Batteries, Energy Storage, News, O&M

Energy storage: Preventing thermal runaway before it begins

Li-ion active primary control system. Courtesy Nexceris.

Utility, industrial, commercial, and government facilities are increasingly turning to Battery Energy Storage Systems (BESSs) in a variety of sizes and power outputs for reliable back-up power, to avoid peak demand charges and to store energy generated by renewable power systems for use at a later time. The market is expected to exceed more than $9 billion by 2024 at a compound annual growth rate of 34 percent, according to a Market Research Engine report.

Given the expected growth in this sector, coupled with the fact that most BESSs are designed by interconnecting a series of lithium-ion (li-ion) batteries, the industry is continuing to focus attention on measures designed to all-but-eliminate any potential for dangerous thermal runaway conditions.  

Thermal runaway occurs when excess heat caused by defects, mechanical failures from damage or improper operation of the system creates a reaction that further increases the temperature.  If left unchecked by built-in system protections or the Battery Management System (BMS), this process can continue to drive up temperature and pressure until the battery cell ruptures, which can cause fires in affected and adjacent cells. 

Fortunately, early detection systems have been developed that can now detect a unique pre-cursor event to thermal runaway – an off-gassing in the battery cell that occurs up to 30 minutes prior to a cascading failure.  This distinctive and recognizable early warning sign enables the problem to be mitigated or the system shut down before thermal runaway can even begin. 

Although virtually all quality BMS equipment monitors temperature and other variables to prevent thermal runaway by triggering protections before temperatures change in an unplanned way, the early detection of off-gassing provides a critical additional layer of protection for the entire system, the facility and even personnel.

Early detection

The early detection of thermal runaway relies on four sequential stages of li-ion battery failure.

A li-ion battery cell first begins to fail when it is subjected to an abuse factor like heat, overvoltage, etc.  The second step is off-gassing.  The third step is smoke, and the fourth step is fire.  But smoke and fire often occur almost simultaneously.  So, by the time smoke is detected, thermal runaway has typically already begun.

Off-gassing usually occurs due to a breakdown of a li-ion battery cell electrolyte, as a result of pressure buildup.  Later, temperature increases, smoke is emitted and then fire breaks out.

To enable off-gassing detection at the earliest stage of a battery event after the initial abuse, a li-ion specific system can significantly improve safety.  The off-gassing detection technology for li-ion BESSs, compatible with all Li-ion chemistries, was originally developed for the U.S. Navy.

Because the system can detect off-gassing at the parts-per-million (ppm)-level concentration range, it can detect individual cell failures without contacting the cells.  This enables action to prevent thermal runaway and its spread to adjacent cells as soon as a single battery cell begins to fail.

Early detection could help prevent thermal runaway before it starts, particularly those related to failure of the active primary control system.

When working with Tier 1 providers, it is typical to have a well-integrated BMS with enough sensors to properly monitor various aspects of li-ion usage, including the state of charge of individual cells and temperature in the system.  However, this may not be the case when dealing with Tier 2 or 3 providers that sell batteries only and leave the BMS to the Engineering, Procurement, Construction (EPC) contractor.

Lower end, offshore battery manufacturers have generally gone from monitoring every small cell group to having one thermocouple run an entire module, monitoring dozens of cells. 

As a result, the Battery Management System is completely dependent on a voltage indication to detect any problems with the cells.  But in many cases, voltage is not the perfect indicator of something potentially going wrong in the cell.

Even in the case of qualified Tier 1 battery suppliers, having an independent, redundant li-ion specific system can help improve safety.  In the same way that seatbelts and airbags combined can reduce the risk of injury in a car crash, such an approach can help a qualified EPC integrate additional complementary safety measures into a safer, more comprehensive solution.

The detection system’s sensitivity to li-ion electrolyte off-gassing is much greater than traditional sensors for other kinds of gas monitoring.

Most sensors that detect gases are either looking for a generic hydrocarbon gas or generic level of gas, and are only detecting at one physical location.  The value of a li-ion specific sensor is that it is looking for the gas emitted from li-ion batteries.  It is incredibly sensitive and able to detect it at much lower levels than any other sensor.

Another advantage of such an advanced li-ion specific system is that it is designed to function as a network of sensors, which enhances its effectiveness in li-ion off-gas detection.  Because it also allows multiple sensors to be deployed, it is much more effective at detecting gas as it disperses through the container.  So, when off-gassing occurs, it is detected immediately and an alarm is sent.

About the author: Steve Cummings is director of the sensors business unit at Nexceris, a developer of gas sensors and monitors.  The company worked with the U.S. Navy a decade ago to develop an off-gassing detection technology for li-ion BESSs that would later be commercialized in a product called Li-ion Tamer, compatible with all Li-ion chemistries.