Optimization of a Partially Evaporating Organic Rankine Cycle with thermal non-equilibrium expansion.

Abstract

The Trilateral Flash Cycle (TFC) is an alternative to the Organic Rankine Cycle (ORC). The TFC have been shown to have greater exergy efficiencies compared to the ORC in low-grade heat-to-power conversion. This is a result of the more efficient heat transfer from the heat carrier to the working fluid, even though the TFC has inherently a lower thermal efficiency. However, these results are based on cycle modeling assuming equilibrium conditions, but the actual two-phase expansion process is a nonequilibrium process. Thermal non-equilibrium between vapour and liquid occurs during the evaporation (or flashing) of the working fluid during the two-phase expansion process. The liquid phase has a temperature greater than the corresponding pressure’s saturation temperature and the equilibrium assumption overestimates the actual vapour quality. In a previous work, this non-equilibrium expansion was modeled and predicted based on the homogeneous relaxation model (HRM). In this work, the impact of the non-equilibrium process on the efficiency of the TFC is investigated and compared to the equilibrium model. The overall power recuperation is around 86% lower when including the thermal non-equilibrium nature of the expansion process. Therefore, the thermodynamic non-equilibrium losses should thus be incorporated when predicting the performance of a TFC.