The widespread adoption of battery energy storage systems (BESSs) serves as an enabling technology for the radical transformation of how the world generates and consumes electricity. The paradigm is shifting from a centralized grid delivering one-way power flow from large-scale fossil fuel plants to new approaches that are clean, renewable, flexible, resilient, distributed, and cost-effective.

Hot, but not too hot

The dramatic growth of the electric vehicle (EV) market has accelerated the adoption of stationary battery storage, with enormous investments in battery research and development (R&D) and improved manufacturing economies of scale.

The market for BESSs is projected to grow at a compound annual growth rate (CAGR) of 30% from 2023-2033, according to IDTechEx. That means the global cumulative stationary battery storage capacity is expected to reach 2 TWh within 10 years.

However, the hot market for BESSs is challenged by the basic fact that electrochemical energy storage is notoriously vulnerable to overheating. From phones to EVs to large BESSs, overheated batteries can lead to sudden fire and explosion in addition to causing degraded performance and shortened lifetime.

What’s driving the rapid adoption of BESSs?

The growth of solar and wind-generated renewable energy is one driver of the rapid BESS adoption. BESSs complement these renewable sources by providing buffering and time-shifting and by facilitating remote and off-grid use cases.

Renewable energy is not the only driver. Large-scale BESS installations are also incorporated into electrical grid networks to provide energy demand balancing and resilience to grid failure. For example, the Pillswood project in Yorkshire, U.K., went live in November with a 98-MW/196-MWh BESS facility — that’s enough capacity to power 300,000 homes for two hours.

Businesses are also installing BESSs for backup power and more economical operation. These “behind the meter” (BTM) systems facilitate energy time-shift arbitrage, in conjunction with solar and wind, to manage and profit from fluctuations in the pricing of grid electricity. BESSs are a cost-effective method of powering large dynamic loads, such as big compressors, motors, and generators, without the need to build out electricity infrastructure and grid connections to accommodate load spikes and peak demand.

Evolving battery technology

New battery technologies, architectures, and chemistries are being developed every day. Nevertheless, Lithium-ion batteries continue to dominate energy storage systems due to falling battery costs and increased performance with minimal weight and space requirements.

Alternative battery technologies are emerging. Sodium-sulfur (Na-S) provides high energy and power density and a long lifetime, but it's hazardous, flammable, and explosive, making it most suitable for standalone renewable energy storage applications where these dangers can be isolated.

Flow batteries store energy in liquid electrolyte solutions and are gaining market share in very large-scale applications. They offer long life spans, fast response times, high scalability, and low risk of fire, but they provide relatively low energy capability and a slow charging/discharging rate.

Lithium-ion will continue to be the most common BESS technology for the foreseeable future. More than 90% of large-scale BESSs in the U.S. use lithium-ion batteries, according to the U.S. Energy Information Administration (EIA), a penetration rate that is typical around the globe.

Thermal stability and uniform temperature

In general, it's best to keep batteries at a moderate, consistent temperature to ensure optimal performance and longevity. Exposure to extreme temperatures, either hot or cold, can damage batteries and cause hazardous events.

The specific temperature range that batteries require to operate safely can vary depending on the type of battery and its design. The safe operating temperature range is typically between minus 20° and 60°C for lithium-ion batteries, minus 20° and 45° for nickel-metal hydride batteries, and minus 15° and 50° for lead-acid batteries. It is important to carefully consult the manufacturer's specifications for the specific type of battery being used to determine its precise safe operating temperature range.

According to the U.S. National Renewable Energy Laboratory, the optimal temperature range for lithium-ion is between 15° and 35°. Research shows that an ambient temperature of about 20° or slightly below (room temperature) is ideal for lithium-ion batteries. If a battery operates at 30°, it’s lifetime is reduced by 20%. At 40°, the losses in lifetime approach 40%, and if batteries are charged and discharged at 45°, the lifetime is only half of what can be expected at 20°.

Thermal stability and maintaining uniform system temperature are critical to performance, longevity, and safety. Avoiding hot spots is crucial to prevent damage and mitigate the risk of triggering a chain reaction that leads to catastrophic thermal runaway.

Internal and external causes of overheating

Several factors contribute to overheating.

Applications that require rapid charging/discharging are referred to as having a high C-rate, which is defined as the charging or discharging current divided by the capacity (the amount of energy the battery can hold). With high C-rate and frequent cycling, internal resistances to the higher currents result in the generation of heat.

High ambient temperature also damages batteries in several ways. One problem is that elevated temperatures lead to an increased rate of side reactions, causing attrition of active material and a buildup of resistance at the electrode surface. Operating lithium-ion batteries at high temperatures will also accelerate the aging process and lead to degradation of performance. Two types of aging occur in combination due to the complex composition and working process of lithium-ion batteries: cycle aging takes place, while charging and discharging, and calendar aging occurs over time, while a battery is inactive.

Paradoxically, low ambient temperatures can cause more problems with internal overheating than high ambient temperatures. One reason is that cold temperatures can result in viscosity changes in the electrolyte that lead to sluggish ion transport, resulting in higher resistance and heat generation.

Designing an optimal cooling solution

A variety of thermal management solutions are available, and the choice of the optimal solution is informed by the C-rate of the application and the environmental conditions, among other factors. At the high end, the most demanding thermal management applications, such as large-scale BESSs installations and high C-rate applications, require active liquid cooling. On the other end of the spectrum, smaller installations with low C-rate applications can be safely and efficiently operated at peak performance with air cooling.

Energy storage plays an important role in the transition toward a carbon-neutral society. Balancing energy production and consumption offers a positive means for integrating renewable energy sources into electricity systems while improving overall energy efficiency. This new paradigm increasingly depends on BESSs, which circles back to thermal stability — cooling systems play a crucial role in providing optimal battery performance, durability, and safety.