The Exergetic Performance of a Solar Hot Water Pre-heating System

This paper discusses the possibility of asimple solar hot water pre-heating system fordomestic hot water supply. In Japan, the fossilfuel use for hot water supply amounts to about30% of the total in residential buildings. The useof conventional solar hot water heaters has beendecreased draniatically over the last 30 years,although the extensive use should have beenexpected. They are regarded to be notnecessarily cheap and moreover theirappearance has not been preferred by quite afew building designers. Their rather high costand bad appearance must have been the mainreasons of declining the use of solar hot waterheaters. Therefore, it is important to investigatethe possibility of a solar hot water pre-heatingsystem which is easy to install with low priceand can even be hidden, e.g, as a part ofbuilding envelopes, upon necessity.We built a simple solar hot water heaterwhich is assumed to be installed at acondominium balcony, and made an experimentin summer, 2007. The measured average watertemperature turned out to be 41.9 C. The exergybalance model of this system was developed andits numerical calculation was performed. Wefound that the warm exergy obtained from thispre-heating system with area of 3m2 for onefamily house reaches 140kJIday and thiscontributes very much to the reduction in theuse of fossil fuel for hot water supply.The exergy balance of the balcony space insummer was also calculated and we found thatthe water carries off the warm exergy to the bath and thereby the warm radiant exergy emitted from the balcony surface to the outdoorenvironment decreased by 20-60%, comparingto a case without solar hot water pre-heatingsystem. The solar hot water pre-heaterembedded in vertical walls performs not onlypre-heating but also mitigating the thermalenvironment outdoors and balcony space. Theseresults of experiment together with simulationsuggest a new direction of the development of ahot water heating system in the future.