Thermo-economic-environmental optimization design of dual-loop organic Rankine cycle under fluctuating heat source temperature

Abstract

Fluctuating heat source temperature leads to off-design operation of organic Rankine cycle systems, which is a challenge for its optimal design. For a dual-loop organic Rankine cycle, this study proposed a multi-objective optimization design framework, which integrates off-design optimization into design optimization to determine the optimal design point of the system under heat source temperature fluctuations. In the optimization design framework, we developed a multi-objective optimization model to maximize the exergy efficiency and annual CO2 emission reduction of the dual-loop organic Rankine cycle and minimize its payback period. Then, the effects of design heat source temperature Td,h on the lifetime performance of dual-loop organic Rankine cycle were investigated. To balance the conflict between different indexes, we proposed a comprehensive evaluation index based on the entropy method. The results show that the influence of actual heat source temperature on the performance of dual-loop organic Rankine cycle is greater than the Td,h. Compared with traditional research that ignore the off-design performance, the proposed design method can avoid 35.8% annual CO2 emission reduction overestimation and 58.7% payback period underestimation at Td,h of 573.1 K. In addition, the optimal Td,h is significantly influenced by the selected index. The optimal Td,h is 513.1 K for the dual-loop organic Rankine cycle to minimize its lifetime payback period. To maximize the comprehensive index, the optimal Td,h for the dual-loop organic Rankine cycle is 573.1 K, which is the temperature at 0.9 of the cumulative distribution curve of the heat source. The average exergy efficiency and annual CO2 emission reduction in the lifetime of the dual-loop organic Rankine cycle at the optimal Td,h of 573.1 K is 4.1% and 6.0% higher than that at the average Td,h of 529.1 K, respectively.