Performance comparison of an endoreversible closed variable-temperature heat reservoir intercooled regenerated Brayton cycle under maximum power and maximum power density conditions

Abstract

In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as the objective for performance analysis of an endoreversible closed intercooled regenerated Brayton cycle coupled to variable-temperature heat reservoirs from the viewpoint of finite-time thermodynamics (FTT) or entropy generation minimization. The analytical formulae for the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers, the intercooler, and the regenerator. Some results in the recent FTT literature are replicated. The intercooling pressure ratio is optimized for dimensionless power density. The effects of component (the intercooler, the regenerator, and the hot- and cold-side heat exchangers) effectivenesses, the thermal capacity rate of the working fluid, the heat reservoir inlet temperature ratio, and the inlet temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir on maximum power density and its corresponding efficiency and intercooling pressure ratio are analysed using detailed numerical examples. The obtained results are compared with those obtained by using the maximum power criterion, including the cycle efficiencies, the normalized maximum specific volumes, and the normalized maximum specific volume differences at two design objectives. The advantages and disadvantages of maximum power density design are obtained via comparison of the main parameters of the cycle at maximum power and maximum power density conditions. The regeneration makes the maximum dimensionless power density decrease, and the intercooling makes the maximum dimensionless power density increase. The finite thermal capacity rates of the heat reservoirs do affect the performance of the variable-temperature heat reservoir cycle even for reversible cycles free of heat transfer irreversibility in general. The maximum power density design has the advantage of smaller size, but it requires a higher pressure ratio and has a lower efficiency than the maximum power design.