Immersion Cooling Fluids – Selection Guide
What to look for in a dielectric fluid for battery energy storage, data centres, e-motors and power electronics.
How immersion cooling works
In immersion cooling, components are submerged directly in a dielectric fluid. The fluid insulates electrically and carries heat away from battery cells, chips, windings or power semiconductors. In single-phase systems the fluid stays liquid and is pumped through a heat exchanger. In two-phase systems a low-boiling fluid evaporates at the hot surface and condenses again.
Single-phase systems mainly use hydrocarbon fluids, synthetic esters or natural esters. Two-phase systems have mostly relied on fluorinated fluids, which are increasingly affected by the planned EU restriction of PFAS.
Selection criteria
| Criterion | Why it matters |
|---|---|
| Dielectric strength | Insulates live parts at the voltages in the system |
| Viscosity | Determines heat transfer and pump power; low viscosity suits server and battery tanks |
| Flash point and fire point | Fire safety of the installation |
| Material compatibility | Seals, cable insulation, plastics, adhesives and thermal interface materials must not swell, leach or degrade |
| Oxidation and thermal stability | Long service life without sludge or acid formation |
| Pour point | Start-up in cold environments |
Battery energy storage systems (BESS)
Battery storage is a key building block of solar and wind projects. Immersion cooling keeps cell temperatures uniform and reduces hot spots inside the modules. Cell chemistry, module design and housing materials differ from project to project, so the fluid has to be qualified for each system – including its behaviour in safety tests of the complete system.
Data centres
Single-phase immersion allows high rack densities and reduces the need for air cooling of the IT hardware. Low viscosity, high dielectric strength and long-term compatibility with cables, connectors and thermal interface materials are decisive.
E-motors, power electronics and bearings – IsoTec EC 68
IsoTec EC 68 is a synthetic polyol ester (ISO VG 68) for dielectric immersion cooling of electric motors and power electronics and for bearing lubrication – one fluid instead of separate cooling and lubrication circuits.
| Typical value | IsoTec EC 68 |
|---|---|
| Viscosity grade | ISO VG 68 |
| Viscosity index | ≥ 115 |
| Pour point | −39 °C |
| Flash point (COC, ASTM D92) | above 260 °C |
| Breakdown voltage (IEC 60156) | above 40 kV |
| FZG scuffing load stage (DIN ISO 14635-1) | ≥ 12 |
More on the IsoTec immersion cooling page.
Laboratory testing for your system
Our laboratory tests material compatibility and fluid properties with the components of your system – seals, cables, plastics and cell or server components – before a fluid is used. This applies to battery storage and data centre projects in particular, where the fluid is selected for the specific design.
Frequently asked questions
Single-phase or two-phase immersion?
Single-phase systems are simpler and use non-boiling fluids such as esters or hydrocarbons. Two-phase systems use low-boiling fluids, mostly fluorinated.
Why are esters used for immersion cooling?
Esters combine high dielectric strength with a high flash point and good thermal stability. Viscosity and material compatibility have to match the application.
Can IsoTec EC 68 be used for battery or server tanks?
IsoTec EC 68 is designed for e-motors, power electronics and bearings. For battery storage and data centres we select and test a fluid for your specific system.
Planning an immersion cooling project?
Tell us about your system – we select a suitable fluid and test it in our laboratory. IsoTec products are supplied to customers outside Germany.