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issue: September 2006 APPLIANCE Magazine

The Open Door
Researching Beyond Refrigerants

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by Reinhard Radermacher, Center for Environmental Energy Engineering (CEEE), University of Maryland

For almost two decades, the environmental impact of refrigerants and resulting regulations has siphoned off resources from other research tasks and opportunities. R134a, R410A, R407C, and possibly a few others have displaced an older generation of man-made fluids. Natural refrigerants such as hydrocarbons and carbon dioxide are making inroads in the market in some parts of the world. And just when a new regulation of the European Union (EU) seems to be the starting point for further reduction in the number of available refrigerant choices, some refrigerant manufacturers are proposing new fluids that may reconcile the advantages of man-made fluids with acceptable environmental impact. Thus, the search for new refrigerants and the need of their characterization in terms of performance, reliability, stability, and environmental impact may just be starting another round.
Meanwhile, other opportunities for the advancement of air-conditioning and refrigeration technology offer important and far-reaching opportunities that have potentially larger environmental benefits than refrigerant selections.
While one can argue that the demands of cost reduction and energy efficiency improvement often force conflicting and competing design choices, there are design tools and techniques that can address and possibly reconcile these issues. Software using heuristic optimization methods, for example, uses the concept of evolution to develop successive generations of improved designs. While there is no guarantee that an optimum is achieved, practical experience shows that very often, significantly better and sometimes quite creative designs are found. The results lead to new design options of lower cost, higher efficiency systems.
One R&D area that can provide energy efficiency gains is improving the performance of cooling system components. In larger compressors, the efficiency of the compression component itself can reach values of 90 percent. The same holds for motor efficiency. Thus, the potential for additional performance increase is limited, and it will only be available at rapidly increasing cost. One approach that possibly can lead to significantly increased compressor performance is changing the compression process from an almost adiabatic process to a more isothermal process. This requires extensive compressor cooling and comes with significant design challenges. However, the rewards can be quite high, especially for refrigerants of small molecular weight, which usually leads to high discharge
The development of oil-free compressors offers another important opportunity. While a huge challenge in terms of compressor reliability, oil-free technology simplifies system chemistry and eliminates the need for extensive reliability testing to qualify new oils, oil additives and other system modifications. The elimination of oil has the potential to significantly improve heat exchanger performance and will allow engineers to design a new generation of heat exchangers that go beyond flat-tube technology, with much smaller flow channels. Advantages include increased compactness while improving performance.
The opportunities described so far pertain to the traditional vapor compression system. Alternative cooling technologies such as thermoelectrics, thermoacoustics, acoustic compression, magnetic cooling, and gas cycles such as the Stirling cycle open up even more possibilities. A considerable research effort is underway to make these technologies successful. Traditional thinking seems to imply that these technologies will substitute vapor compression. For example, the Stirling cycle seems to be emerging as a vapor compression alternative in the range of smaller capacities and high temperature lifts. Thermoelectric systems may displace vapor compression or open up new applications when simplicity, compactness, the lack of moving parts, and smaller temperature lifts are required.
But there may also be creative new opportunities that have only just started being explored. When considering these alternative cooling technologies and what they do best, the initial success may come from skillfully integrating them with vapor compression rather than displacing vapor compression. One example may be the use of thermoelectrics for subcooling the refrigerant in a traditional vapor compression system. With a small temperature lift and high COP, significant simultaneous improvements in terms of both capacity and efficiency can be obtained.
These are just a few of the R&D opportunities that could make a huge impact on the environment and create higher performance cooling systems. And there are many more exciting possibilities just waiting to be explored. So why all the focus on refrigerants? In reality, selecting new refrigerants has little impact on total global warming. Many studies actually show conflicting results among refrigerant choices, indicating that there is no clear gain to be made. A clear and convincing reduction in global warming potential results from aggressively pursuing energy efficiency improvements. We at CEEE are dedicated to doing just that. Are you?

About the Author

Reinhard Radermacher is a professor of Mechanical Engineering and director of the Center for Environmental Energy Engineering (CEEE) at the University of Maryland (College Park, Maryland, U.S.).
If you would like to contact Radermacher, please e-mail editor@appliance.com


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