Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

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1 Develoment of a thermoelectric refrigerator with two-hase thermosyhons and caillary lift J.G. Vián, D. Astrain To cite this version: J.G. Vián, D. Astrain. Develoment of a thermoelectric refrigerator with two-hase thermosyhons and caillary lift. Alied Thermal Engineering, Elsevier, 2009, 29 (10), < /j.althermaleng >. <hal > HAL Id: hal htts://hal.archives-ouvertes.fr/hal Submitted on 13 May 2011 HAL is a multi-discilinary oen access archive for the deosit and dissemination of scientific research documents, whether they are ublished or not. The documents may come from teaching and research institutions in France or abroad, or from ublic or rivate research centers. L archive ouverte luridiscilinaire HAL, est destinée au déôt et à la diffusion de documents scientifiques de niveau recherche, ubliés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires ublics ou rivés.

2 Acceted Manuscrit Develoment of a thermoelectric refrigerator with two-hase thermosyhons and caillary lift J.G. Vián, D. Astrain PII: S (08) DOI: /j.althermaleng Reference: ATE 2625 To aear in: Alied Thermal Engineering Received Date: 20 November 2006 Revised Date: 10 Setember 2008 Acceted Date: 21 Setember 2008 Please cite this article as: J.G. Vián, D. Astrain, Develoment of a thermoelectric refrigerator with two-hase thermosyhons and caillary lift, Alied Thermal Engineering (2008), doi: /j.althermaleng This is a PDF file of an unedited manuscrit that has been acceted for ublication. As a service to our customers we are roviding this early version of the manuscrit. The manuscrit will undergo coyediting, tyesetting, and review of the resulting roof before it is ublished in its final form. Please note that during the roduction rocess errors may be discovered which could affect the content, and all legal disclaimers that aly to the journal ertain.

3 DEVELOPMENT OF A THERMOELECTRIC REFRIGERATOR WITH TWO-PHASE THERMOSYPHONS AND CAPILLARY LIFT. J.G. Vián, D. Astrain Deartment of Mechanical, Energy and Materials Engineering Public University of Navarre, UPNA. Pamlona SPAIN. Tel: , Fax: , vian@unavarra.es Abstract A thermoelectric domestic refrigerator has been develoed, with a single comartment of 0.225m 3, for food reservation at 5ºC. The cooling system is made u of two equal thermoelectric devices, each comosed of a Peltier module (50W) with its hot side in contact with a two-hase and natural convection thermosyhon (TSV) and a two-hase and caillary lift thermosyhon (TPM), in contact with the cold side. The entire refrigerator has been simulated and designed using a comutational model, based on the finite difference method. Subsequently an exerimental otimization hase of the thermosyhons was carried out, untilthermal resistance values of R TSV =0.256K/W and R TPM =0.323K/W were obtained. These values were lower than those obtained with finned heat sinks. Finally, a functional rototye of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal dro (Ambient Temerature Inside Temerature) of 19ºC. The electric ower consumtion at nominal conditions was 45W, reaching a COP value of The study demonstrated that by incororating these two-hase devices into thermoelectric refrigeration increases the COP by 66%, comared with those which use finned heat sinks. Keywords: Thermoelectricity, simulation, refrigeration, Peltier, two-hase, caillarity 1. INTRODUCTION In all refrigeration systems, heat exchange lays a fundamental role in the COP value of the devices. In vaour comression and absortion systems hase changing fluids are used, taking advantage of the good heat transfer coefficients roduced, not only during the condensation but also during the evaoration. Thermoelectric refrigerators are comosed of the so-called Peltier modules, whose most notable hysical henomena are: Seebeck, Peltier, Joule, Thomson and Fourier effects [1]. Aluminium heat sinks, with or without fins and with one or several fans for moving the air, are used for heat transfer, both in the cold and hot sides of the Peltier module. This is shown in alications such as those resented in references [2] and [3]. There are also heat 1

