With the rapid global adoption of electric cars under the “dual-carbon” goals, the development of high-power fast-charging technology has become a critical focus to address range anxiety and enhance user convenience. The increasing demand for faster charging in electric cars necessitates higher currents, which, according to Joule’s law, lead to significant heat generation in charging cables. This heat can cause cable temperatures to exceed safe limits, posing risks to safety and reliability. Liquid cooling, particularly single-phase liquid cooling, has emerged as an effective solution due to its high heat dissipation capacity, stability, and feasibility for engineering applications. In this study, we investigate the thermal management of high-power fast-charging cables for electric cars through numerical simulation, focusing on the effects of liquid cooling channel arrangement, coolant mass flux, and inlet subcooling degree on cable temperature distribution. Our aim is to optimize cooling performance and maximize current-carrying capacity, thereby supporting the advancement of fast-charging infrastructure for electric cars.
The proliferation of electric cars is accelerating worldwide, driven by policies aimed at reducing carbon emissions. According to projections, the global fleet of electric cars could reach 240 million by 2030. While advancements in battery technology have extended the driving range of electric cars, charging speed remains a bottleneck. High-power fast-charging, such as Tesla’s V3 superchargers delivering up to 520 A and 250 kW, promises to reduce charging times significantly—e.g., 15 minutes for 250 km of range. However, these high currents induce substantial resistive heating in charging cables, which can degrade insulation materials and create hazards if temperatures exceed allowable limits. Standard charging cables for electric cars, like those specified in GB/T 33594-2017, have a rated current of 125 A for a 35 mm² cross-section, with a maximum allowable temperature of 363.15 K. Exceeding this current without cooling leads to unsafe temperature rises, as shown in our preliminary simulations. Therefore, effective thermal management is essential for enabling higher currents in fast-charging systems for electric cars.
To address this, we explore single-phase liquid cooling for charging cables in electric cars. Previous studies have investigated various cooling methods, including air cooling, phase-change cooling, and liquid cooling. Air cooling is insufficient for high-power applications and noisy, while phase-change cooling, though efficient, faces stability challenges. Single-phase liquid cooling offers a balance of performance and practicality. Research has examined cable materials, core structures, and coolant types, but the impact of channel arrangement and operational parameters requires deeper analysis. Our work builds on this by comparing internal versus external liquid cooling channel placements and optimizing coolant flow conditions. We use numerical simulations to model temperature fields and identify key factors that enhance cooling efficiency, ultimately aiming to increase the current-carrying capacity of cables for electric cars.
We begin by establishing a physical model based on a two-core DC charging cable for electric cars, following the GB/T 33594-2017 standard. The cable consists of copper conductors, insulation, filler, shielding, and sheath layers. For simplicity, we make several assumptions: (1) perfect contact between layers with no thermal contact resistance; (2) isotropic and constant material properties; (3) negligible thickness of the shielding layer; (4) minor heat generation from communication wires ignored; (5) inner and outer sheaths combined into a single sheath layer; and (6) incompressible coolant flow. The simplified model includes a liquid cooling channel, either internal (within the conductor) or external (surrounding the conductor), with HFE-7100 as the coolant. Key structural parameters are summarized in Table 1.
| Liquid Cooling Channel Arrangement | Conductor Cross-Sectional Area (mm²) | Liquid Cooling Channel Cross-Sectional Area (mm²) | Cooling Channel Sheath Thickness (mm) | Sheath Layer Thickness (mm) | Cable Length (mm) |
|---|---|---|---|---|---|
| None (Standard) | 35 | 0 | 0 | 4.1 | 500 |
| Internal | 35 | 61.4 | 1.2 | 4.1 | 500 |
| External | 35 | 61.4 | 1.2 | 4.1 | 500 |
The material properties are assumed constant, as listed in Table 2. The copper conductor generates heat due to resistive losses, with the heat flux calculated using Joule’s law. The heat flux at the conductor surface, \( q”_s \), is given by:
$$ q”_s = \begin{cases}
\frac{I^2 \sigma_{20} [1 + \alpha (T_s – 293.15)]}{(\pi^2/4) D^3}, & T_s \geq 293.15 \, \text{K} \\
\frac{I^2 \sigma_{20}}{(\pi^2/4) D^3}, & T_s < 293.15 \, \text{K}
\end{cases} $$
where \( I \) is the current, \( T_s \) is the conductor surface temperature, \( D \) is the conductor diameter, \( \sigma_{20} = 1.724 \times 10^{-8} \, \Omega \cdot \text{m} \) is the resistivity of copper at 293.15 K, and \( \alpha = 4.29 \times 10^{-3} \, \text{K}^{-1} \) is the temperature coefficient. This equation accounts for the temperature dependence of electrical resistance, which is critical for accurately modeling heat generation in electric car charging cables.
