What Factors Constrain Thermal Management in Power Batteries?

This article explores the constraints on power battery cooling through experimental and numerical studies, focusing on temperature and velocity distribution within microchannel cold plates on square lithium-ion battery cells. The analysis investigates cooling performance at 1C and 2C discharge rates under working temperatures of 5°C, 15°C, and 25°C, using both experimental setups and simulations.

Abstract

To ensure optimal performance and longevity, lithium-ion batteries require effective thermal management systems (BTMS). This study combines experiments and computational simulations using ANSYS Fluent’s k-epsilon model to investigate the thermal performance of microchannel cold plates. Key findings include:

1. Temperature differences within the microchannel increase with higher C-rates.
2. Discharge rates (1C and 2C) elevate temperatures across all sensor locations on the battery surface.
3. Thermocouples near the electrodes record higher temperatures compared to those at the center of the battery cell.

1. Introduction

The automotive industry’s shift toward sustainable vehicles such as electric vehicles (المركبات الكهربائية), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and fuel cell vehicles (FCVs) highlights the significance of lithium-ion batteries. These batteries dominate due to their high energy and power density, long cycle life, and minimal self-discharge rates. لكن, maintaining battery performance requires proactive measures to prevent overheating, thermal runaway, and reduced efficiency.

Effective thermal management is critical for ensuring safety, كفاءة, and extended battery life. Operating within the standard temperature range of 20°C to 40°C (extendable to -10°C and 50°C) is essential. Various BTMS techniques include:

• Air cooling: Lightweight but limited by low thermal conductivity.
• Liquid cooling: More efficient but comes with higher complexity, weight, and costs.
• Phase Change Materials (PCMs): Passive systems offering simplicity and weight reduction.

This study focuses on liquid cooling using microchannel cold plates, comparing experimental data with simulations to analyze performance under varying discharge rates and temperatures.

2. Experimental Study

2.1 Experimental Setup

The experimental system consisted of two commercial cold plates placed on the top and bottom surfaces of a 20Ah LiFePO4 prismatic pouch cell. A total of 19 thermocouples monitored temperatures at key locations:

• 10 thermocouples on the primary surface.
• 3 thermocouples near the anode, cathode, and mid-region.
• 2 thermocouples at the electrodes.
• 4 thermocouples measuring inlet and outlet temperatures of the cold plates.

2.2 Experimental Procedure

The study evaluated cooling performance at three coolant temperatures (5°C, 15°C, 25°C) and two discharge rates (1C and 2C). The process included:

1. Pre-heating the system to stabilize at the target temperature for two hours.
2. Using LabVIEW to control and monitor charging and discharging parameters.
3. Collecting thermal data, including surface and coolant temperatures.

3. Key Findings

3.1 Heat Generation in Lithium-Ion Batteries

Heat in lithium-ion batteries arises from:

• Electrochemical Reactions: Heat generated or absorbed during charging/discharging.
• Joule Heating (Ohmic Losses): Internal resistance contributes significantly to heat production.

The rate of heat generation depends on current, resistance, and entropy changes, modeled using:

Q=I2R+IΔS⋅nFQ = I^2R + I \Delta S \cdot nFQ=I2R+IΔS⋅nF

where $I$ is the current, $R$ is resistance, $\Delta S$ is entropy change, $n$ is electron flow, و $F$ is Faraday’s constant (96,485 C/mol).

3.2 Cooling Efficiency

1. Microchannel Design: Increased channel numbers and flow rates reduced temperature gradients but faced diminishing returns at higher flow rates.
2. Flow Direction: The effect of flow direction on cooling diminished as mass flow increased.
3. Optimal Parameters: A flow rate of 5 × 10−4^{-4}−4 kg/s balanced cooling performance and system efficiency.

4. Numerical Simulations

Using the Reynolds-Averaged Navier-Stokes (RANS) method, turbulent flows in the microchannels were simulated. Two turbulence models, k-epsilon and k-omega, were compared:

• k-epsilon: Robust for general applications with simple computational requirements.
• k-omega SST: Accurate near boundaries, transitioning smoothly between different flow regimes.

The simulations validated experimental findings, emphasizing the importance of optimizing channel design and coolant flow rates for effective thermal management.

5. Literature Comparisons

5.1 Microchannel Cooling Strategies

• Jarrett and Kim: Optimized serpentine channel designs to balance temperature uniformity and pressure drop.
• Zhao et al.: Demonstrated that increasing channels beyond a threshold offers negligible benefits.
• Jin et al.: Introduced slanted fin designs to improve heat transfer efficiency in liquid cooling plates.

5.2 Air Cooling

Saw et al. analyzed large battery packs using air cooling, deriving correlations between Nusselt and Reynolds numbers to predict thermal performance under varying conditions.

5.3 Internal Cooling

Mohammadian et al. proposed internal cooling using liquid electrolytes, showing significant reductions in temperature gradients.

6. خاتمة

This study underscores the significance of microchannel cooling plates in managing battery thermal performance. Experimental and numerical results highlight the interplay between channel design, flow rates, and temperature distribution. By optimizing these factors, power batteries can achieve enhanced safety, كفاءة, and longevity, supporting their critical role in electric vehicles.

Key Takeaways:

• Higher flow rates reduce temperature extremes but with diminishing returns.
• Channel design significantly impacts thermal uniformity.
• Numerical simulations are essential for refining BTMS designs.

By advancing cooling technologies, we pave the way for safer and more efficient energy storage solutions in sustainable transportation.