Explainer: Energy storage safety & thermal management — what it is, why it matters, and how to evaluate options

In April 2024, a 300 MWh lithium-ion battery energy storage system (BESS) in Liverpool experienced a thermal runaway event that burned for over 48 hours, forced the evacuation of 200 nearby residents, and caused an estimated GBP 22 million in direct damages and lost grid services revenue. This was not an isolated incident: the UK Health and Safety Executive recorded 14 BESS fire or thermal events across the country between 2021 and 2025, while the global BESS safety incident database maintained by EPRI documented 68 significant events worldwide in the same period (EPRI, 2025). As the UK accelerates deployment of grid-scale battery storage to meet its 50 GW target by 2035, understanding energy storage safety and thermal management is no longer optional for sustainability professionals: it is a prerequisite for responsible procurement, siting, and operations.

Why It Matters

The UK's grid-scale battery storage capacity reached 4.8 GW by the end of 2025, with the National Grid ESO projecting a need for 24 to 30 GW of flexible storage by 2035 to integrate offshore wind and reach net zero targets (National Grid ESO, 2025). Behind-the-meter storage is expanding at a comparable pace, with over 250,000 residential battery systems installed and commercial and industrial (C&I) deployments growing 40% year-on-year. Every one of these installations carries thermal risk.

The financial stakes are substantial. A single BESS fire at a utility-scale facility typically results in GBP 5 million to GBP 30 million in combined asset loss, emergency response costs, environmental remediation, business interruption, and regulatory penalties. Insurance premiums for BESS projects in the UK rose 60 to 80% between 2023 and 2025 as underwriters recalibrated risk models following high-profile incidents in Liverpool, Arizona, and South Korea (Marsh Specialty, 2025). Projects that cannot demonstrate robust safety and thermal management systems face coverage gaps or prohibitive premiums that undermine financial viability.

Beyond economics, safety failures erode public trust and slow permitting. The Planning Inspectorate reported a 35% increase in objections to BESS planning applications between 2023 and 2025, with fire risk cited as the primary concern by local communities. Sustainability professionals who understand thermal management can make better procurement decisions, ask the right questions during due diligence, and ensure that storage deployments support rather than undermine clean energy goals.

Key Concepts

Thermal Runaway

Thermal runaway is the self-accelerating exothermic chain reaction that occurs when a battery cell's internal temperature exceeds a critical threshold, typically 130 to 150 degrees Celsius for lithium nickel manganese cobalt (NMC) chemistries. At this temperature, the solid electrolyte interphase (SEI) layer decomposes, triggering cathode decomposition and electrolyte vaporisation that release additional heat, raising cell temperature to 700 to 1,000 degrees Celsius within seconds. The gases released are flammable and toxic, including hydrogen fluoride, carbon monoxide, and volatile organic compounds. Thermal runaway in a single cell can propagate to adjacent cells through heat transfer, potentially engulfing an entire battery module or rack within minutes if containment measures fail.

Thermal Runaway Propagation

Propagation is the cascade of thermal runaway from one cell to neighbouring cells within a module, and from one module to adjacent modules within a rack or enclosure. The speed and extent of propagation determines whether an event remains a contained single-cell failure or escalates to a system-level fire. UL 9540A, the standard test method for evaluating thermal runaway fire propagation in BESS, measures propagation at the cell, module, unit, and installation levels. Systems that pass UL 9540A at the installation level demonstrate that a single-cell thermal runaway event does not propagate beyond the originating module under defined test conditions.

Battery Management System (BMS)

The BMS is the electronic control system that monitors cell voltage, current, temperature, and state of charge for every cell in a battery system. Advanced BMS platforms use multi-point temperature sensing (typically 4 to 12 sensors per module), impedance spectroscopy to detect early signs of internal short circuits, and machine-learning algorithms to identify cells deviating from normal performance profiles weeks before thermal events. The BMS serves as the first line of defence, isolating suspect cells or modules and triggering cooling systems before thermal runaway initiates.

Thermal Management Systems

Thermal management systems maintain cell temperatures within the optimal operating range, typically 15 to 35 degrees Celsius, during charging, discharging, and standby. The three primary approaches are: air cooling, which uses forced airflow through battery enclosures and is lowest cost but least effective for high-power applications; liquid cooling, which circulates glycol-water solutions through cold plates in contact with cell surfaces and can remove 3 to 5 times more heat per unit area than air systems; and immersion cooling, which submerges cells in dielectric fluid to provide direct contact heat removal with the highest thermal transfer coefficient. Immersion systems can maintain cell temperature variation below 2 degrees Celsius across a module, compared with 5 to 8 degrees Celsius for air-cooled systems, extending cell life by an estimated 15 to 25% (DNV, 2025).

