Thermal Analysis and Optimization of Energy Storage Battery Box Based on Air Cooling

For energy storage batteries, thermal management plays an important role in effectively intervening in the safety evolution and reducing the risk of thermal runaway. Because of simple structure, low cost, and high reliability, air cooling is the preferred solution for the thermal management. Based on a 50 MW/100 MW energy storage power station, this paper carries out thermal simulation analysis and research on the problems of aggravated cell inconsistency and high energy consumption caused by the current rough air-cooling design and proposes the optimal air-cooling design scheme of the energy storage battery box, which makes the structure of battery module more reasonable and the temperature distribution between the batteries more uniform. It has a certain guiding significance for energy saving, consumption reduction, and stable operation of energy storage systems.


Introduction
Energy structure transformation is the key path to achieving carbon neutrality, and energy storage plays a significant role in energy transformation [1].Because of the effect on grid security and stability, energy storage technology efficiently increases the power grid's ability to accept clean energy, and have important applications on the power generation side, transmission system, and power consumption side [2][3][4][5].The National Development and Reform Commission and the National Energy Administration pointed out that by 2025, new energy storage will have large-scale development.Five years later, the comprehensive market-oriented development will become true.It is expected that in 2025, 2030, and 2060, the new energy storage capacity is expected to reach about 30 million, 100 million, and 300 million kilowatts respectively [6].As the fast-growing of energy storage, the frequent occurrence of various accidents at home and abroad has sounded the alarm for the industry.According to incomplete statistics from the Zhongguancun Energy Storage Industry Technology Alliance, from 2011 to September 2021, there were over 50 global energy storage safety accidents.The thermal runaway of lithium batteries is the main factor which leads to safety accidents.Perfect thermal management of energy storage system is vital in effectively intervening in the evolution of battery safety characteristics and reducing the risk of thermal runaway [7].
At present, the vast majority of lithium-ion battery thermal management research is oriented to the field of electric vehicles [10], but less research on energy storage systems [8].Compared with the power battery system, the energy storage system gathers more batteries, higher capacity and power, and the existing power battery thermal management research results cannot be fully generalized to energy storage batteries.Air cooling is the earliest and most widely used cooling technology in the energy storage temperature control industry, and it is also the preferred solution for the energy storage system.Henke et al. [9] pointed out that although air cooling is the most common thermal management method for energy storage systems, there is a lack of detailed research on the optimization and performance, as well as comparative study of active and passive air cooling.The design of heat dissipation systems on the market is uneven, and the heat dissipation efficiency and uniformity are also different.For a long time in the future, production developers and users still need to carry out optimization research on the basis of various existing heat dissipation system designs according to actual usage [10].
Based on the system structure of a 50 MW/100 MWh energy storage power station, this paper adopts a modular processing method to establish a lithium-ion battery pack model, uses ANSYS Fluent to perform thermal simulation calculations on the battery module, analyzes and studies the influence of different factors on temperature distribution, optimizes the structure, and proposes a more reasonable thermal design scheme for the battery box.

Model building and meshing
In this study, a module consisting of 14 lithium iron phosphate batteries was established and arranged in a 2×7 longitudinal arrangement.The battery cell in the module is a typical 280 Ah lithium iron phosphate battery, with geometric dimensions of 60 mm×163.1 mm×180.5 mm, a working voltage of 2.5-3.65 V, a standard charging voltage of 3.65 V, and discharge cut-off voltage of 2.5 V. Cold air enters the module from the inlet grille.The fans are arranged on the front panel.The simulation condition is 0.5 C current discharge, and subsequent simulations are carried out under this condition.
The module was simulated using ANSYS Fluent.Structured meshes are suitable for calculations such as fluid and surface stress concentrations.In this study, structured meshing was used, and the resulting mesh is shown in Figure 1, with a 680127 node count and a mesh count of 3.63 million.

Boundary condition setting
In order to simplify the calculation, the following assumptions are made: (1) The module operates under stable conditions, which is regarded as a steady-state process; (2) The air intake is uniform air intake; (3) The battery is simplified to a uniform heating element; (4) There is no energy and material transmission between the surrounding walls of the module and the outside world.
According to the battery size and heat dissipation-related data, it can be calculated that the battery heat dissipation area is 0.1116 m 2 , and the heat flux density of the heat dissipation surface is 81.61W/m 2 .The inlet speed is 3 m/s, the inlet temperature is set to 15°C, the hydraulic diameter is 0.012 m, while the turbulence intensity is 5.36%.The outlet pressure is the ambient pressure.The remaining surfaces are set as walls, which no heat exchange exists.After the Fluent calculation results converge, the initial heat dissipation surface temperature cloud and flow field streamline map can be obtained.

Results and discussion
Under the reference operating conditions, Figure 2 shows the temperature distribution of the module.The temperature of the first few rows of batteries near the air outlet is relatively low overall, and the highest temperature is 341 K, which is located on the inside of the 7 th -row battery.
Figure 2 Module temperature cloud under reference working conditions.

Influence of air inlet position
When the air inlet is located on the lower middle side of the battery box, from the temperature cloud of the battery module we can see that when the wind speed increases from 5 m/s to 6 m/s, although the maximum temperature does not change significantly, the temperature consistency of the first few rows of batteries is improved, and after further increasing the wind speed to 7 m/s, the temperature uniformity of the 7 th -row batteries away from the air outlet is significantly improved.

The effect of changing the position and number of air inlets
Based on the above research, this section changes the air inlet position and number simultaneously to examine the comprehensive impact of them.As shown in Figure 4, when the wind speed is 4 m/s and 5 m/s, the maximum temperature of the battery module is 311 K and 301 K, respectively, which is significantly lower than the benchmark working condition and single-factor action, indicating that the structure of the battery module is more reasonable with the temperature distribution becoming more uniform.The situation is similar when the speed is 6 m/s and 7 m/s.

Conclusion
Based on the system structure of the energy storage power station that has been put into operation, this paper simulates and analyzes the influence of different factors on the distribution of the battery temperature, and realizes the structural optimization of the module.Under the theoretical flow rate condition, the temperature of the first few rows of batteries near the air outlet is relatively low overall, and the maximum temperature is located on the inside of the 7 th -row battery, which can reach 341 K.The position of the air inlet is located on the lower middle side of the battery box, which is more conducive to heat dissipation.Furthermore, on reducing inlet height and increasing inlet number at the same time, the maximum temperature of the battery pack decreases significantly compared with the benchmark working conditions and the single-factor action situation, indicating that the battery module structure is more reasonable and the temperature distribution between the batteries is more uniform.
(a) 4 m/s (b) 5 m/s Figure 4 The effect of changing the position and number of air inlets simultaneously.