ISCCS - Integrated Solar Combined Cycle System

Conventional combined cycle (CC) power plants are a very attractive configuration where a suitable fossil fuel (natural gas is preferred though fuel oil can be used) is available due to excellent performance, cost and emission characteristics. The CC plant consists of a combustion (gas) turbine (GT), heat recovery steam generator (HRSG) and steam turbine (ST). Fuel is combusted in the gas turbine in the normal way, and the hot exhaust gases pass through the HRSG. Here the energy from the gases generates and superheats steam to be used in the ST bottoming cycle. Hence, the energy in the gas, or other fossil fuel, is used much more efficiently than in a GT alone. Modern cycles can achieve overall thermal-to-electric efficiencies of up to 55%.

Solar energy from a parabolic trough solar field can be integrated with a CC to increase the efficiency ever further and to decrease the already low emissions. This is accomplished in an integrated solar-combined cycle system (ISCCS). The ISCCS calls for part of the heat recovery steam generator (HRSG) to be either replaced or paralleled by equipment serviced by solar thermal energy to supplement turbine exhaust gases. This approach increases thermal energy input which produces more electrical output. The system design may be thermodynamically optimized by having the exhaust gas and/or the solar  thermal energy give up its heat at points in the steam cycle where local heat transfer fluid temperatures are compatible with the local steam temperature, i.e., all heat transfer processes take place at reasonably small temperature differences. This design philosophy optimally integrates the solar heat source into the combined cycle HRSG. The integration seeks to achieve efficient operation even though solar energy intensity varies according to weather and time of day.