True nature of the Curzon-Ahlborn efficiency

The Curzon-Ahlborn efficiency has long served as the definite upper bound for the thermal efficiency at maximum output power, and has thus shaped the development of finite-time thermodynamics. In this paper, we repeal the ruling consensus according to which it has a genuine universal character that can be derived from linear irreversible thermodynamics. We demonstrate that the Curzon-Ahlborn efficiency should instead properly be associated with a particular case of nonlinear heat engines, and we derive a generalized expression for the efficiency at maximum power beyond the restrictive case of linear models.

Figure 1

Thermodynamic picture of a generic heat engine, where sources of dissipation are found in the heat exchangers as well as within the energy converting part of the engine viewed as a global system.

Figure 2

Operating curves for the three different types of engine: (a) linear, (b) sublinear, and (c) superlinear.