applied sciences Article
Methodological Considerations of Using Thermoelectrics with Fin Heat Sinks for Cooling Applications Kok Seng Ong, Choon Foong Tan and Koon Chun Lai * Faculty of Engineering and Green Technology, UniversitiTunku Abdul Rahman, Kampar 31900, Malaysia;
[email protected] (K.S.O.);
[email protected] (C.F.T.) * Correspondence:
[email protected]; Tel.: +60-5-468-8888 Academic Editor: Michel De Paepe Received: 25 September 2016; Accepted: 4 January 2017; Published: 7 February 2017
Abstract: An experimental investigation was conducted to evaluate the performance of a thermoelectric (TE) module fitted to a conventional fin heat sink with a similar sized heat source. Experiments were conducted with power inputs at 10 W and 20 W under natural convection (NC) and forced convection (FC) air cooling. The results showed that the use of the TE module is not effective under the NC cooling mode. With the present TE module employed, under FC cooling at 20 W, the applied voltage (Vte ) to the TE module should be >4 V and at 10 W, it should be >1 V. A simple iterative method of predicting the hot side temperature of the TE module was presented. Agreement between predicted and experimental values was better than 2%. Keywords: thermoelectric cooling; fin heat sink; thermal resistance; natural convection; forced convection
1. Introduction A thermoelectric (TE) module consists of a number of thermocouples connected electrically in series and thermally in parallel. TEs are used with fin heat sinks (FHS) to dissipate the heat to the atmosphere. During operation, heat is generated within the thermoelectric cooler (TEC) device due to the Joule heating effect. As a consequence more heat is transferred to the heat sink than absorbed from the heat source. Typical thermal cooling with an air-cooled FHS is shown in Figure 1a and a water-cooled plate heat sink in Figure 1b. Natural convection (NC) or forced convection (FC) by air could be employed. A typical FHS consists of a flat metal base with an array of cooling fins on top with aluminum or copper as the most common metals used. Thermal contact resistances occur at the interfacing metal contacts. In addition, thermal heat spreading resistance exists whenever there is a difference between the heat sink and heat source surface areas. As semiconductor chips become more powerful and compact, they require greater heat dissipation. Hence, more effective thermal cooling systems have to be devised. A thermal cooling device consisting of an air-cooled FHS coupled to a TE module is shown in Figure 2. Thermal heat spreading has been investigated by various researchers. Lee et al. [1] investigated the spreading resistances of various flat rectangular plates with various surface heat transfer coefficients. They compared their simulations with other known existing theories and found that for relatively thick plates, the constriction thermal resistance is insensitive to plate thickness and Biot number. It was solely dependent upon the relative contact size between the heat sink and heat source. Simons [2] simulated thermal heat spreading resistance using exact partial differential heat transfer equations and compared them with those obtained by Lee et al. [1] using aluminum and copper flat plate heat spreaders and under forced convection air cooling. Hasan et al. [3], on the other hand, applied the conductive thermal
Appl. Sci. 2017, 7, 62; doi:10.3390/app7020062
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and found that for relatively thick plates, the constriction thermal resistance is insensitive to plate thickness and Biot number. It was solely dependent upon the relative contact size between plate thickness and Biot number. It was solely dependent upon the relative contact size between the heat sink and heat source. Simons [2] simulated thermal heat spreading resistance using the heat sink and heat source. Simons [2] simulated thermal heat spreading resistance using exact partial differential heat transfer equations and compared them with those obtained by Lee exact partial differential heat transfer equations and compared them with those obtained by Lee et al. [1] using aluminum and copper flat plate heat spreaders and under forced convection air et al. [1] using aluminum and copper flat plate heat spreaders and under forced convection air Appl. Sci. 2017, 7, 62 2 of 11 cooling. Hasan et al. [3], on the other hand, applied the conductive thermal interface material to cooling. Hasan et al. [3], on the other hand, applied the conductive thermal interface material to reduce the resistance. Ellison [4] derived an exact three‐dimensional solution to determine the reduce the resistance. Ellison [4] derived an exact three‐dimensional solution to determine the maximum heat spreading resistance for rectangular heat sources centered on larger sized plane interface material to reduce the resistance. Ellison [4] derived an exact three-dimensional solution to maximum heat spreading resistance for rectangular heat sources centered on larger sized plane heat sinks. Muzychka et al. [5] presented a general solution based on the separation of variables determine the maximum heat spreading resistance for rectangular heat sources centered on larger heat sinks. Muzychka et al. [5] presented a general solution based on the separation of variables heat sources on a on rectangular flux of method for the thermal Muzychka spreading et resistances of eccentric sized plane al. [5] presented a general solution based the separation heat sources on a rectangular flux method for heat the sinks. thermal spreading resistances of eccentric channel. General expressions of an isoflux rectangular source on the surface of finite isotropic variables method for the thermal spreading resistances of eccentric heat sources on a rectangular flux channel. General expressions of an isoflux rectangular source on the surface of finite isotropic and compound rectangular channels were presented, as the well as the for the channel. General expressionsflux of an isoflux rectangular source on surface of solution finite isotropic and compound rectangular flux channels were presented, as well as the solution for and the temperature at the surface of a rectangular flux channel. compound rectangular flux channels were presented, as well as the solution for the temperature at the temperature at the surface of a rectangular flux channel. surface of a rectangular flux channel.
