Thermal management of electronic devices and concentrator photovoltaic systems using phase change material heat sinks: Experimental investigations

Accepted Manuscript Thermal management of electronic devices and concentrator photovoltaic systems using phase change material heat sinks: Experimental investigations Mohamed Emam, Shinichi Ookawara, Mahmoud Ahmed PII:

S0960-1481(19)30472-0

DOI:

https://doi.org/10.1016/j.renene.2019.03.151

Reference:

RENE 11423

To appear in:

Renewable Energy

Received Date: 13 December 2018 Revised Date:

23 March 2019

Accepted Date: 31 March 2019

Please cite this article as: Emam M, Ookawara S, Ahmed M, Thermal management of electronic devices and concentrator photovoltaic systems using phase change material heat sinks: Experimental investigations, Renewable Energy (2019), doi: https://doi.org/10.1016/j.renene.2019.03.151. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Thermal management of electronic devices and concentrator photovoltaic systems using phase change material heat sinks: Experimental investigations Mohamed Emam a, b, c, Shinichi Ookawara a,b and Mahmoud Ahmed b,d,* a

Corresponding author

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Department of Energy Resources Engineering, Egypt-Japan University of Science and Technology (E-JUST), Alexandria 21934, Egypt b Department of Chemical Science and Engineering, Tokyo Institute of Technology, Tokyo 1528552, Japan c Department of Mechanical Engineering, Faculty of Engineering Shoubra, Benha University, Benha, Qalubiya 11629, Egypt d Mechanical Engineering Department, Assiut University, Assiut 71516, Egypt

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E-mail address: [email protected] , or [email protected] (M. Ahmed)

Abstract

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The present experimental study focuses on the passive thermal regulation of electronic devices

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and concentrator photovoltaic (CPV) systems using phase change material (PCM). Therefore, a

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fabricated system was attached to a silicon rubber heater to imitate the heat dissipation from

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electronic devices or CPV cells. Several sets of experiments were performed to investigate the

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melting of three different PCMs (RT25HC, RT35HC, and RT44HC) at three distinct heat flux

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values of 2000, 2950, 3750 W/m2. Results revealed that using RT25HC, RT35HC, and RT44HC

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PCMs decreased the average front wall temperature associated with the electronic component or

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CPV cells by about 69.8, 80.44, and 74.44 ºC when compared to that of the system without

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PCM. The solidification of RT44HC PCM was studied at different ambient temperatures of 20,

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25, and 30 °C. It was noticed that, the time to reach the complete solid phase increases from 407

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min. to 501, and 585 min. when the ambient temperature rises from 20 to 25, and 30 °C,

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respectively. Finally, the effect of cavity formation inside solid PCMs due to solidification

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shrinkage on their cooling performance was investigated. Results indicated that air cavities

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formation inside solid PCMs has insignificant effect on their cooling performance.

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Key words: Phase change materials, Electronic cooling, CPV cooling, Air cavities.

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1. Introduction

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Cooling of temperature sensitive appliances such as electronic components and concentrator

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photovoltaic (CPV) systems has recently become imperative in their design and progression to 1

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mitigate the negative effect of excessive temperature increase on performance [1]. A survey

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performed by the US air force revealed that more than half of electronic breakdowns are a result

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of surpassed safe operating temperatures [2]. Moreover, the failure rate of such components

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doubles for the temperature rise of 10-20 °C. However, a 1 °C reduction in a component

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temperature may decrease its failure rate by about 4 % [3]. Similarly, the over-heating of CPV

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cells decreases the electrical power gain and rises the risk of potential damage to the CPV

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systems which poses a challenge for the efficient and reliable use of this technology [4]. It was

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reported that the obtained electrical power of crystalline silicon solar cells that are currently

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utilized for CPV decreases by 0.4-0.5 % for each 1 °C increase in their operating temperature

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over the characteristic power conversion temperature, 25 °C defined by manufacturers [5].

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Hence, thermal management of electronic components and CPV systems is essential to boost up

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their overall performance and lengthen service life. In view of that, numerous investigations

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including active and passive cooling methodologies have been performed to introduce an

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operative cooling system with the aim of lessening the influence of excessive temperature rise on

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the performance of each kind of these devices [6].

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Based on the available literature, active cooling techniques are able to achieve an acceptable

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heat removal rate from electronic components or CPV cells via applying a continuous air or

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water flow [7,8]. This is accomplished by utilizing mechanical fans or pumps which necessitate

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electrical power from external source and in return rise the installation and maintenance costs

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[9,10]. Alternatively, passive cooling techniques are costless compared to active cooling

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methods, powerless operation and noiseless [11]. One of the emergent passive cooling methods

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is achieved by integrating phase change materials (PCMs) with electronic components or CPV

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cells for thermal regulation. The key concept behind employing PCMs is their capability to

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absorb a large quantity of thermal energy as latent heat during their solid-liquid phase change

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process which is expected to considerably alleviate the unfavorable temperature rise during

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system operation [12]. Moreover, the integration of PCMs with CPV systems offer the ability of

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storing low-cost thermal energy that could be utilized for other applications such as heating

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services of houses [13]. Accordingly, thermal energy is considered as an extra product beside the

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main production of electricity which in return will rise the overall CPV system efficiency [14].

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Phase change material-based cooling method is thus considered as one of the currently research

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interests in electronics and CPV thermal regulation field and it is the focus of the current work.

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Several previous studies associated with the incorporation of PCMs with electronic devices

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and PV systems with and without concentrators have been conducted. It was stated that, there are

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several factors that affect the thermal behavior of the PCM-based heat sink such as the container

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design and orientation, PCM quantity, intensity of heat source and thermophysical properties of

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the employed PCM (i.e., phase change temperature and thermal conductivity) [15]. Table 1 and 2

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summarize key features and the most important findings from selected previous studies that

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dealing with the utilization of PCMs for passive thermal regulation of electronic devices and PV

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systems, respectively.

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Table 1

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Summary of previous studies on the utilization of PCMs for passive thermal regulation of electronic devices. Methodology

Description

El Omari et al. [16]

Numerical

• A passive cooling system using PCMfilled enclosures with five different geometries was introduced. • The geometries which contained the same quantity of PCM were rounded or rectangular, thick or thin, centered relative to the cooled surface or shifted upward vertically.

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Experimental • The thermal behavior of a small-scale PCM-based heat sink with a single cavity and two-series cavity configurations was assessed. • The effects of different pattern arrangements of PCMs in multiplePCM heat sink, and PCM thicknesses on the thermal behavior of heat sink were explored.

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Al Siyabi et al. [17]

Findings

Avci and Yazici [18]

• Their results revealed the high impact of varying the enclosure geometry on the system cooling performance. • The best efficiency was achieved for an enclosure shifted vertically relative to the cooled surface. • However, the effect of the geometry choice on the inverse process (solidification), wasn’t considered in their analysis.

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Reference

Experimental • The effect of inclination angle on the performance of a PCM-based flat-type heat sink was investigated. • Heat sinks without and with N-eicosane PCM were examined and compared at different orientations ranging from 0° (vertical) to 90° (horizontal).

