As the global energy structure accelerates its transition toward low-carbon systems, renewable energy generation technologies such as wind, photovoltaic, hydropower, and biomass have become the core driving forces of power system innovation. These clean energy sources, while offering significant environmental benefits and renewability, suffer from inherent distributed characteristics and intermittency—such as diurnal fluctuations in photovoltaic generation and seasonal variations in wind power—leading to unstable grid power quality. The breakthrough development of energy storage technology has changed this situation by enabling energy time-shifting through “peak shaving and valley filling,” dynamically matching renewable generation with grid load. Among diverse energy storage technology routes, electrochemical energy storage, particularly lithium-ion battery energy storage, has gradually become the mainstream choice for grid-scale energy storage stations due to its modular design, fast response characteristics, and flexible deployment. However, lithium-ion batteries generate heat during charge and discharge processes due to internal resistance and chemical reactions. If this heat cannot be effectively dissipated, the temperature of the energy storage cell rises, potentially degrading performance and even leading to thermal runaway. Furthermore, temperature inconsistency among cells within a battery pack causes uneven aging, which over time reduces capacity, safety, and charge-discharge efficiency. The optimal operating temperature range for lithium-ion batteries is 293–313 K, with a temperature uniformity requirement within 5 K. When the internal temperature difference of a battery pack exceeds 5 K, its cycle life can decrease by more than 30%. Given that lithium intercalated in graphite reacts with electrolyte components and binders above 393 K, this temperature is regarded as a high-risk threshold for thermal runaway. Therefore, the battery thermal management system is critically important for the economy and reliability of energy storage systems.
Notably, when renewable energy generation power suddenly increases, grid energy storage cells must possess high-rate charging capability to rapidly store excess electricity and prevent grid overvoltage; conversely, during peak demand, they must discharge at high rates to supplement grid power and maintain stability. Without effective thermal management, instantaneous high-rate charge and discharge processes inevitably cause sharp temperature changes and large temperature gradients within grid energy storage cells. To ensure that lithium-ion energy storage cells operate within a safe temperature range, efficient cooling is required. Common cooling technologies include air cooling, liquid cooling, and phase-change cooling. Liquid cooling, with advantages such as fast heat transfer, strong stability, significant effectiveness in reducing maximum temperature, and improving temperature uniformity, has been widely adopted.
However, most existing studies on liquid cooling schemes focus on conventional charge/discharge rates of lithium-ion batteries. Research on liquid cooling technologies for grid energy storage cells under high-rate and ultra-high-rate charge/discharge conditions is relatively scarce. In this work, we design a bidirectional counter-flow heat exchange plate to address the issues of excessive temperature and large temperature differences in lithium-ion energy storage cells under high-rate and ultra-high-rate charge/discharge scenarios.
Numerical Model
Geometric Model
The energy storage cell pack model constructed in this study consists of ten blade-type energy storage cells connected in series. The physical properties and geometric parameters are as follows: cell material is lithium iron phosphate (LiFePO₄); cell capacity is 135 Ah; nominal voltage is 3.2 V; thermal conductivities are 18.3 W/(m·K) in the in-plane direction (x and z), and 1.1 W/(m·K) in the through-plane direction (y); specific heat capacity is 1100 J/(kg·K); density is 1715 kg/m³; cell dimensions are 945 mm × 14 mm × 90 mm. Due to the electrolyte-separator layered structure of lithium-ion cells, the overall thermal conductivity of the pack is anisotropic, with in-plane conductivity significantly higher than through-plane conductivity.
To demonstrate the advantage of placing the bidirectional counter-flow heat exchange plate on the side faces of individual energy storage cells, we compare three cooling schemes: Scheme 1 (bottom-mounted unidirectional flow plate, as shown in the figure below), Scheme 2 (side-mounted unidirectional flow plate), and Scheme 3 (side-mounted bidirectional counter-flow plate). In all schemes, a layer of thermally conductive silicone grease is applied between the cooling plate and the cell to ensure heat transfer. In Scheme 1, the unidirectional flow plate has an 8-channel straight flow configuration; coolant flows from Inlet1 to Outlet2. The plate is placed at the bottom of the battery pack, which is a classic arrangement. In Scheme 2, the unidirectional flow plate is placed between the side faces of adjacent energy storage cells, with coolant flowing from Inlet1 to Outlet2. Our proposed bidirectional counter-flow plate (Scheme 3) features dual flow channels, with two inlets at the bottom and two outlets at the top. Coolant enters from Inlet1 and Inlet2 and exits from Outlet1 and Outlet2, creating cross-counter flow within the plate for enhanced heat exchange. Additionally, an air insulation layer is provided between the bidirectional counter-flow plates to thermally isolate adjacent cells. This unique dual-inlet (bottom) and dual-outlet (top) layout, combined with the intermediate air insulation layer, achieves both efficient heat dissipation and thermal insulation.
