THERMO-ECONOMIC ANALYSIS OF ZEOTROPIC MIXTURES AND PURE WORKING FLUIDS IN ORGANIC RANINKE CYCLES FOR WASTE HEAT RECOVERY

asme-orc2015 Tracking Number 7

We present a thermo-economic analysis of an Organic Rankine Cycle (ORC) for waste heat recovery using a low-temperature heat source. A case study for a heat source temperature of 150 °C and a subcritical saturated cycle is performed. As working fluids R245fa, isobutane, isopentane and the zeotropic mixture of isobutane and isopentane are considered. The minimal temperature difference in the preheater and condenser as well as mixture composition are chosen as variable parameters. The aim is to identify the most suitable working fluid in combination with optimal process parameters under thermo-economic criteria. In the present study, shell and tube heat exchanger are designed based on simulation results. Depending on the capacity respectively heat transfer surface, cost estimations for the major components of the ORC system are conducted. Finally, specific costs of the power plant and of the generated electricity are determined. In general, the most cost effective systems show a high minimal temperature difference ΔTPP,C at the pinch-point of the condenser and a low minimal temperature difference ΔTPP,E at the pinch-point of the evaporator. Exemplarily, the design parameters ΔTPP,E = 1 K and ΔTPP,C = 13 K lead to minimal costs of 56.82 €/GJ for R245fa. Choosing isobutane as working fluid leads to the lowest specific costs of electricity with 51.97 €/GJ at ΔTPP,E = 1.2 K and ΔTPP,C = 14 K. Considering only the major components, specific costs for the ORC module range between 1150 €/kWel and 2250 €/kWel. In case of the mixture isobutane/isopentane as working fluid, a mole fraction of 90 % isobutane leads to lowest specific costs of electricity. Although a higher power output is obtained, the specific costs are 2 % higher compared to isobutane. An overcompensation of the additional expenses for heat exchange equipment occurs for higher process integration costs. Finally, a sensitivity analysis for an ORC system using isobutane as working fluid is performed to identify particularly relevant boundary conditions. Based on standard conditions, the specific costs of electricity show the highest sensitivity for a variation of process integration costs and isentropic efficiency of the turbine. Especially for the evaluation of fluid mixtures as potential working fluids, a comprehensive analysis of fluid properties and heat transfer characteristics is required to minimize uncertainties of heat exchanger design and cost estimations.