The Many Avatars of Curzon-Ahlborn Efficiency

Efficiency at maximum power output of irreversible heat engines has attracted a lot of interest in recent years. Curzon-Ahlborn (CA) efficiency is a well-known result in this field, given by the expression $$1 - \sqrt {{T_c}/{T_h}}$$ , where Tc and Th are the cold and hot reservoir temperatures. We discuss the occurence of this particularly simple and elegant formula in different models and contexts.