For the thermodynamic analysis of the steam cycle the commercial simulation program IPSEpro® is used. IPSEpro® has a model library containing all components necessary for the simulation of a conventional power plant. This library has been extended by DLR by all relevant solar components.

Conceptual Design of the Power Plants

Collector fields for the superheated steam driven power cycle and the saturated steam cycle are presented and investigated. The design calculations are performed for the site in Seville for the 21st of June at noon with a direct normal irradiation of 850 W/m2.

Power Plant with Superheated Steam

The schematic diagram of the power plant with superheated steam is displayed in figure 6. Seven collector loops are connected in parallel. Due to the symmetry of the collector field only 4 loops are displayed and calculated. The mass flux is multiplied by 1.75. The collector field is operated in recirculation mode. In the superheating section there are two collectors connected in series, while in the evaporation section eight collectors are connected in series. The two sections are subdivided by a separation drum. One injection nozzle per superheater row is used to control the outlet temperature of each loop. A fraction of the feed water is fed in front of the recirculation pump in order to cool down the recirculated water to avoid cavitation. The mass flux feeded in front of the recirculation pump is controlled to maintain a temperature of approx. 30 K below the according saturation temperature. The steam quality at the separator inlet is set to 0.85. This is enough to guarantee a sufficient cooling of the absorber tubes during steady state operation. During cloudy periods where frequent transients are expected the steam quality should be significantly lower. The steam temperature at the collector outlet (not displayed in figure 6) is set to 400°C. The mechanical efficiency of all pumps is set to 0.97 and the pump efficiency to 0.62. The electrical and mechanical efficiency of all driving motors is set

to 0.98. The isentropic efficiency of the high pressure turbine stage is 0.71 and that of the low pressure stage 0.78. The generator efficiency is 0.96.

Evaporator [Superheater

Power Plant with Saturated Steam

The schematic diagram of the saturated steam power plant is displayed in figure 7. In this case nine parallel collector loops with eight collectors in series are needed. Again due to the symmetry of the collector field only five loops are displayed and calculated. In the end the mass flux is multiplied by the factor 1.8.

The outlets of the parallel rows collectors are connected to a main header entering a separator drum. Again the steam quality at the separator inlet is set to 0.85. The saturated water from the separator is recirculated to the collector inlet whereas the saturated steam is fed to the steam turbine. For the simulation the same component efficiencies have been used as for the superheated steam process.

Design Condition

The design calculations are performed for both options for the specified site for the 21st of June at noon and a DNI of 850 W/m2. The incidence angle for that time and site is 14°. The most important design efficiencies for the two investigated options are listed in table 2.

The thermal efficiency of the solar field is the thermal output of the solar field divided by the DNI on its total aperture area. In case of the saturated steam process the highest fluid temperature in the collector field is the saturation temperature (285°C at 70 bar). Since the collectors of the superheating section of the superheated steam process are operated at
temperatures above the saturation temperature the thermal losses of the superheated steam collector field are higher. Accordingly the efficiency of the saturated steam collector field is 1.8 % higher.

Evaporator

The second row displays the net efficiency of the power block that is defined as the net electrical output of the power plant divided by the thermal output of the solar field. Due to the higher life steam parameters of the superheated steam process the net efficiency of the power block is 1 % higher. The low values of the power block efficiencies are mainly caused by their simplicity. Finally the net efficiency of the superheated steam power plant is 0.2 % higher compared to the saturated steam process. For a final assessment of both options their part-load performance will be investigated in the next section.