ABSTRACT
An obvious use of solar energy is for heating air and water. Dwellings in cold climates need heated air for comfort, and in all countries hot water is used for washing and other domestic purposes. For example, about 30% of the UK’s energy consumption is beneficial for heat in buildings and of Australia’s energy consumption, about 20% is used for heating fluids to ‘low’ temperatures <100 C. Because of this, the manufacture of solar water heaters has become an established industry in several countries, especially Australia, Greece, Israel, USA, Japan and China. The great majority of solar water heaters are for domestic properties, despite large volumes of hot water being used for process heat in industry. For solar energy systems, if the insolation is absorbed and utilised without
significant mechanical pumping and blowing, the solar system is said to be passive. If the solar heat is collected in a fluid, usually water or air, which is then moved by pumps or fans for use, the solar system is said to be active. This chapter concentrates on active solar water heaters, since they are common worldwide, they allow practical experiments in teaching and their analysis can provide a step-by-step appreciation of fundamentals for both active and passive applications. The general principles and analysis that apply to solar water heaters apply
also to many other systems which use active and passive mechanisms to absorb the Sun’s energy as heat, e.g. air heaters, crop driers, solar ‘power towers’, solar stills for distilling water, solar buildings. These other applications will be dealt with in Chapter 6. In this chapter we discuss only water heating, starting with essentials and then discussing successively the various refinements depicted in Figure 5.1. These refinements either increase the proportion of radiation absorbed by the heater or decrease the heat lost from the system. Analysis progresses, step by step, to a surprisingly complex heat transfer problem. Table 5.1 shows that although each successive refinement increases efficiency, it also increases the cost. The approximate ‘price’ in Table 5.1 indicates the cost of manufacture plus some profit. For the institutional reasons discussed in Chapter 17, the monetary cost may
not be the ‘true’ cost to society or the actual price paid by a consumer in a particular economic framework. Section 5.8 briefly examines such issues, together with social and environmental aspects of the technology. The main part of a solar heating system is the collector, where solar radia-
tion is absorbed and energy is transferred to the fluid. Collectors considered in this chapter do not concentrate the solar irradiance by mirrors or lenses; they are classed either as flat plateor as evacuated collectors, in contrast to the focusing collectors discussed in Section 6.8. Non-focusing collectors absorb both beam and diffuse radiation, and therefore still functionwhen beam radiation is cut off by cloud. This advantage, together with their ease of operation and favourable cost (Table 5.1), means that non-focusing collectors are generally preferred for heating fluids to temperatures less than about 80 C. We purposely consider the technology step by step for ease of under-
standing. The simpler collectors (Figure 5.1(a-e)) hold all the water that is to be heated. The more refined collectors, Figure 5.1(f-i), heat only a little water, with the heated water then usually accumulated in a separate storage tank. As discussed in Section 5.5, refinements improve efficiency by reducing the heat losses from the system as a whole. Therefore many solar water heaters heat the water indirectly with the collected heat being transferred to potable water in a storage tank through a heat exchanger. A separate fluid in such solar collectors, e.g. an oil or antifreeze solution, is chosen to reduce corrosion, and which does not freeze in winter or boil in normal operation. The analysis of such heaters continues that in Sections 5.4 and 5.5, though with slightly different fluid properties, and is not given separately here.