Mathematical Model
The heat generation rate of the lithium-ion energy storage cell is calculated using the Bernardi model:
$$
q = \frac{I}{V_b}\left[(E_0 – U) – T\frac{dE_0}{dT}\right] = \frac{1}{V_b}\left(I^2 R – IT\frac{dE_0}{dT}\right)
$$
where \(V_b\) is the cell volume, \(E_0\) the open-circuit voltage, \(U\) the terminal voltage, \(T\) the instantaneous temperature, \(I\) the current, and \(R\) the internal resistance.
The anisotropic heat conduction inside the cell is described by:
$$
\rho C_p \frac{\partial T}{\partial t} = \lambda_x \frac{\partial^2 T}{\partial x^2} + \lambda_y \frac{\partial^2 T}{\partial y^2} + \lambda_z \frac{\partial^2 T}{\partial z^2} + Q
$$
where \(\rho\) is density, \(C_p\) specific heat capacity, \(\lambda_x, \lambda_y, \lambda_z\) thermal conductivities in three orthogonal directions, and \(Q\) the volumetric heat generation rate.
For the coolant flow inside the heat exchange plate, the governing equations include continuity, momentum, and energy equations:
$$
\frac{\partial v}{\partial t} + (v \cdot \nabla) v = -\frac{\nabla p}{\rho_w} + \frac{\mu}{\rho_w} \nabla^2 v + g
$$
$$
\frac{\partial \rho_w}{\partial t} + \nabla \cdot (\rho_w v) = 0
$$
$$
\frac{\partial}{\partial t}(\rho_w C_{p,w} T_w) + \nabla \cdot (-k_w \nabla T_w) + \rho_w C_{p,w} v \nabla T_w = 0
$$
The energy conservation equation for the solid heat exchange plate is:
$$
\frac{\partial}{\partial t}(\rho_n C_{p,n} T_n) + \nabla \cdot (-k_n \nabla T_n) = 0
$$
Here, \(v\) is velocity, \(p\) pressure, \(\rho_w\) coolant density, \(\rho_n\) plate density, \(\mu\) coolant viscosity, \(g\) gravitational acceleration, \(C_{p,w}\) and \(C_{p,n}\) specific heats of coolant and plate, \(T_w\) and \(T_n\) temperatures of coolant and plate, and \(k_w\) and \(k_n\) thermal conductivities.
Boundary Conditions
Initial cell temperature and ambient temperature are both set to 298 K. The coolant inlet is defined as a velocity inlet boundary condition; the outlet is a pressure outlet. The battery pack is subjected to heat generation corresponding to conventional (1C), high (3C), and ultra-high (5C) charge/discharge rates. The total coolant flow rate at the inlet of the heat exchange plate is 12 L/min, with an inlet temperature of 293.15 K. The coolant is a 50% (mass fraction) ethylene glycol aqueous solution. Steady-state simulations are performed to compare the cooling performance of the three schemes under different cell heat generation conditions. The heat generation parameters are obtained from literature. Mesh independence is verified, and a mesh size of 4.23 million elements is selected for computation.
Results and Discussion
Performance under Conventional Rate (1C)
Figure 4 (not shown) illustrates the temperature distribution for Scheme 1 (bottom-mounted plate). The cell temperatures range from 294.5 K to 301 K, which is within the acceptable range. However, the maximum temperature difference across the pack reaches 6.5 K, exceeding the 5 K uniformity requirement. The primary temperature gradient occurs between the top and bottom of the cells. Since the bottom is in direct contact with the cooling plate, it is effectively cooled, but the low through-plane thermal conductivity hinders heat transfer from the top to the bottom, leading to a large vertical gradient.
For Scheme 2 (side-mounted unidirectional plate), the maximum cell temperature is 294.7 K, and the maximum temperature difference is about 1.5 K, meeting the uniformity requirement. The side placement increases the contact area and shortens the thermal path, especially enhancing heat removal in the vertical direction. Scheme 3 (side-mounted bidirectional counter-flow plate) yields a maximum temperature of 294 K and a temperature difference of about 1 K, also satisfying the requirement. Under 1C conditions, Schemes 2 and 3 both perform well, while Scheme 1 fails to achieve temperature uniformity.
