Environmental concerns and enactments of Montreal and Kyoto Protocol for sustainable growth is a welcome impetus for researchers towards a quest for ecologically safe and natural refrigerants and cost effective designs of refrigeration systems. Carbon dioxide (CO2) is one such natural refrigerants that, although was abandoned once due techno-economic reason, has been receiving tremendous attention and is viewed as a strong candidate for long term alternative of synthetic refrigerants. The commercial success of CO2 as a refrigerant and its universal acceptance, however demands cost effective and widely accepted technology operable under various environmental conditions. In this chapter, the use of CO2 as refrigerant in trans-critical vapor compression system is discussed in detail along with its unique challenges associated with operating in tropical region. Further the opportunities for using these systems in tropical region with specific systematic modification are explored. Discussions on component design and system level performance analysis are also included in the chapter.
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Extensive research is in progress now a day towards the design of newer and environmentally safer synthetic refrigerants as well as towards the use of natural refrigerants like Ammonia, Water, Propane, Carbon dioxide, etc. It is found that CO2 is one of the most suitable and viable options among the natural refrigerants. It has a wider operating range, unlike water which cannot be used below 0oC, it is non-toxic, unlike NH3 which has known toxicity and pungent odor, and it is non-inflammable unlike hydrocarbons. However, the technological shortcoming that leads to failure of CO2 as refrigerant earlier are due to its high operating pressure & temperature and absence of suitable material and technology at that time. There was no suitable material and manufacturing technology for high pressure compact heat exchangers, high pressure compressors, etc. – the problems that have now been mostly solved.
In general, it is always better to select a refrigerant which is in harmony with the nature and already exists in abundance in the biosphere. Further, the cost of such refrigerants is expected to be much lower compared to synthetically produced compounds. In terms of environmental sustainability, technical acceptability, cost and safety, CO2 can be one of the best choices for both heating and cooling applications, if the system is designed appropriately. Norwegian Professor Gustav Lorentzen was the first to resurrect the old refrigerant CO2 for its use in refrigeration, air conditioning and heat pump applications in the early 1990s, (Lorentzen & Pettersen, 1993), (Lorentzen, 1994), (Lorentzen, 1995) proposed Automobile Air Conditioning (AAC) system suggesting a trans-critical cycle. This was due to CO2 having a lower critical temperature. While high ambient temperatures in the car AC system and the high side pressure were regulated by throttle valve. Being a trans-critical cycle, heat rejection takes place in the super-critical region; hence the condenser gets replaced by a gas cooler. Suitability of the CO2 cycle for automobile air conditioning, a sector that dominates the global CFC emission, depends largely on its high working pressure. With today's advancement in technology, the high design pressure of CO2 systems is no longer a critical issue. On the contrary, high pressure systems may give considerable practical and economical advantages, due to reduction in dimensions and weight. (Lorentzen & Pettersen, 1993) concluded that considering all the practical factors, the performance of the CO2 system was at par with R12 system with equal heat exchanger dimension and design capacity under some environmental conditions. These results had a significant impact on the business & research community who took renewed interest in CO2 systems almost dramatically.
These works were carried out for the heat pump application, however, in tropical region the major applications are cooling. Operating a CO2 system for cooling application has its own challenges due to the high pressure of heat rejection in super-critical region as discussed in (Kim, Pettersen, & Bullard, 2004) and further explored by (Gupta & Dasgupta, 2010), for a wide range of environmental temperature (25oC to 45oC) in typical Indian condition as one of the tropical region. The authors have elaborated those challenges and also the possible opportunities, if it used in tropical region for the best possible performance.