Figure 1. Thermal cooling with an air-cooled FHS (fin heat sinks) and water-cooled plate heat sink. Figure 1. Thermal cooling with an air‐cooled FHS (fin heat sinks) and water‐cooled plate heat sink. (a) Figure 1. Thermal cooling with an air‐cooled FHS (fin heat sinks) and water‐cooled plate heat sink. (a) (a) Air-cooled FHS; (b) Water-cooled plate heat sink. Air‐cooled FHS; (b) Water‐cooled plate heat sink. Air‐cooled FHS; (b) Water‐cooled plate heat sink.
Figure 2. Thermal cooling with an air-cooled FHS and TE (thermoelectric). Figure 2. Thermal cooling with an air‐cooled FHS and TE (thermoelectric). Figure 2. Thermal cooling with an air‐cooled FHS and TE (thermoelectric).
An overview of applications of TE devices for thermal cooling could be found in Rowe [6], An overview of applications of TE devices for thermal cooling could be found in Rowe [6], An overview of applications of TE devices for thermal cooling could be found in Rowe [6], Riffat and Ma [7], and Lee [8]. Sasaki et al. [9] obtained hot side temperatures between 40 ◦ C and Riffat and Ma [7], and Lee [8]. Sasaki et al. [9] obtained hot side temperatures between 40 °C and Riffat and Ma [7], and Lee [8]. Sasaki et al. [9] obtained hot side temperatures between 40 °C and 62 ◦ C and cold side temperatures between 8.4 ◦ C and 14.8 ◦ C at a power input of less than 80 W with 62 °C and cold side temperatures between 8.4 °C and 14.8 °C at a power input of less than 80 W 62 °C and cold side temperatures between 8.4 °C and 14.8 °C at a power input of less than 80 W cascaded or stacked TEs. Uemura [10] presented a brief description of commercial TE modulesTE and with cascaded or stacked TEs. Uemura [10] presented a brief description of commercial with cascaded or stacked TEs. Uemura [10] presented a brief description of commercial TE showedand howshowed graphs provided by manufacturers be utilizedcould to select module to forselect a specific modules how graphs provided by could manufacturers be autilized a modules and showed how graphs provided by manufacturers could be utilized to select a application. Webb et al. [11] developed equations that connect heat loads with material properties module for a specific application. Webb et al. [11] developed equations that connect heat loads of module for a specific application. Webb et al. [11] developed equations that connect heat loads the TE module and the cold and hot side operating temperatures. They went on to determine the with material properties of the TE module and the cold and hot side operating temperatures. with material properties of the TE module and the cold and hot side operating temperatures. optimum sink resistancesthe foroptimum a fixed set heat of operating conditionsfor anda considered existing They went heat on to determine sink resistances fixed set various of operating They went on exchangers to determine the optimum heat requirement sink resistances for a sink. fixed Riffat set of operating heat sink heat to meet the resistance of the heat and Ma [12] conditions and considered various existing heat sink heat exchangers to meet the resistance conditions and considered various existing heat sink heat exchangers to meet the resistance developed a computer model to show the relationship between the cooling and heating requirements and the optimum TE properties. Chein and Huang [13] presented a theoretical model to determine the performance of a TE device for electronic cooling by assuming values of the hot and cold junctions and then determined the resistance of micro-channel heat sinks. Kim et al. [14] presented a computer simulation for a thermosyphon heat sink. Huang et al. [15] designed a test rig to determine the physical properties of a TE module. These results were then used to determine the optimum design of a TE cooler system using a thermal resistance network model for the heat sink. Additionally,
determined the resistance of micro‐channel heat sinks. Kim et al. [14] presented a computer simulation for a thermosyphon heat sink. Huang et al. [15] designed a test rig to determine the physical properties of a TE module. These results were then used to determine the optimum design of a TE cooler system using a thermal resistance network model for the heat sink. Appl. Sci. 2017, 7, 62 3 of 11 Additionally, Palacios et al. [16] presented a methodology to extract TE internal parameters from information provided by performance curves to simulate the behavior of a commercial system. Vian, J.G. and Astrain [17] developed a heat module etfor TE presented power generation Palacios al. a [16] a methodology to extract TE internal parameters from information exchanger for the cold side of a TE module. provided by performance curves to simulate the behavior of a commercial module for a TE power It is apparent that thermal aperformance of TE generation system. from Vian, the J.G. literature and Astrain [17]the developed heat exchanger formodules the cold could side ofbe a TE module. affected by the physical properties, e.g., the Seebeck coefficient, the electrical resistance, and the It is apparent from the that theapplications. thermal performance of TE modules affected in literature the cooling This paper presents could the be empirical thermal conductivity by the physical on properties, e.g., the Seebeck coefficient, thecooler electrical resistance, and the thermal investigations the performance of a thermoelectric system and, subsequently, to conductivity in the cooling applications. This paper presents the empirical investigations on the evaluate the thermal cooling performance of a TE module fitted to a conventional FHS and performance of a thermoelectric cooler system and, subsequently, to evaluate the thermal cooling cooled under NC and FC air cooling. performance of a TE module fitted to a conventional FHS and cooled under NC and FC air cooling.
