A new reverse cycle defrost design concept for refrigerators.INTRODUCTION
Refrigerator designs today are very different from the past. It has received attention in the U.S. because it consumes about 7% (DOE 2008) of the total primary energy in residences, and it is becoming a primary target in new designs to cut household electricity and also to make refrigerators more green than before (Simpson 2007). Their energy efficiency has improved rapidly in recent years. Vineyard et al. (1997) mentioned that energy consumption of a 1.86 [m.sup.3] (20 [ft.sup.3]) refrigerator, has dropped from 1726 kWh/y in 1972 down to around 460 kWh/y in 2001 with improving gaskets, insulation, compressors, and other technologies to maximize efficiency. They showed the feasibility to reduce the power consumption of the aforementioned refrigerator down to 1 kWh/day. Inverter controlled variable speed compressors have also been laboratory tested with impressive results (Chang 2006; Chang et al. 2009). However, there has been little research work in improving refrigerator defrosting efficiency.
This paper discusses the design and initial testing of a novel reverse cycle defrosting scheme. Reverse cycle defrosting schemes are not new--this approach is commonly used to defrost the outdoor heat exchangers of air-to-air heat pumps. Condensers for the majority of the refrigerators are located at the bottom of the refrigerator, along with a condenser fan. Applying a heat pump type reverse cycle defrost scheme for this kind of design is relatively simple. A 4-way valve is needed. During the defrosting period, the refrigeration cycle is reversed, heating the evaporator and melting the frost. One challenge for this approach, however, is to reduce the hot refrigerant temperature (superheat) before it enters the evaporator located in the freezer compartment to avoid possible damage to the coil due to thermal shock. For typical Asian refrigerator designs the condenser coils are located on one or both refrigerator side panels as shown in Figure 1. This design is aimed to save space and cost, though it is not very energy efficient. There are at least two reasons why the Asia type refrigerators are less efficient: (1) The condenser coils are attached to the refrigerator side panels. The side panels are warm whenever the compressor is running, and part of the heat is transferred into the refrigerator fresh food compartment. (2) The insulation materials are thinner than that of U.S. refrigerators. A heat pump type reverse cycle defrost scheme is not good for this design. As heat is absorbed from the refrigerator side panels during defrost operation, the temperature of the panels will drop, possibly leading to moisture condensation on the panel and water puddling on the floor. A new reverse cycle defrosting concept has been developed to avoid this moisture condensation problem and to reduce thermal shock to the evaporator.
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New Defrosting Scheme
Figures 2 and 3 show the refrigerant circuit for a refrigerator employing the new defrost scheme for normal operation and for defrosting operation, respectively. A test heat exchanger (HX) was handmade for purposes of experimentally evaluating the new concept. The test HX (tube-in-tube design) was 1 m (3.3 ft) long with outer and inner tube diameters of 1.27 and 0.64 cm (1/2 and 1/4 in.) as shown in Figure 4. The purposes of the additional heat exchanger are to (1) reduce, or eliminate, the superheat of compressor discharge refrigerant vapor before it enters the evaporator coil, and (2) evaporate liquid refrigerant before it enters into the compressor. The new defrost scheme does not take heat from the surroundings. Instead, it is exchanging heat internally. The maximum efficiency of the new design is 1.0. However, because it provides relatively cool, saturated vapor refrigerant to the evaporator, defrosting takes place faster than with the conventional defrost scheme (external electric heating of the coil) and less heat is transferred to the freezer compartment during defrost.
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Laboratory Test Procedures
Because of the lack of an environmental chamber, the laboratory tests were meant for qualitative results only to prove the functionality of the new defrosting scheme and provide an initial estimate of its energy saving potential. The test results presented here were under controlled room temperature at 25[degrees]C (77[degrees]F). A container with 4500 cc (0.16 [ft.sup.3]) of hot water at 98[degrees]C (208[degrees]F) was placed inside the refrigerator every night at 7:00 P.M. to provide a moisture source for evaporator frosting.
The temperature controls for the refrigerator's freezer and fresh food compartments were set at the high-high points. For the conventional electric heating defrost tests, the compressor power was shut off and a 190 W heating element was energized manually. Pictures were taken every 5 minutes to evaluate the coil frost accumulation until the coil was free of frost. For the reversed cycle defrosting test, the compressor was again shut off manually, and the cycle was reversed by adjusting the four 2-way valves (Figures 3 and 4), and then the compressor was turned on again. Pictures were taken as before. The temperatures of the interior compartments, and refrigerator power consumption were recorded for both normal and defrost operation. After defrosting, the refrigerator was returned to its normal operation for an additional three on/off cycles because it was found that it took several minutes for the freezer compartment to recover back to its original temperature setting of -17[degrees]C (1.4[degrees]F). The comparison of power consumption for defrosting by the two defrosting methods was based on the sum of the power consumption during defrost plus that for the three subsequent on/off cycles.
