Numerical modeling and experimental validation of a phase change material-based compact cascade cooling system for enhanced thermal management

https://doi.org/10.1016/j.applthermaleng.2019.114470Get rights and content

Highlights

  • The multi-stage PCM structure facilitates cooling via hierarchical heat exchange.

  • The peak temperature was decreased because of the larger heat capacity and energy circulation.

  • Improved cooling performance was observed in continuous heating–cooling cycles.

  • The PCM nanocomposite can reduce the heat accumulation due to successive thermal loads.

  • The cascade structure allows instantaneous heat absorption of short-term energy output spikes.

Abstract

The thermal performance of phase change material (PCM)-based compact cascade cooling systems with an integrated heat sink was experimentally evaluated using heat-transfer measurements under constant heat flux. Numerical calculations were also performed to investigate the fundamental mechanism of the cascade cooling approach using multiple PCMs (i.e., paraffin wax) with different melting points. This structure facilitated cooling via hierarchical heat exchange without additional energy consumption. The experimental results of the cascade cooling system demonstrated that the peak temperature within a fin decreased from 123.4 to 107.2 °C in one heat-supply cycle owing to the latent heat adsorption during a phase change in the PCMs. Particularly, the cascade cooling system reduced the peak temperature by approximately 13.1% compared with natural convection in air. In addition, the time taken to reach the maximum allowed temperature from the peak temperature was decreased by 45.0% because of the larger heat capacity and cascading heat exchange of PCMs. This implies that the lifetime of a system can be increased and failure can be prevented. Improved thermal performance was demonstrated after repetitive heating–cooling cycles. Furthermore, it was numerically demonstrated that a PCM nanocomposite can reduce the heat accumulation because of the low thermal conductivities of PCMs.

Introduction

Highly integrated, high performance electronic devices demanding more energy must be stable to ensure uniform operating temperature in the system. Devices such as MOSFETs and IGBTs dissipate more heat than conventional devices, and the system can fail if they are not cooled properly. In the case of semiconductors, when the temperature exceeds 85 °C, thermal fatigue rapidly increases, and the life expectancy of devices is remarkably reduced [1], [2]. When the operating temperature of devices exceeds their maximum allowed temperature, the environmental conditions should be controlled quickly to lower the peak temperature to prevent system failure. Various cooling strategies (such as liquid, air, or phase change cooling) can be used to prevent system failure caused by excessive thermal loads. The type of cooling fluid (e.g., air, water, or oil) used for each method is determined by the degree of thermal load [3]. However, they require additional equipment (e.g., a fan or pump) to remove the heat and consume additional energy to circulate fluids. In addition, these additional devices make it difficult to optimize the limited space utilization. Therefore, a new cooling approach must be developed for enhanced cooling efficiency by reducing energy consumption and space optimization, through removal of the auxiliary devices. Recently, investigations of the cooling approach using phase change materials have been conducted to overcome the limit of the cooling system based on the circulation of the cooling media [4], [5], [6], [7].

In particular, because a large amount of thermal energy is released into the air during the charge/discharge processes of an electric vehicle, a cooling system using a PCM was proposed to reduce the maximum thermal load and prevent a local temperature increase in the battery pack [8], [9], [10], [11], [12], [13]. There was also a case study of a PCM-based cooling system for thermally stable operation in the transient state of the electric motor. A PCM with high thermal storability was used to maintain a low operating temperature in a harsh environment, such as in an environment where heat accumulates in an engine room due to insufficient outside air circulation [14], [15], [16], [17]. Small electronic devices require higher cooling performance than large devices, and a hot spot due to excessive local heat can occasionally cause a critical problem during normal operation. Several researchers have explored the application of PCMs to enhance thermal performance by means of latent heat adsorption by the PCM [18], [19], [20], [21], [22], [23]. In particular, the PCM was used to increase the thermal stability of high-power semiconductor devices [24]. These examples illustrate the possibility that a PCM can be used to enhance the thermal performance of electronic devices and replace existing passive cooling methods. In addition, other investigations [25], [26], [27], [28], [29], [30] on the combination between passive cooling devices and PCMs show that the PCM can be used to optimize the cooling performance by integrating the PCM system with a heat sink. Recently, various studies to improve the thermal properties of PCMs have been conducted by means of synthesis with nano-materials [31], [32], [33].

