In recent years, the rapid advancement of electrochemical energy storage systems has highlighted the critical need for efficient thermal management solutions. As energy storage cells, particularly lithium-ion batteries, become more powerful and densely packed, their heat generation during operation poses significant safety and performance challenges. Traditional air-cooling methods often fall short in maintaining uniform temperatures and preventing thermal runaway, a dangerous condition where energy storage cells overheat and potentially lead to fires or explosions. To address this, immersion cooling technology has emerged as a promising approach, where energy storage cells are fully submerged in a dielectric coolant for direct and efficient heat dissipation. This article details the development of a novel immersion oil-based coolant, focusing on its formulation, performance evaluation, and compatibility with energy storage cells and associated materials.
The increasing demand for high-capacity energy storage cells in applications such as grid storage and electric vehicles has accelerated research into advanced cooling techniques. Immersion cooling offers superior thermal management by enabling direct contact between the coolant and energy storage cells, facilitating rapid heat transfer and minimizing temperature gradients. However, the success of this method relies heavily on the properties of the coolant, including its thermal conductivity, electrical insulation, and chemical stability. In this work, I developed a mineral-based immersion coolant tailored for lithium-ion battery energy storage systems, ensuring it meets stringent requirements for safety and longevity. The following sections outline the technical specifications, manufacturing process, and experimental validation of this coolant, with an emphasis on its ability to enhance the reliability of energy storage cells.
Technical Specifications and Production Process
To establish a baseline for the immersion coolant, I defined key quality control indicators based on the operational demands of energy storage cells. These indicators ensure the coolant maintains optimal performance under varying conditions, such as high temperatures and electrical stresses. Table 1 summarizes the primary technical specifications for the coolant, referred to as EBC160, which includes parameters like viscosity, flash point, and dielectric strength. These criteria were selected to align with the need for efficient heat removal and electrical isolation in energy storage cells, preventing short circuits and degradation.
| Parameter | Control Limit | Test Method |
|---|---|---|
| Kinematic Viscosity at 40 °C (mm²/s) | 9–11 | GB/T 265 |
| Flash Point (Open Cup, °C) | ≥160 | GB/T 3536 |
| Pour Point (°C) | ≤-50 | GB/T 3535 |
| Acid Value (mg KOH/g) | ≤0.02 | NB/SH/T 0836 |
| Water Content (μg/g) | ≤30 | SH/T 0207 |
| Breakdown Voltage (kV) | ≥50 | GB/T 507 |
| Rotating Oxygen Bomb (min) | ≥400 | SH/T 0193 |
| Evaporation Loss (%, 50 °C, 168 h) | ≤0.5 | In-house Method |
| Acute Dermal Irritation/Corrosion | Non-irritating | GB/T 21604 |
| LD50 (μg/g) | >2,000 | OECD 423 |
The production of EBC160 coolant involves a meticulous process starting with the selection of high-quality crude oil from specific regions, followed by fractionation using molecular refining techniques. This approach allows for the isolation of hydrocarbon components with ideal carbon numbers and boiling ranges, ensuring the base oil possesses low viscosity for efficient flow and high flash point for safety. The process flowchart in Figure 1 illustrates the steps: crude oil selection, distillation to obtain suitable fractions, hydrotreatment to remove impurities, and blending with additives. The additive package, including antioxidants, is optimized to enhance oxidation stability and compatibility with energy storage cells. This streamlined production ensures the coolant can withstand the harsh environments typical of energy storage systems, where energy storage cells are subjected to cyclic charging and discharging.
In developing the base oil, I evaluated several candidate fractions to identify the optimal balance of properties. The kinematic viscosity, for instance, is critical for heat transfer efficiency; too high a viscosity can impede flow, while too low may reduce cooling capacity. The viscosity index (VI) is calculated using the standard formula:
$$ VI = \frac{L – U}{L – H} \times 100 $$
where (U) is the kinematic viscosity of the oil at 40 °C, (L) and (H) are reference values based on the oil’s viscosity at 100 °C. A higher VI indicates less viscosity change with temperature, which is desirable for energy storage cells operating in fluctuating conditions. Table 2 presents the key properties of the base oil fractions considered, with Fraction 1 ultimately selected for its superior combination of low pour point and high flash point, essential for protecting energy storage cells in cold and hot climates.
