Innovative hybrid nano/dielectric fluid cooling system for the new cylindrical shaped Li-ion batteries

https://doi.org/10.1016/j.ijthermalsci.2023.108634Get rights and content

Abstract

Nowadays, the cylindrical 21700-type lithium-ion batteries (LIB) are increasingly being implemented to enhance the distance covered by electric vehicles. In parallel, practical battery thermal management systems should be taken into consideration for battery packages during high current rate discharge operations. In the present work, an innovative hybrid nano/dielectric fluid cooling system that included indirect/direct coolant cooling strategies is numerically studied. The mediums for indirect and direct cooling systems are selected Al2O3-water nanofluid and HFE-7100, respectively. In this investigation, the influences of different C-rates (3C, 5C, and 7C) and coolants, different inflow velocities (0.2 m/s, 0.25 m/s, and 0.3 m/s), geometrical changes (include separator plates and corner curvatures), and real driving cycles for the 21700 cylindrical shaped batteries’ thermal performance are widely studied. Additionally, a comparison between 21700 and the new model of LIBs, 4680-types was carried out to comprehend the effects of various energy-related factors on the behavior of the LIB packs in terms of temperature. According to the numerical results, 4%VF Alumina nanofluid with 0.3 m/s inlet flow rate has much better thermal performance compared to the water with 0.2 m/s inlet flow rate as the coolants for the indirect cooling channels. Moreover, results exhibited that using curved cooling channels and integrated separator plates inside the channels reduced maximum and non-homogeneity temperatures of the LIB bundle by 0.61 °C, and 0.16 °C, respectively. Eventually, two different types of LIBs (21700 and 4680) are compared using a similar number of cells and a cooling system. The comparison indicates that not only using a lower number of 4680-format LIB can provide enough energy but also its thermal performance has better results in comparison with 21700-format LIB.