2. Materials and Methods
2. Materials and Methods
2.1. Thermal Resistance Model 2.1. Thermal Resistance Model
Figure 3a 3a shows shows a cross‐section of FHS-TE the FHS‐TE assembly. TE module is sandwiched Figure a cross-section of the assembly. The TEThe module is sandwiched between between a conventional FHS and a heat source. An aluminum block is added to the assembly to a conventional FHS and a heat source. An aluminum block is added to the assembly to distribute distribute the heat evenly from the heating element. The associated thermal resistance network the heat evenly from the heating element. The associated thermal resistance network is shown in is shown in Figure 3b. All interface temperatures are expected to be uniform since the heat flow Figure 3b. All interface temperatures are expected to be uniform since the heat flow is one-dimensional. ) supplied to the TE module creates a low temperature at the cold is one‐dimensional. Power (P Power (Pte ) supplied to the TE temodule creates a low temperature at the cold side (Tc ) of the TE and c) of the TE and a high temperature (T h) at the hot side. Assuming perfect insulation as aside (T high temperature (Th ) at the hot side. Assuming perfect insulation as shown, heat absorbed at EH) from the heat source. shown, heat absorbed at the cold side is equal to the heat supplied (P the cold side is equal to the heat supplied (PEH ) from the heat source. Heat from the cold surface is dissipated at the hot surface. Heat from the cold surface is dissipated at the hot surface.
Figure 3. Thermal resistance model of the FHS-TE assembly. Figure 3. Thermal resistance model of the FHS‐TE assembly.
The heat transfer rates at the cold (q ) and hot sides (q h) of the TE module are given by: The heat transfer rates at the cold (qc ) cand hot sides (qh ) of the TE module are given by: qc = ααte Ite Tc −
1 12 I Rte − Kte ∆T∆te 2 2te
qh = αte Ite Th +
1 2 I Rte − Kte ∆Tte 2 te
and
and
(1)
(2)
where αte , Ite , Rte , and Kte are the Seebeck coefficient, current, internal resistance, and thermal conductivity of TE, respectively. The temperature difference across the TE module is: ∆Tte = Th − Tc
(3)
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where Th and Tc define the hot side temperature and cold side temperature of TE, respectively. An energy balance (Pte ) gives the power to be supplied to the TE module: 2 Pte = qh − qc = αte Ite ∆Tte + Ite Rte
(4)
The applied TE voltage (Vte ) is: Vte = αte ∆Tte + Ite Rte
(5)
The cooling coefficient of performance of the TE is: COPc = qc /Pte
(6)
In Figure 3b, the 1-D thermal resistance of the FHS is shown as Rf1D and the thermal resistance of the aluminum block as Ral . Contact thermal resistances Rcr between the aluminum block and TE module, as well as between TE and FHS, are assumed to be negligible. From the thermal resistance network the thermal resistance of the FHS is: R f 1D = ( Th − Ta )/qh
(7)
where Ta is the ambient temperature. The internal electrical resistance of the TE depends upon the mean operating temperature of the TE. This can be represented by: Rte = a + bTmte
(8)
where a and b are constants in internal electrical resistance of TE. The mean TE temperature is calculated by: Tmte = ( Th + Tc )/2 (9) From the above we obtain: 2 T − 2aI 2 − 4K T − 4T R f 1D −bIte c te c a te Th = 2 R f 1D bIte + 4αte Ite − 4Kte − 4
(10)
Figure 4 shows the flow chart to simulate the TE hot side temperature (Th ) given the cold side temperature (Tc ). The procedure involves an iteration process. Ambient (Ta ) and cold side temperatures, and applied current (Ite ) to the TE module are first specified together with the characteristics of the TE module, such as the Seebeck coefficient (αte ), thermal conductance (Kte ), internal electrical resistance (Rte ), and the resistance constants a and b. From the manufacturer’s datasheet, αte = 0.053 V/K, Kte = 0.66 W/K, Rte = 0.016, a = −0.36, and b = 0.006. First, a guessed value of Th is input into the program. Next, the simulated value of Th ’ is calculated from Equation (10). The difference between the predicted and guessed values is then calculated. If the difference is more than 1%, the process is repeated until the difference is less than 1%. The thermal resistance Rf1D under NC and FC is then evaluated using the new value of Th ’.
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Figure 4. Flowchart to simulate the TE hot side (Th ) temperature. Figure 4. Flowchart to simulate the TE hot side (Th) temperature.
