Thermal management solutions for battery energy storage systems

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

Hot, but not too hot

The dramatic growth of the electric vehicle market has accelerated the adoption of stationary battery storage, with enormous investments in battery R&D and improved manufacturing economies of scale. The market for BESS is projected to grow at a CAGR of 30% from 2023-2033 according to IDTechEx. The global cumulative stationary battery storage capacity is expected to reach 2 TWh within ten years.

However, the hot market for BESS is challenged by the basic fact that electrochemical energy storage is notoriously vulnerable to overheating. From phones to EVs to large BESS systems, overheating of batteries risks sudden fire and explosion in addition to causing degraded performance and shortened lifetime.  Therefore, cooling systems serve as a critically important enabling technology for BESS, providing the thermal stability that is crucial for battery performance, durability, and safety.

Figure: Pfannenberg chillers

What’s driving the rapid adoption of BESS?

The growth of solar and wind-generated renewable energy is one of the drivers of the rapid adoption of battery energy storage systems. BESS complements 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, UK, went live in November with a 98MW/196MWh BESS facility, enough capacity to power 300,000 homes for two hours.

The residential market is expanding rapidly as consumers seek energy independence in the face of massive grid outages and rising prices, as well as a clean energy approach that includes the integration of solar, EVs, and heat pumps. BESS systems have been installed in 31,000 homes in Australia and 100,000 in Germany, and the California Public Utilities Commission (CPUC) is offering $1 billion in rebates for residential battery storage through 2024.

Businesses are also installing battery energy storage systems 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. BESS is 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 less weight and space requirements giving better energy density compared to other battery types.  Alternative battery technologies are emerging. Sodium-sulfur (Na-S) provides high energy and power density, and a long lifetime, but it is hazardous, flammable, and explosive making Na-S 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 very long lifespan, fast response time, high scalability, and very low risk of fire, but they provide relatively low energy capability and 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 BESS systems in the US use lithium-ion batteries, according to the US Energy Information Administration, a penetration rate that is typical around the globe.

Thermal stability and uniform temperature

In general, it is best to keep batteries at a moderate, consistent temperature to ensure their 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 -20°C and 60°C for lithium-ion batteries, between -20°C and 45°C  for nickel-metal hydride batteries, and between -15°C and 50°C 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 US National Renewable Energy Laboratory, the optimal temperature range for Lithium-Ion is between 15 °C and 35 °C. Research shows that an ambient temperature of about 20°C or slightly below (“room temperature”) is ideal for Lithium-Ion batteries. If a battery operates at 30°C, it’s lifetime is reduced by 20 percent. At 40°C, the losses in lifetime approach 40 percent, and if batteries are charged and discharged at 45°C, the lifetime is only half of what can be expected at 20°C.

Not only is thermal stability critical to performance, longevity, and safety, but also equally important is maintaining uniform temperature throughout the system. Avoiding hot spots is crucial to preventing damage and mitigating the risk of triggering a chain reaction that leads to catastrophic thermal runaway.

Figure: Pfannenberg compact chillers can be mounted on a cabinet door, for a a small footprint that allows for easy integration.

Internal and external causes of overheating

Several factors contribute to overheating. 

Applications. 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 results in the generation of heat.

Ambient Temperature. 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 resulting in a build-up of resistance at the electrode surface. Operation of 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.

Figure: Pfannenberg DTS units can be used for air cooling BESS containers. Source: Pfannenberg USA Inc.

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 BESS installation 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.

These thermal management technologies have evolved over the decades to meet the increasingly demanding cooling requirements in the electrotechnical industry. For example, in the 1950s, Pfannenberg, a global manufacturer of thermal management products, began developing products, such as the first filter fan, to manage the temperature in electrical enclosures. Over the decades, its portfolio has expanded to include air cooling and liquid cooling solutions for manufacturing processes and data centers. These various proven thermal management solutions meet the performance requirements and environmental conditions of the diverse range of BESS applications.

Liquid cooling is extremely effective at dissipating large amounts of heat and maintaining uniform temperatures throughout the battery pack, thereby allowing BESS designs that achieve higher energy density and safely support high C-rate applications. Source: Pfannenberg USA Inc.

Liquid cooling

Active water cooling is the best thermal management method to improve BESS performance. Liquid cooling is extremely effective at dissipating large amounts of heat and maintaining uniform temperatures throughout the battery pack, thereby allowing BESS designs that achieve higher energy density and safely support  high C-rate applications. As the BESS market evolves with a wide diversity of designs and applications, multiple versions of chillers are available to optimize the layout of the cooling system.

Air cooling

At the other end of the spectrum, air cooling systems provide a cost-effective cooling solution for smaller stationary energy storage systems operating at a relatively low C-rate.00

For example, Pfannenberg’s cooling unit seals out the ambient air, and then cools and re-circulates clean, cool air through the enclosure. The closed loop design isolates the external ambient air from the internally conditioned air eliminating the risk of contaminants entering the cabinet. The hermetically sealed compressor guarantees 100% cooling capacity efficiency.

For applications with a relatively low cooling load, where the ambient air is always cooler than the temperature required inside the enclosure, filter fans provide an extremely cost-effective solution that uses natural convection of the air to circulate air and dissipate heat.

The crucial role of cooling technology

Energy storage plays an important role in the transition towards a carbon-neutral society. Balancing energy production and consumption offers positive means for integrating renewable energy sources into electricity systems while improving overall energy efficiency. This new paradigm increasingly depends on battery energy storage systems.

BESS systems, in turn, depend on cooling systems that provide the thermal stability that is crucial for battery performance, durability, and safety. and if applied correctly, will reduce battery degradation and damage, and minimize downtime.

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Pfannenberg Inc. is a global manufacturer of thermal management, liquid cooling solutions and signaling technologies.

https://www.pfannenbergusa.com