Solar-mechanical systems utilize a solar-powered prime mover to drive a conventional air-conditioning system. This can be done by converting solar energy into electricity by means of photovoltaic devices, then utilizing an electric motor to drive a vapor compressor. The photovoltaic panels, however, have a low field efficiency of about 10-15%, depending on the type of cells used, which results in low overall efficiencies for the system.
The solar-powered prime mover can also be a Rankine engine. In a typical system, energy from the collector is stored, then transferred to a heat exchanger, and finally energy is used to drive the heat engine (see Chapter 10). The heat engine drives a vapor compressor, which produces a cooling effect at the evaporator. As shown in Figure 6.18, the efficiency of the solar collector decreases as the operating temperature increases, whereas the efficiency of the heat engine of the system increases as the operating temperature increases. The two efficiencies meet at a point (A in Figure 6.18), providing an optimum operating temperature for steady-state operation. The combined system has overall efficiencies between 17 and 23%.
Due to the diurnal cycle, both the cooling load and the storage tank temperature vary through the day. Therefore, designing such a system presents
appreciable difficulties. When a Rankine heat engine is coupled with a constant-speed air conditioner, the output of the engine seldom matches the input required by the air conditioner. Therefore, auxiliary energy must be supplied when the engine output is less than that required; otherwise, excess energy may be used to produce electricity for other purposes.
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