Thermal performance of direct two-phase refrigerant cooling for lithium-ion batteries in electric vehicles

doi:10.1016/j.applthermaleng.2020.115213

Applied Thermal Engineering

Volume 173, 5 June 2020, 115213

https://doi.org/10.1016/j.applthermaleng.2020.115213 Get rights and content

Highlights

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Direct two-phase refrigerant cooling is proposed for batteries in electric vehicles.
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Thermal performance of the refrigerant cooling is measured under vehicle conditions.
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The most effective performance is observed at the vapor quality of 0.85.
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The cell temperature limit of 45 °C is satisfied under harsh environments.
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During aging, the refrigerant cooling provides 16.1% higher battery capacity.

Abstract

The lithium-ion battery has been considered as a power source of electric vehicles (EVs). An efficient battery thermal management system is essential for lithium-ion batteries with high cooling performance and long lifetime. The objective of this study is to investigate the performance improvement of a novel direct two-phase refrigerant cooling over a conventional liquid cooling for traction batteries of EVs. Based on full-scale experiments under actual vehicle conditions, the thermal performance of the two-phase refrigerant cooling is compared with that of the conventional liquid cooling with the same outer size. The two-phase refrigerant cooling satisfies the maximum cell temperature limit of 45 °C even under harsh environmental conditions. During aging, the two-phase refrigerant cooling provides 16.1% higher battery capacity and 15.0% lower internal resistance compared with the liquid cooling under harsh environmental conditions. Overall, the two-phase refrigerant cooling is considered a preferred alternative to the conventional liquid cooling owing to its reliable performance even with a simple and lightweight structure.

Introduction

Recently, lithium-ion batteries, which have high energy densities and power performances, have been recommended as the most suitable energy storage solution for electric vehicles (EVs), including hybrid electric vehicles (HEVs), plug-in HEVs, and battery EVs. However, the operation characteristics of lithium-ion batteries are strongly influenced by the temperature range. During the charging and discharging processes, the heat generation in the battery cells reduces the battery performance and lifetime [1]. Lithium-ion batteries should be maintained within the temperature ranges of 0–45 °C and −10 to 60 °C under charging and discharging conditions, respectively [2]. In addition, as an uneven temperature distribution can cause a localized deterioration of each cell, the temperature distribution between the battery cells should be maintained below 5 °C under various operating conditions [3]. Therefore, an efficient battery thermal management system (BTMS) is required to maintain the allowable temperature range.

In the initial commercialized HEVs, the air cooling system was preferred owing to its low cost, availability, and simplicity. However, the low heat transfer coefficient of the air hinders its adoption as a BTMS for EVs [4]. Thus, the liquid cooling system has been widely used for the BTMS in the recently released EVs. However, liquid cooling has the shortcomings of complex structure, heavy weight, and coolant leakage owing to the additional components required for the liquid circuit [5]. Furthermore, the conventional BTMS, which has been applied in EVs, is inadequate to limit the rise of the maximum cell temperature during fast charging [6]. Phase change material (PCM) cooling and heat pipe cooling have potential as BTMSs for EVs. PCM cooling has the thermal capacity to absorb a large amount of heat through latent heat transfer. However, after completion of the phase change of the PCM, it can no longer perform battery cooling. Meanwhile, heat pipe cooling has a better heat transfer rate than PCM, but it is limited in contact with heat sources, gravity, and passive control [5]. Hence, the BTMSs using the PCM or heat pipe cooling methods have not yet been commercialized in EVs [7].

Two-phase refrigerant cooling has been proposed to overcome the problems in the existing BTMSs. This cooling can effectively remove a large amount of heat from the batteries compared to the existing BTMS, owing to the higher heat transfer rate of two-phase refrigerant. Two-phase refrigerant cooling has been studied primarily for electronic devices, due to the increased heat generation with the miniaturization and multi-functionality of devices [8]. Several studies on the two-phase refrigerant cooling have been performed for applications in polymer electrolyte membrane fuel cells (PEMFCs). Furthermore, the two-phase refrigerant cooling has been regarded as a reliable cooling method for large-scale PEMFCs [9], [10]. However, in the cases of electronic devices and PEMFCs, an additional refrigeration unit, such as a vapor compression system [11] or a pump-driven two-phase flow loop [12], [13], should be installed for the two-phase refrigerant cooling. In the case of EVs, the two-phase refrigerant cooling can be configured simply by expanding the refrigeration unit used in the existing air conditioning (AC) system.

