Thermodynamic analysis of solar powered triple combined Brayton, Rankine and organic Rankine cycle for carbon free power

Abstract

The present study focuses upon the solar powered triple combined cycle consisting of Brayton cycle running on air, Rankine cycle running on steam and organic Rankine cycle on R245fa as working fluid. The absence of combustion in the topping cycle yields zero emission and carbon free power. Topping cycle receives the heat from the molten salt heat exchanger run on Concentrating Solar Power (CSP) technology employing heliostat field solar collectors for providing high temperature to compressed air for being expanded in gas turbine. Discharge from topping cycle is used for providing working fluid for Rankine cycle, and organic Rankine cycle through respective heat recovery systems. This theoretical study includes thermodynamic analysis based on the first and second law of thermodynamics and aims at the evaluation of performance parameters along with identification of optimal operating conditions and estimation of second law efficiency of major constituent components of triple combined cycle configuration. Results obtained from the study can be used for creating a prototype for field trial and future improvements in it. The total power of 331.59 kW per kg of air entering in the topping cycle is produced by the triple combined cycle having gas turbine, steam turbine and organic Rankine cycle turbine along with the thermal efficiency up to 33.15% at cycle pressure ratio of 24 in Indian context. Second law analysis of the considered triple combined cycle helps in identifying the sources of inefficiencies in the cycle and the second law efficiency of the major constituent components is found to be 97.07% for gas turbine followed by 96.05% for the compressor, 81.33% for ORC turbine, 80.31% for LPST and 79.5% for HPST.