Mitigating the Hazards of Lithium-Ion Battery Energy Storage Systems (BESS)
Battery Energy Storage Systems (BESS) use a layout of batteries and other electrical devices to store electrical energy. These installations are increasingly used in residential, commercial, industrial, and utility applications for peaking or grid support, ranging from large outdoor and indoor sites (e.g., warehouse-type buildings) to modular systems. Containerized systems, a form of modular design, have become a popular means of efficiently integrating BESS projects.
Due to their fast response times, Li-Ion BESSes can be used to stabilize the grid, regulate grid frequency, provide emergency power or industrial-scale peaking services, and reduce the cost of power for end users. However, high power and fast charging cycles can stress batteries and cause them to degrade over time, which is not conducive to safety.
Over the past four years, more than three dozen large-scale battery energy storage systems have failed around the world, causing fires and, in some cases, explosions. With these issues in mind, professionals and authorities need to develop and implement strategies to prevent and mitigate BESS fire and explosion hazards.The guidelines provided in NFPA 855 (Standard for the Installation of Energy Storage Systems) and the International Fire Code Chapter 1207 (Electrical Energy Storage Systems) are the first step.
Prevention and mitigation measures should address thermal runaway, which is by far the most serious BESS failure mode. If thermal runaway cannot be stopped, the most serious consequences are fire and explosion.
Thermal runaway in Li-ion battery cells is essentially the primary cause of fire or explosion in Li-ion BESS. In a variety of situations that lead to short circuits, batteries may undergo thermal runaway, in which stored chemical energy is converted to heat. If the process cannot be cooled sufficiently, temperatures can rise, which can exacerbate the reaction, leading to battery rupture and the release of flammable and toxic gases. The most common triggering events for thermal runaway include:
Battery manufacturing defects
Overcharging (e.g. inverter failure)
Overheating (e.g. poor cooling capacity or cooling system failure)
Mechanical abuse (e.g., seismic events or impacts)
Battery Management System as a Barrier to Thermal Runaway
One of the most important barriers in a battery energy storage system is the battery management system (BMS), which provides primary thermal runaway protection by ensuring that the battery system operates within safe parameters (e.g., state of charge, temperature). In a UL 9540 certified BESS, the BMS monitors, controls, and optimizes the performance of the battery module and disconnects it from the system in the event of an abnormal condition.The BMS also provides battery charge and discharge management.
The BMS will alarm and limit the charging and discharging current or power in the event of undervoltage, overvoltage, overtemperature, or overcurrent conditions. In the event of an emergency, the BMS will stop operation and disconnect the electrical connections to each battery enclosure. This assumes that the BMS is not damaged and is operating normally. However, if an internal cell breakdown occurs, the BMS will not be able to prevent thermal runaway.
Explosion Control
Thermal runaway leading to fire or explosion is the most serious hazard to prevent or mitigate. While some guidance on fire control and suppression already exists, many BESS manufacturers, integrators, and end-users struggle with explosion control requirements. Explosion control can be accomplished by providing one of the following:
Explosion protection systems designed, installed, operated, maintained, and tested in accordance with NFPA 69 (Standard on Explosion Protection Systems)
Installation and maintenance of deflagration ventilators in accordance with NFPA 68 (Standard on Explosion Protection for Deflagration Ventilators)
If an explosion-proof system is implemented in accordance with NFPA 69, combustible concentrations should be maintained at or below 25% of the LFL for all foreseeable changes in operating conditions and material loading. One option for accomplishing these requirements is ventilation or air dilution. This can be accomplished by installing a forced ventilation system that can be automatically activated by a gas detection system when the gas concentration level exceeds a predetermined set point.
In addition, deflagration venting creates a pathway for rapidly expanding gases to exit the enclosure in the event of a deflagration. Protecting the BESS enclosure can be challenging in situations where there is little free air and a high level of internal obstruction. In this case, performance-based engineering methods such as computational fluid dynamics (CFD) may be required.
