In 2022, my country had approximately 8.2215 million kilowatts of installed capacity for lithium-ion battery energy storage projects. According to the "Action Plan for Carbon Peaking before 2030," if lithium-ion battery energy storage projects maintain their share of new energy storage capacity at 94.5% by 2025, this figure will exceed 28.35 million kilowatts.
Two major pain points of electrochemical energy storage
Currently, cost and safety are the two core pain points restricting the development of lithium battery energy storage.
In terms of cost, according to data from the Jiangsu Energy Storage Association, the cost per kilowatt-hour of lithium-ion battery energy storage systems in my country was approximately RMB 1.66/Wh (0.5C system) in 2022. In the future, with the expansion of lithium-ion battery energy storage system manufacturing scale, technological advancements, and timely material supply, costs will continue to decline.
In other words, in the long run, improving the safety of lithium battery energy storage systems is the most critical constraint on the development of lithium battery energy storage.
Numerous factors influence the safety of energy storage systems. From an external perspective, these factors primarily concern the operation and maintenance of energy storage power stations, including power station monitoring, operation, inspection, abnormal operation, and fault handling. From an internal perspective, the main reasons are the inherent characteristics of lithium-ion battery modules and the incomplete, untimely, and inaccurate data collection by the BMS.
Among these measures, improving the working characteristics of the lithium-ion battery module itself in the energy storage system is the fundamental solution to enhancing the safety of the energy storage system.
Energy storage lithium batteries are usually assembled inside the battery compartment. During charging and discharging, they continuously generate heat, which is difficult to conduct to the outside environment in time. However, the optimal operating temperature of energy storage lithium batteries is 25-40℃. Prolonged operation at high temperatures can have irreversible effects on the working efficiency, performance and lifespan of the energy storage system, or even lead to thermal runaway and safety accidents.
Regarding the cooling problem of energy storage batteries, the market has offered suggestions and solutions. Currently, the mainstream methods for heat dissipation of battery packs are air cooling and liquid cooling.
air cooling
As is well known, there are three ways of heat transfer: heat conduction, heat radiation, and heat convection.
Thermal conduction refers to the phenomenon of heat being transferred from one part of a system to another or from one system to another; thermal convection refers to the process by which the temperature of a liquid or gas tends to be uniform through circulation between the hotter and colder parts; thermal radiation refers to the emission of energy outward by an object due to its own temperature.
The internal structure of the containerized lithium battery energy storage system is as follows: the container is 12 m long, 2.4 m wide, and 2.8 m high. A total of 12 battery clusters are placed in the battery compartment, with 6 clusters on each side. Each battery cluster consists of 15 battery modules.
Taking cooling as an example, the temperature of the battery module increases during charging and discharging. When using air cooling, heat is first transferred to the air inside the chamber through thermal radiation. Then, the air inside the chamber and the air outside the chamber transfer heat through thermal convection, thereby reducing the temperature inside the chamber.
There are two main types of air convection: natural wind and mechanical wind. Air flows into the container from the air inlet, and after heat exchange, it flows out from the air outlet.
Natural wind is less efficient and is rarely used except under specific conditions; mechanical wind, on the other hand, needs to be used in conjunction with an air conditioning system to efficiently regulate air temperature.
Currently, the market focuses on three types of research regarding this heat exchange process: changing the forced air cooling conditions, changing the air duct design, and adding a deflector plate inside the battery compartment.
First, forced air cooling can be achieved in two ways: increasing the heat dissipation surface area of the heat source components and increasing the airflow rate per unit time. The former can be achieved by adding heat sinks to the surface of the battery module, while the latter can be achieved by installing a fan (blower).
Modifying the air duct design requires significant expertise. The air duct includes the main air duct connecting to the air conditioning outlet, the baffles within the main air duct, the air duct outlet, and the baffles at both ends of the battery rack, arranged symmetrically according to the characteristics of the container. Existing air duct designs are generally quite efficient, making further improvements difficult.
The addition of deflectors inside the battery compartment will change the airflow distribution, temperature distribution, and cooling effect depending on the number and location of the deflectors.
Although the focus is different, the three methods have the same goal: to increase the airflow speed in the battery compartment, increase the turbulence range of the heat exchange air in the compartment, and ultimately improve the heat exchange efficiency in the energy storage compartment, so as to keep the battery module temperature within the normal operating temperature.
In summary, air cooling has advantages such as simple principle, convenient installation, and low cost, and is widely used in energy storage scenarios where battery energy density is low and charging and discharging speed is slow.
Liquid cooling
Liquid cooling consists of internal and external circulation. The internal circulation is located inside the battery compartment, while the external circulation is located outside the battery compartment.
In terms of internal circulation, the battery pack is in direct contact with the liquid cooling plate. Heat is transferred from the battery module to the interior of the heat-conducting medium through thermal conduction. The heat-conducting medium flows from inside the compartment to outside the compartment, and heat is transferred from inside the battery compartment to outside the battery compartment through thermal convection.
For external circulation, the temperature of the heat transfer medium can be adjusted to a suitable range using temperature control equipment such as air conditioners.
Liquid cooling and air cooling differ in many ways, such as the different heat transfer media and the different media flow devices.
Regarding the heat transfer medium, air cooling uses air as the heat transfer medium, while liquid cooling uses a wider variety of heat transfer media, including water, ethanol, and refrigerants. The specific heat of air is 1.4 kJ/(kg*K), while the specific heat of water in liquid media is 4.2 kJ/(kg*K), which is three times that of air. Therefore, liquid cooling is more efficient than air cooling.
In terms of the medium flow device, liquid cooling requires a main pipe to run through the energy storage battery system inside the container, and each battery cluster is connected to a thinner branch pipe. Each branch pipe is connected to a liquid cooling plate arranged on each battery PACK. Heat exchange is achieved by changing the type and flow rate of the heat transfer medium inside the liquid cooling plate.
There are many types of liquid cooling plates, including parallel mini-channel cooling plates, serpentine channel structure cooling plates, streamlined channel cooling plates, double-layer reverse channel cooling plates, parallel divergent channel cooling plates, and biomimetic fin-like channel cooling plates, etc. The heat dissipation efficiency varies depending on the type.
Immersion liquid cooling refers to immersing the energy storage battery directly in a special insulating coolant. The heat generated during the charging and discharging process is absorbed by the coolant and then circulated externally for cooling.
In summary, the advantages of liquid cooling are high heat exchange efficiency, while the disadvantages are high initial cost. It is suitable for energy storage environments with high battery energy density, fast charging and discharging speed, and high power requirements.
In conclusion
In the specific deployment of an energy storage system, the choice between air cooling and liquid cooling needs to be considered based on factors such as budget, installed capacity, type of energy storage battery, geographical environment, and required heat exchange efficiency.
From the perspective of the entire energy storage market, the only difference between air cooling and liquid cooling is the level of penetration and suitability.
Currently, in addition to air cooling and liquid cooling, many engineers are researching hybrid modes of phase change materials and liquid or air cooling, but these are not yet mature.
