Thermal Management, The Next Big Question in Energy Storage?

Because of the thermal characteristics of the battery, thermal management has become a key link in the electrochemical energy storage industry chain. From the industrial chain value split point of view, the battery cost in the energy storage system accounts for about 55%, PCS accounts for about 20%, BMS and EMS together account for about 11%, thermal management accounts for about 2% -4%.

01.  Overview

The temperature control system is an important part of the electrochemical energy storage system. As one of the key links in the new energy system, electrochemical energy storage plays an important role in increasing the proportion of renewable energy consumption and ensuring the safe and stable operation of the power system.

The electrochemical energy storage system consists of several key components, including cabinet, heat dissipation system, PCS energy storage converter, battery pack, EMS energy management system, energy storage high-voltage box, fire protection system and safety auxiliary system.

The heat sink system plays an important role in electrochemical energy storage systems. Its main function is to ensure that the energy storage system is always in the optimal operating temperature range and to prevent the system from overheating and going out of control.

02.  Incident statistics

Temperature control systems play a vital role in electrochemical energy storage systems, especially with the rapid growth of installed energy storage capacity. With the increase in the capacity and power of the energy storage battery cells, the high power density requirement puts higher demands on the heat dissipation system.

At the same time, problems such as battery heat production and uneven temperature distribution may occur within the energy storage system, which increases safety risks.

According to incomplete statistics from GGII, there were nearly 90 global energy storage safety accidents in the past 10 years, of which 17 energy storage safety accidents occurred in 2022 and more than 10 in 2023. Therefore, temperature control systems are critical to the longevity and safety of energy storage systems.

03. Causes of the accident

(1) Failure of the internal electric core, triggering thermal runaway of the battery and module, and finally causing the whole energy storage system to catch fire or explode.

This kind of failure is usually presented as a fire first, followed by an explosion. For example, the accidents that occurred at the McMicken power station in Arizona, USA, in 2019 and at the Fengtai power station in Beijing, China, in 2021 both occurred after a fire followed by an explosion.

This phenomenon occurs when a single cell fails, triggering an internal chemical reaction that releases heat (exothermic reaction), leading to a continuous rise in temperature that spreads to nearby batteries and modules, ultimately triggering a fire or even an explosion.

Cell failure modes are typically caused by overcharging or control system failures, thermal exposure, external short circuits, and internal short circuits. These failures can be triggered by a variety of conditions, such as indentations or dents, material impurities, penetration by external objects, etc.

2) Energy storage system failure caused by external auxiliary system failure

Typically, energy storage system failures triggered by auxiliary system failures occur outside of the battery system and may result in burning or smoking of external components. However, if the system can be monitored and responded to in a timely manner, these failures do not have the effect of failing or thermal runaway on the battery system's cells.

In the 2021 Vistra Moss Landing Phase I and 2022 Phase II incidents, smoke and fire resulted from fault monitoring and electrical fail-safes that were turned off and unable to respond in a timely manner because they were in the commissioning phase.

This flaming combustion usually starts on the outside of the battery system and eventually spreads to the inside of the cells. As a result, it does not cause a violent exothermic reaction or a buildup of flammable gases and is usually not explosive. Additionally, if the sprinkler system can be activated in a timely manner, it can also prevent extensive facility damage from occurring.

04. Nature of runaway batteries

The essence of thermal runaway of Li-ion battery is the chain reaction inside the battery under abusive conditions. The thermal runaway mechanism of lithium-ion batteries for energy storage can be summarized as follows: under abusive conditions, the temperature of the battery rises abnormally, which firstly leads to the decomposition of the protective layer on the surface of the negative electrode (SEI film) inside the battery, and the electrolyte undergoes reduction/oxidation reaction, thus causing the temperature inside the battery to rise gradually.

When the temperature reaches 150-160°C, the diaphragm begins to shrink and melt, and then an internal short circuit occurs, leading to an increase in the vapor pressure of the electrolyte, which in turn causes the safety valve to open. As the battery temperature rises, a chain exothermic reaction occurs, with the active lithium at the negative electrode reacting with the electrolyte and releasing a tremendous amount of heat, while the lithium phosphate positive electrode releases oxygen and reacts with the electrolyte, further releasing heat.

At higher temperatures, the remaining lithium reacts with the binder, further increasing the battery temperature until thermal runaway occurs. Although changes in the battery material system and thermal runaway triggering mode and other factors may lead to differences in the reaction order and reaction temperature range of the above chain reaction, it is essentially triggered by the alternation and superposition of the above chain reactions.

05. Optimal battery operating temperature

To ensure the high efficiency and safety of lithium batteries, the optimal operating temperature is considered to be within the range of 10~35℃. Within this temperature range, Li-ion batteries are able to operate normally, and too low a temperature will not lead to electrolyte solidification and increased impedance, while too high a temperature will not reduce the capacity, life and safety of the battery.

Energy storage systems are characterized by large capacity, high power and high heat dissipation requirements. At the same time, there are problems within the energy storage system in which the battery is susceptible to thermal influences and uneven temperature distribution.

These characteristics require effective thermal management of the energy storage system to ensure normal battery performance, avoid thermal runaway and prevent safety accidents.