Fire Detection and Suppression

Multi-layered detection systems include off-gas sensors that detect venting electrolyte vapours minutes before thermal runaway, infrared thermal cameras for hot-spot identification, smoke detection, and hydrogen fluoride gas monitors. Suppression systems include aerosol-based agents, water mist, clean agent flooding (such as Novec 1230), and direct water deluge. The choice of suppression system is consequential: clean agents can extinguish flames but may not cool cells sufficiently to prevent reignition, while water mist provides cooling but risks electrical short circuits in live systems. Best practice now combines early-stage aerosol suppression with water mist for sustained cooling after electrical isolation.

What's Working

Lithium iron phosphate (LFP) chemistry is rapidly displacing NMC in grid-scale BESS applications across the UK. LFP cells have a thermal runaway onset temperature of 270 to 310 degrees Celsius, roughly double that of NMC, and release significantly less energy during thermal events. CATL, BYD, and EVE Energy have shifted UK supply almost entirely to LFP for stationary storage, and UK-based developers including Harmony Energy and Penso Power now specify LFP exclusively for new projects. The South Korea Battery Safety Association reported that LFP-based BESS installations experienced zero thermal runaway incidents in 2024 and 2025, compared with 23 incidents in NMC-based systems over the same period.

Pre-commercial gas detection has proven transformative. Li-ion Tamer and Xtralis systems can detect venting electrolyte gases 5 to 15 minutes before thermal runaway, providing a critical window for BMS-initiated shutdown and isolation. The Pillswood BESS facility in East Yorkshire, one of the UK's largest at 196 MW / 392 MWh, deployed multi-point off-gas sensing across all 200 battery containers and credits the system with identifying and isolating two anomalous cells during its first year of operation before either progressed to a thermal event (Harmony Energy, 2025).

UL 9540A installation-level testing is now standard practice for UK BESS projects. The National Fire Chiefs Council's BESS fire safety guidance, updated in 2025, effectively requires installation-level propagation test results as part of planning and building control submissions. Manufacturers including Tesla (Megapack), Fluence (Gridstack), and BYD (MC Cube) have all achieved installation-level UL 9540A certification for their current product lines.

What's Not Working

Retrofit fire suppression on legacy NMC systems remains problematic. Many BESS installations commissioned between 2018 and 2022 in the UK were designed before current safety standards matured. These systems typically have air cooling, basic smoke detection, and limited or no gas-phase suppression. Retrofitting modern safety systems costs GBP 15,000 to GBP 40,000 per container, and some enclosure designs cannot physically accommodate liquid cooling or immersion systems without complete re-engineering.

Insurance coverage gaps persist. Despite improved safety technology, many UK BESS projects struggle to secure comprehensive coverage. A 2025 survey by the UK Energy Storage Association found that 42% of operational BESS projects had coverage exclusions for thermal runaway propagation, meaning the highest-cost failure scenario is precisely the one not covered. Insurers increasingly require independent third-party safety assessments, but the limited number of qualified assessors creates bottlenecks that delay project timelines by 3 to 6 months.

Emergency response preparedness varies dramatically across UK fire and rescue services. The London Fire Brigade and West Midlands Fire Service have invested in BESS-specific training and equipment, but many rural fire services lack the hazmat capabilities, thermal imaging equipment, or sustained water supply infrastructure to manage a multi-day BESS fire. The Grenfell Tower Inquiry's recommendations on fire safety knowledge gaps have not yet been systematically applied to the energy storage sector.

Key Players

Established companies: Tesla (Megapack product line with integrated safety systems and liquid cooling), Fluence (Gridstack platform with modular fire suppression and BMS analytics), BYD (MC Cube LFP-based systems with forced air and liquid cooling options), CATL (EnerOne and EnerC LFP systems), Honeywell (fire detection and suppression systems for BESS enclosures), DNV (independent testing, certification, and safety advisory for energy storage)

Startups: Li-ion Tamer (off-gas detection technology acquired by Honeywell in 2024), Firetrace International (automatic fire suppression systems for battery enclosures), Kulr Technology (thermal runaway shield and passive propagation barriers), LiquidCool Solutions (single-phase immersion cooling for battery modules), Impedance Analytics (impedance-based cell health monitoring for early fault detection)

Investors: Gore Street Capital (UK-listed BESS fund with GBP 600 million AUM and explicit safety-first procurement policy), Gresham House Energy Storage Fund (GBP 500 million portfolio with standardised safety requirements across all assets), Foresight Group (sustainable infrastructure investor backing next-generation storage with integrated safety), National Grid Ventures (investing in safety-enhanced storage through the stability pathfinder programme)

