Figure 7.4 Relationship Between the Concentration Ratio and Temperature of the Receiver Operation [Duffie,1980] Power Conversion Unit The power conversion unit converts thermal energy into mechanical energy which is then be converted into electrical energy using a generator. SDS based on heat engines use closed-cycle conversion systems of the Rankine, Brayton and Stirling type. The energy is extracted from the working fluid by a mechanical device, usually either a turbine or piston. In order to compare the merits of different dynamic power cycles it is useful to compare the efficiency and peak cycle temperature as shown in Figure 7.6 This shows as the peak cycle temperature is increased the efficiency of the cycle also increases. However as the temperature increases so does the use of non-conventional materials and hence the cost of the system. This figure is for terrestial systems and assumes a fixed sink temperature. Rankine Cycle This thermodynamic cycle uses a two phase working fluid in a closed cycle and is shown in Figure 7.7. The liquid phase is pumped to operating pressure, vaporized and expanded across a turbine. This turbine drives a generator to produce electric power. The discharge vapor from the turbine is condensed and the waste heat is rejected. If an organic fluid such as toluene is used as the working fluid, the discharge from the turbine is still superheated. A regenerator is incorporated in this cycle to remove the superheat and transfer the heat to the high-pressure liquid before entering the vaporizer.
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