Each year there are more and more hybrid or fully electric ships navigating waters worldwide, including ferries transporting thousands of people daily. These ships increasingly rely on lithium energy storage as their power source, with modern designs containing over 1,000 individual modules (batteries).
Safety concerns still linger and should be kept at the utmost of considerations for this new technology.
One of the biggest risks for batteries is thermal runaway. Thermal runaway occurs if the lithium-ion cells used in marine batteries are subjected to mechanical abuse, suffer from internal manufacturing defects, or operate over or under the correct voltage or temperature. It means that heat is generated within the lithium-ion cells and causes a reaction between the cathode material and electrolyte. This can result in the cells’ temperature increasing until the cell vents toxic and flammable gasses. If ignition occurs, these gasses can create an unpredictable fire, which can be very difficult to extinguish.
The minimum requirement by the Norwegian Maritime Authority (NMA) for batteries used in commercial vessels in Norway is the Propagation Test Type 1. This is intended to prevent propagation of the thermal event from one module to the next. This test simply means that if a cell in a single module enters thermal runaway and ignites, fire will consume the module but will not ignite the other modules in the pack, and thus the larger system remains safe.
Approval is granted when a single module in a battery pack is tested in a lab situation by putting it into thermal runaway and the adjacent modules in the pack do not ignite. However, in the event of an overcharge situation where it is most likely a fault of charging software, or in event of catastrophic mechanical damage, the likelihood that only one cell or module in the pack be affected by itself is extremely unlikely.
It is much more probable that a) the entire system was damaged or b) any number of individual modules were damaged. In a multi module event, it is my assertion that NMA Propagation Test Type 1 may not prevent propagation from module to module. In my opinion, this renders Propagation Test Type 1 an ineffective method to ensure the safety of the vessel.
Currently there are many battery solutions on the market that use an air cooling system in to try to maintain safe internal temperatures. The effectiveness is questionable and the reliance on a thin-layer fire-resistant separator between cells only reduces the fire risk from thermal runaway - it does not prevent it. Almost all these manufacturers claim this “inherent” and “passive” system prevents propagation from one module to the next. This is the minimum requirement by the NMA Test Type 1 for batteries used in commercial vessels in Norway.
I assert that adherence to this standard alone endangers the vessel, crew, passengers, cargo and environment. It is far more sensible to take all reasonable precautions to eliminate thermal runaway from occurring in the first place.
Prevent thermal runaway
Liquid cooling is the only safety system currently tested and proven to prevent thermal runaway. Liquid cooling prevents batteries from entering thermal runaway by extracting more heat than the cells can produce. A low pressure, high volume closed loop of chilled water is circulated through the battery. PBES has developed a proprietary cooling system, CellCool that takes the idea one step further and circulates coolant through the alloy core of the battery, around each individual cell in every battery. The PBES CellCool is able to remove more thermal energy than the cells can produce when in an overcharge or damage scenario.
In comparison, forced air cooling only cools the external surfaces of the module and is ineffective at eliminating hot spots in the cells.
Grant Brown is vice president of marketing at PBES. In 2017 PBES installed 15.3 MWh of lithium storage on 12 vessels. The PBES energy storage system is DNV GL type- approved and the company is ISO 9001:2015 certified.