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How planners and operators of battery energy storage systems (BESS) limit thermal propagation between cell, module, rack and container — codes and standards, material selection, installation, testing and maintenance from a refractory contractor's perspective.
A thermal runaway begins in a single cell: an internal short circuit, mechanical damage, overcharging or a manufacturing defect raises the cell temperature to the point where the decomposition of the electrolyte becomes self-accelerating. Within seconds, several hundred degrees Celsius are reached, along with flammable gases (hydrogen, carbon monoxide, hydrocarbons) and a jet of hot particles. The cell itself cannot be extinguished in this state — the reaction runs until the stored energy is consumed.
What endangers the installation is not the single cell but propagation: the heat from the first cell warms the neighbouring cells until the threshold is exceeded there as well. Without barriers, the chain runs through the module, the rack and finally the container. Every stage that delays transmission by minutes buys the battery management system time to shut down, the gas venting system time to exhaust, and the fire brigade time to arrive.
The Core Task of Refractory Materials
Not to extinguish, but to separate: a thermal barrier between the levels keeps the temperature on the unexposed side below the ignition threshold of the neighbouring cells long enough for propagation to stop or slow to a manageable rate. The benchmark is the time to temperature breakthrough — not the classification temperature of the material alone.
In Germany there is no single, definitive law for stationary battery energy storage — the requirements are assembled from product standards, test methods, insurers' guidelines and building regulations. For material selection inside the container, the decisive elements are above all the test methods by which resistance to propagation is demonstrated.
UL 9540A is the internationally established test method that examines the spread of a thermal runaway in stages at cell, module, unit and installation level. Many tender specifications and insurers now demand a UL 9540A report; the temperatures and times documented in it are the input values for designing the thermal barriers. NFPA 855, as the installation standard, governs separation distances, fire compartments, and suppression and venting concepts. In Europe, IEC 62933-5-2 describes the safety requirements for grid-connected electrochemical storage systems. The VdS 3103 guideline summarises the expectations of German property insurers regarding lithium-ion storage systems.
For the refractory materials themselves, the classifications under DIN EN 13501-1 (reaction to fire, Class A1 for non-combustible products) and the fire resistance tests under DIN EN 1363 and DIN EN 1364 apply. For fibre materials, occupational safety comes on top: alumino-silicate wool (ASW) falls under TRGS 558, bio-soluble AES wool does not — a point that almost always tips the balance in battery storage applications.
Practical Advice
Ask the system supplier for the UL 9540A report at module and unit level before discussing materials. The surface temperatures measured there and the duration of the event determine what barrier performance you need — anything else is designing in the dark.
An effective barrier concept follows the structure of the storage system from the inside out. Each level has its own task, its own temperature load and therefore its own appropriate material. Anyone who installs the same material at every level either pays too much or provides too little protection at the critical point.
Then there are the penetrations: cables, cooling lines, busbars and ventilation openings pass through every level. A partition wall is only as good as its weakest penetration — here, fibre ropes, paper gaskets and fire collars become part of the concept, not accessories.
In an industrial furnace, temperature dictates the material. In battery storage, three additional requirements shape the selection more strongly: installation space, occupational safety and the customer's bill of materials.
Bio-soluble AES wool (SOLUT BW Blanket, BW Board, BW Paper) is exempt from classification as carcinogenic under Note Q of the CLP Regulation and does not fall under TRGS 558. For storage system manufacturers this means: no exposure categories during installation, no labelling obligation in the product, no SVHC notification under REACH Art. 33. With a classification temperature of 1200 °C it covers the temperatures occurring at wall and ceiling during a thermal runaway. Its practical continuous-service limit below roughly 900 °C is irrelevant here — the load case is an event lasting minutes, not continuous operation.
Microporous boards (SOLUT MP Board) offer the lowest thermal conductivity of all insulating materials — around 0,02 to 0,03 W/mK at 200 to 400 °C — and thus the greatest barrier performance per millimetre. That makes them the material of choice between modules, where every millimetre of installation space costs energy density. They are fibre-free, non-combustible (A1) and available faced with aluminium foil, glass cloth or PE film so that they do not release dust during handling.
Calcium silicate boards (SOLUT CAL Board) go where the barrier has to carry load: beneath racks, as partition walls with fixings, as bases for components. Compressive strengths of 13 to 27 N/mm² at a classification temperature of 1000 °C, fibre-free and workable with woodworking tools.
Why Not ASW?
Alumino-silicate wool does withstand 1260 to 1430 °C, but is classified as a Category 1B carcinogen. In a product that manufacturers ship to many countries and that service technicians open, this entails labelling, information and occupational safety obligations that hardly any storage manufacturer wants to bear. The temperature reserve of ASW is not needed in battery storage — the bio-soluble alternative is sufficient for the load case.
