Immersion Fluids as a Core Safety and Performance Decision
As battery energy storage systems continue to scale in size and move closer to people, buildings, and critical infrastructure, expectations around safety and reliability are rising fast. Long-term performance now carries equal weight. Thermal runaway, fire propagation, and siting limitations are central design constraints. At the same time, asset owners expect these systems to operate reliably for decades, not just a few years.
At EticaAG, we see immersion cooling as a direct response to these pressures. More specifically, we see the choice of immersion liquid as one of the most important safety and performance decisions in a modern BESS.
Among the available options, synthetic ester fluids stand out as purpose-built materials that address fire prevention and battery longevity at the same time. Their engineered chemistry allows them to support cell-level thermal control, resist ignition under extreme conditions, and maintain stability over long operating lifetimes.
Why the Immersion Liquid Matters in Battery Energy Storage
Growing Focus on Safety, Thermal Runaway, and Siting Constraints
Lithium-ion batteries store an enormous amount of energy in a compact footprint. That energy density is what makes BESS economically valuable, but it also introduces risk. Elevated temperatures accelerate degradation. Localized heating can trigger thermal runaway, and once a fire starts, it can propagate rapidly if conditions allow.
These risks directly affect where BESS can be deployed. Urban sites, rooftops, commercial buildings, and mission-critical facilities demand higher safety margins than remote utility locations. Regulators, insurers, and fire authorities are increasingly scrutinizing not just system-level protections, but the fundamental materials used inside the system.
Fluid Selection as a Core Design Variable
Traditional BESS design places heavy emphasis on hardware. Enclosures, HVAC systems, fire detection, and suppression equipment all play important roles. Immersion cooling changes that equation.
When battery cells are submerged in a dielectric liquid, the fluid itself becomes a core part of the safety and thermal architecture. Heat is removed directly from the cell surface, and fires are stopped at the cell level. The chemical and physical properties of the liquid now matter as much as the mechanical design of the enclosure.
How Immersion Liquid Shapes Safety and Reliability
In an immersion-cooled system, the dielectric liquid performs three critical functions simultaneously.
- It provides direct and uniform thermal management. Heat is absorbed at the point of generation, reducing temperature gradients between cells.
- It contributes to fire prevention. By surrounding the cells, the liquid limits oxygen availability and dissipates heat that would otherwise support ignition and propagation.
- It supports long-term system reliability. Stable operating temperatures and chemically robust fluids slow degradation of both cells and supporting components.
Because of this, immersion liquid selection is a safety-critical design choice.
What Is a BESS Immersion Liquid?
Immersion Cooling in Battery Energy Storage Systems
Immersion cooling refers to a thermal management approach in which battery cells are fully submerged in a dielectric liquid. Because it is electrically non-conductive, the fluid can safely surround energized components. This allows direct contact with battery cells without the risk of an electrical short circuit.
This is fundamentally different from indirect cooling methods, where air or liquid flows through channels, plates, or coils that are thermally coupled to the battery but physically separated from it.
Role of the Dielectric Liquid
Within a BESS immersion cooling architecture, the dielectric liquid enables:
- Direct heat removal from battery cells, including hotspots that are difficult to address with indirect methods
- Fire prevention and propagation control by limiting ignition conditions and absorbing thermal energy
- More uniform temperature distribution across modules and packs
Immersion liquid does not stop a defective cell from failing, but it fundamentally changes how that failure behaves. The liquid removes heat and displaces oxygen. As a result, the surrounding environment no longer supports combustion, and the event is typically contained to the originating cell.
Why Not All Immersion Liquids Are the Same
A common misconception is that any non-conductive liquid can function as an immersion fluid. In reality, BESS immersion liquids must meet a demanding set of criteria simultaneously.
A true BESS immersion liquid must deliver:
- High dielectric strength to ensure electrical insulation
- Effective thermal conductivity and heat capacity for continuous heat removal
- Chemical and oxidation stability under elevated temperatures
- High fire-point and fire-resistant behavior to support fire prevention
- Favorable environmental and regulatory characteristics
When evaluated against these requirements, three fluid categories are commonly discussed for BESS immersion applications: mineral oil, natural ester fluids, and synthetic ester fluids.
Mineral Oil in BESS Applications
Legacy Use in Electrical Equipment
Mineral oil has a long history in electrical equipment, particularly in transformers. It is familiar, widely available, and relatively inexpensive. For decades, it served its purpose well in applications with lower power density and slower thermal dynamics.
Limitations Under Modern BESS Operating Conditions
Battery energy storage systems present a very different operating profile. High C-rate cycling, frequent thermal transients, and long service expectations expose the limitations of mineral oil.
From a safety perspective, mineral oil has a lower fire-point than ester-based fluids. It is also more flammable under sustained heat exposure. Under controlled flammability testing, mineral oil ignites more readily and sustains combustion once ignited. This behavior increases the risk of fire propagation if an external ignition source is present.
Mineral oil is also fossil-based, with a weaker environmental profile. Its oxidation stability under continuous high-temperature operation is limited, which can lead to fluid degradation over time.
As regulatory scrutiny increases and siting moves closer to people and assets, these limitations translate into tighter siting restrictions, higher insurance scrutiny, and fewer viable deployment locations.
Natural Ester Fluids in BESS Applications
Natural ester fluids are typically derived from vegetable oils. Chemically, they are triglycerides composed of fatty acids. Their renewable origin and biodegradability make them attractive from an environmental standpoint.
Benefits Compared to Mineral Oil
Natural esters offer higher fire-points than mineral oil and improved biodegradability. These properties reduce ignition risk and improve environmental acceptability compared to legacy transformer oils.
