Clodic, D.

Zoughaib, A.

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Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2000Technical and Economical Evaluation of VacuumInsulated Panels for A European FreezerD. ClodicEcole des Mines de ParisA. ZoughaibEcole des Mines de ParisFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlClodic, D. and Zoughaib, A., "Technical and Economical Evaluation of Vacuum Insulated Panels for A European Freezer" (2000).International Refrigeration and Air Conditioning Conference. Paper 501.http://docs.lib.purdue.edu/iracc/501TECHNICAL AND ECONOMICAL EVALUATION OF VACUUM INSULATED PANELS FOR A EUROPEAN FREEZER Assaad ZOUGHAIB, Denis CLODIC Ecole des Mines de Paris, Centre d'Energetique 60 Boulevard Saint Michel, F-75272 Paris Cedex 06 Phone +33 1 40 5192 49 Fax +33 146 34 24 91 e-mail: denis.clodic@cenerg.ensmp.fr, assaad.zgheib@ cenerg.ensmp.fr Abstract: Vacuum insulated panels (VIP) is an emerging technology that may constitute an alternative to rigid polyurethane (RPU) foam as insulation in refrigerators and freezers. The "skin effect" caused by the aluminum layer in the envelope complicates the evaluation of the global thermal resistance. This article present a new method of VIP characterization. Values of the global thermal resistance obtained are used in a dynamic simulation tool ENEREp® to perform predictive calculations. Direct experimental measurements permitted to validate calculations. Finally, a life cycle cost analysis is performed and payback period of the technical option is calculated. Nomenclature k P,W e Thermal conductivity, W/m.K Electrical power, heat flux, W Thickness, m A AT G Area, m2 Temperature difference, K Specific heat loss coefficient, W/m3.K 1. STATE OF ART The Montreal Protocol phased out CFCs refrigerants. R-11 (CCL3F) being used as an expanding gas in the rigid polyurethane foams (PUR) is replaced by alternative fluids such as R-14lb, cyclopentane and C02. As a consequence, when using cyclopentane, the thermal resistance of PUR foams is reduced by about 33%. In parallel, international and particularly U.S. and European efforts [1] are focused on the reduction of appliance energy consumptions. The contradiction between lower thermal resistance of insulation and lower energy consumption of refrigerating appliances pushed the insulation manufacturers to find alternative solutions such as VIP. 2. VIP STRUCTURE Vacuum panels are composed of three main components: a core material, an envelop and a getter. Many materials can be used as a core material in VIP such as open cell polyurethane, expanded polystyrene (XPS), silica powder, perlite ... therefore, for technico-economical considerations, the open cell PU and the XPS are the most usual [2]. The internal absolute pressure of a VIP is about 50 Pa abs. The thermal resistance of the panel is directly related to this pressure level; i.e. for open cell PU, at a pressure of 10 Pa abs the Eighth International Refrigeration Conference at 345 Purdue University, West Lafayette, IN, USA- July 25-28,2000 thermal conductivity in the center of the panel is about 6 mW/m.K, that increases to 25 mW/m.K when the pressure reaches 1000 Pa abs [3]. The envelop and getter roles are to maintain this pressure during the panel lifetime. The getter is a chemical adsorbent composed of barium and lithium alloy that adsorbs nitrogen in large quantities. Additions of calcium and cobalt oxides permit the adsorption of humidity, hydrogen, R-141b ... The envelop is a multilayer polymer (figure 1 ). The frrst layer is in nylon that stops water molecules. The polyethylene teraphtalate (PET) and aluminum layer prevent the passage of smaller molecules, and finally, the polyethylene (PE) layer permits the welding [4]. This envelop guarantees the leak tightness. Therefore, the aluminum layer creates a "skin effect", which is the heat transfer along the envelop. NylonlSJ!m PET l2J!m AI 6J!m 4-----l PE SOJ!m Figure 1 -Envelop composition Considering the high thermal resistance of the panel in the normal direction, the heat transferred through the aluminum layer is not negligible and the equivalent thermal resistance is dimension dependent. 3. VIP CHARACTERIZATION 3.1. Standard Methods For Thermal Conductivity Characterization The standard methods of thermal conductivity characterization are described in ISO 8301 and 8302 [5] and called respectively the "heat flow meter apparatus method" and "the guarded hot plate". Both methods are not convenient for the VIP characterization because of the apparatus size constraint. Only small specimen can be measured. Because of the "skin effect", the VIP characterization shall be performed in real dimensions. The "guarded hot box" method described in ASTM [6] is used in general to characterize building assemblies. The method uses a measurement cell in which the temperature is maintained and a test cell where a thermal flux is generated. The tested material constitutes the partition wall between the two cells. As it is described, this method cannot be used to characterize super insulation since the estimated heat loss through the test cell walls are not negligible compared to the heat flow going through the VIP. An adaptation of the method is necessary to permit the exact measurement of the VIP equivalent thermal resistance. Eighth International Refrigeration Conference at 346 Purdue University, West Lafayette, IN, USA- July 25-28,2000 3.2. Adaptation Of "The Guarded Hot Box Method" The test bench used for the VIP characterization consists of two identical cells in XPS separated by the tested panel (figures 2a and 2b ). Each cell