CO2 Refrigeration and Heat Pump Systems—A Comprehensive Review

Abstract

An increased awareness of the impacts of synthetic refrigerants on the environment has prompted the refrigeration industry and researchers worldwide to seek better alternatives in terms of technical, economic and environmental performance. CO2 refrigerant, also known as R744, has re-emerged as a potential alternative to existing refrigerants with its zero ozone depletion potential (ODP) and impressively low global warming potential (GWP). A refrigeration system utilising this refrigerant, however, suffers performance degradation when it operates in warm or hot climatic regions due to its inevitable operation in the supercritical region. In addition, the CO2 refrigerant properties necessitate the need for components designed to withstand very high operating pressures. These challenges have not been let unnoticed; related industries and researchers are actively involved in research and development of various components and systems which in turn encourages increased applications of these systems. In this paper, a comprehensive review of CO2 refrigeration systems and the state of the art of the technology and its applications in various industries is presented. In particular, the paper reviews recent research and developments on various aspects of CO2 systems including cycle modifications, exergy analysis of the systems, system modelling, transcritical operation consideration and various existing and potential applications.

Application of transcritical CO2 heat pumps to boiler replacement in low impact refurbishment projects

“…While optimum gas cooler pressure, exit temperatures and suction superheat levels are reported in optimisation studies by Bruno et al [ 16 ] and others, these are interim variables in a physical model, controlled indirectly. They are affected by the heat pump hardware, gas charge levels, expansion valve settings and compressor speeds as well as ambient and target output temperatures.…”

“…Significant research has been completed concerning the development and optimisation of transcritical CO 2 heat pumps with several helpful literature reviews published [ [14] , [15] , [16] , [17]] ]. However, the focus is often on refrigeration or water heating and only recently has there been significant research into using the heat pumps for space heating.…”

Archives of Thermodynamics

“…Studies of Baek et al [36], Ferrara et al [37], Kohsokabe et al [38], Ma et al [30] reported that the implementation of an expander unit, in replacement of the valve, leads to an improvement in COP of a CO 2 cycle under transcritical operation, in some cases similar to traditional heat pumps.…”

Improving the Energy Efficiency, Limiting Costs and Reducing CO2 Emissions of a Museum Using Geothermal Energy and Energy Management Policies

“…For the sake of keeping this study unbiased, two widely recognized ASHRAE standards are considered in order to define respectively thermal comfort and microclimate conditions: ANSI/ASHRAE Standard 55-thermal environmental conditions for human occupancy [16] (along with ISO 7730:2005 [17]) and the "Museums, Galleries, Archives and Libraries" chapter 21 in the American Society of Heating, Refrigeration, and Air Conditioning Engineers Inc. (ASHRAE) Handbook [18]. With regards to thermal comfort of people, it is an obvious assertion that the buildings do not need to be air conditioned for comfort in closure time when the visitors are not present.…”