Analysis of cooling technology of power battery of new energy vehicles

To effectively deal with the energy crisis, air pollution and other problems, various countries and regions have increasingly strict emission standards for automobile exhaust. New energy vehicles have been produced and vigorously developed. The power battery is a vital part of new energy vehicles, and the battery’s operating temperature needs to be precisely controlled to achieve the smooth functioning of new energy vehicles. This paper will analyze the current application status, principles and application scenarios of different cooling technologies for power batteries of new energy vehicles by examining the characteristics of various cooling technologies, contrasting their cooling capacities, summarizing their corresponding ways of improvement, and identifying the development trend. Despite the extensive use of air cooling systems in the past decades, it has been gradually replaced by liquid cooling in recent years, and It has been discovered that heat pipe cooling works greatest when the starting temperature is constant. However, the heat pipe cooling method is still in development and has not been applied to real vehicles. Therefore, heat pipe cooling may become the most potentially efficient cooling method in the future.


Introduction
Due of the nation's economy's rapid growth, there has been the problem of energy shortage and fuel depletion.The key to solving the problem is to develop new energy sources-powered vehicles as an alternative.Consumers prefer electric vehicles to traditional fuel vehicles due to their advantages, such as zero-emission and low noise.New energy vehicles also can actively respond to national policies and reduce greenhouse gas emissions.
The power battery is the power supply source of the new energy vehicles.Due of lithium-ion batteries'(LIB) high specific energy, LIB outperform other batteries in cycle life and battery life.Therefore LIB are frequently applied in electric vehicles.The temperature can affect the activity of lithium ions, further affecting the life and performance of LIB and even threatening the driver's safety.A battery thermal management system (BTMS) can direct the battery to operate at the proper temperature and save battery life [1].Under driving conditions, the battery pack emits heat and increases the temperature.If the heat is too intense, the temperature rises steadily, shortening the battery's lifespan and interfering with the operation of vehicles.The four primary categories of cooling technology at the moment are air cooling (AC), liquid cooling (LC), heat pipe cooling (HPC), and phase change material cooling (PCMC).Tang et al. changed the shape of the cooling plate, and the cooling impact of LC BTMS was enhanced [2].Lu et al. created a Tesla valve-type cooling plate for rectangular LIB, which has better heat dissipation effect than a Z-type channel [3].
This paper first introduces the main application status of power batteries in new energy vehicles.Secondly, the paper explains how the cooling system works on the battery.Thirdly, four cooling technologies' application scenarios and current research progress are analyzed and compared, namely, AC technology, LC technology, HPC technology, and PCMC technology.Further, the paper compares the cooling capabilities of the different technologies and describes their characteristics.Finally, improvement methods corresponding to different technologies are proposed at the end.This paper hopes to provide knowledge for better management of batteries, extending the lifetime of batteries, creating income for the enterprise and ensuring driving safety.

Frequency multiplexing
Ternary lithium battery (TLB) is ternary polymer lithium battery [4].Its pack energy density is around 200Wh per kg, while the individual energy density is approximately 250Wh per kg [5].The battery has a high energy density and can support new energy vehicles for a specific period.It can also release almost 75% of its capacity in a -20 °C environment [5].

Lithium iron phosphate battery
Lithium iron phosphate battery (LIPB) is a kind of LIB with lithium iron phosphate as cathode material [8].During charging, lithium ions enter the electrolyte from the crystal of lithium iron phosphate, proceed through the diaphragm, then embed in the grid of graphite.Following the removal of lithium ions from lithium iron phosphate, Iron phosphate is produced from lithium iron phosphate.During discharging, lithium ions are removed from the graphite crystal, then back into the lattice of lithium iron phosphate [9].
LIPB has benefits of being inexpensive and having a long lifespan [8].Due to its high safety, LIPB is mainly used in larger vehicles.Nowadays, the LIPB is used in BAIC EV160, JAC iEV4, SAIC E50, BYD e6, Tengshi and some other vehicles [6]. Figure 2 shows the charging mechanism of the LIPB. Figure 3 shows the discharging mechanism of the LIPB.

