Recent Advances in Transcritical CO2 (R744) Heat Pump System

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energies Review Recent Advances in Transcritical CO2 (R744) Heat Pump System: A Review Rajib Uddin Rony 1, Huojun Yang 1,*, Sumathy Krishnan 1 and Jongchul Song 2 1 College of Engineering, North Dakota State University, Fargo, ND 58108, USA; rajib.rony@ndsu.edu (R.U.R.); sumathy.krishnan@ndsu.edu (S.K.) Architectural Engineering and Construction Science, Kansas State University, Manhattan, KS 66506, USA; jonsong@ksu.edu 2 * Correspondence: huojun.yang@ndsu.edu; Tel.: +1-701-231-7194 Received: 7 January 2019; Accepted: 29 January 2019; Published: 31 January 2019 􏰰􏰱􏰲􏰰􏰳 􏰵􏰶􏰷 􏰸􏰹􏰺􏰻􏰼􏰲􏰽 Abstract: Heat pump (HP) is one of the most energy efficient tools for address heating and possibly cooling needs in buildings. Growing environmental concerns over conventional HP refrigerants, chlorofluorocarbons (CFCs), and hydrofluorocarbons (HFCs) have forced legislators and researchers to look for alternatives. As such, carbon dioxide (R744/CO2) has come to light due to its low global warming potential (GWP) and zero ozone depleting characteristics. Even though CO2 is environmentally benign, the performance of CO2 HP has been of concern since its inception. To improve the performance of CO2 HP, research has been playing a pivotal role in developing functional designs of heat exchangers, expansion devices, and compressors to suit the CO2 transcritical cycle. Different CO2 HP cycles coupled with auxiliary components, hybrid systems, and refrigerant mixtures along with advanced control strategies have been applied and tested. This paper presents a complete overview of the most recent developments of transcritical CO2 HPs, their components, and applications. Keywords: CO2; heat pumps; transcritical cycle; COP 1. Introduction The natural refrigerant CO2 was one of the first refrigerants used in the mechanical refrigeration systems. Subsequently, synthetic refrigerants such as CFCs and HFCs took over due to their superior performance in heating and cooling applications. However, environmental concerns over using CFCs and HFCs have urged researchers to identify alternatives that are environmentally benign and can serve as an effective replacement to the conventionally used working fluids in the heat pumps (HPs) [1]. Additionally, the fluorocarbons are to be phased out, according to the Montreal Protocol [2]. Although there exist several natural refrigerants, not many meet the technical and safety requirements. For example, ammonia (R717, ASHRAE safety class B2) is toxic and flammable and, therefore, not suitable for several applications [3]. On the other hand, water (R718, ASHRAE safety class A1) is non-toxic [3]. However, due to its low density and low working pressure, it cannot serve as an ideal candidate for vapor compression refrigeration cycles [4]. Another drawback of water is that it has a very low heating coefficient of performance (COP) and is not cost-effective [5]. Table 1 represents a comparison of the properties of CO2 with some of the most commonly used refrigerants. Among natural refrigerants, CO2 (ASHRAE safety class A1) is one of the few that is non-toxic and non-flammable [3] and can be released to the environment without the need to be recovered from any dismissed equipment. CO2 has very low GWP (i.e., 1) compared to other commercially available refrigerants. Moreover, CO2 has no regulatory liability since it has zero ozone depletion potential (ODP). The abundance of CO2 in the environment (0.04% of the atmospheric air) makes it cost-effective. Energies 2019, 12, 457; doi:10.3390/en12030457 www.mdpi.com/journal/energies

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