devices protect batteries from excessive charge when the modules produce more electricity than the batteries can store.  They also keep batteries from releasing electricity if their charge is too low.  Without charge controllers, batteries suffer extreme wear-and-tear and become less effective, last a shorter amount of time and possibly even short circuit.

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AZ Dept. of Commerce Report - From Calculators to Power Plants

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From Calculators to Power Plants: PV Systems in Action

Photovoltaics systems are quite different from traditional methods of generating electricity. Their power production is directly affected by the weather and the time of day – I. e. they can’t produce electricity without sunshine. Ironically, photovoltaic cells are also affected by the sun’s heat becoming less efficient at high temperatures.

One important quality of photovoltaics is their flexibility. Unlike nuclear plants, for example, PV systems can be made small enough to power a hand-held calculator, or large enough to power an entire community. When the demand for electricity increases, a PV system can simply be enlarged, provided the owner can afford it.

Photovoltaic is a young technology and important questions remain as to how it should best be used. Among these questions are:  What kind of backup should be provided for nighttime or cloudy days?  should solar systems be installed at homes or at special generating stations?  and should utilities or individuals own and operate solar electric systems?

Despite these questions, photovoltaics are already used in hundreds of different ways.  Those applications fall into four broad categories: stand-alone systems, grid-connected units, central utility stations, and consumer products.  Each category is discussed below.

  • Stand-alone systems

As the name implies, stand-alone photovoltaic systems are virtually self-sufficient.  They provide all the electricity for a particular application, operating without a utility-line backup.  They are typically small systems, generating less than 10 kilowatts of electricity.

Stand-alone systems are most commonly found in areas far from power lines.  Ranchers, for example, often install solar-powered water pumps to replenish livestock watering holes in distant grazing areas.  These solar systems do not require constant refueling like diesel generators and often cost half as much as power line extensions.

More than 1200 Arizona homes or cabins rely on stand-alone photovoltaics as their main source of electricity.  In particular, PV powered homes are becoming a common sight on Arizona’s Native American reservations.  More than 140 homes on the Navajo Reservation get their electricity from the sun, and the Hopis are working toward 350 such homes.  The Hopis do not allow power lines to enter their villages and have relied for years on diesel generators or batteries, or have simply lived without electricity.

“Four villages have (utility) power lines running 1/8 to 1/4 miles from the village”, said Doran Dalton, sales manager for the Hopi Solar Electric Enterprise.  “We don’t hook up to the power lines because of our long-standing tradition of self-sufficiency.  We don’t have an objection to electricity, we simply want to own the source by which we get it.

Most Hopis live with very little electricity, and rely on small PV systems that power only a few lights and perhaps a television set.  Families that consume more energy install larger PV systems that provide electricity for all the conveniences of a modern home – microwave ovens computers, stereo systems, washing machines, evaporative coolers and lights.  Home PV systems can also power energy-consuming air conditioners and clothes dryers but these sizeable PV systems can cost more than $40,000.

Despite their vast electricity resources, utility companies also use stand-alone photovoltaic systems instead of extending costly power lines.  Many warning sirens at Palo Verde Nuclear Generating Station draw their power from solar panels.  Photovoltaics also furnish electricity for mountaintop microwave repeater stations owned by Salt River Project.

Stand-alone photovoltaics are used for numerous other applications in Arizona, including:  nearly 100 monitoring stations owned by the U.S. Geological Survey, some fire watch towers, a commercial radio station transmitter near Prescott, numerous emergency roadside telephones, billboards, lights and fans at the Picacho Peak Rest Area, and many irrigation or watering system controls.  At a recreation area near Roosevelt Lake, photovoltaics furnish all the electricity for indoor and outdoor lights and toilet fans.

  • Grid-connected systems

Homes or devices connected to photovoltaic systems as well as utility power lines are called “grid-connected systems.”  The utility can supply power at night, when electricity is cheapest, or simply serve as a backup. Federal law mandates that utility companies must purchase any excess power produced by the PV system, although at a reduced rate.

Homes or devices that employ both types of power are not common in Arizona, and the biggest reason is probably money.  Photovoltaics can be extremely cost-effective when compared to the price of extending power lines.  However, for individuals who live in cities or already have grid electricity, the choice to install PV is more difficult.  Homeowners must weigh the cost of utility bills – a few hundred dollars a month at the most – against the price of a photovoltaic system that can cost thousands of dollars for a typical house and a typical energy lifestyle.

As the price of PV decreases, and the cost of utility power increases, PV systems will compare more favorably for grid-connected applications.  In fact, utility companies like Salt River Project are already preparing for that possibility.  SRP has been researching a house in Chandler that employs electricity from both the grid and a small PV system.  “Our experience is that (the PV system) is reliable.  It produces power day after day with very little maintenance,” said Biff Hoffman, manager of SRP’s Research and Development division.

Individuals who are not connected to the utility grid sometimes install utility lines to power only one or two home items.  One Scottsdale couple lived without utility power for seven years, relying entirely on a PV system and propane generator backup.  This off-grid system was sufficient to power an evaporative cooler and the other electrical devices in their home.  When in 1990, temperatures reach 122 degrees Fahrenheit, they decided to install an air conditioner.  To meet the unit’s electrical demand, the couple had to choose between doubling the size of the solar system (another $17,000), or paying for a power line at less than $8,000.  The rest, as they say, is history.

The same couple discovered an interesting problem that sometimes occurs with photovoltaic systems. Electricity provided by their PV system did not follow perfect sine-wave form, as does utility power.  That change in the quality of electricity ruined their computer printer two times before the couple discovered the reason—a faulty inverter.

Because of the occasional problem in power quality, some utility companies require “power conditioning systems” between the home PV system and the grid.  These systems serve two functions: they ensure that electricity entering the grid is of the same quality as power produced by the utility company, and, they can serve as an off switch so that line workers are not hurt by electricity traveling from the photovoltaic system.  SRP determined that power-conditioning systems were necessary from its research on the Chandler house.

Some photovoltaic systems are connected to the grid for reasons not related to power requirements.  Bus stops in downtown Phoenix, for example, have a utility line back up to satisfy the city’s insurance requirements.

  • Central Power Plants

One day, enormous fields of photovoltaic arrays may stand among the Saguaro and Ocotillo of Arizona’s deserts. Connected to the utility grid, their combined power may equal production at large coal plants such as the Navajo Generating Station near Page.

Today, however, the world’s largest central photovoltaic power plant generates a maximum of only six megawatts of electricity – 125 times less than each of three units at the Navajo station.  This plant, called the Carrisa Plant project, was built in 1984 by ARCO Solar Corporation (now called “Siemens Solar”) and is connected to the grid owned by Pacific Gas & Electric Company.

