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.
Highlights
The natural refrigerant CO2 was one of the first refrigerants used in the mechanical refrigeration systems
Kim et al [32] developed a steady-state model to analyze the thermodynamic performance of an internal heat exchanger (IHX) in a geothermal CO2 Heat pump (HP), where a counter-flow multi-tube heat exchanger (HX) with several tubes encompassed in a larger tube was used as the IHX
Ituna-Yudonago et al [112] analyzed the effect of transient thermal effectiveness of an IHX on a CO2 system, and they reported that increasing the CO2 temperature by 10 ◦ C at the IHX inlet of the gas cooler side could decrease the coefficient of performance (COP) of about 3% due to reduced heat transfer coefficient (HTC)
Summary
The natural refrigerant CO2 was one of the first refrigerants used in the mechanical refrigeration systems. 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]. The lower compression ratio of CO2 compared to fluorocarbons results in higher isentropic efficiency in a CO2 HP system [8] Such advantages make CO2 an excellent candidate to serve as an effective replacement to the conventional working fluids like CFCs and HFCs. significant challenges of using CO2 are its low critical point (31.1 ◦ C and 7.38 MPa) [6] and high operating pressure (8.0 to 11.0 MPa) [9], which require careful attention while designing the system components. Recent modifications of each HP component and their impact on heating and cooling applications concerning different heat sources are detailed and discussed
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