  The energy conversion efficiency of far-from-equilibrium systems is generally
limited by irreversible thermodynamic fluxes that make contact with different
heat baths. For complex systems, the states of the maximum efficiency and the
minimum entropy production are usually not equivalent. Here we show that the
proper adjustments of the interaction between the energy and matter currents
offer some important criteria for the performance characterizations of thermal
agents, regardless of the system types and transition protocols. The universal
thermodynamic coupling rule plays a critical role in irreversible processes. A
double quantum dot system is applied to demonstrate that the performances of
heat engines or refrigrators can be enhanced by suitably adjusting the coupling
strength between thermodynamic fluxes.
