THERMODYNAMIC ANALYSES OF SINGLE BRAYTON AND COMBINED BRAYTON-RANKINE CYCLES FOR DISTRIBUTED SOLAR THERMAL POWER GENERATION

This paper explores the theoretical efficiencies of single Brayton and combined Brayton-Rankine thermodynamic power cycles for distributed solar thermal power generation. Thermodynamic analyses are conducted for the nominal solar power input to the receiver of 75 kW, concentration ratio in the range 50-100 suns, and for selected heat transfer fluids including air, argon, carbon dioxide, helium, and hydrogen for the Brayton cycle and for the topping cycle of the combined system. C6-fluoroketone, cyclohexane, n-pentane, R-141b, R-245fa, HEE-7000, and steam are examined as working fluids in the bottoming segment of the combined cycle. A single Brayton cycle is found to reach a peak efficiency of 13.3% with carbon dioxide and 100 suns solar input. The four top-performing Brayton cycle fluids are examined as topping cycle fluids in the combined cycle. Each of the four fluids is paired with seven potential bottoming fluids, resulting in 28 heat transfer fluid configurations. The combination of the Brayton topping cycle using carbon dioxide and the Rankine bottoming cycle using R-141b gives the highest thermal efficiency of 22.3% for 100 suns.