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transfer in equally sized exchanges can, to some extent, be compensated by comparing R410A with CO2 systems that have a smaller temperature pinch. It has been claimed that the efficiency of CO2 ejectors is generally in the range of 20 β 30 %, while the efficiency of ejectors for low-pressure refrigerants such as R410A typically is less than 20 % (Elbel and Lawrence, 2016). Table 3 also shows that there is little to gain by including an ejector in the R410A cycle. Due to the small improvement, less than 3 % gain in COP, R410A-based integrated heat pumps are unlikely to be designed with an ejector. This is supported by Minetto et al. (2016) who compared reversible heat pumps based on CO2 and R410A, but only included ejectors in the CO2 systems. Figure 10 shows that if both the CO2 and R410A systems have a 1 K temperature pinch, the CO2 heat pump is most efficient for ππ β€ 0.8. However, because low pinch temperatures are both more important and easier to achieve in CO2-based systems, it is likely that such systems would be designed for a tighter approach than an equivalent R410A system, therefore, the performance heat advantage illustrated by this work could be considered conservative. For example, if the CO2 systems pump is the most efficient for ππ β€ 1.0. As mentioned in the introduction, the use of high GWP have a 1 K temperature pinch, while the R410A system have a 5 K temperature pinch, the CO2 heat heat refrigerants like R410A will be reduced through regulations. The results show that CO2 is an excellent replacement refrigerant for small heating ratios ππ . However, since CO2 systems are inefficient heat compared to R410A for ππheat larger than 1.0, it is important to investigate other low GWP replacement refrigerants for such heating scenarios. 4.4.2 The Impact of Variations in Heat Demand The heat demand will typically variate both on a daily and a yearly basis, which means that the heat pump can be expected to sometimes operate in both pure space heating and DHW mode. Figure 8 only switches between two modes, ππ = 0 and ππ = 5, Figure 11 shows that a heat pump must heat heat shows that it is important to include an ejector if the system is running in DHW mode. If a system generate more than 81 % of the heat in the ππheat = 0 mode before CO2 is the best alternative. If an ejector is used, CO2 is best if the system produces more than 72 % of the heat in the ππheat = 0 mode. However, such variations in heat demand are modelled more realistic with a system having fixed heat exchangers. 5 Conclusions This study presents integrated heat pump performance at a variety of operating conditions, using the assumption that the design is optimized for those conditions. The modelling work is based on a minimum allowed temperature approach in the heat exchangers and realistic compressor efficiency data. Multivariable optimization techniques based on temperature pinches have earlier been used in sensitivity studies for LNG cascade processes, but this is a new approach when applied to the modelling of integrated heat pumps. This approach enables a better understanding of the impact of important performance factors than presented in previous studies of integrated heat pumps. The systems modelled here show that CO2 can outperform R410A when the ratio between space and water heating is lower than 0.6 β 1.0, and the feedwater is colder than 20 Β°C (for example in Northern Europe) and the exchanger designs have tight pinch temperatures below 10 K. However, it is believed that these are conservative estimates with respect to the CO2 heat pumps, since differences in real equipment performance are not accounted for, as explained in the discussion. Efficient compressors and the use of ejector also benefits CO2-based systems. However, the sensitivity study shows that R410A processes always can be assumed to be more efficient for space to DHW heating ratios above 1. 16PDF Image | CO2 Heat Pump Performance
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