The Carissa Plain plant covers dozens of acres of land with photovoltaic arrays mounted on two-axis trackers. Mirrors, placed next to the arrays, help reflect light to increase the potential power output.  Unfortunately, the intense reflected light has partially destroyed the protective module coatings and has actually decreased production of electricity.  Further development of better coatings should solve this problem.

For those reasons, the Carissa Plain plant is slated to be dismantled. Since, 1984, its only revenue has come from electricity sales to Pacific Gas and Electric Company.  The electricity is purchased at PG&E’s “avoided cost” of producing electricity – a price even lower than wholesale.  In the meantime, worldwide demand for photovoltaic panels has dramatically increased and their value has risen.  Consequently, it is more economical for the owners of Carissa Plain to sell the individual modules than to sell the electricity they produce.

Near Sacramento, California is another photovoltaic central power plant that has operated since 1984. The plant was built in two stages, called SMUDPV-1 and SMUD PV-2 and together they generate up to two megawatts of electricity at maximum production – enough to power 400 to 500 homes.  The total plant employs more than 58,000 photovoltaic modules and occupies more than 20 acres of land.

So far, three photovoltaic central power plants have been built in Arizona.  One completed in 1982 at Phoenix’s Sky Harbor Airport, was once the world’s largest grid-connected photovoltaic power plant.  It was designed to produce 225 kilowatts of power using concentrator solar modules.  The plant was dismantled in 1987, when the lease was not renewed.

APS has donated many of the panels from the plant to Arizona high schools and others are still being researched at the Solar Test and Research Center (STAR Center) in Tempe.

The Solar Test and Research Center was established in 1998 to test the effectiveness of different photovoltaic equipment in the Arizona climate.  It features five photovoltaic arrays, each producing 2 kilowatts of power used in the APS grid.  Many different types of photovoltaic cells are represented in the arrays.

Data obtained from the STAR Center provides valuable information about photovoltaic systems and how they operate.  For example APS has found that single-axis sun tracking systems improve electrical output by about 20 percent and double-axis trackers improve output another 20 percent.  They have also found that output decreases about 10 percent in midsummer, when the weather is hottest.

Arizona’s other photovoltaic central power plant is not utility-operated.  It belongs to a 24-home subdivision in Glendale, Arizona called “Solar One.”  The first-of-its-kind subdivision was constructed by John F Long Homes, with a photovoltaic field along its south side.  The 2600-panel system provides 192 kilowatts of electricity at peak output and provides much of the electricity used by homeowners during daylight hours.  The utility company provides nighttime power and purchases any excess produced by the PV system.  Until rates changed in 1991, some Solar One homeowners actually received refund checks from the utility company.  Photovoltaic central power plants have also received attention abroad.  They have been constructed in Denmark, Greece, Spain, Germany, Saudi Arabia and Japan.

Number cells or modules needed to power various applications

Number cells or modules

Item powered by photovoltaics

Electricity (in watts) produced at peak output

1 Small Cell

(1” X 2”

Calculator

.1 Watt

1 Standard Cell

(4” X 4”)

Small Yard Light

.5 Watt

Module  4’ x 1.5”

 

Color TV for 3 hours

60 Watts

110 Modules

(47 watts each)

A 1500-1800 square foot house with an evaporative cooler, not air conditioning

5.2 kilowatts

169 Modules

Same house with an air conditioner

8 kilowatts

2,600 Modules

Solar One, 24-home subdivision in Glendale, Arizona

192 kilowatts (PV system provides only part of total power used here)

58,000 Modules

SMUD PV-1 and PV-2 photovoltaic central power plant in California

2 megawatts

  • Consumer Products

From toys to security systems, ever-growing arrays of consumer products operate on electricity supplied by photovoltaic cells.  These products are available through catalogs, at many Arizona photovoltaic companies, and even in some department stores.

An estimated 200 million people already own PV-powered calculators and wristwatches. Other solar-powered devices include: portable camping lights, Frisbee-sized pool cleaners, small fans that roll up in car windows, and hats with tiny fans for extra cooling.

Car manufacturers such as Mazda and Audi now offer a “solar sunroof” option in some new car models.  These sunroofs, incorporating see-through photovoltaic cells, power ventilation systems that help cool a parked car as much as 20 degrees.  PV cells have also been used to power entire electric cars.

Landscape lights and security systems are practical, photovoltaic-powered products that are growing in popularity.  Depending upon the size and complexity of these systems, owners can avoid hundreds of dollars in expenses for digging power line trenches or hooking-up to a power line.

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AZ Dept. of Commerce Report - Solar Cells

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Single – Crystalline Cells

The oldest and most efficient type of photovoltaic cell is made from single-crystalline Silicon.  It is called single-crystalline because the atoms form a nearly perfect, regular lattice – if you could see into the cell, it would look exactly the same in almost every spot.  In these cells, electrons released during the photovoltaic effect have clear, unobstructed paths on which to travel.

Most silicon comes from ordinary sand and several steps are required to turn it into a crystalline solar cell.  The silicon must first be separated from the oxygen with which it is chemically bound.  Then it must be purified to a point where the material includes less than one non-silicon atom per billion.  The resulting semiconductor grade silicon is one of the world’s purest commercial materials and has a price tag of $40 to $50 per kilogram.

The process of growing crystalline silicon begins with a vat of extremely hot, liquid silicon.  A “seed” of single-crystal silicon on a long wire is placed inside the vat.  Then, over the course of many hours, the liquid silicon is cooled while the seed is slowly rotated and withdrawn.  As they cool, silicon atoms inside the vat bond with silicon atoms of the seed.  The slower and smoother the process, the more likely the atoms are to bond in the perfect lattice structure.

When the wire in fully removed, it holds a crystal about 8 inches in diameter and 3 feet long – the size of long salami.  It is cut into wafers, 8/1000 to 10/1000 of an inch thick with a diamond-edge blade and much of the silicon crystal, now worth hundreds of dollars per kilogram, is turned into dust in the process.  The wafers are polished, processed into cells, and mounted in modules.

More than a hundred industry and university research teams have worked to upgrade and automate the manufacture of crystalline silicon solar cells.  They try to further reduce the cost of purified silicon, to develop high-speed crystal pullers and water-slicing techniques, and to improve the overall design of modules.

One of the main objectives of PV research, however, has been to increase the efficiency with which photovoltaic modules convert sunlight into electricity.  Commercial solar modules typically turn 10 to 14 percent of the sunlight that strikes them into electricity.  In the laboratory, module efficiencies of more than 20 percent have been achieved.

NOTE: Photovoltaic conversion efficiency is generally based on module output rather than cell output. Modules include many connections and tiny wires in which electricity is lost.  Consequently, they give lower efficiencies than individual cells.

