Thermodynamic Analyses of Single Brayton and Combined Brayton–Rankine Cycles for Distributed Solar Thermal Power Generation

Abstract

This paper reports theoretical efficiencies of single Brayton and combined Brayton–Rankine thermodynamic power cycles for distributed solar thermal power generation. Thermodynamic analyses are conducted with a nominal heat input to the cycle of 150 kW and component parameters for a 50 kWe gas microturbine for selected working fluids including air, Ar, CO2, He, H2, and N2 for the Brayton cycle and for the topping cycle of the combined system. Cycle parameters including maximum fluid temperature based on solar concentration ratio, pressure loss, and compressor/turbine efficiencies are then varied to examine their effect on cycle efficiency. C6fluoroketone, cyclohexane, npentane, R141b, R245fa, and HFE7000 are examined as working fluids in the bottoming segment of the combined cycle. A single Brayton cycle is found to reach a peak cycle efficiency of 15.31% with carbon dioxide at design point conditions. Each Brayton cycle fluid is examined as a topping cycle fluid in the combined cycle, being paired with six potential bottoming fluids, resulting in 36 working fluid configurations. The combination of the Brayton topping cycle using carbon dioxide and the Rankine bottoming cycle using R245fa gives the highest combined cycle efficiency of 21.06%.