Abstract
We present a thermo-economic analysis of an Organic Rankine Cycle (ORC) for waste heat recovery. 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 mixture of isobutane and isopentane are considered. The minimal temperature difference in the evaporator and the condenser, as well as the mixture composition are chosen as variables in order to identify the most suitable working fluid in combination with optimal process parameters under thermo-economic criteria. In general, the results show that cost-effective systems have 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. Choosing isobutane as the working fluid leads to the lowest costs per unit exergy with 52.0 €/GJ (ΔTPP,E = 1.2 K; ΔTPP,C = 14 K). Considering the major components of the ORC, specific costs range between 1150 €/kW and 2250 €/kW. For the zeotropic mixture, a mole fraction of 90% isobutane leads to the lowest specific costs per unit exergy. A further analysis of the ORC system using isobutane shows high sensitivity of the costs per unit exergy for the selected cost estimation methods and for the isentropic efficiency of the turbine.
Highlights
Organic Rankine Cycle (ORC) systems for waste heat recovery have a high growth potential [1].Numerous investigations are performed in order to maximize the efficiency of such power plants by working fluid selection with respect to the heat source temperature
Under the consideration of zeotropic mixtures as potential ORC working fluids, we provide a thermo-economic analysis of waste heat recovery ORC, in order to clarify if an efficiency increase overcompensates the additional heat transfer requirements
R245fa is examined as benchmark, since this working fluid is commonly used in existing ORC power systems at the considered to the specific models for For eachthe equation of state and the property models regarding each temperature level considered mixture, thetransport composition is varied in discrete steps of working are provideddifference in Appendix
Summary
Organic Rankine Cycle (ORC) systems for waste heat recovery have a high growth potential [1].Numerous investigations are performed in order to maximize the efficiency of such power plants by working fluid selection with respect to the heat source temperature. The use of zeotropic fluid mixtures is a promising optimization approach due to a good glide match of the temperature profiles at phase change. In this context, Angelino and Colonna di Paliano [2] show for a low-temperature application that mixtures of natural hydrocarbons (n-butane/n-hexane) lead to an efficiency increase of 6.8% compared to the pure working fluid n-pentane. Heberle et al [12] show high second law efficiencies for mixture compositions which lead to a good match of the temperature profiles at condensation These concentrations show a significant increase in heat exchange capacity. Similar results are obtained by Energies 2016, 9, 226; doi:10.3390/en9040226 www.mdpi.com/journal/energies
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