Zeotropic working fluid selection for an organic Rankine cycle bottoming with a marine engine

Organic Rankine cycle (ORC) is an effective approach for low-grade energy utilization. It is important to improve the efficiency of an ORC when the heat source temperature varies. In this study, a method of selecting zeotropic mixture for a bottoming ORC with changeable heat source temperature is presented. The effects of the operation conditions of the marine engine and the ambient temperature are investigated. First, the optimal performances of the ORC using 40 pure working fluids are determined and compared. Then, two zeotropic mixtures benzene/m-xylene and cyclopentane/toluene are selected and the mechanism of temperature match with the heat source and sink is explored. Finally, the performance improvement with benzene/m-xylene using the composition adjustment method is evaluated. The results indicate that the suitable pure working fluids are isopentane and R245ca for a low exhaust temperature while toluene and m-xylene are the best when the exhaust temperature is high. Using the zeotropic mixtures benzene/m-xylene and cyclopentane/toluene can obtain a high performance over the operation range of the marine engine. When the exhaust temperature is 225 degrees C, the net power and exergy efficiency of benzene/m-xylene are improved by 6.9%-21.9% and 6.5%-22.0%, respectively, compared with the pure fluids benzene and m-xylene. When the exhaust temperature increases to 380 degrees C, these improvements decrease to 1.9%-6.8%. If a zeotropic mixture is used, the critical temperature of the component with a high-boiling point should be close to the maximum operation temperature of the heat source, and the temperature glide should first match with the temperature increase of the heat sink. If the heat sink temperature is fixed, it is impossible to enhance the ORC performance using the composition adjustment method. However, the composition adjustment method is effective when the temperature of the heat sink varies with the ambient temperature. The net power and exergy efficiency of the ORC are improved by up to 21.9% and 22% in winter using benzene/m-xylene. However, these improvements are less than 6.8% in summer. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Organic Rankine cycle (ORC) is an effective method for utilizing low-grade energy. This study presents a method for improving the efficiency of a bottoming ORC with variable heat source temperature by selecting zeotropic mixtures. The effects of the marine engine's operation conditions and ambient temperature are investigated, and the optimal performances of pure working fluids and zeotropic mixtures are compared. It is found that using zeotropic mixtures can enhance the performance of the ORC over a range of operating conditions.