In recent years, the widespread adoption of lithium-ion batteries in electric vehicles and energy storage systems has highlighted the critical need for effective thermal management. During charge and discharge cycles, electrochemical reactions within these energy storage cells generate heat, leading to temperature rise. Excessive temperatures can degrade battery efficiency, accelerate aging, and even trigger thermal runaway, posing significant safety risks. Traditional cooling methods, such as air cooling and phase change materials, often fall short in extreme conditions, prompting increased research into liquid cooling systems. Among these, immersion cooling, where the coolant directly contacts the energy storage cells, has emerged as a promising approach due to its superior temperature uniformity and heat dissipation capabilities. However, current immersion cooling systems primarily rely on fluorinated liquids, which suffer from high costs, poor thermophysical properties, and environmental concerns. Alternative media like silicone oils offer better thermal performance and cost-effectiveness but introduce flammability risks at high temperatures. This study addresses these challenges by proposing a novel dual-loop dynamic switching system that utilizes modified silicone oil for everyday cooling and rapidly switches to a fire suppression fluid during thermal emergencies. Through comprehensive simulation analysis, we demonstrate that this design maintains excellent temperature homogeneity, with maximum temperature differences within 1.8°C, meeting national standards while reducing costs and enhancing safety for energy storage cells applications.
The selection of an appropriate cooling medium is paramount for the efficiency and safety of immersion cooling systems in energy storage cells. Commonly used coolants include mineral oils, silicone oils, and fluorinated liquids. Mineral oils, while electrically insulating, often exhibit high viscosity, leading to increased pump power requirements and potential impacts on system performance. Moreover, their flammability introduces safety concerns, making them less suitable for high-density energy storage cells. Fluorinated liquids, such as AC6000, are non-flammable due to their stable molecular structure but have limitations in thermal conductivity and specific heat capacity, hindering heat dissipation efficiency. Additionally, their high cost and environmental persistence, with potential decomposition into toxic gases like HF at elevated temperatures, limit their practicality. In contrast, modified silicone oils, exemplified by ICL-1000, present a compelling alternative with superior thermophysical properties and lower costs. The key performance parameters of AC6000 fluorinated liquid and ICL-1000 modified silicone oil are compared in Table 1, highlighting advantages in thermal management for energy storage cells.
| Performance Indicator | AC6000 Fluorinated Liquid | ICL-1000 Modified Silicone Oil | Comparative Analysis |
|---|---|---|---|
| Flash Point (°C) | None (Non-flammable) | ≥320 | Fluorinated liquid has no flash point, but modified silicone oil achieves an ultra-high flash point through modification, significantly exceeding that of mineral oils (150–200°C). |
| Thermal Conductivity (W/m·K) | 0.06–0.07 | 0.15–0.18 | Modified silicone oil offers over 2.5 times the thermal conductivity of fluorinated liquid, facilitating faster heat extraction from energy storage cells. |
| Specific Heat Capacity (kJ/kg·K) | 1.05–1.10 | 1.65–1.75 | Modified silicone oil has approximately 60% higher heat storage capacity, slowing temperature rise rates in energy storage cells. |
| Viscosity @40°C (10−6 m²/s) | 0.6–0.8 | 18–22 | Fluorinated liquid’s low viscosity reduces pump power, whereas modified silicone oil requires optimized flow channel design for efficient circulation around energy storage cells. |
| Dielectric Strength (kV/mm) | ≥35 | ≥45 | Modified silicone oil provides better insulation, reducing short-circuit risks in energy storage cells assemblies. |
The thermophysical advantages of modified silicone oil are further quantified through fundamental equations. The heat transfer rate in immersion cooling can be described by Newton’s law of cooling: $$q = h \cdot A \cdot \Delta T$$ where \(q\) is the heat flux, \(h\) is the heat transfer coefficient, \(A\) is the surface area, and \(\Delta T\) is the temperature difference. For energy storage cells, a higher thermal conductivity \(k\) of the coolant enhances \(h\), as approximated by $$h \propto \frac{k}{L}$$ for convective flows, where \(L\) is a characteristic length. With modified silicone oil’s thermal conductivity of 0.15–0.18 W/m·K compared to fluorinated liquid’s 0.06–0.07 W/m·K, the heat transfer coefficient increases significantly, improving cooling efficiency. Similarly, the specific heat capacity \(C_p\) influences the temperature rise during operation, given by $$\Delta T = \frac{Q}{m \cdot C_p}$$ where \(Q\) is the heat generated by energy storage cells and \(m\) is the mass of coolant. The higher \(C_p\) of modified silicone oil (1.65–1.75 kJ/kg·K) results in a smaller \(\Delta T\) for the same heat load, enhancing thermal stability. Cost analysis, as summarized in Table 2, underscores the economic benefits of modified silicone oil for large-scale energy storage cells systems, where coolant expenses can dominate overall costs.
