This article combines its own construction experience and relevant standards and specifications in the industry to discuss the safety aspects of lithium battery energy storage systems, and jointly promote the safe development of the energy storage industry.
Energy storage fire protection systems are mainly used in large-scale and distributed energy storage power stations, mobile energy storage vehicles, and backup power storage stations. Covering the entire industry chain of power generation, transmission and transformation, electricity sales and electricity consumption.
1. Composition of energy storage fire protection system
The energy storage fire protection system is mainly composed of a detection control part and a fire extinguishing part, which can realize automatic detection, alarm and fire extinguishing protection functions for the protective area or battery box.
The detection control part mainly includes detectors, fire control hosts, emergency start and stop buttons, audible and visual alarms, and deflation indicators. Among them, the detector is a dedicated lithium battery fire detector.
The fire extinguishing part mainly includes gas fire extinguishing devices, pipelines, nozzles, etc. Fire extinguishing devices usually use cabinet-type heptafluoropropane fire-extinguishing devices, hanging heptafluoropropane fire-extinguishing devices, etc.
The fire control host is an intelligent automatic fire alarm equipment that combines fire alarm and gas fire extinguishing into one. It uses a Chinese LCD display and is mainly designed for medium and small fire engineering projects. On the one hand, the fire control host receives the sensor data from the detector, makes logical judgments, and automatically and/or manually outputs active and/or passive control signals to start the execution device;
On the other hand, the fire control host transmits status data to the host computer control cabinet, and accepts control instructions from the host computer to execute corresponding fire extinguishing starts or other actions.
2. Analysis of hazard sources of lithium battery energy storage system
Fire hazards in lithium battery energy storage systems are roughly divided into two aspects: out-of-control internal reactions of lithium batteries and fire hazards in electrical equipment. According to fire protection regulations, the location of the battery (hereinafter referred to as the battery compartment) and the location of the high and low voltage electrical equipment (hereinafter referred to as the equipment compartment) are isolated designs.
3.1 The internal reaction of the lithium battery is out of control
- Lithium battery
Lithium battery charging and discharging are mainly completed by chemical reactions. During the charging and discharging process, heat energy will inevitably be generated. If the heat energy generated by the battery itself exceeds the battery’s heat dissipation capacity, the lithium battery cannot be dissipated in time, and the heat will Accumulation causes the battery to overheat.
Chemical reactions occur between the internal materials of the battery, such as the decomposition of the SEI film, the decomposition of the electrolyte, the decomposition of the positive and negative electrodes, etc. During the decomposition, a large amount of heat and gas will be generated, causing the battery to heat up and bulge.
The heat will cause the battery to lose control and the battery will The temperature rises rapidly, causing internal combustion of battery materials, and the decomposition of battery fluid will produce flammable gases. When the concentration of flammable gases reaches a certain level, an explosion will occur when encountering an open flame.
Therefore, the surface phenomena of lithium-ion battery reaction out of control mainly include battery heating, bulging, smoke, open flame, burning, and even explosion.
The above problems are mainly caused by the internal chemical reaction of the battery being too fast and uncontrollable, and external overcharging, puncture, External factors such as fire sources, extrusion, short circuits, etc. will all cause the above problems.
Moreover, the battery module of the lithium battery energy storage system is designed to connect multiple battery packs in series, which undoubtedly increases the safety risks of lithium batteries.
Moreover, once a fire occurs due to unstable performance of a certain battery, it will inevitably affect the surrounding lithium batteries. safety and further expand the scope of the fire.
- Electrical equipment
There are a large number of electrical equipment in the lithium battery energy storage system. The stability and safety of the electrical equipment will also affect the performance and stability of the battery, such as the impact of high voltage, large current and lightning surge due to unexpected operation.
Since there are many weak current systems and communication systems in the lithium battery energy storage system, the high voltage and high current in the system will inevitably have a great impact on the energy storage system.
At the same time, it will also cause damage to the protection components of the energy storage system, making it impossible to proceed. Protective actions can even cause fires.
4. Fire safety measures
Lithium battery fire safety is the key prevention content in the design of the entire energy storage system. The following will discuss the fire safety measures of lithium battery energy storage systems from multiple aspects.
- Fire alarm system
The entire process of spontaneous combustion of lithium batteries due to out-of-control internal chemical reactions caused by the lithium battery itself or stimulated by external conditions will result in the release of gas, energy and the generation of smoke. In severe cases, open flames will occur.
That is, the location where the lithium battery is placed is isolated from other equipment, and special smoke, temperature, light and other detectors are installed. Corresponding gas detectors can be added in a targeted manner according to the characteristics of different lithium batteries.
Ensure that the gas change alarm in the battery compartment is detected early when the battery is out of control, the electrical equipment in the linked equipment room is disconnected from the power supply, and the alarm information is uploaded to the background monitoring personnel.
