Battery safety: Is the industry doing enough?
Current safety requirements for battery installations on ships are ineffective, argues Grant Brown, vice president marketing at energy storage company PBES.
Each year there are more and more hybrid or fully electric ships navigating waters worldwide. Some estimate that all modern commercial vessels will soon have some form of energy storage on board. These ships range from ferries transporting thousands of people daily, to offshore supply vessels that maintain safety in critical oil rig operations.
These ships increasingly rely on lithium energy storage as their power source, with modern designs containing over 1000 individual modules. The technology has proven itself reliable and powerful. However, safety concerns linger and should be kept at the utmost of considerations for this new technology.
Not all battery systems are equipped with the same safety systems. Testing and certification for battery systems aboard ships has increased, but room remains to raise the bar higher.
One of the biggest risks for batteries is thermal runaway. This 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. In these cases, heat is generated within the lithium-ion cells and causes a reaction between the cathode material and electrolyte. This can result in the cell’s 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 for batteries used in commercial vessels in Norway is the Propagation Test Type 1. NME Propagation Test Type 1 is intended to prevent propagation of the thermal event from one module to the next. This test 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 either the entire system was damaged, or any number of individual modules were damaged. In a multi-module event, the NMA Propagation Test Type 1 may not prevent propagation.
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.
ABOVE: Baseline test to show the energy contained in a single cell (75Ah); industrial lithium-ion modules contain 24 times this amount or more.
PREVENTING 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 that takes the idea one step further. CellCool circulates coolant through the alloy core of the battery, around each individual cell in every battery. It is able to remove more thermal energy than the cells can produce when in an overcharge or damage scenario.
Testing shows that the PBES system is so effective that is works even if the coolant pump is disabled, meaning that in the event of catastrophic damage to the vessel, the system will still protect the batteries. Due to its patented design, CellCool also eliminates hot spots on the cells and maintains optimal cell temperature thus increasing lifespan.
By comparison, forced air cooling only cools the external surfaces of the module and is ineffective at eliminating hot spots in the cells. An air-cooled battery requires around 3,500 times more air flow volume than water flow volume to achieve the same heat removal. To try to compensate, the battery room for an air-cooled system requires a robust HVAC system - an extra cost not typically included in the battery price.
THERMAL BARRIERS AND VENTING
Effective internal thermal barriers are an essential part of lithium battery safety systems. PBES’s Thermal-Stop is a metal barrier integral to the structure of the battery that works like a firewall. It prevents an overheated, overcharged or damaged cell from propagating to the adjacent cell. The event is therefore isolated to one cell and does not affect the others.
In the event of a damaged cell within a module, dangerous gases can be released. It’s important for every supplier to create safe venting for battery systems. E-Vent is a PBES-patented system to vent flammable gasses from a damaged module safely away from the battery area. It reduces risk of a secondary explosion and allows the crew to re-enter the vicinity of the battery system sooner to make repairs and restore power.
In independently observed tests o PBES systems, no measurable amount of gas could be captured. In fact, when the test modules were disassembled and inspected, the cells that had been overcharged looked virtually identical to untested units, save a small rupture on the top of the cell near the terminals. Even though the cells were not damaged and no appreciable gas has been detected, we understand the potential risks. Every system we install has an integrated E-Vent system.
Given the rapid uptake of large format lithium-ion batteries in commercial marine vessels, energy storage system safety is lacking. Current standards leave significant potential for hazardous situations to arise. The industry has responded to price pressure from owners and operators by reducing costs and safety systems to meet the bare minimum of requirements. It is the responsibility of industry and regulators to do everything possible to ensure a major incident does not occur.
ABOVE: Thermal runaway test with cooling system disabled. Graph shows the voltage and temperature as it increases to cell failure, and subsequent cooling. No thermal runaway event occurs.
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