Passive Solar Design

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Passive solar design

Looking into the sky on a sunny day, the notion that humans could have an "energy crisis" seems absurd. Each day, the earth receives 1.78 x 10¹⁴ kilowatts of energy, more than 10,000 times the amount needed by the whole world this year. All that is required is a way to collect and harness the energy of sunlight.

Humans have explored systems for the capture of solar energy for centuries. The Roman architect Vitruvius described a plan in the first century b.c. for building a bathhouse heated by sunlight. He explained that the building should "look toward the winter sunset" because that would make the bathhouse warmer in the late afternoon. More recently, water heaters operated by solar energy were built and widely sold in the early 1990s, especially in California and Florida.

Most historical examples illustrate the principles of passive solar heating, namely constructing a building so that it can take advantage of normal sunlight without the use of elaborate or expensive accessory equipment. A home built on this principle, for example, has as much window space as possible facing toward the south, with few or no windows on other sides of the house. Sunlight enters the south-facing window and is converted to heat, which is then trapped inside the house. To reduce loss of heat produced in this way, the window panes are double- or triple-glazed, that is, consist of two or three panes separated by air pockets. The rest of the house is also as thoroughly insulated as possible. In some cases, a house can be built directly into a south-facing hill so that the earth itself acts as insulation for the north-, east-, and west-facing walls.

Adjustments can also be made to take account of changing sun angles throughout the year. In the winter, when the sun is low in the sky, solar heat is needed most. In the summer, when the sun is high in the sky, heating is less important. An overhang of some kind over the south-facing window can provide the correct amount of sunlight at various times of the year. Changing seasonal temperatures can also be managed by installing insulating screens on the south-facing window. When the screens are open, they allow solar energy to come in. When they are closed (as at night), they keep heat inside the building.

The primary drawback to the use of solar energy is its variability. Often the energy supply and the energy requirements are out of balance for weeks or months depending on season, amount of cloud cover, latitude, etc. Thus, there is frequently the need for storage of energy for later use. Several technologically-simple methods for storing solar energy are commonly used in passive solar design. A large water tank on top of the house, for example, provides one way to store this energy. Sunlight warms the water during the day and the water can then be pumped into the house at night as a heat source. A heat sink such as a dark-colored masonry wall or concrete floor near the south facing windows can also enhance a passive solar system by absorbing heat during the day and slowly radiating it into the room during the night. A Trombe wall may also be installed. This masonry wall, 6-18 inches (15-46 cm) thick with gaps at the top and bottom, faces the sun. The space in front of the wall is enclosed by glass. Air between the glass and the masonry wall is heated by the sun, rises and passes into the room behind the wall through the upper gap. Cooler air is drawn from the room into the space through the lower gap. The masonry wall also acts as a heat sink.

[David E. Newton ]