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Description
INTRODUCTION
Over the last 25 or so years, the worldwide phase-out of chlorofluorocarbons (CFCs) and hydrofluorochlorocarbons (HCFCs) has led to active research and development programs by the HVAC&R industry, manufacturers, universities, and governments with the aim of identifying and developing new fluids and equipment. The most commonly pursued short-term solution is to retrofit CFC applications with appropriate HCFCs and hydrofluorocarbons (HFCs), and longer-term solutions include replacing CFC and HCFC applications with appropriate HFCs or "natural" (e.g., water, air, ammonia, carbon dioxide, hydrocarbon) refrigerants and/or to develop new equipment and cycles. The twin environmental concerns of ozone depletion and global warming are the principal driving forces behind these changes. While the Montreal Protocol and its amendments have addressed the ozone depletion problem, more recent initiatives (for example, the Kyoto Protocol) are attempting to address the global warming problem. As a response, the industry has developed measures that account not only for a refrigerant's direct contribution but also a refrigerant's indirect contribution to global warming. The total equivalent warming impact (TEWI) is one such measure that accounts for a refrigerant's direct contribution through leakage of the refrigerant into the atmosphere and its indirect effect related to the amount of carbon dioxide generated from burning fossil fuels to power the refrigeration system. Another similar measure is the life-cycle climate performance (LCCP), which, in addition to including the effects captured by TEWI, also attempts to account for other indirect effects related to the manufacturing, transporting, recycling, etc., of the refrigerant.
Even though CFCs are scheduled for phase-out, many will still be used for some time. For example, R-11, R-12, R-113, and R-114 are presently used in industrial processes operating under high ambient conditions and in high-temperature heat pumps, with R-114 being the most widely used refrigerant in these applications. However, as this equipment is being considered for replacement or for new applications, alternative heating technologies are being considered since no known and viable non-CFC retrofit refrigerants are available. Moreover, with the recent increase in primary energy costs and expected price pressures for primary energy persisting into the near future, it is anticipated that there will be a renewed interest in high-temperature heat pumps for many possible applications. Therefore, a strong need exists to identify and develop new refrigerants and technologies to allow for the continued application and expanded use of high-temperature heat pumps despite the scheduled phase-out of CFC refrigerants. The scope of this paper, however, is somewhat more limited than finding direct replacements for these CFC refrigerants. In particular, what is addressed is this question: What should be the thermodynamic parameters of a replacement refrigerant for R-114 in high-temperature heat pumping applications?
When considering replacement refrigerants, either for existing or new applications, a good starting point for the performance potential of the various alternative refrigerants in an idealized vapor-compression refrigeration cycle. This approach avoids unnecessary, costly, and time-consuming experimentation and detailed system modeling. In particular, simplified analytical approaches (see, for example, Brown [2007]) can indicate a few promising refrigerants from a much longer list of potential refrigerants. This shortened list can then be investigated in depth using conventional approaches such as experimentation or detailed system modeling. Regardless of which approach one chooses to pursue, typical measures of performance potential for heat pumping applications include the heating coefficient of performance ([COP.sub.H]) and the volumetric heating capacity (VHC), where [COP.sub.H] is a measure of energy efficiency (operating costs [Didion 1999]) and VHC indicates equipment size (capital costs [Didion 1999]).
It is known from previous studies (e.g., McLinden [1990], Morrison [1994], Didion [1999]) that properties and characteristics such as cost, stability, toxicity, flammability, environmental impact, liquid viscosity, liquid thermal conductivity, materials compatibility, solubility with lubricants, etc., are all extremely important. As a first-cut approximation, however, the thermodynamic properties critical point temperature and vapor molar heat capacity can provide an indication of how a particular refrigerant will perform in an idealized vapor-compression refrigeration cycle. In particular, increasing the critical point temperature implies a higher... |
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