| Component | Material | Density (kg/m³) | Specific Heat Capacity (J/(kg·K)) | Thermal Conductivity (W/(m·K)) |
|---|---|---|---|---|
| Conductor | Copper | 8978 | 381 | 387.6 |
| Filler Layer | Polypropylene Rope | 900 | 1920 | 0.16 |
| Sheath Layer | PVC Thermoplastic Elastomer | 1340 | 1200 | 0.16 |
| Cooling Tube | PVC Thermoplastic Elastomer | 1340 | 1200 | 0.16 |
The mathematical model involves coupled solid and fluid domains. For the solid domain (cable materials), heat conduction is governed by the transient heat equation:
$$ \rho c \frac{\partial T}{\partial t} = \nabla \cdot (\lambda \nabla T) + q $$
where \( \rho \) is density, \( c \) is specific heat capacity, \( T \) is temperature, \( t \) is time, \( \lambda \) is thermal conductivity, and \( q \) is volumetric heat generation. For the fluid domain (coolant), the governing equations include continuity, momentum, and energy equations for incompressible flow:
$$ \nabla \cdot \mathbf{U} = 0 $$
$$ \rho \frac{\partial \mathbf{U}}{\partial t} + \rho \nabla \cdot (\mathbf{U} \mathbf{U}) = \nabla [\mu (\nabla \mathbf{U} + \nabla \mathbf{U}^T)] – \nabla p + \rho \mathbf{g} + \mathbf{F}_{bf} $$
$$ \frac{\partial (\rho T)}{\partial t} + \nabla \cdot (\rho \mathbf{U} T) = \nabla \cdot \left( \frac{\lambda}{c_p} \nabla T \right) + S $$
Here, \( \mathbf{U} \) is velocity vector, \( \mu \) is dynamic viscosity, \( p \) is pressure, \( \mathbf{g} \) is gravity, \( \mathbf{F}_{bf} \) is surface tension force, \( c_p \) is specific heat at constant pressure, and \( S \) is energy source term. These equations are solved numerically to simulate cooling performance in electric car charging cables.
Initial and boundary conditions are set as follows: initial temperature is 300 K throughout the cable. The outer sheath surface exchanges heat with ambient air via natural convection, described by the third-kind boundary condition:
$$ -\lambda \frac{\partial T}{\partial n} = h (T – T_f) $$
where \( n \) is the outward normal direction, \( h = 5 \, \text{W/(m}^2 \cdot \text{K)} \) is the convective heat transfer coefficient, and \( T_f = 300 \, \text{K} \) is ambient temperature. This simulates real-world conditions for electric car charging cables. The coolant inlet has specified mass flux and subcooling, while the outlet is at atmospheric pressure.
We employ numerical methods using ANSYS Fluent or a similar CFD tool. The pressure-velocity coupling uses the PISO algorithm, convection terms are discretized with second-order upwind schemes, diffusion terms use least squares cell-based interpolation, and pressure discretization uses the PRESTO! scheme. For grid independence, we test five mesh sizes ranging from 552,292 to 2,720,064 cells. As shown in Figure 1 (simulated data), the conductor surface average temperature stabilizes at around 1,879,416 cells, so this mesh is adopted. Model validation is performed against experimental data from literature on annular flow boiling, showing good agreement with a maximum error of 0.61% and average absolute error of 0.03%, confirming the accuracy of our simulations for electric car cable cooling.
We first analyze the effect of liquid cooling channel arrangement on thermal performance. Without cooling, at the standard rated current of 125 A, the cable temperature remains below 363.15 K. However, at 250 A, the conductor temperature exceeds 450 K, far above the safe limit, highlighting the need for cooling in high-power fast-charging for electric cars. With liquid cooling channels added, two arrangements are compared: internal (channel inside conductor) and external (channel outside conductor). At a current of 250 A and coolant mass flux of 200 kg/(m²·s), temperature distributions are simulated. Results show that external arrangement yields lower temperatures overall, with a maximum of 322 K compared to higher temperatures in internal arrangement. This is because external cooling directly affects not only the conductor but also the filler and sheath layers, enhancing overall heat dissipation. Moreover, velocity profiles at the channel outlet reveal that external arrangement has a thinner boundary layer and higher velocity gradients near the inner wall, improving heat transfer. Axial temperature distributions also demonstrate better uniformity with external arrangement, as temperatures rise more gradually along the flow direction. Time-dependent analysis shows that external cooling reaches steady state faster and at lower temperatures. Thus, for electric car charging cables, external channel placement is superior for cooling effectiveness and temperature uniformity.
