Equivalent combined cycle modeling and performance optimization for a three-heat-reservoir thermal Brownian heat transformer with external heat-transfer

Three-heat-reservoir (THR) heat transformers can upgrade temperature gradient of thermal energy, and lots of instructive research has been conducted in the last decades. However, study of THR heat transformer cycle theory in micro systems remains lacking. By employing macro equivalent combined cycle method, this paper builds a finite-time thermodynamic model of THR thermal Brownian heat transformer, which is a combination of a two-reservoir thermal Brownian heat pump driven by a two-reservoir thermal Brownian engine. Expressions of performance parameters are deduced, and operating temperatures and external load ratio are determined by solving heat balance equations. Maximal heating load and corresponding coefficient of performance (COP) are given by modulating external load, heat exchanger inventory allocations and barrier height synchronously, and the optimal thermal conductance allocations and optimal working temperatures are identified. Results indicate that external thermal resistances affect heat flow transmission and the coupling of combined cycle, ultimately shaping the cycle performance. Half of the overall heat exchanger inventory should be arranged in middle heat exchanger under maximal heating load objective. Heating load exhibits an extremum with respect to COP. Equivalent combined cycle modelling is an effective and efficient method for performance optimization of THR thermal Brownian heat transformers with external heat-transfer.