Optimization of a decoupled combined cycle gas turbine integrated in a particle receiver solar power plant

The Next-CSP project aims at improving the performances of central receiver Solar Thermal Electric (STE) plants through the development and integration of a new technology based on high temperature (800°C) particles used as both heat transfer fluid and storage medium. The objective of the present paper is to define the best combined cycle configuration by exploring various design options where Turbine Exhaust Temperature (TET) and compressor outlet temperature vary. Regarding the Dense Particle Suspension Heat eXchanger (DPS-HX) train that provides the heat input to the gas turbine’s working air, a particular attention is paid to the particle-side layout. A decent net cycle efficiency can be achieved by a two-reheat topping cycle with a TET of 600°C, with equal expansion ratios and a 3 pressure-reheat Rankine cycle at 160-20-3 bars / 585 / 575°C. Further increasing the low pressure (LP) turbine expansion ratio would result in a less bulky and cheaper LP DPS-HX, at the expense of cycle efficiency. Adjusting the HP and IP pressure ratios can simplify the particle-side layout of the DPS-HX train, again at the expense of efficiency (at least 0.5% pt). With a two-reheat topping cycle, the particles cannot be expected to enter the receiver at a temperature far below 600°C. The impact of that high receiver inlet temperature and of the resulting moderate storage density on the plant’s efficiency and economics should be discussed further. Similarly, a detailed techno-economic optimization of the DPS-HX train should be performed in order to define the optimal pressure drops and temperature differences.