OPTIMISATION OF MINIMUM BACKUP SOLAR WATER HEATING SYSTEM D. Mills and G.L. Morrison* Department of Applied Physics University of Sydney Sydney Australia. E-mail: d.mills@physics.usyd.edu.au *School of Mechanical and Manufacturing Engineering University of New South Wales Sydney Australia 2052 E-mail: g.morrison@unsw.edu.au Current 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 the design approach taken to use existing commercial flat plat 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 rise to approximately 80-90% in Sydney using a standard model of family usage specified in Australian Standard AS4234. Pollution from fuel use drops to as little as 40% of that of conventional solar systems. These new designs use one absorber plate instead of two and a lighter tank. Comparisons of solar fraction are offered for a range of international sites. An important insight is that with such a performance optimised system, the ultimate solar fraction is limited by 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 system area. However, some additional backup fuel reductions can be made through manual control of backup energy, because these allow 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 solar systems in this way. 1. INTRODUCTION In clear climates, the annual percentage of thermal water heating load supplied by a solar water heater can be very high, often in excess of 90% as an annual average. However, in more temperate latitudes this figure falls because of a combination of lower seasonal solar irradiation and lower ambient temperatures leading to higher collector thermal losses, and lower input water temperatures in winter. For example, a typical flat plate collector system in the relatively good Eastern Australian climate is limited to an annual solar fraction of about a 60-70% in the largest potential markets such as Sydney and Brisbane. In earlier work, (Rabl, 1976; Mills and Guitronich, 1978; Mills and Guitronich, 1979 Mills, Monger and Morrison, 1994), it was suggested that collector performance could be tuned using non-imaging asymmetrical reflector elements so that winter collection ability was boosted to improve annual solar contribution. However, the physical size and appearance of many such systems constitutes a significant market barrier. Furthermore, these systems can overheat significantly under certain conditions. This paper describes the design approach taken to use relatively inexpensive commercial flat plate elements 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. The evaluation of the design changes made during optimisation is carried out using a standard model of family usage specified in