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energies Article Carbon Dioxide Mixtures as Working Fluid for High-Temperature Heat Recovery: A Thermodynamic Comparison with Transcritical Organic Rankine Cycles Abubakr Ayub 1 , Costante M. Invernizzi 1,* , Gioele Di Marcoberardino 1 , Paolo Iora 1 and Giampaolo Manzolini 2 1 Department of Mechanical and Industrial Engineering, University of Brescia, via Branze, 38, 25123 Brescia, Italy; a.ayub@unibs.it (A.A.); gioele.dimarcoberardino@unibs.it (G.D.M.); paolo.iora@unibs.it (P.I.) Energy Department, Politecnico di Milano, 20156 Milan, Italy; giampaolo.manzolini@polimi.it 2 * Correspondence: costante.invernizzi@unibs.it; Tel.: +39-030-3715-569 Received: 30 June 2020; Accepted: 24 July 2020; Published: 4 August 2020 Abstract: This study aims to provide a thermodynamic comparison between supercritical CO2 cycles and ORC cycles utilizing flue gases as waste heat source. Moreover, the possibility of using CO2 mixtures as working fluids in transcritical cycles to enhance the performance of the thermodynamic cycle is explored. ORCs operating with pure working fluids show higher cyclic thermal and total efficiencies compared to supercritical CO2 cycles; thus, they represent a better option for high-temperature waste heat recovery provided that the thermal stability at a higher temperature has been assessed. Based on the improved global thermodynamic performance and good thermal stability of R134a, CO2-R134a is investigated as an illustrative, promising working fluid mixture for transcritical power cycles. The results show that a total efficiency of 0.1476 is obtained for the CO2-R134a mixture (0.3 mole fraction of R134a) at a maximum cycle pressure of 200 bars, which is 15.86% higher than the supercritical carbon dioxide cycle efficiency of 0.1274, obtained at the comparatively high maximum pressure of 300 bars. Steam cycles, owing to their larger number of required turbine stages and lower power output, did not prove to be a suitable option in this application. Keywords: transcritical cycles; waste heat recovery; fluid mixtures; carbon dioxide; Organic Rankine Cycles 1. Introduction The increase of conversion efficiency and promotion of energy recovery represent two effective strategies for primary energy savings. In particular, energy recovery from waste industrial heat represents an interesting option which is currently widely accepted and applied where possible [1]. According to its temperature level, waste heat can be used in a wide range of applications: from heat sources for heat pumps in district heating systems [2] to its direct utilization for preheating air or for steam generation and finally for power production by thermodynamic cycles [3–5]. According to [6], the waste heat recovery potential in industry in the EU accounts for about 920TWh, with Germany, Italy, France and UK contributing about 50% of the overall potential. Moreover, about 30% of the theoretical waste heat is available at temperatures higher than 270 ◦C. Energies 2020, 13, 4014; doi:10.3390/en13154014 www.mdpi.com/journal/energiesPDF Image | CO2 Mixtures as Working Fluid for High-Temperature Heat Recovery
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