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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!

What's cooking? A Comprehensive Look at Solar Cooking

WHAT'S COOKING - SLIDE 01 Cooking - the event that occurs every day in every home (except for fast food runs or special dinners out) and is taken totally for granted as a part of daily life. We cook breakfast, lunch and a dinner (unless you are on a cold salad kick). We outfit our kitchens with the latest in appliances, from microwaves to double convection oven, to stovetops with a multitude of tradable tops for grilling, broiling and deep frying.




Did you know that a kitchen is one of the primary rooms for remodeling - both for owner desire and investment?

WHAT'S COOKING - SLIDE 05Cooking shows fill public TV on Saturdays with Jaques and Julia and America's Test Kitchen, and store racks are full of monthly magazines like Good Housekeeping, Bon Appetit and Food & Wine and we take our love affair with cooking and the equipment it comes with to our backyards with bar-b-cues.

Cooking equipment, just like other types of equipment in the house, uses energy resources to operate and with energy use comes consumption of resources (oil and gas) and costs.


WHAT'S COOKING - SLIDE 11 In the summer, Arizonans know that cooking outdoors is "the thing to do" - men return to their primal selves, and women know that it keeps the heat out of the kitchen thereby keeping it comfortable (and an additional benefit is that it helps keep summer utility bills down). WHAT'S COOKING - SLIDE 12In Arizona, we are blessed with an abundance of sunshine. There is actually more energy in the sunlight that falls upon a house than the total energy that whole house uses over the course of a day. There it is - a resource untapped, underutilized, and available to anyone who wants it.

How about cooking?





We know about cooking with the sun - you've all heard, and probably used, the phrase "it was so hot you could fry an egg on the sidewalk". There is a great history and record of solar cooking ranging from the Age of Inquiry to the present.

WHAT'S COOKING - SLIDE 19During the 18th Century, scientific investigation was in full bloom, looking at natural phenomena and developing an understanding regarding how and why things work. The understanding of natural phenomenon applied to technology was in the forefront of scientific and industrial activity and the utilization of created materials, like glass, was a source of creativity, invention and observation.

WHAT'S COOKING - SLIDE 20There was great fascination with the sun - its composition, its relationship to the weather and cycles, and its impact upon plants, animals and people. Experiments abounded, from the development of magnifying lenses to the creation of highly polished specially shaped mirrors used to focus the sun's rays to melt metals and set distant objects on fire, to creating steam to run a printing press to the development of cooking devices using the sun.

WHAT'S COOKING - SLIDE 21In the late 1700's, experiments with glass and trapped solar heat by French-Swiss naturalist Horace deSaussure lay the foundation for not only solar cooking but also for passive solar heating of buildings and active heating of water by use of solar collector boxes. Le Journal de Paris received from deSaussure descriptions of experiments and observations.

WHAT'S COOKING - SLIDE 22"It is a known fact, and a fact that has been know for a long time, that a room, a carriage, or any other place, is hotter when the rays of the sun pass through glass"

WHAT'S COOKING - SLIDE 23At the time there was little empirical research regarding temperatures and glass covered heat traps, so in 1767 deSaussure executed a series of experiments to determine the nature of this phenomena by constructing a miniature greenhouse - a series of bottomless glass boxes, nesting within the other with air between, and all sitting on a black base.

WHAT'S COOKING - SLIDE 24 He then aimed it to the sun, and measured the temperature inside each concentric box. He discovered that the outermost box registered the lowest of t the temperatures (which was still higher than the outside air temperature) while the innermost box registered the highest at 185 degrees F.

WHAT'S COOKING - SLIDE 25With a little inventiveness, a lot of inquisitiveness, and further experimenting he observed that "fruits... exposed to this heat were cooked and became juicy, "

WHAT'S COOKING - SLIDE 26 deSaussure continued his inquiries by insulating the "hot box" sides with black cork and leaving the glass top, which resulted in increased captured heat, and increasing internal temperatures.

