With the rapid expansion of the global electric vehicle market, particularly in China where EV adoption is accelerating, the transportation of these vehicles via roll-on/roll-off (ro-ro) ships has become a critical logistics chain component. However, the inherent risks associated with lithium-ion batteries, especially thermal runaway events leading to combustion and explosion, pose significant safety challenges. In this study, we investigate the combustion and explosion characteristics of electric vehicles equipped with ternary lithium batteries during ro-ro ship transport using computational fluid dynamics (CFD) simulations. We focus on gas diffusion behaviors, explosion overpressure distributions, and the influence of pressure relief areas on these phenomena, providing insights for enhancing safety protocols in maritime logistics.
The proliferation of electric vehicles, driven by advancements in battery technology and supportive policies, has made China a leader in the EV industry. As the number of China EV units increases, so does the frequency of their transport across oceans, primarily via ro-ro ships. These vessels, designed for carrying wheeled cargo, present unique confined spaces where thermal runaway incidents can escalate into catastrophic events. Thermal runaway in lithium-ion batteries involves exothermic reactions, gas generation, and potential explosions, which are exacerbated in enclosed environments like ship holds. Understanding these dynamics is crucial for developing effective mitigation strategies. Our research aims to address this by modeling a single-layer ro-ro ship cabin and an electric vehicle to simulate real-world scenarios, analyzing key parameters such as gas concentration, temperature, and overpressure under varying conditions.
To model the ro-ro ship environment, we developed a computational domain based on standard large ocean-going vessels, with a cabin length of 199.0 m, width of 39.0 m, and a height of 2.5 m. The cabin includes two rectangular passages, each 25 m long and 8 m wide, for vehicle access. For the electric vehicle model, we simplified a typical China EV to focus on the battery compartment, which houses a ternary lithium-ion battery with a nominal voltage of 3.7 V and capacity of 50 Ah. The battery module is positioned in one corner of the compartment, with a distance of 0.5 m from each wall, and features pressure relief valves and vents. The vehicle is placed with a spacing of 30 cm in the x-direction and 50 cm in the y-direction from adjacent vehicles, as per common guidelines. We installed measurement points around the vehicle to monitor pressure and temperature during simulations, with points located 0.1 m above the vehicle底盘 to capture battery-level conditions.
Mesh generation was performed using ANSYS Fluent meshing software, with sensitivity analysis to determine optimal grid sizes. We tested large, medium, and small grid configurations, as summarized in Table 1. The medium grid size was selected for its balance between computational accuracy and efficiency, as it showed minimal deviation in peak overpressure and temperature compared to finer grids. The boundary conditions were set to reflect real-world scenarios: cabin walls were defined as no-slip boundaries, the ramp passages as pressure outlets, and the battery pressure relief valve as a mass inlet with a pressure of 0.5 MPa and a mass flow rate of 1 g/s, consistent with stable gas emission during thermal runaway. The gas composition was based on typical ternary lithium battery emissions, including hydrogen, carbon monoxide, and hydrocarbons, while air consisted of 23% oxygen and 77% nitrogen by volume. For explosion simulations, we used the species transport model, realizable k-epsilon turbulence model, and the pressure-based solver with a semi-implicit method for pressure-linked equations (SIMPLE). The time steps were set to 0.01 s for gas diffusion and 0.0001 s for explosion phases to ensure numerical stability.
| Grid Type | Non-Combustion Zone | Combustion Zone | Local Refinement |
|---|---|---|---|
| Large Grid | 0.15 m × 0.15 m × 0.15 m | 0.075 m × 0.075 m × 0.075 m | 10 partitions |
| Medium Grid | 0.10 m × 0.10 m × 0.10 m | 0.05 m × 0.05 m × 0.05 m | 10 partitions |
| Small Grid | 0.05 m × 0.05 m × 0.05 m | 0.025 m × 0.025 m × 0.025 m | 10 partitions |
We defined multiple simulation cases to explore the effects of pressure relief configurations, as detailed in Table 2. These included single-side single-vent scenarios with vent lengths ranging from 0.5 m to 1.5 m (all with a width of 0.1 m) and double-side single-vent scenarios with similar dimensions. The state of charge (SOC) for the ternary lithium battery was set to 50%, as this level represents a common operational state and has been shown to produce significant gas emissions and explosion risks. The primary goal was to assess how vent area influences gas diffusion, combustion temperature, and overpressure during thermal runaway events.
