Renewable Energy

The Solar Slab Heat Exchanger

James Kachadorian opens his book The Passive Solar House with the following statement: "All houses are solar [9]." If a house has windows and the sun is shining, there is an opportunity to gain free passive solar heat. But, not everyone avails themselves of the opportunity. As a home builder, Kachadorian acknowledges that "home building plans were typically insensitive to the position of the sun. Our prefabricated home packages were labeled simply 'front, back, right side, left side,' not 'south, east, west, north.' We offered little or no advice on siting [the position of the home in relation to the sun], except that we needed enough room to get a tractor-trailer to the job site [10]." He describes how potential home buyers showed little interest in the insulating qualities (R-values) of their homes and were less interested in energy efficiency than the attractiveness of the house. After oil prices rose in the 1970s, he acknowledged that the home buyer, contractors, and design team must all work together to provide a more comprehensive plan for home construction and energy efficiency [11].

In January of 1976, Kachadorian opened Green Mountain Homes, the first company in the United States to focus on the design and manufacturing of solar homes in kit form. His company created simple changes in construction which invited the free energy of the sun into the home. He did meet resistance. People were afraid the house would become too hot or too cold. They feared it would have to be weird looking, use strange fickle gadgetry and materials, and require huge expanses of glass [12]. His designs are far from that. They incorporate traditional designs and use the materials required for normal construction. The only downside is that a basement cannot be included in the home as a ground level foundation is necessary for the absorption of heat into the slab.

It was stated earlier that traditional home building kits did not mention the orientation of the front of the house: south, east, west, or north. This becomes the first step in building any home which will use passive solar heat. The major windows at the front of the house should face south to grab the rays of the sun. The back of the house, with few windows, will face north. According to Kachadorian, solar principal #1 is to orient the house properly with respect to the sun's position as it relates to the building site. "Use a compass to find true south, and then by careful observation site the house so that it can utilize the sun's rays from the east, south, and west during as much of the year as possible [13]."

The best solar gain design for any construction style presents as much surface area as possible to the sun. This is achieved when the longest axis, which is usually the front of the house, is facing true south [14]. For example, a house in Hartford,

Connecticut facing true-south will have a 100% solar benefit at high noon. If the house is rotated 22 1/2 degrees to the southeast or southwest, there is a 92% solar benefit. Rotating from true-south by 45 degrees provides a 70% solar benefit and a rotation of 67 1/2 degrees results in a gain of only 36% [15]. Staying within the range of a 20 degree shift from true-south would still provide ample solar gain while giving some leeway as to the orientation of the front of the house. If possible, a windbreak of some type is desirable on the north side. This can be provided by an outcropping, hillside, or group of evergreen trees to protect the house from north winds [16]. Deciduous trees on the east, west, and south provide shade in summer and, after the loss of their leaves in the fall, allow the sun's rays to penetrate into the house during the winter. We know that the sun's rays are angled above the front of the house in summer, thus lowering the amount of solar energy directly entering the front windows. This does not hold true for the sun as it rises in the morning and drops in the afternoon. Therefore, houses in warmer summer climates may need less east and west facing windows as they will not stop performing as solar collectors in summer [17]. Another option would be to plant deciduous trees or use insulating window coverings.

James Kachadorian's Solar Slab is constructed with concrete blocks (cinder blocks) and poured concrete (see Figure 2). This slab is easy to build, reasonably priced, and provides an effective thermal mass to absorb heat from the sun. A layer of 1" Styrofoam is first laid on the ground. This is topped with 12" of compacted sand or gravel which is covered with a poly vapor barrier. Next come the concrete blocks which make this slab unique. They are laid so that the holes in the blocks line up horizontally forming air passages running north and south. Finally a rebar enforced slab is poured over the blocks [18]. It is this slab which will absorb the solar gain and conduct it into the air passages of the cinder blocks. At the north and south ends of the slab, wooden spacers are inserted before pouring the slab to create air vents. These vents will allow the circulation of air between the house and the concrete "radiator" slab [19].

Figure 2. Reprinted with permission of the publisher from The Passive Solar House by James Kachadorian: Chelsea Green Publishing.

This is how it works. Light rays from the sun, which are short-wave energy, pass through transparent glass. As soon as the light rays strike an object, for instance the floor covering above the slab, light changes form to long-wave energy or heat. The concrete slab's temperature rises as heat is absorbed and the air in the vents of the cinder blocks is warmed [20].

During winter days, the south wall will be warmer than the north wall. Heated air along the south wall will rise. As it rises, it pulls warmed air up through the floor vent from the slab. Cooler air along the north wall will drop down into the floor vent where it circulates through the slab, is warmed and rises back into the house at the south wall. This is another example of the thermosiphoning circulation of air which was first utilized in the Trombe Wall [21].

But, not all days are sunny, and we want our homes to be warm at night when the sun is not shining. Heat storage is accomplished by using thermal mass to capture the heat of the sun during the day. "Thermal mass is comprised of building materials that absorb heat effectively, charging up like a thermal battery and then yielding this heat back into the home's living space through periods of time when the building is not actively gaining heat from the sun or from some other source [22]." In simple terms, the heat stored in the mass of the slab radiates back into the home during times when the sun is not shining. When discussing solar heating it is important to remember that heat wants to move into the cooler spaces. As the temperature drops, the slab radiates energy back into the house. It acts like a battery being discharged. The next day when the sun returns, the slab is recharged and another day of heating begins. If backup heat is needed, it is only necessary to provide the difference between the thermal mass temperature and the desired room temperature, thus saving on energy bills [23].