Taking the Heat out of Energy Storage

The global supply of stationary energy storage systems has expanded dramatically as the number of solar and wind farms increase – but ensuring the safe operation of larger systems is critical to their long-term success.

Taking the Heat out of Energy Storage


By Tim Grejtak, Lux Research


South Korea offers an object lesson on how overlooking safety can have a chilling effect on markets – in its case, resulting in around US$400 million in lost sales of stationary energy storage systems.


Three years ago, South Korea was the largest stationary energy storage market in the world with more than 1 gigawatt hours (GWh) of deployments, surpassing Germany, the United States, and even China. However, after a series of battery fires, the South Korean government ordered stationary storage systems to cease operations, froze installations of new systems, and launched an investigation and inspection effort. 


Some 35% of such facilities were reportedly suspended in 2019 due to the fallout from the 23 fires that occurred over a 21-month period. 


The South Korean government was quick to clear battery cell manufacturers of blame for the fires, noting that while some cells had manufacturing defects, none would have caused the level of damage that could have led to the fires. 


Instead, the installation, operation, and maintenance of the systems were to blame. Battery systems were improperly shielded from sparks or shocks, poor weatherisation led to dust and moisture that caused equipment to fail, and systems were either improperly installed or installed without adequate safety and control measures. 


The government responded with a more rigorous and frequent inspection process, new liability laws, and specific codes and standards, such as locating outdoor energy storage systems in dedicated enclosures and limiting behind-the-meter energy storage capacity to 600kWh. 


While these standards may help reduce the severity of behind-the-meter storage fires, they may not address the challenges of utility-scale energy storage systems installed in solar-heavy regions like Australia, California, and Arizona, which are much larger than 600kWh and often in remote sites. 


In April 2019, while the South Korean fires were being investigated, another fire broke out at a 2MW system installed at an Arizona Public Service substation that injured four firefighters. When external readings indicated temperatures inside the battery compartment were below 40°C, the firefighters opened the battery compartment door, and there was an explosion. 


Reducing risk

The fundamental issue with stationary storage systems is that they are larger and contain more energy than consumer devices or even electric vehicles. When a fire breaks out, therefore, larger systems increase the complexity and severity of the situation. 


Managing these large, complex devices requires multiple levels of safety. The stationary storage safety framework illustrated in this article is meant to convey the critical need for a thorough systems analysis when evaluating the safety of stationary storage products. 

The key components in the Stationary Storage Safety Framework are four-fold: system design, pack design, cell design and Li-ion cell chemistry.

Ensuring the safe operation of a large stationary storage system means dissipating any build-up of thermal, electrical, or chemical energy. For a device with the sole purpose of storing energy, this clearly presents a challenge. 


Choice of battery materials is central to this process. Due to the need for a long cycle life, stationary energy storage systems tend to use less energy-dense cathodes such as lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC), which have both higher thermal runaway onset temperatures and lower peak thermal runaway temperatures. 


This should in theory make stationary storage systems safer, but because the systems are bigger with more cells, the odds of any one cell experiencing a failure are much higher. Should a cell fail and start to generate heat, it is up to the higher-level systems to either isolate that energy or dissipate it before it causes failures in other cells or systems.


While the cathode is a considerable source of chemical energy in stationary energy storage systems, it is not the only source. The organic electrolyte contained in the cells as well as the plastics in cell, module, pack housing, and wiring can also contribute additional energy in the event of a fire. 


Furthermore, the gases generated during combustion, such as carbon monoxide, methane, and hydrogen, can pose an additional toxicity or explosion threat if not vented adequately. Ultimately, an effective safety strategy means addressing heat generation by cells during thermal runaway, fires from flammable components as well as explosive gas build-up. 


In a study conducted by DNV GL and Con Edison, the two companies recommended a staged approach using water and other fire suppressants to cool and extinguish Li-ion battery fires. 

An energy storage system in nature
Addressing heat generation by cells during thermal runaway is a key aspect in improving the safety of stationary energy storage.

Differing paths

Stationary storage system safety is complex and faces requirements on multiple levels, from cell chemistry choice to system-level safety features. As a result, different regions have taken different approaches to safety. 


Stakeholders have voiced concerns, but regulators are confident the improvements needed to reach these standards will be completed on time. While safety is a crucial element in large stationary storage systems, it is essentially a solvable problem with today's technology. 


There are limited opportunities for advanced thermal materials or battery management systems to improve some specific elements of stationary storage safety, but safety codes and standards are being drafted without these being inherently necessary. 


Instead, incremental improvements in controls, battery management systems, and thermal materials will provide differentiated, tangible benefits to safety, so long as cost increases are kept to a minimum.