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

摘要

This paper discusses the possibility of a simple solar hot water pre-heating system for domestic hot water supply. In Japan, the fossil fuel use for hot water supply amounts to about 30% of the total in residential buildings. The use of conventional solar hot water heaters has been decreased dramatically over the last 30 years, although the extensive use should have been expected. They are regarded to be not necessarily cheap and moreover their appearance has not been preferred by quite a few building designers. Their rather high cost and bad appearance must have been the main reasons of declining the use of solar hot water heaters. Therefore, it is important to investigate the possibility of a solar hot water pre-heating system which is easy to install with low price and can even be hidden, e.g, as a part of building envelopes, upon necessity.We built a simple solar hot water heater which is assumed to be installed at a condominium balcony, and made an experiment in summer, 2007. The measured average water temperature turned out to be 41.8°C. The exergy balance model of this system was developed and its numerical calculation was performed. We found that the warm exergy obtained from this pre-heating system with area of 3m~2 for one family house reaches 140kJ/day and this contributes very much to the reduction in the use of fossil fuel for hot water supply.The exergy balance of the balcony space in summer was also calculated and we found that the water carries off the warm exergy to the bathand thereby the warm radiant exergy emitted from the balcony surface to the outdoor environment decreased by 20~60%, comparing to a case without solar hot water pre-heating system. The solar hot water pre-heater embedded in vertical walls performs not only pre-heating but also mitigating the thermal environment outdoors and balcony space. These results of experiment together with simulation suggest a new direction of the development of a hot water heating system in the future.