With the increasing application of large lithium-ion battery packs in the power grid, fire safety based on lithium-ion battery energy storage systems is becoming increasingly important. Based on the thermal runaway experiment of lithium-ion batteries, a small amount of combustible gas leakage was observed. By analyzing the composition of the leaked gas, CO content and system temperature can be used as the main basis for system warning, and a thermal runaway protection mechanism for lithium-ion batteries can be established.

The lithium-ion battery thermal runaway warning judgment has been added to the energy storage system, and combined with multi-level protection mechanism and safety linkage technology, the overall framework of the fire safety system based on the lithium-ion battery energy storage system has been designed. The components, communication, and personnel safety in the fire safety system have been elaborated. This system can accurately monitor the thermal runaway status of lithium-ion batteries, and can quickly link fire safety devices, significantly improving the safety and stability of the battery energy storage system during operation.

1 Research on identification methods for thermal runaway

The research on the fire safety system of lithium-ion battery energy storage systems is based on the identification of thermal runaway characteristics of lithium-ion batteries. At present, the mainstream identification methods for early thermal runaway at home and abroad mainly include:

① Obtaining key data such as temperature, voltage, and current of the battery through the battery management system BMS for judgment and research

② Method of measuring the pressure on battery modules using strain gauge sensors;

③ Thermal runaway identification method for detecting the internal resistance value of batteries;

③ A method for determining thermal runaway by collecting leaked gas from batteries and analyzing the gas composition and content.

Method ①: Relying on the battery management system, once it hangs up, it loses the ability to identify thermal runaway. Due to the importance of fire protection, an independent fire safety system should be set up as much as possible; Compared to methods ② and ③, gas detection is the most effective early warning method for thermal runaway of batteries, which can provide timely and rapid warning when a single battery experiences thermal runaway. Therefore, the preferred method for analyzing and identifying thermal runaway gases is gas detection.

Thermal runaway gas analysis and research

The leakage of combustible gases and chemical reactions have been accompanied by the entire process of thermal runaway of the battery. The environment in which the batteries in the energy storage system are located is relatively stable under normal circumstances, but once the batteries generate thermal runaway, abnormal parameters such as temperature, gas, and light intensity in the energy storage system will inevitably occur.

The key issue in the safety mechanism of battery thermal runaway is how to accurately extract the gas values in the early stages of thermal runaway. Gas parameters can accurately display the thermal runaway state of the battery and effectively avoid misjudgment caused by the working environment of the battery itself. The early effective warning method for battery thermal runaway is to monitor gas, and the selection of gas detectors is the focus of research.

2 Thermal runaway gas extraction experiment

(1) Heating leads to thermal runaway gas extraction experiment.

1) Scheme design.

Using a pair of 400W heating pads to heat a 150Ah lithium battery, during the heating process, in order to prevent the expansion and deformation of the lithium battery shell from causing insufficient contact between the heating pad and the battery shell, resulting in heat loss of the heating pad and the inability of the battery temperature to reach the thermal runaway standard, which affects the experimental effect, it is necessary to use fixtures to make the battery and heating pad fit together throughout the entire experimental process; Utilize temperature sensors to monitor the real-time temperature of the heating element and battery, and arrange battery thermal runaway monitoring equipment around and on top of the experimental box to monitor and store key parameters such as gas, smoke, and temperature in real-time in the experimental box; In order to have a visual understanding of the thermal runaway state of the battery during the entire experimental process and to control the experimental process, a micro camera was used to record the experiment. When the explosion-proof valve of the battery was observed to open due to high temperature, heating was immediately stopped; Use a sampling pump to discharge enough gas into a safe area, and after the gas cools down and stabilizes, collect and seal the gas.

2) Experimental results.

From the gas concentration data in the experiment, it can be seen that in the early stage of battery heating, due to the temperature not reaching the threshold of the lithium battery pressure relief valve, a smooth increase in gas concentration was detected; When the temperature of the lithium battery reaches the threshold of the pressure relief valve, the gas concentration sharply increases. Through analysis, it can be determined that a certain type of gas concentration value can be used to judge the early state of thermal runaway of lithium batteries. The selected gas type needs to meet the following requirements: low proportion in air, low cost of quantitative detection, and the ability to detect significant changes in the concentration of this type of gas after thermal runaway of the battery occurs

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(2) Overcharging leads to thermal runaway gas extraction experiment.

Experimental plan. This experiment overcharged a 150A • h lithium battery using a constant current charging device. Using the gas collection device shown=, the gas generated at intervals of 10 ℃ between 60~100 ℃ was sampled and collected, and the changes in battery temperature and state were recorded in detail.

