A study of the optimal conditions for organic Rankine cycles coupled with vapour compression refrigeration using a rigorous approach based on the Helmholtz energy function

Abstract

VCR-ORC systems combine an organic Rankine cycle (ORC) to produce energy from low-moderate temperature sources and a standard refrigeration cycle (VCRC). Accurate objective functions for the optimal performance of the system are developed based on the Helmholtz energy function and its derivatives. Moreover, the utility of two objective functions is analysed in the search for the optimal conditions of a VCR-ORC system. Finally, it presents a comparative analysis of several working fluids to give the directives in selecting an optimal working fluid and its operation optimal conditions. The method described above uses a set of refrigerants and the PC-SAFT EOS as the prediction model. The results presented in this work show that variables such as the condenser temperature, T1, are fundamental for the optimisation of VCR-ORC systems. Pentane and R1233zd have the best performance for this system. However, the fourth-generation refrigerant shows the best operational condition in all the indicators among the studied fluids despite its wet expansion in optimal operation. In the idealised VCR-ORC cycle, the optimal ORC efficiency links with the optimal COP of the cycle while the extracted heat links with the optimal mechanical work produced by the ORC.