If not otherwise defined the assumptions of table 1 apply.

Results

In figure 10 the levelised electricity costs are plotted as a function of solar field and burner size.

For all observed burner sizes an optimal solar field size exists resulting in a minimal LEC (convex shape of the surface).

It was taken into account that the gas fraction of the produced thermal energy should not exceed a certain amount for legal, respectively ecological reasons. The Californian SEGS plants are allowed to use 25% gas. In Spain it is being discussed at present to allow small amounts of co-firing but probably only in combination with a thermal storage.

without boiler

The results for the examined plant configurations are given in the next table.

Due to more full load hours and comparably low gas prices the levelised electricity costs can be reduced by 8% respectively 14% with increasing gas share in comparison with the Solar Only plant. A further and important advantage of hybrid mode is more reliable operation. The investment costs of all three variants are approximately the same (around 100 M€ for 50 MWel).

Conservative assumptions for O&M-costs have been taken into account and vary between 2.6 and 3.2 ct/kWh (depending on the solar field size). These values are slightly higher than the values of the trough plants in California (2.5 ct/kWh) [6].

An important issue is how efficiently precious natural gas is used in such a hybrid steam plant compared to a high efficiency combined cycle (CC) plant. To answer the question corresponding efficiencies have to be defined:

Пі gives the ratio of plant electricity output and fuel input. r|2 is defined as the ratio of the supplementary electricity yield due to hybrid operation (in excess of a Solar Only plant) divided by the gas input. For p2 it was assumed that the plants are designed optimally according to the LEC (see plant variants in table 3). That means that the hybrid plants have smaller solar fields than the Solar Only plant. In case the optimally designed Solar Only plant was equipped with a gas burner providing for 15 resp. 25% co-firing, the gas converting efficiency would be higher for both variants. For further interpretation of the efficiencies see summary and outlook.