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• They concluded that using a multiple-PCM heat sink with PCMs arrangement of RT50-RT55 elongated the thermal regulation period by 110 min and 130 min compared with using only RT50 and RT55, respectively. • Moreover, as the PCM thickness (quantity) increased from 30 mm to 60 mm, the thermal-regulationperiod extended by about 50 min. • Their measurements revealed that the heat sink inclination angle has a significant effect on the system cooling performance while its effect is negligible for the one without PCM. • Increasing the inclination angle from 0° to 90° improved mixing within the liquid PCM and resulted nearly uniform temperatures at the back side of the heat sink surface which consequently enhanced the system cooling performance.

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Gharbi et al. [21]

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Experimental • They designed and fabricated a new • Their experimental measurements cooling technique by integrating a indicated that the proposed system liquid-metal PCM module with a flat limited the electronic chips heat pipe thermal spreader for temperature rise which consequently electronics cooling applications. extended their operation time. Experimental • This study aimed at maximizing the critical time which is time required by one of the electronic components before reaching the critical temperature. • The melting process of a PCM in a rectangular enclosure with three discrete heat sources was investigated. • Moreover, the effect of heat flux repartition between sources and intermittent periodic heat flux on the system cooling performance was evaluated.

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• It was noticed that the cooling performance depends strongly on the heat flux repartition and the maximum heat transfer was seen for the lower source. • Locating the higher power component at middle or lower section seems the best manner to extend the critical time.

Summary of previous studies on the utilization of PCMs for passive thermal regulation of PV systems. Reference

Methodology

Hachem et al. [22]

Experimental

Description

Findings

• The influence of employing pure (White petroleum jelly) and combined PCM (white petroleum jelly, copper, and graphite) on the overall performance of a PV panel. • A transient energy balance was also introduced to analyze the system thermal behavior and correlate it to the electrical gain. Experimental • The integration of a paraffin based PCM with melting range of 38-43 °C with a flat PV panel was examined in dry hot weather of United Arab Emirates through the entire year.

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• They concluded that the selection of a PCM with an appropriate melting temperature strongly depends on the applied thermal conditions. • The use of a PCM with a higher melting point can prolong the protection time of the target devices from overheating. • A lower melting temperature, however, is not always unfavorable because in some cases, depending on the heating power, it enables a prompt protection of the target devices.

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Li et al. [20]

Experimental • They explored the transient performance of a PCM-based heat sink employing two different PCMs (neicosane and 1-hexadecanol) with close thermophysical properties but different phase change temperatures.

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Fan et al. [19]

Hasan et al. [23]

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• It was observed that using pure and combined PCM improved module electrical efficiency by an average of 3 % and 5.8 %, respectively. • Therefore, the use of combined PCM is more promising than pure PCM.

• They found that the PCM helped to enhance the PV annual electricity output by about 5.9 % with a maximum decrease in the temperature of the cell up to 13 °C.

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• Their measurements showed that the maximum temperature of the PV panel without PCM was 35.6 °C higher than that of the panel with PCM. • Moreover, the predicted results revealed that the annual electricity production of PV-PCM panel was increased by about 7.3%. • Based on the obtained results, it was concluded that neglecting PCM convective and radiative heat transfer will cause significant errors. • It was also noted that an optimal performance can be achieved when the melting temperature of PCM is slightly higher, such as 5 °C, than the ambient temperature.

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Ma et al. [25]

Experimental • The study aimed at studying the effect and of latent cooling on the electric numerical performance of a conventional PV panel by utilizing RT28HC as the PCM. • A simulation of PV panels with and without PCM in TRNSYS software was conducted, and the obtained results were compared with the experimental data. Numerical • In this study, an improved thermal resistant model of the PV-PCM system was introduced. • The model consumes less computation time than CFD method and can incorporate the convective heat transfer effect within melted PCM through applying the enhanced conductivity method.

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Stropnik and Stritih [24]

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• The selection criteria for climates • It was found that the climates having suitable for PCM integration with PV less variations in the ambient panels were reported in this study. temperature are more suitable for PCM integration. • A mathematical model for analyzing the transient behavior of the PV-PCM • The utilization of PCM with PV for system incorporating the effect of cooling improved electricity climate was developed. generation by 9.7% and this value reduced to 6.6% for the climate • having large variations. • Finally, heat extraction by PCMsystems is more effective in warm climates in comparison to cold climates.

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Khanna et al.[26]

One of the main challenges of employing PCMs in cooling applications is their typically

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low thermal conductivities which result in a slow charging and discharging rates [27]. In addition

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to literature in Table 1, various techniques had been applied to enhance the heat transfer process

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in PCMs and accordingly improve their cooling performance. The utilization of thermal

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conductivity enhancer (TCE) such as metallic fins, nano particles, and metal foams in boosting

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the performance of PCM-based heat sinks used for thermal management of electronics has been

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reviewed by Kumar et al. [28]. Kamkari and Shokouhmand [29] experimentally examined the

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influence of incorporating partial fins on the transient melting behavior of lauric acid (PCM) in a

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rectangular cavity. Their measurements showed that increasing the number of partial fins

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reduced the required time to reach the complete melting phase and improved the total heat

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transfer rate. Ali et al. [30] examined the thermal behavior of two different configurations of pin-

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fin heat sinks (2 mm square and 3 mm circular) integrated with two distinct PCMs (paraffin wax

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and n-eicosane) for thermal regulation of electronic components. Based on the comparison of

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heat sinks base temperature, they indicated that using a 3 mm diameter of circular pin-fins

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achieves a better cooling performance when compared to the 2 mm square pin-fin heat sink. An

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experimental and numerical investigation of the solid-liquid phase change of n-octadecane

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(PCM) with CuO nanoparticle additives as a TCE was carried out by Dhaidan et al. [31]. It was

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concluded that dispersing an appropriate weight fraction of nanoparticles in conventional PCM

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has a significant potential for enhancing its thermal conductivity which augments the heat

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transfer rate and improves the energy storage performance. Rehman et al. [2] evaluated and

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compared the performance of a copper foam based heat sinks with and without PCM used for

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thermal regulation of electronic systems. Results revealed that employing a copper foam with

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0.83 PCM volume fraction achieved a maximum temperature reduction of 25 % compared with

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that without any PCM at a heat flux of 0.8 KW/m2.

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Although, thermal management of the conventional PV system using phase change materials

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was widely inspected as reported in the literature in Table 2, there is still a large chance for

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research in their applications in CPV, specifically at relatively high solar incident irradiance.

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Based on authors’ knowledge, the emerging CPV-PCM system has not been widely investigated.

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Extensive research is needed to quantify and evaluate the applicability of combining PCMs with

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CPV systems for thermal regulation. Emam and Ahmed [32] proposed anew integrated CPV-

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PCM system including various designs of the PCM heat sink to overcome the low thermal

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conductivity of most of PCMs and improve their cooling performance. Results showed that the

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integrated CPV-PCM system with a five-parallel cavity configuration PCM heat sink attained the

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best CPV performance due to the enhanced heat transfer inside the PCM domain. Another study

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by the same authors [33] was conducted to assess the thermal behavior and electrical

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performance of a new integrated CPV system using PCM and a water jacket. Their results

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demonstrated that using PCM based heat sink with a water jacket attained a considerable

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decrease in the CPV cell temperature, with reasonable temperature uniformity. Lu et al. [34]

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designed and simulated a new BICPV system incorporated with PCM to mitigate its temperature

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increase as a result of the concentrated solar irradiance. It was noticed that due to the latent heat

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absorption of the utilized PCM, the temperature of CPV system was reduced by over 20 °C on

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average which translated to an increase in the electrical efficiency by about 10 % compared to

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the system without PCM.

