On April 26, the Tucson Solar Potluck will be held in the desert north of Tucson. It is the 32nd straight year for the gathering of solar enthusiasts, a streak that some believe qualifies it as the longest continuously held solar event in the U.S.
Solar Building Design in Arizona
Solar utilization has a long history, beginning with some of the earliest structures in which humans lived. The early inhabitants of what we now call Arizona probably did not think of their homes as passively heated and cooled. They built them in response to the climate, to social and cultural standards and to their need for adequate shelter. They did not have available to them abundant energy resources or mechanical devices for moderating the indoor climate of their homes. So they used what was available - the sun, wind, caves, fire and available materials such as branches and sticks, and mud and stone. If necessary, they built several dwellings, including one for summer and one for winter.
Some of the earliest buildings in Arizona which took advantage of the sun were the cliff dwellings which, in many cases, faced south. While archeology shows that many cliff dwellings built during the same period and later, did not face south, those with the correct orientation provided a better level of potential comfort than those that did not orient to the south. Thus the low winter sun could enter and heat the people directly as well as heat the mud and stone walls of the apartments which remained warm in the cool nights, and during the summers, the cave roof shaded the dwelling from the direct rays of the sun keeping both people and structures cool.
Early desert dwellings included pit houses semi nestled into the earth with earthen berms which took advantage of the coolness and thermal stability of the earth; ramadas - outdoor, shaded work structures (cooking, etc.) which allowed breezes to blow through. Subsequently, multi-family dwellings called pueblos also incorporated ramadas. These two types of structures gave the inhabitants a choice between using the high mass adobe structure as a shelter from extreme heat and cold, or the low mass shelter (ramada) when it was comfortable outside. The ramada was also often used as an outdoor kitchen to keep the house from getting too smoky or warm.
Arizona's Environmental Diversity
All solar buildings are climate and site responsive. Arizona is a composite of differing patterns of elevation, temperature, solar radiation, humidity, wind conditions, vegetation, and terrain and even within the general climate zones of the state, there are local variations and factors to be taken into consideration. Arizona is defined into 4 general climate zones -
Arizona's environmental diversity leads to differing building design strategies and expressions, but also contain some shared design aspects. In areas of severe winter conditions (Flagstaff, Northern Arizona, etc. ) solar buildings can meet wintertime heating needs by capturing the sun's rays (and heat) in ways where warming can be immediate or stored for later use, while in the summer, cooling is easily provided by cross ventilation and shading. In areas of severe summer conditions (Phoenix, Yuma, Tucson, etc.) winter heating needs are easily met using the sun in a similar manner, and cooling can be achieved by proper site planning (orientation, landscaping, shading), materials selection and placement, space planning, building form, and use of the diurnal (day/night) cycle of heat flow coupled with passive and active solar equipment. Those temperate areas of Arizona, as well as those temperate times of the season in the areas of extreme temperature, can attain comfort by the use of fundamental solar planning, building materials, and ventilation.
Arizona has two rainy seasons, one in the winter and one in the summer. However, most days have some sunshine, and Arizona receives an average of 80-90% of the possible sunshine over a year's period.
The desert zones are characterized by long, hot summers with high temperatures over I 00'F and lows in the 70's and 80's and diurnal (day/night) temperature swings of 30'. Winters are mild with highs in the 60's and 70's and lows in the 30's and 40's with occasional nighttime drops to freezing and below. Average January temperatures are 51.2 ' F in Phoenix, 55.4' F in Yuma and 50.9' F in Tucson. July high temperatures for the purposes of cooling system design are 107 'F in Phoenix, 109 'F in Yuma and 102 ' F in Tucson. Most of the year the air is very dry in the desert zones except during the height of the summer rainy season, when high humidity can cause much discomfort, particularly in the Phoenix and Yuma areas.
Passive (non-mechanical) heating potential in the desert zones is great and can easily provide 80-100% of the heating load. Passive cooling potential is greatest during the dry summer periods, but usually must be augmented with mechanical cooling during the most humid times. Heating and cooling designs must be considered in conjunction with one another.
