Organic, Rankine, Waste, Heat, Zeotropic
An organic Rankine cycle (ORC) is a thermodynamic cycle that is particularly well-suited for waste heat recovery. It is generally employed for waste heat with temperatures in the range of 80 °C – 300 °C. When the application is strictly to convert waste heat into work with no restrictions on heat source exit temperature, thermal efficiency is not as relevant as other aspects of the cycle performance. In such an application, maximization of net power may be the objective rather than maximization of thermal efficiency. An air-cooled ORC for waste-heat conversion is studied in the present work. Two alternative cycle configurations which could increase the net power produced from a heat source with a given temperature and flow rate are proposed and analyzed. These cycle configurations are: • An ORC with two-phase flash expansion • An ORC with a zeotropic working fluid mixture (ZRC) A simplified ORC model is introduced which calculates the pinch point in the heat exchangers based on a specified minimum temperature difference. This model is used to assess the merits of each cycle configuration with respect to a baseline ORC when the finite capacity of the heat source and heat sink fluids is considered. The finite capacity of the heat sink fluid is incorporated into the model in terms of a condenser fan power requirement. Of all working fluids studied for the baseline ORC, R134a and R245fa result in the highest net power. The ORC with two-phase flash expansion offers the most improvement over the baseline cycle provided the expander can handle two-phase flow at the same isentropic efficiency as in the baseline case. Relative improvements are highest at low source temperatures. The maximum increase in net power is 84% over the baseline ORC when water is the working fluid at a source temperature of 80 °C. At low source temperatures, the improvements decrease with increasing condenser fan power requirements. The improvements of the ZRC are also higher for low heat source temperatures. The ZRC shows improvement between 20% and 40% over the baseline as long as the condenser fan power is not negligible. At the highest estimated condenser fan power, the ZRC shows up to 92% improvement at a source temperature of 100 °C, while the ORC with flash expansion is no longer beneficial. This work represents a first step toward identifying a more optimal ORC configuration for waste heat recovery. Other data, including experimental validation, operating experience, and economic analysis particular to an application will be required to support a final recommendation.