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Based on the above literature survey, it is evident that the design and selection of practical

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and effectual PCM-based thermal regulation systems necessitate a comprehensive understanding

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of the heat transfer mechanisms that rule the melting and solidification processes of the

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employed PCM. In addition, experimental studies associated with the PCM re-solidification

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issue is limited which needs to be further investigated for the success of this cooling

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methodology [35]. In view of that, the present study aims to provide more information about

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such integrated thermal regulation systems via exploring their behavior during melting and

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solidification processes. In the current experiments, a fabricated thermal system was attached to

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a silicon rubber heater to imitate the heat dissipation from electronic devices or CPV cells in

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CPV-PCM systems. The system was designed and assembled to provide a comprehensive

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understanding of the heat transfer mechanisms that rule the melting and solidification processes

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of the PCM. The transient temperature distribution within the PCM at the vertical mid-plane of

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the container as well as the system front wall temperature at three different positions were logged

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throughout the experimental run.

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Firstly, several sets of experiments were performed to investigate the melting of three

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different PCMs at three distinct heat flux values of 3750, 2950, and 2000 W/m2. This is to

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quantify the potential improvements of utilizing PCMs with different thermo-physical properties.

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The selected PCMs were RT25H, RT35H, and RT44H (organic paraffin wax based). The data

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sheet provided by the manufacturer (RUBITHERM®, Germany) designates the employed PCMs

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possess a long lifetime with stable performance even after up to 2000 phase change cycles [36].

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Secondly, the solidification of RT44HC PCM was investigated at different ambient temperatures

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of 20, 25, and 30 °C. Finally, for the first time, the effect of cavity formation inside solid PCM

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due to solidification shrinkage on its cooling performance was also investigated. The current

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findings can open doors for further research towards the development and commercialization of

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novel passive cooling technology for electronic appliances and CPV systems. Furthermore, the

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observed phenomena and obtained experimental measurements provide a pivotal source for

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validation of numerical approaches.

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2. Experimental setup and procedures

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2.1 Design and fabrication of experimental setup The experimental setup used in the current study with the main components is schematically

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shown in Fig. 1. A rectangular cavity with internal dimension of 50 mm in width, 120 mm in

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height, and 100 mm in depth was employed as the PCM container. A silicon rubber heater with a

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maximum power density of 0.5 W/cm2 was used as a constant heat flux source to mimic the heat

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dissipation from electronic components or CPV cells in CPV-PCM systems. The electric power

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was supplied to the heater via a volt slider, (AS ONE Corp., Model RSA-5), to control the

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amount of heat flux, while the wall heat flux was detected by using a heat flux sensor (Captec

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Enterprise, HF-10). To mount the heater and attain uniform heat flux on the system surface, the

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front wall of the enclosure was formed from a 10 mm thick high conductive aluminum plate (k =

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218 W/mK). The remaining five walls of the PCM containment were fabricated from a 20 mm

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thick acrylic Plexiglass clear sheets. This is to facilitate direct visualization and photography of

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the PCM phase transition process and to reduce heat loss to the environment by their low thermal

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conductivity (k = 0.19 W/m K). Acrylic plates were assembled together using M4 stainless steel

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bolts, while silicon rubber thin sheets (0.5 mm thickness) were placed between the plates to

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prevent leakage. For further insulation, a 30 mm thick foaming Urethane sheets (k = 0.026

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W/mK) were used on all sides of the enclosure with the aim of minimizing heat losses to the

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environment.

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Sixteen calibrated K-type micro-thermocouples, (AS ONE Corp., Model No. T1-SP-K), of

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0.127 mm bare wire diameter were used for temperature measurements. Twelve thermocouples

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were installed inside the PCM cavity (vertical mid-plane) for monitoring the transient

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temperature distribution within the PCM. Three more thermocouples were attached to the front

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aluminum plate to measure the transient local temperature variation of the system front wall as

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shown in Fig. 1. The remaining thermocouple was placed outside the test cell for recording the

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ambient temperature. Four multichannel temperature recorders were employed to record the

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temperatures at every 1 s.

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In the current study, the melting and solidification of a three commercially available PCMs

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with different phase transition temperatures were experimentally inspected. These PCMs were

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RT25H, RT35H, and RT44H (organic paraffin wax based). The data sheet provided by the 8

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manufacturer (RUBITHERM®, Germany) designates the employed PCMs possess a remarkable

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latent heat capacity in narrow temperature ranges, a stable performance throughout repeated

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phase change cycles and neglected super-cooling effect. Moreover, they are chemically inert,

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friendly to environment, non-toxic, easy handling, relatively low cost (about US$ 7.5/kg), and

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have an unlimited lifetime. Table 3 lists the thermo-physical properties of the PCMs and other

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components.

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Table 3

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Manufacturer-provided thermo-physical properties for the employed PCMs, aluminum and Plexiglass.

Heat of fusion, (kJ/kg) Thermal conductivity Solid, (W/m°C) Liquid, (W/m°C) Density Solid, (kg/m3) Liquid, (kg/m3) Specific heat capacity Solid, (kJ/ kg K) Liquid, (kJ/ kg K) Volumetric expansion, ∆V (1/kg) % Sub-cooling, (°C)

0.2 0.2 880 (at 15 °C) 770 (at 40 °C) 2 2 12.5 Negligible

0.2 0.2

RT44HC [37] 41-44 (main peak: 43) 250

Aluminum [38] Plexiglass [38] N/A N/A

0.2 0.2

N/A

N/A

211 N/A

0.19 N/A

880 (at 25 °C) 770 (at 40 °C)

800 (at 25 °C) 700 (at 80 °C)

2675 N/A

1180 N/A

2 2 12 Negligible

2 2 12.5 Negligible

0.903 N/A N/A N/A

1.464 N/A N/A N/A

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N/A: Not Applicable.

RT35HC [37] 34-36 (main peak: 35) 240

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Melting temperature (range), (°C)

RT25HC [37] 22-26 (main peak: 25) 230

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Thermo-physical properties

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Fig. 1. (a) Schematic diagram of the experimental setup, (b) The highlighted thermocouples

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locations, (c) Real photos of the PCM enclosure and (d) A three-dimension schematic view of the enclosure assembly. 173

2.2 Experimental procedures and data reduction

Initially, the solid PCM was melted completely in a hot water bath and then stirred well

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before being poured gradually into the container in a layer by layer method to reduce the

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formation of air gaps inside the solid PCM. This manner was continued until about 83.3% (100

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mm height) of the total volume of the enclosure filled with the solid PCM, giving 16.7% (20 mm

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height) free volume for the PCM expansion during the phase change process. Furthermore, three

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longitudinal holes (4 mm diameter) were drilled through the upper wall of the enclosure which

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were used for ventilation (release the pressure due to volume expansion to avoid leakage) and to

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facilitate thermocouples fixation as shown in Fig. 1.

