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Feature system typically has an operating limit of 31 bar , while a new trans-critical CO 2 system has an operating limit of 120 bar .”
He lists one of the biggest challenges as being the skills required to design , build and maintain trans-critical systems . “ It ’ s more complex than HFC system and a solid understanding is needed . Other challenges associated with using CO 2 in heat exchangers include managing high operating pressures and addressing the difficulties in designing for a wide range of operating loads , which are concerns not unique to CO 2 applications .
“ CO 2 enhances the energy efficiency of heat exchangers primarily through its latent heat of vapourisation . This metric measures the amount of heat absorption possible when the high-pressure liquid refrigerant changes phase to vapour upon exposure to heat energy . For R744 ( CO 2
), the latent heat of vapourisation is 350 kJ / kg at 1 bar , which is significantly higher compared to 217 kJ / kg for R134a and 248 kJ / kg for R407C . This higher latent heat of vapourisation allows CO 2 to absorb more heat energy per unit of surface area , improving the overall efficiency of heat exchangers .”
He lists an example / case study of where CO 2 has significantly improved energy efficiency as including the Proklima ‘ Natural Refrigerant ’ work by Dr . Volkmar Hasse , which serves as an excellent reference . The scientific metrics presented in this study remain relevant and are a great starting point for those new to working with CO 2
. “ The long-term energy savings associated with using CO 2 in heat exchangers are significant enough to drive the shift toward
CO 2 systems , although contractors and owners often keep specific figures confidential . Since a refrigeration system operates dynamically , energy savings must be evaluated in context . While a CO 2 system might not be as energy efficient as an HFC system during extremely hot afternoons , a year-round review of energy consumption reveals that CO 2 systems generally have a small percentage of operational hours where they are less efficient compared to HFC installations .” He notes that using CO 2 in heat exchangers contributes to reducing greenhouse gas emissions in several indirect ways . “ A
CO 2 evaporator can be smaller than those required for other refrigerants , leading to a reduction in the amount of copper needed and the associated indirect global warming potential ( GWP ) from raw material production . Additionally , CO 2 gas coolers , which are often made with durable stainless-steel tubing , are less likely to develop refrigerant leaks compared to copper tube condensers . Energy savings from CO 2 systems also contribute to lower indirect GWP , as less coal is needed for power generation , or if solar power is used , the indirect GWP is further reduced . The TEWI ( Total Equivalent Warming Impact ) index is a useful tool for combining these environmental factors into a single , comparable metric ,” says La Grange .
He says that the lifecycle impacts of CO 2
-based systems compared to traditional refrigerant systems are still being assessed , but they appear to be at least on par with , if not better than , traditional HFC systems . “ Older trans-critical
CO 2 systems , which have been in operation for around 10 years , suggest a potential for longer lifespan in the future . The higher operating pressures in CO 2 systems lead to more robust design , installation , and commissioning processes , resulting in fewer leaks compared to historically poorly installed HFC systems . Well-maintained CO 2 systems are likely to last longer , and the significant capital investment in these systems emphasises the importance of doing things correctly from the start . However , direct comparisons are challenging due to the variability in the installation quality of many HFC systems , which often suffer from leaks .
“ Current research trends and future developments in CO 2 technology for heat exchangers focus on several key areas . Manufacturers are continuously competing to improve the coefficient of performance ( COP ) of compressors . Additionally , significant development efforts are being directed towards advancing electronic system controls and expansion devices , which are crucial for optimising the efficiency and performance of
CO 2 systems .” He notes that potential breakthroughs and technological advancements that could enhance CO 2 system performance include improvements in compressor technology . “ Manufacturers have successfully increased operating pressures , which has significantly reduced the CO 2 temperature limits and expanded the feasibility of CO 2 systems in high ambient temperature areas where they were previously considered impractical .
“ Analysing both initial investment and long-term operational costs reveals evolving trends in CO 2 systems . While past research extensively covered the feasibility of R744 systems , recent studies are increasingly focused on new components and their specific benefits in various applications . Although the capital outlay for a trans-critical CO 2 system is higher compared to an HFC system of similar capacity , the premium has been steadily decreasing and continues to shrink , making CO 2 systems a more competitive option over time ,” says La Grange . RACA
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