Passive solar technologies convert sunlight into usable heat, cause air-movement for ventilation or cooling, or store heat for future use, without the assistance of other energy sources. Technologies that use a significant amount of conventional energy to power pumps or fans are classified as active solar technologies. Some passive systems use a very small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, and use.
Passive solar technologies include direct gain and indirect gain for space heating, solar water heating systems based on the thermosiphon, use of thermal mass and phase-change materials for dampening indoor air temperature swings, solar cookers, the solar chimney for enhancing natural ventilation, and earth sheltering.
Solar panels are used in passive and active solar hot water systems
More widely, passive solar includes technologies such as the solar furnace and solar forge, but these typically require some external energy for aligning their concentrating mirrors or receivers, and historically have not proven to be practical or cost effective for wide-spread use. 'Low-grade' energy needs, such as space and water heating, have proven, over time, to be better applications for passive solar energy utilization.
Advantages and comparisons to active solar
Passive solar systems have little to no operating costs, often have low maintenance costs, and emit no greenhouse gases in operation. They do, however, need to be optimized to yield the best performance and economics. Energy conservation reduces the needed size of any renewable or conventional energy system, and greatly enhances the economics, so it must be performed first. Passive solar technologies often yield high solar savings fractions, especially for space heating; when combined with active solar technologies or photovoltaics, even higher conventional energy savings can be achieved.
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