(3) Experimental Conclusion

All gas samples collected in this chapter of the experiment were analyzed for gas composition content. From the data, it can be seen that the values of nitrogen and oxygen do not change significantly during the experimental process. Carbon dioxide is one of the main constituent gases in the atmosphere, so these three types of gases do not have significant reference value. The analysis and monitoring cost of olefinic gases is relatively high and not suitable for promotion. Therefore, carbon monoxide gas, which has significant changes in content before and after the experiment and is easy to monitor and analyze, is suitable for early warning of thermal runaway monitoring in lithium-ion batteries. In order to improve alarm accuracy and avoid system false positives and omissions, carbon monoxide content and temperature data are considered in combination.

3 Application Design of Fire Safety System

3.1 Multilevel protection mechanism

Multi level protection refers to the partition detection and protection method for various parts of the battery, specifically from the internal part of the battery, enclosed battery cluster, and battery compartment, with the aim of timely and rapid warning in case of thermal runaway of a single battery. When a lithium-ion battery experiences thermal runaway, it may be accompanied by electrolyte leakage, which can lead to high voltage, insulation failure, indirect electric shock, fire and other hazards in power grid equipment.

The monitoring inside the individual battery pack can alarm as early as possible in case of abnormal battery conditions such as electrolyte leakage and thermal runaway, and can control dangerous situations in a timely manner before thermal diffusion occurs, improving the system’s prevention and warning performance. When a single battery experiences thermal runaway, the detector inside the battery pack can be installed in a fire extinguishing system where thermal diffusion does not form a linkage. For lithium iron phosphate batteries, the initial single fire is easy to extinguish or suppress. Installing and using a detection controller in the battery pack is particularly important for lithium-ion energy storage stations. The fire protection system of energy storage power plants needs to implement a hierarchical warning mechanism, adopt multi-level fire treatment control, reduce the risk of large-scale fire in the energy storage system, and effectively ensure the safety of the energy storage system.

3.2 Detector reference threshold

The gas fire detector selects two parameters: the battery pack meter and the carbon monoxide concentration inside the battery pack for composite detection, to make a comprehensive judgment on the thermal runaway and fire situation of the lithium battery, and to avoid system false alarms and omissions. When the surface temperature of the battery reaches 60 ℃, the gas sensor detects carbon monoxide gas, and the concentration gradually increases, causing the battery to experience slight bulging. Gas detection can balance gas concentration and concentration rise rate, improving detection accuracy.

3.3 Main components of the fire safety system

(1) Control the host. The control host is one of the core components of the fire safety system, responsible for the linkage of the fire safety system, real-time analysis and processing of collected data; Provide at least four types of communication interfaces: Ethernet, controller area network (CAN), RS485 communication network, and dry contact.

(2) Sensors. Sensors are responsible for collecting parameters such as battery temperature, carbon monoxide gas concentration, and smoke concentration, and transmitting the data to the host to make comprehensive judgments on the thermal runaway and fire conditions of lithium batteries.

(3) Alarm facilities. When there is a thermal runaway of the battery or even a fire hazard, the host can timely alert workers through sound and light alarms and gas spraying indicator lights deployed inside and outside the station.

(4) User operated switch. The user operated switches include emergency start/stop and automatic manual state switching switches.

3.4 Fire safety system communication design

The design of communication lines includes the following aspects:

① There must be a line between the fire safety system and the BMS that can communicate in case of a fire;

② There is a communication line between the detection device and the backend display system to display the collected data to the staff;

③ There is a communication line between the station air conditioner and BMS to ensure that the air conditioner is turned off when the fire extinguishing equipment is activated.

3.5 Safety protection measures for staff

(1) Manual automatic mode. The fire safety system can choose manual or automatic mode independently. When the system is in automatic mode, the data obtained by the host determines whether to start the fire extinguishing equipment; When the system is in manual mode, the fire extinguishing equipment is controlled by staff to ensure the safety of maintenance personnel in the station.

(2) Delay start mode of fire extinguishing device. When the fire extinguishing facilities are activated, the delay work will be adjusted appropriately based on the on-site situation, providing necessary time for staff evacuation.

4 Conclusion

On the basis of analyzing the characteristic parameters of thermal runaway of lithium-ion batteries, this article proposes a gas detection and warning device based on the lithium-ion battery energy storage system, and designs a multi-level warning and protection linkage system to ensure that while quickly and accurately detecting the thermal runaway state of lithium-ion batteries, it can be linked with fire extinguishing equipment, significantly improving the safety of the battery energy storage system.