2.2. Experimental Procedure 2.2. Experimental Procedure An experimental apparatus to determine the thermal performance of a FHS-TE assembly is shown An experimental apparatus to determine the thermal performance of a FHS‐TE assembly is in Figure 5. The 40 mm × 40 mm × 4 mm thick TE module used is the HT8, 12, F2, 4040 manufactured shown in Figure 5. The 40 mm × 40 mm × 4 mm thick TE module used is the HT8, 12, F2, 4040 by Laird Technologies (Penang, Malaysia). The five-fin aluminum FHS measured 45 mm × 45 mm, manufactured by Laird Technologies (Penang, Malaysia). The five‐fin aluminum FHS with a 10 mm thick base. The heating element measured 40 mm × 40 mm × 4 mm, and the aluminum measured 45 mm × 45 mm, with a 10 mm thick base. The heating element measured 40 mm × 40 block 40 mm × 40 mm × 5 mm. The electrical circuit for this is also shown. Thermal insulation was mm × 4 mm, and the aluminum block 40 mm × 40 mm × 5 mm. The electrical circuit for this is provided at the bottom and sides of the assembly up to, and including, the TE module to minimize heat also shown. Thermal insulation was provided at the bottom and sides of the assembly up to, loss to the ambient environment. The insulation consisted of a composite layer of 10 mm thick cork and including, the TE module to minimize heat loss to the ambient environment. The insulation board (Trusco, Osaka, Japan) and 95 mm thick rockwool (Rockwool, Selangor, Malaysia). Heating was consisted of a composite layer of 10 mm thick cork board (Trusco, Osaka, Japan) and 95 mm provided from an AC power supply (Shanghai MCP, Shanghai, China). Power to the heat source (PEH ) thick rockwool from (Rockwool, Selangor, Malaysia). was Taipei, provided from AC power was determined the product of AC voltmeter (VEHHeating ) (GW Instek, Taiwan) andan ammeter (IEH ) EH) was determined from supply (Shanghai MCP, Shanghai, China). Power to the heat source (P readings (GW Instek, Taipei, Taiwan). Power input to the TE (Pte ) was determined from the product of EH) (GW Instek, Taipei, Taiwan) and ammeter (I EH) readings (GW the product of AC voltmeter (V DC voltmeter (Vte ) (GW Instek, Taipei, Taiwan) and ammeter (Ite ) readings (GW Instek, Taipei, Taiwan). Four thermocouples (Tf1–Tf4) (Graphtec, Tokyo, Japan) inserted into 3-mm diameter holes drilled ) was determined from the product of DC Instek, Taipei, Taiwan). Power input to the TE (Ptewere in a row through the base of theTaipei, heat sink to measure FHS/TE temperature distribution. Instek, Taiwan) and the ammeter (Iteinterface ) readings (GW Instek, Taipei, voltmeter (Vte) (GW TE hot surface (Th ) was obtained from the arithmetic mean were of these four temperatures. Taiwan). Four temperature thermocouples (Tf1–Tf4) (Graphtec, Tokyo, Japan) inserted into 3‐mm Thermocouples in 1.5-mm deep machined onsink the top and bottom surfaces of diameter holes centrally drilled located in a row through the grooves base of the heat to measure the FHS/TE the aluminum block measured the mean temperature at the of the aluminum block was obtained from the interface temperature distribution. TE cold hot surface surface temperature (Th) top (T ) and of the heating surface (T ). The TE cold junction temperature (T ) was assumed equal to T s c alm alm . arithmetic mean of these four temperatures. Thermocouples centrally located in 1.5‐mm deep The mean operating temperature (T ) across the TE module was calculated by taking the arithmetic m grooves machined on the top and bottom surfaces of the aluminum block measured the mean mean temperatures between hot (T ) and cold surfaces (Tc ). Additional thermocouples were employed h alm) and of the heating surface (Ts). cold surface temperature at the top of the aluminum block (T to measure the external surface temperature of the insulation (Tins ) and ambient temperature (Ta ). The TE cold junction temperature (Tc) was assumed equal to Talm. The mean operating All thermocouples were connected to a data logger and logged every minute. Experiments were temperature (Tm) across the TE module was calculated by taking the arithmetic mean performed with heater power inputs at 10 W and 20 W (nominal). At each power input setting, temperatures between hot (Th) and cold surfaces (Tc). Additional thermocouples were experiments were carried out with increasing TE voltages from 1 V to 6 V. The results are tabulated in employed to measure the external surface temperature of the insulation (Tins) and ambient Table 1 for NC (Runs D1–D2) and for FC (Runs D3–D4). temperature (Ta). All thermocouples were connected to a data logger and logged every minute. Experiments were performed with heater power inputs at 10 W and 20 W (nominal). At each power input setting, experiments were carried out with increasing TE voltages from 1 V to 6 V. The results are tabulated in Table 1 for NC (Runs D1–D2) and for FC (Runs D3–D4).
