  The three-terminal heat device consisting of a cavity and coupled to a heat
bath is established. By tuning the temperatures of the electrodes and the
phonon bath, the device can function as a heat engine or a refrigerator. We
study the characteristic performance in the linear and nonlinear regime for
both setups. It is our focus here to analyze how the efficiency of the heat
engine and coefficient of performance of the refrigerator are affected by the
nonlinear transport. With such considerations, the maximum efficiency and power
are then optimized for various energy levels, temperatures and other
parameters.