Test Results and Discussion
Figures 5 and 6 show the comparison of the evaporator coil defrost speed by the two defrost schemes. With the conventional electric method it took about 25 minutes to defrost vs. 19 minutes for the reverse cycle method. It was later found that during the reverse cycle test, the evaporator fan was inadvertently left on (it was off for electric defrost test). If the fan had been off as intended during the reverse cycle test, it is estimated coil defrosting would have taken only 15 minutes. Figure 7 shows the power consumption for the two defrost schemes. The average power consumption rate was about the same for both methods during the defrost period but since the reverse cycle method had a much shorter defrosting time its total defrost energy consumption was less. Figure 8 shows the energy consumption for three on/off cycles after defrosting was completed for each defrost method. The first cycle for each method shows that the compressor running time for the electric defrost was much longer than that of the reverse cycle defrosting. The trend was the same even for second and third cycles, an indication that the new reverse cycle defrost concept is more efficient than the conventional electric method.
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Table 1 shows the data collected for two comparison tests between the novel reverse cycle defrost and conventional electric defrost. The reverse cycle defrosting method clearly outperformed the electrical heating element defrosting scheme by consuming an average of 27% less energy during defrost. Total energy savings over a defrost and 3 subsequent on/off cycles was at least 8% less for the reverse cycle defrosting method.
Table 1. Data Collected for Both Defrosting Methods Data Electric Heating Reverse Cycle Set Element A. Defrosting time 1. 25 min. 19 min. 2. 27 19 B. Total defrost 1. 85.1 W-h 64.1 W-h (24% less) energy Consumption 2. 86.3 60.4 (30% less) C. Total energy 1. 465.6 W-h 382.4 W-h used for three on/off cycles 2. 485.7 466.9 D. Compressor 1. 90 min. 64 min. running Time first "on" cycle 2. 103 89 E. Freezer 1. 11.1[degrees]C 5.6[degrees]C compartment (52[degrees]F) (42[degrees]F) temperature after 2. 15.3[degrees]C 8.6[degrees]C defrosting (60[degrees]F) (47[degrees]F)
It must be noted that the reverse cycle defrost tests were done using an initial design for the defrost HX (shown in Figure 4). With an optimized defrost HX design to assure that the refrigerant returning to the compressor was saturated vapor, the reverse cycle defrosting efficiency could be higher. The defrost HX was difficult to study because the refrigerant flow rate was difficult to measure. However, we did add a small low cost accumulator before the compressor inlet, which was not shown on the refrigerant schematic. The one meter long tube-in-tube HX was based on an educated guess. If it was too short, we should see frost on the suction line all the way to the compressor inlet, which did not happen. If it was too long, the effectiveness of defrosting would be reduced. We had the plan to study the design of the tube-in-tube HX in the future, if funding permits.
A novel reverse cycle refrigerator defrosting cycle was presented. The novel defrosting system worked as expected. The refrigerator did not suffer severe thermal shock because the refrigerant entering the evaporator was not superheated. Because the defrosting energy was supplied by condensing cool refrigerant vapor inside the evaporator tubes, it defrosted the coil faster than did the conventional resistance heating element. Also, it dissipated less heat to the freezer compartment so the compressor took less running time to recover to the compartment temperature before defrosting.
The Energy and Environment Laboratory of Taiwan's Industry Technology Research Institute provided the research funding for this study. The authors appreciate the Mechanical Engineering students that worked diligently in collecting the data and providing the figures for this paper.
Chang, W-R. 2006. Implementation and energy-saving analysis of inverter-driven refrigerators/freezers with vacuum insulation panels. Proceedings of the 3rd Asian Conference on Refrigeration and Air-conditioning, May 21-23, 2006, Gyeongju, Korea.
Chang, W.-R., T.-S. Shaut, C.-H. Lin, and J.-Y. Lin. 2009. Experimental study of HC Isobutane to replace refrigerant HFC-134a for inverter-driven household refrigerator-freezer. Proceedings of the 4th Asian Conference on Refrigeration 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 Efficiency and Renewable Energy.
Simpson, D. 2007. Far more than an efficient refrigerator. October issue, Appliance Magazine.
Vineyard, E.A., J.R. Sand, C.K. Rice, R.L. Linkous, and C.V. Hardin. 1997. DOE/AHAM refrigerator advanced technology development project, ORNL/CON-441 report, Oak Ridge National Laboratory, Oak Ridge, TN.
C.T. Yang is associate professor in the Department of Mechanical and Computer-Aided Engineering, St. John's University Taiwan, ROC. V.C. Mei is guest professor in the School of Automation and Mechatronics, St. John's University Taiwan, Taiwan, ROC. W.R. Chang is researcher and J.Y. Lin is senior researcher and deputy director at Residential & Commercial Energy Conservation Technology Div., Industrial Technology Research Institute, Taiwan, ROC.