5 - Efficiency of heat engines at maximum power

Abstract

This chapter explains the efficiency of an internally reversible heat engine when producing maximum power output. It explains the thermal efficiency of a Carnot cycle operating between high temperature (TH) and low temperature (Tc) reservoirs. This cycle is extremely idealized. It requires an ideal, reversible heat engine (internally reversible) but, in addition, the heat transfer from the reservoirs is also reversible (externally reversible). To achieve external reversibility it is necessary that the temperature difference between the reservoirs and the engine is infinitesimal, which means that the heat exchanger surface area must be very large or the time to transfer heat must be long. The former is limited by size and cost factors whilst the latter will limit the actual power output achieved for the engine. It is possible to evaluate the maximum power output achievable from an internally reversible (endoreversible) heat engine receiving heat irreversibly from two reservoirs at TH and Tc.