RATIONAL ORGANIZATION OF HEAT EXCHANGE IN THE CHAMBER OF THERMOELECTRIC COOLER

Transcription

1 THERMOELECTRIC PRODUCTS RATIONAL ORGANIZATION OF HEAT EXCHANGE IN THE CHAMBER OF THERMOELECTRIC COOLER S.O.Filin (Sczecin Technical University, Poland) S.О. Filin Comparitive analysis of constructions and spatial arrangement of thermoelectric cooling units is made from the standpoint of efficiency of heat exchange between the cold sink and air in the chamber of cooler. The results of experimental study of the influence of fan operating mode in the chamber on such temperature characteristics of cooler as minimum temperature in the chamber, temperature non-uniformity along chamber height, air-sink temperature difference are presented. Introduction In recent years, as compared to general dynamics of progress in thermoelectric cooling, the manufacture of stationary chamber-type thermoelectric coolers (TEC) of volume from 15 to 200 litres has been growing at a rapid pace [1-3]. In addition to Russian and Ukrainian companies that gave a mighty impetus to this line of applied thermoelectricity, in the European market of stationary TEC leading positions are occupied by such companies as Supercool (Sweden), CampingGaz (France), AC United (Germany-Italy), Аtlant (Byelorussia). Depending on the purpose, thermoelectric coolers of the above mentioned type maintain the following temperatures in the chamber: general-purpose domestic coolers without a freezer 0 5ºС, coolers for short-term storage of food (including mini-bars) 2 6ºС, medium-temperature cooling shop-windows 4 8ºС, high-temperature cooling shop-windows, wine coolers 8 12ºС. Of great importance in the design of stationary thermoelectric coolers is the stage of choosing a means for heat removal from heat exchanger in the chamber that in thermoelectricity is commonly called cold sink. Rationalization of heat exchange in the chamber means solution of the following basic tasks: minimization of temperature difference between medium air temperature in the chamber and the surface of cold sink Т c ; provision of uniformity of air temperature field along the chamber height expressed by temperature difference between the lower and upper control point Т h = Т 1 Т 3 (Fig.1а); provision of heat sink non frosting-up or so-called tear dropping mode of its operation with a view to exclude the necessity of defrosting process. In the process of their solution a designer has the opportunity to choose from several variants of cold sink design related, in particular, to the following: cold sink shape: sheet, finned with flat fins, finned with needle fins; cold sink location: bottom, side (for example, on real wall), top (on the ceiling), at an angle; 82 Journal of Thermoelectricity 4, 2004 ISSN

2 availability of a fan (forced air convection), or its absence (natural convection); quantitative characteristics of elements used: heat exchange surface, capacity of a fan; structurization: for example, division of heat sink into sections, the use of several smaller fans instead of a large one, the use of two-axial fan for simultaneous servicing the hot and cold side of thermopile, etc. For all the evidence of qualitative effect of selected design on TEC parameters, the literature practically lacks experimental data on the quantitative relationships between air convection intensity and cooler characteristics. At the same time, reported design analysis of heat exchange effect on TEC characteristics [4] is difficult to be directly applied to contemporary models of coolers due to fast progress in structural elements of TEC and change in their design principles. The purpose of research described in this paper is partial compensation of this gap and analytical generalization of the author s and literary experimental data accumulated in the course of creating thermoelectric coolers of different volume. 1. Arrangement of TEC unit In TEC chambers with a volume up to l forced convection inside the chamber is rarely used so far, with the exception of coolers-thermostats where high precision of temperature maintenance and its regularity along the volume are required. Typical schemes of unit arrangement in TEC case are shown in Fig.1. With equal height of cabinet -type TEC, the highest intensity of natural convection is observed with the unit arrangement in the top wall (Fig.1b), and this scheme is matched with the lowest air temperature gradient along the chamber height. Considerable distortions in the air circulation schemes in the chamber can be introduced by the availability of shelves (especially continuous, for example, of acrylic plastic) and the way of products arrangement. 1 2 T 1 h/ h T 2 T 3 h/4 h/4 T 1 T 2 T 3 T 4 h/4 6 a) b) c) d) Fig.1. Typical schemes of units arrangement in TEC housing illustrating air circulation loops with natural convection and thermocouple locations: а) lateral location, b) horizontal top location, с) top location at an angle, d) bottom location; 1 hot side fan, 2 hot sink, 3 module, 4 transition element, 5 cold sink, 6 cold sheet heat exchanger. In bin -type TEC with bottom unit arrangement (Fig. 1d) the task of «stretching» cold along the chamber height is usually performed by П-shaped (3 walls) or five-walled heat exchanger of aluminum sheet of thickness 1-2 mm. Despite this structural peculiarity, temperature non-uniformity along the chamber height is considerably higher than with the top arrangement of cooling unit and can ISSN Journal of Thermoelectricity 4,