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All tests were performed indoors in an air-conditioned space where it was possible to control

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the ambient temperature value. The experiments were initiated by supplying power to the electric

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heater via a volt slider to adjust the heat flux value. Concurrently, the PCM, front wall and

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ambient temperatures were logged using the multichannel temperature recorders. Moreover, it is

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worth mentioning that part of the insulation of one side of the container was being periodically

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removed for a time span of 8 to 9 s every 5 min. and photographs were taken. This is to visualize

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the liquid-solid interface evolution and facilitate the calculation of liquid fraction of PCM during

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melting process. The viewing area used for taking photographs only equals 50 × 100 mm2 10

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(corresponding to the initial solid PCM domain), while the remaining part which equals 50 × 20

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mm2 (corresponding to the initial air gap) was permanently covered with the insulation during

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melting process to minimize heat loss to the surrounding. In the current study, the captured digital images (color images) were processed and analyzed

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in order to estimate the instantaneous liquid fraction value. Firstly, the color image was

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converted to a binary image which consists of only black and white colors corresponding to

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liquid and solid phases, respectively as shown in Fig. 2. Secondly, based on the binary image, the

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liquid-solid interface which separates the solid and liquid phases was digitized using a MATLAB

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plot digitizing toolbox. Consequently, the area representing the liquid (black) and solid (white)

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phases was calculated. The observable liquid fraction (λo) was assessed based on the following

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equation:

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λo =

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Al At

(1)

where: Al and At are the area corresponding to the liquid phase (mm2) and the total viewing area

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(50 × 100 mm2), respectively. Nevertheless, the expansion of the PCM during melting process

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must be considered to calculate the real liquid fraction value (λr). Thus, Eq. 1 is modified as

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below [39]:

λr =

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λo (1 + ∆V ) (1 + ∆V ⋅ λo )

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(2)

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where: ∆V is the PCM solid-liquid volume exchange during phase change process.

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Fig. 2. Digitizing of the solid-liquid interface and calculation of the area corresponding to liquid

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and solid phases.

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The total thermal regulation enhancement achieved using PCM (Λ) was assessed as the

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difference between the system front surface temperature evolution without PCM and that of the

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system with PCM as illustrated in Appendix A (Fig. A.1). Precisely, Λ for a given experimental

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condition is calculated by subtracting the integral of the system front wall temperature evolution

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using PCM (Tw,PCM) with time from the integral of that of the system without PCM (Tw,no-PCM)

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with time at the same operating conditions (heat flux and ambient temperature) as shown in Eq.

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3: tf

tf

ti

ti

Λ = ∫ Tw , no − PCM dt − ∫ Tw, PCM dt

220 (3)

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Several sets of experiments were performed for each PCMs (RT25H, RT35H, and RT44H)

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during melting and solidification processes using three distinct heat flux values of 2000, 2950,

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and 3750 W/m2. Each stage of experiment was repeated to verify the measurement accuracy and

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attain the required reproducibility of the results. The obtained results were compared, and the

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deviation was found to be insignificant. The specifications and accuracy of the employed

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instruments as reported in their datasheets are presented in Appendix A (Table A.1).

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3. Results and discussion

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Thermal management of electronic components and CPV systems using PCMs is influenced

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by the thermal behavior of the utilized PCM during its phase transition process. Thus, the design 12

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and selection of practical and efficient PCM based thermal system regulations necessitate a

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comprehensive understanding of the heat transfer mechanisms that rule the melting and

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solidification processes of the PCM. In the current experiments, the fabricated thermal system

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was attached to a silicon rubber heater as previously mentioned to imitate the heat dissipation

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from electronic devices or CPV cells. The transient temperature distribution within the PCM at

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the vertical mid-plane of the container as well as the system front wall temperature at three

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different locations were logged throughout the experimental run. Initially, the melting of three

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different PCM (Table 1) have been tested at different heat flux values. This is to quantify and

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evaluate the applicability of utilizing PCMs with different thermo-physical properties for thermal

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regulation of electronics and CPV systems. Secondly, the solidification of RT44HC PCM was

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inspected at different ambient temperature of 20, 25, and 30 °C. Finally, the effect of cavity

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formation inside solid PCM during solidification process on its cooling performance was

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investigated.

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3.1 Thermal characterization of PCM-based heat sinks during melting process

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3.1.1 Liquid-solid interface evolution

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Better understanding of the governing heat transfer mechanisms and flow patterns that rule

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PCMs melting behavior and consequently their cooling performance can be achieved by direct

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visualization of the PCM phase change process. For that reason, the developed system was

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designed and built to provide accurate measurements of the instantaneous liquid-solid interface

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evolution during the PCM melting process. Figure 3 illustrates successive photographs of the

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solid-liquid phase change process of RT44HC PCM when a heat flux of 3750 W/m2 is applied on

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the system front wall and at an ambient temperature of 25 ± 1 °C. Based on the figure, some

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important observations can be made.

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254

At the very early stages of the experiment, heat is conducted through the front aluminum

255

plate (hot wall) to the adjacent solid PCM which results in a steep rise in the temperature with

256

time until reaching to the employed PCM melting point. At that time, a very thin layer of liquid

257

PCM appears alongside the hot wall and the solid-liquid interface is a vertical straight line as

258

shown in Fig. 3(a). This behavior reveals that heat is transferred mainly by pure conduction

259

during that period and normally to the hot wall. Further increase in time allows the buoyancy

260

force to grow enough to overcome the molecular viscous force which allows the hot molten 13

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liquid to flow upward along the hot wall. At that time, buoyancy-driven convection begins in the

262

liquid region and the small concavity in the top portion of solid-liquid interface (Fig. 3(b)) gives

263

a significant proof of that. With continued energy input, the melt layer becomes more

264

pronounced and the growing natural convection currents forms a circulating current in the melted

265

liquid as shown in Fig. 3(c). The hot buoyant liquid firstly ascends to the upper part of the

266

system, adjacent to the heated wall, then turns around and flows toward the liquid-solid interface

267

where part of its energy is consumed for melting the solid PCM. Consequently, the liquid

268

becomes colder and denser and then descends alongside the interface to the bottom of the

269

container. This sort of flow pattern speeds up the local heat transfer and accordingly the rate of

270

melting at the top part of the container when compared to the lower part, forming a manifest

271

curvature of the melting interface (Fig. 3(d)). As melting continues (Fig. 3(e-i)), the solid PCM

272

begins to vanish gradually from the top part of the container allowing the molten liquid to touch

273

the opposite back wall and the interface shape at the top part of the solid PCM develops a convex

274

curvature. This manner remains until reaching to the complete liquid phase.

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Figure 4 displays the evolution of the solid-liquid interface during the melting process of

276

RT44HC PCM for different heat flux values of 3750, 2950, and 2000 W/m2 and an ambient

277

temperature of 25 ± 1 °C. It is noticed that applying different values of heat flux on the front

278

aluminum plate results in almost identical trends for the evolution of the solid-liquid interface.

