Exergy analysis of domestic-scale solar water heaters

The study in this paper is based on three procedure theory. Exergy analysis is conducted with the aim of providing some methods to save cost and keep the efficiency of domestic-scale solar water heater to desired extent and at the same time figuring out related exergy losses. From this survey, it is shown that for an ordinary thermally insulated domestic-scale solar water heater, Dju (exergy losses due to imperfectly thermal insulation in collector) and DjR (exergy losses due to imperfectly thermal insulation in storage barrel) cannot be avoided. Dku (exergy losses due to irreversibility in collector) is mainly caused by irreversibility of heat transfer and DkR (exergy losses due to irreversibility in storage barrel) is dominated by the mixing of water at different temperature. Dku acts as the driving force for the system while DkR is of little contribution. A good design of storage barrel with little DkR will go a long way in improving exergy efficiency. An equation for computing DkR is presented. For the collector, which is the core of the domestic-scale solar water heater, a judicious choice of width of plate W and layer number of cover is necessary. We define collector exergy efficiency ηxc to be ηxc=Exo/Exu. The relation between collector exergy efficiency and width of plate together with layer number of cover is also analysed.