Increasing the efficiency and generated electricity of organic rankine cycles by using zeotropic mixtures as working fluids

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

We present a systematic and comparative study of the potential of zeotropic mixtures as working fluids in ORCs, considering most of the commonly used hydrocarbon and siloxane substances as components in various concentrations. We investigate the impact on the operation, the efficiency and power output of an ORC. The ORC cycle is realistically simulated in steady state conditions, taking into account all elements of an actual cycle (including an internal heat exchanger). By performing a pinch analysis, the power output is maximized, given the heat profiles of both the heat source and heat sink. The use of suitable zeotropic mixtures as working fluids has a positive effect on the ORC performance in all investigated cases. The potential increase in cycle efficiency and generated electricity is larger for lower temperature heat sources and when the temperature drop over the heat source exchanger is larger. When the ORC is optimized for operation with a low temperature heat source, the potential for increase in electricity production (maximum reported value approximately 23%) by using mixtures is particularly remarkable.