- Automatic fire extinguishing device
The lithium battery itself contains chemical substances, and there are many electrical equipment connected in the warehouse. Once a fire occurs, if the fire cannot be extinguished in time, the losses caused will become greater and greater as time goes by.
Therefore, the energy storage system configuration The fire extinguishing device must be a physical method that quickly suffocates the fire by isolating oxygen.
5. Fire extinguishing gas
The most widely used fire extinguishing gas in the energy storage system industry is heptafluoropropane.
Heptafluoropropane is a colorless, odorless, non-conductive gas. Its fire extinguishing principle is to inhibit the reaction of the chemical reaction chain. It is stored in a liquid and releases gas. After spraying, the ventilation system can be used to discharge the gas out of the room to extinguish the fire.
There will be no residue left after completion, and it will not cause work loss or pollution to the electrical equipment on site. When installing a hexafluoropropane fire extinguishing device in an energy storage system, you need to pay attention to the following points.
- Discharge concentration
The spray concentration of hexafluoropropane needs to be designed based on the actual protection area. If the fire extinguishing agent is not filled enough, the fire extinguishing concentration in some small protection areas may be too high, while the fire extinguishing concentration in other protection areas is smaller, and the ideal fire extinguishing situation will not be achieved.
In the system design, the filling amount of fire extinguishing agent should be 9% higher than the actual concentration ratio to meet the complete fire extinguishing requirements.
- Fully enclosed space design
Generally speaking, gas fire extinguishing requires a closed space to ensure the fire extinguishing effect of the fire extinguishing agent. During the release process of heptafluoropropane, it is important to note that when operation and maintenance personnel operate in the battery compartment, fires may occur due to unexpected accidents or misoperation. Sometimes, a fire door needs to be set up to ensure the safety of personnel.
Since the liquid state of heptafluoropropane turns into a gaseous state after injection, the indoor pressure will increase rapidly, and a corresponding drain port needs to be set up in the battery compartment. The pressure relief device should be specifically designed based on the pressure that the indoor building or container structure can withstand to prevent cracks in the closed building or container.
- Determination of injection time
Under normal circumstances, if the pre-ignition time of the solid surface of lithium batteries or other equipment is long, a deep fire will form inside it, which is very detrimental to fire safety, although the fire extinguishing device can extinguish the external open flame.
But there is still combustion inside, and when the re-ignition conditions are reached, it will re-ignite. If the pre-ignition time is long enough, dangerous accidents such as explosions may even occur. For this reason, based on relevant regulations and actual operating conditions, it is more reasonable to set the actual injection time of heptafluoropropane to 8 seconds.
- Air conditioning unit
When charging and discharging lithium batteries, it is a process of continuously dissipating heat to the outside world, and the ambient temperature will produce obvious changes in the performance of lithium batteries.
The temperature characteristics of lithium batteries of -40°C to 60°C mean that the capacity of lithium batteries decreases as the temperature decreases. Attenuation is accelerated, and high temperature does not have a great effect on the capacity change of lithium batteries.
Moreover, in high temperature environments, the electrical equipment and batteries in the battery compartment will increase the probability of fire hazards. Therefore, the optimal temperature of the battery compartment of the energy storage power station is 25℃, at this temperature, the battery capacity and safety are optimal.
- Linkage device
Lithium battery energy storage systems usually require daily inspections by operation and maintenance personnel. In order to prevent casualties due to misoperation by operation and maintenance personnel or unexpected accidents, fire doors and exhaust systems must be installed in the battery compartment as well as emergency stop buttons placed outside.
And form a multi-level safety linkage device with fire alarm systems, automatic fire extinguishing devices, fire doors, vents and other fire equipment.
- Electrical Design
For problems such as high voltage and large current that may occur in electrical equipment, install lightning protection and surge protection, communication lightning protection and surge protection equipment and corresponding lightning arresters on the high-voltage side circuit.
During the battery energy storage system design, the lightning protection and grounding facilities of the original building were reasonably used to reduce the risk of high voltage and high current as a whole.
When laying out and designing the energy storage system, it is necessary to consider the convenience of daily inspections of operation and maintenance and the situation of emergencies. There should be no less than 2 entrances and exits in the mobile energy storage equipment area.
A fire lane with a width of at least 3.5m must be reserved around the energy storage system placed on the ground. An escape channel with a width of at least 1.5m must be reserved around the energy storage system placed in the basement, and corresponding firefighting facilities must be provided around it.
In order to ensure the stability of the fire power supply of the lithium battery energy storage system, the power used by the fire protection equipment must be separately connected from the mains, and the cables must be fire-resistant cables. Electrical system supplies power.
6. Summary
As lithium battery energy storage technologies continues to improve, its corresponding lithium battery energy storage systems will also be applied on a large scale. And the innovation of the top BESS factory is driving this process. How to ensure the safety of lithium battery energy storage systems is the first factor affecting development.
As the last passive defense system, fire protection’s reasonable design, material selection, layout and construction directly affect the healthy development of the energy storage industry.
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