Power density analysis and optimization of a regenerated closedvariable-temperature heat reservoir Brayton cycle

Abstract

In this paper, the power density, defined as the ratio of poweroutput to the maximum specific volume in the cycle, is taken as theobjective for performance analysis and optimization of an irreversibleregenerated closed Brayton cycle coupled to variable-temperature heatreservoirs from the viewpoint of finite time thermodynamics (FTT) orentropy generation minimization (EGM). The analytical formulae about therelations between power density and pressure ratio are derived with theheat resistance losses in the hot- and cold-side heat exchangers and theregenerator, the irreversible compression and expansion losses in thecompressor and turbine, the pressure drop losses at the heater, cooler andregenerator as well as in the piping, and the effect of the finite thermalcapacity rate of the heat reservoirs. The obtained results are comparedwith those results obtained by using the maximum power criterion, and theadvantages and disadvantages of maximum power density design are analysed.The maximum power density optimization is performed in two stages. Thefirst is to search the optimum heat conductance distribution correspondingto the optimum power density among the hot- and cold-side heat exchangersand the regenerator for a fixed total heat exchanger inventory. The secondis to search the optimum thermal capacitance rate matching corresponding tothe optimum power density between the working fluid and thehigh-temperature heat source for a fixed ratio of the thermal capacitancerates of two heat reservoirs. The influences of some design parameters,including the effectiveness of the regenerator, the inlet temperature ratioof the heat reservoirs, the effectiveness of the heat exchangers betweenthe working fluid and the heat reservoirs, the efficiencies of thecompressor and the turbine, and the pressure recovery coefficient, on theoptimum heat conductance distribution, the optimum thermal capacitance ratematching, and the maximum power density are provided by numerical examples.The power plant design with optimization leads to a smaller size includingthe compressor, turbine, and the hot- and cold-side heat exchangers and theregenerator. When the heat transfers between the working fluid and the heatreservoirs are carried out ideally, the pressure drop loss may beneglected, and the thermal capacity rates of the heat reservoirs areinfinite, the results of this paper then replicate those obtained in recentliterature.