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Figure Figure 5.5. Experimental Experimental apparatus apparatus (a); (a); and and layout layout (b) (b) to to determine determine the the thermal thermal performance performance of of the the FHS-TE assembly. FHS‐TE assembly. Table 1. Experimental results for FHS-TE (fin heat sinks-thermoelectric) assembly under natural (NC) Table 1. Experimental results for FHS‐TE (fin heat sinks‐thermoelectric) assembly under natural (NC) and forced convection (FC). and forced convection (FC). Run almT TThh PEH PEH Vte Vte Ite Ite Pte Pte Ta Ta Ts Ts Tc ≈ T Tc ≈ alm NC/FC NC/FC Run # (◦ C) (◦ C) # (W) (W) (V) (V) (A) (A) (W) (W)(°C) (◦ C)(°C) (◦ C) (°C) (°C) 10.1 10.1 1.01 1.01 0.52 0.520.52 0.5220.6 20.6 65.7 65.7 64.7 64.8 64.7 64.8 61.9 69.2 10.0 10.0 2.00 2.00 0.85 0.851.69 1.6920.3 20.3 62.9 62.9 61.9 69.2 10.0 3.00 1.16 3.50 20.3 63.8 62.8 77.1 D1 D1 10.0 10.0 3.00 4.01 1.16 1.473.50 5.8820.3 20.2 63.8 66.3 62.8 77.1 65.3 85.9 74.6 106.9 10.0 10.0 4.01 6.00 1.47 2.025.88 12.1320.2 20.0 66.3 75.7 65.3 85.9 NC 10.0 19.9 6.00 1.00 2.02 0.6112.13 0.6120.0 20.9 75.7105.4 74.6 106.9 103.2 96.0 NC 102.3 102.0 19.9 19.8 1.00 2.00 0.61 0.900.61 1.8120.9 20.1105.4104.5 103.2 96.0 19.8 3.00 1.19 3.57 20.2 102.9 100.7 107.1 D2 19.8 19.8 2.00 4.00 0.90 1.471.81 5.8820.1 20.1104.5104.8 102.3 102.0 102.5 115.0 D2 19.8 - 3.00 6.00 1.19 - 3.57 - 20.2 - 102.9 100.7 107.1 19.8 10.3 4.00 1.00 1.47 0.635.88 0.6320.1 19.9104.832.9 102.5 115.0 31.9 30.8 33.0 ‐ 10.0 6.00 2.00 ‐ 1.04 ‐ 2.08 ‐ 20.3 ‐ 28.2 ‐ 27.1 ‐ 9.9 3.00 1.45 4.36 20.5 24.8 23.8 36.1 D3 10.3 10.0 1.00 4.00 0.63 1.840.63 7.3719.9 20.7 32.9 23.3 31.9 30.8 21.3 39.6 19.6 48.8 10.0 10.0 2.00 6.00 1.04 2.622.08 15.720.3 20.7 28.2 20.7 27.1 33.0 FC D3 9.9 20.1 3.00 1.00 1.45 0.804.36 0.8020.5 20.1 24.8 51.7 23.8 36.1 49.5 40.5 45.1 42.9 10.0 20.3 4.00 2.01 1.84 1.197.37 2.3820.7 20.1 23.3 47.3 21.3 39.6 20.0 3.00 1.57 4.72 20.1 43.6 41.5 45.7 D4 10.0 20.0 6.00 4.00 2.62 1.9615.7 7.8420.7 19.9 20.7 41.3 19.6 48.8 39.2 49.1 FC 38.0 58.3 20.1 20.3 1.00 6.00 0.80 2.700.80 16.220.1 20.3 51.7 40.1 49.5 40.5 20.3 2.01 1.19 2.38 20.1 47.3 45.1 42.9 D4 20.0 3.00 1.57 4.72 20.1 43.6 41.5 45.7 3. Results and Discussion 20.0 4.00 1.96 7.84 19.9 41.3 39.2 49.1 20.3 7 show 6.00 the 2.70 16.2 temperatures 20.3 40.1 38.0 Figures 6 and transient recorded for58.3 the
ΔTtete ∆T (◦(°C) C)
Th,theory Th,theory TmTm COP COP c c (◦ C)(°C) (◦ C) (°C)
0.1 0.1 7.2 7.2 14.3 14.3 20.6 32.3 20.6
64.8 64.8 65.5 65.5 69.9 69.9 75.6 90.7 75.6
−32.3 7.2 −−7.2 0.4 6.4 −0.4 12.5 6.4
90.7 32.57 0.82 93.6 107.5 99.6 102.1 99.6 10.97 32.57 99.393.6 103.9 5.55 105.1 102.1 3.37 10.97 113.3 99.3 108.8 -103.9 -5.55 - 105.1
−12.5 1.1 5.9‐ 12.3 −1.1 18.3 29.2 5.9
108.8 16.36 3.37 31.4 30.0 ‐ 4.80 ‐ 30.0 2.28 31.4 1.36 16.36 30.4 34.2 30.0 0.63 4.80
113.3 30.6 33.2 ‐ 36.5 40.030.6 48.933.2
−12.3 9.0 −18.3 2.2 4.2 29.2 9.9 20.3 −9.0
30.0 45.0 44.0 30.4 43.6 34.2 44.2 48.2 45.0
39.336.5 41.740.0 44.5 48.048.9 57.039.3
−2.2 4.2 9.9 hot20.3 (T
h)
19.34 63.863.8 19.34 5.89 5.89 68.868.8 2.86 76.6 2.86 85.976.6 1.70 0.82 1.70 107.5 85.9
2.28 25.00 8.50 1.36 4.25 0.63 2.55 1.25 25.00
44.0 8.50 43.6 4.25 44.2 2.55 48.2 cold1.25 and (Tc )
41.7 44.5 48.0 57.0 of sides
the TE and also for the heat source (Ts ) surface temperature at a power input (PEH ) of 10 W and under 3. Results and Discussion NC. The following results are observed:
Figures 6 and 7 show the transient temperatures recorded for the hot (Th) and cold (Tc) • Temperatures of Th , Ts , and Tc are plotted for the first 120 min and, every 60 min thereafter. s) surface temperature at a power input (PEH) of sides of the TE and also for the heat source (T The results showed that steady state is achieved after 2 h at the low TE voltage run of 1 V while it 10 W and under NC. The following results are observed: took only about an hour at the higher TE voltage >2 V. • Heat source temperature (Ts ) cis higher than cold side temperature (Tc ) by about 1 ◦ C. This was Temperatures of T h, Ts, and T are plotted for the first 120 min and, every 60 min thereafter. The attributed to the temperature diffference across the thin aluminum block, calcualted to be results showed that steady state is achieved after 2 h at the low TE voltage run of 1 V while it ◦ C. around 1 took only about an hour at the higher TE voltage >2 V. • Temperature difference between hot and cold sides of the TE (Th −c) by about 1 °C. This was Tc ) is about 36 ◦ C at the Heat source temperature (T s) is higher than cold side temperature (T ◦ low-power input of 10W and about 12 C at the higher power input of 20W. Temperature difference attributed to the temperature diffference across the thin aluminum block, calcualted to be decreases with TE power input (Pte ). around 1 °C. • Heat source temperature (Ts ) increases with power input (PEH )h − T from 73 ◦ C at 10 W to 104 ◦ C Temperature difference between hot and cold sides of the TE (T c) is about 36 °C at the at 20 W. low‐power input of 10W and about 12 °C at the higher power input of 20W. Temperature difference decreases with TE power input (Pte).
