R-152a Refrigeration System for Mobile Air Conditioning

Transcription

1 SAE TECNICAL PAPER SERIES R-152a Refrigeration System for Mobile Air Conditioning James A. Baker, Mahmoud Ghodbane, Lawrence P. Scherer and Prasad S. Kadle Delphi William R. ill General Motors Corporation Stephen O. Andersen U.S. Environmental Protection Agency Reprinted rom: Advances in Automotive Climate Control (SP-1733) 2003 SAE World Congress Detroit, Michigan March 3-6, Commonwealth Drive, Warrendale, PA U.S.A. Tel: (724) ax: (724) Web:

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3 R-152a Refrigeration System for Mobile Air Conditioning James A. Baker, Mahmoud Ghodbane, Lawrence P. Scherer and Prasad S. Kadle Delphi William R. ill General Motors Corporation Stephen O. Andersen U.S. Environmental Protection Agency Copyright 2003 SAE International ABSTRACT In recent years, climate protection has become as important as ozone layer protection was in the late 1980 s and early 1990s. Concerns about global warming and climate change have culminated in the Kyoto Protocol, a treaty requiring its signatories to limit their total emission of greenhouse gases to pre-1990 levels by The inclusion of hydrofluorocarbons (Cs) as one of the controlled substances in the Kyoto Protocol has increased global scrutiny of the global warming impact of C-134a (called R-134a when used as a refrigerant), the current mobile air conditioning refrigerant. Industry s first response was to begin improving current R-134a systems to reduce leakage, reduce charge, and increase system energy efficiency, which in turn reduces tailpipe CO 2 emissions. An additional option would be to replace the current R-134a with a refrigerant of lower global warming impact. This paper documents the use of another C, R-152a, in a mobile A/C system. It compares its cooling and energy performance with that of a comparable R-134a system. This paper also highlights the environmental benefit of R-152a and pinpoints safety issues that need to be addressed in the future. INTRODUCTION This work is the product of a cooperative effort sponsored by General Motors, Delphi, and the U.S. EPA. The intent was to demonstrate the potential use of R-152a in a mobile air conditioning system. The work included the building and testing of a prototype system, wind-tunnel testing on a production vehicle for performance, and ride-testing the vehicle at the 2002 SAE Alternate Refrigerants Symposium [1]. This paper is complemented by a prior SAE publication [2] in 1999 by one of the authors wherein the potential of R-152a was assessed via system simulation techniques. That paper provides valuable supporting information on many of the properties and characteristics of R-152a that make it suitable for use in mobile A/C systems. Chemically Similar Environmentally Very Different C C C C Why R-152a? R-134a Molecular Weight = 102 1,1,1,2 Tetrafluoroethane GWP = 1300 R-152a Molecular Weight = 66 1,1 - Difluoroethane Similar Materials Compatibility GWP = % Reduction in Climate Impact from Refrigerant Change (not including energy savings) Table 1 R-134a and Chemical / Environmental Comparison The use of R-152a was discussed in a cursory manner during the search for an alternative to CC-12 for mobile A/C in the 1980 s in response to concerns about protection of the Earth s ozone layer. Such discussions, however, quickly moved to selecting and developing systems suitable for R-134a. Recent concerns about R- 134a emissions in Europe have led to unsuccessful attempts to ban all Cs in several European countries. Some have pending regulations aimed at reducing both

