1、242 2010 ASHRAEABSTRACTRefrigerators have been becoming more energy efficientin recent years. However, there is little research work on refrig-erator defrosting. Refrigerators generally still use electricalresistance heating elements for defrost. In Asia, particularly inJapan and Taiwan, a lot of re
2、frigerator condensers areattached to the refrigerator side panels, with heat dissipationthrough the panels. This design saves refrigerator space, butis not very energy efficient. Applying a typical heat pump typecycle reversal for defrosting would cause the condenser panelsto become cold, and moistu
3、re might condense on the surfaceof the side panels and run down to the floor. A novel reversecycle system was designed to completely by-pass the con-denser, thus avoiding moisture condensation on the side pan-els. Qualitative laboratory test data indicated that the reversecycle defrosting concept wo
4、rked as expected, and it was foundto be more energy efficient than the conventional direct electricheating defrost method.INTRODUCTIONRefrigerator designs today are very different from thepast. It has received attention in the U.S. because it consumesabout 7% (DOE 2008) of the total primary energy i
5、n resi-dences, and it is becoming a primary target in new designs tocut household electricity and also to make refrigerators moregreen than before (Simpson 2007). Their energy efficiency hasimproved rapidly in recent years. Vineyard et al. (1997) men-tioned that energy consumption of a 1.86 m3(20 ft
6、3) refriger-ator, has dropped from 1726 kWh/y in 1972 down to around460 kWh/y in 2001 with improving gaskets, insulation, com-pressors, and other technologies to maximize efficiency. Theyshowed the feasibility to reduce the power consumption of theaforementioned refrigerator down to 1 kWh/day. Inver
7、ter con-trolled variable speed compressors have also been laboratorytested with impressive results (Chang 2006; Chang et al.2009). However, there has been little research work in improv-ing refrigerator defrosting efficiency.This paper discusses the design and initial testing of anovel reverse cycle
8、 defrosting scheme. Reverse cycle defrost-ing schemes are not newthis approach is commonly used todefrost the outdoor heat exchangers of air-to-air heat pumps.Condensers for the majority of the refrigerators are located atthe bottom of the refrigerator, along with a condenser fan.Applying a heat pum
9、p type reverse cycle defrost scheme forthis kind of design is relatively simple. A 4-way valve isneeded. During the defrosting period, the refrigeration cycle isreversed, heating the evaporator and melting the frost. Onechallenge for this approach, however, is to reduce the hotrefrigerant temperatur
10、e (superheat) before it enters the evap-orator located in the freezer compartment to avoid possibledamage to the coil due to thermal shock. For typical Asianrefrigerator designs the condenser coils are located on one orboth refrigerator side panels as shown in Figure 1. This designis aimed to save s
11、pace and cost, though it is not very energyefficient. There are at least two reasons why the Asia typerefrigerators are less efficient: (1) The condenser coils areattached to the refrigerator side panels. The side panels arewarm whenever the compressor is running, and part of the heatis transferred
12、into the refrigerator fresh food compartment. (2)The insulation materials are thinner than that of U.S. refriger-ators. A heat pump type reverse cycle defrost scheme is notgood for this design. As heat is absorbed from the refrigeratorside panels during defrost operation, the temperature of thepanel
13、s will drop, possibly leading to moisture condensation onA New Reverse Cycle Defrost Design Concept for RefrigeratorsC.T. Yang, PhD V.C. Mei, PhD, PEW.R. Chang J.Y. Lin, PhDMember ASHRAEC.T. Yang is associate professor in the Department of Mechanical and Computer-Aided Engineering, St. Johns Univers
14、ity Taiwan, ROC. V. C .Mei is guest professor in the School of Automation and Mechatronics, St. Johns University Taiwan, Taiwan, ROC. W.R. Chang is researcherand J.Y. Lin is senior researcher and deputy director at Residential & Commercial Energy Conservation Technology Div., Industrial Technol-ogy
15、Research Institute, Taiwan, ROC.OR-10-026 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or
