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Energies 2021, 14, 4103 3 of 28 CO2 heat pumps are suitable to produce DHW, as the temperature glide in the refrigerant side of the gas cooler follows nearly perfectly the relatively large temperature difference in the waterside, reaching up to 90 ◦C. Additionally, Nekså (2002) [19] mentioned other applications for CO2 heat pumps that, with the course of years, were realized, e.g., SH and residential heat pumps and heat pump dryers. Stene (2005) [20] investigated the efficient integration of SH and DHW to maximize the use of the gliding heat rejection of CO2 heat pumps. The concept is based on splitting the gas cooler into three parts connected in series: the warmest and coldest parts to reheat and preheat DHW, and the intermediate part to produce SH. Thus, it is possible to minimize CO2 temperature downstream of the gas cooler, reducing expansion losses and improving the performance. Tosato et al. (2020) [21] performed an experimental and numerical investigation of a newly developed CO2 air/water reversible heat pump, intended for household applications. The system was evaluated at a range of ambient temperatures (−2.0 to 11.2 ◦C), and at DHW setpoint temperatures ranging from 60 to 80 ◦C. The results illustrated that the highest COP was achieved at DHW setpoint temperature of 60 ◦C, due to an increase in DHW mass flow rate through the gas cooler. However, charging time was significantly reduced in comparison to when setpoints of 70 and 80 ◦C were applied. Dai et al. (2019) [22] suggested using mechanical subcooling in CO2 transcritical heat pump cycle to reduce the gas cooler outlet temperature. They found that the primary energy consumption was reduced compared to a conventional transcritical single-stage CO2 heat pump, which resulted in additional reductions in emissions of around 16%. Emissions were reduced by approximately 18–33% and 62–69% compared to a coal-fired boiler and direct electric heating, respectively. Another measure to increase the system efficiency of CO2 heat pumps is simultaneous production of DHW and cooling. Byrne et al. (2009) [23] investigated a CO2 heat pump lay- out for simultaneous production of heating and cooling aimed at hotels, luxury dwellings, or smaller office buildings. The system design is based on a division of the gas cooler into three parts: a DHW heat exchanger, a SH heat exchanger, and a subcooler that heats water to defrost a backup air evaporator. This air evaporator is necessary to balance the system when the space cooling demand is an insufficient heat source to achieve the heating demand. The authors performed a numerical study to compare this heat pump architecture operating with CO2 and with R407C, and observed that CO2 can outperform the HFC in terms of environmental impact. Diaby et al. (2019) [24] is a continuation of the previous work, as the authors present heat pump models for either simultaneous cooling, SH, and DHW or desalination. The numerical results in both cases are satisfactory, and the authors conclude that CO2 is an exceptionally suited refrigerant for multipurpose heat pumps compared to “standard” refrigerants. This statement is supported by the conclusions in the study from Liu et al. (2016) [25], where the purpose of the heat pumps would be cooling and heating processes in food processing industries. An experimental study of combined AC and DHW production with a CO2 heat pump is introduced in Adriansyah (2004) [26]. The results revealed a combined (heating and cooling) coefficient of performance (COP) as high as 8 when all the heat available in the gas cooler can be recovered. Farsi et al. (2016) [27] delved into the use of heat pumps for combined cooling and desalination, and the authors indicate the potential of CO2 to improve desalination. Moreover, energy savings are maximized and the total annual cost is reduced when compared to separate systems (CO2 refrigeration system and a desalination unit using steam from a boiler run with methane). Singh et al. (2020) [28] presented numerical simulations of a planned installations of a 140 kW transcritical CO2 heat pump for a centralized kitchen in Banga- lore, India. The heat pump will preheat hot water to 90 ◦C for steam production while supplying AC cooling for the entire building and utilizing thermal storage to compensate for asynchronous thermal demands. Simulations illustrated that the system can achieve a COP above 6 when operating in combined heating and cooling mode. The total energy consumption is expected to be reduced by 33% compared to the current solution, which will reduce yearly CO2-eq emissions by about 300 tonnes.PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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