1 Air cooling and heat dissipation design of industrial and commercial energy storage system

Air cooling is the use of air as a heat exchange medium, the use of air to circulate in the battery pack, the use of the temperature difference between the battery module and the air for heat transfer, generally divided into passive air cooling and active air cooling. The main factors affecting its cooling efficiency are cooling mode, air flow field design, battery arrangement and air inlet wind speed.

1.1 Battery Dimensions and spacing
Battery spacing is a key factor affecting the performance of air cooling. Suitable battery spacing can not only improve the efficiency of air cooling, but also maintain the uniform distribution of battery temperature. The front and back spacing and left and right Angle of adjacent cells in the battery module were converted into 6 independent variables, and the corresponding CFD simulation output (maximum temperature and temperature difference) was used to train the Bayesian neural network, and the optimal battery spacing arrangement was obtained. The study showed that the front and back spacing had less effect on the battery temperature than the left and right spacing. Increasing the spacing from left to right in the middle area of the battery pack can improve the temperature uniformity of the entire battery pack. A parallel air supply optimization strategy that reduces the spacing around cooler cells by increasing the distance between warmer cells. The efficiency of the air cooling system is optimized. Under constant heating power, the maximum temperature of the battery pack is reduced by 0.8K and the maximum temperature difference is reduced by 2.9K (by 42%). This optimization strategy has a significant effect in controlling temperature difference. Under the unsteady heating rate, the maximum temperature difference during 4 C and 5 C discharge is reduced by 39% and 37%(1.5 and 1.8 K), respectively, and both the maximum temperature and the maximum temperature difference are reduced. The influence of different battery spacing and inlet length in the air-cooled system of the storage container is studied. The simulation condition is 1 C current discharge, and the ambient temperature and inlet gas temperature are 25 and 15 ℃. The effects of battery module spacing of 10, 20, 30 mm and air inlet length of 80, 130, 180 mm on the system temperature were compared. The results are shown in Table 2, the best effect is achieved when the spacing is 20 mm and the length of the air inlet is 80 mm. It can be seen that the increase of the spacing of the battery can play a role in the optimization of the system within a certain range, and the effect becomes better with the shortening of the length of the air inlet.

1.2 System design and thermal management strategy

The goal of system design and thermal management strategy is to control the temperature of the battery module in a timely and effective manner, so that the battery can work in a suitable environment. The existing research mainly includes the design of control strategy, the type of air flow field and the optimization of air inlet wind speed, so as to ensure the efficiency of the thermal management system.
(1) System control strategy

Aiming at the thermal management of megawatt container energy storage system, a set of temperature control strategy of energy storage system including air conditioner and fan is designed. The system will control the operation and shutdown of air conditioner and fan according to the real-time battery temperature and ambient temperature. When the ambient temperature is lower than 12 ° C, the air conditioner will heat the battery, and when the temperature is higher than 28 ° C, the air conditioner will cool the battery. When the BTMS detects that the temperature of a BBU is higher than 33 ° C, the fan of the BBU starts independently. When the temperature of the BBU is lower than 31 ° C, the fan of the BBU stops running. The data show that the operating temperature of the battery is kept below 40 ℃ and the temperature consistency is good under the condition of low power rate. A multi-outlet thermal management system is designed for 5×5 battery modules, which is different from the previous one in one out of the outlet layout, and its heat dissipation performance is more effective. In this study, 1 air inlet is located in the top center, and 4 air outlet is located in the lower right corner of the four sides, which has the best cooling effect. Compared with the original model, the maximum temperature, maximum temperature difference, average temperature and temperature standard deviation are reduced by 16.4%, 48.7%, 10.5% and 43.1%, respectively. When the battery is discharged at 3 ° C, the temperature of the battery module can be kept below 40 ° C by providing at least 2 m/s air inlet speed, which can be seen that the strategy ensures that the battery can also run well under large rate conditions.

9 kinds of air flow field design diagram

Temperature field of storage containers under different structures

Maximum temperature and temperature difference of batteries under different air flow fields

(2) System layout design
In the air cooling system, by choosing the right flow pattern, the cooling efficiency can be further improved. The influence of different air flow fields on the temperature of the battery module is studied. It is found that the temperature of the fluid will rise in sequence during the flow process when using serial air supply, resulting in a large temperature difference between the two sides of the battery. The wedge-shaped parallel air supply (Z-type) can effectively ensure the consistency of the battery temperature. The maximum temperature and temperature difference of the battery in the battery pack under 9 different flow field designs with the same air velocity and heat dissipation efficiency were studied, as shown in Figure 2. The results of Table 3 show that the cooling effect of flow field No.3 is the worst, and the corresponding Tmax and ΔTmax are 329.33 K and 8.22 K, respectively. The lowest Tmax (324.91 K) and the lowest ΔTmax (2.09K) appear on the 9th and 7th, respectively. It can be seen that the position of the inlet and outlet have a significant impact on the convection pattern, and different flow paths lead to different air distribution. The higher the air velocity on both sides of the battery, the better the cooling effect, the closer the air velocity in each channel, and the better the temperature consistency of the battery.

(3) Air inlet wind speed

Wind speed is very important for air cooling system, reasonable wind speed can improve the cooling performance of the system, while ensuring low energy consumption. The cooling performance of BTMS at different inlet wind speeds was studied. BTMS with air conditioning, when the ambient temperature > 20 ° C, the inlet air temperature is 20 ° C, when the ambient temperature is equal to 20 ° C, the use of ambient air direct cooling. Research shows that at ambient temperatures of 30 ℃ and 50 ℃, the average temperature and maximum temperature difference of the battery in a complete cycle decrease with the increase of the wind speed. As can be seen from Table 4, when the wind speed is equal to 1 m/s, the battery can maintain a reasonable temperature, and the wind speed continues to increase, but the benefits will gradually decrease, and the energy consumption will increase. Therefore, the selection of wind speed in practical applications should be balanced between the two. The study also found that increasing the wind speed can reduce the operating temperature and maximum temperature difference of the battery, and the capacity loss rate of the battery is also slowed down.

End of cycle temperature at different wind speeds