16、digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 243the panel and water puddling on the floor. A new reverse cycledefrosting concept has been developed to avoid this moisturecondensation problem and to reduce thermal shock to theevaporator.New Defrosting Sc
17、hemeFigures 2 and 3 show the refrigerant circuit for a refrig-erator employing the new defrost scheme for normal operationand for defrosting operation, respectively. A test heat ex-changer (HX) was handmade for purposes of experimentallyevaluating the new concept. The test HX (tube-in-tube design)wa
18、s 1 m (3.3 ft) long with outer and inner tube diameters of1.27 and 0.64 cm (1/2 and 1/4 in.) as shown in Figure 4. Thepurposes of the additional heat exchanger are to (1) reduce, oreliminate, the superheat of compressor discharge refrigerantvapor before it enters the evaporator coil, and (2) evapora
19、teliquid refrigerant before it enters into the compressor. The newdefrost scheme does not take heat from the surroundings. In-stead, it is exchanging heat internally. The maximum effi-ciency of the new design is 1.0. However, because it providesrelatively cool, saturated vapor refrigerant to the eva
20、porator,defrosting takes place faster than with the conventional defrostscheme (external electric heating of the coil) and less heat istransferred to the freezer compartment during defrost.Laboratory Test ProceduresBecause of the lack of an environmental chamber, thelaboratory tests were meant for q
21、ualitative results only toprove the functionality of the new defrosting scheme andprovide an initial estimate of its energy saving potential. Thetest results presented here were under controlled room temper-ature at 25C (77F). A container with 4500 cc (0.16 ft3) of hotFigure 1 Condenser coil arrange
22、ment for the refrigeratortested.Figure 2 Novel defrosting refrigeration cyclenormaloperation (2-way valves A,C open, B, D close).Figure 3 Novel defrosting refrigeration cycledefrostoperation (2-way valves A,C close, B, D open).Figure 4 Tube-in-tube heat exchanger. 2010, American Society of Heating,
23、Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 244 A
24、SHRAE Transactionswater at 98C (208F) was placed inside the refrigerator everynight at 7:00 P.M. to provide a moisture source for evaporatorfrosting.The temperature controls for the refrigerators freezer andfresh food compartments were set at the high-high points. Forthe conventional electric heatin
25、g defrost tests, the compressorpower was shut off and a 190 W heating element was ener-gized manually. Pictures were taken every 5 minutes to eval-uate the coil frost accumulation until the coil was free of frost.For the reversed cycle defrosting test, the compressor wasagain shut off manually, and
26、the cycle was reversed by adjust-ing the four 2-way valves (Figures 3 and 4), and then thecompressor was turned on again. Pictures were taken asbefore. The temperatures of the interior compartments, andrefrigerator power consumption were recorded for bothnormal and defrost operation. After defrostin
27、g, the refrigera-tor was returned to its normal operation for an additional threeon/off cycles because it was found that it took several minutesfor the freezer compartment to recover back to its originaltemperature setting of 17C (1.4F). The comparison ofpower consumption for defrosting by the two d
28、efrosting meth-ods was based on the sum of the power consumption duringdefrost plus that for the three subsequent on/off cycles.Test Results and DiscussionFigures 5 and 6 show the comparison of the evaporatorcoil defrost speed by the two defrost schemes. With theconventional electric method it took
29、about 25 minutes todefrost vs. 19 minutes for the reverse cycle method. It was laterfound that during the reverse cycle test, the evaporator fan wasinadvertently left on (it was off for electric defrost test). If thefan had been off as intended during the reverse cycle test, it isestimated coil defr
30、osting would have taken only 15 minutes.Figure 7 shows the power consumption for the two defrostschemes. The average power consumption rate was about thesame for both methods during the defrost period but since thereverse cycle method had a much shorter defrosting time itstotal defrost energy consum