Air cooling
The principle of AC technology is that air enters from the air inlet of the vehicle.Then flows into the battery cooling channel to achieve the heat emission.Take Z-type BTMS as an example to explain.
There are eight lithium-ion batteries in the center.The air enters the channel's entrance and rises through the batteries.Finally, the air is released from the channel's outlet.This process can achieve the purpose of heat dissipation.Figure 4 is a Z-type BTMS structure diagram.Figure 5 is the basic schematic diagram of air cooling.

Liquid cooling
There are direct and indirect versions of the LC system, but both use coolant as the medium.A direct contact cooling system dissipates heat through circulation by submerging the batteries directly in the coolant.For the indirect cooling system, add a cooling plate around the battery [12].Heat is also lost when the liquid moves through the flow channel.The LC structure is displayed in Figure 6.

Heat pipe cooling
The phase change is the foundation of the HPC system and allows for heat conduction.There are three parts to the heat pipe: condensation part, evaporation part and thermal insulation part.Typically, the heat source is connected to the evaporation section [12].The medium in the sealed pipe absorbs heat when passing through the evaporation section, and then emits heat when passing through the condensation section [14].This procedure has the potential to reduce the temperature.Figure 8 shows the structure of HPC, and Figure 9 displays the working principle of HPC.

Phase change material cooling
The working principle of the PCMC technology is to judge the current temperature through the phasechange material.When the material varies within a specific temperature range, the state of the material changes [16].During this change process, the heat is absorbed and released to accomplish the goal of cooling [12].Figure 10 shows the cooling structure model of phase change materials, and Figure 11 shows the cooling schematic diagram of phase change materials.

Common cooling forms
The two main categories of cooling methods are forced AC and passive AC [18].The vehicle generates airflow when driving.Passive air cooling uses this airflow to dissipate heat, while forced air cooling increases the flow rate by adding fans [18].The heat dissipation in the forced air cooling mode is more adequate than it in passive air cooling.Automobiles with small battery capacities currently use aircooling technologies [18].For example, the early Nissan Leaf, Kia Soul Ev [11], as well as the current Wuling Hongguang MINI, Toyota Prius, Euler Black Cat, Nezha, and other models all use air cooling technology [18].Li et al. studied an AC BTMS including 32 cylindrical LIB and reduced the maximum temperature from 314.58K to 314.22K by replacing the model [1].At present, the cooling medium used in LC technology is mainly a mixture of ethanol and water [19].Furthermore, a cooling method that is currently occupying a prominent trend is LC technology.For example, the Volt of General Motors, the Model X of Tesla [19], the Model S of Tesla, the MENLO of Chevrolet [13], the BMW i3, the Geely Emgrand EV [11] and other models all adopt LC.Wu et al. created a novel kind of LC LIB method [20].The immersion liquid is silicone oil [20].Through simulation, the highest temperature increase and the greatest temperature variations of the direct contact LC method are 20% -30% of the peak temperature increase and the greatest temperature variations of the indirect contact LC method [20].Ma et al. studied the nonlinear method of liquid cooling for vehicle power batteries [13].Through simulation, under the combined condition of NEDC-HWFET-US06, compared with the PID method, the maximum deviation between the battery temperature and the optimum temperature was reduced by 0.8K, and the energy consumption was reduced by 6.3% under the nonlinear cooling optimization [13].
Although heat pipe cooling has a higher effectiveness, it is more expensive and has a complicated design [14], and mass production and commercialization remain a problem [18].Nonetheless, with the advancement of technologies, heat pipe cooling would be more appropriate for practical applications.He et al. created a BTMS that incorporates a heat pipe and a liquid-cooled plate [21].Through numerical simulation, it was found that when the height of the aluminum plate is greater than 50mm, the thickness is greater than 2mm, the coverage angle with the battery is greater than 75°, and the coverage angle with the heat pipe is greater than 60°, the battery can work within the normal temperature range [21].Gan et al. used cylindrical battery packs and designed a heat dissipation structure combining heat pipes and heat conduction elements [15].Through simulation, it was shown that the greater the circumferential angle between the heat conduction elements and the battery surface, the better the effect of heat dissipation of the battery.However, the result has little effect when the circumferential angle is greater than 95 ° [15].
The key to PCMC technology is the selection of phase-change materials.Paraffin is usually selected because of its relatively suitable temperature range, low toxicity and high latent heat of phase change [17].Paraffin's limited thermal conductivity is another drawback, too.Compound cooling is the prevailing trend in PCMC.Give examples, the thermal management system of PCMC coupled with AC, PCMC coupled with LC, and PCMC coupled with HPC is also being studied.At present, this technology is still in the experimental stage and has not gained popularity. Yang et al. carried out experiments on square LIB made of phase change materials and adopted the composite method of phase change materials and aluminum flat tubes with micro-pass tubes [22].The outcomes demonstrate the composite's superior cooling capability.

