  We present results obtained by using nonlinear irreversible models for heat
devices. In particular, we focus on the global performance characteristics, the
maximum efficiency and the efficiency at maximum power regimes for heat
engines, and the maximum coefficient of performance (COP) and the COP at
maximum cooling power regimes for refrigerators. We analyze the key role played
by the interplay between irreversibilities coming from heat leaks and internal
dissipations. We also discuss the relationship between these results and those
obtained by different models.
