Short-Term Performance of Integral Solar Water Heaters

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Abstract
The 8-hr daily test of solar integral or collector/storage domestic water heaters is considered. For the case of no draw-off, it is shown that a computer simulation gives results closely in agreement with an integral equation which involves the collector parameters, the total insolation and the daily temperation variation, but does not require the actual daily ambient temperature. The relationship of this test to integral unit performance when water is drawn off according to various demand patterns is investigated, and it is shown that the short-term efficiency is typically reduced to two thirds of the daily test result, but can be as low as one third. Thus a simple thermal test such as that specified by the South African Bureau of Standards, and other standards organizations, should at best be regarded as a comparison between various units of similar type, although even there the importance of optical efficiency is overemphasized. Simple tests do not provide a guide to performance in actual service.

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