Solar Building Design in Arizona Continued

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.

Heat Transfer

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.

1) 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.

2) 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..

3) 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.