Optimum performance analysis of a combined thermionic-thermoelectric refrigerator with external heat transfer

An irreversible combined thermionic-thermoelectric refrigerator model with external finite rate heat transfer is established. The general expressions for cooling load and coefficient of performance (COP) versus applied voltage are derived. The performance of the irreversible combined refrigerator, in which the heat transfer between the device and the heat reservoir obeys Newton's heat transfer law, is analyzed and optimized by using the combination of finite time thermodynamics and non-equilibrium thermodynamics. The influence of external heat transfer is investigated by comparing the performance of irreversible combined refrigerator with the traditional analysis without heat transfer losses. The performances of the combined refrigerator device with external heat transfer are further compared with those of an independent vacuum thermionic refrigerator with and without considering external heat transfer, respectively. Moreover, for the fixed total heat transfer surface area of four heat exchangers, the allocations of the heat transfer surface area among the four heat exchangers are optimized for maximizing the cooling load and the COP. The effect of total heat transfer surface area on the optimum performance of the irreversible refrigerator is explored by numerical examples. The results obtained herein may provide guidelines for the design and application of practical combined thermionic-thermoelectric refrigeration devices.