4 exchanges with a fluid inside (water or water with ethylene glycol). In both cases there is no two-hase, and the efficiency in the heat transfer is oor, achieving very high thermal resistances, even when secific otimizations have been carried out, such as those resented in references [4] and [5]. This is mainly due to the roblem caused by the high heat flow roduced in the hot and cold sides of the Peltier module (u to W/m 2 ). Consequently, the thermal resistances roduced in thermoelectric refrigeration are very high, and this causes much lower COP values than the ones obtained in vaour comression refrigeration. The influence of heat dissiation on the COP of Peltier modules was shown in reference [6]. In this regard, a two-hase thermosyhon system (TSF), with incororated fan, for the hot side of the Peltier module was develoed, as shown in reference [7]. It imroved the thermal resistance by 36% in comarison with a finned heat sink. Also another thermosyhon with two-hase and caillary lift (TPM) for the cold side of the Peltier module has been develoed, imroving its thermal resistance by 37% in comarison with a finned heat sink, as shown in reference [8]. Hongxia has conducted a review [9] of thermoelectric refrigeration devices, where Riffat et al s work is worth noting [10], in which a heat ie is used in the hot side and a PCM thermosyhon in the cold side of the Peltier module. The imrovement of these devices in order to transfer heat from the Peltier module, and its alication to a domestic refrigerator, is the rincial aim of this work. A two-hase thermosyhon, without moving arts, has been designed to imrove the heat transfer from the hot side of the Peltier module, as well as an original assembly system, which allows for easier assemblies and avoids the aearance of thermal bridges which are roduced in the screw joint of finned heat sinks. 2

5 2. OBJECTIVES The overall aim of this work is to design a domestic refrigerator, without moving arts, whose cooling system is based on thermoelectric technology and two-hase thermosyhons. In order to achieve this overall aim the following secific objectives have been ut forward: : - Numerical simulation of the entire thermoelectric domestic refrigerator. - Alication of the thermosyhon with two-hase and caillary lift (TPM) design, on the cold side of a thermoelectric module. - Design and otimization of a thermosyhon, with two-hase and without fan (TSV) for the hot side of a Peltier module. - Design of an assembly system which will break thermal bridges and make assembly easier. - Design and construction of a thermoelectric domestic refrigerator rototye, which incororates TPM systems, Peltier modules, TSV devices and assembly system. Exerimental analysis of the imrovement obtained in its COP. 3. THERMOELECTIC REFRIGERATOR DESCRIPTION This is a domestic refrigerator with a single refrigerator comartment of m 3 whose cold roduction system uses thermoelectric modules couled to thermosyhons (TPM and TSV) without moving arts. The refrigerator is secifically comosed of two identical thermoelectric devices, as shown in Figure 1, which consists of: - A 40*40mm and 50W maximum ower Peltier module, Marlow 6L model. - A thermosyhon, with two-hase and caillary lift (TPM), with the cold extender incororated, for the cold side of the Peltier module. This device was 3

6 resented in [8]. - A thermosyhon, with two-hase without moving arts (TSV), to dissiate the heat from the hot side of the Peltier module. TSV TSV Assembly system Peltier module TPM TPM Figure 1. Thermoelectric device comosed of: Peltier module, TPM, assembly system and TSV. 4

7 TPM Assembly system TSV Figure 2. Photograhs of the thermoelectric domestic refrigerator rototye with its thermosyhons. For the construction of this thermoelectric refrigerator, we started with a vaour comression commercial domestic refrigerator from Bosch-Siemens, BOSCH KGP39. The entire cooling system (evaorator, comressor, condenser and valves) was removed, leaving only the aliance casing. Two hotograhs, of the thermoelectric domestic refrigerator rototye, are shown in Figure 2, where the TPM, the TSV and the assembly art incororated in the refrigerator furniture can be seen Two-hase thermosyhon without fan (TSV) The oerating rincile of the TSV is based on a steel tank, whose volume is a right angle rism, with a fluid inside (see Fig. 1). The Peltier module is laced on the outside art of this tank s back wall. The hot side of the Peltier module, which is in contact with 5

8 this wall, transfers the thermal ower roduced by the module, to the tank. The heat dissiated is thus transmitted to the inner fluid, which leads to its boiling. The vaour which is generated by the boiling rises by natural convection towards the to of the thermosyhon where it comes into contact with a coil comosed of steel tubes, through which the vaour flows. When the vaour makes contact with this coil (which is located on the outside of the refrigerator), it condenses and returns by way of gravity in a liquid state to the bottom, returning once again to the tank and forming, in this way, a closed and self-owered cycle. The TSV uses the secific latent heat of two-hase, in the boiling rocess as well as during condensation, in order to dissiate the heat efficiently, enabling it to sread from a small hot surface (40*40mm of the Peltier module) to a large one (the entire coil of the thermosyhon). This effect is shown in the thermograhs of Figure 3, taken from both the TSV and a finned heat sink without thermosyhon. These thermograhs demonstrate how the TSV device sreads the heat flow over the entire area, while in the finned heat sink the heat flow is concentrated in the area near the Peltier module. TSV Finned heat sink 31.1 C C 32 SP C C Peltier module Figure 3. Comarison of TSV and finned heat sink thermograhs, with a 50W Peltier module 6

