Analysis of Air-Cooling Battery Thermal Management System for Formula Student Car

Designing a good energy storage system represents the most important chal-lenge for spreading over a large scale of electric mobility. Proper thermal management is critical and guarantees optimum working temperature in a battery pack. In the various battery thermal management technologies, air cooling is one of the most used solutions. The following work analyzes the cooling performance of the air-cooling thermal management system by choosing appro-priate system parameters and analyzes using CFD simulations for accurate thermal modeling. These parameters include the influence of airflow rate and cell spacing on the configuration. The outcome of the simulations is compared using parameters like maximum temperature, and temperature distribution in the battery module to obtain optimum results for further applications. Fi-nally, the simulations of the optimal solution will be compared to experimen-tal results for validation.

Pouch cell-The cells are contained with a soft plastic-aluminum package, with a typically thin foil form factor. Current collectors are welded internally to terminal tabs that protrude through seals to allow the external connection.

Thermal Issues of Li-Ion Batteries
Li-ion batteries produce heat during the charge and discharge operations and high temperature will decay the performance. The temperature of the cells must be strictly maintained within optimum working temperature range 15˚C -45˚C across the entire battery pack. Thermal issues like overheating and uneven temperature distribution can lead to rapid cell degradation and shorten battery life.
In extreme cases, thermal runaway may also occur when the cell temperature rises above the limit, in which this high temperature triggers the exothermic reactions in the operating cells. These reactions release more heat led to an uncontrolled heat generation causes fire or explosion; this incident initiates at about 90˚C when the SEI decomposes. Thus considering, one of the major concerns in the development of lithium-ion battery packs is thermal management. As we discussed above, thermal issues for the battery will give a negative impact on battery performance, lifespan, and safety of the batteries. The thermal management is required to solve these issues,

Thermal Management Technologies
Over the last two decades, many battery thermal management technologies have been developed to maintain the optimum temperature range in the battery packs namely.
1) The Air-cooling method classifies into Natural & Forced Convection; 2) The Liquid-cooling method classifies into Indirect & Direct cooling; 3) Thermal storage method such as Phase Change Material; 4) The Heating method classifies into Self-internal heating, Convective heating, and Mutual Pulse heating.
Compared with the above technologies, the air and liquid cooling are most widely using methods to fulfill the requirements of automotive industries. This work intended to focus on the air-cooling method due to its simple structure, lower manufacturing cost, lighter, easier to maintain, and higher reliability of the system. However, the air has much smaller heat capacity than liquid, and thus a much higher volumetric flow rate is required to achieve the same cooling performance.

Air-Cooling Method
In the Air-cooling battery thermal management solution, the air is the cooling medium and removes the heat generated by the battery cell. This cooling method classified into Passive cooling (Natural convection) and Active cooling (Forced

Battery Pack
The Formula Student electric car is equipped with a storage system placed is in  Table 1 the whole battery pack specifications are reported and in Figure 2 and Figure 3 the cell specifications and performance graphs are reported. In Figure   4 and Figure 5 CAD drawing and photo of the battery pack are presented.

Battery Module
Initially, to perform CFD analysis on the Air-cooling battery thermal management solution method has been studied using ANSYS FLUENT software. Li-ion cells and battery module modelled in workbench. The battery module has 126 Cylindrical Li-ion cells of 18,650 format, positioned linearly in 9 rows of 14 each.
And each battery cell has a diameter of 18 mm and a length of 65 mm.
The thermal properties of single cell are reported in Table 2, where the most important parameters used for CFD thermal model are axial and radial thermal conductivities.

Air-Cooling Methodology
In this battery module configuration, there are two inlets and two outlets in which the air enters and exits through these sections (see Figure 6). When the air enters from the both the inlets, this flow is used to cool down the batteries arranged in series, so its temperature rises due to its low heat capacity, and this leads to higher cell temperatures at the pack outlet.

Heat Generation inside the Batteries
Battery cooling is directly proportional to the heat generated inside the battery.
The most common equation describing heat generation in a battery cell during an electrochemical process (charge or discharge) is given by Equation (1). Journal of Transportation Technologies where q is the heat generation in the battery cell, V o is the open-circuit voltage, V is the cell voltage, I is the applied current and T is the temperature of the cell.
From the Equation (1), the first term of the equation represents the overpotential due to the ohmic losses and charge transfer at the interface. The Second term represents the reversible entropic heat from the reaction [6] [7]. This heat generation can be expressed as overpotential heat Q: Q represents heat generation during both charging and discharging. Express-Journal of Transportation Technologies ing the difference between V and V o by IR. R is the overpotential resistance related to Q [8] [9].
If the heat generation rate per unit time and volume, the Equation (2) represents: where V is the effective calculating volume of the battery module. In Table 3 the single cell heat generation data are collected.