Polycrystalline Silicon Cells

Polycrystalline photovoltaic cells are exactly what the name implies – a patchwork quilt of single-crystalline silicon molecules.  Connections between these molecules are random and do not form a perfect lattice structure. Polycrystalline cells are less efficient than single-crystalline cells because released electrons cannot follow clear paths.

These cells are produced by pouring hot, liquid silicon into square molds or casts.  The silicon is cooled to form solid blocks, which are sliced like single-crystalline silicon.

These cells are less expensive to produce than single-crystalline cells because their manufacturing process does not require many careful hours of cooling and rotating silicon material.

The main challenge of polycrystalline cells is attaining a sufficiently high efficiency.  Typically, the boundaries between crystals impede the flow of electrons, resulting in module efficiencies of only 7 to 10 percent.

Concentrator cells

Concentrator cells employ lenses and mirrors to focus the sun’s light onto a high-efficiency, single-crystalline cell. Concentrators help gather sunlight so that a smaller-than-normal cell can produce the same amount of electricity as a standard module.  Efficiencies range from 15 to 20 percent with efficiencies as high as 26 percent for a single cell.

Although they use less of the costly photovoltaic material, other elements increase their cost.  Because of their lenses and mirrors, for example, concentrator cells must air directly at the sun.  A tracking system is crucial for effective operation.

Thin-film technologies

In the past decade, much progress has been made in developing and refining thin-film photographic cells.  These cells are created by depositing hot, liquid silicon or other semi-conductor materials onto glass, metal or plastic.

One thin-film technology, which is already employed in many PV modules, is called “amorphous silicon”. It is composed or randomly arranged atoms, forming a dense, noncrystalline material resembling glass.  The silicon layer is less than a millionth of a meter (a micron) thick requiring considerably less pure silicon then other cell types.

Researchers are working to obtain higher efficiency from this material, which lacks the ordered structure and inherent photovoltaic properties of crystalline silicon.  Today’s commercial efficiency average 5 to 6 percent but efficiencies as high as 14.5 percent have been exhibited in laboratories.

Tandem Cells

These cells are still in the developmental stage but offer great potential for the future of photovoltaics.  Tandem, or multiple-junction cells, are actually several cells stacked on top of each other.  Each cell layer is able to convert a different wavelength, or color, of the light spectrum into electricity.

Tandem cells have displayed efficiencies higher than 14 percent in the laboratory and theorist predict efficiencies as 35 to 40 percent.

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AZ Dept. of Commerce Report - Sizing a Photovoltaic System

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Sizing a Photovoltaic System

(What follows is a quick and easy method for sizing photovoltaic systems. However, the formula is not intended

to be a sizing procedure for design purposes, but is offered only as an aid to readers seeking preliminary sizing information.  For accuracy in system design, the guidance of an experienced photovoltaic system designer is highly recommended.)

The size, and subsequently the cost, of a photovoltaic system depend upon two factors: the electrical requirements of the devices (loads) relying on the system and the amount of sunshine available to power the system.  Both factors determine the quantity and size of panels, batteries, and other components.

  • The Load

The load is defined as the amount of electric power being consumed at any given moment.  To determine the load, it is necessary to select the units that will rely on the system for power.  If the system is used to power a home, the load will consist of appliances, lights and other common home items.

The next step is to determine the wattage of each item.  The wattage of a device is usually stamped or printed on a nameplate or identification plate on the rear of the unit.  If the unit lists VA (volts x amps), that is the wattage.  If only amps are listed, multiply the amps by the volts listed to find the wattage.

Finally, decide how many hours per day (average) each item is to be used.  The load estimate must be as precise as possible to avoid oversizing or undersizing the system. If design is oversized, money is wasted on excess capacity. If it is undersized, power shortages during operation may result.

The average daily load then, is found by the following formula:

Daily load = wattage X time in use (DL = W x T) DL = 100 watts x 2 hours DL = 200 watt-hours

The load profile, together with the amount of sunshine, can be used to determine the size of the array.

  • Available Sunshine

Sunshine is rated in peak hours, the hours of the day at which you can expect the maximum rated performance from a solar panel.  On average, Arizona has six peak hours of sun daily.

Panels are rated in peak watts, the amount of electricity they can produce at peak sun. Consequently, the number of watt-hours available from a panel is found by this formula:

Panel                 Number hours peak sun
Watt-hours = x rated panel output
(Wh = Sp x P)
Wh = 6 hours x 40 watts
Wh = 240 watt-hours per panel

Size of the Array

Because batteries and inverters consume a certain amount of the power generated by the solar cells, it is wise to allow for at least a 20 percent safety factor over and above the exact calculated load needs.

The number of panels is thus calculated as follows:

number of panels =

 

(Daily load x 1.2)

needed

Watt-hours

Number of Batteries

To determine the number of batteries required for the daily load, the owner must decide how many days of reserve he or she desires.  Storage batteries must be capable of operating the load during periods of little or no sun, without any electricity generated by the photovoltaic array.  (A PV system that requires one to five days’ storage capacity should be outfitted with special, deep-cycle batteries.)

To determine the number of batteries required, the designer must know how much energy the batteries can store (energy capacity) and compare that to the daily load and desired reserve.  Batteries are normally rated in amp-hours instead of watt-hours.  To convert to watt-hours, use the following formula. (Battery capacity and discharge average voltage are usually stamped on the battery)

Battery Daily load (watt-hours) x Capacity =  discharge average voltage in watt hours

Once the battery capacity is knows, the number of batteries required can be calculated from the figures previously determined as follows:

 

Number

Batteries=

Watt-hours       Days

required     x   reserve

by load

 

needed

Energy capacity

(watt-hours)

per battery

 

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AZ Dept. of Commerce Report - Glossary

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Glossary of Terms

Alternating Current:  Electric Current in which the direction of flow is reversed at frequent intervals, 60 cycles per second.  The type of electrical supplied by utility companies.

Ampere:  A measure of electric current; the flow of electrons.

Amorphous:  The condition of a solid in which the atoms are not arranged in an orderly pattern.

Atoms: The smallest particle of an element that can exist either alone or in combination.

Battery:  Rechargeable electric storage unit that operates on the principle of changing electrical energy into chemical energy by means of a reversible chemical reaction.

Break-even:  The cost of a photovoltaic system at which the cost of the electricity it produces exactly equals the price of electricity from a competing source.

Concentrators:  A photovoltaic module that includes an optical component such as a lens or focusing mirror to direct incident sunlight onto a solar cell or panel.

Direct current:  Electric current that always flows in the same direction: positive to negative. Batteries and photovoltaic cells are all DC devices.

Efficiency:  The percentage of available sunlight converted to electricity by a module or cell.