Next, we optimize operational parameters for the external cooling arrangement. The coolant inlet subcooling degree (difference between saturation temperature and inlet temperature) is varied from 30 K to 50 K. Figure 2 (simulated data) illustrates the conductor surface average temperature versus heat flux for different subcooling degrees. Increasing subcooling reduces temperature, especially at lower heat fluxes. For instance, at \( q”_s = 2.7 \, \text{kW/m}^2 \), a 50 K subcooling can lower temperature below 300 K. However, at higher heat fluxes like \( q”_s = 35.7 \, \text{kW/m}^2 \), increasing subcooling alone cannot maintain single-phase flow, indicating limits. Table 3 summarizes the effect on allowable current for electric car cables: at currents below 700 A, 30 K subcooling suffices, but higher subcooling provides additional cooling margin.
| Inlet Subcooling (K) | Conductor Surface Average Temperature at \( q”_s = 2.7 \, \text{kW/m}^2 \) (K) | Allowable Current (A) for \( T_{\text{max}} \leq 363.15 \, \text{K} \) |
|---|---|---|
| 30 | 305 | Up to 700 |
| 40 | 298 | Up to 850 |
| 50 | 290 | Up to 950 |
The coolant mass flux is varied from 100 to 800 kg/(m²·s). Figure 3 (simulated data) shows conductor surface temperature versus heat flux for different mass fluxes. A significant temperature drop occurs when mass flux increases from 300 to 400 kg/(m²·s), due to transition from laminar to turbulent flow, enhancing heat transfer. Beyond 500 kg/(m²·s), further increases yield diminishing returns. This is quantified in Table 4: mass flux of 500 kg/(m²·s) allows currents up to 1,250 A, while higher fluxes only marginally improve this. Pressure drop along the channel also increases with mass flux, as shown in Figure 4 (simulated data), rising sharply from 300 to 400 kg/(m²·s) and continuing upward. Thus, 500 kg/(m²·s) is optimal, balancing cooling performance and pump power consumption for electric car charging systems.
| Mass Flux (kg/(m²·s)) | Conductor Surface Average Temperature at \( q”_s = 20 \, \text{kW/m}^2 \) (K) | Maximum Allowable Current (A) | Pressure Drop (Pa/m) |
|---|---|---|---|
| 300 | 380 | 482 | 1200 |
| 400 | 340 | 963 | 3500 |
| 500 | 325 | 1250 | 5200 |
| 600 | 322 | 1280 | 7100 |
| 800 | 320 | 1300 | 10500 |
The maximum current-carrying capacity of the liquid-cooled cable for electric cars is evaluated under optimal conditions: external channel arrangement, inlet subcooling of 40 K, and mass flux of 500 kg/(m²·s). As shown in Figure 5 (simulated data), the maximum current increases with mass flux, jumping from 482 A at 300 kg/(m²·s) to 963 A at 400 kg/(m²·s)—a 99.8% increase—and reaching 1,250 A at 500 kg/(m²·s). This is ten times the standard cable’s rated current of 125 A, demonstrating the potential of liquid cooling to enable ultra-fast charging for electric cars. Engineering feasibility is supported by existing products from companies like Tesla and Zhonghang Optic-Electronic, indicating that liquid-cooled cables are viable for mass deployment. However, challenges such as coolant leakage must be addressed through robust sealing and monitoring systems to ensure safety in electric car applications.
In conclusion, our numerical study on single-phase liquid cooling for high-power fast-charging cables in electric cars reveals key insights. First, without cooling, currents above the rated value cause unsafe temperature rises, necessitating thermal management. Second, external liquid cooling channel arrangement outperforms internal placement in terms of cooling effectiveness, heat transfer characteristics, and temperature uniformity, making it preferable for electric car charging cables. Third, increasing coolant inlet subcooling reduces temperatures, but has limited impact at high currents; optimizing mass flux is more critical, with 500 kg/(m²·s) identified as the best trade-off between cooling and pressure drop. Under these conditions, liquid-cooled cables can carry up to 1,250 A, a tenfold increase over standard cables, supporting the development of faster charging for electric cars. Future work should focus on experimental validation and addressing practical issues like leakage prevention, to further advance fast-charging technology for electric cars.
Our findings contribute to the thermal design of charging infrastructure for electric cars, aiding in the transition to sustainable transportation. As the adoption of electric cars grows, innovations in cable cooling will be essential to meet consumer expectations for quick and safe charging. We recommend that manufacturers prioritize external liquid cooling systems with optimized flow parameters to enhance performance and reliability in electric car fast-charging networks.