Effect of design parameters on performance of built-in-storage solar water heaters

Abstract

Solar domestic water heaters are usually forced circulation or thermosyphon heaters. A new design of heater in which the storage and collector are combined in one unit has been tested. The construction of the heater involves a rectangular box-like structure with the top face painted black and enclosed behind a single or double sheet of glass. The back surface and sides are insulated, with the assembly inclined at a suitable angle. The main advantages of such a heater are higher collection efficiency, due to direct contact of water with the absorber plate, and the absence of pipes connecting the collector to storage. In this study, the effect of storage volume/collector area ratio, number of glazings and mode of operation on the heater's performance was experimentally investigated. The work involved construction of three built-in-storage heaters, two of which had depths of 8 cm and one of 6 cm. The heaters were operated concurrently for 5 months in which measurements of heated water temperature, ambient temperature and insolation were made. Experimentation was carried out to study the effect of storage tank depth (volume/collector area) over a 4-month period by side-by-side operation of 6-cm and 8-cm heaters. The maximum and minimum temperatures recorded for the 6-cm heater were 76 and 46°C, respectively, while the corresponding temperatures for the 8-cm heater were 70 and 41°C, respectively. The day-average maximum efficiencies of the 6-cm and 8-cm heaters were 65 and 73%, respectively, with a difference of 8–10% throughout the observation period. The study of stratification on the two heaters showed no significant influence of the depth. The effect of glass covers was studied by tests on two heaters, both of 8-cm depth. One of these had a single glass cover while the other was double-glazed. The double-glazed heater performed better with a difference of 1–10°C in water temperature. Different modes of operation were studied by concurrent tests on two heaters of 801. capacity. The results showed that best performance could be obtained by drawing off all heated water into a separate storage as soon as it reached a pre-set temperature level (in this study 35°C). The heater was also found to be capable of handling intermittent loads quite easily. Finally, it is concluded that the built-in-storage heater performs quite well under Benghazi conditions, and the choice of 8-cm depth with double-glazing seems a good design optimum.