3 reach 7-8ºС. However, for portable thermoelectric coolers other advantages of the bottom arrangement dominate its disadvantages. Another distinctive feature of this TEC type is that food products laid in the bottom of the cooler and adjacent to metal walls have direct contact to cooled surface. In so doing, the share of food products directly cooled is from 30 to 70 % of the entire mass, therefore, test method of bin -type TEC in the determination of average temperature in the chamber utilizes four thermocouples, instead of three, as required by attachment to standard. The fourth point of temperature measurement is in the geometrical centre of the chamber bottom in contact with the surface, as shown in Fig.1d. In case free convection is used in TEC with large volume of chambers (starting approximately from 80 liters), one has to take into account the necessity of respective increase of heat exchange surface with mandatory distribution of unit sections on several walls, which is often additionally caused by heat exchange conditions on the hot side as well. Example is provided by commercial balcony thermostat ТТ-200, whose unit is divided into 12 sections and arranged on the three side walls of the product [6]. Fig.2. Fig.3. In portable TEC by a series of European manufacturers a tendency has been to use two-axial fans providing air circulation on both unit sides. It applies to TEC constructions with the unit location in the lid. With certain deterioration of heat exchange on the hot side, this design allows minimizing the surface and mass of cold sink. It is important not only for the cooler in general, but also for convenience of using the lid that must be lightweight. Example can be provided by a standard series of coolers by English company Gio`Style (Fig. 2). A similar unit design is used in the Icetime TE36 model of cooler by French company Camping Gas (Fig.3). This cooler can be installed and used in two positions: both as a bin and a cabinet. Depending on the location of TEC, the unit is in the lid or in the door. It is evident that location of the unit in the door is only possible under forced air convection on the cold side, the air outlet being directed to the door hanging axis. At the same time, for stationary TEC the task of reducing mass and price is not so relevant, owing to which for this class of coolers it is preferable to use schemes with two and more separate fans on both sides of cooling unit. 2. Temperature diagrams of TEC for various operating conditions As early as during pre-evaluation of required cooling capacity of the unit, in addition to basic TEC characteristics, i.e. the value of temperature difference created by the cooler, it is necessary to 84 Journal of Thermoelectricity 4, 2004 ISSN

4 have information on two other temperature differences: between the cold sink and average temperature in the chamber Т cold and between the hot sink and ambient air Т hot. The recommended values Т cold and Т hot are represented in Table 1. Reduction of these values against those indicated above, though providing energy gain in power consumption, is related to excessive increase of heat exchange surface, hence, results in the degradation of mass-dimensional and cost parameters of TEC. Recommended values of Т cold and Т hot according to [6]. Heat exchange type Т cold Т hot Natural air convection (without a fan) Forced air convection (with a fan) Cooling with running water Table 1. T, C o T hot hot 1 hot 2 hot 3 T hot T hot outside T mod T ref T mod T ref T mod T ref T cold cold 1 T cold cold 2 T cold a) b) c) inside cold 3 Fig.4. Examples of temperature distribution in thermoelectric cooler at constant ambient temperature of 25ºС: T mod temperature difference on thermoelectric module junctions, T ref temperature difference created by cooler, T cold temperature difference on the cold side (air in the chamber heat sink), T hot temperature difference on the hot side (heat sink ambient air); а) natural convection in the chamber, b), с) forced convection in the chamber; а), b) at temperature stabilization in the chamber at the level of +5ºС, а), с) at stabilization of supply voltage for thermoelectric unit. With correctly designed cold sink calculated for work under free convection, the value of Т cold is 8 10ºС. A reduction of this value by 5 С, as is shown in Fig. 4, i.e. transition from variant а) to variant b) with the use of forced convection in TEC chamber, allows almost the same reduction of temperature difference on the junctions of thermoelectric module. Hereby the same average temperature in the chamber is provided with a lower unit supply voltage and, hence, lower power consumption. For a cooler of volume 14 liters these figures make, accordingly, 15V and 12V, 40 и 26 W of power requirement, i.e. power consumption of the cooler is reduced by more than 30%. In constructions of TEC with a parallel schemes of power supply to the hot side fan and module (group of modules), simultaneously with reduction of Т cold, a slight increase in Т hot can be observed, generally not exceeding 1 С. For the represented three variants of TEC operating conditions under comparison the inequality Т hot2 > Т hot3 > Т hot1 holds true. ISSN Journal of Thermoelectricity 4,