Gao et al. [14] proposed an emergency thermal safety management system coupled with the direct refrigerant cooling that sprays R410A directly into the battery pack. Al-Zareer et al. [15], [16] numerically analyzed a refrigerant-based BTMS. The batteries were submerged into a battery pack filled with R134a liquid. However, these methods encounter problems with a large amount of refrigerant and space limitation due to the installation of the nozzle and refrigerant tank for each battery. Cen et al. [17] proposed a finned-tube heat exchanger with round configuration for a battery cooling module using R134a. The round configuration has inherent shortcomings of thermal contact resistance and low thermal performance [18]. Park et al. [19] simulated the cooling performance of a two-phase refrigerant cooling BTMS using R134a with minichannel heat sinks. They concluded that the two-phase refrigerant cooling exhibited an excellent performance owing to its weak dependence on the ambient temperature.

Although the direct two-phase refrigerant cooling is considered promising for the BTMS in EVs owing to its simplicity, better thermal performance, and lower cost [7], comprehensive studies on the performance of the direct two-phase refrigerant cooling using minichannel modules by full-scale experiments have not yet been reported. Additionally, in the previous analytical studies, the battery cell module assembly (CMA) and cooling module were extremely simplified, which is not applicable to commercial EVs. For practical applications, full-scale experiments for the two-phase refrigerant cooling are required under actual vehicle conditions due to the limited database with measurement difficulties in actual EVs.

In this study, a novel direct two-phase refrigerant cooling method is proposed as an alternative to the conventional BTMS in EVs. The refrigerant cooling module using minichannels was newly designed to solve the problems of the direct refrigerant spray cooling. The minichannels exhibited improved thermal performance compared to the round tubes especially at fast charging conditions. The performance of the two-phase refrigerant cooling was measured by full-scale experiments with the same outer size as the conventional liquid cooling module under actual vehicle conditions [20]. In the two-phase refrigerant cooling, the effects of temperature variations on the cooling channels, cell temperature, and temperature difference between the battery cells were measured and analyzed by varying the charging rate and environmental conditions. Furthermore, the variations in the battery capacity and internal resistance in the battery cells during aging were estimated. Finally, to propose a preferred battery cooling module in EVs, the maximum cell temperature, maximum temperature difference between the battery cells, and heat transfer coefficient were compared between the two-phase refrigerant cooling and conventional liquid cooling under actual vehicle conditions.

This study comprises four sections including the introduction. In Section 2, the configurations of the test setups for the newly proposed direct two-phase refrigerant cooling and commercialized liquid cooling are illustrated. The geometries of the tested cooling modules are also described in detail. In Section 3, the thermal characteristics of the two-phase refrigerant cooling are presented at various battery charging rates and outlet refrigerant states. Furthermore, the cooling performance improvement of the two-phase refrigerant cooling over the conventional liquid cooling is evaluated under actual vehicle conditions. In addition, the variations in the battery capacity and internal resistance of the battery cells during aging are estimated to investigate the effects of the cell temperature profile on the battery lifespan. The last section concludes this study by summarizing all the test results and giving brief insights on each of them.

Section snippets

Battery cooling systems

Fig. 1 shows schematics of the liquid cooling and direct two-phase refrigerant cooling systems for batteries in EVs. As shown in Fig. 1(a), the liquid cooling system comprises a refrigerant loop and a liquid coolant loop. A battery chiller is installed to transfer the heat between the liquid coolant to the refrigerant. The liquid coolant flow rate is controlled by a three-way valve according to the cooling load of the battery pack [5]. Meanwhile, as shown in Fig. 1(b), the two-phase refrigerant

Performance characteristics of the two-phase refrigerant cooling

Fig. 5 illustrates the temperature distribution on the battery cells and cooling channels in the two-phase refrigerant cooling at various outlet vapor qualities and superheats. The refrigerant pressure and battery charging rate were maintained at 380 kPa and 2.0 C, respectively. At the vapor qualities of 0.85 and 0.95, the temperature variations on the channels were substantially lower than those at the superheats of 5 and 10 °C owing to the large temperature fluctuation in the superheat

Conclusion

In this study, the performance improvement of the direct two-phase refrigerant cooling with minichannels blazed on a flat cooling plate over the conventional liquid cooling was experimentally evaluated under actual environmental conditions for traction batteries in EVs. The thermal characteristics of the direct two-phase refrigerant cooling were analyzed under various battery charging rates and outlet refrigerant states. In the direct two-phase refrigerant cooling, the weight of the cooling

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the Energy Technology Development Program (No. 20173010013220) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy, Republic of Korea.

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Thermal performance of direct two-phase refrigerant cooling for lithium-ion batteries in electric vehicles

Highlights

Abstract

Introduction

Section snippets

Battery cooling systems

Performance characteristics of the two-phase refrigerant cooling

Conclusion

Declaration of Competing Interest

Acknowledgment

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