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Heat source temperature (T s) increases with power input (PEH) from 73 °C at 10 W to 104 °C at Heat source temperature (T s) increases with power input (PEH) from 73 °C at 10 W to 104 °C at Appl. Sci. 2017, 7, 62 7 of 11 20 W. 20 W.
Figure 6. Transient temperatures for the FHS-TE assembly (NC (Natural convection), = 10 W, Run PEH = 10 W, Figure 6. Transient temperatures for the FHS‐TE assembly (NC (Natural convection), P Figure 6. Transient temperatures for the FHS‐TE assembly (NC (Natural convection), PEH EH= 10 W, Run Run D1). D1). D1).
Figure 7. Transient temperatures for the FHS‐TE assembly (NC, P EH= 20 W, Run D2). Figure 7. Transient temperatures for the FHS-TE assembly (NC, PEH = 20 W, Run D2). Figure 7. Transient temperatures for the FHS‐TE assembly (NC, P EH= 20 W, Run D2).
Figures 8 and 9 show the transient temperatures recorded for the hot side (T h), cold (Tc) side, Figures 8 and 9 show the transient temperatures recorded for the hot side (T Figures 8 and 9 show the transient temperatures recorded for the hot side (Thh),), cold (T cold (Tcc)) side, side, s) under FC power input (PEH) of 10 W and 20 W, respectively. The following and heat source (T and heat source (T and heat source (Ts )s) under FC power input (P under FC power input (PEH )EH of) of 10 W and 20 W, respectively. The following 10 W and 20 W, respectively. The following results results can be seen: results can be seen: can be seen: • • • • • • •
Steady‐state is achieved earlier under FC compared to NC, i.e., after about 30 min, compared Steady‐state is achieved earlier under FC compared to NC, i.e., after about 30 min, compared Steady-state is achieved earlier under FC compared to NC, i.e., after about 30 min, compared to to 120 min and 60 min under NC. to 120 min and 60 min under NC. 120 min and 60 min under NC. Heat source temperature (T s) is higher than the cold side temperature (Tc) by about 1 °C. A Heat source temperature (T ) by about 1 °C. A Heat source temperature (Ts )s) is higher than the cold side temperature (T is higher than the cold side temperature (Tc ) cby about 1 ◦ C. A similar similar difference is observed under NC showing that the contact thermal resistance does not similar difference is observed under NC showing that the contact thermal resistance does not difference is observed under NC showing that the contact thermal resistance does not change change with convection. change with convection. with convection. Cold side temperature (T c) decreases with an increase in TE applied voltage (Vte) as expected. Cold side temperature (T Cold side temperature (Tcc) decreases with an increase in TE applied voltage (V ) decreases with an increase in TE applied voltage (Vtete) as expected. ) as expected. The temperature difference (T The temperature difference (Thh − T − Tcc) between hot and cold sides increase with TE applied ) between hot and cold sides increase with TE applied The temperature difference (Th − Tc ) between hot and cold sides increase with TE applied voltage voltage (V voltage (Vtete) as expected. ) as expected. (V te ) as expected. Heat source temperature (T Heat source temperature (Tss) wasbrought below, or to,ambient temperature (T ) wasbrought below, or to,ambient temperature (Taa) when power ) when power Heat source temperature (Ts ) wasbrought below, or to, ambient temperature (Ta ) when power input (P EH) to the heat source was at 10 W and TE input voltage (V input (PEH) to the heat source was at 10 W and TE input voltage (Vtete) was at 6 V. ) was at 6 V. input (PEH ) to the heat source was at 10 W and TE input voltage (Vte ) was at 6 V. Heat source temperature (T ) was Heat source temperature (Tss) roseto about 40°C when the heat source power input (P ) roseto about 40°C when the heat source power input (PEH EH) was Heat source temperature (Ts ) roseto about 40 ◦ C when the heat source power input (PEH ) was increased to 20 W. increased to 20 W. increased to 20 W. The temperature difference between TE hot (T The temperature difference between TE hot (Thh) and cold (T ) and cold (Tcc) sides was lower at the higher ) sides was lower at the higher The temperature difference between TE hot (T ) and cold (T ) sides was lower at the higher power power input of 20 W, comapred to 10 W. c h power input of 20 W, comapred to 10 W. input of 20 W, comapred to 10 W.
Heat source temperature (Ts) wasbrought below, or to,ambient temperature (Ta) when power input (PEH) to the heat source was at 10 W and TE input voltage (Vte) was at 6 V. Heat source temperature (Ts) roseto about 40°C when the heat source power input (PEH) was increased to 20 W. Appl. Sci. 2017, 7, 62 8 of 11 The temperature difference between TE hot (T h) and cold (Tc) sides was lower at the higher power input of 20 W, comapred to 10 W.