279

This is mainly due to the fact that the main heat transfer mechanisms and flow field structures in

280

the liquid PCM are the same within the investigated range of heat flux. However, increasing the

281

value of heat flux rises the rate of melting which decreases the required time to reach complete

282

liquid phase. For instance, increasing the heat flux value from 2000 to 2950 and 3750 W/m2

283

decreases the complete melting time from 345 to 203 and 127 min., respectively (see Fig. A.2 in

284

Appendix A). For further verification, the evolution of the solid-liquid interface during the

285

melting process of RT44HC, RT35HC, and RT25HC PCMs for the heat flux value of 3750

286

W/m2 and the ambient temperature of 25 ± 1 °C were compared and presented in Fig. 5. Based

287

on the figure, there is no significant difference in the solid-liquid interface shapes when different

288

PCMs with different melting points are employed. Indeed, using PCMs with a lower melting

289

point such as RT25HC speeds up the evolution of the solid-liquid interface. It is noticed that the

290

required time to reach the complete melting phase while using RT25HC PCM is about the half of

291

that when RT44HC is employed and this trend is true at any heat flux value (see Fig. A.3 in

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14

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Appendix A). Another important observation is that unlike the expected trend, the rate of melting

293

using RT35H PCM is very close to that of RT44H PCM, although their melting temperatures and

294

latent heats of fusion are different. This can be attributed to the fact that the solid and liquid

295

densities of the RT44HC PCM is lower than those of RT35HC PCM as can be seen in Table1.

296

Therefore, the absorbed heat per unit volume in case of RT44HC PCM is slightly less than that

297

of RT35HC PCM.

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298 299

Fig. 3. Sequential photographs of the solid-liquid phase change process of RT44HC PCM at a

300

heat flux value of 3750 W/m2 and an ambient temperature of 25 ± 1 °C.

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Fig. 4. The evolution of the solid-liquid interface during the melting process of RT44HC PCM

304

for different heat flux values and an ambient temperature of 25 ± 1 °C

305 306

Fig. 5. The evolution of the solid-liquid interface during the melting process of RT44HC,

307

RT35HC, and RT25HC PCMs for a heat flux value of 3750 W/m2 and an ambient temperature of

309

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25 ± 1 °C.

3.1.2 PCM temperature distribution and temperature history

310

The transient temperature distributions within the PCM at the vertical mid-plane of the

311

container were logged throughout the experimental run to provide further qualitative and

312

quantitative assessment for the PCM cooling performance. Figure 6 illustrates the RT44HC PCM

313

temperature distributions during melting recorded from four distinct thermocouple rows for a

314

heat flux of 3750 W/m2 and at an ambient temperature of 25 ± 1 °C. Each row is represented by 17

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three thermocouples which are positioned at the same horizontal level, but at different locations

316

as shown in Fig. 6. It can be observed that the temperature distributions for all thermocouple

317

rows during the early period of melting in which the temperature is lower than the melting limit

318

(43 °C) are nearly the same. This behavior is most likely attributed to the sensible heating of the

319

solid PCM by conduction heat transfer during this period as previously stated. Once the natural

320

convection current is originated in the melting region, thermocouples sited at the top part of the

321

container (3rd and 4th rows) exhibit a sharper temperature rise compared to those at the bottom

322

part of the container (1st and 2nd rows). This sudden increase in temperature is observed as the

323

melt front passes the tip of each thermocouple or between two neighboring thermocouples which

324

augments the local heat transfer rate between the liquid and solid PCM and results in the

325

temperature jump. Therefore, the higher the temperature jump, the greater the local heat transfer

326

rate is observed. Furthermore, it is noticed that the fourth thermocouple row (Fig. 6(d)) shows a

327

nearly uniform temperature distributions during the time period of 55-125 min. This can be

328

attributed to the presence of a region in the melted liquid with stratified thermal layers and poor

329

convection currents. As previously clarified, with the elapse of time the solid PCM begins to

330

vanish gradually from the top part of the container due to the higher rate of melting at this part.

331

Once, there is no solid PCM (cold source) at the top part, the temperature gradient as the source

332

of the natural convection current diminishes and the convection current is weakened in this

333

region. Similar trend is observed for the 2nd and 3rd thermocouple rows during the time period of

334

105-125, and 75-125 min., respectively. This reveals that the region with poor convection which

335

initiates at the top part of the container extends downward with melting progress until reach to

336

the complete melting state. Only the results of RT44HC at a heat flux of 3750 W/m2 is presented

337

since the same thermal behavior can be observed for the RT35HC and RT44HC PCMs. In

338

addition, this trend is true for any heat flux value.

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339

Furthermore, the transient temperature variation of the thermocouples installed at the first

340

and third thermocouples columns during the melting of RT44HC are presented in Fig. 7 for three

341

different heat flux values of 3750, 2950, and 2000 W/m2. It is clearly shown that during the early

342

stage of melting when the PCM is completely solid, all thermocouples shows a progressive rise

343

in temperature until reaching to the employed PCM melting point (43 °C). This is caused by

344

conduction heat transfer in the solid PCM which is much stronger near to the front heated wall as

345

indicated by the greater temperature rise of the first thermocouple column when compared to 18

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those of the third column. Subsequently, abrupt rise in temperatures are observed revealing that

347

the melted front is passing the thermocouples tips and then the temperatures increase gradually

348

until reaching to the full liquid phase. These observations are nearly the same for all

349

thermocouples. However, it is worth noticing that the temperature values at the end of the sharp

350

temperature rise when the liquid PCM impinging the thermocouples are associated with the

351

liquid PCM temperatures at the end of the thermal boundary layer passes each thermocouple.

352

These values are different and decrease from higher to lower thermocouples in each column as

353

illustrated in the figure which implies the decreased temperature of the liquid PCM as it descends

354

beside the interface as explained before. Moreover, the temperature values at the end of the sharp

355

temperature rise of the third column of thermocouples are greater than those of the first column

356

of thermocouples. This is mainly due to the increased temperature of the liquid PCM with time

357

due to the sensible heating from the hot wall side. There are no significant differences when

358

different values of heat flux are applied on the front wall except that the liquid PCM bulk

359

temperature is reduced with the decreased heat flux value (Fig. 7(b) and Fig. 7(c)).

360

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( ) Conduction

( ) Strong Convection

( ) Poor Convection

Fig. 6. RT44HC PCM temperature distributions during melting recorded from four distinct

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thermocouple rows for a heat flux of 3750 W/m2 and at an ambient temperature of 25 ± 1 °C.

