Potential of transcritical recompression Rankine cycle operating with CO2-based binary mixtures

CO2 transcritical Rankine cycle (CO2-TRC) has a great potential for many applications like concentrating solar power (CSP) and waste heat recovery due to advantages of using low- or medium-temperature heat sources and compact structures. However, the relatively low critical temperature of CO2 and the simple regenerative cycle layout limit the applicability to hot arid environmental conditions. In this work, a transcritical recompression Rankine cycle (TRRC) applying CO2-based binary mixture as working fluid is proposed to improve thermal performances. A thermodynamic model is developed and applied to estimate the potential of the cycle for CSP or waste heat recovery applications with ambient temperature in the range of 0-35 degrees C. The results show that adding gaseous media (i.e. propane, H2S, R32 and R161) into CO2 increases the critical temperature and thus extends the condensation temperature range. The decreasing condensation pressure mainly contributes to the improvement of the optimal thermal efficiency, exergy efficiency and specific work of TRRC. Among the selected working fluids, the TRRC applying CO2-R32 shows the greatest potential under the medium-temperature heat source (e.g. turbine inlet temperature of 400 degrees C) and relatively high ambient temperature (e.g. 15 similar to 35 degrees C). When the ambient temperature is 35 degrees C, the corresponding thermal efficiency, exergy efficiency and specific work of the TRRC applying CO2-R32 are 32.77%, 58.99% and 36.93 kJ/kg, respectively, which are comparable to those of the pure CO2 TRRC at a lower ambient temperature of 15 degrees C.

Automated summary

The CO2 transcritical Rankine cycle has a lot of potential applications, but is limited by its low critical temperature and simple cycle layout. This study proposes a transcritical recompression Rankine cycle using CO2-based binary mixture to enhance thermal performance. Adding other gaseous media to CO2 increases the critical temperature and extends the condensation temperature range. Among the selected working fluids, the cycle using CO2-R32 shows the greatest potential under medium-temperature heat sources and relatively high ambient temperatures.