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Cascaded dual-loop organic Rankine cycle with alkanes and low global warming potential refrigerants as working fluids

Abbas, Wameedh Khider Abbas; Vrabec, Jadran

FG Thermodynamik und Thermische Verfahrenstechnik

A cascaded dual-loop organic Rankine cycle (CD-ORC) consisting of a high-temperature ORC (HT-ORC) and a low-temperature ORC (LT-ORC) with a shared heat exchanger is studied. Thermal efficiency is investigated as a key indicator for system performance over a wide range of heat source temperature ranging from 170 °C to 330 °C. The relation between the thermal efficiency and the critical temperature of the working fluids is explored. For this purpose, six alkanes and 31 refrigerants with a low GWP (≤150) are considered as working fluids in the HT-ORC and the LT-ORC, respectively. Cyclohexane and cyclopentane are found to be suitable for the HT-ORC with a maximum thermal efficiency of 19.13% and 18.03%. The thermal efficiency of both loops is highly affected by the working fluids since it increases with the critical temperature. As a whole, the CD-ORC may achieve a total thermal efficiency of 25.24%, 24.88% and 24.60% when using the working fluid combinations cyclohexane-R1366mzz(Z), cyclohexane-R1233zd(E) or cyclohexane-butane. This indicates that low GWP refrigerants are well-suited for LT-ORC. Compared to regular ORC, CD-ORC systems may achieve a thermal efficiency that is better by around a quarter.