One important consideration concerning the storage tank is the decision as to the best location for the auxiliary heater. This is especially important for solar space-heating systems because larger amounts of auxiliary energy are usually required and storage tank sizes are large. For maximum utilization of the energy supplied by an auxiliary source, the location of this energy input should be at the load, not at the storage tank. The supply of auxiliary energy at the storage tank will undoubtedly increase the temperature of fluid entering the collector, resulting in lower collector efficiency. When a water-based solar energy system is used in conjunction with a warm-air space heating system, the most economical means of auxiliary energy supply is by the use of a fossil fuel-fired boiler. In case of bad weather, the boiler can take over the entire heating load.
When a water-based solar energy system is used in conjunction with a water space heating system or to supply the heated water to an absorption air-conditioning unit, the auxiliary heater can be located in the storage-load loop, either in series or in parallel with the storage, as illustrated in Figure 6.15. When auxiliary energy is employed to boost the temperature of solar energy heated water, as shown in Figure 6.15a, maximum utilization of stored solar energy is achieved. This way of connecting the auxiliary supply, however, also has the tendency to boost the storage tank temperature because water returning from the load may be at a higher temperature than the storage tank. Increasing the
storage temperature using auxiliary energy has the undesirable effect of lowering the collector effectiveness, in addition to diverting the storage capacity to the storage of auxiliary energy instead of solar energy. This, however, depends on the operating temperature of the heating system. Therefore, a low-temperature system is required. This can be achieved with a water-to-air heat exchanger, either centrally with an air-handling unit or in a distributed way with individual fan coil units in each room to be heated. This system has the advantage of connecting easily with space-cooling systems, as for example with an absorption system (see Section 6.4). By using this type of system, solar energy can be used more effectively, because a high-temperature system would imply that the hot water storage remains at high temperature, so solar collectors would work at lower efficiency.
Another possibility is to use under-floor heating or an all-water system employing traditional heating radiators. In the latter case, provisions need to be made during the design stage to operate the system at low temperatures, which implies the use of bigger radiators. Such a system is also suitable for retrofit applications.
Figure 6.15b illustrates an arrangement that makes it possible to isolate the auxiliary heating circuit from the storage tank. Solar-heated storage water is used exclusively to meet load demands when its temperature is adequate. When the storage temperature drops below the required level, circulation through the storage tank is discontinued and hot water from the auxiliary heater is used exclusively to meet space-heating needs. This way of connecting the auxiliary supply avoids the undesirable increase in storage water temperature by auxiliary energy. However, it has the disadvantage that stored solar energy at lower temperatures is not fully utilized, and this energy may be lost from the storage (through jacket losses). The same requirements for a low-temperature system apply here as well in order to be able to extract as much energy as possible from the storage tank.
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