Electric current:  The volume of electricity propelled by the voltage; a flow of electrons.

Electrons:  An elementary particle consisting of a charge of negative electricity.

Grid-connected:  A house or device that receives power from a utility company.

Inverter:  This appliance converts independent DC-power into AC power, or regular household current.

Load:  The lights or appliances run by your electrical system. The amount of electrical power being consumed at any given moment.

Kilowatt:  A thousand watts

Megawatt:  One million watts, 1,000 kilowatts

Off-grid:  See stand-alone.

Peak watts:  The amount of power a photovoltaic device will produce at noon on a clear day with sun approximately overhead when the cell is faced directly toward the sun.

Photovoltaic:  Capable of producing a voltage when exposed to radiant energy, especially light.

Photovoltaic array:  A group of solar electric panels connected together.

Photovoltaic cell:  The basic building block in photovoltaic systems. Sometimes called “Solar Cells.”

Photovoltaic effect:  The conversion of sunlight absorbed by a solar cell directly into electricity.

Photovoltaic module:  A solar electric panel

Semiconductor:  Any material that has limited capacity for conducting an electric curre.

Stand-alone:  An isolated photovoltaic system not connected to a grid.

Voltage:  A measure of the force or “push” given the electrons in an electric circuit, a measure of electric potential.

Voltage regulator:  An electrical device used to keep voltage at predetermined levels.

Watt-hours:  Sometimes called watt. The amount of watts used by an appliance is an hour.



A publication of the Arizona Department of Commerce Energy Office 3800 N. Central Ave, Suite 1200, Phoenix, Arizona  85012, (602) 280-1402, or call toll-free in Arizona (800) 352-5499. Fourth Edition:  July 1992.

Editor - Norma Dulin Gurovich , Written by Norma Dulin Gurovich and Jim Arwood.  Much of this information was Adapted from the following publications: Bringing Solar Electricity to Earth, Electric Power Research Institute, June, 1990. Photovoltaics: Entering the 1990s, National Renewable Energy Laboratory, November 1989.  Solar Energy:  A Powerful Ally, Salt River Project, 198?.  Today’s Photovoltaic Systems:  An evaluation of Their Performance, Sandia National Laboratories, 1987.

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AZ Dept. of Commerce Report - Birth of a Technology

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The Birth of a Technology

French scientist Edmund Becquerel in 1839 discovered the photovoltaic effect – that light falling on certain materials can produce electricity.  Later physicist, including Albert Einstein, found that tiny photons, or particles of light, could interact with electrons surrounding the nucleus of an atom.  That interaction causes a free stream of electrons - the basis of electricity.

Scientists in the 1930s developed a theory for the electrical properties of silicon and other crystalline materials, substances that came to be known as “semi-conductors.”  Primitive photovoltaic cells were developed using selenium, but they were very expensive, only one percent efficient, and little more than a scientific curiosity.

Early in 1954, a small team of scientists at Bell Laboratories tried to find a practical way to generate electricity for telephone systems in rural areas not connected to power lines.  Using crystalline silicon, they fashioned an enormous solar cell capable of turning six percent of the sunlight that struck it into electricity.  Soon the efficiency was raised to eleven percent and scientists realized that the new devices could have practical applications. They had another reason for optimism; the material they were using, silicon, is the world’s second most abundant element, comprising 28 percent of the earth’s crust.

These achievements were greeted with much fanfare amid the technological developments of the 1950s.  Solar cells seemed to promise an unlimited supply of electricity and roused considerable excitement.  A 1957 article in “Business Week” envisioned an automatically controlled solar car in which “all riders could sit comfortably in the back seat and perhaps watch solar-powered TV.”

It was an unfavorable time, however, to develop a new energy technology.  Oil was priced at less than $2 per barrel, and large fossil-fuel power plants were built at a record pace.  Moreover, in 1954 construction began on the world’s first commercial nuclear reactor.  Nuclear power was envisioned as a source of electricity “too cheap to meter,” and most government energy funds were devoted to that technology.

Photovoltaic researchers also faced an unsettling economic reality. Silicon cells developed in the 1950s were extremely expensive, with costs as high as $600 per watt (compared to $5 or $6 today).  Funding for research to reduce the cost was not available in an era with falling electricity prices and minimal concerns about the environment.

The space program rescued photovoltaics from the technological scrap heap.  American scientists in the late 1950s went searching for a lightweight, long-lasting power source for satellites.  Photovoltaic cells, which could take advantage of the continuous sunlight of space, were their choice. In 1958 just four years after the Bell laboratory breakthrough, silicon solar cells were boosted into orbit aboard Vangard I, the second U.S. Satellite.

With the help of large contracts from the National Aeronautics (NASA) four U.S. commercial companies enter the photovoltaics business and by the late 1960’s were producing hundreds of thousands of solar cells a year.  Amid the heady competition of the post-sputnik space race, the Soviet Union began equipping its satellites with photovoltaics as well.  Today, solar cells power virtually all satellites.

Achievements in photovoltaic cell research during the peak years of the space program included a major increase in efficiency and reduction in cost of more than 400 percent.  However, the space-related PV market leveled-off and photovoltaic cell production declined.

In 1973, America was jolted by its first oil crisis – gasoline prices soared and interest in alternative fuels was awakened.  Beginning in 1975, the U.S. government funded a steadily growing research and development program aimed at making photovoltaics economical for terrestrial use.  Japan and European countries followed suit.  Perhaps a dozen private companies entered the solar cell research or production business.

Since the 1970s, the overall market for photovoltaics has increased more than ten-fold.  In the same period, the cost of PV modules has dropped from about $50 per watt to $5 or $6 per watt.  Production has increased to more than 250 times the highest level during the peak years of the space program, and can barely keep up with demand.

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AZ Dept. of Commerce Report - From Calculators to Power Plants

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From Calculators to Power Plants: PV Systems in Action

Photovoltaics systems are quite different from traditional methods of generating electricity. Their power production is directly affected by the weather and the time of day – I. e. they can’t produce electricity without sunshine. Ironically, photovoltaic cells are also affected by the sun’s heat becoming less efficient at high temperatures.

One important quality of photovoltaics is their flexibility. Unlike nuclear plants, for example, PV systems can be made small enough to power a hand-held calculator, or large enough to power an entire community. When the demand for electricity increases, a PV system can simply be enlarged, provided the owner can afford it.

Photovoltaic is a young technology and important questions remain as to how it should best be used. Among these questions are:  What kind of backup should be provided for nighttime or cloudy days?  should solar systems be installed at homes or at special generating stations?  and should utilities or individuals own and operate solar electric systems?

Despite these questions, photovoltaics are already used in hundreds of different ways.  Those applications fall into four broad categories: stand-alone systems, grid-connected units, central utility stations, and consumer products.  Each category is discussed below.

