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experimentally based performance studies have been conducted in the past on integrated CO2 heat pumps (Nekså, 2002; Stene, 2005; Heinz et al., 2010; Minetto et al., 2016; Stene and Alonso, 2016). The data presented in these references is based on a few real operational cases that are all influenced by a large number of parameters related to the actual exchanger designs and sizes. As a result, these studies are not able to discuss sensitivity to important factors in detail. There are only a few theoretical studies modelling integrated heat pumps, and they all model state solutions for fixed systems using different heat exchanger models like the log mean temperature difference (LMTD) and the number of transfer units (NTU) method (Byrne et al., 2009; Blanco et al., 2012; Minetto et al., 2016). Byrne et al. (2009) simulated the annual coefficient of performance (COP) for CO2 heat pumps with three gas coolers designed for simultaneous heating and cooling. Blanco et al. (2012) modelled integrated heat pumps with one condenser and one desuperheater for subcritical R410A processes. Minetto et al. (2016) studied annual performance of reversible CO2 heat pumps with two gas coolers and an ejector, created a model based on experimental data and compared the performance with a state-of-the-art R410A unit. Heinz et al. (2010) simulated and conducted experiments on integrated CO2-based heat pumps for low-heating-energy buildings, and Stene and Alonso (2016) presented a report on field measurements of heat pump systems in net zero energy buildings. These studies show that CO2 units can be very efficient when the ratio between space and water heating is low, but are inefficient for pure space heating and cooling service. In this work, the number of design dependent variables are reduced through multivariable optimization, and by modelling the gas coolers using a minimum allowed temperature difference between the two fluids exchanging heat, which is also known as the temperature pinch. Parameters related to heat exchanger sizing, and optimal high pressure control of the backpressure valve set point, disappear as input parameters in the optimized model, which makes this method suitable as a basis in a comparative study of performance. This is a new approach to investigate performance of integrated heat pumps with multiple gas coolers, e.g. used in sensitivity studies of mixed refrigerant liquefied natural gas (LNG) cascade processes (Ding et al., 2017). To the best of the authors' knowledge, neither the sensitivity to important factors in integrated heat pump systems, nor the performance improvement realizable through the inclusion of an ejector is well documented in earlier literature. The scope of this study is, therefore, to conduct a sensitivity study which describes the situations where CO2 outperforms conventional HFC-based systems and vice versa, and present process data that can contribute to more efficient system designs and increase our understanding of how the equipment effects the performance. The methods used in this study are described in Section 2. The results are presented in Section 3, and the discussion in Section 4. The main conclusions from this study are summarized in Section 5. 2 Method This chapter explains the development of the multivariable optimization model that forms the basis for this study. 2.1 System Design and Performance Modelling This article studies the integrated heat pump described in Figure 1, using CO2 or R410A as refrigerant. The design is similar to the CO2-based prototype investigated by Stene (2005). However, an electrical heater has been included to make the model more general, so that the processes can be optimized for any heating scheme or compressor model. The subcritical R410A systems are assumed to operate similarly to the transcritical CO2 heat pump, as discussed by Blanco et al. (2012), in order to set up a fair basis for comparing the two refrigerants. 3PDF Image | CO2 Heat Pump Performance
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