KEY CONCEPTS
• A thermal management process is most concerned with controlling and estimating precise temperature profiles in the BESS. 
• A choice of thermal management system impacts safety, degradation, stability and performance of the BESS. 
• Degradation of batteries is largely dependent on ambient temperatures, and it is the most understudied challenge due to multiple variables that BESS operate under.

The U.S. Energy Storage Association plans to add 100 gigawatts (GW) of new energy storage to grid installations to help transition from fossil fuels to sustainable energy sources.¹ Among the key technologies for the successful integration of reliable energy sources is an effective thermal management of battery energy storage systems (BESS) (see Figure 1). 


Figure 1. BESS example. Figure courtesy of Nicolasrodel, CC BY-SA 4.0, via Wikimedia Commons.

In addition to sustainability related goals, utility-side battery energy storage is becoming valuable to the existing grid system by providing an aid for load balancing, real time response, frequency standards, power quality, reducing peak hours and load transferring, reliability improvement and system conservation (see Energy and Net Zero Targets).² The market for the utility use cases along with some commercial/industrial and residential use cases could grow the BESS market to 150 gigawatthours (GWh) in 2030 and bring the installed base to 700 GWh BESS by 2030 (see Figure 2).³


Figure 2. Battery energy storage system capacity is likely to quintuple between now and 2030.³

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Energy and net zero targets
Energy is moving toward a 100% renewable energy source.^A The following countries/regions with net zero targets cover 88% of global emissions. 

• EU: Climate neutrality by 2050
• U.S.: Carbon-free electricity production by 2035; net zero emissions by 2050
• China: Carbon neutral by 2060

The International Energy Agency (IEA) World Energy Outlook 2024 predicts electricity generation would need to grow by almost three times and renewables by eight times to reach net zero targets by 2050.

A. IEA (2024), “World energy outlook.” Available at www.iea.org/reports/world-energy-outlook-2024.

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However, there are technical and economic challenges with regards to their widespread deployment (see Figure 3). The objective of this article is to examine various technical solutions, strategies and testing methods that are currently in use, and planned to be used in the future, to prevent and improve performance of thermal management systems and battery capacity degradation challenges. 



Figure 3. Mind map of BESS optimization.⁴

BESS technology overview 
Presently BESS technology includes the use of lithium-ion (Li-ion), lead-acid (LA), sodium-sulfur (NaS), zinc-bromine (ZBB), nickel-cadmium (Ni-Cd), vanadium-redox (VRB) and polysulfide bromine (PSB) batteries. These are typically used for load leveling, power quality, grid extension and support, demand management and voltage regulation. Although the Li-ion batteries have high energy and power densities with long-lasting life cycle and excellent efficiency, they can be an expensive investment compared to other emerging chemistries. This battery type is also manufactured as packs, organized in series or parallel to realize the necessary current, voltage and power.⁴ An emerging technology of interest that may address the cost issue is the sodium-ion (Na-ion) battery, due to the vast abundance of sodium in nature (>20 times of lithium, see Figures 4 and 5) and theoretical performance being similar to its Li-ion counterpart.


Figure 4. Examples of present BESS technologies.⁴


Figure 5. Abundance of elements in the Earth crust. Figure adapted from Zhao, L., Zhang, T., Li, W., Li, T., Zhang, L., Zhang, X. and Wang, Z. (2023), “Engineering of sodium-ion batteries: Opportunities and challenges,” Engineering, 24, pp. 172-183.

Ron Turi, principal of Element 3 Battery Venture, shares the following data:
• Electric vehicle (EV) battery cell production in 2024 global ~876 GWh
• Global projection 2030 ~2,500 GWh (prior to global market disruption, uncertainty and disarray) 

Turi says: “Smaller battery market segments—like BESS—will benefit from the previously mentioned growth, coming as a direct result of spillover from the dominant EV battery end-use industry and market.”

He adds: “One thing worth clarifying is the difference between GW and GWh when it comes to BESS batteries. GW often refers to peak power and not the size of the battery. However, GWh is the size of the battery and reflects how long the battery can run at a rated power level—not the peak power—and GWh relates to battery cost. This can mean lower GWh when peak GW is specified and higher GWh when nominal GW is specified. Also, the magnitudes of individual battery sites are most often on the megawatt (MW) and megawatthour (MWh) scale. Just as important, reports on BESS markets almost always report the cumulative installation figures over the years. So, 100 GWh of BESS by 2030 equals 1.6 million EV batteries cumulative over the five-year period, which averages to ~300k EV batteries per year over this period—so, still small compared to EV battery production.”