| Fraction | Kinematic Viscosity at 40 °C (mm²/s) | Flash Point (°C) | Density at 20 °C (kg/m³) | Pour Point (°C) | Acid Value (mg KOH/g) |
|---|---|---|---|---|---|
| Fraction 1 | 10.75 | 172 | 830.8 | -64 | 0.01 |
| Fraction 2 | 9.25 | 168 | 850.5 | -63 | 0.01 |
| Fraction 3 | 7.18 | 150 | 881.9 | -56 | 0.01 |
| Fraction 4 | 9.08 | 158 | 864.8 | -62 | 0.01 |
Formulation and Antioxidant Evaluation
The formulation of EBC160 coolant required careful selection of antioxidants to prevent oxidative degradation, which can lead to sludge formation and reduced cooling efficiency in energy storage cells. I incorporated T501 (2,6-di-tert-butyl-p-cresol), a common antioxidant in transformer oils, and conducted rotating oxygen bomb tests to determine the optimal concentration. The test measures the induction time in minutes, indicating how long the oil resists oxidation under accelerated conditions. The relationship between antioxidant concentration and oxidation stability can be modeled using an exponential decay function:
$$ t = A(1 – e^{-kC}) $$
where (t) is the induction time, (C) is the antioxidant concentration, (A) is the maximum achievable induction time, and (k) is a rate constant. As shown in Table 3, increasing the T501 content from 0% to 0.5% significantly extended the induction time, with diminishing returns beyond 0.3%. Based on this, I selected 0.4% as the ideal concentration, providing robust protection for energy storage cells without unnecessary cost.
| Sample | Antioxidant Content (%) | Induction Time (min) |
|---|---|---|
| 0 | 0 | 61 |
| 1 | 0.1 | 304 |
| 2 | 0.2 | 441 |
| 3 | 0.3 | 504 |
| 4 | 0.4 | 508 |
| 5 | 0.5 | 515 |
After finalizing the formulation, I analyzed the EBC160 coolant to verify it meets all specifications. Table 4 displays the typical quality analysis data, confirming excellent properties such as low acid value, high breakdown voltage, and minimal evaporation loss. These characteristics are crucial for maintaining the integrity of energy storage cells, as they prevent corrosion, electrical failures, and coolant loss over time. The coolant’s non-irritating nature and high LD50 value further ensure safety during handling and operation.
| Parameter | Control Limit | EBC160 Result |
|---|---|---|
| Kinematic Viscosity at 40 °C (mm²/s) | 9–11 | 9.36 |
| Kinematic Viscosity at 100 °C (mm²/s) | Report | 2.31 |
| Flash Point (°C) | ≥160 | 166 |
| Pour Point (°C) | ≤-50 | -69 |
| Acid Value (mg KOH/g) | ≤0.02 | 0.01 |
| Water Content (μg/g) | ≤30 | 21 |
| Breakdown Voltage (kV) | ≥50 | 62 |
| Rotating Oxygen Bomb (min) | ≥400 | 540 |
| Evaporation Loss (%) | ≤0.5 | 0.08 |
| Acute Dermal Irritation/Corrosion | Non-irritating | Non-irritating |
| LD50 (μg/g) | >2,000 | >2,000 |
Material Compatibility Assessment
Compatibility with materials used in energy storage cells and their enclosures is paramount to avoid degradation, leaks, or electrical issues. I conducted accelerated aging tests by immersing various metals and polymers in EBC160 coolant at 85 °C for 336 hours, simulating long-term exposure. The materials included components like terminals, seals, and insulators commonly found in energy storage cells. After testing, I measured mass changes and inspected for visual alterations, while also analyzing the coolant for changes in key properties. The mass change percentage is calculated as:
$$ \text{Mass Change (\%)} = \frac{W_f – W_i}{W_i} \times 100 $$
where (W_i) and (W_f) are the initial and final masses, respectively. As shown in Table 5, all materials exhibited minimal mass changes (within ±0.5%), indicating no significant swelling or dissolution. For instance, polymer films used in energy storage cells showed negligible effects, ensuring structural integrity.
