The United Nations Development Programme (UNDP) world energy assessment estimates the global use of energy to heat water to be 10,000 PJ.1 This technology is especially important for rapidly growing cities like Mexico City and Sao Paulo which have a severe pollution problem as well as an energy problem. There are two basic systems for solar heating:
• flat bed collectors
• vacuum tube collectors.
Flat bed collectors consist of metal plates coated matt black behind glass or plastic. They are tilted to maximize uptake of solar radiation. Behind the plates are pipes carrying the heat-absorbing medium, either water or air. Water for indirect systems contains antifreeze. The underside of the plates is insulated. Flat bed collectors realize temperatures around 35°C and are best employed to supply pre-heated water for a gas boiler or immersion heater.
It is increasingly becoming the case that the collector system incorporates a photovoltaic (PV) module to provide power for the circulating fan, making it a true zero fossil energy option.
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