5 3. Brief description of the object and method of study Of practical interest is to compare experimental temperature characteristics of the same sample of TEC with and without a fan on the cold side, obtained under various operating conditions, as well as to determine experimental dependences of these characteristics on the capacity of a fan. With this aim the necessary investigations were performed on the experimental sample of thermoelectric cooler of volume 13.9 l for cooling milk products. The cooler unit comprised two thermoelectric modules of МТ type, heat sinks made of commercial aluminum profiles, and axial fans with rated DC supply voltage 12 V. Arranged on the hot and cold unit sides were fans with dimensions mm and mm, respectively. The unit was mounted on the side wall of the chamber, so that the distance from the upper end face of cold sink to the top wall be 10 mm, and its fins be vertically oriented. The heat sink surface was calculated for work under natural convection conditions and made m V - M 2 R1 R2 M V - Fig.5. Scheme of electric power supply to TEC unit used in testing. In the course of each series of experimental investigations the temperature in the room under study was kept by air conditioning system at the constant level of 22ºC to an accuracy of +0.5 С. The temperature at control points was controlled by chromel-copel thermocouples connected to digital device ЦР7702. Electric power supply to the unit and fan on the cold side was realized by means of two independent direct current sources with controlled output (Fig.5). The tests were performed with empty cooler chamber. Experiments under natural convection were performed both with disconnected fan and in its absence. In the course of experiments under forced convection, the fan capacity control was realized by change in its supply voltage U v in the range of 0 to 10 V. According to the analysis, to expand the above range of change in supply voltages is inexpedient because of the absence of essential positive results. Minimum possible capacity of a fan was matched with its supply voltage 4.0 V, below which there was a stop in rotation of fan blades. At the same time, guaranteed start of a fan called for a voltage of 5.0 V. Preliminary studies were made of 6 fan models by various manufacturers with about equal rated capacity distinguished by a lower threshold of rotation voltage within 4-6 Volt. Selected fan type KDE1206PHS2 by Sunon company was characterized by minimum value of threshold voltage and minimum power requirement. Measurement of aerodynamic characteristics, including air velocity, was not performed. For simplification, a relation between fan supply voltage U v and air velocity V over the entire range of U v change can be represented as follows: 2/3 V = A U V, were А is empirical coefficient depending on structural parameters of the sink and fan and their mutual arrangement. 86 Journal of Thermoelectricity 4, 2004 ISSN

6 4. Investigation results Investigation results are represented in Fig. 6 and 7. Experiments were performed with three values of unit supply voltage U a 9, 12 and 15 V. Qualitative character of all dependences was practically independent of this voltage. Thus, for example, at U a = 12V the use of forced convection resulted in reduction of average temperature in chamber from 8.03ºС to 4.77ºС. Respectively, Т cold was reduced from 10.7ºС to 3.7ºС. T, o C U a =9 V V 12 V 15 V 2 12 V 2 15 V U V, V Fig.6. Dependences of average temperature Т inside in the cooler chamber (1) and temperature difference Т cold chamber-heat sink (2) on fan supply voltage in the chamber for various values of thermoelectric unit supply voltage (9V, 12V, 15V); the value U v = 0 corresponds to conditions of natural air convection in the chamber. One can clearly see optimal value of fan supply voltage, hence its optimal capacity when minimum average temperature Т inside is reached in TEC chamber. For U a =9V and U a =15V the voltage U v made 7, and for U a = 12V U v = 6V. This small divergence is attributable to error of measurement. In terms of thermal physics, the presence of extreme function Т inside (U w ) can be interpreted as follows. Despite the fact that with increase in U v the temperature difference Т cold is monotonously decreasing, which is a positive factor, the rate of its decrease is falling. With further increase in U v by the value of Т inside, three other negative factors start to have a decisive effect, namely: increase in heat inflow through chamber insulation, due to growth of air motion velocity inside and air-chamber walls heat transfer coefficient; increase in thermal power released by the fan; increase in temperature difference Т hot on the unit hot side, which is related to temperature dependence of module electrical resistance. Combined effect of the above three factors leads to growth of average temperature in the chamber. ISSN Journal of Thermoelectricity 4,