  • Stand-alone systems

As the name implies, stand-alone photovoltaic systems are virtually self-sufficient.  They provide all the electricity for a particular application, operating without a utility-line backup.  They are typically small systems, generating less than 10 kilowatts of electricity.

Stand-alone systems are most commonly found in areas far from power lines.  Ranchers, for example, often install solar-powered water pumps to replenish livestock watering holes in distant grazing areas.  These solar systems do not require constant refueling like diesel generators and often cost half as much as power line extensions.

More than 1200 Arizona homes or cabins rely on stand-alone photovoltaics as their main source of electricity.  In particular, PV powered homes are becoming a common sight on Arizona’s Native American reservations.  More than 140 homes on the Navajo Reservation get their electricity from the sun, and the Hopis are working toward 350 such homes.  The Hopis do not allow power lines to enter their villages and have relied for years on diesel generators or batteries, or have simply lived without electricity.

“Four villages have (utility) power lines running 1/8 to 1/4 miles from the village”, said Doran Dalton, sales manager for the Hopi Solar Electric Enterprise.  “We don’t hook up to the power lines because of our long-standing tradition of self-sufficiency.  We don’t have an objection to electricity, we simply want to own the source by which we get it.

Most Hopis live with very little electricity, and rely on small PV systems that power only a few lights and perhaps a television set.  Families that consume more energy install larger PV systems that provide electricity for all the conveniences of a modern home – microwave ovens computers, stereo systems, washing machines, evaporative coolers and lights.  Home PV systems can also power energy-consuming air conditioners and clothes dryers but these sizeable PV systems can cost more than $40,000.

Despite their vast electricity resources, utility companies also use stand-alone photovoltaic systems instead of extending costly power lines.  Many warning sirens at Palo Verde Nuclear Generating Station draw their power from solar panels.  Photovoltaics also furnish electricity for mountaintop microwave repeater stations owned by Salt River Project.

Stand-alone photovoltaics are used for numerous other applications in Arizona, including:  nearly 100 monitoring stations owned by the U.S. Geological Survey, some fire watch towers, a commercial radio station transmitter near Prescott, numerous emergency roadside telephones, billboards, lights and fans at the Picacho Peak Rest Area, and many irrigation or watering system controls.  At a recreation area near Roosevelt Lake, photovoltaics furnish all the electricity for indoor and outdoor lights and toilet fans.

  • Grid-connected systems

Homes or devices connected to photovoltaic systems as well as utility power lines are called “grid-connected systems.”  The utility can supply power at night, when electricity is cheapest, or simply serve as a backup. Federal law mandates that utility companies must purchase any excess power produced by the PV system, although at a reduced rate.

Homes or devices that employ both types of power are not common in Arizona, and the biggest reason is probably money.  Photovoltaics can be extremely cost-effective when compared to the price of extending power lines.  However, for individuals who live in cities or already have grid electricity, the choice to install PV is more difficult.  Homeowners must weigh the cost of utility bills – a few hundred dollars a month at the most – against the price of a photovoltaic system that can cost thousands of dollars for a typical house and a typical energy lifestyle.

As the price of PV decreases, and the cost of utility power increases, PV systems will compare more favorably for grid-connected applications.  In fact, utility companies like Salt River Project are already preparing for that possibility.  SRP has been researching a house in Chandler that employs electricity from both the grid and a small PV system.  “Our experience is that (the PV system) is reliable.  It produces power day after day with very little maintenance,” said Biff Hoffman, manager of SRP’s Research and Development division.

Individuals who are not connected to the utility grid sometimes install utility lines to power only one or two home items.  One Scottsdale couple lived without utility power for seven years, relying entirely on a PV system and propane generator backup.  This off-grid system was sufficient to power an evaporative cooler and the other electrical devices in their home.  When in 1990, temperatures reach 122 degrees Fahrenheit, they decided to install an air conditioner.  To meet the unit’s electrical demand, the couple had to choose between doubling the size of the solar system (another $17,000), or paying for a power line at less than $8,000.  The rest, as they say, is history.

The same couple discovered an interesting problem that sometimes occurs with photovoltaic systems. Electricity provided by their PV system did not follow perfect sine-wave form, as does utility power.  That change in the quality of electricity ruined their computer printer two times before the couple discovered the reason—a faulty inverter.

Because of the occasional problem in power quality, some utility companies require “power conditioning systems” between the home PV system and the grid.  These systems serve two functions: they ensure that electricity entering the grid is of the same quality as power produced by the utility company, and, they can serve as an off switch so that line workers are not hurt by electricity traveling from the photovoltaic system.  SRP determined that power-conditioning systems were necessary from its research on the Chandler house.

Some photovoltaic systems are connected to the grid for reasons not related to power requirements.  Bus stops in downtown Phoenix, for example, have a utility line back up to satisfy the city’s insurance requirements.

  • Central Power Plants

One day, enormous fields of photovoltaic arrays may stand among the Saguaro and Ocotillo of Arizona’s deserts. Connected to the utility grid, their combined power may equal production at large coal plants such as the Navajo Generating Station near Page.

Today, however, the world’s largest central photovoltaic power plant generates a maximum of only six megawatts of electricity – 125 times less than each of three units at the Navajo station.  This plant, called the Carrisa Plant project, was built in 1984 by ARCO Solar Corporation (now called “Siemens Solar”) and is connected to the grid owned by Pacific Gas & Electric Company.

The Carissa Plain plant covers dozens of acres of land with photovoltaic arrays mounted on two-axis trackers. Mirrors, placed next to the arrays, help reflect light to increase the potential power output.  Unfortunately, the intense reflected light has partially destroyed the protective module coatings and has actually decreased production of electricity.  Further development of better coatings should solve this problem.

For those reasons, the Carissa Plain plant is slated to be dismantled. Since, 1984, its only revenue has come from electricity sales to Pacific Gas and Electric Company.  The electricity is purchased at PG&E’s “avoided cost” of producing electricity – a price even lower than wholesale.  In the meantime, worldwide demand for photovoltaic panels has dramatically increased and their value has risen.  Consequently, it is more economical for the owners of Carissa Plain to sell the individual modules than to sell the electricity they produce.

Near Sacramento, California is another photovoltaic central power plant that has operated since 1984. The plant was built in two stages, called SMUDPV-1 and SMUD PV-2 and together they generate up to two megawatts of electricity at maximum production – enough to power 400 to 500 homes.  The total plant employs more than 58,000 photovoltaic modules and occupies more than 20 acres of land.