| Cost Item | AC6000 Fluorinated Liquid | ICL-1000 Modified Silicone Oil |
|---|---|---|
| Medium Cost (per MW·h) | 98,000 USD | 27,000 USD |
Despite the superior thermal and economic properties of modified silicone oil, its flash point, though high, poses a risk under extreme conditions such as thermal runaway in energy storage cells. To mitigate this, we designed a dual-loop dynamic switching system that integrates cooling and fire suppression functionalities. This system operates based on temperature thresholds detected by sensors embedded within the energy storage cells pack. Under normal operating conditions, modified silicone oil circulates through the flow channels, directly contacting the energy storage cells to dissipate heat via forced convection. The flow dynamics are governed by the Navier-Stokes equations: $$\rho \left( \frac{\partial \mathbf{v}}{\partial t} + \mathbf{v} \cdot \nabla \mathbf{v} \right) = -\nabla p + \mu \nabla^2 \mathbf{v} + \mathbf{f}$$ where \(\rho\) is density, \(\mathbf{v}\) is velocity, \(p\) is pressure, \(\mu\) is dynamic viscosity, and \(\mathbf{f}\) represents body forces. For energy storage cells cooling, the Reynolds number \(Re = \frac{\rho v D}{\mu}\) determines flow regime, with laminar flow preferred for minimal pressure drop. When temperatures exceed 250°C, indicating a potential thermal event, the control system activates valves to disconnect the silicone oil loop and connect a fire suppression fluid loop, such as perfluorohexanone. This fluid rapidly fills the cavity, displacing the silicone oil, cooling the energy storage cells, and inerting the environment to prevent combustion. The switching mechanism ensures that energy storage cells are protected without compromising daily cooling performance. After temperatures drop below 80°C, the system reverts to silicone oil circulation, and the fire suppression fluid is purified and recycled for reuse.
The structural design of the immersion cooling battery pack for energy storage cells includes an installation chassis, isolation frames, separation plates, and multiple sealing strips. The chassis features cooling grooves along its length, with sealing strips on the sidewall tops to ensure leak-proof operation. Inlet and outlet pipes are positioned between the sidewalls, and multiple energy storage cells are mounted on the isolation plates. This compact design facilitates efficient coolant flow around each energy storage cell, maximizing heat exchange area. The dual-loop system shares flow channels, reducing structural complexity and cost compared to traditional separate cooling and fire suppression systems. This integration is crucial for scalable applications in energy storage cells, where space and economics are constraints. The shared channel approach minimizes pressure losses and ensures rapid fluid switching, enhancing overall system reliability for energy storage cells protection.
To validate the thermal performance of the proposed system, we conducted computational fluid dynamics (CFD) simulations focusing on flow field and thermal field analyses. For the flow field simulation, a model of the internal flow channels was developed, assuming the use of ICL-1000 modified silicone oil as the coolant. The material properties and boundary conditions are summarized in Table 3 and Table 4, respectively. The inlet flow rate was set to 4 L/min, resulting in a Reynolds number of 583, indicating laminar flow, which is desirable for energy storage cells cooling to avoid excessive shear stresses. The simulation solved the continuity and momentum equations to assess pressure drop and velocity distribution. The results showed an inlet-outlet pressure drop of 3.8 kPa, and the velocity contour at the inlet cross-section revealed no significant flow dead zones, ensuring uniform coolant distribution around the energy storage cells. This is expressed by the continuity equation: $$\nabla \cdot \mathbf{v} = 0$$ and the energy equation for incompressible flow: $$\rho C_p \frac{\partial T}{\partial t} + \rho C_p \mathbf{v} \cdot \nabla T = \nabla \cdot (k \nabla T) + \dot{q}$$ where \(\dot{q}\) is the heat generation rate from energy storage cells.
