Optimal analyses and performance bounds of the low-dissipation three-terminal heat transformer: The roles of the parameter constraints and optimization criteria

Heat transformers, also referred to as temperature boosters, draw considerable attention for the great potential in exploiting low-grade thermal energy. In this paper, a generic model of three-terminal heat transformer without reference to any specific heat-transfer law is established on the basis of the low-dissipation assumption. Accordingly, the optimum behaviors and parametric choices under two different parameter constraints are investigated and compared. Notably, the connection between overall time constraint and the presence of external heat leak for the three-terminal heat transformer is revealed. In addition, the Omega function based on the trade-off consideration is introduced to provide more practical evaluations. The performances of the three-terminal heat transformer operated at maximum Omega regime are derived and found to be less powerful but more efficient comparing with the associated maximum heating load regime. Moreover, the influences of dissipation symmetry on several key performance indicators are elaborated by using numerical calculation, which leads to the important results of the present paper, namely the performance bounds of coefficient of performance (COP) at maximum heating load and maximum Omega regimes. Finally, the reported COPs from previous researches are collected to illustrate the validity and practical significance of the proposed model and associated performance bounds.