Global Warming Impacts of Ozone-Safe Refrigerants and Refrigeration, Heating, and Air-conditioning Technologies. Steve Fischer Jim Sand Van Baxter

Transcription

1 Global Warming Impacts of Ozone-Safe Refrigerants and Refrigeration, Heating, and Air-conditioning Technologies Steve Fischer Jim Sand Van Baxter Oak Ridge National Laboratory Oak Ridge, Tennessee To be presented at the 1997 International Conference on Ozone Protection Technologies Baltimore, Maryland November 12-13,1997 Document prepared by the Energy Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee , managed by Lockheed Martin Energy Research Corporation, for the U.S. Department of Energy under contract number DE-.AC05-960R22464.

2 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, proctss. or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, m m- mendation. or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not neccssarily state or reflect those of the United States Government or any agency thereof.

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4 Global Warming Impacts of Ozone-Safe Refrigerants aiid Refrigeration, Heating, and Air-Conditioning Technologies Steve Fischer Jim Sand Van Baxter Oak Ridge National Laboratory Oak Ridge, Tennessee Abstract The Montreal Protocol along with the London and Copenhagen amendments mandate the phase-out of mpy chlorine containing compounds that are used as the working fluid in refrigeration, a$-conditioning, and heating equipment. Many of the chemical compounds that have been proposed, and are being used in place of the class of refigerants eliminated by the Montreal Protocol are now being questioned because of their possible contributions to global warming. N&GI~ re-efrigerants are put forth as inherently superior to manufactured refrigerants because they have very low or zero global warming potentials (GWs). Questions are being raised about whether or not these manufactured refrigerants, primarily hydrofluorocarbons (HFCs), should be regulated and perhaps phased out in much the same manner as CFCs and HCFCs. Several of the major applications of refrigerants are examined in this paper and the resuits of an analysis of their contributions to greenhouse warming are presented. Supermarket refrigeration is shown to be an application where alternative technologies have the potential to reduce emissions of greenhouse gases (GHG) significantly with no clear advantage to either natural or HFC refrigerants. Mixed results are presented for automobile air conditioners with opportunities to reduce GHG emissions dependent on climate and comfort criteria. GHG emissions for hermetic and factory built systems (Le. household refrigerators/fieezers, unitary equipment, chillers) are shown to be dominated by energy use with much greater potential for reduction through efficiency improvements than by selection of refrigerant. The results for refiigerators also illustrate that hydrocarbon and carbon dioxide blown foam insulation have lower overall effects on GHG emissions than HFC blown foams at the cost of increased energy use. Introduction Provisions of the Montreal Protocol, and its various amendments, require the phase-out of chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) compounds that have been used extensively as refrigerants in heat pumps, air conditioners, and refrigeration systems. Much of this equipment has been, or is being, redesigned to use hydrofluorocarbons (HFCs) or natural

5 refrigerants and new technologies that have a reasonable potential of becoming con~mercial products for these five applications before to The study included applications in Europe, Japan, and North America and used representative data for each region for equipment size and efficiency, weather and climate, and carbon dioxide einissions from power generation. The results of that study are the focus of this paper. TEWI Although TEWI is finding its way into common usage, it is probably best to spend a IittIe time explaining the concept. Figure 1 is a graph of the radiative forcing (temperature change) from the release of GHGs horn a typical low temperature refrigeration system in an American supermarket. The lower curve shows how carbon dioxide from electrical use accumulates in the atmosphere for the 15 years the system is used, and then how the carbon dioxide is gradually removed through natural processes (e.g. plants, ocean, rain water). The upper curve in Fig. 1 illustrates the..additional effect resulting from refi-igerant leaking from the refrigeration system while it is us& and then its gradual decomposition in the atmosphere and removal. The total effect from energy use and refrigerant eqissions is proportionalrto the area under the curves; energy use corresponds to the area under the bottom curve and refrigerant emissions to the area between... the curves. TEWI must be stated on the basis of some integration time horizon; how far out in time is the integration carried. Using a short period of time omits a great deal of the overall impact by leaving out a lot of the area under the curves, using an extremely long period of t h e Time (years) overlooks the near term effects, that these releases will have on Figure 1 Emission of greenhouse gases from a supermarket climate change. A 100 year time refrigeration system (frozen food cases in North America). period is commonly used. Results and Discussion TEWI should be used as a tool or technique for comparing the global wanning impacts of alternative refrigerants or cooling technologies. The results from the ORNL study are presented as bar charts reflecting the indirect effects of energy use on global warming with the direct effects from refrigerant emissions. A hypothetical example is shown in Fig. 2. The TEWI of a baseline system is represented as the sum of two segments, a darkly shaded portion for energy 3