| Case | Vent Configuration | Vent Dimensions (Length × Width) | Vent Area (m²) |
|---|---|---|---|
| 1 | Single-side single-vent | 1.5 m × 0.1 m | 0.15 |
| 2 | Single-side single-vent | 1.25 m × 0.1 m | 0.125 |
| 3 | Single-side single-vent | 1.0 m × 0.1 m | 0.10 |
| 4 | Single-side single-vent | 0.75 m × 0.1 m | 0.075 |
| 5 | Single-side single-vent | 0.5 m × 0.1 m | 0.05 |
| 6 | Double-side single-vent | 1.5 m × 0.1 m | 0.15 (per side) |
| 7 | Double-side single-vent | 1.0 m × 0.1 m | 0.10 (per side) |
| 8 | Double-side single-vent | 0.5 m × 0.1 m | 0.05 (per side) |
The gas diffusion behavior during thermal runaway was a critical aspect of our analysis. For a ternary lithium battery at 50% SOC, the hydrogen volume fraction inside the battery compartment rapidly increased, reaching a peak of 32.5% within 40 seconds, as shown in Figure 3. This rapid accumulation is governed by Fick’s law of diffusion, where gas molecules move from high-concentration regions to low-concentration areas. The diffusion process can be described by the equation:
$$ \frac{\partial C}{\partial t} = D \nabla^2 C $$
where \( C \) is the concentration, \( t \) is time, and \( D \) is the diffusion coefficient. Initially, molecular diffusion dominated, but as hydrogen, being lighter than air, accumulated at the top of the compartment due to buoyancy-driven natural convection. This led to stratification, with hydrogen volume fractions exceeding 20% in upper regions, while lower areas maintained lower concentrations. The pressure relief valves initially released air, but as gas diffusion progressed, hydrogen emission rates peaked at 0.9 L/s within 10 seconds before stabilizing around 0.15 L/s. This behavior highlights the rapid gas generation and the potential for explosive mixtures to form in confined spaces, emphasizing the need for timely ventilation in electric vehicle transport on ro-ro ships.
Combustion and explosion characteristics were evaluated based on overpressure and temperature distributions. In Case 1 (single-side single-vent with 1.5 m × 0.1 m vent), the overpressure at measurement points exhibited a sharp rise to a peak of 12.509 kPa within 0.08 seconds, followed by a rapid decay and stabilization, as illustrated in Figure 6. This pattern is typical of shock waves in confined explosions. The maximum overpressure varied with vent area; for instance, in Case 5 (vent area of 0.05 m²), the peak overpressure reached 34.9 kPa, whereas in Case 1 (vent area of 0.15 m²), it dropped to 12.5 kPa. This inverse relationship between vent area and overpressure can be expressed using the following empirical formula for explosion overpressure in confined spaces:
$$ \Delta P = k \cdot \left( \frac{E}{V} \right) \cdot \left( \frac{1}{A_v} \right) $$
where \( \Delta P \) is the overpressure, \( k \) is a constant, \( E \) is the energy released, \( V \) is the volume, and \( A_v \) is the vent area. Double-side vent configurations further reduced overpressure; for example, in Case 6, the peak overpressure was only 3.5 kPa, demonstrating the effectiveness of symmetric venting in mitigating blast forces. These findings are summarized in Table 3, which compares maximum overpressures across different cases. The reduction in overpressure with larger vent areas is crucial for protecting the battery compartment and adjacent electric vehicles from structural damage, a key consideration in the design of ro-ro ship safety systems for China EV transport.
