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Feb 7, 2017 - at 20 W, the applied voltage (Vte) to the TE module should be >4 V and at 10 W, it should be >1 V. ..... Wiley: Hoboken, NJ, USA, 2010. 9.
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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. 

Appl. Sci. 2017, 7, 62 

 

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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 

9 of 11  9 of 11 

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 

Appl. Sci. 2017, 7, 62

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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.

References  1. 2. 3.

4.

Lee, S.; Song, S.; Au, V.; Moran, K.P. Constriction/spreading resistance model for electeronic packaging. In  Proceedings of the ASME/JSME Engineering Conference, New York, NY, USA, 19–24 March 1995.  Simons, R.E. Simple formulas for estimating thermal spreading resistance. Electron. Cool. Mag. 2004, 10, 2.  Hasan, A.; Hejase, H.; Abdelbaqi, S.; Assi, A.; Hamdan, M.O. Comparative effectiveness of different phase  change  materials to improve  cooling  performance  of  heat sinks  for  electronic devices. Appl. Sci. 2016,  6,  doi:10.3390/app6090226.  Ellison,  G.N.  Maximum  thermal  spreading  resistance  for  rectangular  sources  and  plates  with  nonunity  aspect ratios. IEEE Trans. Compon. Packag. Technol. 2003, 26, 439–454. 

Appl. Sci. 2017, 7, 62

11 of 11

References 1. 2. 3.

4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16.

17.

Lee, S.; Song, S.; Au, V.; Moran, K.P. Constriction/spreading resistance model for electeronic packaging. In Proceedings of the ASME/JSME Engineering Conference, New York, NY, USA, 19–24 March 1995. Simons, R.E. Simple formulas for estimating thermal spreading resistance. Electron. Cool. Mag. 2004, 10, 2. Hasan, A.; Hejase, H.; Abdelbaqi, S.; Assi, A.; Hamdan, M.O. Comparative effectiveness of different phase change materials to improve cooling performance of heat sinks for electronic devices. Appl. Sci. 2016, 6. [CrossRef] Ellison, G.N. Maximum thermal spreading resistance for rectangular sources and plates with nonunity aspect ratios. IEEE Trans. Compon. Packag. Technol. 2003, 26, 439–454. [CrossRef] Muzychka, Y.S.; Culham, J.R.; Yovanovich, M.M. Thermal spreading resistance of eccentric heat sources on rectangular flux channels. Trans. ASME 2003, 125, 178–185. [CrossRef] Rowe, D.M. General Principles and basic considerations. In Thermoelectrics Handbook–Macro to Nano; Taylor & Francis: Boca Raton, FL, USA, 2006. Riffat, S.B.; Ma, X. Improving the coefficient of performance of thermoelectric cooling systems: A review. Int. J. Energy Res. 2004, 28, 753–768. [CrossRef] Lee, H.S. Thermal Design Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells; Wiley: Hoboken, NJ, USA, 2010. Sasaki, K.; Shindoh, T.; Itoh, Y. Enhancement of energy use efficiency by simultaneous of cooling and heating action during thermoelectric conversion. In Proceedings of the 22nd International Conference on Thermoelectrics, La Grande Motte, France, 17–21 August 2003; pp. 637–640. Uemura, K.I. Commercial Peltier Modules. In Handbook of Thermoelectrics; Rowe, D.M., Ed.; CRC Press: Boca Raton, FL, USA, 1995. Webb, R.L.; Gilley, M.D.; Zarnescu, V. Advanced heat exchange technology for thermoelectric cooling devices. Trans. ASME 1998, 120, 98–105. [CrossRef] Riffat, S.B.; Ma, X. Optimum selection (design) of thermoelectric modules for large capacity heat pump applications. Int. J. Energy Res. 2008, 28, 1231–1242. [CrossRef] Chein, R.; Huang, G. Thermoelectric cooler application in electronic cooling. Appl. Therm. Eng. 2004, 24, 2207–2217. [CrossRef] Kim, W.T.; Kim, K.S.; Lee, Y. Design of a two-phase loop thermosyphon for telecommunications system. KSME Int. J. 1998, 12, 942–955. [CrossRef] Huang, B.J.; Chin, C.J.; Duang, C.L. A design method of thermoelectric cooler. Int. J. Refrig. 2008, 23, 208–218. [CrossRef] Palacios, R.; Arenas, A.; Pecharroman, R.R.; Pagola, F.L. Analytical procedure to obtain internal parameters from performance curves of commercial thermoelectric modules. Appl. Therm. Eng. 2009, 29, 3501–3505. [CrossRef] Vian, J.G.; Astrain, D. Development of a heat exchanger for the cold side of a thermoelectric module. Appl. Therm. Eng. 2008, 28, 1514–1521. [CrossRef] © 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).