DNV GL Study Evaluates Fire Risk for Shipboard Battery Systems
DNV GL has released a new study on managing the fire risks involved with lithium ion battery installations, and its findings point to important new safety considerations.
Like diesel engines, batteries come with fire risks, and they require special safety measures to reduce the hazard to the ship and crew in the event of a fire. In its new study, DNV GL examined what happens when lithium-ion battery cells are overheated to the point of failure (thermal runaway), and it evaluated several common methods of avoiding or minimizing harm. The biggest takeaway is that ventilation alone is not enough to prevent an explosion if a very large number of battery modules (totaling 4,000 amp hours or more) fail in the same compartment at once.
"In addition to fire suppression and ventilation, the battery design must have preventative safety barriers so that the 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 advised that:
- With early detection via a purpose-built gas sensor or smoke detector, a problem cell can be disconnected before thermal runaway, halting the process and avoiding a fire.
- When the gases produced in thermal runaway are combusted, the explosion risk goes down substantially. Cells in thermal runaway with visible fire appear to produce much less hazardous gas than cells without any flame - as little as half as much.
- Adequate ventilation is required to reduce the risk of compartment overpressure and explosion from the gas produced in a thermal runaway, but ventilation alone will not be enough if a significant portion of the battery system ignites. If batteries totaling 4000 amp hours (Ah) are failing at once, even ventilation rates of 100 air changes per hour (ACH) will not be enough to avoid an explosion-magnitude overpressure.
- It is critical to take these ventilation requirements into consideration with regard to gas-based fire-suppression systems (CO2 or Novec 1230), which require ventilation shutdown. Shutting off ventilation can increase the toxic and explosive battery gas concentration in the room until ventilation can start again.
- Tested fire suppression systems provide different benefits but no "silver bullet" solution. For all options, early fire detection and early release of a fire suppression medium greatly increase a fixed firefighting system's effectiveness.
- Hi-Fog provides good heat control at the module level in addition to providing full battery space protection from external fires. It also shows good gas absorption and gas temperature reduction capabilities.
- In general, direct injection of fire suppression media into a burning battery module (via a special-purpose fixed firefighting system) is much more effective at heat mitigation compared to external media application. This method had the highest potential for module-to-module fire spread control.
- Conventional sprinklers control heat at the module level, but since water can displace the gas into pockets with high concentrations, the explosion risk 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. Given the toxicity, following a lithium-ion battery fire there should be no re-entry into the space without sufficient PPE.
- Lithium Iron Phosphate (LFP) cathode cells are generally harder to force into thermal runaway compared to Nickel Manganese Cobalt Oxide (NMC) cells. The temperature increase rate is also lower for NMC cells.
The three-year study involved collaboration with a wide variety of stakeholders in government and in industry, including the Norwegian Maritime Authority, Danish Maritime Authority, U.S. 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 full picture as we work to promote safety,” said Denis Cederholm-Larsen, senior ship surveyor at the Danish Maritime Authority.