Thermodynamic analysis of subcritical/transcritical ORCs with metal–organic heat carriers for efficient power generation from low-grade thermal energy

Abstract

Recovery of low-grade energy for power generation by organic Rankine cycle (ORC) can effectively alleviate energy crisis and reduce CO2 emission, but the low heat source temperature severely affects the thermal efficiency and output power of the system. Using new working fluids with enhanced thermal properties is one promising approach to improve cycle performance. Based on the concept of nanofluids with conventional refrigerants and metal–organic heat carriers (MOHCs), thermodynamic models of subcritical/transcritical basic ORC and regenerative ORC (RORC) with R134a/Cr-MIL-101 and R134a/Mg-MOF-74 nanofluids are constructed. Thermodynamic analysis results reveal that adsorption of R134a from MOFs increases pump work input and the turbine work output, while desorption increases the evaporator heat input. Also, MOHCs improve the thermal efficiency of both ORC and RORC, and the thermal efficiency improvement ratio of transcritical cycle is maximized at high evaporation pressure and low turbine inlet temperature. By adding a 0.9% mass fraction of Mg-MOF-74 to R134a, the maximum thermal efficiency of RORC is improved from 13.66% to 16.61%, and its maximum specific net work output is increased from 27.82 kJ/kg to 34.42 kJ/kg. Furthermore, both exergy efficiency and economic benefit are improved with MOHC-added systems.