The following overview compares the four most important barrier materials — with the properties that really matter in battery storage: thermal conductivity in the event case, installation space requirement, mechanical load capacity and regulatory status.
| Material | Application | Strength | Limitation | Regulatory status |
|---|---|---|---|---|
| AES fibre blanket / board (SOLUT BW) | Container internal lining, wall lining | 1200 °C classification, flexible, no labelling required | needs installation space (25–50 mm), not load-bearing | not covered by TRGS 558, Note Q |
| Ceramic fibre paper (SOLUT BW Paper) | Cell interlayers 1–3 mm | thin, flexible, easy to cut | low mechanical strength | not covered by TRGS 558 |
| Microporous board (SOLUT MP Board) | Module to module, tightest installation space | highest insulating performance per mm (λ ≈ 0,02–0,03 W/mK) | pressure-sensitive, facing required | fibre-free, fire classification A1 |
| Calcium silicate board (SOLUT CAL Board) | Load-bearing partition walls, bases | 13–27 N/mm² compressive strength, can be screw-fixed | heavier, thicker than microporous | fibre-free, fire classification A1 |
Rule of Thumb
Where installation space is the scarcest resource: microporous. Where load is carried or fixings are screwed: calcium silicate. Where surfaces are lined: bio-soluble fibre. Where it has to be thin: fibre paper. ASW only where insulation is permanently above 900 °C — in battery storage, practically never.
The best material selection is worthless if the barrier has gaps. From our installations we know four recurring weak points: open butt joints between boards, unsealed penetrations, steel fixings that act as thermal bridges through the insulation, and insulation layers that are crushed during assembly and thereby lose their thickness and insulating performance.
Verification is carried out in two stages: the material properties come from the data sheets and the fire classifications, and the barrier performance of the system from a propagation test at module or unit level — sensibly in accordance with UL 9540A, because the results are then also usable for insurers and approval authorities. We supply the material data and support the manufacture of test specimens so that the tested system matches what is installed later.
Most delays in storage projects arise not during installation but before it — because the fundamentals are missing when quotations are to be compared. These twelve points should be clarified before the first enquiry goes out:
Our Offer to Planners
Send us the UL 9540A data and the installation space budget — we will propose the layer build-up for each level, supply the material properties for the safety verification and, on request, manufacture test specimens and series kits.
A thermal barrier does not work in normal operation — it waits. Which is exactly why it is easily damaged during maintenance and modification: boards are removed for access and reinstated incompletely, seals at penetrations are lost during cable replacement, insulation layers are crushed by retrofitted components. The maintenance documentation should therefore include a barrier plan that identifies every barrier, every penetration and every seal.
From a Single Source
We supply the materials with data sheets in seven languages, cut boards and blankets to drawing, install with our own fitters and, in the event of modification or decommissioning, handle the dismantling including proof of disposal. For manufacturers producing storage systems in series, we prefabricate barrier kits.
| Standard | Description | Relevance |
|---|---|---|
| UL 9540A | Test method for evaluating thermal runaway propagation at cell, module, unit and installation level | Provides temperatures and times for barrier design; increasingly demanded by insurers and tender specifications |
| NFPA 855 | Installation standard for stationary energy storage systems — separation distances, fire compartments, suppression and venting concepts | Framework for the arrangement of partition walls and distances between units |
| IEC 62933-5-2 | Safety requirements for grid-connected electrochemical energy storage systems | European basis for safety verification of the overall system |
| VdS 3103 | Guideline of the German property insurers for lithium-ion batteries and storage systems | Insurers' expectations regarding siting, thermal barriers and fire protection |
| DIN EN 13501-1 | Classification of the reaction to fire of construction products (Class A1: non-combustible) | Verification of the non-combustibility of the insulating and partition materials used |
| DIN EN 1363-1 / EN 1364 | Fire resistance tests, general requirements and non-loadbearing elements | Verification of the fire resistance of partition walls and linings |
| TRGS 558 | Activities involving high-temperature wool — exposure categories and protective measures | Applies to ASW, not to bio-soluble AES wool; decisive for material selection |
| CLP Regulation, Note Q | Exemption of bio-soluble mineral wools from classification as carcinogenic | Basis for fibre materials in battery storage that require no labelling |
Anton Brem
Managing Director
Refractory construction, high-temperature insulation and fire protection in industrial plants
Everything you need to know about Thermal Barriers in Battery Energy Storage: Refractory Materials Against Thermal Runaway
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