So far, three photovoltaic central power plants have been built in Arizona.  One completed in 1982 at Phoenix’s Sky Harbor Airport, was once the world’s largest grid-connected photovoltaic power plant.  It was designed to produce 225 kilowatts of power using concentrator solar modules.  The plant was dismantled in 1987, when the lease was not renewed.

APS has donated many of the panels from the plant to Arizona high schools and others are still being researched at the Solar Test and Research Center (STAR Center) in Tempe.

The Solar Test and Research Center was established in 1998 to test the effectiveness of different photovoltaic equipment in the Arizona climate.  It features five photovoltaic arrays, each producing 2 kilowatts of power used in the APS grid.  Many different types of photovoltaic cells are represented in the arrays.

Data obtained from the STAR Center provides valuable information about photovoltaic systems and how they operate.  For example APS has found that single-axis sun tracking systems improve electrical output by about 20 percent and double-axis trackers improve output another 20 percent.  They have also found that output decreases about 10 percent in midsummer, when the weather is hottest.

Arizona’s other photovoltaic central power plant is not utility-operated.  It belongs to a 24-home subdivision in Glendale, Arizona called “Solar One.”  The first-of-its-kind subdivision was constructed by John F Long Homes, with a photovoltaic field along its south side.  The 2600-panel system provides 192 kilowatts of electricity at peak output and provides much of the electricity used by homeowners during daylight hours.  The utility company provides nighttime power and purchases any excess produced by the PV system.  Until rates changed in 1991, some Solar One homeowners actually received refund checks from the utility company.  Photovoltaic central power plants have also received attention abroad.  They have been constructed in Denmark, Greece, Spain, Germany, Saudi Arabia and Japan.

Number cells or modules needed to power various applications

Number cells or modules

Item powered by photovoltaics

Electricity (in watts) produced at peak output

1 Small Cell

(1” X 2”

Calculator

.1 Watt

1 Standard Cell

(4” X 4”)

Small Yard Light

.5 Watt

Module  4’ x 1.5”

 

Color TV for 3 hours

60 Watts

110 Modules

(47 watts each)

A 1500-1800 square foot house with an evaporative cooler, not air conditioning

5.2 kilowatts

169 Modules

Same house with an air conditioner

8 kilowatts

2,600 Modules

Solar One, 24-home subdivision in Glendale, Arizona

192 kilowatts (PV system provides only part of total power used here)

58,000 Modules

SMUD PV-1 and PV-2 photovoltaic central power plant in California

2 megawatts

  • Consumer Products

From toys to security systems, ever-growing arrays of consumer products operate on electricity supplied by photovoltaic cells.  These products are available through catalogs, at many Arizona photovoltaic companies, and even in some department stores.

An estimated 200 million people already own PV-powered calculators and wristwatches. Other solar-powered devices include: portable camping lights, Frisbee-sized pool cleaners, small fans that roll up in car windows, and hats with tiny fans for extra cooling.

Car manufacturers such as Mazda and Audi now offer a “solar sunroof” option in some new car models.  These sunroofs, incorporating see-through photovoltaic cells, power ventilation systems that help cool a parked car as much as 20 degrees.  PV cells have also been used to power entire electric cars.

Landscape lights and security systems are practical, photovoltaic-powered products that are growing in popularity.  Depending upon the size and complexity of these systems, owners can avoid hundreds of dollars in expenses for digging power line trenches or hooking-up to a power line.

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AZ Dept. of Commerce Report - Fundamentals of Photovoltaics

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The Fundamentals of Photovoltaic Systems

The basic element of a photovoltaic system is the solar cell. Modern solar cells are approximately 4 inches square and are most often made from silicon, a semiconductor.

The photovoltaic effect occurs when sunlight shines on the silicon, freeing electrons and generating an electric current. The electricity is collected and transported by metal contacts on the top and bottom of the cell. The current flows through a wire to provide electricity.

Groups of cells are mounted on a rigid, rectangular plate and wired together to form a module often called a” panel” or “flat-plate collector.”  The module is sealed with plastic or glass for protection. Two or more modules connected together form an “array.”

Single cells produce little power and are not often used individually.  They can be found, however, in some items such as small yard lights. Individual panels, which usually produce 40 to 60 watts of electricity, power larger devices such as downtown Phoenix bus stop lights.  Arrays, depending on the number of panels used, can provide all the electricity for a home or even create a huge generating station.

Modules or arrays are sometimes mounted on tracking systems, which follow the sun across the sky.  These devices help maximize electricity production because sunlight shines directly on the PV modules throughout the day.  Single-axis trackers move as the sun changes position from the east to west.  Two-axis trackers not only follow the suns east to west movement, but also allow for its apparent change in attitude with different seasons. Trackers can increase the energy production of a photovoltaic system by nearly 40 percent.

Electricity storage is a critical component of many PV systems. If power is needed at night or on cloudy days, solar-generated electricity can be stored in batteries.  In large PV-powered homes, for example, it is not uncommon to find banks of 50 or more batteries.  These battery banks usually store sufficient electricity to power the home through one or two cloudy days and nights.

All photovoltaic, cells produce direct current (dc) electricity.  That electricity can be used immediately if the PV cell is connected to a device designed for dc power – many refrigerators in recreational vehicles, for example. However most homes and appliances are designed to operate on alternating current (ac) electricity provided by utility companies.  For applications, an inverter, which changes dc electricity to ac must be added to the PV system.

Charge controllers are also important components of many PV systems.  These devices protect batteries from excessive charge when the modules produce more electricity than the batteries can store.  They also keep batteries from releasing electricity if their charge is too low.  Without charge controllers, batteries suffer extreme wear-and-tear and become less effective, last a shorter amount of time and possibly even short circuit.

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AZ Dept. of Commerce Report - Glossary

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Glossary of Terms

Alternating Current:  Electric Current in which the direction of flow is reversed at frequent intervals, 60 cycles per second.  The type of electrical supplied by utility companies.

Ampere:  A measure of electric current; the flow of electrons.

Amorphous:  The condition of a solid in which the atoms are not arranged in an orderly pattern.

Atoms: The smallest particle of an element that can exist either alone or in combination.

Battery:  Rechargeable electric storage unit that operates on the principle of changing electrical energy into chemical energy by means of a reversible chemical reaction.

Break-even:  The cost of a photovoltaic system at which the cost of the electricity it produces exactly equals the price of electricity from a competing source.

Concentrators:  A photovoltaic module that includes an optical component such as a lens or focusing mirror to direct incident sunlight onto a solar cell or panel.

Direct current:  Electric current that always flows in the same direction: positive to negative. Batteries and photovoltaic cells are all DC devices.

Efficiency:  The percentage of available sunlight converted to electricity by a module or cell.

Electric current:  The volume of electricity propelled by the voltage; a flow of electrons.

