New energy power storage, grid side energy storage, large-scale off grid and microgrid energy storage power stations often use containerized energy storage, with tens of thousands of battery cells installed in series/parallel inside the container.
There is only a thin membrane insulation between the positive and negative electrodes of lithium-ion batteries. Electrical isolation mainly relies on insulation materials and electrical switches. Insulation materials may carbonize at high temperatures and become conductive materials. Isolation switches may also break down under high voltage. Power device switching tubes may also conduct abnormally under reverse high voltage and surge impact.
In the long-term thousands of charge and discharge cycles, especially in the state of overcharging and overheating, it is possible to cause short circuit faults and local loss of control in the battery cells. If any one of the battery cells has a safety problem and there is no strict safety protection measure to deal with it in advance, it may cause a chain reaction in the system, resulting in an explosion accident.
Increasing insulation materials and strength, and building a copper and iron wall for energy storage power stations, may solve the safety issues of energy storage power stations, but it will increase the cost of power stations and is not conducive to the large-scale promotion and application of energy storage.
The safety issues of containerized energy storage need to be addressed from multiple aspects such as system design, material selection, and security design, in order to comprehensively balance the two important indicators of safety and cost.
At present, the main safety technologies and measures adopted by the energy storage power station include: new modular energy storage technology, aerogel insulation materials, traditional electrical protection, thermal management and efficient fire safety systems.
1. Modular energy storage technology
The first generation of lithium batteries simply connected battery packs in series into clusters, while the second generation of lithium batteries added some intelligent battery management units on the basis of the first generation of lithium batteries. However, a series of problems such as high DC bus voltage and battery insulation risk, uneven discharge current between clusters, and inability to mix cascaded batteries in lithium battery systems cannot be completely solved, which raises questions about the safe and stable application of lithium batteries.
The new modular energy storage system corresponds to a BMS battery management system for each battery module. It is equipped with multiple functions such as electrical and physical dual isolation, automatic fault module exit, and battery insulation failure warning, ensuring the safety and reliability of lithium batteries. The module adapts to active current sharing, supports the mixing of hierarchical batteries and different brands of batteries, and can be expanded in stages and maintained in minutes, solving many application problems of lithium batteries in one fell swoop.
2. Aerogel
Aerogel is a kind of solid material with nano porous network structure and filled with gaseous dispersion medium in the pores. It is the lightest solid in the world. Aerogel is recognized as the lightest solid material in the world, and it is a new generation of high-efficiency energy-saving thermal insulation material. Aerogel has the characteristics of high flame retardancy, light volume and low consumption, and has become the best choice of thermal insulation materials for power battery cells. At present, it has been adopted by battery enterprises and new energy vehicle manufacturers.
The air gel fireproof and heat insulation materials are used between the electric cores and the upper cover of the module and the PACK. The main security design at the module level is isolation, which means that problem units are "divided and treated" through isolation. This is the insulation and fire prevention design of modules.
Thermal runaway management of module mainly depends on aerogel between single batteries. The aerogel is encapsulated by PET, and its overall thermal conductivity is small, which can well delay the heat transfer between monomers. By isolating individual problematic cells, it can prevent the impact on other monomer cells, thus ensuring the safety of the battery module level.
3. Electrical protection for energy storage power stations
Protection zones for energy storage power stations: The DC side is divided into DC energy storage unit protection zone, DC connection unit protection zone, and convergence zone; The communication side is divided into an AC filter protection zone and a transformer protection zone. There are overlapping parts between adjacent protected areas, ensuring that all electrical equipment is within the protection range.
The division of protected areas is closely related to the configuration of relay protection. On the one hand, the types of electrical equipment in the protected areas are different, and the characteristics of electrical and non electrical quantities after faults occur are also different;
On the other hand, there are significant differences in the coordination between adjacent protected areas depending on the division of the protected areas. Therefore, the configuration and coordination of energy storage power station protection are based on the protection zoning.