is equiped with an electrical heating device. The cell temperatures and the temperatures of the panel side surfaces are measured by PTlOO resistive sensors. The test is performed in two steps and the ambiance temperature is kept constant. • A first step is needed to characterize the cell heat losses. Both cells are heated to have the same mean temperature on both surfaces of the tested panel. Since the separation wall is adiabatic, the heating power of each cell (P adiab) corresponds to the cell heat loss. • In the second step, one cell is removed and the remaining cell is heated up to the same temperature reached in the first step. In this case, the heating power (Pspec) is equal to the heat loss measured in the first step and to the heat flow passing through the tested panel. The difference between the two electrical powers measured in steps 1 and 2 is the heat flux that went through the tested panel. Both VIP side mean surface temperatures are measured and the geometric characteristics are known. The VIP equivalent thermal conductivity can be calculated using equation (1). (~pee - padiab ) e k=---=----At1T (1) When the tested VIP does not fit exactly in between cells, a previously tested material is used as a frame, as shown in figure 2a. The heat flux (Prrame) passing through the frame can be calculated and equation (2) is used to calculate the VIP equivalent thermal conductivity. k = (Pspec - padiab - Pframe ) e At1T (2) The novelty of the method consists in the measurement of the cell heat loss rather than trying to estimate or to calculate them; the frrst step permits this by keeping the separation wall adiabatic. Eighth International Refrigeration Conference at 347 Purdue University, West Lafayette, IN, USA- July 25-28, 2000 3.3. Results The method is validated by measuring the thermal conductivity of a known material. A test is performed on RPU. Comparison between the given and measured values of the thermal conductivity is presented in the table 1. Material Usual PU Measured thermal conductivity (mW/m.K) 20 Table 1 - Compared values of thermal conductivity. Given thermal conductivity (mW/m.K) 21 Difference (%) 5 Measurements are performed on two types of VIP using different core material. The first is an open cell PU VIP and the second is using XPS. Temperature distribution on the VIP surface for both panels shows the existence of "the skin effect". On the VIP cold side, the lowest temperature is in the center and the highest one is located on the borders (figure 3 ). Figure 3- Temperature distribution on the cold side of the VIP The equivalent thermal conductivity of VIP panels and the conductivity in the center (the one measured in the center by the mean of a heat flow meter apparatus) are given in the table 2. Core material Open cell PU XPS Dimensions (mm) 450x450x20 450x450x20 Table 2 -Equivalent thermal conductivity of the tested VIP Equivalent thermal conductivity (mW/m.K) 12.7 9 Conductivity in the center (mW/m.K) 9 5.5 4. EVALUATION OF VIP INTEGRATION IN A EUROPEAN UPRIGHT FREEZER By using VIP in the insulation of a refrigerator/freezer, the energy consumption can be reduced without changing the thickness of walls. The freezer is the most advantageous model Eighth International Refrigeration Conference at 348 Purdue University, West Lafayette, IN, USA- July 25-28,2000 when using super insulation since the temperature difference is much higher than in a refrigerator. 4.1. Simulations of Different Integration Scenarios A European upright freezer has been chosen for evaluation. A dynamic simulation software ENERE:FID [7] (figure 4) is used to calculate the appliance dynamic behavior when adding VIP to the insulation. The reference is simulated and results are compared to the measured annual consumption measured and the specific cabinet heat loss (table 3). Figure 4 - ENERE[iB> output screen. Table 3- Reference simulation. Measured Calculated Difference Energy consumption (kWh/an) 535 502 6% 4.73 4.82 2% Differences are acceptable and the simulation is validated for the reference. The modification of the reference permits to estimate the VIP integration impact in one wall or more. The appliance used can integrate VIP in the door, the top and partially in the side walls because of the compressor niche. In order to maintain the mechanical rigidity of the cabinet, the VIP does not occupy all the wall thickness; the use of 60% of the thickness is a good compromise. Two walls cannot be easily modified: the back wall because of the condenser, and the bottom because of the appliance weight. Different combinations of modifications are simulated and the results are presented in table 4. Table 4- Simulation results for different scenarios o integration. Scenario G (W/m .K) COP Energy saving (%) Reference 4.82 1.08 Door only 4.5 1.1 8 Side walls only 4.25 1.1 13 Side walls and top 3.92 1.13 21 Side walls, top & door 3.68 1.15 26 Eighth International Refrigeration Conference at 349 Purdue University, West Lafayette, IN, USA- July 25-28,2000 Table 4 indicates that the energy saving potential of the VIP option is significant. This gain cannot be calculated using a steady state simulation of heat loss because of the strong dynamic behavior of the cycle coefficient of performance (COP). The mean COP of the cycle decreases with the compressor running time. The energy saving due to the heat loss reduction is amplified by the compressor running time reduction thus the mean cycle COP increases. 