4 C refrigerant emissions and tailpipe emissions from the fuel burned to power the air conditioning. The potential use of R-152a as a replacement for R- 134a presumes that nations will ultimately differentiate greenhouse gases on the basis of their environmental performance, not simply by their chemical class, as they seek to reduce emissions. R-152a offers chemical properties and refrigerating characteristics very similar to those of R-134a but with dramatically lower climate impact. While Table 1 provides a brief introductory comparison of R-152a and R-134a, reference [2] offers more in depth analysis of both refrigerants. Temperature ( 0 ) Discharge A/C Temperature Comparison x 40% R.. (38 0 C x 40% R..) - ull Size Sedan - Soak Temperature Average Breath R-152a R-134a ( 0 C) 21 Compressor Average Discharge Time (min.) City Traffic 50 kph 100 kph (Outside Air) (Outside Air) 10-1 Accumulator Suction Line Solenoid Shut off Valve (normally closed) Liquid Line Solenoid Shut off Valve (normally closed) Condenser igure 2 Performance Comparison Evaporator Shut-off valves Expansion Device igure 1 System Description COOLING PERORMANCE COMPARISON igure 2 represents a typical wind tunnel test data that simulates the cooling down of a parked, hot vehicle, soaking in the sun. The soak temperature is the interior breath temperature reached after parking the vehicle for more than one hour in the hot sun. Average breath temperature is the average temperature at the driver and passenger head levels. The average discharge temperature is the average temperature of the air coming out of the A/C vents. The data shows that the R- 152a system has equal or better cooling performance than the R-134a system. A full size sedan production vehicle served as the base vehicle for this testing. The vehicle was first tested with the original equipment (OE) R-134a system, then equipped and tested with an R-152a system. Components used to convert from R-134a to R-152a were all current production R-134a components chosen to represent more up-to-date technology, and to reduce the total system refrigerant charge. The expansion device was modified to account for the different thermalphysical properties of R-152a. In addition, solenoid-operated, normally-closed, shut-off valves were placed on the inlet and outlet to the evaporator and electrically connected to the air bag system and compressor circuit. igure 1 is a pictorial representation of the prototype R-152a system. eat exchanger face areas and airflows for all testing were identical for both systems and all testing was performed in the same wind tunnel. The system refrigerant charges, including reserve charge, were 0.908kg (2 lbs.) for the R-134a system and kg (1 lb.) for the R- 152a system kpa Pressure (psig) Compressor Pressure and Discharge Temperature Comparison x 40% R.. (38 0 C x 40% R..) - ull Size Sedan - R-152a Discharge Pressure Suction Pressure R-134a Discharge Temperture Time (min.) City Traffic 50 kph 100 kph (Outside Air) (Recir. Air ) (Outside Air) ( 0 ) ( 0 C) igure 3 Compressor Thermal Characteristics

5 igure 3 depicts the thermal characteristic behavior of the two refrigerants during the tests. As expected from its saturation properties, an R-152a system shows lower suction and discharge pressures than R-134a system, while it exhibits higher compressor discharge temperatures than the R-134a baseline system. Lower discharge pressures would allow longer idles without A/C cut-offs due to high discharge pressures. owever, it may be necessary to assess the effect of higher operating temperatures on current discharge hose materials, joint seals, and compressor durability. In summary, R-152a systems can provide the same or better comfort cooling as current OE R-134a systems. ENERGY COMPARISON A full size sedan production vehicle equipped with an original equipment (OE) R-134a system served as the base vehicle for this testing. constraints. As expected, the critical charges for these systems were proportional to their molecular weights. igures 4 & 5 compare the energy use of R-134a versus R-152a expressed as their respective mechanical coefficients of performance () versus ambient temperature and road speed. Note that, in all cases, the of R-152a exceeded that of R-134a, indicating that less energy was required for operation. It should also be noted that the computation is based on the measured compressor torque instead of the change in refrigerant thermal properties across the compressor. The measured torque is a more accurate means of determining the compressor energy requirements. Table 2 (a) and (b) show the cooling performance of the R- 152a system to be superior to that of the R-134a system at road speeds and equal at idle. The R-152a system outperformed the R-134a on cooling performance, and did while using less energy. The environmental importance of this energy saving is discussed in the next section. : 38 0 C x 40% R.. (100 0 C x 40% R..) Grille Air 0 C ( 0 ) Comp. Out kpag (psig) Comp. Out 0 C ( 0 ) Blower Air Out 0 C ( 0 ) Average A/C ront 0 C ( 0 ) Evap. Out kpag (psig) 48 kph (30 mph) 80 kph (50 mph) R-152a R-134a R-152a R-134a 37 (99) 37 (99) 38 (100) 37 (99) 1323 (192) 1452 (210.6) 1361 (197) (211) 88 (190) 76 (169) 97 (207) 79 (174) 41 (106) 41 (106) 40 (104) 40 (104) 13 (56) 15 (59) 12 (54) 14 (58) 233 (33.8) (42) 214 (31) (39.8) Sweep of Mechanical at Different Vehicle Speeds in Outside Air Mode 81 0 x 55% R.. (27 0 C x 55%) +8.4% +3.5% +2.6% +1.8% x 40% R.. (38 0 C x 40%) +12.1% +9.7% +9.7% +17.3% : 38 0 C x 40% R.. (100 0 C x 40% R..) Grille Air 0 C ( 0 ) Comp. Out kpag (psig) Comp. Out 0 C ( 0 ) Blower Air Out 0 C ( 0 ) Average A/C ront 0 C ( 0 ) Evap. Out kpag (psig) (a) 113 kph (70 mph) R-152a R-134a R-152a R-134a 37 (99) 37 (99) 50 (121) 54 (129) (193) 1541 (224) 1655 (240) (315.7) 112 (233) 98 (208) 89 (193) 89 (193) 39 (103) 39 (103) 58 (137) 61 (142) 12 (54) 13 (56) 23 (73) 26 (79) 204 (29.6) 233 (33.8) (56.8) (73) (b) 30 mph (48 kph) 50 mph (80 kph) 70 mph (113 kph) Vehicle Speed x 25% R.. (46 0 C x 25%) % +11.4% % mph (48 kph) 50 mph (80 kph) 70 mph (113 kph) Vehicle Speed 30 mph (48 kph) 50 mph (80 kph) 70 mph (113 kph) Mechanical = Vehicle Speed System Air Cooling Capacity Measured Compressor Shaft Power igure 4 Energy Comparison at Various s Table 2 igh Load Tests (All Testing in Ouside Air Mode) The vehicle was first tested with the OE R-134a system, then evacuated, recharged and tested with R-152a in the same wind tunnel. In this test, the components were kept the same for both systems except for the desiccant and the expansion device. As previously stated, the heat exchangers and their respective airflows remained identical throughout both series of testing. These tests were primarily intended to directly compare the energy usage of the two refrigerants when tested using identical components and subjected to the same conditions and ENVIRONMENTAL CONSIDERATIONS The substitution of R-152a for R-134a would result in a dramatic improvement in the climate impact of mobile A/C systems. The much lower greenhouse gas global warming impact of R-152a (GWP = 120 for R-152a versus GWP = 1300 for R-134a) combined with its lower system charge (35 to 40% less than R-134a system) would reduce the system s climate impact by almost 94%. According to the Mobile Air Conditioning Society (MACS) [3], R-134a emissions are estimated to be approximately 96 grams per year with refrigerant recycling during service and recovery during scrap. Assuming R-152a emissions would be the same, 96

