High temperature Cesium-sCO2 combined cycle for concentrating solar power applications

The temperature potential of concentrating solar power plants is not fully utilized using existing steam cycles. The increase of turbine inlet temperature is limited by the materials used due to the high pressures, temperatures, and corrosion potential. The use of alkali metals with high-temperature boiling points allows for lower pressure in high-temperature parts of the cycle. A novel approach using standard SRK EoS with Twu alpha function and Rackett equation for calculating the thermodynamic properties of Cs, K, and Na is proposed and compared with experimental data and property tables showing maximum deviation of vapor pressure, heat of vaporization, and vapor specific heat capacity below 10 %, vapor density below 15 %, and saturated liquid density below 4 % between 600 and 1000 °C. The resulting conversion efficiency of the novel combined cycle comprised of the topping cesium Rankine cycle and bottoming recompression CO2 cycle is 53.50 % at 800 °C, to be compared with 52.83 % of the reference recompression cycle. Preliminary designs of cesium and sCO2 turbines along with the Cs-sCO2 heat exchanger were carried out according to the cycle assumptions. Based on the literature review it is possible to design a combined cycle with 900 °C TIT using the same materials as the 800 °C TIT sCO2 recompression cycle and reach a thermal efficiency of 56.8 %. After additional tests, it might be possible to significantly reduce the equipment cost by using advanced austenitic or even stainless steel in the high-temperature part of the cesium cycle.