WHAT'S COOKING - SLIDE 27He tried the apparatus in the plains and in the mountains, and found that while external temperatures were significantly different, the internal temperature of the box remained generally the same. This reinforced deSaussure's idea that while the same amount of solar energy struck the earth, both at the plains and the Alps, the cooler mountain conditions had more to do with qualities of the air and atmosphere, than a difference in solar radiation. Similar to many of today's modern cookers, deSaussure's unit allowed the sunlight to pass through the glass cover which was then captured as heat, within a sturdy, insulated box. The captured thermal energy heated the contents of the box - in this case, food which became cooked.

WHAT'S COOKING - SLIDE 28British astronomer Sir John Herschel, while in Africa, experimented with the "hot box", so called because of the heat it retained. Herschel was able to cook numerous kinds of food while in various African locations, much to the delight of his guests and colleagues, on who was Samuel Pierpoint Langley, who later became head of the Smithsonian Institute; the American astrophysicist who became intrigued with Herschel's demonstrations and joyfully built hot boxes of his own.

Frenchman Augustine Mouchot combined the "hot box" with the intriguing mirror experiments of the time. he developed an over - made of a tall blackened copper cylinder surrounded by a cylinder of glass, capturing a 1" airspace between the two. Then he used a solar mirror to reflect and concentrate the sun's energy onto the cooker. The solar mirror reflected sunlight onto the oven cylinder, heating the contents inside to such a rapid degree that the food was cooked in a very short time. Improvement of his "vertical oven" into a 20 x 20 inch box weighing less than 30 pounds that could bake a pound of bread in 45 minutes, and a stew in 3 hours, led to its use by the French Foreign Legion.

WHAT'S COOKING - SLIDE 32W. A. Adams, developed a solar cooking unit very similar to today's popular contemporary ovens. A glass fronted box mounted on a small tiltable platform, and using an eight sided mirror to focus the sun's energy to the center of the cooker, this design reached temperatures high enough to cook a four pound turkey within 4 hours.

The early 1900's saw scientists, and backyard tinkerers alike, developing designs that improved upon deSaussure's original "hot box", and that development continues to present day and Arizona has a very significant role in that development.

WHAT'S COOKING - SLIDE 33The First National Solar Cook-off in Phoenix, Arizona on September 19, 1981, sponsored by the Arizona Solar Energy Association was the first event of its kind in the world. held at the Phoenix Civic Center Plaza, it showcased designs from throughout the nation and worlds, from the largest to the smallest of cookers. The largest cooked 60 pounds of food at a tine; the smallest was used by backpackers in the Himalayas.

Today the solar cooking tradition continues in grand style at Arizona's Annual Solar Potluck, an annual Tucson event put on by Citizens for Solar, a not for profit organization of solar cooking enthusiasts. Hundreds of people, cooking enthusiasts, and the curious alike, get together preparing and eating breads, vegetables, lasagna, chickens and meats and even pizza. Cookers of all shapes and sizes are set up and through the day provide tasty food is shared with all attendees. The Potluck shows just how easy and effective solar cooking is in preparing delicious foods, and the public sees first hand just how this natural no-cost-energy cooking approach provides for a more comfortable kitchen environment and lowers home energy costs in the intense Arizona summers.

Today's solar cookers function much as the early predecessors, but with the advantages of past experience and contemporary materials. The components are the same. An insulated container with a transparent top to allow sunlight to the interior. The light rays impact the interior surface, are transformed to heat energy, which is absorbed by the cooker interior and its contents. While a little heat escapes back through the glass most is contained within the box and/or the cooking utensil. Additional sunlight can be directed to interior by the use of reflectors or winged additions which provide additional area of sunlight gathering potential. These reflectors serve a dual purpose in allowing the regulation of the sun's energy that travels to the box thereby allowing for some temperature control.

Solar cookers come in all sorts of sizes, shapes, and construction. There are commercial products and there are home made ovens. The simplicity of a solar cooker reflects the simplicity of its use and designs fall into some basic categories.

