O ptimisation of minimum backup solar water heating system

Current flat plate solar water heaters overproduce slightly in summer and have poor performance in winter at the time of maximum load. They use an expensive absorber plate over the entire absorbing aperture of the collector and fail to use the backside of the absorber. They often have under insulated tanks and are not optimised as integrated systems. This paper describes a design approach taken to use existing commercial flat plate absorber and tank components in a new way to maximise solar contribution and minimise material usage in the construction of the system. The design criterion used is not maximum peak efficiency, but minimum annual backup energy supplied to the system to meet an annual load. This corresponds to meeting a minimum greenhouse emissions requirement in both invested pollution during manufacture and pollution from backup energy supplied. Two new designs are shown which allow the solar fraction of systems to be increased to approximately 80–90% in Sydney Australia using a standard model of domestic hot water usage specified in Australian Standard AS4234. Pollution from fuel use drops to as little as 40% of that of conventional flat plate solar water heaters. These new designs use one absorber plate instead of two and a smaller and better insulated tank. Comparisons of solar fraction are evaluated for a range of climatic conditions. An important insight is that with such a performance optimised system the ultimate solar fraction is limited by occasional long duration cloud cover at the site of installation and making the system larger only increases dumped energy, not utilisable energy. Technical efficiency improvements only reduce the required collector area. However, some additional backup fuel reductions can be made through manual control of backup energy use, because this allows finer control of backup relative to real demand. Pollution from backup fuel usage may be able to be reduced to 1/4 that of current flat plate solar water heaters.  2003 Elsevier Ltd. All rights reserved.