General aspects of carbon dioxide as a refrigerant (original) (raw)
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A review of carbon dioxide as a refrigerant in refrigeration technology
South African Journal of Science, 2015
Tough environmental laws and stringent government policies have revolutionised the refrigeration sector, especially concerning the cycle fluid known as the refrigerant. It has been observed that only natural refrigerants are environmentally benign. When other refrigerant qualities are considered, especially those relating to toxicity and flammability, carbon dioxide emerges as the best among the natural refrigerants. However, carbon dioxide based refrigerants are not without drawbacks. Even though the use of R744 – a carbon dioxide based refrigerant gas – has solved the direct effect of emissions on the environment, studies to investigate the indirect effects of these systems are needed. Improvement in existing technical solutions and the formulation of additional solutions to existing R744 refrigeration problems is paramount if this technology is to be accepted by all, especially in areas with warm climates. National policies geared to green technologies are important to clear ...
Carbon dioxide as a natural refrigerant
International Journal of Low-Carbon Technologies, 2007
In the beginnings of mechanical refrigeration, at the end of the nineteenth century, carbon dioxide was one of the fi rst refrigerants to be used in compression-type refrigerating machines, later gaining widespread application mainly onboard refrigerated ships, but common in other sectors of refrigeration as well. It was only immediately after World War II that CO 2 was rapidly eclipsed as a refrigerant, due to the advent of the synthesised halogenated working fl uids, addressed as safe and ideal refrigerants at that time. Because of the stratospheric Ozone depletion environmental issue, CFC and HCFC working fl uids are now in the process of being phased out of use under the Montreal Protocol. The Global Warming environmental issue casts concern over the use of the new HFC fl uids as substitute refrigerants, because of their high GWP values, which make them subject to regulations under the Kyoto Protocol. In this mixed situation, CO 2 is being revisited as a fully environmentally friendly and safe refrigerant. An intense research activity on its prospective applications is underway in many research establishments in Europe, Japan and North America, and important results have already been reached in exploiting the peculiar characteristics of this high-pressure fl uid operated with a transcritical cycle. In some applications CO 2 systems have already been commercialised; this applies to heat pump water heaters, as a brine in indirect systems and in the low temperature stage of cascade systems. The paper critically analyses the prospects for the future return of CO 2 as a working fl uid, or sometimes as a brine with change of phase, in important application areas. These include air conditioning and heat pump systems in the residential and commercial sectors, commercial and transport refrigeration and mobile air conditioning.
Study & Simulation of Co2 Refrigeration System
Carbon-dioxide is not new to refrigeration. Its use began in the mid-nineteenth century and steadily increased, reaching a peak in the 1920s. Its use declined with the introduction of chlorofluorocarbons (CFCs) that operated at much lower pressures. Use of CO 2 continued, but chiefly in cascade systems for industrial and process applications. Recently, strong interest has been shown in CO 2 as a refrigerant by vending machine manufacturers. There are also possibilities for other light commercial refrigeration applications, as well as for residential air conditioning also including the industrial sector applications that involves cold storage.
Efficiency enhancement by subcooling the carbon dioxide process with water as refrigerant
2018
As a consequence of the F-Gas regulation, R404A is no longer an option for commercial refrigeration applications. Therefore, this paper focuses on natural refrigerants. There are a few options like carbon dioxide, which has an efficiency loss with increasing ambient temperatures. A promising option is subcooling of the carbon dioxide process with a chiller using water as the refrigerant. This will result in a new optimized high pressure of the carbon dioxide process depending on the ambient temperature. Finally the annual COP values of the standard transcritical and subcooled system will be discussed.