Introduction

In the present age, conserving energy and protecting the environment are highly mandatory due to the abrupt climate change and its effects on natural disasters [1]; thus, power alternatives are broadly proposed for different applications [2]. The most effective strategy for having an eco-friendly environment is developing electric vehicles (EVs) [3]. After several years of studying on discovering a desirable energy storage system for EVs, automotive industries selected various lithium-ion batteries' (LIBs) types for their EVs. The most considerable advantages of LIBs are providing a long driving range and cycle life, fast charging, a high level of safety, and energy density [4,5]. Nonetheless, the critical challenge of LIBs is being highly susceptible to high temperature fluctuations. The dynamic performance of EVs can be influenced due to unacceptable LIB pack temperatures throughout the discharge or charge processes [6]. The standard temperature range for the operation of LIBs is in the middle of −10 °C and 50 °C; however, the range of temperatures that provide the best operating conditions is from 20 °C to 40 °C. Moreover, the thermal non-homogeneity of the LIB package should be kept below 5 °C [7,8]. At high working temperatures of LIBs, some malfunctions emerge in the package such as battery degradation, thermal runaway, and capacity/power fade. A substantial number of LIBs must be taken into consideration to provide enough power for the driving system of EVs. Hence, the limitation of the installation space in the battery pack makes heat aggregation and local hot spots of LIBs the major problems to the LIB pack's safety and energy efficiency [9,10]. Additionally, the heat transfer behavior of the LIB package remarkably relies on the heat distribution of LIBs placed in a pack [11,12]. Thus, Having a well-designed thermal management system (TMS) is imperative for improving the safety of LIBs and ensuring that they operate within an acceptable temperature range throughout the charging and discharging processes [13].
In general, LIB thermal management systems are categorized into three primary strategies such as passive [14,15], active [16,17], and hybrid, which combines various methods [18,19]. The passive TMS method defines as a cooling system that includes no moving mechanical parts, for instance phase change material (PCM) [20] and heat pipe [21]. Contrary, the active TMS always has moving mediums such as air [22] or liquid [23]. Each cooling strategy has its advantages and disadvantages. For example, because of forced convection air cooling strategy privilege, such as operational reliability, manufacturing expenses, and preservation, they are broadly utilized by EV industries [24]. However, in large thermal oscillations and high working temperatures of LIBs in the pack throughout stressful operations, namely fast charging/discharging, the forced convection air cooling method cannot deliver good thermal behavior because of the low thermal conductivity of air. In addition, the LIB pack maximum and non-homogeneity temperatures are reduced by employing PCM for TMSs [25]. Nonetheless, throughout long discharge/charge cycles, PCM only can absorb aggregated heat of LIBs. Hence, the PCM cooling system will fail to transfer the heat into the environment [26]. Therefore, PCM cooling systems usually should be integrated with active thermal cooling systems, including liquid cooling systems to improve the temperature efficiency of LIB packages at the cost of increasing the systems’ weight, expense, and complexity [27].
In the liquid cooling method, the heat transfer coefficient and the specific heat capacity are effective in high heat removal as compared with other cooling systems [28]. Based on the outcomes of many investigations, the thermal behavior of a LIB package using a liquid cooling mechanism extremely depends on several variables such as inflow velocity, density, thermal conductivity, specific heat capacity, and the viscosity [29]. Many EV producers, such as BMW, Tesla, and Chevrolet, have selected the liquid cooling strategy for EVs' battery packs during the past years [30,31]. The liquid cooling system is divided into two main methods, namely indirect and direct systems. In indirect cooling systems using various channels [[30], [31], [32], [33]] and coolants [23,34], mediums should not contact LIBs throughout the charge and discharge processes. Several studies have used an indirect liquid cooling system as a TMS for their LIB pack to facilitate the geometrical design of cooling channels and mediums' features, leading to the better thermal efficiency of LIBs. Tousi et al. [35] investigated a novel indirect AgO nanofluid cooling channel integrated separator plates that enhance the temperature homogeneity of Lithium-ion cylindrical shaped batteries. Their results illustrated that the LIB pack maximum and non-homogeneity temperatures throughout the 7C discharge rate kept below 32.3 °C, and 1.07 °C, respectively. To improve the thermal efficiency of the LIB packs with different numbers of cells, Sarchami et al. [36,37] used two different types of channels (wavy/stair) for their indirect liquid cooling systems. Their results indicated that the LIB bundle's maximal temperature was profoundly diminished by using a stair channel during the charge/discharge operations. Although, indirect cooling strategies are complicated and suffer from leakage problems in the LIB pack.
The direct liquid cooling system that is commonly known as immersion cooling, is another TMS which has been utilized for LIB packages due to its advantages, such as being dielectric, non-flammable, and non-toxic. In order to decrease the thermal resistance between the LIBs and the liquid, the cooling fluid completely or partially submerges the LIBs. Thus, the coolant can efficiently disperse the heat produced by LIBs. The immersion cooling strategy has received much attentions due to advantages such as reducing thermal resistance, optimum LIB pack temperature homogeneity, simplifying the package design, and reducing the complexity of the battery TMS [38,39]. Additionally, thermal runaway is prevented in the immersion cooling method which significantly enhances the level of safety in EVs [28]. Recently, Jithin and Rajesh [17] numerically studied different types of dielectric coolants such as deionized mineral oil, engineered fluid, and water which utilized in the thermal management of LIBs through a liquid immersion cooling system that operates in a single phase. They concluded that the immersion cooling method maintained excellent temperature uniformity during the discharge process. Their results also indicated that using engineer produced fluids which has lower viscosity can be more advantageous compared to mineral oils. A direct cooling system was designed by Patil et al. [40]. As stated by results, the immersion cooling system showed superior thermal behavior compared to the indirect cooling system in the LIB packages. Furthermore, it is essential to achieve an appropriate balance of many aspects, ensuring that the BTMS concurrently satisfies the requirements of temperature management, given the straightforward structural design of this cooling system with the objective of lightweight and material-saving [41,42].
After careful evaluation of different immersion cooling method studies, it is concluded that coolant features are the most important factors in the direct systems to obtain thermal stability and improve cooling efficiency. Based on features of the most used coolants in the immersion method such as being dielectric, low viscosity, non-flammable, non-toxic, high boiling point, availability, high heat capacity, and thermal conductivity, HFE-7100 has been selected for this work. In this numerical study, different effects such as C-rate, geometrical variation, and inflow velocity of cooling channels on the thermal effectiveness of 21700-type LIB bundle are evaluated. Moreover, a real driving cycle condition has been used to study the heat transfer performance of the LIB bundle by creating a novel cooling channel with lightweight and material-saving that included both indirect and direct cooling methods. Finally, to understand the difference between the 21700 and 4680 LIB formats during discharge operation, the maximum and non-homogeneity temperatures of both LIB packages have been widely studied.

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Section snippets

Numerical model

This section explains the numerical simulation of indirect and direct liquid cooling strategies employing Alumina nanofluid and HFE-7100, respectively, in an innovative cooling channel system. Hence, battery pack details, coolant flow, and LIB's governing equations, boundary and initial conditions, and numerical model are presented in the next subsections.

Result and discussion

A complex multi-physics model of a 21700-type Lithium-ion cylindrical shaped battery package with an innovative hybrid dielectric/nanofluid cooling system is proposed for this numerical investigation. Different influences like C-rate, geometrical changes, and inlet flow velocity of channels on the thermal behavior of 21700-type LIB bundle are assessed. Additionally, by proposing an innovative cooling channel that merged both indirect and direct cooling methods, a real driving cycle condition

Conclusion

In the present work, a novel hybrid dielectric/nanofluid cooling system as a combined direct/indirect liquid-cooled thermal management system is proposed. This innovative hybrid strategy improved the LIB packs’ thermal efficiency by using 2% and 4% volume fractions Alumina nanofluids which have a high heat capacity and thermal conductivity. Nonetheless, after careful evaluation of the results, the main issue of this hybrid cooling system was revealed. During the high C-rate operations, the

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.

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