9 The main contribution of this TSV device, as regards its redecessor TSF, which has been resented in [7], is the fact that the change of lane geometry for tubular geometry has allowed us: on one hand, to use ammonia as refrigerant, given that this geometry withstands higher ressures, and, on the other hand and more imortantly, the fluid condensation area has been greatly increased. This enables it to be oerated without an exterior fan. This makes the system cheaer and quieter, by removing all the moving arts in a domestic refrigerator Assembly system. The assembly system, which allows us to join the heat exchanger of the cold side (TPM) with the Peltier module and the heat exchanger of the hot side (TSV) in the thermoelectric domestic refrigerator rototye, is based on an injected lastic art, which lodges the Peltier module. This lastic art, which contains the three elements mentioned above, is incororated in the foam of the rototye furniture. This way, the thermosyhons and the Peltier module can be ut in and removed easily, as shown in Figure 1. The designed lastic art not only fulfils the assembly function, but it also breaks the thermal bridge roduced in the current assembly systems, because in these systems, the hot side of the heat sink is joined to the cold side of the dissiater by screws. 4. METHODOLOGY A numerical simulation was made with a comutational model, which was develoed in [11], for the urose of determining the number of Peltier modules required for this alication. This model simulates the entire refrigerator, that is, the aliance casing, Peltier modules and its heat sinks, as well as the thermal bridges roduced by conventional assembly systems with screws. The model rovides us with information about the total 7

10 electric ower consumtion of the thermoelectric refrigerator and, thus, of the exected COP also by incororating the above-noted thermosyhons instead of the conventional finned heat sinks Methodology for the exerimental analysis of the hot side thermosyhons A series of tests were carried out for the exerimental study of the TSV, whose rimary objective was to learn about its thermal resistance, based on the following exression: ( Th T ) R = (1) Q amb h To obtain these thermal resistance values, it is necessary to know both the temerature of the hot side of the Peltier module ( T ) and the ambient temerature (T h amb ), as well as the thermal ower dissiated by the hot side of the Peltier module ( Q ). Thermocoules were laced to measure these temeratures. However, the recise measurement of the heat flow resents greater technical difficulties. Its calculation is based on the first law of thermodynamics regarding energy and steady state, alied to the Peltier module: Q = Q + W (2) h c e h Insulating the cold side of the thermoelectric module, Q = 0 is achieved, and so the exression (2) becomes: h e c Q = W (3) Where: e = V I (4) W. whose magnitudes are easily measured. 8

11 4.2. Methodology for the exerimental analysis of the thermoelectric refrigerator rototye The main objective of the thermoelectric refrigerator study was to determine the maximum thermal dro the refrigerator was able to achieve, the electric ower consumtion and the COP, in different oerating conditions. To this end, the rototye was ut into a standard climatic chamber, in order to control the ambient conditions. Both the thermal dro and the electric ower consumtion can be measured directly, however several calculations are necessary to know the COP. The COP calculation for this refrigerating device was based on the following exression: Q COP = (5) W c e To determine the refrigeration ower of the thermoelectric module, the heat flow entering the enclosure is calculated, since in a steady state it is equal to the refrigeration ower. That is:. Q c = S. U.( T ) (6) amb T int Where: U = 1 1 e h k int h ext (7) Convection coefficients are calculated by using the exression suggested by Parmelee et al. in [12] for a flat late, disregarding the viscous heating and considering laminar flow, given that the air seeds are small: Nu L = Pr.Re 1/ 3 1/ 2 L 0. 6 Pr 50 5 Re < Re x, c 5 10 (8) In the event that a fan is not used inside, i.e. natural convection, the exression suggested by Churchill et al. in [13] is used to calculate the convection coefficient: 9

12 Nu L = Ra 1/ 4 L Pr 9 [ 0 < Ra < 10 ] L 9 /16 4 / 9 (9) 5. RESULTS AND DISCUSSION An exerimental study has been carried out with the TSV which was built, comaring it with its redecessor TFS, whose geometry was flat instead of tubular. The study was carried based on tests erformed with both thermosyhons, in natural convection as well as in forced convection, using a fan and a wind tunnel. The results are shown in Figure 4, where it can be observed that both thermosyhons give similar results in forced convection, while in natural convection the thermal resistance of the TSV is much lower than that of the TSF. In addition, the thermal resistance value of the TSV in natural convection is good enough for it to be used in the thermoelectric refrigerator, since this avoids the use of two fans. 0,111 0,121 TSF TSV 0,267 0,570 Figure 4. Comarative exerimental study of both thermosyhons, in natural and forced convection 10

