Catching the thermal runaway: Testing safety of energy storage systems
By Rod Walton -6.29.2020
As energy storage projects grow around the world, driven by an increase in renewable energy generation and the integration of such into grid operations a troubling phenomenon needs to be addressed. Fires – many of them linked to lithium-ion batteries – are occurring with troubling frequency.
This article was originally published in Smart Energy International Issue 3-2020.
Over the course of the last 12 months, more than 20 energy storage systems in Korea have caught fire, and in April last year, a 2MW battery array in Arizona caught fire and eventually exploded. Fires linked to lithium-ion batteries also occurred in Europe and Australia.
Comprised of an interconnected series of Lithium-ion (Li-ion) batteries, Battery Energy Storage Systems (BESSs) help utilities provide reliable back-up power, avoid peak demand charges, and store energy generated by renewable power systems.
“Once a Li-ion BESS goes into thermal runaway you cannot stop it [in that cell] –your goal is to try to stop the propagation of heat and thermal runaway to adjoining cells,” explains retired NYC firefighter Paul Rogers. He is co-founder of Energy Storage Response Group (ESRG), a national fire safety consultancy with nearly 50 years of combined experience that specialises in the risk assessment, investigation, and safety testing of energy storage systems.
The Electric Power Research Institute (EPRI) has chosen to focus attention on evaluating and testing technologies to increase the safety of BESSs in response to this challenge.
Thermal runaway conditions can occur when excess heat caused by defects or 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.
EPRI is conducting trial tests with multiple utilities of an advanced early warning system that can detect an off-gassing event as a precursor to thermal runaway 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.
According to Rogers, while a BMS can provide data and shut the BESS down, depending on the safety integrity levels of the BMS component, there are times when the BMS can miss critical indicators or malfunction. Because of this, Rogers recognizes the need for a separate, redundant system that can indicate the potential of thermal runaway before it occurs so the BESS can be shut down to prevent it.
“You need layers of protection to reduce your risk,” says Rogers, who recommends incorporating an additional early warning system besides the BMS.
One such option is a product called Li-ion Tamer that provides advanced warning before thermal runaway in lithium-ion BESSs. The early detection of thermal runaway relies on four sequential stages of Li-ion battery failure, according to Steve Cummings, Director of the Sensors Business Unit at Nexceris.
“A Li-ion battery cell first begins to fail when it is subjected to an abuse factor like heat, overvoltage, etc.,” says Cummings. “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, the system offers a battery off-gas monitor and sensor network designed specifically for lithium-ion batteries.
EPRI is currently testing the system at several test sites, including Duke Energy’s Emerging Technology Innovation Center in North Carolina, which has validated the system’s ability to detect even minute traces of off-gassing as a precursor to thermal runaway minutes before it occurs, allowing action to deter potential cascading failure.
Says Tom Fenimore, currently Duke Energy Business Development Manager assisting in deploying new leading-edge technologies: “We were educating ourselves in lithium-ion energy storage, in deterring thermal runaway events, when we had the opportunity to … install off-gassing detection technology [in a BESS], which provides early detection of thermal runaway so it can be deterred,” says Fenimore.
They are currently testing the system in a 20-foot container that is professionally developed and built for energy storage. The BESS is rated at 650kWh and can discharge roughly 350kWh of energy. It has six racks of lithium-ion batteries that are used daily in the operation of the micro-grid that supports the Mount Holly Technology Innovation Center.
“We take a proactive approach to energy storage safety that involves implementing layers of protection,” says Fenimore.
Fenimore states the first line of defence is the battery management system (BMS) because it is the system that manages the charging and discharging of the batteries. That is the first system that can sense whether an out of bounds condition exists within a battery cell. Since it monitors voltage and temperature, it has the first opportunity to put the battery into a safe state should an event occur.
“The next level of technology deployed inside our BESS systems is an off-gasser system, which is deployed inside the container to sense whether a battery cell has gone into a condition where it off-gasses,” says Fenimore. “That off-gas event is a direct indication that the battery is in a compromised, abnormal condition. At that point, we take that signal and shut the battery down, disconnect the battery from the grid, ventilate and cool the battery, as well as alert people so proper maintenance can be done to address the problem.”
Fenimore says that Duke Energy has participated in offsite disruptive testing by actively placing battery cells into a condition of thermal runaway under controlled conditions.
“We have seen from testing that we typically have early notification in the order of minutes that an event has occurred [before thermal runaway] when even seconds can make a difference,” says Fenimore. “We want to take advantage of that capability wherever we can to protect the public and the capital asset we are deploying.”
According to Fenimore, Duke Energy has projects built or planned across six states that use the technology.
“[This] technology is standard in our BESS safety specifications and we are specifying it moving forward,” says Fenimore. “The BESSs are typically deployed alongside solar, wind, or combinations of solar and traditional generators. The BESSs meet regulatory mandates for a NonWires Alternative because they do not require building additional distribution or transmission lines to get power to where it needs to go.”
For utilities and other stakeholders, when it comes to safely integrating new energy storage technology into municipal code, national safety consultant Rogers highlights the importance of adding adequate safeguards to avoid any incidents that could derail achieving more widespread support.
According to Rogers, back in the 1970s in NYC there was an explosion from a liquefied natural gas (LNG) facility that had nothing to do with LNG directly “Still, since then to this day, NYC has had a moratorium on LNG that prohibits new projects to be installed into the city without a variance because the fire department will not allow it,” says Rogers.
The lesson, according to Rogers, is one that the battery energy storage industry should take to heart when it comes to rolling out lithium-ion BESS technology: taking the time to fully test and implement safe equipment, methods and procedures always pays off in the end.