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1. Introduction 1.1. Potential of the ORC for waste heat recovery Low and medium grade waste heat utilization for electricity production has attracted significant interest as a means to alleviate the energy shortage and environmental pollution problems associated with excess CO2 and other air pollutants (NOx, SOx etc) emissions [1]. Most typical sources of abundant waste heat can be found in industrial processes of steel, cement, glass, oil and gas [2]. Schuster et al. report [3] that the heat rejected by various industrial processes can surpass 50% of the total initial heat produced. Additionally, remarkable amounts of waste heat are carried by exhaust gases that are rejected from internal combustion engines [4]. Campana et al. [2] have assessed the potential of electricity generation from waste heat in Europe and have estimated that a total of 21.6 TWhe can be produced on an annual basis, with savings amounting to 1.95 billion € and 8.1 million tonnes of Greenhouse Gas (GHG) emissions. Considering the above, it becomes evident that, apart from its positive environmental impact, waste heat recovery (WHR) for the production of electricity can lead to considerable economic benefits for the relevant industries, by helping reduce energy demands and subsequently allow to decrease fuel consumption costs. The Organic Rankine Cycle (ORC) has been repeatedly proposed and extensively investigated as an appealing technology for WHR-to-power applications [5-9]. It is also already a widespread technology as a bottoming cycle for geothermal applications and biomass fired power plants [10, 11]. Hajabdollahi et al. [6] modelled and optimized an ORC for diesel engine waste heat recovery and identified R123 and R245fa as the best working fluid candidates from a thermo-economic standpoint. Quoilin et al. [7] developed a thermo-economic model for evaluating WHR-ORCs. By comparing a number of refrigerants under varying operational conditions, he stressed that the thermo-economic and the thermodynamic optimization of WHR-ORC systems may give different results. Wei et al. [8] underlined the importance of maximizing the utilization of the heat source for increasing the power output of the ORC. Dai et al. [9] compared the conventional water-steam Rankine cycle with the ORC for low grade (145 oC) heat sources. He concluded that the ORCs are more efficient, with R236EA exhibiting the highest exergy efficiency among the working fluids that he examined. Some of the ORC’s primary advantages against the conventional steam-water Rankine cycle include its capability to operate at very low temperatures, reduced cost, smaller component size and simplicity of construction and operation [1, 5, 12]. Two of the main operating parameters affecting the overall thermodynamic efficiency and cost competitiveness of ORC applications are the working fluid and its evaporation pressure/temperature [7]. 1.2. Working fluids and efficiency improvement strategies There is a wide variety of organic fluid candidates for ORC systems, including hydrocarbons, hydrofluorocarbons, hydrochlorofluorocarbons, perfluorocarbons, siloxanes etc [12]. Although many studies have focused on the selection of the most appropriate ones [1, 13, 14], no single fluid has been identified as ultimately optimal for all ORCs. This is in part due to the variability of the characteristics (temperature profile ranges, physicochemical properties etc.) of the several heat sources considered. Furthermore, it can be also attributed to the different cycle working conditions that are assumed in each case, as well as to the different indicators used by various researchers for evaluating the system performance [14]. So far a number of refrigerants have been already banned or are to be phased out in the proximate future, in accordance with legislation (e.g. Montreal Protocol [15] and Kyoto Protocol [16]) aiming to restrict the use of high Ozone Depletion Potential (ODP) and Greenhouse Warming Potential (GWP) fluids. The European Union through a series of regulations [17, 18] aims to progressively decrease the production of certain fluorinated gases (F-gases) by 80% of today’s level until 2030. Because of the above legal restrictions, the price of typically used ORC-fluids, such as R245fa is expected to increase dramatically. The result is a shift of interest towards the use of low ODP and 3PDF Image | Thermodynamic investigation of waste heat recovery
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