The Optimal Evaporation Temperature of Subcritical ORC Based on Second Law Efficiency for Waste Heat Recovery

Chao Liu,Hong Gao,Xiaoxiao Xu,Jinliang Xu,Chao He

doi:10.3390/e14030491

Chao Liu, Hong Gao + Show 3 more

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https://doi.org/10.3390/e14030491

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Abstract

The subcritical Organic Rankine Cycle (ORC) with 28 working fluids for waste heat recovery is discussed in this paper. The effects of the temperature of the waste heat, the critical temperature of working fluids and the pinch temperature difference in the evaporator on the optimal evaporation temperature (OET) of the ORC have been investigated. The second law efficiency of the system is regarded as the objective function and the evaporation temperature is optimized by using the quadratic approximations method. The results show that the OET will appear for the temperature ranges investigated when the critical temperatures of working fluids are lower than the waste heat temperatures by 18 ± 5 K under the pinch temperature difference of 5 K in the evaporator. Additionally, the ORC always exhibits the OET when the pinch temperature difference in the evaporator is raised under the fixed waste heat temperature. The maximum second law efficiency will decrease with the increase of pinch temperature difference in the evaporator.

Highlights

Over the past years, with the increasing consumption of fossil fuels, more and more low-grade waste heat is directly released into the environment and some severe environmental problems, such as global warming, ozone depletion and thermal pollution, have arisen
Based on the analysis in this paper, the following conclusions could be made: The optimal evaporation temperature (OET) of the Organic Rankine Cycle (ORC) is related to the waste heat temperature, the critical temperature of working fluids and the pinch temperature difference in the evaporator
When the critical temperature of working fluids is lower than the temperature of waste heat by 18 ± 5 K and the pinch temperature difference in the evaporator is fixed at 5 K, OET in ORC will appear for the temperature ranges investigated

Summary

Introduction

With the increasing consumption of fossil fuels, more and more low-grade waste heat is directly released into the environment and some severe environmental problems, such as global warming, ozone depletion and thermal pollution, have arisen. To solve the problems mentioned above, recovering the low-grade waste heat is important. The use of conventional steam power cycles to recover low-grade waste heat, such as exhaust gas from engines and waste heat from industrial processes, is economically infeasible [1,2]. The Organic Rankine Cycle (ORC) has the potential to use low-grade waste heat and renewable energy sources, such as geothermal energy and solar energy [3,4,5,6,7,8]. Wang et al [9] investigated the effect of mass flow rate of working fluids on the performance of the cycle for pure and zeotropic mixtures working fluids in a low-temperature solar

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