Thermodynamic screening of organic rankine cycle working fluids and architectures: application to automotive internal combustion engines

Abstract

In the presented work, three promising ORC architectures are thermodynamically investigated for application on internal combustion engines for long-haul trucks. The cycles examined are the subcritical ORC (SCORC), the partial evaporating ORC (PEROC) and the transcritical ORC (TCORC). The employed screening approach has previously been developed by the authors and is now adapted for this particular application. In total 67 working fluids are considered. Four specific cases are postulated. These include various heat source (350°C, 500°C) and heat sink (25°C, 50°C, 75°C, 100°C) conditions and two levels of maximum cycle pressure (32 bar and 50 bar). Additionally, the effect of selecting a volumetric machine as expander type is examined. The results show that the PEORC and the TCORC give the highest second law efficiencies. However, when a simple low expansion ratio volumetric expander is selected, subcritical ORCs gave the highest second law efficiencies. Furthermore methanol and ethanol, operating under subcritical conditions, give generally good results for all cases in the study.