4.2 Demonstrator Realization And Experimental Results The predictive calculations presented promising energy savings. The realization of a demonstrator permitted to confirm the results. For this purpose, the VIP location is cut out in the walls of the reference upright freezer (figures Sa & 5b). Figure 5a -VIP location Figure 5b -Integration of VIP in a side wall Three different integration scenarios are tested using the modified demonstrator. Test results compared to the calculations are presented in table 5. Table 5 -Experimental results for three scenarios of integration. Scenario Measured energy saving(%) Calculated energy saving(%) VIP in the door 6.8% 8% Door and top 8.3% 10.3% Door, top & one side wall 14% The frrst and the second results are very close to calculations. The difference observed may be caused by the bad contact between the VIP and the residual RPU. This will not happen if the RPU is foamed with the VIP installed. The last scenario could not be simulated because the actual version of ENEREF® does not permit non symmetric side walls. Validation of the frrst two tests allows to generalize the simulation results. The energy saving calculated is technically possible. Eighth International Refrigeration Conference at 350 Purdue University, West Lafayette, IN, USA- July 25-28, 2000 4.3 Life Cycle Cost Analysis An economical evaluation of the VIP is performed in this section. The main difficulty consists in the evaluation of the over cost. Two approaches are available. For both approaches, assumptions are as follows. • Appliance lifetime: 15 years, • Discount rate: 5% and • Electricity price: 0.55 FF/kWh (which is the price in France). The first approach is proposed by AHAM [8].1t considers a material over cost of 15$/m2 and an investment over cost of 10$/unit. The life cycle cost analysis based on this approach is performed and results are given in table 6. Table 6- Life cycle cost analysis (first approach). Description Purchase price Purchase Energy Energy saving Life cycle Payback period ~rice diff. consum~tion cost FF FF kWh/an (%) FF (Year) Reference 3000 0 502 0 5866 0 Door 3136 136 462 8 5774 7,6 Side walls 3162 162 439 13 5667 5,45 Door and top 3232 232 398 21 5504 4,65 Door, top 3259 259 371 26 5377 4,06 &side walls With this approach, the payback period is acceptable and the VIP option seems to be viable. The second approach is more conservative. It is proposed by CECED to be used in the European project SAVE II. The proposition is to increase the manufacturing cost with an additional value of 15$/m2 for the use of VIP. When applying the usual margins, this over cost becomes 45$/m2 added to the purchase price. The calculations corresponding to this assumption are given in table 7. Table 7- Life l}l.cle cost analysis (second ape,roach). Description Purchase Purchase price Energy Energy saving Life cycle Payback period Qrice diff. consumption cost FF FF kWh/an (%) FF (Year) Reference 3000 0 502 0 5866 0 Door 3216 216 462 8 5853 13.83 Side walls 3302 302 439 13 5808 11.73 Door and top 3518 518 398 21 5790 12.36 Door, top 3596 596 371 26 5714 10.94 &side walls By using this assumption, the payback periods have doubled. This assumption allows to judge that the VIP option is not viable. Eighth International Refrigeration Conference at 351 Purdue University, West Lafayette, IN, USA- July 25-28, 2000 5. CONCLUSION In this paper, the new method presented to characterize thermal resistance of VIP allows to have real data to be used in predictive calculations. These data are used to evaluate VIP technology in a European freezer. The calculated energy savings are significant (about 26%) for a coverage of 4 walls. With appropriate design a higher coverage may be possible and thus more energy savings. Two opposite economical approaches are used to calculate the payback period. The two assumptions can lead to contradictory conclusions. The first approach considers the VIP option as viable but the second rejects it. The VIP technology allows to reduce energy consumption without changing the appliance geometry, which is a real advantage compared to options like increasing wall thickness. 6. ACKNOWLEDGEMENTS The results presented in this paper are extracted from a study funded by ADEME, the French Egency for Environment and Energy Management. The tested VIPs have been supplied by SAES Getters spa and DOW Chemicals. 7. REFERENCES [1] European Council Directive 95/97 of September 3 1996 concerning requirements for energy efficiency of refrigerators, freezers and combined domestic electric appliances. Official Journal n°L236, 1996, pp 36-43. [2] Dow Chemicals, technical documents, 1999. [3] Paolo Manini, The COMBOGETTER as a key component m the vacuum insulated panels technology, Vuoto, 4-5/1997. [4] Toyo Aluminium, technical documents,1999 [5] Francesco De Ponte, Sorin Klarsfeld, Thermal conductivity of insulating materials, Techniques de l'Ingenieur. 1991. [6] Standard test method for thermal performance of building assemblies by means of a guarded hot box. ASTM C236-89 (1993)e1, 1999 [7] ENEREF® a software for the simulation of energy consumption of refrigerators, freezers and combined refrigerators-freezers. Center for Energy Studies, ARMINES. [8] Joint comments of the AHAM, NRDC, ACEEE, NYEO , CEC, PGE and SCE relating to energy conservation standards for refrigerator/freezers. Docket No. EE-RM-93-801 Eighth International Refrigeration Conference at 352 Purdue University, West Lafayette, IN, USA- July 25-28,2000 

Technical and Economical Evaluation of Vacuum Insulated Panels for A European Freezer

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Technical and Economical Evaluation of Vacuum Insulated Panels for A European Freezer

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