The Organic Rankine Cycle: Thermodynamics, Applications and Optimization

New energy conversion technologies are required in order to ensure the production of electricity without generating environmental pollution. Among them, low-grade heat sources or low power heat sources present an important potential of development.
The Organic Rankine Cycle (ORC) is a well known technology since the early 80’s. A large amount of ORC power plants have been built, mainly for geothermal, waste heat recovery and combined heat and power applications. This technology shows a number of advantages over the traditional steam Rankine cycle that make it more profitable for power plants with a limited electrical output power (typically lower than 1 MWe), despite a lower efficiency. The optimization of the ORC is quite different from that of the steam cycle, mainly because of the heat source temperature limitation, and because there is usually no constraint regarding the vapour quality at the end of the expansion.
This chapter presents an overview of the current state of the art in the ORC technology and exposes the main target applications. The modelling of such a cycle is described and issues such as fluid selection, optimization or control of the cycle are thoroughly reviewed.