In previous investigations, the multi-stage design was proposed to increase the heat capacity for thermal energy storage. The different PCMs connected in series were installed along the flow path of the heat transfer fluid, and the optimization of their configuration design was the main goal of these studies [34], [35], [36], [37]. However, compact multi-stage PCM cooling systems without physical circulation of cooling media such as air or water among the PCMs need to be developed for wide applications, such as power electronics or electric motors in electric vehicles.

Here, we propose a PCM-based cascade system for effective cooling performance in a more compact manner. First, a comparative study between a single PCM and multi-stage PCMs was experimentally and numerically performed to confirm the thermal energy circulation between PCMs with different melting points. Paraffin waxes were selected as the PCMs used in this study, because they are chemically stable and have tunable temperature ranges for the phase change by changing the carbon content. Second, a parametric study on the use of a multi-stage configuration and the system performance under successive heating/cooling cycles was conducted. Finally, the effect of a PCM nanocomposite (e.g., a mixture of paraffin wax and carbon nanotubes) on the thermal conductivity and heat transfer inside the heat sink was numerically analyzed to increase the applicability of our PCM-based cascade cooling systems.

Section snippets

PCM-based cascade cooling system

The combination of a heat sink and PCMs facilitates cooling, as a large amount of latent heat is adsorbed by the PCMs during a phase change near their melting points. Fig. 1(a) shows the fundamental mechanism of heat removal in a PCM-based cascade cooling system. When the electric/electronic devices are subjected to thermal loads during their operation, the PCM in direct contact with a fin array (i.e., PCMl, where subscript l indicates lower) starts adsorbing heat from the heat source. Once the

Numerical method

A numerical investigation was performed to verify and understand the fundamental heat transport mechanism in a PCM-based cascade cooling system. In particular, a thermal energy transport phenomenon based on convective and conductive heat transfer was analyzed by comparing single and multi-stage PCMs. In the numerical analysis, the heat flux value supplied by a heat source was calculated by assuming the efficiency of a cartridge heater was 0.9 [39]. The convective heat transfer coefficient in

Experimental setup

Fig. 3(a) shows the experimental apparatus used to demonstrate enhanced cooling performance provided by the PCM-based cascade cooling system, and to validate the numerical simulation results. The size of the entire experimental setup was approximately 215.0 × 150.0 × 189.9 mm3 (L × H × D). This apparatus was placed in an environmental chamber made from polycarbonate (10-mm thickness) for thermal insulation. In addition, to minimize heat loss due to thermal resistance resulting from an air gap

Experimental validation

The appropriateness of the numerical models was validated against the experimental temperature values at specific positions without PCMs (i.e., HS, S1, S2, and S3) and the heating time (i.e., 8 h). The steady and transient state simulation results and temperature values measured at each position are shown in Fig. 4. The convective heat transfer coefficient depends on the surface temperature values or geometrical characteristics. However, because the convective heat transfer coefficient in the

Summary and conclusion

The thermal performance of a PCM-based cascade cooling system was numerically and experimentally evaluated based on heat transfer analysis under a constant heat flux. In particular, the numerical model was validated against experimental measurements using a heat sink integrated with cascade cooling materials. In one heating and cooling cycle, it was observed that the peak temperature in a single PCM with a fin decreased from 123.4 to 107.2 °C compared to natural convection in air, which

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgment

This Research was supported by grant (17RTRP-C137546-01) from Railroad Technology Research Program (RTRP) funded by Ministry of Land, Infrastructure and Transport of Korean government. This work was also supported by the 2018 Yeungnam University Research Grant.

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