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Figure 8. Transient temperatures for the FHS-TE assembly (FC (forced convection), PEH = 10 W, Figure 8. Transient temperatures for the FHS‐TE assembly (FC (forced convection), P EH= 10 W, Run Run D3). D3). Figure 8. Transient temperatures for the FHS‐TE assembly (FC (forced convection), PEH= 10 W, Run
D3).
= 20 W, Run D4). Figure 9. Transient temperatures for the FHS-TE assembly (FC, PEH Figure 9. Transient temperatures for the FHS‐TE assembly (FC, P EH= 20 W, Run D4). Figure 9. Transient temperatures for the FHS‐TE assembly (FC, PEH= 20 W, Run D4).
Interface temperatures (T , and T ) under NC and FC conditions and heat source power Interface temperatures (Th , hT, T Ts ) sunder NC and FC conditions and heat source power input c , cand (PEH )Interface temperatures (T areEHcompared in Figures 10 and 11. The results show that: ) are compared in Figures 10 and 11. The results show that: input (P h, T c, and T s) under NC and FC conditions and heat source power input (PEH) are compared in Figures 10 and 11. The results show that: • All temperatures are lower under FC cooling because of higher cooling heat transfer rates All temperatures are lower under FC cooling because of higher cooling heat transfer rates during during FC air cooling mode as expected. All temperatures are lower under FC cooling because of higher cooling heat transfer rates FC air cooling mode as expected. • Hot side (T during FC air cooling mode as expected. Hot side (Thh) temperatures increase with an increase in the applied TE voltage (V ) temperatures increase with an increase in the applied TE voltage (Vtete). ). • Cold side (T c ) temperatures decrease with an increase in the applied TE voltage (V te). Hot side (T te). Cold side (Th) temperatures increase with an increase in the applied TE voltage (V ) temperatures decrease with an increase in the applied TE voltage (V c te ). • As a result, the temperature difference between hot and cold sides (T h − Tc) increase with Cold side (T c) temperatures decrease with an increase in the applied TE voltage (V ). applied As a result, the temperature difference between hot and cold sides (Th − Tc ) increasetewith applied TE voltage (V te). As a result, the temperature difference between hot and cold sides (T h − Tc) increase with TE voltage (Vte ). At low TE applied voltage (V te), the TE cold side (Tc) could be higher than the hot side (Th). This applied TE voltage (V te). • At low TE applied voltage (Vte ), the TE cold side (Tc ) could be higher than the hot side (Th ). This is is because of the insufficient TE power supply to generate a sufficient temperature differential At low TE applied voltage (V c) could be higher than the hot side (Th). This because of the insufficient TEte), the TE cold side (T power supply to generate a sufficient temperature differential to to enable the TE to function. is because of the insufficient TE power supply to generate a sufficient temperature differential enable the TE to function. to enable the TE to function.
Figure 10. Comparison of interface temperatures for the FHS‐TE assembly (NC and FC, P EH= 10 W, Figure 10. Comparison of interface temperatures for the FHS-TE assembly (NC and FC, PEH = 10 W, Runs D1 and D3). Figure 10. Comparison of interface temperatures for the FHS‐TE assembly (NC and FC, P EH= 10 W, Runs D1 and D3). Runs D1 and D3).
Figure 10. Comparison of interface temperatures for the FHS‐TE assembly (NC and FC, P EH= 10 W, Appl. Sci. 2017, 7, 62 9 of 11 Runs D1 and D3).
Figure 11. Comparison of interface temperatures for the FHS‐TE assembly (NC and FC, P EH = 20 W, Figure 11. Comparison of interface temperatures for the FHS-TE assembly (NC and FC, PEH = 20 W, Runs D2 and D4). Runs D2 and D4). Appl. Sci. 2017, 7, 62 Appl. Sci. 2017, 7, 62
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A comparison between experimental and predicted TE hot side temperature (T h) is shown A comparison between experimental and predicted TE hot side temperature (Th ) is shown in hot side temperature increases with both the heat source and TE power inputs. It is also higher ) and under NC and FC modes. It can be seen that the in Figure 12 at different power inputs (P hot side temperature increases with both the heat source and TE power inputs. It is also higher Figure 12 at different power inputs (PEH ) EH and under NC and FC modes. It can be seen that the hot under theNC air flow condition. The predicted values are lower than experimental values by under theNC air flow condition. The predicted values are lower than experimental values by side temperature increases with both the heat source and TE power inputs. It is also higher under less than 2 °C °C or about about 2%. Keeping in are mind that the experimental accuracy of 2the the ◦C less than 2 or Keeping in mind experimental of theNC air flow condition. The2%. predicted values lowerthat thanthe experimental valuesaccuracy by less than °C, the agreement between experimental and theoretical is thermocouples is about +0.5 ◦ or about 2%. Keeping in mind that thethe experimental of the thermocouples about +0.5 is C, °C, agreement accuracy between experimental and is theoretical thermocouples is about +0.5 extremely good. the agreement between experimental and theoretical is extremely good. extremely good. c) and temperature difference The experimental coefficient coefficient of cooling performance (COP The experimental ofof cooling performance (COP difference across c) temperature and temperature difference The experimental coefficient cooling performance (COP c ) and te) are compared in Figure 13 at various power inputs to both the across the TE module (ΔT the TE module (∆T ) are compared in Figure 13 at various power inputs to both the heating element te across the TE module (ΔTte) are compared in Figure 13 at various power inputs to both the EH ) and the applied voltage (V te ) to the TE and also under NC and FC cooling heating element (P (PEH ) and the applied voltage (Vte ) to the TE and also under NC and FC cooling modes. It can be seen EH) and the applied voltage (V te) to the TE and also under NC and FC cooling heating element (P c decreases with TE modes. It can be seen that the temperature difference increases while COP that the temperature difference increases while COP decreases with TE power input. The NC air flow c c decreases with TE modes. It can be seen that the temperature difference increases while COP results in a higher temperature difference and COP . A higher COP and lower temperature difference c. A higher power input. The NC air flow results in a higher temperature difference and COP c c c. A higher power input. The NC air flow results in a higher temperature difference and COP are obtained at higher heat source power input. c and lower temperature difference are obtained at higher heat source power input. COP c and lower temperature difference are obtained at higher heat source power input. COP
Figure 12. Comparison of experimental and predicted T for the FHS‐TE assembly. Figure 12. Comparison of experimental and predicted Thhh for the FHS‐TE assembly. for the FHS-TE assembly. Figure 12. Comparison of experimental and predicted T
Figure 13. Coefficient of performance COP and temperature difference ΔT for the FHS‐TE Figure 13. Coefficient of performance COPccc and temperature difference ΔT and temperature difference ∆Ttetete for the FHS‐TE for the FHS-TE assembly. Figure 13. Coefficient of performance COP assembly. assembly.
