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4 Discussion The main focus of the article is to investigate performance of CO2-based integrated heat pumps, and compare them with systems based on conventional R410A-based systems. 4.1 Modelling Basis Performance of integrated heat pumps are calculated in MATLAB by combining models of the main components in the system, i.e., heat exchangers, compressor and ejector. The compressor outlet pressure is optimized for the operative conditions in each case. The heat pump model is based on multiple variables optimization algorithms (ga and fminsearch), since also the duty of each exchanger is optimized. 4.1.1 Exchangers The approach in this article is based on a model using the minimum allowed temperature approach as an input, assuming that the heat exchanger design is optimized for the given operating condition. Earlier literature is influenced by heat exchangers design, either by fixed exchangers in experimental setups, or by applying heat exchangers models with many input parameters in theoretical studies. These heat exchanger parameters increase the complexity in previous work, and make it difficult to identify the conditions where e.g. CO2 outperform R410A-based systems. 4.1.2 Compressor In this study, compressors are either modelled with a constant efficiency, or the Dorin CD1000H (CO2) compressor vendor model. The difference between the two approaches is small, as illustrated by almost parallel lines in Figure 8. Figure 9 shows that the outlet pressure is always between 78 and 95 bar, which corresponds to an isentropic compressor efficiency between 67 and 69 % (see Figure 5). As shown in Figure 6, the CO2-based GEA HGX2/90-4 compressor typically operates with an isentropic efficiency between 69 – 70 %, and the Bitzer 4VDC-10Y-40P (R410A) compressor between 70 – 71 %. That is, both the most efficient R410A and CO2 compressors identified in this study are modelled relatively accurate by a 70 % isentropic efficiency. 4.2 Impact of System Design Figure 8 presents results that show the improvements going from two gas coolers (AB or BC) to three (ABC) are significant, which support the selection of the ABC design as the base case for this study. 4.2.1 The Impact of Compressor Efficiency Figure 11 shows that CO2 systems benefit more by having a compressor with a high isentropic efficient for heating ratios 𝑟𝑟 ≤ 0.6, while the CO2 unit is more efficient for 𝑟𝑟 ≤ 0.8 if the units efficiency, e.g. if both systems have a compressor with a 55 % efficiency, the CO2 unit is more heat heat have 75 % isentropic compressor efficiency. Differences in real equipment performance is discussed in Section 4.3.4. 4.2.2 Conventional Expansion Valve vs. Ejector Figure 8 shows the ejector gain, compared to conventional expansion valve systems, in CO2-based systems with 25 % ejector efficiency. COP increases from 3 % (in DHW mode) to 10 % (in space Table 3 shows that the increase in COP is in the range 3.6 – 7.4 % for the CO2 cases with 𝑟𝑟 = 1.0, heat heating mode), i.e., a large gain is obtained in cases where CO2 is inefficient compared to R410A. where CO2 and R410A-based systems have similar COP. The results in Table 3 compare well with the findings of Banasiak et al. (2012) showing that an ejector could increase COP with 8 %. Table 3 also shows that there is little to gain by including an ejector in the R410A cycle, less than 3 % gain in COP. R410A-based integrated heat pumps are therefore unlikely to be designed with an ejector, which is supported by Minetto et al. (2016). 13PDF Image | CO2 Heat Pump Performance
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