Since the solar thermal power plant will operate in part load most times of the year the knowledge of the part load behaviour is crucial. In a first step the system performance is calculated as a function of the load. This gives an impression of the part-load behaviour of each option. To assess the performance of both options at the site specified a yearly calculation using the according meteorological data is performed.

The system performance strongly depends on the operation mode of the solar thermal power plant. For the following comparison three different operation modes are considered: fixed pressure, gliding pressure and modified gliding pressure mode.

In case of the fixed pressure mode the operation pressure is 65 bar for all mass fluxes lower than the design mass flux. For mass fluxes higher than the design mass flux the operation pressure is increased appropriately. In gliding pressure mode the operation
pressure is a direct function of the load and thus of the mass flux. A third option is the modified gliding pressure mode. Here the operation pressure is also a function of the load but it is always higher than 30 bar. This pressure limit of 30 bar is chosen since so far the DISS test facility has only been operated at an operation pressure higher than 30 bar. No reliable operation experience is available for an operation pressure lower than 30 bar.

It is assumed that the inlet temperature difference at the air cooled condenser is 28 K and the inlet temperature difference 14 K[14]. For the first part-load calculation the ambient temperature is set to 25°C.

Part-Load Calculation

Figure 8 displays the efficiency of the collector field as a function of the corrected DNI. The solar field efficiency for the saturated steam option (solid lines) is always higher than that of the superheated steam option. As described above this is caused by the lower fluid temperature in the solar field and the according lower thermal losses.

Comparing the efficiency characteristic of the different operation modes for a given process option, the gliding pressure mode has the superior part-load performance followed by the modified gliding pressure and the fixed pressure mode. In case of the fixed pressure mode the operation pressure and thus the operation temperature of the evaporation section is fixed whereas the pressure and thus the temperature and the according thermal losses decreases in the other cases. The efficiency of the gliding pressure and the modified gliding pressure mode are identical as long as the operation pressure of the modified gliding pressure mode is higher than 30 bar. For a lower irradiation the pressure and thus the operation temperature will remain at this level and accordingly the efficiency will fall below that of the gliding pressure mode.

The efficiency characteristic of the power block is displayed in figure 9 for both options. As explained above the efficiency of the superheated steam option is higher than that of the saturated steam option. And again the gliding pressure mode is superior to the other operation modes. The lower efficiency of the fixed pressure and the modified gliding pressure mode is caused by the throttling losses in the first turbine stage of these options.

Yearly Calculation

For the final assessment of the system performance a yearly calculation of the power plant is performed using the satellite data for the DNI and the ambient temperature described in a previous section. As described above it is assumed that the plant is only operated for a DNI multiplied by the cosine of the incident angle higher than 250 W/m2, lower values are neglected. This threshold is reached for 2770 hours per year. The performance calculation is performed using IPSEpro. As an example figure 11 displays the results for the gross and net efficiency for each hour of the year for the saturated steam option operated in fixed pressure mode.

This calculation has been performed for both process options for the three different operation modes. From the results presented in figure 11 the peak and mean net efficiencies, the net electricity production and the equivalent full load hours have been derived. The according results are presented in table 3.

According to table 3 the net electricity production of the saturated steam option for the specified site is approx. 4% higher for all operation modes. Although the absolute values given in table 3 have to regarded as preliminary it comes out that the saturated steam option is an interesting option for small size DSG solar thermal power plants.