Passive solar architecture principles and applications

¶ … passive solar architecture is nothing new. Socrates observed, more than 2400 years ago, that when a house is facing to the south, the sun's rays penetrate through the windows of the house in the winter, giving heat to the inside, and shines right above the house in the summer, giving shade and cooling on the inside. At that time, this house probably lost heat as fast as it was collected, because of the convective and radiation losses. But the idea was there, which the Romans built upon, by using windows, covered with glass, to trap the solar energy. This caused the internal temperature to stay constant throughout the night.

Passive solar architecture has come a long way since the time of Socrates. Most passive solar homes have the same characteristics in each that make them more energy efficient than they were back then. In the Northern Hemisphere, the windows are facing south, and in the Southern Hemisphere, the windows face north, in order to get the most efficient use of the sun.

Solar radiation passes through solar-oriented glass windows, or solar spaces, and is absorbed by surfaces of the materials inside the insulated envelope of the building. These heated surfaces will re-radiate energy into the interior of the house; the air temperature rises, but the heat does not get rerouted back outside, nor can it escape, resulting in trapped thermal energy.

Light strikes the interior surfaces that are made from high density materials, such as concrete, brick, stone, or adobe. These materials can absorb energy and re-radiate it over time, which results in a very smooth temperature swing curve for the building and reduces the possibility of overheating the air in the house, or building. When a passive solar house is well-designed and well-insulated, air leakage rates are reduced and solar heat stays inside the building envelope. Open floor plans facilitate movement of the solar heat from the south side to the rest of the house. If a mechanical device is needed during extended cloudy periods, smaller, direct vented units or a woodstove work nicely.

There are two types of solar heat used in passive solar architecture design: Direct Gain and Indirect Gain. Direct gain is the simplest type as they rely on receiving solar heat directly from south facing windows, called solar windows. Some of this heat is used immediately, while the rest of the heat is stored in the walls, floors, ceilings, and furniture. This is then radiated through the house during the day, and into the night.

Performance and comfort levels will be increased if the thermal mass, such as concrete, concrete block, brick, or adobe is increased within the space. This is so that the thermal energy can be absorbed and spread out evenly. Recommended mass surface-to-glass area is 6:1. These surfaces should be covered with carpet, cork, wallboard, or any other materials with R-values greater than 0.5, to effectively insulate the mass from the solar energy being collected.

Indirect gain collects and stores energy in one part of the house, and uses natural heat movement to warm the rest of the house, which is done by placing a thermal storage wall 3-4 inches inside an expanse of the south facing glass. This wall is made of high density materials, such as masonry, stone, brick, adobe, or water-filled containers, then is painted a dark color to effectively absorb the solar radiation.

So, what are the advantages of having a passive solar house? A well-designed passive solar house can reduce energy bills by at least 75%! A solar house does cost an additional 5-10% to build, but that cost is recovered by the energy savings. Without the need for mechanical or auxiliary resources, there is not a need for extensive heating or cooling costs, and in many parts of the United States, passive solar houses do not need additional energy to heat or cool them. With the current and projected fuel costs, the additional construction costs are recovered very quickly. In fact, the state of Arizona offers a one time tax incentive of up to 25% for up to $4,000 of passive solar enhancements made to the house.

There are also disadvantages to a solar house. The intense glare of the sun in direct gain type designs can fade furniture fabrics, and other surfaces. To some people, this is a big disadvantage. Privacy is another problem with a solar house -- windows. If the occupants can see out of the house, then the rest of the world can see in. It might be best, while planning the house, to put the bedrooms and bathrooms out of sight of the windows. Back in the 70's, when passive solar houses started becoming popular, the media largely contributed to the myth that overheating of the houses in the summer is common.