While more cooling than heating is needed in the desert zones, heating is the primary need in the basin and range and the high mountain zones. Summer nighttime temperatures drop low enough in these zones to allow adequate passive cooling if prevention of unwanted heat gains in the summer is incorporated. Summer high temperatures in the basin and range zone are in the mid 90's and the lows are in the high 50's and low 60's. The January average temperature is 37.1 ' F in Prescott and 32.6 ' F in Winslow. Summer high temperatures in Flagstaff normally reach the mid 80's but can get higher while summer lows are in the low 50's. The January average temperature for Flagstaff is 28 ' F.
Because of the high solar input, passive heating can provide 70 to 100% of the heating needs in the cooler Arizona climates. If passive heating and cooling are combined with proper means of controlling heat gains provided, little or no backup cooling is necessary.
Modern Solar Homes
Historically there are many examples of solar uses, strategies, and techniques In Arizona, but solar houses, as we think of them today, were not built until the 1940's. One of the earliest Arizona solar home designers was architect Arthur T. Brown who was instrumental in the design of an earth integrated passive solar home in Florence, Arizona in 1940, and other solar homes in the southern part of the state. One of his best known solar homes in Tucson incorporates a mass wall behind glass which stored solar heat in winter, keeping the house warm late into the evening. Brown provided for summer cooling with deep overhangs to keep the sun out; low vents on the north side and high vents near the south side ceiling for cross-ventilation; and the incorporation of evaporative cooling.
Definitions and Concepts
First and foremost, there is a great difference between an energy efficient building and a solar building. Solar buildings purposefully utilize the building's attributes of orientation, form, materials, and equipment to use the sun and other natural elements (earth, wind, water) to interact with solar and environmental conditions and resources to provide a unified, comprehensive approach to heating, cooling, lighting, water heating, cooking, etc.. A solar building, by definition, incorporates and builds upon energy efficient attributes, in its aggressive use and/or mitigation of environmental resources and conditions. An energy efficient building, while highly insulating and even efficient in its' energy consumption may not utilize the environmental resources that are available to provide for human comfort.
Solar building design approaches range from Passive Solar Buildings, (the building, form shape and materials are used to meet human comfort needs with little or no other power resources required) to Active Systems (mechanical devices powered by conventional and alternative energy sources are used to help collect, store, and distribute solar and renewable energy energy resource benefits and/or electricity to meet needs) to Hybrid Systems (a composite of the two).
Passive solar homes are those that use natural means -the sun - along with the heat transfer mechanisms of convection, conduction, radiation and evaporation to provide comfort. A passive building is designed to stay comfortable both winter and summer with little or no need for additional energy. Systems that depend on fans and pumps for their operation are called active. If a small amount of energy is used to run a fan and distribute heat (or coolness) throughout the house, the system is called hybrid. All require careful siting, spatial planning, and correct orientation to optimize effectiveness.
Solar design always considers the location of the building and the location of the sun. Since there are some basic rules of physics, and the sun's impacts change as it moves across the sky and is at differing angles to the earth's surface during the seasons, there are some fundamental rules of thumb for solar building design.
Sun LocationThe sun is our greatest ally in solar design. As the seasons of the year change, the sun's location in the sky changes. In the winter, the sun is very low in the sky. It rises in the southeast and sets in the southwest. In the summer, it is very high in the sky, rising in the northeast and setting in the northwest. These differences in solar location throughout the year are one of the keys to solar design. It means we can take advantage of the winter sun to heat our homes while we can keep the summer sun out.
WINTER ORIENTATION - Optimum orientation to the sun in the wintertime will accommodate heating, in both the severe winter conditions of the high mountains and milder conditions of the low desert. Since the sun is generally always to the south of us (high in the horizon during the summer and low in the winter) maximum exposure is to the south for purposes of passive solar heating of a building and orientation of solar equipment (water heaters, photovoltaic modules, cookers, etc.). The "natural" form of the building to allow for direct solar access, would be elongated in the east/west direction. (DIAGRAM)
SUMMER ORIENTATION - Optimum orientation of a building in the summer tends to be the same - with minimization of east and west facades (due to intense early morning and afternoon low horizon sun impacts) and a major south facing facade with strong overhangs in response to the high angle of the sun during the main part of the day.
COOL COURTS/WARM COURTS - There are cold (north) and a warm (south) spaces adjacent to a building, and the use of cool courts and warm courts does much to mitigate negative climatic impacts upon a building as well as enhances the outdoor lifestyle that defines Arizona.
