Considerations on alternative organic Rankine Cycle congurations for low-grade waste heat recovery

Abstract

When organic Rankine cycles (ORC) are employed to convert waste heat into work, the thermal efficiency is not recommended as a key performance metric because waste heat recovery and power output are not generally maximized at the point of peak efficiency. In such an application, maximization of the net power output should be the objective. Two alternative cycle configurations that can increase the net power output from a heat source with a given temperature and flow rate are analyzed and compared to a baseline ORC. These cycle configurations are: an ORC with two-phase flash expansion (TFC) and an ORC with a zeotropic working fluid mixture (ZRC). A design-stage ORC model allowed a consistent comparison of multiple ORC configurations with finite capacity of the source and heat sink fluids. Simulation results indicated that the TFC offered the most improvement over the baseline ORC, but required a highly efficient two-phase expansion. The ZRC shows improvement over the baseline as long as the condenser fan power requirement is not negligible. At the highest estimated condenser fan power, the TFC is no longer beneficial. Finally, a partial-evaporating ZRC (PE-ZRC) has also been considered to reduce the volume ratio requirements of a flash expansion.