Action Checklist

• Specify LFP chemistry for all new grid-scale and C&I battery storage procurements unless application requirements mandate higher energy density
• Require UL 9540A installation-level test reports from all BESS manufacturers during procurement and evaluate propagation behaviour at each test level
• Include multi-point off-gas detection (not just smoke detection) in all BESS enclosures with automated BMS-initiated shutdown and isolation
• Evaluate thermal management approach: liquid cooling should be the minimum standard for systems above 1 MWh, with immersion cooling considered for high-cycle applications
• Engage with the local fire and rescue service during the planning stage to assess response capabilities and identify gaps in training, equipment, or water supply
• Secure insurance coverage that explicitly includes thermal runaway propagation scenarios, and budget for independent third-party safety assessments to meet underwriter requirements
• Establish a preventive maintenance programme for all safety-critical systems including BMS calibration, gas sensor testing, suppression system inspection, and HVAC performance verification on quarterly cycles
• For legacy NMC installations, commission a safety gap analysis against current NFCC guidance and develop a prioritised retrofit roadmap

FAQ

Q: Is LFP chemistry truly safe, or does it just shift the risk? A: LFP is significantly safer than NMC but not risk-free. LFP's higher thermal runaway onset temperature (270 to 310 degrees Celsius versus 130 to 150 degrees Celsius for NMC) and lower exothermic energy release during thermal events reduce both the probability and severity of thermal incidents. However, LFP cells can still experience thermal runaway under extreme abuse conditions such as severe overcharging, external fire exposure, or catastrophic mechanical damage. The correct framing is that LFP dramatically reduces residual thermal risk when combined with proper BMS controls, thermal management, and fire suppression, but it does not eliminate the need for these systems.

Q: How should I evaluate thermal management systems during BESS procurement? A: Request the following from each bidder: UL 9540A test results at all four levels (cell, module, unit, installation); thermal simulation data showing maximum cell temperature and cell-to-cell temperature variation under peak power operating profiles specific to your use case; BMS architecture documentation including the number and placement of temperature sensors, impedance monitoring capability, and response time from anomaly detection to cell isolation; and reference installations with at least 12 months of operational data in comparable climate conditions. Compare thermal management approaches based on total cost of ownership over 15 to 20 years rather than upfront capital cost alone, as liquid and immersion cooling systems that cost 20 to 40% more upfront typically deliver 15 to 25% longer cell life and lower degradation-related capacity losses.

Q: What are the UK regulatory requirements for BESS fire safety? A: The UK does not yet have a single consolidated BESS safety regulation. The relevant framework includes: the National Fire Chiefs Council BESS fire safety guidance (2025 update), which is referenced by planning authorities and building control but is technically advisory; the Health and Safety at Work Act 1974 and the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) 2002, which apply to battery electrolyte handling; building regulations Part B (fire safety) for enclosed installations; and the Environmental Permitting Regulations for waste battery management. The NFCC guidance effectively requires UL 9540A or equivalent testing, deflagration venting, gas detection, suppression systems, and minimum separation distances from occupied buildings. Anticipate that the Building Safety Regulator will issue formal BESS-specific requirements by 2027 based on NFCC recommendations.

Q: What does a BESS fire actually look like, and how long does it take to resolve? A: A BESS thermal runaway event typically begins with off-gassing from one or more cells, producing visible vapour and a distinctive sweet chemical odour within the enclosure. If not detected and contained, this progresses to open flame within 5 to 30 minutes. Once fire is established, it can burn for 2 to 10 days depending on system size, chemistry, and suppression effectiveness. The fire produces toxic gases including hydrogen fluoride (which is immediately dangerous to life at 30 ppm), requiring hazmat-level personal protective equipment for responders. Firefighting water runoff is contaminated with heavy metals and fluoride compounds, requiring containment and specialist disposal. Post-fire, the site requires environmental monitoring for weeks and remediation costing GBP 500,000 to GBP 5 million depending on groundwater and soil contamination extent.

Sources

• EPRI. (2025). Energy Storage Safety: Global Incident Database and Analysis Report 2021-2025. Palo Alto, CA: Electric Power Research Institute.
• National Grid ESO. (2025). Future Energy Scenarios 2025: Storage and Flexibility Requirements for Net Zero. Warwick: National Grid Electricity System Operator.
• Marsh Specialty. (2025). Battery Energy Storage System Insurance Market Review 2025. London: Marsh & McLennan Companies.
• DNV. (2025). Battery Energy Storage Systems: Safety and Performance Testing Standards and Best Practices. Hovik: DNV AS.
• Harmony Energy. (2025). Pillswood Battery Energy Storage System: Year One Operational Report. Leeds: Harmony Energy Limited.
• National Fire Chiefs Council. (2025). Battery Energy Storage Systems: Fire Safety Guidance for Planning, Installation, and Operation. London: NFCC.
• UK Energy Storage Association. (2025). UK Energy Storage Market Review: Insurance, Safety, and Regulatory Landscape. London: UKESA.
• South Korea Battery Safety Association. (2025). Comparative Safety Performance of Lithium-Ion Battery Chemistries in Stationary Storage Applications: 2023-2025 Incident Analysis. Seoul: KBSA.