A substantial fraction of world energy use is as low temperature heat. For example, Figure 16.2 shows the demand in Britain for total energy and for space heating. Although details change from year to year, the conclusion remains that in winter over half of the national energy consumption is for space heating in buildings at temperatures of about 18 ± 3 cC. It is usually not sensible to meet this demand for heat from the best thermodynamic quality energy supplies (Section 1.4.3), since these should be saved for electricity generation, engines and motor drives. Thus, for example, it is better to capture solar heat gains (Section 6.3), and then to keep buildings within comfortable temperatures using the averaging and heat-storage characteristics of the building mass. Heat storage also provides a way of fruitfully using 'waste' energy utilised or recovered from other processes, e.g. by load control devices (Section 1.5.4).
In the higher latitudes, solar heat supply is significantly greater in summer than in winter, see Figures 4.7 and 4.10, yet the demand for heat is greatest in winter. Therefore the maximum benefit from solar heat requires heat storage for at least 3 months, say in hot water in an underground enclosure. To consider this possibility, we estimate the time, tloss, for such a heat store to have 50% of its content withdrawn while maintaining a uniform temperature Ts. Assume that the immediate environment, e.g. the soil temperature, has constant temperature Ta. The heat balance equation is it1 T -y mc =--^^^ (16.5)
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