Passive solar heating basics - Understanding how it works

Passive solar heating is one of the most cost effective ways of heating a home or other building. In most locations, the amount of solar energy that the roof of a home receives is greater than the amount of energy needed to keep it warm. In addition to that, most passive solar applications add little to nothing to the cost of creating a building, but can help reduce the cost of heating it and of replacing or repairing equipment. It's reliable, mechanically simple, and a valuable asset to a home.

In passive solar heating, the heating and cooling system is integrated into the building materials and elements. Windows, floors, and even the walls and roof are used to collect, store, release, and distribute heat. These same elements also work in passive solar cooling. It's important to understand that passive solar design doesn't necessarily mean that standard mechanical systems have to be eliminated. Recent designs that couple passive solar heating with high efficiency backup heating systems have been able to reduce greatly the size of the traditional heating systems required. This means that the amount of nonrenewable fuel required to maintain a comfortable temperature indoors can be reduced significantly.

To use passive solar heating in a home, two elements have to be present. These are: a transparent southern exposure that allows energy from the sun to enter the home, and a material that allows heat to be stored and released later. These two basic elements allow a number of different approaches to passive solar design to be used. It's important to remember, however, that a passive solar heating approach that works in one home or climate will not necessarily be effective in all situations.

Direct gain is the simplest of approaches to passive solar heating. Sunlight comes in through south facing glass, and is almost entirely converted to thermal energy. Usually, the walls and floors are used for thermal storage and solar collection by directly encountering sunlight, and by absorbing reflected energy. As long as the temperature in the room stays high, heat will be conducted into the center of the thermal mass. However, as soon as the temperatures drop, the heat flow is reversed. This strategy can be implemented in many ways, dependent on materials used and local topography.

Indirect gain uses basic elements of heat storage and collection in combination with convection. This approach places storage materials between the sun and the interior habitable space. This means that there is no direct heating. Instead, the heat is transferred between one side of the thermal storage wall to the other. In most cases, the thermal mass can't absorb solar energy as fast as it enters, making the temperatures in this space rise quickly. This heat buildup can be used to help warm a space by creating a vent at the top and bottom of the thermal mass, to allow air to circulate.

Isolated gain uses a fluid, such as liquid or air, to collect heat in what is referred to as a flat plate solar collector, which is attached to the structure. Heat is then transferred by natural convection through pipes or ducts, and collected in a storage area, such as a bin or tank. The cooler air or water is then displaced, and forced back to the collector to become warm again.