Turi also shares the following U.S. Energy Information Administration (EIA) description of electricity generation capacity of energy storage systems:⁵

Two basic ratings for energy storage system (ESS) electricity generation capacity are:

• Power capacity—the maximum instantaneous amount of electric power that can be generated on a continuous basis and is measured in units of watts (kilowatts [kW], MW or GW)
• Energy capacity—the total amount of energy that can be stored in or discharged from the storage system and is measured in units of watthours (kilowatthours [kWh], MWh or GWh)

The U.S. EIA collects and publishes data on two general categories of ESSs based on the size of power generation capacity:

• Utility scale or large scale have at least 1 MW of net generation capacity and are mostly owned by electric utilities or independent power producers to provide grid support services (see Table 1).
• Small scale has less than 1 MW of net generation capacity, and many are owned by electricity end-users that use solar photovoltaic systems to charge a battery. EIA publishes data only for small-scale battery ESS.



Table 1. U.S. utility-scale energy storage systems for electricity generation, 2022.⁵

Turi states: “BESSs are not primary electricity generation sources. They must use electricity supplied by separate electricity generators or from an electric power grid to charge the storage system, which makes ESSs secondary generation sources. ESSs use more electricity for charging than they can provide when discharging and supplying electricity. Because of this difference, EIA publishes data on both gross generation and net generation by ESSs. Gross generation reflects the actual amount of electricity supplied by the storage system. Net generation is gross generation minus electricity used to recharge the storage system and the electricity consumed to operate the ESS itself. Net generation from ESSs is reported as negative in EIA data reports to avoid double counting the generation from charging sources for ESSs and the generation from ESSs. The difference between gross and net generation varies widely by type of ESS (see Figure 6).”


Figure 6. General locations of energy storage technologies for electricity generation on an electricity grid.⁵

Turi states: “The vast majority of BESS use lithium iron phosphate (LFP) chemistry for cost-effectiveness. Because the containerized versions control battery temperature, the ill effects of LFP at lower temperatures are avoided." 

In addition, Turi states, the trend for BESS is toward lithium manganese iron phosphate (LMFP) and a serious emerging technology with Na+ that is already in production. Both LMFP and Na+ improve cost-effectiveness and many think that they are the future of BESS.

Thermal management practices and methods
Despite advancements in BESS technologies, critical knowledge gaps remain, hindering the smooth integration and operation of the systems. For example, improvements that address battery thermal management optimization can improve battery performance by decreasing its capacity degradation (see Figure 3).

Controlling the temperature of a BESS ensures optimal performance and long-term health of the system, and effective thermal management systems (TMSs) provide temperature control by managing heat dissipation and mitigating temperature fluctuations.⁶ In cold ambient conditions, the same factors apply to warming up the battery to optimize its performance. 

Complex electrochemical reactions and electric-to-thermal conversion determine the thermal characteristics of a battery. The production of heat by, e.g., Li-Ion batteries, is a complex process that involves a knowledge of how the rate of electrochemical reaction varies with time and temperature, in addition to how current flows within the battery.⁴

Mark Rabin, a founder of Portable Electric, explains: “BESS systems require thermal management at the component level and ultimately within the enclosure. These system temperatures are heavily influenced by the external environmental temperatures (i.e., hot sun in the desert to cold in the north) and how the system is being operated. When power electronics and batteries alike are heavily used, they naturally generate heat. When lightly used they generate much less heat. Well thought through and innovative system design can greatly reduce waste heat due to more efficient topology and innovative engineering.” He continues: “There are several methods for thermal management: 

1. Air cooled, which is basically done by using intake fans pulling air from the outside environment into the system and forced out of the system. This is sufficient for many systems, especially if they aren’t being pushed to the limit, and space isn’t that big of a constraint. 
2. Air cooling with an air conditioning unit, which cools down air from the outside and cools the air inside of the sealed enclosure. This could involve refrigerants within the cooling system; however, it does not circulate cooling liquid throughout the system. 
3. Liquid cooling is significantly more effective and more complex. This is where a fine network of typically water-glycol mixtures (like ethylene glycol or propylene glycol) is circulated throughout the system directly cooling components and recirculating the heat out of the components. This enables system design to be significantly more compact and efficient. In EV batteries, liquid cooling is now the norm and considered inherently safer.”