Eighth International Conference on Advances in Applied Science and Environmental Engineering - ASEE 2018, 2018
In many large office buildings, hospitals and hotels AC cooling is affected by chilled water generated by a refrigerating plant which rejects its heat to the environment by means of cooling tower water or, increasingly so, by air cooled condensers and adiabatically assisted air-cooled condensers. Hot water for space heating, AC reheat and sanitary purposes is usually provided by a boiler. Using a COAG, the Council of Australian Governments, data base on annual energy consumption per m 2 of building, it is shown that significant reductions in the electrical energy, natural gas and cooling water consumption may be achieved when using transcritical CO 2 refrigerating systems for building cooling, heating and sanitary hot water, thus saving gas and attendant emissions, and cooling water. It is shown that the specific energy consumption measured in kWh/m 2 .annum reduces about 30% when retrofitting trans-critical CO 2 systems to existing Australian buildings and about 55% in new buildings. These numbers range from about 10% to 50% in hospitals. Supermarkets with all CO 2 refrigeration would save about 37%. The natural refrigerant CO 2 is non-toxic, non-flammable, low cost and energy efficient. CO 2 also has superior heat transfer properties and its GWP=1. As such CO 2 is a sustainable refrigerant and will future proof any system against the serious implications of the impending phase down, and ultimate phase out, of the high GWP HFC refrigerants by the amendment to the Montreal Protocol, known as the Kigali amendment, where it was agreed to by the 197 signatories to the Montreal Protocol on 15 October 2,016.
Applied Thermal Engineering, 2008
This work presents the experimental evaluation of the energy efficiency and optimal gas-cooler pressures of a single-stage refrigerating plant working with carbon dioxide as refrigerant in transcritical conditions. The performance of the plant was tested at three different evaporating temperatures (-0.9, -10.1 and -18.1 ºC), for three gas-cooler refrigerant outlet temperatures (31.2, 33.6 and 40.0 ºC.) at each evaporating temperature and in a wide range of gas-cooler pressures (74.4 to 104.7 bar).
Applied Energy, 2010
Carbon dioxide is an interesting solution for commercial refrigeration and in perspective for air-conditioning systems. In this paper a newly developed carbon dioxide transcritical air cooled chiller for refrigerating propylene glycol down to À8°C supply temperature is described. The aim of the project was at optimising the cycle energy efficiency while assuring reliable operation and simple management of the unit. The carbon dioxide optimal pressure issue is addressed with an innovate system architecture and control logic. Using a flash tanks and two electronic valves, the optimal cycle upper pressure was maintained in transcritical operation mode. The managing of the valves allows the refrigeration machine efficiency improvement when the gas cooler inlet air allows subcritical working conditions. A simulation model of the chiller was developed and its results validated with experimental data. A measurement campaign was carried out, testing the chiller at external temperatures ranging from 18 to 35°C, the unit energy efficiency ranging from 3.1 to 2.0.
Subcooling methods for CO2 refrigeration cycles: A review
International Journal of Refrigeration, 2018
CO 2 subcooling has resulted a method to upgrade the performance of CO 2 refrigeration plants in the recent years, with overall improvements up to 12% with internal heat exchangers, 22% with economizers, 25.6% with thermoelectric systems and 30.3% with dedicated subcooling methods. This paper comprehensively reviews the recent studies that consider subcooling as a way to upgrade the performance of CO 2 refrigeration cycles. The review is limited to CO 2 refrigeration cycles with accumulation receiver for commercial purposes and does not consider air conditioning or MAC systems. It is organized as follows: first, the thermodynamic aspects of subcooling in CO 2 refrigeration cycles are described and discussed; second, the main results and conclusions of the recent investigations are analysed inside two big groups: subcooling internal methods and subcooling external methods. Finally, the review synthesizes the current state of the art and points out the lines of research that deserve future developments. Highlights Research using subcooling as way to improve CO 2 refrigeration is analysed. COP improvements up to 37.8% of CO 2 base systems have been reported. State-of-the art subcooling systems are presented and discussed New opportunities for research are highlighted in the review.
Carbon dioxide as refrigerant for tap water heat pumps: A comparison with the traditional solution
International Journal of Refrigeration, 2005
Increased concern about the environmental impact of the refrigeration technology is leading toward design solutions aimed at improving the energy efficiency of the related applications, using eco-friendly refrigerants, i.e. ozone-friendly and with the least possible global warming potential (GWP). In this respect, carbon dioxide (ASHRAE R744) is seen today as one of the most promising refrigerants and is raising great interest in industrial and scientific fields. In the present work, the plant options are investigated, which are related to the design of air/water heat pumps for tap water using CO 2 . A comparison is made, in terms of energy efficiency, between a system working with CO 2 and a similar one working with HFC R134a; such a comparison is carried out by means of a simulation model of a refrigerating machine/heat pump, characterized by a detailed representation of the heat exchangers, based on their subdivision into elementary volumes. Results show that carbon dioxide is an interesting substitute for synthetic fluids, if the design of the system is focused to take advantage of its properties. q