Limitations in Immersion-Cooled BESS
For immersion-cooled battery systems, natural esters introduce several challenges.
Their molecular structure includes unsaturated bonds, which makes them more susceptible to oxidation. Over long operating lifetimes and elevated temperatures, this can lead to chemical degradation.
Natural esters also tend to have higher viscosity at low temperatures, which can complicate cold-climate operation. In addition, because they are derived from agricultural feedstocks, their properties can vary depending on source and processing.
These characteristics make natural esters safer than mineral oil, but less predictable over long operating lifetimes, increasing uncertainty around maintenance, fluid management, and lifecycle cost in high-cycling BESS environments.
Synthetic Esters: A Simplified Chemistry Overview
Synthetic esters are engineered fluids created through controlled chemical reactions between acids and alcohols. Unlike natural esters, their molecular structure is intentionally designed rather than inherited from biological sources.
Synthetic esters are formed through an esterification reaction, where a carboxylic acid reacts with an alcohol to produce an ester molecule and water as a byproduct. By selecting specific acids and alcohols, chemists can control the resulting molecular structure and tailor the fluid’s performance characteristics.
This molecular control allows properties such as thermal stability, viscosity, oxidation resistance, and fire behavior to be built into the fluid itself rather than relying heavily on additives.
One important design variable is saturation. Synthetic esters can be engineered with saturated molecular structures that resist oxidation far better than vegetable-based triglycerides, improving long-term stability and reducing degradation under sustained heat.
For BESS immersion cooling, synthetic esters are purpose-built materials designed through controlled esterification for safety-critical energy storage environments.
Why Synthetic Esters Are the Best Choice for BESS Immersion Cooling
Fire Prevention and Fire Resistance
Fire safety is a key reason we advocate for synthetic ester immersion liquids in battery energy storage systems.
Synthetic esters used in BESS applications exhibit very high fire-points and auto-ignition temperatures. For example, advanced synthetic ester fluids used in BESS immersion applications typically demonstrate flash points above 250°C (482°F) and ignition points above 300°C (572°F), values that are significantly higher than those of mineral oil and directly reduce ignition risk under thermal stress.
| Coolant Type | Flash Point °C | Ignition Point °C |
|---|---|---|
| Mineral Oil (MIC) | 199.2 ± 0.4 | 218.4 ± 0.2 |
| Silicone Oil (SIC) | 212.8 ± 1.0 | 224.5 ± 0.6 |
| Ester-Based (EIC) | 261.3 ± 2.8 | 301.3 ± 2.3 |
High fire-point behavior delays ignition and resists sustained combustion. Experimental testing shows ester-based immersion coolants (EIC) require substantially more thermal energy to ignite than mineral oil (MIC) and silicone oil (SIL). Under controlled flammability conditions, synthetic esters can withstand prolonged exposure to heat sources without sustaining combustion, while mineral oil ignites and continues to burn.
In an immersion-cooled system, the liquid fully surrounds the battery cells, limiting oxygen availability at the cell surface while continuously dissipating heat. If a cell fails and enters thermal runaway, the synthetic ester helps prevent ignition and limits the event to the originating cell by suppressing the conditions required for fire propagation.
Thermal Management at the Cell Level
Synthetic esters also excel at thermal management.
Because the liquid is in direct contact with the cells, heat is removed evenly across all surfaces. Temperature gradients within modules and packs are reduced. This uniformity improves control during high C-rate charging and discharging, as well as during abnormal conditions.
Better thermal control directly supports safety by limiting peak temperatures. Lower thermal stress on individual cells reduces the likelihood of cascading failures.
The figure below illustrates how direct contact between the immersion liquid and battery cells enables more uniform heat removal compared to indirect cooling approaches, which is why fluid properties play such a central role in immersion-cooled BESS.
Oxidation Stability and Long-Term Robustness
BESS assets are expected to operate for 10 to 20 years or more. Over that timeframe, fluid stability matters.
Synthetic esters are engineered for oxidation resistance under continuous elevated temperatures. This stability reduces fluid degradation, minimizes byproduct formation, and supports consistent performance over the life of the system.
Fewer fluid replacements translate into lower lifecycle risk and more predictable operating costs. This stability supports predictable performance over the full asset life, which simplifies risk assessment for owners, insurers, and permitting authorities.
Environmental and Regulatory Advantages
Synthetic ester formulations used in BESS immersion cooling are readily biodegradable and exhibit low toxicity. These characteristics simplify handling, reduce environmental risk, and align with modern permitting expectations.
As siting moves into urban and sensitive environments, these advantages become increasingly important.
From Fluid Properties to System-Level Safety: EticaAG and Shell
When evaluated specifically for battery energy storage immersion cooling, synthetic ester fluids consistently outperform mineral oil and natural esters across the criteria that matter most. Fire prevention behavior, thermal performance, chemical stability, and environmental profile all favor synthetic esters when assessed as part of a complete BESS architecture.
System-level safety outcomes are ultimately driven by how materials behave under thermal stress. Synthetic esters resist ignition, limit the energy available to sustain combustion, and enable uniform cell-level cooling that supports long-term, predictable battery operation.
At EticaAG, we treat immersion liquid selection as core system engineering. Our collaboration with Shell reflects this approach. Shell brings deep expertise in engineered synthetic ester fluids developed for demanding electrical and energy applications. By aligning fluid chemistry with immersion architecture and fire prevention objectives, we design systems that prioritize failure containment, operational stability, and asset longevity.
Immersion cooling, combined with purpose-built synthetic ester fluids, represents a shift from reactive protection to safety by design. By addressing the thermal and chemical conditions that allow failure to propagate, this architecture enables safer deployment of battery energy storage systems in increasingly demanding environments.
