In recent years, the rapid growth of the electric car industry, particularly in the China EV market, has highlighted the critical need for efficient thermal management systems for power batteries. As a researcher focused on advancing electric car technologies, I have dedicated efforts to addressing the challenges posed by low-temperature environments, which significantly impact battery performance, safety, and longevity. Lithium-ion batteries, commonly used in electric cars, are highly sensitive to temperature variations; excessive cold can lead to increased internal resistance, reduced capacity, and even thermal runaway in severe cases. This study aims to develop and evaluate a novel thermal management system using flat ultra-thin micro heat pipes, specifically designed for electric car applications in China EV contexts. By integrating segmented thermal resistance models with battery thermal dynamics, we seek to optimize both heating and cooling processes, ensuring reliable operation under diverse conditions. The findings from this research are expected to contribute to the broader adoption of electric cars by enhancing battery efficiency and safety.
The importance of thermal management in electric car batteries cannot be overstated, especially as the China EV sector expands globally. In low-temperature environments, batteries experience sluggish ion mobility, leading to performance degradation. Traditional cooling methods, such as air or liquid cooling, often fall short in maintaining uniform temperature distributions. Therefore, we propose the use of heat pipes, which offer high thermal conductivity, compact structure, and bidirectional heat transfer capabilities. This approach aligns with the growing demand for sustainable electric car solutions in the China EV industry, where energy efficiency and environmental concerns are paramount. Our investigation focuses on a 3.2V 50Ah lithium-ion battery pack, typical in many electric cars, to simulate real-world scenarios and provide actionable insights.
To design an effective flat ultra-thin micro heat pipe for electric car batteries, we first selected appropriate working fluids based on their thermal properties. Water was chosen as the primary fluid due to its high latent heat, favorable surface tension, and compatibility with copper materials, which are commonly used in China EV components. The heat pipe structure includes a sintered copper wick, a wall thickness of 0.16 mm, and a maximum vapor chamber diameter of 5.51 mm, optimized for a length of 168.3 mm. This design ensures efficient heat absorption and dissipation, critical for electric car batteries operating in low-temperature conditions. The maximum heat transfer capacity is set at 8.0 W, with a liquid filling ratio of 25% relative to the evaporator volume, balancing performance and practicality for China EV applications.
The operational principle of the heat pipe involves phase change processes: at the evaporator section, heat from the electric car battery causes liquid vaporization, and the vapor moves to the condenser section, where it releases heat and condenses back to liquid. This cycle is governed by capillary forces, and the condition for stable operation is given by the inequality: $$ \Delta P_c \geq \Delta P_p + \Delta P_l + \Delta P_g $$ where $\Delta P_c$ is the capillary pressure head, $\Delta P_p$ is the pressure drop due to vapor flow, $\Delta P_l$ is the liquid flow resistance, and $\Delta P_g$ is the gravitational pressure difference. This equation ensures that the heat pipe can effectively manage heat in electric car systems, even under varying loads in China EV environments.
We compared different working fluids to validate our selection for electric car batteries, as summarized in Table 1. This analysis highlights water’s advantages in low-temperature ranges relevant to China EV operations, such as its boiling point at 100°C and freezing point at 0°C, making it suitable for environments down to -20°C. Other fluids, like ammonia or methanol, were considered but rejected due to toxicity or lower thermal stability, which could compromise electric car safety.
| Working Fluid | Temperature Range (°C) | Boiling Point (°C) | Freezing Point (°C) |
|---|---|---|---|
| Ammonia | -60 to 1100 | -33 | -78 |
| Methanol | 10 to 130 | 64 | -98 |
| R113 | -10 to 100 | 48 | -35 |
| Acetone | 0 to 120 | 57 | -95 |
| Water | 30 to 250 | 100 | 0 |
| Sodium | 600 to 1200 | 892 | 98 |
Next, we developed a segmented thermal resistance model coupled with a battery thermal model to accurately predict temperature dynamics in electric car batteries. Unlike integrated models that oversimplify heat transfer, our segmented approach accounts for variations in evaporator and condenser sections, reducing computational errors. The thermal resistance for the evaporator section, $R_e$, is defined as: $$ R_e = R_{e,\text{wall}} + R_{e,\text{wick}} + R_{e,\text{eva}} $$ where $R_{e,\text{wall}}$ is the wall thermal resistance, $R_{e,\text{wick}}$ is the wick resistance, and $R_{e,\text{eva}}$ is the evaporation thermal resistance. The corresponding heat transfer coefficient, $K_e$, is given by: $$ K_e = \frac{L_e}{R_e A_e} $$ Here, $L_e$ is the axial length and $A_e$ is the cross-sectional area of the evaporator, parameters crucial for electric car battery designs in China EV systems.
For the condenser section, the thermal resistance is split into finned and plain tube parts to enhance accuracy. The resistances are: $$ R_{c1} = R_{\text{con1}} + R_{c,\text{wick1}} + R_{c,\text{wall}} $$ and $$ R_{c2} = R_{\text{con2}} + R_{c,\text{wick2}} + R_{\text{tin}} $$ with heat transfer coefficients: $$ K_{c1} = \frac{L_{c1}}{R_{c1} A_{c1}} $$ and $$ K_{c2} = \frac{L_{c2}}{R_{c2} A_{c2}} $$ This model allows for precise simulation of heat distribution in electric car batteries, addressing the unique demands of China EV applications where temperature uniformity is vital for battery longevity.