The effects of employing a TE module for cooling is shown in Figure 14, which compares The effects of employing a TE module for cooling is shown in Figure 14, which compares alm),with and without the TE module under the surface temperature of the aluminium block (Talm ),with and without the TE module under the surface temperature of the aluminium block (T EH), and with various applied TE NC and FC conditions, at 10 W and 20 W power inputs (P (PEH ), and with various applied TE NC and FC conditions, at 10 W and 20 W power inputs te). The values shown in dashed lines represent the temperature (Talm) obtained voltage (V voltage (Vte). The values shown in dashed lines represent the temperature (Talm) obtained
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The effects of employing a TE module for cooling is shown in Figure 14, which compares the surface temperature of the aluminium block (Talm ),with and without the TE module under NC and FC conditions, at 10 W and 20 W power inputs (PEH ), and with various applied TE voltage (Vte ). The values shown in dashed lines represent the temperature (Talm ) obtained without the TE while the solid lines show the results obtained with the FHS-TE assembly. The temperature of the surface of the aluminium block is about 1–2 ◦ C lower than the heating surface temperature (Ts ). The results show the following:
•
The surface temperature of the aluminium block is higher under NC when compared to FC, as expected, because of less efficient cooling. • Lower power input (PEH ) results in lower temperature. • Under the NC cooling mode, the temperature is lower without the use of the TE at both TE applied voltages (Vte ) of 10 W and 20 W. Appl. Sci. 2017, 7, 62 10 of 11 • In other words, the use of the TE module is not effective under the NC cooling mode. The heat Under the FC cooling at 20 W, the applied voltage (V te) to the TE module should be >4 V and at transfer resistance of the TE module has a negative effect on the natural heat loss. 10W, it should be >1 V.
Figure 14. Comparison of surface temperature of aluminum block with and without TE. Figure 14. Comparison of surface temperature of aluminum block with and without TE.
Under the FC cooling at 20 W, the applied voltage (Vte ) to the TE module should be >4 V and at 4. Conclusions 10 W, it should be >1 V.
An experimental investigation was conducted to evaluate the thermal cooling performance of a TE module fitted to a conventional FHS and cooled under NC and FC air flows. The results 4. Conclusions showed that the use of the TE module is not effective under the NC cooling mode. With the An experimental investigation was conducted to evaluate the thermal cooling performance of a present TE module employed, under FC cooling at 20 W, the applied voltage (Vte) to the TE TE module fitted to a conventional FHS and cooled under NC and FC air flows. The results showed module should be >4 V, and at 10 W, it should be >1 V. A simple iterative method of predicting that the use of the TE module is not effective under the NC cooling mode. With the present TE module the hot side temperature of the TE module was presented. Agreement between predicted and employed, under FC cooling at 20 W, the applied voltage (Vte ) to the TE module should be >4 V, and at experimental values was better than 2%. 10 W, it should be >1 V. A simple iterative method of predicting the hot side temperature of the TE module was presented. and experimental was better than 2%. Acknowledgments: The Agreement study was between supported predicted by university internal fund values (UTARRF/2015‐C1/L01) and Collaborative Research in Engineering, Science and Technology (CREST) under project number P22C2‐13. Acknowledgments: The study was supported by university internal fund (UTARRF/2015-C1/L01) and Collaborative Research in K.S.O. Engineering, Scienceconceived and Technology (CREST) under project number P22C2-13. Author Contributions: and K.C.L. and designed the experiments; C.F.T. and K.C.L. performed the experiments; K.S.O. and C.F.T. analyzed the data; K.C.L. contributed Author Contributions: K.S.O. and K.C.L. conceived and designed the experiments; C.F.T. and K.C.L. performed reagents/materials/analysis tools; K.S.O. wrote the paper. the experiments; K.S.O. and C.F.T. analyzed the data; K.C.L. contributed reagents/materials/analysis tools; K.S.O. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.
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