what is active solar?
What is passive solar?
Which is best? Passive solar heating or active solar heating?
Passive solar heating
Solar heating of trapped air, water and solids has been used for centuries, but modern architectural design can enhance all three effects for space heating, hot water supply and heat storage.
Such passive solar heating relies on short-wave radiation being absorbed by materials so that they heat up and then slowly re-emit long-wave radiation.
The most obvious example of passive solar heating is inside a greenhouse, where solar radiation that passes through the glass heats the inside air to temperatures well above those outside.
Glass is more transparent to short-wave radiation than to long wavelengths, so a glazed room absorbs and retains heat, especially if the windows face the Sun.
Passive solar heating systems first appeared during the 19th century, particularly in the southern states of America, where water filled tanks were warmed behind glazed walls. As cheap and more convenient oil and gas came to dominate domestic heating during the 20th century this bulky approach all but disappeared. Since the late 1960s, growing environmental consciousness has brought renewed interest in solar heating.
By 2001, 57 million m2 of simple solar collecting panels had been installed globally, mainly in industrialized countries. That they are not even more widely used reflects their high price, driven by high demand relative to a restricted number of suppliers. Despite current high costs of installation, in the long-term almost totally free energy input makes passive solar heating a good investment.
Active solar heating
A more technically advanced exploitation of insolation uses mirrors to focus solar radiation.
Active solar heating is increasingly used in developing countries for cooking, thereby reducing the demand for fuelwoods and the risk of cancers from smoke inhalation.
Similar, roof-mounted devices heat water for domestic use or radiators to much higher temperatures than does passive solar heating.
On a larger scale, arrays of large reflectors boil water to power turbines and generate electricity, or create extremely high temperatures in solar furnaces.
The mirrors are motorized and controlled so that they track the movement of the Sun.
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