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Fig. 7. The transient temperature variation of the thermocouples installed at the first and third thermocouples columns during the melting of RT44HC for three different heat flux values. 364

3.1.3 Variation of average hot wall temperature

365

The transient variation of the average hot wall temperature during the melting process

366

associated with the operating temperature of the electronic component or CPV cell is the key

367

parameter to evaluate the thermal regulation performance of PCM based heat sinks and 22

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understand how the PCM could be employed as a cooling method. Therefore, the average hot

369

wall temperature throughout the experimental run was calculated using three thermocouples

370

attached to the hot wall as shown in Fig. 1. To emphasize the importance of using PCMs, the

371

developed thermal system with and without PCM was tested at different heat flux values and the

372

results were compared and presented. Figure 8 shows the average hot wall temperature variation

373

with time for all selected PCMs (Table 1) compared with the same system without PCM (no-

374

PCM) at different heat flux values of 2000, 2950, and 3750 W/m2 and at an ambient temperature

375

of 25 ± 1 °C. In case of the no-PCM system and for all heat flux values, the average hot wall

376

temperature shows a rapid increase with time until reaching a steady state when the amount of

377

the absorbed heat balances the heat losses to the surrounding. For instance, the no-PCM system

378

attains steady state temperatures of 159, 123, and 89 °C after 83, 123, 175 min. at heat flux

379

values of 3750, 2950, and 2000 W/m2, respectively. On the other hand, in case of the thermal

380

system with PCM, there are in general several stages of the hot wall temperature variation related

381

to the melting process of the employed PCMs. Initially, the average hot wall temperature

382

increases rapidly (like the no-PCM case) until reaching to the utilized PCM melting point which

383

is mainly due to the sensible heating of the solid PCM by conduction heat transfer. Subsequently,

384

the gradient of temperature increase reduces and the average hot wall temperature of the system

385

with PCM diverges from that of the no-PCM system. This discloses that the solid PCM adjacent

386

to the hot wall starts melting and large amount of thermal energy is absorbed as a latent heat. As

387

time elapses, natural convection is initiated in the melted region allowing more heat to transfer

388

from the hot wall to the solid PCM resulting in a reduction in the average hot wall temperature,

389

which afterward remains almost constant for a while. Later, the wall temperature starts to rise

390

slowly until the PCM completely melts as shown for all tested PCMs. This variation is observed

391

when the solid phase of PCM gradually begins to transfer to liquid phase from the top part of the

392

container where the convection current is weakened in that region. As previously clarified, the

393

region of weak convection currents extends with time from up to down which results in less heat

394

transfer from the hot wall to the solid PCM and the wall temperature increases gradually until the

395

end of melting. Once all solid PCM completely melts and the latent heat has depleted, the

396

gradient of temperature rises gradually with time until the end of the experimental run.

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Fig. 8. The transient variation of the average hot wall temperature of the system without and

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with PCM (no-PCM) at different heat flux values and at an ambient temperature of 25 ± 1 °C. 398

To provide further details, Fig. 9 compares the deviation, ∆T of the average hot wall

399

temperature with PCM, Tw, PCM from that of the system without PCM, Tw,no-PCM, versus time for

400

all tested PCMs at a heat flux of 3750 W/m2. It is noticed that, for all PCMs the deviation of

401

Tw,PCM from Tw,no-PCM steeply rises until reaching a peak of 69.8, 80.44, and 74.44 ºC after 71,

402

82.7, and 84.7 min. from the starting of melting process for the RT25HC, RT35HC, and

403

RT44HC PCMs, respectively. This trend is most likely due to the large amount of thermal energy

404

absorbed as a latent heat during the melting process of the PCM which maintains the wall 24

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temperature at a considerably lower value compared to that of the system without PCM. Further

406

increase of time results in a decrease in ∆T until the end of the experimental run and this trend is

407

true for all employed PCMs. This variation is most likely due to the increase in the average hot

408

wall temperature at the end of melting process as previously explained, while the wall

409

temperature without PCM reaches a steady state temperature and remains constant. It is also

410

observed that, during the first hour of the experimental run (t < 60 min.) using the lower melting

411

point RT25HC PCM achieves the highest temperature deviation compared with the higher

412

melting PCMs, RT35HC and RT44HC. This can be clarified by the fact that in case of the

413

system filled with the RT25HC PCM, melting and consequently latent heat absorption occurs

414

earlier and at a lower temperature compared with those of RT35HC and RT44HC PCMs filled

415

systems. Accordingly, the wall temperature using RT25HC PCM is kept lower than that of the

416

higher melting PCMs, RT35HC and RT44HC during that period. However, with increasing in

417

time (t ≥ 60 min.), this trend is changed and RT25HC PCM attains the lowest temperature

418

deviation until the end of the experimental run. This is mainly caused by the higher rate of

419

melting when using the lower melting point RT25HC PCM as previously shown in Fig. 5 which

420

results in the solid phase PCM to fast melting in the container and the wall temperature starts to

421

increase sharply until the end of the experimental run. In general, this confirms that the lower

422

melting PCMs performed better than the higher melting PCMs in thermal regulation but for a

423

limited period. Figure 10 illustrates the total thermal regulation enhancement, Λ calculated based

424

on Eq. 3 for all selected PCMs at different heat flux of 2000, 2950, and 3750 W/m2. Based on the

425

figure, throughout the experimental run, using RT35HC PCM achieves the highest Λ which

426

results from the reasonable temperature reduction compared with the no-PCM system achieved

427

for a prolonged period of time. For additional verification, the obtained temperature reduction

428

using PCMs can be translates to an increase in the CPV systems output power. This is

429

accomplished by assuming the temperature coefficient of maximum power point as -0.5 %/°C [5]

430

which means that every 2 °C reduction in its average operating temperature will theoretically

431

result in a 1 % increase in the output power. Table 4 presents the average values throughout the

432

experimental run of the hot wall temperature, the deviation, ∆T, and the percentage of output

433

increment. The obtained results further prove the advantages of employing PCMs as an operative

434

passive cooling method for electronics and CPV systems.

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435 25

Fig. 10. The total thermal regulation

wall temperature with PCM, Tw,PCM from that

enhancement, Λ for all selected PCMs at

of the system without PCM, Tw,no-PCM, versus

different heat flux values of 2000, 2950, 3750

time for all tested PCMs at a heat flux of

W/m2

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Fig. 9. The deviation, ∆T of the average hot

3750 W/m2 Table 4

The average values throughout the experimental run of the hot wall temperature, the deviation, ∆T, and the percentage of CPV power output increment. 3750W/m2

No-PCM

127

RT25HC

77.1 71.52 77.11

∆T (°C) ∆P elec (%) Tw (°C) ∆T (°C)

2000W/m2

∆P elec (%) Tw (°C) ∆T (°C)

∆P elec (%)

0

0

105.53

0

0

77.86

0

0

49.9

24.95

70.79

34.74

17.37

54.58

23.28

11.64

55.48

27.74

64.38

41.15

20.58

52.19

25.67

12.84

49.89

24.95

67.73

37.8

18.9

56.71

21.15

10.58

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RT35HC RT44HC

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Tw (°C)

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3.2 Thermal characterization of PCM-based heat sinks during solidification process

440

Phase change materials can achieve a considerable reduction in the operating temperature of

441

electronic components and CPV cells, which significantly enhances their performance and saves

442

them from damage caused by excessively high temperatures. However, their cooling effects are

443

much less efficient after their latent heat has dissipated as previously revealed. Therefore, for

444

long-term use of PCMs, a sufficient time for recovery of the latent heat between two cycles to

445

achieve a full transition into its solid phase at the starting of the new cycle is essential. In view of 26

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that, the efficient PCM re-solidification is an important issue which needs further investigation

447

for the success of this technology. In the current work, the solidification process of RT44HC

448

PCM is inspected at different ambient temperatures. Figure 11 shows temperature histories of the

449

1st, 2nd and 3rd columns of thermocouples during the solidification of RT44HC PCM at ambient

450

temperature of 30 ºC. During solidification process the heat supply from the electric heater was

451

ended and the insulation layer was removed to facilitate heat losses to the surrounding. Initially,

452

all the thermocouples racks exhibit a sharp decrease in the temperature caused by the sensible

453

heat dissipation from the liquid PCM to the environment. The reduction in the thermocouple’s

454

temperature is continued until it reaches the solidification limit of the employed PCM (43 °C)

455

which afterward remains almost constant for a while. This variation indicates the starting of the

456

latent heat recovery and the liquid PCM begins to solidify at a nearly constant temperature. Once

457

the liquid PCM surrounding the thermocouple tip is completely solidified, the temperature starts

458

to decease gradually until the end of the experimental run and this trend is true for all

459

thermocouples. However, the time span during which the thermocouple temperature remains

460

constant and the solidification of the liquid PCM occurs depends on the thermocouple position.

