Editorial: expansion work recovery machine: worth using or not?
To address the global warming effects of the air-conditioning and refrigeration systems, researchers worldwide are focusing on enhancing the efficiency of vapor compression cycles and the use of low global warming potential (GWP) refrigerants such as HFO-1234yf, HFC-152a, and natural refrigerants. Among natural refrigerants, carbon dioxide ([CO.sub.2]) is already applied in commercial heat pump water heaters and thrives in Japan. Researchers are intensely scrutinizing the capacity for using [CO.sub.2] for mobile air conditioning and small-capacity refrigeration applications. Hydrocarbons (HFCs) are already being used, primarily in small-capacity equipment, and their application to larger systems is under discussion. The use of ammonia is under investigation as an option for commercial applications. Among low GWP refrigerants that are currently available, only [CO.sub.2] is nontoxic and nonflammable. One technical issue related to the [CO.sub.2] transcritical cycle is that it is inherently less efficient than conventional HFC subcritical cycles at high ambient temperatures--due to the large difference in high- and low-side pressures. One method for increasing the efficiency of a [CO.sub.2] cycle is to approach isentropic expansion with a work-recovery machine (expander) rather than isenthalpic expansion with an orifice-type expansion device.
Recovering throttling losses by use of expander technology was first proposed by Horst (1911) and Plank (1912). After about a hundred years of silence due to failure in early works, research on the expander experienced a resurgence in the early twenty-first century as the [CO.sub.2] transcritical cycle's large throttling losses garnered scientists' attention. Most of the recent expander research includes enhancement of the efficiency of the various types of expanders and integration of the compressor and expander. In 2002, Stosic and his colleagues developed a twin screw combined compressor and expander in a low-temperature application for [CO.sub.2]. The isentropic efficiency of the expander section was about 65%. In 2008, Matsui and his colleagues combined a scroll compressor with a two-stage rotary expander in a heat pump water heater for [CO.sub.2] and achieved an isentropic efficiency of up to 55%. In summary, the efficiency of the various types of expanders has improved and has been reported to be in the range of 55% to 70%. When employing an expander, the potential for improvement of system energy efficiency is in the order of 20% to 50%, depending on assumptions and operating conditions for the system.
Severe fluctuation in energy cost and worsening global warming trends require us to take immediate actions in enhancing the efficiency of HVAC equipment and to use low GWP refrigerants. There are many pathways toward achieving these goals. As the efficiency of the expander evolves, the application of the expander certainly becomes one such pathway. The development of sustainable air-conditioning and refrigeration equipment is our duty and will contribute to improving the climate change caused by ozone depletion and global warming.
The Henry Ford. 2004. Annual Report, The Henry Ford Organization, Dearborn, MI.
Horst L. 1911. Verbesserte [CO.sub.2]-Expansions-Kaeltemaschine. Zeitschrift fuer Sauerstoff- und Stickstoff-Industrie, Heft 12.
Matsui, M., M. Wada, T. Ogata, and H. Hasegawa. 2008. Development of high-efficiency technology of two-stage rotary expander for [CO.sub.2] refrigerant. The International Compressor Engineering Conference at Purdue University, July 14-17, Purdue University, West Lafayette, IN.
Plank, R. 1912. Verbesserung der Kohlensaeure-Kaeltemaschine durch Einfuehrung eines Expansionszylinders. Zeitschrift fuer die gesamt Kaelteindustrie, Heft 12.
Stosic, N., I.K. Smith, and A. Kovacevic. 2002. A twin screw combined compressor and expander for [CO.sub.2] refrigeration systems. The International Refrigeration Conference at Purdue, July 17-20, Purdue University, West Lafayette, IN.
Yunho Hwang, PhD
Yunho Hwang is a research associate professor and associate director of the Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, MD.
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|Publication:||HVAC & R Research|
|Date:||Jul 1, 2009|
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