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Solar energy can be captured passively and actively. In passive solar systems, the key is to allow solar radiation to pass through a selective material, such as glass, without allowing it to re-radiate back out. This is termed thermal storage and it can be used for several applications, including space heating for the home, to produce domestic hot water, and for the heating of swimming pools and spas. A passive solar heating system harvests energy from the sun without the use of electrical or mechanical devices.