We coupled this segmented model with a battery thermal model using a mesh of 3,493,037 elements, 130 contact surfaces, and 568,647 nodes. The density and specific heat of the evaporator and condenser sections were set to 5,059 kg/m³ and 103.6 J/kgK, respectively, reflecting typical materials used in electric car components. Boundary conditions included heat pipe cooling on both sides of the battery or mixed cooling scenarios, with a focus on the y-axis where the highest temperature gradients occur in electric car batteries. This integration enables dynamic predictions of temperature fields, essential for optimizing China EV thermal management systems.
To evaluate the cooling performance of the flat ultra-thin micro heat pipe in electric car batteries, we conducted simulations under constant current discharge rates of 1C to 3C, with initial temperatures matching the ambient environment. Key metrics included maximum battery pack temperature, temperature rise, and temperature difference. For instance, under a 3C discharge rate, the segmented model showed a maximum temperature of 31.2°C and a maximum temperature difference of 6.8°C, whereas the integrated model resulted in 35.7°C and 8.9°C, respectively. This demonstrates the superior accuracy of our segmented approach for electric car applications, particularly in China EV contexts where precise thermal control is needed to prevent hotspots and ensure safety.
Figure 1 illustrates the temperature distribution for both models, highlighting that the highest temperatures occur near the positive terminal of the battery pack. Cell 2 exhibited the most uneven temperature distribution, with maximum differences of 5.2°C for the segmented model and 7.5°C for the integrated model. In contrast, Cells 1 and 3 maintained more uniform temperatures, with differences below 3°C, underscoring the effectiveness of our design for electric car batteries in China EV systems.
We further analyzed temperature trends over time, as shown in Figure 2. The segmented model revealed a gradual increase in maximum temperature rise during initial discharge, accelerating when the state of charge reached 0.65 due to higher heat generation rates. This trend closely matched experimental data, validating the model for electric car applications. Conversely, the integrated model showed abrupt changes, leading to larger deviations. The temperature difference between evaporator and condenser sections decreased to a minimum of 2.0°C, emphasizing the heat pipe’s isothermal properties, which are beneficial for electric car batteries in China EV environments.
Table 2 compares different cooling methods under various discharge rates, including natural convection without heat pipes, forced convection without heat pipes, natural convection with heat pipes, and forced convection with heat pipes. The latter method, using our flat ultra-thin micro heat pipe, achieved the best performance for electric car batteries. For example, at a 3C discharge rate, the maximum battery pack temperature was 21.3°C, with maximum pack and cell temperature differences of 4.3°C and 3.5°C, respectively. In contrast, natural convection without heat pipes resulted in temperatures up to 39.6°C and differences of 8.6°C, highlighting the critical role of advanced thermal management in electric car systems, especially for the China EV market.
| Cooling Method | Discharge Rate (C) | Max Pack Temperature (°C) | Max Pack ΔT (°C) | Max Cell ΔT (°C) |
|---|---|---|---|---|
| Natural convection, no heat pipe | 1 | 26.5 | 6.3 | 4.5 |
| Natural convection, no heat pipe | 2 | 31.6 | 7.8 | 4.7 |
| Natural convection, no heat pipe | 3 | 39.6 | 8.6 | 5.9 |
| Forced convection, no heat pipe | 1 | 23.6 | 5.78 | 4.3 |
| Forced convection, no heat pipe | 2 | 26.8 | 7.1 | 4.4 |
| Forced convection, no heat pipe | 3 | 33.6 | 8.2 | 5.6 |
| Natural convection with heat pipe | 1 | 21.9 | 5.6 | 4.2 |
| Natural convection with heat pipe | 2 | 24.9 | 6.7 | 4.4 |
| Natural convection with heat pipe | 3 | 30.6 | 7.8 | 5.3 |
| Forced convection with heat pipe | 1 | 13.2 | 3.4 | 2.7 |
| Forced convection with heat pipe | 2 | 17.2 | 4.0 | 3.1 |
| Forced convection with heat pipe | 3 | 21.3 | 4.3 | 3.5 |
For heating performance in low-temperature environments, we tested two methods: direct battery heating and heat pipe heating, with initial ambient temperatures of 0°C, -10°C, and -20°C, and a heat source of 180 W. Direct heating showed faster initial warming, taking 163s, 253s, and 419s to reach 20°C for Cell 1 at the respective temperatures, compared to 182s, 283s, and 421s for Cell 2. The average heating power for direct heating was 1.32 times that of heat pipe heating, indicating higher efficiency for electric car batteries in China EV applications. However, heat pipe heating provided more uniform temperature distribution, reducing the risk of localized overheating.
Figure 3 depicts the temperature rise for heat pipe heating at -10°C. The evaporator section reached 20°C in approximately 260s, while the condenser was at 9°C. After 50s, the temperature increase rates aligned between sections, demonstrating effective heat redistribution. For direct heating, the evaporator temperature rose linearly, reaching 20°C in about 300s, with the condenser showing an initial rapid increase of 1°C/s before plateauing at 75°C. These results confirm that our flat ultra-thin micro heat pipe can maintain stable heating for electric car batteries, crucial for China EV operations in cold climates.
In conclusion, our study demonstrates that flat ultra-thin micro heat pipes, combined with a segmented thermal resistance model, significantly enhance the thermal management of electric car power batteries in low-temperature environments. This approach ensures efficient cooling and heating, minimizes temperature gradients, and prevents thermal runaway, supporting the reliability and safety of electric cars. The findings are particularly relevant for the China EV industry, where advancing battery technology is key to sustainable transportation. Future work could explore integration with other thermal management strategies to further optimize performance for electric car applications globally.