Andre Swarts, a staff engineer, Powertrain Engineering Division of Southwest Research Institute, says that there are several types of fluid or other technologies used in thermal management for battery management systems: “Direct liquid immersion cooling is a promising technology for enhanced battery thermal management. It offers vastly improved heat transfer capabilities, and also greater responsiveness with less thermal inertia when compared to indirect liquid cooling solutions with cooling plates, ribbons or channels. For these fluids, the dielectric characteristics and electrical conductivity are critical, in addition to heat transfer properties. Even the electrical conductivity of water-based coolants used in indirect cooling systems are now being scrutinized to protect against electrolysis and hydrogen production in the event of accidental contact between the coolant and a battery cell.”



Swarts says: “This is a big challenge for not only material property tests but also simulated service. The test conditions such as pressure, temperature and flow may not be relevant to the new applications of fluids. Additionally, there are no established pass/fail criteria—at best we can rank products from good to bad without any guarantee that the ranking will hold when the applications change. Most existing test methods and limits within product standards were born out of market challenges. For battery systems, we don’t have a large body of knowledge yet to know what these challenges are. So, when we do tests and get results using existing methods, we don’t know how far away we are from the limits and whether those limits are universal or application specific. One example is electrical conductivity: historical test methods were developed for insulating and conductive fluids using current density and frequencies that considered power transmission and fuel handling safety. The test geometries, voltages and currents for battery systems may be very different.”

Turi shares: “Although a mix of ethylene and propylene glycols has been the ‘go to’ legacy response. However, a thermally stable, nonpolar, high dielectric liquid seems like a better choice.”

Challenges in battery capacity degradation and test methods 
Battery degradation leads to a reduction in its capacity and efficiency and even safety problems. The term cycle life refers to the total number of times a battery can be discharged and charged before it is replaced. Nonlinearity in battery degradation can be traced to a variety of causes, such as state of charge (SOC), high temperature, depth of discharge (DOD) and charge or discharge current rate. One of the issues contributing to the short lifespan of Li-ion batteries, for example, is the highly utilized DOD, which tends to significantly reduce the total number of cycles.⁴

While discussing battery cooling optimization methods that improve battery capacity degradation, Turi states: “There are multiphysics models developed by Idaho National Laboratory (INEL) as well as commercial solutions that predict temperature profiles in cells undergoing different rates of charge/discharge. These models show the thermal dissipation pathway, which is different for cylindrical versus prismatic versus pouch cell formats. The one common feature is high contribution to heat removal through current collector foils to tabs. There is more thermal dissipation when there is more metal pathway in the cell construction. Often, there are too few thermal sensors in a fielded battery to benefit from the modeling directly. And thermal sensors only measure outside the cell—not inside which is going to be higher temperature.” 

He continues: “Cells that spend longer times/higher temperatures experience faster cycle life decline due to the acceleration of degradation reactions in the cell. The side reactions that form solid electrolyte interphase (SEI) layers are one important example. But the rate of other side reactions also increases, and most are exothermic—exacerbating the problem. These reactions build up internal resistance and make the cell appear to have lost capacity. Some reactions do degrade active materials or consume lithium and contribute directly to capacity loss.”

Turi states: “Thermal management systems that keep internal cell temperatures below ~40℃ in a consistent way avert the worst of the damage from the above side reactions.”

Swarts continues: “There are methods that can improve battery capacity degradation in BESS: Temperature influences battery cell degradation significantly, and differential aging rates between cells within a module at even slightly different temperatures can lead to antagonistic interactions, thereby impacting overall capacity retention. Given the increased pack sizes, power density and packaging challenges, the use of simulation tools and laboratory controlled tests to ensure temperature uniformity is becoming a necessity.” 

He adds: “From a safety point of view, the ability to remove heat during the early stages of battery failure is key to limit temperature increases and prevent thermal runaway propagation to adjacent cells and modules. This is enabled by not only good thermal design, such as providing means of cooling and selection of materials with the appropriate properties, but also appropriate routing of vent gasses and hot ejecta.”