C-Rate
C-Rate is a relative measure performance of the battery [10]. Charge and discharge rates are governed by C-rates. For instance, a 1C discharge rate would deliver the battery rated capacity in 1hr, and 2C discharge rate means it will discharge twice as fast in 30 minutes. In this work, 1.5C-rate and 3C-rate were taken into consideration for simulations ( Figure 7).

Cooling Fan
Active cooling technologies, that rely on an external device (cooling fan) to enhance the heat transfer. The fan can be used to cool down the battery and to maintain the optimum temperature range for the battery. In which, the rate of fluid flow increases the heat removal rate. At the top of the module, there are inlet and outlet sections where we can mount a cooling fan to force the air for cooling. Adding a properly sized cooling fan to this battery module will force air across the module and allow the greater thermal transfer. The following specifications of the cooling fan (Table 4) can match the battery module configuration.

Study
All the simulations are performed in steady state, as the goal has been to com-

Results and Discussion
In order to estimate the impact of cells spacing gap (1.5 mm, 2 mm and 2.5 mm) and flow rate supplied by fan system, the various configurations have been analyzed for 1C-rate and 3C-rate, corresponding to 0.3 W and 1.2 W of heat generation per single cell.
In Figure 8 the temperature maps of the module are reported, for 1C-rate and a set of flow rates; as expected, the hottest cells are in central position, but the difference between hottest and coldest cells (ΔT in the following) do not exceed 4˚C, a good value for an automotive application. At higher flow rates, 5 m/s and 6 m/s, this air cooling is able to perform adequate cells cooling and a good temperatures distribution along the module.
In Figure 9 the same module geometric configuration (1.5 mm of air gap between cells) is simulated at a discharge current equal to 3C-rate, for a new set of fan flow rates, starting from 6 m/s (in order to compare the results with 1C-rate case) to 11 m/s.
In this case, the ΔT rise noticeably and reaches the value of 9˚C for the case with an air flow rate of 6 m/s; with an air flow rate of 11 m/s the ΔT decrease to a value of 6˚C, an optimal value for automotive applications. The 3C-rate is equivalent to 28 kW of power supplied by the whole battery pack to the electric drive, that is a overall power higher than the average power utilized during "Endurance" Formula Student event. Thus, this case represents a severe thermal test of the cooling system. Obviously, the ambient temperature must be considered for future applications. In Figure 10 the case of 2 mm of air gap between cells is simulated and reported for 1C-rate case. The increased air gap reduces the air speed in the stream line between cells and, as consequence; the heat removal is less efficient, without any advantage in air flow friction reduction. Similar consideration can be made by considering Figure 11, where the 3C-rate case with 2 mm air gap is reported.
Thus, it is evident that, in the case with 2 mm air gap, ΔT and number of cells at higher temperatures increase.
In Figure 12 and Figure 13 the results of analysis for 2.5 mm air gap configuration are reported, and it is evident the confirmation of a sensible reduction of heat removal efficiency.
In general for all the configuration and C-rate considered, it can be seen in the module, that the cell temperature is low near the inlet, and the cell temperature Journal of Transportation Technologies  is high in the middle area near to the outlet. It is in the situation when the air flows through the battery surface, which has a higher temperature, it will absorb heat from the cells leading to a gradual increase in the temperature of air from the inlet to outlet. But when we observe the cells at outlet, the cell temperature is little bit low compare with the temperature in the middle area, because it depends on the module layout where both the outlets are placed side by side, in which the air is distributed evenly coming from the two paths absorbs the cell heat at the outlet to reduce the temperature.

Influence of Air Flowrate
As reported in Figure 14 and Figure 15, as the increasing air flowrate, the convective heat transfer in the module will be increased, and then the cell temperatures are going to be reduced. From the figures above, the maximum temperature in Journal of Transportation Technologies

Influence of Spacing between the Cells
It is seen that the maximum temperature in the module is rapidly increasing as the gap increases between the cells, which means the thermal performance is deteriorated. As reported in Figure 16, ΔT in the module of 1.5C-rate is slowly decreasing as the gap increases, but it is not the same case when the heat Journal of Transportation Technologies

Conclusions
The main objective of this thesis work is to study the cooling performance of the air-cooling battery thermal management system specific to 18,650 Li-ion cells.
Typical cases are employed on the battery configuration to explore the influences of increasing space between the cells with different flowrates. The comparison of all the simulations at 1.5C -rate shows that the configuration of 1.5mm spacing with the velocity of 6m/s is giving the better result of maximum temperature is 23.7˚C and the maximum temperature difference in the module is 2.5˚C (see Figure 17).

Future Work
In this module, we changed the paths of inlet & outlet like inverted the flow for 1.5C & 3C rate module with the optimal velocity of 6 & 11 m/s. This approach reduced the maximum temperature and temperature difference in the module compare with the previous normal flowrate modules (see Figure 18). This approach can also be used for future work for better results.