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2.3. State of the art of the fourth generation of refrigerants The new restrictions imposed by the F-gas regulation pushed the research of new refrigerants or blends that could be efficient replacements to high GWP refrigerants, with minimal modifications to current system technologies, at the lowest possible cost. Concerning the environmental aspect, it is fundamental to consider both the direct and indirect effect of the substances; e.g. by the means of the TEWI calculation. This way, wrong conclusions can be avoided; in fact, a refrigerant with low GWP could actually have a higher indirect impact because of the poorer performance in the system. Generally, the literature available is rather limited, as some of the upcoming options are new substances and mixtures and still lack vast experimentation. McLinden et al. (2014) conducted studies on over 56 000 molecules, with 15 or fewer atoms and comprising only the elements C, H, F, Cl, Br, O, N, and/or S that were successively screened by their ODP, GWP and various properties, reducing the number of candidates to 1200. When applying further criteria, such as having an appropriate critical temperature (between 300 and 400 K for almost all of the common refrigerating applications), 62 candidates were left. The most promising ones for residential heat pump systems, divided in categories, will be described in this section. A further elimination process will then be performed and motivated, leading to the shortlist of options presented in section 2.4. 2.3.1. HFCs The most relevant for the considered application belonging to this category are R410A, R134a, R152a and R32. The GWP is still not optimal, and thus they should be regarded as medium term solutions. HFC/HC blends The addition of a HC element to the HFC mixture improves the solubility with the lubricant, in extreme cases allowing the use of conventional oils, thus removing the need for retrofitting. Furthermore, the flammability of the HC blends can be reduced by the mixing of a HFC. R134a Due to its high GWP (1300), it has been limited from MAC applications and banned in new vehicles’ models from 2013 with the MAC regulation, and will be phased down in other applications. In fact, it will be allowed until 2022 in commercial hermetically sealed equipment (GWP<2500). In 2022 the limit to GWP will be set to 150, thus making R134a unacceptable. R152a It has a critical temperature lower than 400 K, and presents a flammability level of A2. Usually used as a component in a mixture and can substitute, in a transitional manner, R134a. (McLinden, 2014). The literature available regarding HP applications is limited, proving that the research on this refrigerant is still at an early stage. Ho-Saeng Leea et al. (2012) performed a study in a water source heat pump system on a R152a/R32 mixture, with varying percentual composition of R32. It was found that the tested system required a compressor power up to 13.7% lower when compared to an equivalent R22 system, along with an increased COP (up to 15.8%); the refrigerant charge diminishes of up to 27%. On the other hand, the compressor discharge temperature is increased up to 15.4 °C. -13-PDF Image | Next generation of refrigerants for residential heat pump systems
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