Swarts says: “The biggest challenge is the multitude of factors that impact battery life: All the way from the basic cell chemistry to how a pack is designed, assembled and used. There is no ‘one-size-fits-all’ solution, so understanding the contributions of and interaction between the various factors is critical.”

Rabin shares: “Actually most of the challenges in working with battery systems today aren’t really around degradation, although degradation can happen faster with batteries that are not managed properly. I would say they are first and foremost around finding access to reliable battery pack manufacturers who’ve spent the time and resources developing robust battery management systems (BMS), which are UL/UN certified, and fully tested for thermal runaway. Not all BMSs are the same, and not all battery cells are the same, so creating a robust pack is the key, and then also how these packs are then managed by a ‘pack controller’ or ‘energy management system.’ The cooling will be integrated within these systems. Although still wild west in many respects with regard to industry standards, we are seeing the industry circling around the UL 9540 certification.⁷ Many battery groups big and small claim to have certifications or be ready to certify; however we’re still a long way off from this.”

Innovation in thermal monitoring and battery management systems
Turi shares that some innovative approaches to thermal management are “the use of nonflammable/inert electrolyte solvents to raise the operating temperature of the battery, for example. Also having extra metal pathways helps to dissipate heat from the cell.”

Most of the emerging technologies (real time/live) in thermal monitoring and control might fall under solid state, where cell temperatures can rise with impunity—until the cathode active material (CAM) autothermal temperature, Turi says.

Swarts says “Physics-based models required to describe complex systems are not practical to develop or resolve at large scale, so there is an increasing reliance on artificial intelligence (AI) and machine learning (ML) to reduce large amounts of data to useful and deployable tools. Ensuring that the training data is sufficiently rich and broad is an ongoing challenge to deliver robust models.”

Rabin states: “First and foremost, starting with innovative and efficient component and system design (i.e., solid state) will already help make the system run more efficiently with significantly less heat generation. With the move to solid state everything, we’re seeing way fewer moving parts, and this is a good thing! Secondly, BMS and energy management system (EMS) design are critical to the longevity of the system, useful life of the batteries, safety and more. Third, new and novel battery chemistries and technologies are coming into the market that aren’t based on lithium-ion technology, which is known for thermal runaway. Long-duration batteries like zinc-air, and flow batteries like vanadium, are coming online. The last thing I’ll say is don’t be fooled by every new news article and new research being touted as the next great thing. These are essential for moving the industry forward, however any new technology moving from small lab scale to fully commercialized and certified can take a decade or more, and if you think of buying an EV today, those batteries will still be around in 15 years, so things do move slower than the media would have us believe. When betting our global transportation, communications and grid systems on batteries, it is clear that some level of conservatism will be built into them.” 

When asked about the next big thing for BESS in general, Rabin shares: “Solid state for both batteries and power electronics. We’re also seeing the introduction of less energy dense, yet safer and cheaper battery chemistries used for stationary applications. Lithium-ion batteries are amazing for the energy and power density, and really effective for mobility applications, however, don’t need to be used for all grid applications.” 

Turi adds: “One last point—the types of thermal management for BESS can vary. Most of the containerized BESS units use heating, ventilation and air conditioning (HVAC) systems to control the inside temperature, and the HVAC becomes a substantial part of the equipment and operating costs. Smaller BESS can be placed in underground enclosures in order to even out the extremes of ambient temperature ranges. But it does seem that in most cases, the battery and the thermal management system are mainly developed independently and integrated as separate components. In these cases, there is room for innovation to optimize the entire BESS design.”

Summary
BESS are vital in grid expansion planning, as the use of BESS is adopted to improve power quality, voltage and frequency control, peak shaving, load smoothing and energy arbitrage. The thermal management process, which is a critical component of the battery management system, is most concerned with estimating the precise state of temperature. Based on the discussion in this article, many variables affect battery degradation impacting their efficiency, lifespan and overall system performance. To address these challenges, future research needs to consider these variables while addressing environmental concerns such as disposal and recycling.

REFERENCES
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Dr. Yulia Sosa is a freelance writer based in Peachtree City, Ga. You can contact her at dr.yulia.sosa@gmail.com.