Book - School of Science and Technology

Solar collectors 597SolarcollectorPressurereliefvalveExpansionvesselBack-upheat fromheatingsystemHot water to tapsWater cylinderFigure 20.29 Simple solar hot water pre-heat systemcommercial size applications are few in number. Some experimental systems are underdevelopment aimed at providingspace heatingas well as domestic hot water, althoughthese usually require a top-up source of energy.The main types of solar collector are illustrated in Figure 20.28; (a) flat plate collectors,(b) pipe and fin usually made of copper or copper/aluminium alloy, (c) water sandwichdesign, (d) evacuated tube collectors using a pipe and fin collector within a partial vacuumfixed in groups on a frame with a common header connected to the water heating circuit.Efficiencies of both type of collector have improved with time and are now between50% and 75% dependingon the application. Thus for an average solar irradiance ofaround 3.6 GJ/m 2 pa for the United Kingdom, solar heating systems can now typicallydeliver between 1.6 and 2.7 GJ/m 2 pa.Figure 20.29 illustrates a typical domestic arrangement. In the past solar heating wassupplied to a separate indirect cylinder to the main domestic hot water cylinder. However,a single cylinder with two coils has now become the norm. Modern domestic hot watersystems are typically designed to meet 60% to 70% of the hot water demand. Costsremain high, between £1,400 and £2,500 (2001 prices) for a single dwelling, and suchsystems are usually installed to reduce energy consumption rather than for economicreasons.Concerns are often expressed regarding the health risk arising from high concentrationsof L. pneumophilia within stores of water heated by solar collector. As had been emphasizedpreviously 25±45 C is the temperature range for rapid multiplication of the bacteriaand a residence time of one or two days is adequate for a high concentration to develop.For normal domestic use the arrangement illustrated above can overcome some of theseproblems by ensuringthat water is not stored for any length of time below 55 C.However, the risk of contamination should always be assessed before proposinga solarsystem.