Exergy analysis of domestic-scale solar water heaters

Solar water heater is the most popular means of solar energy utilization because of technological feasibility and economic attraction compared with other kinds of solar energy utilization. Earlier assessments of domestic-scale solar water heaters were based on the first thermodynamic law. However, this kind of assessment cannot perfectly describe the performance of solar water heaters, since the essence of energy utilization is to extract available energy as much as possible. So, it is necessary to evaluate domestic-scale solar water heaters based on the second thermodynamic law. ter the technology process, from the property of energy utilization perspective, we can separate the technology process into three intimately related sub-procedures, namely conversion procedure, utilization procedure, and recycling procedure. An energy analysis entitled ‘Three Procedure Theory’ can be conveniently conducted as presented by Professor Hua Ben. Compared with other theories of energy analysis, three procedure theory exhibits great advantages. The utilization procedure puts forth requirement for the design of parameters in conversion procedure and sets up limits in the consideration of recycling procedure. Of course, under specific conditions, the utilization procedure also receives feedback from other procedures. Three procedure theory furnishes us a good platform to perform energy analysis. udy 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.