31、ption was less. Figure 8 shows theenergy consumption for three on/off cycles after defrostingwas completed for each defrost method. The first cycle foreach method shows that the compressor running time for theelectric defrost was much longer than that of the reverse cycledefrosting. The trend was th
32、e same even for second and thirdcycles, an indication that the new reverse cycle defrost conceptis more efficient than the conventional electric method.Figure 5 Electric heating element defrosting.Figure 6 Reverse cycle defrosting.Figure 7 Comparison of defrosting power consumption(first test). 2010
33、, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs
34、prior written permission. ASHRAE Transactions 245Table 1 shows the data collected for two comparison testsbetween the novel reverse cycle defrost and conventional elec-tric defrost. The reverse cycle defrosting method clearlyoutperformed the electrical heating element defrostingscheme by consuming a
35、n average of 27% less energy duringdefrost. Total energy savings over a defrost and 3 subsequenton/off cycles was at least 8% less for the reverse cycle defrost-ing method.It must be noted that the reverse cycle defrost tests weredone using an initial design for the defrost HX (shown inFigure 4). Wi
36、th an optimized defrost HX design to assure thatthe refrigerant returning to the compressor was saturatedvapor, the reverse cycle defrosting efficiency could be higher.The defrost HX was difficult to study because the refrigerantflow rate was difficult to measure. However, we did add asmall low cost
37、 accumulator before the compressor inlet, whichwas not shown on the refrigerant schematic. The one meterlong tube-in-tube HX was based on an educated guess. If itwas too short, we should see frost on the suction line all theway to the compressor inlet, which did not happen. If it was toolong, the ef
38、fectiveness of defrosting would be reduced. We hadthe plan to study the design of the tube-in-tube HX in thefuture, if funding permits.CONCLUSIONSA novel reverse cycle refrigerator defrosting cycle waspresented. The novel defrosting system worked as expected.The refrigerator did not suffer severe th
39、ermal shock becausethe refrigerant entering the evaporator was not superheated.Because the defrosting energy was supplied by condensingcool refrigerant vapor inside the evaporator tubes, it defrostedthe coil faster than did the conventional resistance heatingelement. Also, it dissipated less heat to
40、 the freezer compart-ment so the compressor took less running time to recover to thecompartment temperature before defrosting.ACKNOWLEDGMENTThe Energy and Environment Laboratory of TaiwansIndustry Technology Research Institute provided the researchfunding for this study. The authors appreciate the M
41、echanicalEngineering students that worked diligently in collecting thedata and providing the figures for this paper.REFERENCESChang, W-R. 2006. Implementation and energy-saving anal-ysis of inverter-driven refrigerators/freezers with vac-uum insulation panels. Proceedings of the 3rd AsianConference
42、on Refrigeration and Air-conditioning, May21-23, 2006, Gyeongju, Korea.Chang, W.-R., T.-S. Shaut, C.-H. Lin, and J.-Y. Lin. 2009.Experimental study of HC Isobutane to replace refriger-ant HFC-134a for inverter-driven household refrigera-tor-freezer. Proceedings of the 4th Asian Conference onRefriger
43、ation and Air Conditioning, May 21-22, 2009,Taipei, Taiwan.DOE. 2008. 2008 Buildings Energy Data Book, September.U.S. Department of Energy, Office of Energy Efficiencyand Renewable Energy.Simpson, D. 2007. Far more than an efficient refrigerator.October issue, Appliance Magazine.Vineyard, E.A., J.R.
44、 Sand, C.K. Rice, R.L. Linkous, andC.V. Hardin. 1997. DOE/AHAM refrigerator advancedtechnology development project, ORNL/CON-441 re-port, Oak Ridge National Laboratory, Oak Ridge, TN.Figure 8 Power consumption of 3 on/off cycles after de-frosting (first test).Table 1. Data Collected for Both Defrost
45、ing MethodsData SetElectricHeatingElement Reverse CycleA. Defrosting time 1. 25 min. 19 min.2. 27 19B. Total defrost energy Consumption1. 85.1 W-h 64.1 W-h (24% less)2. 86.3 60.4 (30% less)C. Total energy used for three on/off cycles1. 465.6 W-h 382.4 W-h2. 485.7 466.9D. Compressor running Time firs
46、t “on” cycle1. 90 min. 64 min.2. 103 89E. Freezer compartment temperature after defrosting1. 11.1C (52F) 5.6C (42F)2. 15.3C (60F) 8.6C (47F) 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.