WHAT'S COOKING - SLIDE 34Box cookers, are simple, insulated boxes with a heat resistant transparent cover which can be a removable lid as the oven "door". The interior is black, for fuller solar absorption.

The box container can be made of virtually any material, from high tech polymers to simple plywood to extremely low cost and light cardboard construction, and the solar face is transparent, usually glass but can be other comparable materials.

WHAT'S COOKING - SLIDE 35It provides slow, even cooking of large amounts of food, with temperatures 140 - 225 degrees depending on construction. Addition of reflectors achieve the higher temperatures.

WHAT'S COOKING - SLIDE 36Slant faced cookers. With the slanted face pointed directly at the sun, this type of cooker puts the collector glass in a more perpendicular orientation to the sun's rays while maintaining a level interior.

WHAT'S COOKING - SLIDE 37These are usually highly efficient and the addition of reflectors will increase performance. These cookers can be made to be portable with collapsible reflectors of aluminum or mirrored foil glued to a sturdy backing, folding onto the cooker for easy transport.

WHAT'S COOKING - SLIDE 38Multi-faceted, cone shaped cookers. A faceted conical shaped interior covered with many small mirrors, this cooker brings more reflected light directly to upon the cooking pots reaching temperatures of 300 - 450 degrees F, and operates the same as a conventional kitchen oven. The larger size (generally 4' in diameter), it is large enough to roast a turkey.

WHAT'S COOKING - SLIDE 39Instead of a totally contained oven and cooking utensils, this cooker, developed in France, has reflector panels, separate from the cooking utensils, directing sunlight directly onto a dark colored pot inside a plastic bag or under a glass bowl. These cookers are extremely portable and easy to assemble and use.

WHAT'S COOKING - SLIDE 40Concentrating cookers. A curved, sometimes concave, reflective surface (aluminum foil, etc.) that focuses the sun's energy to a single focal point at which is placed a pot sitting on a separate stand. Temperatures exceeding 600 degrees F have been attained. This cooker requires more continuous attention in order to keep the focal point on the cooking utensil at all times since the sun angle is continuously changing as the sun moves across the sky.

WHAT'S COOKING - SLIDE 41These cookers can be complicated to make and because of the high heat generated at the focal point, can cause burns and eye injury if not used correctly.

WHAT'S COOKING - SLIDE 42The variety of cookers shows that there is choice for the user - depending on situation, costs, and desire. All work and the common question regarding which is best gains this response from solar cooking enthusiasts.

WHAT'S COOKING - SLIDE 43Today, solar cookers are growing in use, not only in technological, energy rich countries, but also in countries where there is no energy for cooking and depleted resources for even a fundamental cooking fire. Whether motivated by need or by choice, solar cooking is finding its way into the lives and lifestyles of peoples around the world. Community kitchens outfitted with solar cookers provide prepared food for large groups of people, and cookers are used in Arizona backyards net to, and in place of, the American barbeque, and are even being designed as a permanent component of existing and new housing.

Arizona has an abundance of sun; a treasure of solar experience and knowledge, and an affinity for outdoor cooking. Solar cooking is natural fit - not only for celebrating the summer but for year round use, and it can be a significant aspect of keeping summer cooling bills down.

Solar cookers come in a variety of configurations and constructions; are commercially available or can be self-constructed, but have a commonality ---

they are effective


fun to use


reduce energy demands and associated costs


and according to solar enthusiasts, they are healthier.




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...





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,

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.


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.



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


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.


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.


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.


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


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.

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...


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 -




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:


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.



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 

WINDOWS - It’s the clear thing.


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.

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


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.


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.


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.


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.


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 -


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.


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:






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.


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.



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.


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

Full slide show in window below | For 3 page summary in a separate PDF window: Residential PV Stand-alone Brochure {edocs}pdfs/stand-alone.pdf,700,550{/edocs}
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Residential PV Grid-Tie Presentation

Full slide show in window below | For 3 page summary in separate window: Brochure format

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AZ Dept. of Commerce Report - Glossary


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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