| Case | Vent Area (m²) | Maximum Overpressure (kPa) | Peak Temperature (°C) | Temperature at 1.5 s (°C) |
|---|---|---|---|---|
| 1 | 0.15 | 12.5 | 2559 | 1005 |
| 2 | 0.125 | 16.5 | 2470 | 950 |
| 3 | 0.10 | 21.4 | 2380 | 900 |
| 4 | 0.075 | 29.6 | 2250 | 850 |
| 5 | 0.05 | 34.9 | 2100 | 800 |
| 6 | 0.15 (double) | 3.5 | 2600 | 1300 |
| 7 | 0.10 (double) | 5.2 | 2550 | 1250 |
| 8 | 0.05 (double) | 7.0 | 2500 | 1200 |
Temperature analysis revealed that combustion events reached extreme highs, with peak temperatures exceeding 2559°C in some cases. In Case 1, the temperature inside the battery compartment remained above 900°C even after 1.5 seconds, posing a prolonged risk of igniting adjacent materials. For adjacent electric vehicles at a 30 cm spacing, surface temperatures soared to 2080°C within 0.2 seconds and stayed above 200°C for over 1 second, as shown in Figure 8. This sustained high temperature exceeds the ignition points of common combustibles found in vehicles, such as plastics, resins, and fluids, which typically ignite around 250°C. The temperature dynamics can be modeled using the energy equation:
$$ \rho c_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + \dot{q} $$
where \( \rho \) is density, \( c_p \) is specific heat, \( T \) is temperature, \( k \) is thermal conductivity, and \( \dot{q} \) is the heat generation rate. Larger vent areas, while reducing overpressure, extended the duration of high temperatures due to enhanced heat release and gas exchange. In double-side vent cases, temperatures persisted above 1200°C after 1.5 seconds, indicating that symmetric venting might exacerbate thermal hazards by allowing more efficient gas combustion. This trade-off between overpressure reduction and temperature persistence is critical for safety assessments in ro-ro ship designs handling electric vehicles.
The influence of vent area on flame behavior and gas dynamics was further explored through spatial temperature distributions. In single-side vent configurations, smaller vent areas resulted in longer flame jets and higher velocities, driven by increased pressure gradients. For example, in Case 5, the flame jet extended further from the vent, reaching adjacent vehicles more rapidly. In contrast, larger vent areas promoted vortex formation inside the compartment, leading to more uniform temperature mixing but prolonged heat retention. In double-side vent cases, opposing gas flows created a stable high-temperature zone in the compartment center, which could accelerate battery casing failure and electrolyte leakage, fueling further combustion. This feedback loop, described by the relation:
$$ \frac{dm}{dt} = -k m + f(T) $$
where \( m \) is the mass of combustible material and \( f(T) \) is a temperature-dependent leakage rate, underscores the importance of vent design in preventing escalation. The temperature at a point 30 cm from the vent (Measurement Point 2) varied significantly with vent area, as detailed in Table 4. In Case 1, the temperature peaked at 2081°C and remained at 250°C after 1.5 seconds, whereas in Case 5, it reached 1338°C but decayed faster. This suggests that while larger vents reduce immediate blast forces, they increase the risk of fire spread to nearby electric vehicles, necessitating revised spacing guidelines beyond the standard 30 cm.
| Case | Vent Area (m²) | Peak Temperature (°C) | Temperature at 1.5 s (°C) |
|---|---|---|---|
| 1 | 0.15 | 2081 | 250 |
| 2 | 0.125 | 1977 | 240 |
| 3 | 0.10 | 1853 | 230 |
| 4 | 0.075 | 1597 | 220 |
| 5 | 0.05 | 1338 | 210 |
| 6 | 0.15 (double) | 2210 | 300 |
| 7 | 0.10 (double) | 1947 | 280 |
| 8 | 0.05 (double) | 1403 | 260 |
In conclusion, our study demonstrates that thermal runaway in ternary lithium batteries of electric vehicles can lead to rapid gas diffusion and severe combustion events in ro-ro ship environments. The hydrogen concentration can reach explosive levels within seconds, and explosion overpressures are highly dependent on vent area, with larger vents reducing peak pressures but prolonging high-temperature conditions. The standard 30 cm spacing between electric vehicles is insufficient to prevent fire spread, as adjacent vehicle surfaces can sustain temperatures above ignition points for extended periods. Based on these findings, we recommend optimizing vehicle arrangements by increasing spacing or implementing additional fire barriers, particularly for vehicles with larger or double-side vents. For the China EV industry, enhancing vent design and ship safety protocols is essential to mitigate risks during maritime transport. Future work should explore real-time monitoring systems and advanced suppression techniques to further improve safety in this growing sector.