Electrons:  An elementary particle consisting of a charge of negative electricity.

Grid-connected:  A house or device that receives power from a utility company.

Inverter:  This appliance converts independent DC-power into AC power, or regular household current.

Load:  The lights or appliances run by your electrical system. The amount of electrical power being consumed at any given moment.

Kilowatt:  A thousand watts

Megawatt:  One million watts, 1,000 kilowatts

Off-grid:  See stand-alone.

Peak watts:  The amount of power a photovoltaic device will produce at noon on a clear day with sun approximately overhead when the cell is faced directly toward the sun.

Photovoltaic:  Capable of producing a voltage when exposed to radiant energy, especially light.

Photovoltaic array:  A group of solar electric panels connected together.

Photovoltaic cell:  The basic building block in photovoltaic systems. Sometimes called “Solar Cells.”

Photovoltaic effect:  The conversion of sunlight absorbed by a solar cell directly into electricity.

Photovoltaic module:  A solar electric panel

Semiconductor:  Any material that has limited capacity for conducting an electric curre.

Stand-alone:  An isolated photovoltaic system not connected to a grid.

Voltage:  A measure of the force or “push” given the electrons in an electric circuit, a measure of electric potential.

Voltage regulator:  An electrical device used to keep voltage at predetermined levels.

Watt-hours:  Sometimes called watt. The amount of watts used by an appliance is an hour.



A publication of the Arizona Department of Commerce Energy Office 3800 N. Central Ave, Suite 1200, Phoenix, Arizona  85012, (602) 280-1402, or call toll-free in Arizona (800) 352-5499. Fourth Edition:  July 1992.

Editor - Norma Dulin Gurovich , Written by Norma Dulin Gurovich and Jim Arwood.  Much of this information was Adapted from the following publications: Bringing Solar Electricity to Earth, Electric Power Research Institute, June, 1990. Photovoltaics: Entering the 1990s, National Renewable Energy Laboratory, November 1989.  Solar Energy:  A Powerful Ally, Salt River Project, 198?.  Today’s Photovoltaic Systems:  An evaluation of Their Performance, Sandia National Laboratories, 1987.

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AZ Dept. of Commerce Report - Sizing a Photovoltaic System

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Sizing a Photovoltaic System

(What follows is a quick and easy method for sizing photovoltaic systems. However, the formula is not intended

to be a sizing procedure for design purposes, but is offered only as an aid to readers seeking preliminary sizing information.  For accuracy in system design, the guidance of an experienced photovoltaic system designer is highly recommended.)

The size, and subsequently the cost, of a photovoltaic system depend upon two factors: the electrical requirements of the devices (loads) relying on the system and the amount of sunshine available to power the system.  Both factors determine the quantity and size of panels, batteries, and other components.

  • The Load

The load is defined as the amount of electric power being consumed at any given moment.  To determine the load, it is necessary to select the units that will rely on the system for power.  If the system is used to power a home, the load will consist of appliances, lights and other common home items.

The next step is to determine the wattage of each item.  The wattage of a device is usually stamped or printed on a nameplate or identification plate on the rear of the unit.  If the unit lists VA (volts x amps), that is the wattage.  If only amps are listed, multiply the amps by the volts listed to find the wattage.

Finally, decide how many hours per day (average) each item is to be used.  The load estimate must be as precise as possible to avoid oversizing or undersizing the system. If design is oversized, money is wasted on excess capacity. If it is undersized, power shortages during operation may result.

The average daily load then, is found by the following formula:

Daily load = wattage X time in use (DL = W x T) DL = 100 watts x 2 hours DL = 200 watt-hours

The load profile, together with the amount of sunshine, can be used to determine the size of the array.

  • Available Sunshine

Sunshine is rated in peak hours, the hours of the day at which you can expect the maximum rated performance from a solar panel.  On average, Arizona has six peak hours of sun daily.

Panels are rated in peak watts, the amount of electricity they can produce at peak sun. Consequently, the number of watt-hours available from a panel is found by this formula:

Panel                 Number hours peak sun
Watt-hours = x rated panel output
(Wh = Sp x P)
Wh = 6 hours x 40 watts
Wh = 240 watt-hours per panel

Size of the Array

Because batteries and inverters consume a certain amount of the power generated by the solar cells, it is wise to allow for at least a 20 percent safety factor over and above the exact calculated load needs.

The number of panels is thus calculated as follows:

number of panels =

 

(Daily load x 1.2)

needed

Watt-hours

Number of Batteries

To determine the number of batteries required for the daily load, the owner must decide how many days of reserve he or she desires.  Storage batteries must be capable of operating the load during periods of little or no sun, without any electricity generated by the photovoltaic array.  (A PV system that requires one to five days’ storage capacity should be outfitted with special, deep-cycle batteries.)

To determine the number of batteries required, the designer must know how much energy the batteries can store (energy capacity) and compare that to the daily load and desired reserve.  Batteries are normally rated in amp-hours instead of watt-hours.  To convert to watt-hours, use the following formula. (Battery capacity and discharge average voltage are usually stamped on the battery)

Battery Daily load (watt-hours) x Capacity =  discharge average voltage in watt hours

Once the battery capacity is knows, the number of batteries required can be calculated from the figures previously determined as follows:

 

Number

Batteries=

Watt-hours       Days

required     x   reserve

by load

 

needed

Energy capacity

(watt-hours)

per battery

 

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AZ Dept. of Commerce Report - Solar Cells

#azdoc-rprt-hdg#

Single – Crystalline Cells

The oldest and most efficient type of photovoltaic cell is made from single-crystalline Silicon.  It is called single-crystalline because the atoms form a nearly perfect, regular lattice – if you could see into the cell, it would look exactly the same in almost every spot.  In these cells, electrons released during the photovoltaic effect have clear, unobstructed paths on which to travel.

Most silicon comes from ordinary sand and several steps are required to turn it into a crystalline solar cell.  The silicon must first be separated from the oxygen with which it is chemically bound.  Then it must be purified to a point where the material includes less than one non-silicon atom per billion.  The resulting semiconductor grade silicon is one of the world’s purest commercial materials and has a price tag of $40 to $50 per kilogram.

The process of growing crystalline silicon begins with a vat of extremely hot, liquid silicon.  A “seed” of single-crystal silicon on a long wire is placed inside the vat.  Then, over the course of many hours, the liquid silicon is cooled while the seed is slowly rotated and withdrawn.  As they cool, silicon atoms inside the vat bond with silicon atoms of the seed.  The slower and smoother the process, the more likely the atoms are to bond in the perfect lattice structure.

