2 - Power plant thermodynamics

Abstract

This chapter begins with the derivation of expressions for ideal power plant efficiencies when heat is received from a finite heat source and emphasises the importance of Carnot's principle rather than the Carnot cycle. It is shown that it is impossible to maximise power recovery and cycle efficiency simultaneously and that, as a first approximation, the atmosphere can be treated as an infinite heat sink. The importance of work ratio is emphasised and it is shown that, as a consequence, power plant cycles based on ideal gases as working fluids are unsuitable for systems recovering power from low-grade heat. When generating power from an external heat source, the importance of matching the temperature changes of the heat source and the working fluid in the primary heat exchanger is demonstrated, and it is shown that this limits the value of Carnot or Stirling cycles. Cycles based on pressurising, evaporating, expanding and condensing volatile fluids are shown to be superior both because of their very high work ratios and their superior matching characteristics. The principles of working fluid selection are described for selecting volatile fluids that combine good temperature matching and high work ratio in order to maximise the power recovery from single-phase heat sources.