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Energies 2021, 14, 4103 2 of 28 DHW, Nordic hotels have been utilizing conventional thermal energy sources, e.g., electric boilers with inefficient central systems [7], justified by relatively low electricity prices. Within Europe in general, fossil fuel-fired boilers still represent the most applied heating source [5]. In the past decade, district heating and cooling networks have gained solid footing and are becoming important in Scandinavia [8,9]. Yet, separate chillers, i.e., vapor compression units, are generally utilized to fulfill the cooling demands in hotels, even if access to the district cooling network exists at the location. Heat pumps appear as a suitable alternative to meet all the different demands with a single unit while boosting energy efficiency and reducing operational costs. This is achieved through their principle of operation of upgrading heat from one temperature level to another with a considerably low input of work, namely, electricity. Furthermore, a recent five-year study of hotels in Nordic countries has shown that the specific energy consumption in hotels with heat pumps as the primary heat source is lowest compared to those using alternative systems, such as electric boilers or district heating [10]. Heat pumps are vapor compression systems that transfer heat from a heat source at relatively low temperature, e.g., air, water, ground, or chilling water loop, to a heat sink at a higher temperature, such as a SH circuit or hot water tanks. Heat is transferred through a fluid, i.e., refrigerant, which is circulated and adapted to the required temperature levels by means of work input to a compressor. Refrigerant selection has been a hot topic in the last decades, mainly as the historically favored synthetic refrigerants are, or have been, responsible for significant environmental consequences, either destruction of the ozone layer by CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) or global warming by HFCs (hydrofluorocrbons). Natural refrigerants, such as ammonia, CO2, and hydrocarbons, are widely utilized in different applications and have the potential to replace synthetic refrigerants in heat pumps and chillers. The natural refrigerants were among the first utilized in vapor compression systems and have negligible impact on the environment, but can introduce challenges in terms of toxicity, operation at high pressure, or flammability [11]. Natural refrigerants are competing for the niche of heat pumps with the newly developed HFOs (hydrofluoroolefins), which fulfill the requirements of low global warming potential (GWP) dictated by several national or international regulations, e.g., F-gas in Europe. However, recent studies and reports have raised concerns regarding the HFO’s decomposition product trifluoroacetic acid (TFA). Widespread and long-term application of HFOs can result in TFA accumulating in drinking water, which can have severe effects on human health and the environment [12]. In addition, a newly published report demonstrates that one of the most applied HFOs (HFO-1234ze) in current use ultimately decomposes partially into the refrigerant R23; one of the most potent greenhouse gases known (100-year GWP of 14,800) [13]. Although a recent study predicts that HFOs, HFCs and their mixtures will still have a significant market share as far as 2030 [14], it could be agreed that natural refrigerants are the long-term solution, and among them CO2 (GWP = 1) appears as a safe and sustainable choice for commercial heat pumps, e.g., for hotels. CO2 has had a success story in commercial refrigeration (centralized units, condensing units, and plugins). Now, CO2 is becoming a competing alternative in other sectors, such as industrial refrigeration, due to factors like increased efficiency and component size, reductions in operational costs (economy of scale), and legislation [15]. Due to its low critical temperature (31 ◦C), CO2 applications were initially limited to operations where heat rejection (condenser) would happen well below the critical point, such as freezing in cascade refrigeration units. The implementation of CO2 in heat pumps and commercial refrigeration, which can operate with heat rejection or production above CO2’s critical temperature, was realized thanks to the investigations of Gustav Lorentzen and his team. Lorentzen (1994) [16] presented the basic layout of a transcritical CO2 heat pump, based on a system with suction accumulator, high-pressure control through the valve feeding the evaporator, and the application of a gas cooler in place of the condenser. At the time, this unit was suggested as an efficient and environmentally friendly replacement of R12 in mobile air conditioning (AC) [17]. Nekså et al. (1998) [18] stated that transcriticalPDF Image | Evaluation of Integrated Concepts with CO2 for Heating
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