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GWP natural refrigerants (such as butane, pentane, hexane etc.) as working fluids. These refrigerants are available at low prices and are viewed as promising options concerning the overall system efficiency of the ORC. However, a major safety issue concerning their use in ORC systems is their high flammability and the consequent explosion risk. Therefore, the design and operation of such systems requires special attention. Meanwhile, a number of studies have suggested that the ORC energy and exergy efficiency can be further improved when the working fluid is pumped to supercritical pressures before it is heated to its maximum temperature [19-21]. The main concept behind the application of the supercritical ORC relies on the expected minimization of the exergy destruction losses during the heating of the supercritical working fluid, which takes place under a variable temperature (contrary to the evaporation which occurs isothermally). Schuster et al. [19] showed that for a number of working fluids, the supercritical ORC can lead to an increase of the thermal and overall system efficiency of waste heat applications to up to 8%. According to Vetter et al. [20], who performed thermodynamic simulations considering a large number of working fluids, an increase in both thermal efficiency and net power output for low enthalpy (~150-170 oC) processes is achieved under supercritical operation of the cycle. Mikielewicz et al. [21] reports that through the implementation of the supercritical ORC, an overall efficiency improvement of roughly 5% can be attained in micro CHP applications. Despite these potential efficiency benefits, however, it should be noted that in supercritical conditions, the necessary heat exchanger surface is generally larger [22], and thus the heat exchange equipment may be more costly and require rigorous design. Another possible improvement of the ORC is the replacement of pure substances with zeotropic mixtures as working fluids for the cycle [1, 23, 24]. Contrary to pure fluids, the subcritical isobaric phase change of these mixtures takes place non isothermally. The temperature glide of the working fluid during its evaporation and condensation permits a better matching of its temperature with the temperature of the heat source (evaporator) and the cooling medium (condenser). This, in turn, provides the opportunity to reduce the irreversibility that occurs in these process steps of the ORC and increase the exergetic efficiency of the system [23-25]. Heberle et al. [23] examined the potential of R227ea/R245fa and isobutene/isopentane mixtures for low temperature geothermal applications. He estimated that the use of these zeotropic mixtures can lead to an increase in the exergy efficiency from 4.3 to 15% compared to their most efficient pure components. Liu et al. [24] explored the thermodynamic performance of a number of zeotropic mixtures. Like Heberle et al. [23], he emphasized the matching of the temperature glide during the condensation of the working fluid with the temperature change of the cooling medium as an optimization criterion for estimating the ideal concentration of the mixture components. A detailed thermodynamic and economic comparison between the use of fluid mixtures and the supercritical ORC for geothermal waste heat utilization is given in [26]. 1.3. Objectives of the present work The present work and has a dual purpose. Firstly, it aims to evaluate the performance of the WHR- ORC when using natural hydrocarbons as working fluids. The second goal is to investigate the potential increase of the cycle efficiency by using binary zeotropic mixtures consisting of pure natural refrigerants. In both cases, the operation under subcritical and supercritical pressures is simulated by thermodynamically modelling the WHR-ORC system. The significance of this paper can be summarized in the following points. To begin with, given the advantages of the natural hydrocarbons against artificial refrigerants regarding their cost, environmental behaviour and safety, it is very interesting to further evaluate their attractiveness as working fluids candidates for the ORC based on their thermodynamic behaviour. Secondly, the work aims to assess two of the most promising improvements of the ORC for these natural hydrocarbons: the supercritical operation and the use of working fluids consisting of binary zeotropic mixtures. Most importantly, to the knowledge of the authors, the combined effect of supercritical operation pressures and the use of binary mixtures in the ORC has not yet been investigated. 4PDF Image | Thermodynamic investigation of waste heat recovery
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