6 I Direct Expanson achieved for all of the (achievable loss refel R-404A refrigerants by meeting the ARE R-507 R-407A estimates for leakage rates in the R-407C future. I R-404A The bottom two sets of C R-507 R-407A bars in Fig. 3 represent the & R-407C.c TEWI for two alternative R-404A U R-507 technologies for providing R-407A R-407C supermarket refrigeration; R-717 secondary loops and R-404A R-507 distributed refrigeration Refrigerant Emissions R-407A A-407C systems. Both of these 0 1,000,000 2,000,000 3.Mx ,000,000 5,000,000 approaches substitute piping of TEWI (kg COz) brine for the long refrigerant lines used in tha conventional systems. Secondary loop Figure 3 TEWI for low temperature supermarket systems still consist of refrigeration in North America. compressor racks in a machine room, but an iti&mediate heat transfer loop is introduced so a brine is chilled in the machine room and the cold brine is pumped to the display cases. This approach eliminates the major sources of refrigerant leakage as well as greatly reducing the refrigerant charge. Since the refrigerant has been isolated in the machine room, and does not enter the retail sales floor, a toxic or flammable refrigerant can be used safely. Ammonia (R-7 17) is shown as one of the alternative refrigerants for secondary loop systems. These five bars show an increase in energy use compared to the alternatives above them, due to the pumping power to circulate the brine and themlodynamic losses resulting from the intermediate heat exchanger. They also show lower TEWI than both baseline systems (at the top) and the alternatives with reduced leak rates. There are differences between the TEWI for the five refrigerants in secondary loop systems, though these are very small with the possible exception of ammonia. Distributed systems rely on locating compressors close to the dispiay cases they service (e.g. o n the sales floor, on the roof, on top of walk-in coolers). In this instance, a water loop is used to transfer the heat removed from the display cases to a cooling tower or air-cooled heat exchanger on the roof (this system uses a high-side secondary loop while the preceding one uses a low-side secondary loop). There is a small pumping power associated with the distributed systems, particularly for low temp systems, because less energy is required to pump warm or hot water than a cold brine. TEWl for the distributed systems are very similar to those for secondary loops with slightly lower energy use and slightly higher refrigerant emissions. While only one set of results is shown, the TEWI for low temperature and medium temperature supermarket refrigeration systems in North America, Japan, and Europe all show similar comparisons. First, significant reductions in TEWI could be achievedfiom the best attainable iji 1997 through further improvements in leak reduction or by employing either circulating brine in secondary loops or using distributed refrigeration systems. With both secondary loop and distributed systems, there are. only small differences in TEWI between the alternative refrigerants. c 0) L 9) 5