7 Transition from natural to forced convection reduces temperature non-uniformity in the chamber more than 3-fold: from 3-5ºС to ºС. At the same time, with so small chamber volume the relationship Т h (U v ) is poorly expressed. The increase in U v from 4V to 10V gives as low as 0.2ºC reduction of Т h. The additional advantage of option with forced convection is the absence of sink frosting-up with temperature maintenance in the chamber at the level of +4 +6ºС. With natural convection, the same result can be achieved by increasing the surface of heat exchange by a factor of about 4, which eventually proves to be more expensive than the use of a fan. T, o C 4 U a =15 V V 12 V 1 U V, V Fig.7. Dependences of temperature non-uniformity Т h along the cooler height on fan supply voltage in the chamber for various values of thermoelectric unit supply voltage (9V, 12V, 15V); the value U v = 0 corresponds to conditions of natural air convection in the chamber. Conclusions 1. The use of forced air convection in TEC chamber gives essential power advantages, expressed in the reduction of average temperature in the chamber by 3-5ºС (at constant unit supply voltage) or in the reduction by 20-35% of unit power consumption (at constant temperature in the chamber). 2. Transition from forced convection through fan installation on the cold heat exchanger is attended by minimum reduction of TEC cost. The wholesale price of axial fan is from $2 to $5 depending on the manufacturer, purchase amount, type of bearings used (roller or ball), noise level, operating life and the availability of electron protection from reverse switching and current overloads. There is also a minimum increase in the unit power consumption (for the variant of supply voltage stabilization). As long as at 6V the fan does not consume more than 50 ma of current, its power consumption is less than 1% of the unit power. 3. There is optimal value of fan supply voltage, when minimum temperature in TEC chamber is achieved. 4. It is expedient to supply the fan on the cold side of TEC with a less than rated voltage. Extrapolating the results obtained in this paper to the range of TEC chamber volumes from 10 to 100 liters, it can be asserted that the range of recommended voltages is from 0.5 to 0.7 of the rated one, the lower bound of this range corresponding to lower chamber volumes. To realize in practice this 88 Journal of Thermoelectricity 4, 2004 ISSN

8 recommendation without the use of independent fan supply in the chamber, it is necessary to choose the type and quantity of modules used in the unit so as to provide the possibility of parallel fan connection to one of the modules. An example of such a connection is shown in Fig. 8а. M 2 M 2 M 3 R1 R2 R1 R2 R3 R4 M 1 M 1 M V V - a) b) Fig.8. Recommended schemes of electric power supply to one-sectional (a) and two-sectional (b) thermoelectric TEC units with the fan on the cold side. 5. When using a multi-sectional heat sink on the cold side of TEC unit, it is advisable to install a separate fan for each section (Fig.8b). The same recommendation concerns the hot side as well. References 1. Filin S.O. Application areas and efficiency analysis of stationary thermoelectric coolers. //Kholodilny Biznes.-2003, 10.- P Piskunov V.V. Domestic coolers market in //Kholodilny Biznes , 9. P Piskunov V.V. Transport refrigerators. Mini-coolers market. //Kholodilny Biznes , 6.- P Naer V.A., Garachuk V.K. Theoretical fundamentals of thermoelectric cooling.- Odessa: Odessa Polytechnical Institute Publ., p. 5. Inter-state standard ГОСТ Domestic electric cooling devices. General technical specifications. 6. Filin S. Termoelektryczne urządzenia chłodnicze. Masta, Gdańsk, с. Submitted ISSN Journal of Thermoelectricity 4,