A review of thermodynamic cycles used in low temperature recovery systems over the last two years

Abstract

This review explores the potential of low and medium grade heat in different thermodynamic cycles used to transform wasted heat into mechanical work. The aim of this review is to study the state of the art of the thermodynamic cycles used to recover low-grade heat. The relevance of researching low grade heat or waste heat applications is that a vast amount of heat energy is available at negligible cost within the range of medium and low temperatures, with the drawback that existing thermal cycles cannot make efficient use of such available low temperature heat due to their low efficiency. The different types of Organic Rankine Cycles have been reviewed, highlighting their relevant characteristics where Simple Organic Rankine Cycle, Regenerative Organic Rankine Cycle, Cascade Organic Rankine Cycle, Organic Flash Cycles, Other Rankine Configurations and Trilateral Cycles are included. Reviews were conducted of specific applications of the low-grade heat recovery. In contrast, there are no actual publications which summarise the current state of the art of the thermodynamic cycles used to convert wasted heat into mechanical power. This paper offers a different approach and analyses low-grade heat recovery from a thermodynamic point of view and compares their efficiency. The analysis shows that cycles using closed processes are by far the most efficient published thermal cycles for low-grade heat recovery. Rankine cycles reviewed show similar low efficiencies. In contrast, closed process cycles have a configuration, which allows efficient exploitation of low-grade heat.