6 grams of R-152a would be equivalent in global warming impact to 1.3 gallons of fuel use. Compared to R-134a systems of equal weight, the fuel savings due to lower energy requirements of R-152a systems would completely counterbalance the climate impact of R-152a emissions. Thus, climate impact of today s R-134a refrigerant emissions in mobile A/C systems would be completely offset by substituting R- 152a for R-134a. Mechanical Variation with in Outside Air Mode Vehicle 30 mph (48 kph) +3.5% +9.7% +11.1% Vehicle 50 mph (48 kph) +2.6% +9.7% +11.5% means optimized. The same improvements that were applied to the enhanced R-134a system could be applied here as well. These systems are expected to improve for better environmental and cooling performance. A/C SYSTEM CONSIDERATIONS The similarity in chemical properties and vapor pressures between R-152a and R-134a allows the use of the current system components. This can be a significant advantage because it offers the potential for flexible manufacturing on a continuous basis and / or could ease the transition for those wishing to change to R-152a. Other than system control settings, the only modification foreseen at this time is changing the desiccant x 55% R.. (27 0 C) x 40% R.. (38 0 C) x 26% R.. (46 0 C) Vehicle 70 mph (113 kph) % % % x 55% R.. (27 C) 100 x 40% R.. (38 C) 115 x 26% R.. (46 C) % The environmental acceptability and the increase in cooling performance could be further improved with more efficient compressors, more effective heat exchangers, and higher quality fittings to reduce leakage. On the system operating side, the adoption of an energy efficient system [5] is highly recommended. There are significant environmental and economical values in carrying such improvements x 55% R.. (27 0 C) x 40% R.. (38 0 C) x 26% R.. (46 0 C) 81 0 x 55% R.. ( x 40% R.. (38 igure 5 Energy Comparison at Various Road Load Conditions igure 6 shows a Life Cycle Climate Performance (LCCP) analysis of some of the proposed alternatives to the current R-134a system. This is an update of the presentation made in 1999 at the Earth Technologies orum [4] that incorporates R-152a in this analysis. LCCP takes into account the energy and refrigerant emissions from all phases of manufacturing, use, servicing, recycling, and scrap throughout the life cycle of the A/C system. The figure compares the baseline R- 134a system to an enhanced R-134a system, and to systems using R-152a and R-744 (carbon dioxide) in different climates operated under simulated drive cycle conditions for those locations. The drive cycles and ambient profiles are specific for these locales and the same as those used in reference [4]. The enhanced R-134a system assumes a 50% reduction in the leakage rates from the baseline R-134a system used in [4] and a 30% improvement in efficiency. Recently published results of the SAE Alternative Refrigerant Cooperative Research Program [6] indicate a 10 to 47% improvement in between an R-134a production system and an enhanced R-134a system. The R-152a system is shown as having the lowest climate impact of any alternative refrigerant system under consideration to date. This system was by no R744 -enh R744 -enh R744 -enh LCCP Analysis Comparison of R-134a, R-134a-enh., R-152a, and R-744 Vehicle A/C Systems Versus Climate rankfurt Tokyo Direct emissions Indirect emissions Phoenix LCCP [kg. of Equivalent CO 2 ] igure 6 LCCP Analysis (Vehicle A/C Systems Versus Climate) SAETY CONSIDERATIONS R-152a is mildly flammable and is listed by the American Society of eating, Refrigerating and Air-Conditioning Engineers (ASRAE) as a Class A2 refrigerant, defined as being of low toxicity and lower flammability than Class A3, which comprises hydrocarbons such as propane. As a reference, R-134a is Class A1, low toxicity and non-flammable.