THERMAL TRANSITIONS - In areas of temperature extremes ( severe cold or severe heat), thermal decompression should be considered. As one moves from the outdoor temperature (-32 degrees or + 100 degrees), a series of transitions should occur moving a person from hot to warm to cool to comfortable (or conversely from cold to cool to temperate to warm). In hot climes this movement is from the direct sunlight (hot) to filtered shade (trees and vegetation) to a cooler zone of more dense vegetation, shade and fountains, to exterior structural elements (porches, verandas, etc.), to an "air lock" entry, to the heart of the building. This decompression pattern is also practiced, with differing natural and built elements, in cold weather design.
The advantage of this thermal decompression is two fold.
1) it allows for tempering the environment that surrounds the building, reducing the extreme temperature range between the exterior and interior of the building , therefore there is less demand to heat (or cool) a building at any given time, and
2) it allows for the human physiology to acclimate to temperature change in moving from 100 degrees to 74 degrees (or 20 degrees to 74 degrees). The negative problems caused by sudden thermal impacts upon the human body are well known, and mitigation of this condition is a beneficial by-product of good solar building design.
SOLAR BUILDING MATERIAL APPROACHES
HEAT FLOW - Heat always flows to cold, and the rate of flow is directly affected by the temperature difference - i.e. the greater the temperature difference the faster the heat flow and the type and density of a material.
In order to understand how solar design works, it is important to understand the basic physical mechanisms which make solar design possible. They are convection, conduction, radiation and evaporation.
Convection occurs when air or a liquid carries heat from warm surfaces to cool ones. When air or the liquid is heated, it expands, becomes lighter and rises. When it contacts cooler surfaces, it transfers its heat to those surfaces. The air or liquid then cools, becomes more dense and sinks. Thus a circular convective current is set up which moves heated air or liquid from warm objects or surfaces to cooler ones. This principle can be used to heat and/or cool.Conduction describes the passage of heat through a materials such as the walls of a house. Depending on the material composition, the denser the object or material, the more quickly the heat will usually move through it, although a very dense, thick wall can inhibit rapid transfer of heat. Insulation, by its light density and trapping of air, resists heat transfer and thus reduces the amount of heat flowing through walls and roof areas.
Radiation describes the transfer of heat across space without warming the air in between. Sunlight is short wave radiation while heat is long wave radiation. Change in the type of radiation occurs when light (short wave) strikes a dark solid. Dark objects exposed to sunlight will get warm, even on a cold day. If you stand a few feet away from a brick wall that has absorbed solar radiation all day, you will still feel heat radiating from it after the sun has gone down. Heat radiation from a hot wood stove is another example of this mechanism.
|Evaporation is a heat transfer process through which air can be cooled. Water added to nonsaturated (dry) air is absorbed and cools the air. Evaporative cooling processes can either be natural such as when plants give off moisture to the atmosphere or sweat evaporates from your skin, or they can be forced such as in a mechanical evaporative cooler. Evaporative cooling processes are enhanced with ventilation.||
There are three primary solar design approaches to solar building design.
- Thermal Mass - The building structure and materials are utilized to meet the heating and cooling requirements by means of storing warmth and coolth. Materials of high thermal capacity and density are often used for both their characteristics to impede heat flow as well as storage of heat or cold. Typical materials include adobe and its' variations (rammed earth, etc.), brick, concrete, water, and composite thermal storage materials with integrated insulation and thermal breaks, etc.. The advantage of a high mass structure is that it is a part of the heating and cooling system and can carry on for a number of days in the face power failures or inclement weather. This capability also requires much less in the way of mechanical heating and cooling equipment. Enhancement of the high mass capabilities is achieved through the use of "out-sulation", the addition of an insulated external wall barrier.
- Thermal Skin - The building envelope is comprised of a highly efficient thermal barrier, effectively reducing the intrusion of summer heat or loss of wintertime heat. The reduction of unwanted summer heat gain to the interior and/or winter heat lost to the cold translates to a reduction in the need to provide replacement heat, or cooling, thereby requiring less equipment and less energy consumed. Typical materials include highly insulated heavier frame construction; insulation panels with integral frame structure; double envelope systems, straw bale construction, composite materials of insulation and structure, etc..
- Composite - The building envelope is a thermal "skin" approach with much of the building's interior elements of floors (exposed brick, tile, and concrete); walls (high mass thermal storage interior walls, bancos; and structural and decorative elements (masonry and/or encased water) providing the storage for natural heating and cooling.