| Medium Name | ICL-1000 Modified Silicone Oil |
|---|---|
| Density (kg/m³) | 860 |
| Viscosity @40°C (10−6 m²/s) | 22 |
| Specific Heat Capacity (kJ/kg·K) | 1.75 |
| Thermal Conductivity (W/m·K) | 0.18 |
| Boundary Condition | Parameter |
|---|---|
| Inlet Flow Rate (L/min) | 4 |
| Reynolds Number (Re) | 583 |
For the thermal field simulation, a detailed model incorporating the energy storage cells and surrounding components was established. The material properties of various structures are listed in Table 5, and the boundary conditions are provided in Table 6. The energy storage cells were subjected to a cycle of 0.5C charging for 2 hours followed by 0.5C discharging for 2 hours, with heat generation powers of 11.542 W during charging and 10.964 W during discharging. The coolant flow rate was varied between 3 L/min and 4 L/min, and an external cooling power of 500 W was applied, maintaining a minimum inlet temperature of 20°C. The thermal simulation solved the heat conduction equation: $$\frac{\partial T}{\partial t} = \alpha \nabla^2 T + \frac{\dot{q}}{\rho C_p}$$ where \(\alpha = \frac{k}{\rho C_p}\) is the thermal diffusivity. After accounting for adhesive layer thickness, the pressure drop reduced to 3.2 kPa. The temperature distribution results, as shown in the simulation output, demonstrated a maximum temperature difference of 1.814°C across the energy storage cells, well within the industry standard requirements for electric vehicle battery packs. This uniformity is critical for prolonging the lifespan and ensuring the safety of energy storage cells, as it minimizes localized hot spots that could lead to degradation or failure.
| Structure | Material | Density (kg/m³) | Thermal Conductivity (W/m·K) | Specific Heat Capacity (J/kg·K) | Viscosity @40°C (cSt) |
|---|---|---|---|---|---|
| Energy Storage Cell | – | 2161.9 | Normal: 5.13, Transverse: 23.77 | 1000 | – |
| Foamed Rubber | – | 60 | 0.034 | 1000 | – |
| Epoxy Plate | – | 1800 | 0.2 | 550 | – |
| End Plate | AL6063 | 2702 | 218 | 900 | – |
| Cross Beam | AL6063 | 2702 | 218 | 900 | – |
| Reinforcement Rib | AL6063 | 2702 | 218 | 900 | – |
| Thermal Adhesive | – | 2500 | 1.5 | 1000 | – |
| Liquid Cooling Plate | AL3003 | 2702 | 155 | 893 | – |
| Coolant | Modified Silicone Oil | 860 | 0.18 | 1750 | 22 |
| Boundary Condition | Parameter |
|---|---|
| Ambient and Initial Temperature (°C) | 25 |
| Energy Storage Cells Operating Condition | 0.5C charge for 2 h + 0.5C discharge for 2 h; Charge heat generation: 11.542 W; Discharge heat generation: 10.964 W |
| Coolant Flow Rate (L/min) | 4, 3 |
| External Cooling Power (W) | 500 (minimum inlet temperature 20°C) |
The simulation results confirm that the dual-loop dynamic switching system effectively maintains thermal homogeneity in energy storage cells, with the maximum temperature difference not exceeding 1.8°C. This performance meets stringent national standards and underscores the system’s capability to handle variable heat loads from energy storage cells. The use of modified silicone oil as the primary coolant, combined with the rapid switch to fire suppression fluid, provides a balanced solution to the trade-offs between cooling efficiency, safety, and cost. The shared flow channel design reduces material usage and complexity, contributing to lower overall system costs for energy storage cells applications. Furthermore, the fire suppression mechanism enhances safety by quickly addressing thermal runaway scenarios, which are critical concerns in high-density energy storage cells packs. The ability to recycle the fire suppression fluid adds to the sustainability of the system, aligning with environmental goals for energy storage technologies.
In conclusion, our study presents an innovative immersion cooling system for energy storage cells that leverages modified silicone oil for daily thermal management and a dynamic switching mechanism for emergency protection. The dual-loop design addresses the limitations of traditional fluorinated liquids and mitigates the flammability risks associated with silicone oils, offering a cost-effective and safe solution for energy storage cells thermal management. Simulation analyses validate the system’s excellent temperature uniformity and efficiency, with maximum temperature differences within acceptable limits. This approach not only advances the practical application of immersion cooling for energy storage cells but also provides a scalable framework for future energy storage systems, promoting wider adoption of lithium-ion batteries in sustainable energy infrastructures. Future work could focus on experimental validation and optimization of the switching dynamics for various energy storage cells configurations.