7 compartment is several degrees higher than the return air for the HFC-134a system. This affects how quickly the hot air inside a car is drawn down to a comfortable level. A second set of calculations was performed requiring that the CO, system provide the same return air temperature as the HFC-134a system; these results are shown in the bottom bar indicate a TEWI higher than the worst case scenario for HFC-134a in the sout/z~vesternus. Energy use for automobile air conditioning was calculated using regional temperature profiles and corresponding cooling loads. The state point efficiency calculations showed that the transcritical CO, cycle is less efficient than HFC- 134a at high outdoor temperatures, but more efficient than HFC-134a at low outdoor temperatures. Combining the efficiencies with different outdoor temperature profiles leads to very different results for this application. Figure 5 displays the results for+automobileair conditioning in Germany, and they are quitgdifferent than those for th& sb;thwestern U.S. Both the hydrocarbon and transcritical CO, systems have much lower TEWI than the best possible case with HFC-134a. The results for Figure 5 TEWI for automobile air conditioning in most of Japan, Europe, and Germany. northern parts of the U.S. are similar to those for Germany. Unitary Space Conditioning: heat pumps and air conditioners are common applications of vapor compression systems in North America and Japan with growing usage in Europe. In North America these are frequently window mounted or through the wall packaged units or central split systems with the compressor and one heat exchanger mounted on a cement pad outside and another heat exchanger in #e central air duct for the building. Systems in Japan commonly use a central compressor and outdoor heat exchanger with refrigerant pumped to individual heat exchangers in several different rooms in the building. Heat pumps are not commonly used in Europe for residential and commercial buildings and very few buildings have air conditioning. When heat pumps are used, however, a hydronic distribution system (as opposed to forced air commonly used in North America) serves as a secondary Ioop allowing the mechanical package to be situated outdoors. The TEWI for ducted split-system electric heat pumps is shown in Fig. 6 using residential building heating and cooling loads for Atlanta. R-407C and R-41OA are shown as alternative refrigerants beside standard and high efficiency HCFC-22 air-source heat pumps; R-407C and R-410A are both mixtures of HFCs that are being developed to repiace HCFC-22. The bars for the high efficiency HCFC-22 heat pump and geothermal (ground source) heat pump clearly show that reductions in TEWI can be achieved by efficiency gains that exceed the direct effects on 7

8 R will be used outside of industrial applications. There is some disagreement over how much refrigerant chillers lose each year due to leakage, purging (low pressure systems have air leak into them instead of refrigerant leaking out and the air must be purged to maintain proper operation), and maintenance. Prior to the Montreal Protocol refrigerant loss rates were fairly high, but manufacturers responded aggressively in tightening systenis and designing high efficiency purge units that reduced emissions considerably (to virtually 0 for new machines). Figure 7 shows the TEWI for 3500 kw (1000 ton) chillers in Atlanta for several different refrigerants and four different make-up rate scenarios. The I 4% charm loss /veal HCFC.123 direct global warming effect HFC-134. HCFC.22 from refrigerant losses is a R717 small fraction of the TEWI, HCFC-123 E. HFC-1% even at emission rates much 2 HCFCGZ a R717 higher than the %% per year.-a, Z HCFC-123 claimed by the.industry, 2 HFC-1HCFC-22 particularly for HCFC-I 23 B7i7 machines. Ammonia has an energy use eqy wlent to that for HCFC-22, and the difference in TEWI between these two systems only becomes significant at the Figure 8 TEWI for centrifugal chillers in Atlanta. higher emission rates. v) Household Refrigerators: refrigerators have used fluorocarbons as refrigerants since the 1950s and as blowing agents for plastic foam insulation since the early 1970s. Both the refrigeration systems and the insulations have been developed to be very efficient, first with CFC-12 and CFC-11 for the refrigerant and blowing agent and then with HFC-134a and HCFC-14lb in North America and Japan and HFC-I 34a and CFC-I 1 and water in Europe. Much of the European production has proceeded h i t h e r to using propane and isopentane as refrigerants and cyclopentane as the blowing agent. Alternative blowing agents are heeded to replace HCFC-141b in those countries where it is still used because of the phase-out o~hcfcs.many different HFCs have been considered to fill this role, but manufacturers and suppliers have not yet settled on the best replacement. Estimated TEWI for a typical 510 liter (1 8 ft ) North American refrigerator are shown in Fig. 8 for different blowing agents. These bars have segments for the energy use, direct effect of the blowing agent, and direct effect fiom refrigerant. The energy use of each of the alternative blowing agents is larger than that for HCFC-14lb because of the poorer quality of the insulation that can be made with these compounds. High resistivity vacuum panel insulation could be used to improve the insulation (and reduce TEWI), but its use has been limited to date due to high costs and uncertainty over how its insulating value stands up over the lifetime of the appliance. The direct effect of each of the HFC blowing agents is a fairly large fraction of the TEWI for each of the alternatives, and while cyclopentane (or even CO,) blown foam increases energy use relative to most of the HFCs, the TEWI can be lower. The HFC refrigerant adds a small, but not 9