Performance under High Rate (3C)
Under 3C charging, the thermal management performance diverges significantly. Table 1 summarizes the key results for the three schemes at 3C.
| Scheme | Max Temperature (K) | Max Temperature Difference (K) | Meets T ≤ 313 K? | Meets ΔT ≤ 5 K? |
|---|---|---|---|---|
| 1 (Bottom unidirectional) | 357 | 52 | No | No |
| 2 (Side unidirectional) | 307 | 13 | Yes | No |
| 3 (Side bidirectional counter-flow) | 299 | 4.8 | Yes | Yes |
Scheme 1 exhibits a maximum temperature of 357 K, approaching the high-risk threshold for thermal runaway, and a temperature difference of 52 K, making it completely unacceptable. Scheme 2 reduces the maximum temperature to 307 K, well below the risk threshold, but the temperature difference remains 13 K, exceeding the 5 K limit, which would accelerate cell aging and degrade performance. Scheme 3, employing the bidirectional counter-flow plate, achieves a maximum temperature of 299 K and a temperature difference of only 4.8 K, satisfying both the temperature and uniformity requirements. The counter-flow design enables thermal compensation between the two coolant streams, eliminating the temperature rise along the flow direction observed in unidirectional schemes.
Performance under Ultra-High Rate (5C)
Under the extreme 5C rate, the results are even more pronounced. Table 2 compares the three schemes at 5C.
| Scheme | Max Temperature (K) | Max Temperature Difference (K) | Risk of Thermal Runaway? |
|---|---|---|---|
| 1 (Bottom unidirectional) | 470 | ~107 | Extreme (exceeds 393 K) |
| 2 (Side unidirectional) | 332 | 39 | Moderate (below 393 K but high gradient) |
| 3 (Side bidirectional counter-flow) | 308 | 14 | Low (well below 393 K) |
Scheme 1 leads to a catastrophic maximum temperature of 470 K, far exceeding the thermal runaway threshold of 393 K, posing severe safety hazards. Scheme 2 yields a maximum of 332 K, which, although below the critical threshold, still causes a large temperature difference of 39 K, leading to non-uniform aging and potential local hot spots. Scheme 3 keeps the maximum temperature at 308 K and the temperature difference at 14 K. While the temperature difference exceeds the ideal 5 K limit, such extreme 5C conditions are rare and short-lived in practical grid operations; hence the impact on cell longevity is limited. The bidirectional counter-flow plate demonstrates robust thermal management capability even under extreme conditions.
Heat Transfer Analysis of the Heat Exchange Plate
Figure 10 (not shown) presents the coolant temperature distribution within the heat exchange plates under the 3C condition. In Scheme 1, the coolant temperature rises significantly along the flow direction (left to right), with a temperature increase of 15 K, due to heat accumulation in the central region and non-uniform flow distribution. In Scheme 2, the temperature rise is about 10 K, but the vertical (height-wise) temperature variation is relatively small because the plate width aligns with the cell height direction, which has higher thermal conductivity. Scheme 3, the bidirectional counter-flow plate, exhibits remarkable temperature uniformity with a temperature difference of only about 1 K across the entire plate. This stems from the cross-counter flow design: the two coolant streams exchange heat in opposite directions, maintaining a nearly constant coolant temperature. Additionally, the air layer between adjacent bidirectional plates prevents thermal interference, further enhancing uniformity.
The superior thermal management performance of the bidirectional counter-flow plate is attributed to its unique flow channel architecture. By creating a thermal compensation mechanism, it minimizes the temperature gradient along both the flow direction and the plate width, thereby ensuring that the energy storage cell pack experiences both low maximum temperature and excellent temperature uniformity over a wide range of charge/discharge rates.
Conclusions
This paper proposes a bidirectional counter-flow heat exchange plate for thermal management of grid energy storage cell packs. Through numerical simulation, we compare the thermal performance of three cooling schemes—bottom-mounted unidirectional plate, side-mounted unidirectional plate, and side-mounted bidirectional counter-flow plate—under 1C, 3C, and 5C charge/discharge rates. The main conclusions are as follows:
- Scheme 1 (bottom-mounted unidirectional plate) fails to meet the temperature uniformity requirement (ΔT ≤ 5 K) under all tested rates, and under high/ultra-high rates, the maximum temperature exceeds safe limits, posing thermal runaway risks.
- Scheme 2 (side-mounted unidirectional plate) meets the maximum temperature requirement under 1C and 3C but cannot control the temperature difference within 5 K at 3C and 5C. The temperature difference reaches 13 K and 39 K, respectively, which accelerates cell aging.
- Scheme 3 (side-mounted bidirectional counter-flow plate) satisfies both the maximum temperature and temperature uniformity criteria under all three rates. At 3C, it achieves a maximum temperature of 299 K and a temperature difference of 4.8 K; at 5C, the maximum temperature is 308 K and the difference is 14 K, which is acceptable for short-duration extreme events.
The bidirectional counter-flow heat exchange plate effectively suppresses thermal gradients across the large side surfaces of the energy storage cell and enhances heat transfer from the top to the bottom of the cell. Its counter-flow design creates a self-compensating thermal environment, leading to superior temperature uniformity and cooling efficiency. This scheme demonstrates strong adaptability over a wide range of charge/discharge rates, providing a reliable technical pathway for grid-scale energy storage systems to cope with extreme operating conditions.