Lithium-ion batteries offer high energy density in a small space. That makes them highly suitable for stationary electrical energy storage systems, which, in the wake of the energy transition, are being installed in more and more buildings and infrastructures. However, these positive characteristics have unique fire risks. This challenge can be addressed effectively by means of an application-specific fire protection concept for stationary lithium-ion battery energy storage systems, such as the one developed by Siemens through extensive testing. It is the first of its kind to receive VdS approval.
Each lithium-ion battery cell consists of two electrodes: a negative anode and a positive cathode. They are kept apart by a separator. Another essential component is the ion-conducting electrolyte.
However, this functional principle, while successful and generally safe, has designrelated risks. The battery cells are characterized by the presence of a large amount of chemical energy in a small space and a very small distance between the electrodes (separator layer typically ≈ 30 µm). At the same time, the electrolytes used are typically combustible or highly flammable.
For this reason, a battery management system (BMS) not only controls and monitors the state of charge at the cell and system level but also manages the temperature during charging and discharging. This ensures that the cells are kept within the operating range defined as safe.
Thermal runaway as a hazard scenario
Exceeding the safe temperature range can result in what is called “thermal runaway.” When this occurs, the energy stored in the battery is suddenly released, and within milliseconds the temperature rises to many hundred degrees. As a result, the electrolyte ignites or electrolytic gas explodes.
During a thermal runaway event, the electrolyte successively evaporates as the temperature climbs. This causes the pressure inside the cell to increase until the electrolyte vapors are released through a relief valve or a bursting cell wall. Without countermeasures, this results in an explosive gas-air mixture. All it then takes is an ignition source to cause an explosive combustion. In addition, a thermal runaway event in a battery system can spread from cell to cell, leading to a major fire.
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