461

As an instance, the liquid PCM surrounding the 1st column of thermocouples (T1, T2, T3, and T4)

462

shows the highest rate of solidification compared to those of the 2nd and 3rd columns as shown in

463

the figure. This behavior is mainly attributed to the high heat removal rate at this part of the

464

container provided by the high conductive aluminum wall (front wall). Furthermore, the

465

solidification rate of the liquid PCM surrounding the thermocouples located at the top and

466

bottom boundaries is higher than that of those placed at the middle of the container and this trend

467

is true for all thermocouples columns. Consequently, the liquid PCM at the middle of the

468

container and near to the back-Plexiglass wall spends the longest time to reach the complete

469

solid phase. This can be explained by the fact that the solidified PCM layers adjacent to the

470

container walls adds resistance to heat transfer and delays the solidification of the liquid PCM at

471

the middle of the container. This situation increases the potential of air gaps formation in this

472

area due to the solidification shrinkage of the PCM which may affect the cycle process of

473

melting and consequently its cooling effect.

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Moreover, Fig. 12 presents temperature histories of the 1st, 2nd and 3rd columns of

475

thermocouples during the solidification process of RT44HC PCM at different ambient

476

temperatures of 20, 25, and 30 °C. Based on the figure, similar trend is observed for the 27

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temperature histories of all thermocouples during the solidification process of RT44HC PCM at

478

different ambient temperatures. However, rising the ambient temperature prolongs the required

479

time to reach the complete solid phase. As shown in the figure, the time required to reach the

480

complete solid state increases from 407 min. to 501, and 585 min. when the ambient temperature

481

rises from 20 to 25, and 30 °C, respectively. In real applications, the efficient PCM re-

482

solidification can be achieved via fabricating the whole PCM container from high conductive

483

aluminum walls to ensure a rapid heat removal rate from the liquid PCM to the environment.

484

Furthermore, in case the weather is not suitable to allow natural solidification further

485

enhancement could be accomplished by incorporating PCM with forced water convection to

486

boost heat dissipation from the system as described by Emam and Ahmed [33].

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Fig. 11. Temperature histories of the 1st, 2nd and 3rd columns of thermocouples during the solidification of RT44HC PCM at ambient temperature of 30 ºC.

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Fig. 12. Temperature histories of the 1st, 2nd and 3rd columns of thermocouples during the solidification process of RT44HC PCM at different ambient temperatures of 20, 25, and

30

°C.

489

3.3 Effect of air cavities on the thermal behavior of PCM during melting In the real applications, solidification shrinkage due to the density variation from liquid to

491

solid PCM occurs which develops air cavities inside the solid PCM as discussed earlier.

492

Therefore, the effect of cavities formation inside the solid PCM on its cooling performance is an

493

important phenomenon to be studied. For this purpose, the thermal behavior of the developed

494

system during melting of RT44HC PCM with and without air cavities is investigated under the

495

same operating conditions and the obtained results are compared. In the case of filling the system

496

with RT44HC PCM without air cavities, the solid PCM was melted completely before being

497

poured gradually into the container in a layer by layer method allowing the previous layer to

498

completely solidify at each time to avoid the formation of air gaps inside the solid PCM. This

499

was continued until about 83.3 % (100 mm height) of the total volume of the enclosure filled

500

with the solid PCM, giving 16.7% (20 mm height) free volume for the PCM expansion during

501

the melting process. On the other hand, in case of the system filled with RT44HC PCM with air

502

cavities, the same amount of the solid PCM was melted completely before being poured totally

503

into the container and then left to solidify naturally which increases the potential of air gaps

504

formation inside the solid PCM. Fig. 13 shows the instantaneous photographs during melting of

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RT44HC PCM with and without air cavities at a heat flux value of 3750 W/m2 and an ambient

506

temperature of 25 °C. The viewing area used in the current experimental run for taking

507

photographs equals 50 × 120 mm2 to visualize both the liquid-solid interface and the liquid level

508

throughout the melting process.

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Visual observations revealed that at t = 0, the volume occupied by the solid PCM in case of

510

the system with cavities is larger than that of the system without cavities by about 4% for the

511

same quantity of the PCM which confirms the formation of air gaps inside the solid PCM. As

512

expected, the solid-liquid interface in case of the system without cavities shows usual and

513

uniform shapes during the melting process and liquid level rises gradually with time due to the

514

solid-liquid volume change as indicated by the red dashed line in Fig. 13. On the other hand, the

515

system with air cavities exhibits unusual and non-uniform forms of the solid-liquid interface

516

especially during the time period of 30-50 min. Moreover, after about 30 min. form the starting

517

of melting process the liquid level is suddenly decreased and then starts to rise again with time as

518

shown in the figure. These observations are most likely due to the existence of air cavities inside

519

the solid PCM. Once the melt front reaches the cavity, the liquid PCM is withdrawn to fill that

520

cavity and the liquid level is suddenly decreased as shown. Thereafter, the liquid level begins to

521

raise gradually after all cavities have filled with liquid PCM and this trend is continued until the

522

end of the experimental run. Such behavior affects the solid-liquid interface which develops non-

523

uniform shapes as illustrated in Fig. 13. Finally, after about 60 min. form the starting of melting

524

process, there is no significant difference between both systems with and without air cavities. For

525

more verification, the transient variation of the average wall temperature for both systems is

526

compared and presented in Fig. 14 to study the effect of air cavities formation on the cooling

527

performance of the PCM. Based on the figure, the average wall temperature of the system

528

without air cavities is almost identical to that of the system with air cavities. Thus, it can be

529

concluded that air cavities formation inside solid PCMs during the solidification process has no

530

significant effect on their cooling performance.

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Fig. 13. The instantaneous photographs during melting of RT44HC PCM with and without air

533

cavities at a heat flux value of 3750 W/m2 and an ambient temperature of 25 °C.

534 535

Fig. 14. The transient variation of the average wall temperature during melting of RT44HC PCM

536

with and without air cavities at a heat flux value of 3750 W/m2 and an ambient temperature of

537

25 °C.

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3.4. Comparison with previous works.

539

Table 5 presents a summary of the previous obtained results of research works that dealing with

540

the utilization of PCMs for passive thermal regulation of electronic devices and PV systems.

541

Furthermore, these results are compared with the current measurements.

542 543

Table 5

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Comparison with previous research work.

