Experimental comparative performance testing of solar water heaters

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Performance testing of solar heating systems and solar collectors according to International Standard Test procedures require sophisticated and expensive elaborate set-ups. Outdoor collector testing is not feasible in countries with widely fluctuating solar radiation conditions. Indoor testing does not give its true performance when the equipment is situated outdoors. This paper reports on a simple test procedure where the performances of the flat plate, U-tube and heat pipe natural convection solar heaters and the heat pipe force convection solar heater, which were tested on different days, were compared as if they were simultaneously tested side by side. The procedure allowed: (i) the maximum hot water storage temperature that could be achieved by the system over a long period of time without any water draw-off at all, (ii) overnight water temperature drop in the storage tank and (iii) expected end-of-day water temperature and mean system efficiency when water is completely drained down (draw-off) in the evening. Maximum temperatures reached for the natural convection heat pipe, force convection heat pipe, flat plate and U-tube system were 100, 84, 65 and 50°C, respectively. Overnight temperature drops due to standing tank loss and reverse flow were presented and found to be dependent upon initial tank temperature. By pro-rating all the results to reflect on the same area/volume ratio, the expected water temperature rise for the U-tube, forced convection heat pipe, flat plate and natural convection heat pipe systems was 13.6, 17.6, 20.6 and 28.4°C, respectively.

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