13 The grah, in Figure 4, shows a timeline of the temeratures obtained in a test with the final rototye thermoelectric refrigerator. These tests were carried out at different suly voltages of thermoelectric modules. As exected, the higher the voltage, the greater the thermal dro obtained. It is worth noting that the temerature inside the refrigerator remains constant over time, at a given voltage. This hels considerably in reserving food, unlike vaour comression cooling systems, which follow start-sto comressor cycles that reach temerature variations of u to 8ºC. This is one of the imortant advantages of the thermoelectric refrigerator, since it is ossible to kee the temerature constant just by varying the voltage. The difference between the two interior temeratures, reresented in the grah, is due to the stratification caused by the absence of a fan inside, since one of the sensors is laced at the to and the other at the bottom. 25 T amb 20 Temerature [ºC] Inc.T=18ºC Inc.T=17ºC Inc.T=16ºC Inc.T=15ºC T int 1 T int 2 5 7V 7 V 8V 8 V 9V 9 V 10,5 V 10.5V Time [min] Tamb [ºC] Tint1 [ºC] Tint2 [ºC] Figure 5. Temerature timeline in the thermoelectric refrigerator rototye, for different voltage ranges alied on the Peltier modules 11

14 In Figure 6 can be seen the starting u of the refrigerator without load, for a room temerature of 22ºC after the rototye door was ket oen so the inner temerature was the same as the room temerature. It can be noted that after 300 minutes the refrigerator reaches an inner temerature of 6ºC T amb Temerature (ºC) T int 1 T int 2 ambient T int 1 T int Time (min) Figure 6. Exerimental results of the starting u rocess of the thermoelectric refrigerator Table 1 shows the calculated values for steady state. It can be seen that the higher the thermal dro between the inside and the ambient temerature, the higher, understandably, the electric ower consumtion of the refrigerator. The maximum thermal dro reached is 19ºC, so, if the room temerature is higher than 25ºC, the inner temerature will not reach the desired values for good reservation (as an examle, if the room temerature is 30ºC, the inner temerature would be around 11ºC) More interesting is the fact that the COP significantly increases when the Peltier modules sulied voltage decreases. From these values it can be deduced that for nominal working conditions at an ambient temerature of 12

15 22 C and 6ºC inside the refrigerator, the total electric ower consumtion of the refrigerator is 1.08kWh/day (44.9W), and the COP This COP value achieved in the thermoelectric refrigerator with the incororation of the thermosyhons (TSV and TPM), means an increase of 66%, when comared to the value achieved in a thermoelectric refrigerator with finned heat sinks and fan, for the same ambient conditions, which value, COP=0.27, was obtained exerimentally in reference [7]. Another advantage is that with the TSV the fans have been comletely removed, so it results in a comletely noise-free domestic refrigerator. However, the electric ower consumtion of the thermoelectric refrigerator (1.08kWh/day) is still much greater than the vaour comression cooling systems consumtions. The same domestic refrigerator model with vaour comression cooling system and rated A needs 0.5kWh/day. Table 1. Calculated values of the tests with the thermoelectric domestic refrigerator rototye, for steady state V (V) T amb (ºC) T int (ºC) T amb T int (ºC) Q (W) c W (W) e COP 12 25,9 7 18,9 24,7 106,1 0,233 10,5 20,6 3 17,6 23,1 77,4 0, ,8 3,9 16,9 22,2 56,8 0, ,8 4,9 15,9 20,9 44,9 0, ,9 6 14,9 19,7 34,5 0, CONCLUSIONS A thermosyhon with two-hase, to dissiate the heat from the hot side of the Peltier module, has been develoed with a thermal resistance of 0.256K/W and without fan. A thermoelectric domestic refrigerator with an inner volume of m 3 has been designed and built. It is comosed of two Peltier modules (Marlow 6L), two TSV s and two TPM s, which we have develoed. The two TPM s are resented in reference [8]. Its main exerimental results can be summarized as follows: 13

