Carbon footprint analysis of organic rankine cycle system using zeotropic mixtures considering leak of fluid

Abstract

The energy demand and severe environmental pollution problems have drawn global attentions. Organic Rankine cycle (ORC) as a waste heat power generation technology has broad application prospects. Emissions of carbon dioxide equivalent (CO2,eq) for measuring global warming potential and the emission reductions during life-time can directly reflect the environmental impact of ORC. A carbon footprint evaluation method for ORC using zeotropic mixture is developed in life cycle perspective. The security, thermodynamic and environmental criteria of binary mixture in ORC are also incorporated to evaluate the system environmental influence. Zeotropic mixtures, including R134a/R290, R134a/R600, R134a/R600a, R245fa/R600a, R245fa/R290, R227ea/R600a and R227ea/R290, are selected as the working fluids. Results showed that ORC with R245fa/R600a operated under environmental criterion produced the minimum CO2,eq emission of 26.30 g CO2,eq/kWh and the system with R227ea/R600a operated under thermodynamic criterion possessed the highest emission reduction of 5595.76 tons CO2,eq. Compared with the cases operated under security and environmental criteria, ORC operated under thermodynamic criterion generated a higher net power output at the expense of larger emissions. The primary source of CO2,eq emission from equipment is the heat exchangers while the counterpart from working fluid is during leak process. Meanwhile, the implementation of environmental criterion reduces the CO2,eq emissions of working fluid in ORC significantly, compared with cases under security and thermodynamic criteria. In consideration of the maximum emission reductions of CO2,eq, the compositions of the mixtures, i.e. R245fa/R600a and R245fa/R290, are mainly determined according to the environmental criterion. In addition, it is found the emission reductions decrease linearly with the leak rates of working fluids in ORCs in this work.