2017

Ashraf et al. [42]

2017

Zhu et al. [43]

2017

800-2800 Electronic cooling. W/m2 Experimental analysis of the cooling performance of PCMbased circular pin-fin heat sinks integrated with electronic devices 800-1600 Electronic cooling. W/m2 This experimental investigation focuses on the optimization of passive cooling system using extruded finned surfaces with different PCMs as the thermal conductivity enhancers. Six PCMs of varying melting temperature and heat capacities, namely Paraffin wax, RT-54, RT44, RT-35HC, SP-31 and neicosane were employed. 8000Electronic cooling. They experimentally evaluated 160002 W/m the effects of pore size and filling height ratio of the copper foam on the transient thermal performance of a PCM-based heat sink.

0.013225 At 2000 W/m2, the PCM-based m2 circular pin-fin heat sink with 4 mm fin thickness achieved a maximum temperature reduction of about 32 °C compared to the heat sink without PCM. 0.0049 m2

At 1600 W/m2, the SP-31 PCMbased round pin-fin heat sink achieved a maximum temperature reduction of about 29 °C compared to the same heat sink without PCM.

0.0049 m2

At a pore size of 15 ppi, the PCM-based heat sink attained a maximum temperature reduction of about 20 °C and 45 °C for heating powers of 8000 and respectively, 16000 W/m2, compared to the case without copper foam.

0.3339 m2

The use of RT42 PCM attained 10.5 °C decrease in PV temperature on aver age at peak time on yearly basis compared with the PV module without PCM cooling.

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Mahmoud 2013 et al. [40]

Applied Cooled Temperature reduction of the heat loads area target device maximum temperature 2000 0.0025 m2 The Electronic cooling. reduction was 12 °C compared to W/m2 Experimental study of inserting the heat sink without PCM. different configurations and PCM type on the thermal performance of PCM-based heat sinks used for passive cooling of electronic. Application

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Hasan et 2017 al. [23]

About 600 PV cooling. W/m2 The yearly energy performance of a PV-PCM system was examined in extremely hot environment of the United Arab Emirates (UAE).

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Sharma 2017 et al. [5]

-

The use of pure and combined PCM reduced the temperature of PV panels by an average of 2.7 °C and 5.6 °C, respectively, as compared to the reference PV panel without cooling.

About 550 W/m2

1.6 m2

Using RT28HC PCM decreased the maximum temperature of the PV panel by about 35.6 °C compared to that of the PV panel without PCM.

About 900 W/m2

0.0009 m2

The average temperature in the center of the CPV-PCM system was reduced by 10.7 °C using micro-fins with PCM and 12.5 °C using micro-fins with n-PCM as compared to using the micro-fins only.

2000, 2950, and 3750 W/ m2

0.012 m2

Results revealed that at 3750 W/m2, using RT25HC, RT35HC, and RT44HC PCMs attained a maximum reduction in the average front wall temperature associated with the electronic component or CPV cells by about 69.8, 80.44, and 74.44 ºC when compared to that of the system without PCM.

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Current work

About 750 W/m2

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Stropnik 2016 and Stritih [24]

PV cooling. Experiments and transient energy balance were performed in order to improve the performance of photovoltaic cells using pure (White petroleum jelly) and combined PCM (white petroleum jelly, copper, and graphite). PV cooling. Experimental and numerical examination of the effect of RT28HC PCM cooling on the electric performance of a conventional PV panel. CPV cooling. A novel combined passive cooling method for CPV incorporating micro-fins, phase change material and Nanomaterial enhanced PCM was introduced. Electronic and CPV cooling. A fabricated system was attached to a silicon rubber heater to imitate the heat dissipation from electronic devices or CPV cells. Several sets of experiments were performed to investigate the melting and solidification of three different PCMs (RT25HC, RT35HC, and RT44HC) at three distinct heat flux values.

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4. Conclusion

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This study presents an experimental evaluation of the applicability of integrating PCMs with

547

electronic devices and concentrator photovoltaic systems for passive thermal regulation. Several

548

sets of experiments were performed to investigate the melting of three different PCMs (RT25HC,

549

RT35HC, and RT44HC) at three distinct heat flux values of 3750, 2950, and 2000 W/m2.

550

Furthermore, the solidification of RT44HC PCM was inspected at different ambient temperature

551

of 20, 25, and 30 °C. Finally, for the first time, the effect of cavity formation inside solid PCMs

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due to solidification shrinkage on their cooling performance was also investigated. It can be

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concluded that RT35HC PCM achieved a maximum temperature reduction in the operating

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temperature of electronic components or CPV cells, which significantly enhances their

555

performance and saves them from damage caused by excessively high temperatures. In addition,

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rising the ambient temperature prolongs the required time to attain the complete solid phase.

557

Finally, the current experiments showed that air cavities formation inside solid PCMs during the

558

solidification process has no significant effect on their cooling performance. The current findings

559

can open doors for further research towards the development and commercialization of novel

560

passive cooling technology for electronic appliances and CPV systems.

561

Acknowledgments

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The authors would like to thank the Egyptian government specially Ministry of Higher

563

Education (MoHE-Egypt) for providing the financial support to conduct this research at the

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Tokyo Institute of Technology.

565

Appendix A

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Fig. A.1. Transient variation of the system front wall average temperature with and without PCM

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during melting of RT35HC PCM at a heat flux of 3750 W/m2 and an ambient temperature of

569

25 °C.

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Table A.1 The specifications and accuracy of the employed instruments as reported in their datasheets.

Heat flux meter

Volt slider Multichannel temperature recorders

Micro-thermocouples

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Insulation

100 mm × 100 mm 1.5 mm 250 °C 0.5 W/cm2 10 mm × 10 mm 0.4 mm 0.995 µV/(W/m2) 0.00015 °C/(W/m2) ± 3% -200 to 200 °C -50.00 to 50.00 mV ± 0.5% 60,000 data 100 V 0 to 130 V 4 -270 to 1370 °C ± 0.5 °C 0.1 °C 240,000 data/ch. 0.1 s to 30 min K-type Teflon 0.127 mm (Temp. sensor) and 0.32 mm (connector) 0 to 200 °C 30 mm k = 0.026 W/mK

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Heat flux sensor

Specifications Dimensions Thickness Max. temperature Electrical power density Dimensions Thickness Representative sensitivity Thermal resistance Accuracy Temperature range Measurement range Accuracy Recording capacity Input voltage Output voltage Number of channels Measurement range Accuracy Resolution Recording capacity Recording interval Type Covering material Wire diameter Temperature range Thickness Thermal conductivity

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Fig. A.2. Variation of the liquid fraction with

Fig. A.3. Variation of the liquid fraction with

time during melting of RT44HC PCM at

time during melting of RT25HC, RT35HC,

different heat flux values of 2000, 2950, and

and RT44HC PCMs at a heat flux value of

2

3750 W/m2.

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Highlights

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Cooling of electronic components and CPV systems using PCM based heat sinks Heat transfer characteristics during melting and solidification of different types of PCMs PCMs achieved a considerable reduction in the temperature of electronic components and CPV cells The time required to reach the complete solid phase rises with the increase in the ambient temperature Air cavities formation inside solid PCMs has insignificant effect on their cooling performance

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