16 - Maximum thermal dro: 19ºC - Zero noise (due to the absence of external fans), comletely silent - Electric ower consumtion in nominal conditions (6ºC inside and 22ºC outside): 1.08kWh/day - COP value in nominal conditions: 0.45 It has been exerimentally demonstrated that, by including the develoed thermosyhons (TSV and TPM), there is an increase of 66% in the COP of the thermoelectric refrigerators. This refrigerator reresents an imortant ste in thermoelectric refrigerators due to the develoment of the two thermosyhons and their assembly. However, using thermoelectricity in domestic refrigeration is far from reaching the values of vaour comression cooling systems. The same domestic refrigerator with a vaour comression cooling system and rated A needs 0.5kWh/day, which reresents half the consumtion of the thermoelectric refrigerator in the same conditions. As a result of this research carried out by the research grou of the Public University of Navarre, and financed by the multinational Bosch-Siemens, two atents [14] and [15] have been obtained. REFERENCES [1] D.M. Rowe, CRC Handbook of Thermoelectrics, ISBN , 19-25, ; 1995 [2] N.M. Khattab, and E.T. El Shenawy. Otimal oeration of thermoelectric cooler driven by solar thermoelectric generator. Energy Conversion and Management 47(4), (2006), [3] G. Min, and D.M. Rowe. Exerimental evaluation of rototye thermoelectric domestic-refrigerators. Alied Energy 83 (2) (2006),

17 [4] D. Astrain, J.G.Vián. Study and otimization of the Heat Dissiater of a Thermoelectric Refrigerator. Journal of Enhanced Heat Transfer 12 (2) (2005), [5] R.W. Knight, J.S. Gooding and D.J. Hall, Otimal thermal design of forced convection heat sinks-analytical, J Electron Packaging 113 (1991), [6] M.J. Nagy, R.J. Buist. Effect of Heat Sink design on thermoelectric cooling erformance. Proceeding of the XIII International Conference on Thermoelectrics. Kansas City, Missouri, USA (1994) [7] D. Astrain, J.G. Vián, M. Dominguez. Increase of COP in the thermoelectric refrigeration by the otimisation of heat dissiation, Alied Thermal Engineering. 23 (17) (2003), [8]. J.G. Vián, D. Astrain. Develoment of a heat exchanger for the cold side of a thermoelectric module. Alied Thermal Engineering Volume 28, Issues 11-12, August 2008, Pages [9] Xi, Hongxia, Luo, Lingai. Develoment and alications of solar-based thermoelectric technologies. Renewable and Sustainable Energy Reviews, v 11, n 5, June, 2007, [10] S.A. Omer, S.B. Riffat and X. Ma, Exerimental investigation of a thermoelectric refrigeration system emloying a hase change material integrated with thermal diode (Thermosyhons), Renewable Energy 21 (2001), [11] D. Astrain, J.G. Vián, J. Albizua. Comutational model for refrigerators based on Peltier effect alication. Alied Thermal Engineering. 25 (2005), [12] G.V. Parmelee and R. G. Huebscher. Heat Transfer by Forced Convection Along a Smooth Flat Surface, Heat Piing Air Cond. 19 (8) (1974), [13] S. W. Churchill and H. S. Chu. Correlating Equations for Laminar and Turbulent Free Convection, Int. J. Heat Mass Transfer, 18 (1975), [14] J.G. Vián, D. Astrain, J.M. Lamuela, S. García. Kältegerät und Peltier- Kühlvorrichtung dafür, 2006,, Germany. INTERNATIONAL: F25B21/02; F25B21/02. EUROPEAN: F25B21/02 [15] J.G. Vián, D. Astrain, J.M. Lamuela, S. García. Thermosyhon, 2006,, Germany. INTERNATIONAL: 15

18 EUROPEAN:! NOMENCLATURE COP Coefficient of erformance e Thickness M h int Interior convective heat transfer coefficient W/m 2 K h ext Exterior convective heat transfer coefficient W/m 2 K I Peltier electric current A k Thermal conductivity W/mK Nu Pr Nusselt number Prandtl number Q h Peltier module hot side heating ower W Q c Absorbed by the Peltier module cold side heating ower W Ra Rayleigh number R Thermal resistance K/W Re Reynolds number S Surface M 2 T Absolute temerature K T amb Ambient temerature K T h Peltier hot side temerature K T int Inner temerature K TPM TSF TSV Thermosyohon with two-hase and caillary lift Thermosyohon with two-hase in forced convection (fan) Thermosyohon with two-hase in natural convection (without fan) U Heat transfer global coefficient W/m 2 K V Peltier sulied voltage V W Electric ower consumtion of the Peltier module W e 16