DNV GL has published a new study on fire risk management from lithium-ion battery installations, and its findings highlight important new safety considerations.
Like diesel engines, batteries present a fire hazard and require special safety measures to reduce the risk to the ship and crew in the event of a fire. In its new study, DNV GL looked at what happens when lithium-ion battery cells are overheated to the point of failure (thermal runaway), and evaluated several common methods to avoid or minimize damage. The most important thing to remember is that ventilation alone is not sufficient to prevent an explosion if a very large number of battery modules (totaling 4000 amp hours or more) fail in the same compartment at a time.
“In addition to fire suppression and ventilation, the battery design should include preventive safety barriers so that fire and gas emissions are limited to as small a part of the battery system as possible,” said Henrik Helgesen, Project Manager for the Research Project and Senior Consultant at DNV GL.
Among other findings, DNV GL indicated that:
– With early detection via a gas sensor or a specially designed smoke detector, a problematic cell can be disconnected before thermal runaway, stopping the process and preventing a fire.
– When the gases produced in thermal runaway are burned, the risk of explosion decreases significantly. Thermally runaway cells with visible fire appear to produce much less dangerous gas than cells without flame – barely half as much.
– Adequate ventilation is necessary to reduce the risk of compartment overpressure and explosion of gas produced during thermal runaway, but ventilation alone will not be sufficient if a significant part of the battery system ignites. If batteries totaling 4000 ampere-hours (Ah) fail at the same time, even ventilation rates of 100 air changes per hour (ACH) will not be enough to avoid explosively large overpressure.
– It is essential to take into account these ventilation requirements with regard to gas-based fire extinguishing systems (CO2 or Novec 1230), which require the ventilation to be stopped. Stopping ventilation may increase the concentration of poisonous and explosive battery gas in the room until ventilation can resume.
– The fire extinguishing systems tested offer various advantages but no quick fixes. For all options, the early detection of fires and the early release of a fire extinguishing medium greatly increases the effectiveness of a stationary fire fighting system.
– Hi-Fog offers good heat control at the module level in addition to providing complete protection of the battery space against external fires. It also shows good gas absorption and gas temperature reduction capabilities.
– In general, direct injection of fire extinguishing media into a burning battery module (via a fixed special-purpose firefighting system) is much more effective at mitigating heat than the application of external media. This method had the highest potential for controlling the spread of fire from module to module.
– Conventional sprinklers control heat at the module level, but since water can displace gas in pockets at high concentrations, the risk of explosion is considered to become more severe with sprinklers.
– Toxic gases produced in a battery fire include carbon monoxide, nitrogen dioxide, hydrogen chloride, hydrogen fluoride, hydrogen cyanide, benzene and toluene – comparable to burning plastic. Considering the toxicity, following a lithium-ion battery fire, there should be no re-entry into space without sufficient PPE.
– Lithium Iron Phosphate (LFP) cathode cells are generally more difficult to force into thermal runaway compared to Nickel Manganese Cobalt Oxide (NMC) cells. The rate of temperature increase is also lower for NMC cells.
The three-year study involved collaboration with a wide variety of stakeholders in government and industry, including the Norwegian Maritime Authority, Danish Maritime Authority, United States Maritime Administration (MARAD), Corvus Energy, Kongsberg, ABB, Stena, Scandlines and Damen, among others.
“It is very important for us to work closely with all parts of the industry and understand the big picture as we work to promote safety,” said Denis Cederholm-Larsen, Senior Marine Surveyor at the Danish Maritime Authority.