Observations on efficiency of heat engines operating at maximum power

Abstract

The behavior of the efficiency of a general class of cyclic endoreversible heat engines at maximum power point is considered. The functional temperature dependence of heat transfer is considered as a variable, and its physical significance is addressed. It is shown that a broader spectrum of solutions emerges for efficiency at maximum power, as a function of the relevant system parameters, than originally realized. Specifically, it is proved that efficiency at maximum power is in general not independent of the heat-transfer coefficients, and that a type of ‘‘symmetry’’ exists relative to the functional temperature dependence of Newtonian heat conduction. The ‘‘Curzon–Ahlborn efficiency’’ of finite-time thermodynamics is shown not to be a fundamental upper limit on the efficiency of an endoreversible engine at maximum power point. Rather, this upper limit depends both on the functional temperature dependence of the heat transfer and on the relative value of the hot- and cold-side heat-transfer coefficients.