DC energy storage unit protection configuration: overvoltage and undervoltage protection, thermal protection and overcurrent protection, voltage and current change rate protection, charging protection; DC connection unit protection configuration: equipped with fuses, low-voltage DC circuit breakers, low-voltage DC isolation switches, and mid span battery protection. For multiple energy storage units, the DC connection units should be connected separately as much as possible to avoid losing more power supply capacity in case of faults;
Bidirectional Converter (PCS) Protection Configuration: Input and Output Side Overvoltage Protection, Overfrequency and Underfrequency Protection, Phase Sequence Detection and Protection, Anti islanding Protection, Overheating Protection, Overload and Short Circuit Protection.
4. Thermal management of lithium batteries
In order to meet the normal use of the battery pack and supporting equipment under the environmental conditions and system operating conditions of the project site, the container is subjected to thermal management control through the following aspects, mainly including air conditioning, thermal management design, insulation layer, etc. The thermal management system ensures that the temperature inside the container can guarantee the normal operation of the battery pack and supporting electrical equipment.
The temperature control scheme inside the container is as follows: the temperature at each set point inside the container is monitored in real time through a temperature probe. When the temperature at the set point is higher than the set starting temperature of the air conditioner, the air conditioner operates the cooling function and cools the inside of the container through a specially designed air duct. When the temperature reaches the lower limit of the set value, the air conditioner stops working.
When the set point temperature is lower than the set starting temperature of the air conditioner, the air conditioner operates the heating function and heats up the interior of the container through a specially designed air duct. When the temperature reaches 15 ℃, the air conditioner stops working.
During the operation of lithium batteries, the presence of internal electrochemical reactions and the increase in ambient temperature can raise the internal temperature of the battery, exacerbating the reactions; In high-altitude regions, the low temperature of the environment can also reduce the reaction speed inside the battery.
The former may lead to thermal runaway, causing premature battery failure and safety issues, while the latter can also reduce the battery's charging and discharging capabilities and efficiency.
5. Container fire safety
Compared to lead-acid batteries, lithium batteries of the same volume have a higher density and store more energy. After detonation and ignition, their flames form a jet shape and the temperature of the ignition source is higher. At the same time, they also release a large amount of toxic and harmful gases, making them a greater safety hazard.
When extinguishing a lithium battery fire, it is important to promptly extinguish any open flames to prevent the fire from spreading rapidly; Secondly, it is necessary to reduce the rate of thermal runaway reaction, so that the heat generated by the thermal runaway reaction inside the lithium battery can be released in an orderly manner; Thirdly, it is necessary to continuously reduce the temperature of lithium batteries to avoid the recurrence and rapid spread of lithium battery fires.
Integrated fire protection devices in containers often adopt a three-level architecture, including early warning, alarm, and action, fire protection system devices, including detection controllers, fire control boxes, sound and light alarm bells/lights, temperature and salt spray sensors, and perfluorohexane gas fire extinguishing devices.
The installation principle of the detection controller should be selected near the battery pack, and based on the actual structure of the rack, the top space on the battery cabinet can be chosen for installation. The fire extinguisher device adopts cabinet type heptafluoropropane fire extinguisher and aerosol fire extinguishing device. Among them, the cabinet style perfluorohexane is installed in the battery room, and the aerosol automatic fire extinguishing series device is installed in the electrical room.
The container is equipped with a perfluorohexane fire protection device. Once the smoke sensor and temperature sensor detect a high-temperature fire fault signal, the container can notify the user through sound and light alarm and remote communication. At the same time, the running lithium battery equipment can be cut off. After 30 seconds, the fire protection device releases perfluorohexane gas to extinguish the fire. A prominent indication is required on the escape door inside the container: please leave the container within 30 seconds after the fire warning signal sounds.
The aerosol automatic fire extinguishing series device is a new type of hot aerosol fire extinguishing device, which is a breakthrough product in the field of fire protection with ultra-high fire extinguishing efficiency and reliability.
When a fire occurs, the automatic fire extinguishing device of the fire hot aerosol triggers the action of the fire extinguishing agent through electric starting or temperature sensing starting, quickly producing a large amount of sub nanometer solid particles and inert gas mixture, which acts on every corner of the fire in a three-dimensional fully submerged manner in the form of high concentration smoke. Through multiple effects of chemical inhibition, physical cooling, and diluted oxygen, the fire is quickly and efficiently extinguished, and it is non-toxic to the environment and personnel.