| Material | Initial Mass (g) | Final Mass (g) | Mass Change (%) |
|---|---|---|---|
| Terminal Connector | 4.3545 | 4.3615 | 0.16 |
| Polymer Film | 2.0694 | 2.0587 | -0.52 |
| Sealing Material | 0.3622 | 0.3618 | -0.02 |
| Insulator | 0.5966 | 0.5988 | 0.11 |
| Electrode Tab | 3.4385 | 3.4387 | 0.06 |
Post-test analysis of the coolant revealed no adverse effects on its electrical and chemical properties. Table 6 summarizes the results, with parameters like water content, acid value, dielectric dissipation factor, permittivity, and breakdown voltage remaining stable. This confirms that EBC160 coolant does not degrade or interact negatively with materials in energy storage cells, supporting its long-term use in immersion cooling systems. The dielectric dissipation factor, for example, stayed below 0.0001, indicating minimal energy loss and excellent insulation for energy storage cells.
| Material Exposed | Water Content (μg/g) | Acid Value (mg KOH/g) | Dielectric Dissipation Factor | Permittivity | Breakdown Voltage (kV) |
|---|---|---|---|---|---|
| None (Blank) | 22.7 | 0.006 | 0.00006 | 2.13 | 64.8 |
| Terminal Connector | 23.9 | 0.005 | 0.00005 | 2.11 | 63.5 |
| Polymer Film | 22.2 | 0.006 | 0.00006 | 2.12 | 61.8 |
| Sealing Material | 21.8 | 0.006 | 0.00006 | 2.11 | 63.5 |
| Electrode Tab | 24.0 | 0.006 | 0.00006 | 2.13 | 66.8 |
Safety Performance and Thermal Runaway Prevention
One of the most critical aspects of immersion cooling is its ability to mitigate thermal runaway in energy storage cells, a chain reaction that can cause overheating, gas generation, and fires. To evaluate this, I performed nail penetration tests on lithium-ion energy storage cells, comparing their behavior in air versus submerged in EBC160 coolant. The test simulates an internal short circuit by piercing the cell, triggering rapid heat release. The temperature rise during thermal runaway can be described by the Arrhenius equation:
$$ k = A e^{-E_a / RT} $$
where (k) is the reaction rate constant, (A) is the pre-exponential factor, (E_a) is the activation energy, (R) is the gas constant, and (T) is the temperature. In air, the cell surface temperature exceeded 400 °C, accompanied by flames and smoke. In contrast, when immersed in EBC160, the coolant absorbed heat rapidly, limiting the cell surface temperature to 280 °C and the coolant bulk temperature to 48 °C. The heat transfer efficiency can be approximated using Newton’s law of cooling:
$$ \frac{dQ}{dt} = h A (T_{\text{cell}} – T_{\text{coolant}}) $$
where (dQ/dt) is the heat transfer rate, (h) is the heat transfer coefficient, (A) is the surface area, and (T_{\text{cell}}) and (T_{\text{coolant}}) are the temperatures of the energy storage cell and coolant, respectively. The high (h) value of the coolant ensures quick dissipation, preventing propagation to adjacent energy storage cells.
The image above illustrates the nail penetration test setup, highlighting the stark difference in safety outcomes. In air, the energy storage cell erupts in flames, whereas in coolant, only white vapor from electrolyte evaporation is visible, with no fire or explosion. This demonstrates how immersion cooling with EBC160 can fundamentally resolve the safety challenges associated with thermal runaway in energy storage cells, making it a viable solution for large-scale energy storage systems.
Conclusion
Through systematic development and testing, I have successfully created an immersion oil-based coolant, EBC160, that excels in thermal management, electrical insulation, and material compatibility for lithium-ion battery energy storage systems. The coolant’s formulation, derived from carefully selected base oils and antioxidants, ensures long-term stability and performance, even under extreme conditions. Experimental results confirm that it effectively prevents thermal runaway in energy storage cells by rapidly absorbing heat and suppressing fires, addressing a global safety concern. As the demand for reliable energy storage cells grows, this immersion cooling technology is poised to become a mainstream solution, enhancing the safety and efficiency of electrochemical energy storage systems worldwide. Future work will focus on scaling up production and exploring applications in diverse energy storage cell configurations.