Comparison of cooling capacity
Table 1 compares the cooling capacities of various cooling techniques.There are distinct features for every technology.Although air cooling technology is widely used, its thermal conductivity is low, and temperature uniformity cannot be reasonably controlled [14].Therefore it cannot be applied to highpower batteries [18].The most popular cooling method employed is liquid cooling.[12].It benefits from a high heat transfer coefficient, extensive covering, and greater temperature uniformity [18].However, it has drawbacks of complex structure and liquid leaks.The heat pipe cooling method offers a powerful thermal conductivity and cooling effect but is expensive.It takes up much space and could harm the battery's construction [18].Thus, heat pipe cooling technology has not been popularized in new energy vehicles.Further, the selection of materials is where the phase change material cooling technique is most challenging.The focus of materials today is primarily on organic phase change materials, yet these materials have low heat conductivity.
Huang et al. carried out experiments on three heat dissipation modes [23].Specifically, no heat management system, AC and HPC.Compared with no cooling method, the temperature of the battery based on AC decreased by 12.9%, and the temperature of the battery based on HPC decreased by 28.0% [23].Table 2 shows the temperature comparison of batteries with different cooling methods.
To sum up, the cooling effect comparison results are HPC > LC, PCMC > AC.But from the perspective of practicality, the results are LC, AC > PCMC, HPC.

Improvement of cooling technology
Table 3 shows the corresponding improvement methods and effects of different cooling technologies.The effect of air cooling can be varied by altering the distance between batteries, the location, and the quantity of inlets and outlets.The effect of liquid cooling can be changed by changing the medium.
Adding thermally conductive elements to the heat pipe can change the capacity of the HPC or use the specific current discharge to improve the heat dissipation effect.Paraffin and graphite can enhance the cooling capacity of PCMC technology.Alternatively, phase change material can be made into doublelayer versions by modifying the structure, which allows good control of thermal management, no matter the highest temperature or widest temperature range.

Conclusion
At present, the cooling technology of power batteries is widely used in liquid cooling and air cooling, and liquid cooling is the mainstream technology.Although the cooling impact is better than that of air cooling, there are still some disadvantages, such as the limitation of the cooling medium, which can not further improve the cooling effect.The current trend of phase-change material cooling technology combines with other forms to produce a better cooling effect than pure phase-change material.Finding more suitable materials and composite methods and promoting commercial use is also necessary.The heat pipe cooling effect is the highest when the initial temperature is the same.However, how to overcome the shortcomings of high cost and cumbersome layout, which can be applied to the real vehicle, need to be the focus of research in later studies.If heat pipe cooling technology is practical, the cooling of the power battery of new energy vehicles would rise to a new level.
Figure 7 displays the basic logical representation of the indirect cooling system.

Table 3 .
Improvement methods and effects of different cooling technologies.