When the wire in fully removed, it holds a crystal about 8 inches in diameter and 3 feet long – the size of long salami.  It is cut into wafers, 8/1000 to 10/1000 of an inch thick with a diamond-edge blade and much of the silicon crystal, now worth hundreds of dollars per kilogram, is turned into dust in the process.  The wafers are polished, processed into cells, and mounted in modules.

More than a hundred industry and university research teams have worked to upgrade and automate the manufacture of crystalline silicon solar cells.  They try to further reduce the cost of purified silicon, to develop high-speed crystal pullers and water-slicing techniques, and to improve the overall design of modules.

One of the main objectives of PV research, however, has been to increase the efficiency with which photovoltaic modules convert sunlight into electricity.  Commercial solar modules typically turn 10 to 14 percent of the sunlight that strikes them into electricity.  In the laboratory, module efficiencies of more than 20 percent have been achieved.

NOTE: Photovoltaic conversion efficiency is generally based on module output rather than cell output. Modules include many connections and tiny wires in which electricity is lost.  Consequently, they give lower efficiencies than individual cells.

Polycrystalline Silicon Cells

Polycrystalline photovoltaic cells are exactly what the name implies – a patchwork quilt of single-crystalline silicon molecules.  Connections between these molecules are random and do not form a perfect lattice structure. Polycrystalline cells are less efficient than single-crystalline cells because released electrons cannot follow clear paths.

These cells are produced by pouring hot, liquid silicon into square molds or casts.  The silicon is cooled to form solid blocks, which are sliced like single-crystalline silicon.

These cells are less expensive to produce than single-crystalline cells because their manufacturing process does not require many careful hours of cooling and rotating silicon material.

The main challenge of polycrystalline cells is attaining a sufficiently high efficiency.  Typically, the boundaries between crystals impede the flow of electrons, resulting in module efficiencies of only 7 to 10 percent.

Concentrator cells

Concentrator cells employ lenses and mirrors to focus the sun’s light onto a high-efficiency, single-crystalline cell. Concentrators help gather sunlight so that a smaller-than-normal cell can produce the same amount of electricity as a standard module.  Efficiencies range from 15 to 20 percent with efficiencies as high as 26 percent for a single cell.

Although they use less of the costly photovoltaic material, other elements increase their cost.  Because of their lenses and mirrors, for example, concentrator cells must air directly at the sun.  A tracking system is crucial for effective operation.

Thin-film technologies

In the past decade, much progress has been made in developing and refining thin-film photographic cells.  These cells are created by depositing hot, liquid silicon or other semi-conductor materials onto glass, metal or plastic.

One thin-film technology, which is already employed in many PV modules, is called “amorphous silicon”. It is composed or randomly arranged atoms, forming a dense, noncrystalline material resembling glass.  The silicon layer is less than a millionth of a meter (a micron) thick requiring considerably less pure silicon then other cell types.

Researchers are working to obtain higher efficiency from this material, which lacks the ordered structure and inherent photovoltaic properties of crystalline silicon.  Today’s commercial efficiency average 5 to 6 percent but efficiencies as high as 14.5 percent have been exhibited in laboratories.

Tandem Cells

These cells are still in the developmental stage but offer great potential for the future of photovoltaics.  Tandem, or multiple-junction cells, are actually several cells stacked on top of each other.  Each cell layer is able to convert a different wavelength, or color, of the light spectrum into electricity.

Tandem cells have displayed efficiencies higher than 14 percent in the laboratory and theorist predict efficiencies as 35 to 40 percent.

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Solar for Consumers - Electrical Generation - PV & Thermal Solar



Photovoltaics



How a PV Cell Works

 


Power Tower



Dish Stirling



Trough

Electrical Generation

The generation of electricity from solar energy can be achieved through two major technology alternatives. One uses the light from the sun to generate electricity directly, (photovoltaic technologies), and the other uses the heat from the sun to increase the temperature of a working fluid which-in turn can be used to generate electricity, (solar thermal technologies). Each of these major alternatives can, in turn, be subdivided into variants of the major technology. Photovoltaic technologies fall into crystalline, multi-crystalline, thin-film or concentrator variants while the solar thermal technologies fall into trough, power tower, dish engine and thermal electric variants.

Photovoltaics

Generally speaking, How a PV Cell Works use a semiconductor material that is exposed to sunlight. The energy of the incident light displaces electrons from their normal atomic orbits and an electrode grid structure on the surface of the semiconductor collects these electrons and makes them available for use in an external circuit. This is very similar to the way that the chemical reaction and the electrodes in a dry battery cell make electrons available for external use.

The terms crystalline, thin film and concentrator describe the manner in which the semi-conducting material is processed and optimized as a photovoltaic cell. Crystalline cells are fabricated from ingots of the semiconductor material, usually silicon, that are cut into relatively thin slices, processed to optimize the electron collection efficiency and laminated into a protective enclosure. Thin film cells are extremely thin layers of semi-conducting material that are evaporated onto a substrate, and concentrating cells use a plastic lens to concentrate sunlight from a large area onto a much smaller area of crystalline semi-conducting material. All types have their merits and problems and are described in detail in the referenced locations.

Download the Arizona Consumer's Guide - this booklet is designed to guide you through the process of buying a solar electric system.  NOTE: You will need Adobe's Acrobat Reader to open, view, and print this document.  Acrobat is freely available and can be downloaded from Adobe's Web site. Arizona Consumer's Guide (PDF Format)

Visit the National Geographic's web site and take the: PV Quiz


Solar Thermal

Both the trough and power tower solar thermal technologies use mirrors to concentrate the heat from the sun onto a vessel containing a heat transfer fluid. The fluid is then pumped into a steam generator where the heat is transferred to water turn it into steam. The steam can then be used to spin a conventional steam turbine connected to a generator to make electricity.

In the case of the trough, the mirror is a long parabola with a steel tube containing the heat transfer fluid running along the focal axis of the mirror. The axis of the mirror is usually aligned in a North-South direction and the mirror is rotated from East to West as the day progresses so that the energy from the sun is continually focused onto the steel tube. Rows of mirror/tube assemblies are used to form large multi-acre solar fields from which the heated transfer fluid is collected and used in the generation of steam.

The power tower system is a little different in that all of the transfer fluid heating is achieved in a heat receiver on the top of a tower located in the center of a field of computer controlled mirrors, or heliostats. Cold fluid is pumped up to the top of the tower, the heliostats focus the sun's energy onto the receiver and heat the fluid which is subsequently returned to the ground and used in a steam generator in the same way as the heat transfer fluid in the trough system.

Dish/engine systems are somewhat different in that the heat from the sun is used to heat a working fluid within a heat engine. The rotating shaft of the engine is connected to a generator, which produces electricity without the need to go through a steam generation process. The engine is located at the focal point of a parabolic dish mirror, which is made to track the sun across the sky throughout the day.


Good Resources:

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