7 Since there is no prior experience in employing R-152a as a refrigerant in mobile air conditioning systems, practices from other applications of R-152a were examined. In particular, the release of R-152a used in aerosol products is comparable to the quantity and rate of leakage that could occur in a failed or damaged mobile air conditioning. R-152a (referred to by aerosol product manufacturers as C-152a or 1,1-difluoroethane) is commonly used as a propellant and solvent ingredient in aerosol hair, paint, antiperspirant, and shaving cream products. C-152a is used in 100% concentration in dusting aerosols, boat horns, tire inflators, and compressed gas guns. It is sold without product use restriction to the general public. Airline passengers are allowed to travel with personal use products containing C-152a in carry-on and checked luggage. Products are labeled as being notflammable that pass the Consumer Product Safety Commission (CPSC) aerosol flame extension test. This test consists of spraying the vapor across the flame of a candle at a distance of 6 inches (15.24 cm) and observing flame extension (burning beyond the candle flame) and flashback of the candle flame to the aerosol can. Not-flammable labeling criteria require no flashback and no more than 18 inches (45.72 cm) flame extension. Pure C-152a gives no flame extension and no flashback during this test. Thus, to the extent that refrigerant leaks mimic aerosol can sprays, this test confirms that C-152a is safer than hydrocarbons, such as propane and butane that give both flashback and flame extensions in excess of 18 inches (45.72 cm). Additional safety information can be gleaned from the International Electro-Technical Commission s current draft of IEC , Edition3: Part2-40 Particular Requirements for Electrical eat Pumps, Air Conditioners and Dehumidifiers Containing lammable Refrigerants. This document provides criteria for using flammable refrigerants and suggests that refrigerant charges below a refrigerant-dependent threshold mass are not a concern for household appliances. That mass is listed as 520 grams for R-152a. This is greater than the 454 grams needed for the system tested in this study. Safety engineering with follow-up risk assessment will be needed to assure safe systems. Our test vehicle was equipped with shut-off valves to create refrigerant isolation zones to limit the amount of refrigerant that could be released into the passenger compartment. These valves automatically close when the system was not operating and when the vehicle air bags deploy. R-152a systems use less energy than current R- 134a systems. R-152a systems have considerably lower impact on the climate from direct refrigerant emissions. Based on a wide variety of climates and drive cycles, R-152a Systems have the lowest overall climate impact of any alternative refrigerant system under consideration to date. R-152a systems offer the opportunity for flexible manufacturing (i.e., same components and manufacturing processes as R-134a). Additional risk assessment and safety engineering are needed to assure acceptable R-152a system safety. When the safety issues are resolved, the conversion to R-152a would be relatively easy for the industry. Therefore, R-152a merits consideration for use in future A/C systems. REERENCES [1] 2002 Phoenix SAE Alternate Refrigerant Symposium, SAE, Warrendale, PA (USA) [2] Ghodbane, M., An Investigation of R-152a and ydrocarbon Refrigerants in Mobile A/C, SAE Paper , SAE, Warrendale, PA (USA) [3] Baker, James A., Mobile Air Conditioning and the Global Climate - A Summary of the Phoenix Alternate Refrigerant orum-july 15-18, 1998, Earth Technologies orum, October 26-28, 1998, Washington, DC [4] Sumantran, V., et al, Environmental Impact Study of Alternative Automotive Refrigerants, 1999 Earth Technologies orum, 2111 Wilson Boulevard, 8th loor, Arlington, VA [5] orrest, W.O. Energy Efficiency Automotive Air Conditioning System, SAE Paper , SAE, Warrendale, PA (USA) [6] SAE Alternative Refrigerant Cooperative Research Project, November 2002 report. CONCLUSION Several conclusions can be drawn from this investigation: R-152a systems provide equal or better comfort cooling compared to current R-134a systems.