The Semiclosed Recuperated Cycle With Intercooled Compressors

The gas turbine combined cycle (GTCC) is the best currently available choice, if gaps in the renewable electric power supply need being filled with power from fossil fuels. The GTCC manufacturers are in a fierce competition responding to these needs, especially for the best part load efficiency, the fastest load ramp capability and for the lowest low load power parking at an acceptable NO_x and CO emission level. But there is an option outperforming the GTCC technology for the above mentioned requirements, which is theoretically known since years but it has not yet been practically developed. It is the semiclosed recuperated cycle (SCRC). Wettstein (2013) has described this recently in “The Air Breathing Semiclosed Recuperated Cycle and Its Super Chargeable Predecessors,” Gas Turbine World 2013, March/April Issue, Vol. 42, No. 2). The SCRC does not require any component technology, which is not yet proven in operating large commercial GTCC or GT plants. But of course the cycle integration is a different one, requiring a specific design of the components. An inherent side feature of the SCRC is the exhaust gas composition, which corresponds to a near-stoichiometric combustion gas. This allows comparing the SCRC with a (CO₂−) capture ready GTCC having exhaust gas recirculation. The above mentioned article, the thermodynamic performance analysis of a SCRC with an adiabatic compressor is described. But the cycle becomes even more attractive with an intercooling stage in each of the two compressors. Here, this is quantified with another detailed thermodynamic analysis. Additionally, also an ideal case with isothermal compression is analyzed. The latter is of course unrealistic for a practical realization. But it indicates the potential of using more than one intercooling stage per compressor. The aim of this paper is to quantitatively compare the three variants with adiabatic, intercooled and isothermal compressors. In all three cases the same turbine and recuperator temperature limitations are used while some other cycle data assumptions are adapted to the compressor technology in order to achieve an optimal performance level for each variant. The thermodynamic results have been cross-checked with a breakdown of the exergy losses in the three variants. The final results for base load operation indicate that the intercooled variant could become the best choice.