Solar design, having nothing to do with photovoltaic cells (solar panels) is a construction method that utilizes sunlight as a means to provide supplemental heating to a house or structure. The concept is fairly simple and requires no more in materials than building any other home. Early designs however were quite radical consisting of geodesic domes and other space aged shapes or bland and simple boxes and huts. Unfortunately these designs became the stereotype for environmental construction even though they were quickly traded for other more traditional designs such as the New England Saltbox. The only true sacrifice in comparison with other homes is the lack of a basement (but even that has alternatives).

The concept is fairly straightforward and simple to understand. By positioning the long, face side of the house (usually the front) directly toward the south you allow the daytime sun to strike the most surface area of the house throughout the day. To allow the sun to heat the interior of the home and not just the outside walls we add windows (glazing) to the front of the house for daytime sunlight and a few to the east and west to catch morning and evening rays. On a sunny day this is usually more than enough to heat the home in winter and will even overheat your house. In order to save this excess heat energy for nighttime use (when the suns no longer doing the job) we need a heat sink. Nothing more than a concrete slab, the heat sink warms up during the day and then slowly radiates heat throughout the night. Because we want an even heat distribution, warm air rises, and we can't just throw a concrete slab anywhere... the natural choice for our "solar slab" is the first floor. One of the best designs I have found uses concrete blocks as a base with holes laying parallel rather than pointing up. By placing the blocks one by one in front of the other we can create air channels under our floor to serve as the heat ducts. Concrete is then poured over the block base to hold them in place and act as our heat sink.  The thickness of the slab varies from 4-7" depending on the heating requirements of the house, the amount of solar radiation the house receives, and thus the required Mass for a suitable heat sink.

Solar Home Cycle

At first light, sun shines through the eastern windows and begins to warm the floor & air. By noon the sun is entering the front of the house full force, the air temperature remains steady (provided the number of windows and slab thickness were calculated correctly) and the solar slab is gradually raising in temperature. As the sun drops below the horizon the solar slab slowly begins to radiate heat to maintain the air temperature. In a properly designed system the solar slab is able to keep the house from dropping below 60 degrees until morning after a full days "charging" from the sun. We call this "thermal balance" and it is a crucial part of a properly functioning system. As the sun comes up the next morning the cycle starts again, heating the air back up to temperature and storing energy for the evening.

Problems

As mentioned before there are no extra costs to building a passive solar home, only preplanning and minor design adjustments.  The only flaw is the reliability of the sun. Naturally, when the suns not out during the day the solar slab can't collect energy. Weather conditions can vary greatly depending on time of year and location. This is why solar design is meant as a Supplement to normal means and not a replacement. Each area is unique and homes will realize different efficiencies based on local weather conditions. Central New York for example is westsouthwest of the great lakes and therefore encounters elevated precipitation and cloud cover in the coldest winter months (25% sunshine) where as Maine which is much further north has sunshine no less than 50% of the time in Dec, Jan, and Feb.

What does this Mean?

By looking up the average percentage of possible sunshine your area receives (which can be obtained from the US Weather Bureau...or for the same price included in a book on passive solar homes) you can estimate directly the relative percentage of your homes heating costs that you could cover for each month of the year. In layman’s terms, a mean of 59% possible sunshine for Portland Maine in the month of February suggests that a passive solar home could supply up to 59% of its heating costs in that month from FREE solar energy. What’s more is that the solar slab isn't just limited to sunlight to store heat energy. By using a wood stove or hot water radiant heat you can heat your home on dreary days and still charge the solar slab to take over at night. Its truly a versatile system!

Misconceptions

Passive solar design does not mean your house has to look like its out of The Jetsons. Your home can easily be built to suite your style and needs. The interior and solar slab can be covered with anything, even hardwood flooring and contrary to what you may think you won't be living in a "wall of glass". In fact, too many windows can throw the house out of balance and make it less efficient than a normal home.  Another fallacy is that you must tilt your windows toward the sun. The suns path through the sky is closest to the horizon during the winter months. That means its already shinning almost directly through your windows in the months you need it most. "What about the summer, won't it overheat?" In the summer the sun travels high in the sky overhead and will barely shine through. In fact, because the sun doesn't shine through the windows in the summer it doesn't heat your solar slab. This means the slab will cool further through the night when the temperature drops off and then act as a cold source during the day.  It warms in the winter and cools in the summer, the best of both worlds! But the best part about passive solar design is that its integrated straight into the construction of the house. There are no mechanical parts and no maintenance is required.  Simply put its the BEST way to build a home.

Suggested Reading:

The Passive Solar House by James Kachadorian

The Solar House by Dan Chiras

The Solar Living Source Book by John Shaeffer & Real Goods