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Energies 2021, 14, 4103 4 of 28 CO2 heat pumps are increasing their presence in the portfolio of different manu- facturers in Europe, in some cases adapting existing compressor packs for commercial refrigeration. An example of this is shown in Smitt et al. (2020) [29], with a performance analysis based on field measurements of a CO2 heat pump for integrated production of heating and cooling with a 6 m3 thermal storage. The study evaluates how different demands change during the specific year and how they affect the different performance indicators. One of the main conclusions of this study is that COPs improve with DHW charging (compared to SH only), meaning that DHW charging strategy is crucial to boost efficiency. A later numerical study of the same system demonstrated how the energy savings could be reduced with 5.8–13.2% for different seasonal scenarios when charging the DHW storage at low loads [30]. Additional perks of applying the low load charging strategy were reduced peak power usage, operational fluctuations, and ON/OFF cycles. A dedicated and more complex CO2 heat pump architecture is introduced in Tosato et al. (2020) [31], including two-stage evaporation supported by ejector (heat source or AC production, depending on operation mode). Results from a limited period in winter are presented, indicating a good efficiency with DHW production and the benefit of developing control strategies to minimize start and stops. The potential of evaporation in two stages was not fully evaluated with the available data in Tosato et al. (2020) [31]. The first results in summer mode for the aforementioned CO2 heat pump were presented in Hafner et al. (2020) [32], showing COPs around 5 when producing chilled water at 7 ◦C (from 11 ◦C) and DHW at 60 ◦C (from 30 ◦C). The authors concluded that the potential of two-stage evaporation with an ejector could not be fully utilized unless higher waterside temperature differences are allowed in the evaporators. In contrast to most of the previous articles and references, which analyzed or in- troduced rather simple layouts for CO2 heat pumps, this work presents sophisticated architectures applicable for hotels, which are evaluated numerically with transient models. These systems include ejectors, parallel compression, and combined air-to-CO2 evapora- tors/gas coolers, which can be applied as either heat source or sink. Load profiles are established based on previous analysis of a medium-sized hotel, and the performance is determined for each heat pump architecture and according to the climate of different cities in Scandinavia. Additional locations in Central Europe and the Mediterranean are included to evaluate the feasibility of such installations in warmer climates. Two separate DHW charging strategies are implemented to evaluate the influence of charging strategy in comparison to design with respect to performance. The sustainability of each design is investigated in terms of annual global warming contribution at each location. An eco- nomic evaluation is included to discuss whether these CO2 heat pumps are cost-efficient compared to more conventional approaches, including electric boilers and separate chillers for AC cooling applications. 2. System Description The approach of the integrated solution with CO2 is to use a single unit with a flexible design to supply SH, DHW, and AC within the building. The simplified schematic of an integrated CO2 unit with a standard single-stage compression (SC) is shown in Figure 1. The main system components are four compressors, a gas cooler section for heat production, AC evaporators, an intermediate temperature (INT) liquid receiver, and four air evaporators that can be employed as gas coolers for heat rejection when surplus heat is produced. The integrated unit is dimensioned to operate in the Scandinavian ambient- temperature range, from −15 to 35 ◦C. In addition to SC integrated system design, which is described in Section 2.1, two alternative system configurations are presented in Section 2.2 to investigate measures that can enhance efficiency. Note that ambient air has been selected as the main heat source for all configurations. Superior heat sources, such as seawater and boreholes, could be applied in place of ambient air. However, alternative heat sources are highly dependent on the building’s specific location and would not be applicable everywhere. As a result, air heat exchangers are applied in all system configurations.PDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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