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

In this paper, the power density, defined as the ratio of power output to t he maximum specific volume in the cycle, is taken as the objective for perf ormance analysis and optimization of an irreversible regenerated closed Bra yton cycle coupled to variable-temperature heal reservoirs from the viewpoi nt of finite time thermodynamics (FTT) or entropy generation minimization ( EGM). The analytical formulae about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the it-reversible compressi on and expansion losses in the compressor and turbine, the pressure drop lo sses at the heater, cooler and regenerator as well as in the piping, and th e effect of the finite thermal capacity rate of the heat reservoirs. The ob tained results are compared with those results obtained by using the maximu m power criterion, and the advantages and disadvantages of maximum power de nsity design are analysed. The maximum power density optimization is perfor med in two stages. The first is to search the optimum heat conductance dist ribution corresponding to the optimum power density among the hot- and cold -side heat exchangers and the regenerator for a fixed total heat exchanger inventory. The second is to search the optimum thermal capacitance rate mat ching corresponding to the optimum power density between the working fluid and the high-temperature heat source for a fixed ratio of the thermal capac itance rates of two heat reservoirs. The influences of some design paramete rs, including the effectiveness of the regenerator, the inlet temperature r atio of the heat reservoirs, the effectiveness of the heat exchangers betwe en the working fluid and the heat reservoirs, the efficiencies of the compr essor and the turbine, and the pressure recovery coefficient, on the optimu m heat conductance distribution, the optimum thermal capacitance rate match ing, and the maximum power density are provided by numerical examples. The power plant design with optimization leads to a smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the reg enerator. When the heat transfers between the working fluid and the heat re servoirs are carried out ideally, the pressure drop loss may be neglected, and the thermal capacity rates of the heat reservoirs are infinite, the res ults of this paper then replicate those obtained in recent literature.