Evaluation of hybrid Cooling System Models for Battery Pack of Electric Vehicle under Hot Zones Conditions

Abstract

The charges/discharges of Li-ion batteries generate excessive heat, especially at high charge/discharge rates, which represent the main barrier to Li-ion battery usage. Therefore, using an effective battery thermal management system is quite essential to ensure optimal operation of lithium-ion batteries. Furthermore, a hybrid cooling system from an active and passive cooling system can provide more temperature control and uniformly throughout the battery cell than solely a passive or active cooling system.

Therefore, in this study, an experimental and numerical investigation of two novel hybrid cooling systems using phase change material as a passive cooling system combined with other cooling systems is studied. The first cooling model (PCM/Fin-Air-Model), uses phase change material combined with aluminum fins and forced air. While the second model (HP/PCM-Model), uses a new model of flat heat pipes having a finned condenser and double circular grooved evaporator in addition to the usage of phase change material integration in the evaporator section. The HP was charged with acetone as a working fluid with 40 % filling ratio. Phase change material type Rubitherm-RT47 with a melting temperature range of 41-48 °C is selected. The present hybrid models are designed for cooling a battery pack consisting of 12 cylindrical batteries 18650-type LiCoO₂ arranged with three batteries in parallel and four batteries in series. The thermal performance of the novel models are compared with other conventional models of passive phase change material model (PCM-Model), forced air model (Air-Model), and the same heat pipe without the phase change material combination (HP-Model). All models are examined using various discharge rates (C-rate) of 1, 2, and 3C, and various inlet air velocities ( ) of (0.75, 1.5, 2.25, and 3 m/s), and various operating temperatures () of (25, 30, 35 and 40 °C).

Numerical simulation is achieved using three-dimensional models built using COMSOL Multiphysics 5.6 and comprehensively validated with experiment results. The experimental results indicated that the novel hybrid models display a good cooling performance, especially at a high C-rate compared with the conventional models which became insufficient for battery cooling at a 3C. The PCM/HP-Model reduces the maximum operating temperature at 3C by 18.9, 26.6, and 9.8%, while the PCM/Fin-Air-Model reduces it by 15.2, 23.3, and 4.6%, compared to the Air-Model, PCM-Model and HP-Model, respectively. Furthermore, the largest maximum temperature difference for the PCM/HP-Model and PCM/Fin-Air-Model did not exceed 4.2 and 1.9 ℃, respectively. Additionally, the HP/PCM-Model has the lowest pressure losses and fan power consumption than other models.

The numerical result shows that the increase of the air velocity has a slight effect on the battery cooling using the novel hybrid models while it has a significant effect on the battery cooling using Air-Model, where the reduces by 5.3, 12.8 and 30 % as the increases from 0.75 to 3 m/s for PCM/HP-Model, PCM/Fin-Air-Model, and Air-Model respectively. The increasing of from 0.75 to 3 m/s reduces the PCM liquid fraction from 0.877 to 0.772 and from 1.0 to 0.748 at the end of the 3C rate period of the PCM/HP-Model and PCM/Fin-Air-Model, respectively. The usage of PCM-Model is not recommended for a 3C rate even at a low operation temperature of 25 ℃.

Moreover, the result demonstrated that the HP/PCM-Model exhibits a better thermal performance and lower power consumption, especially at a high C-rate, as well as it can offer a safe working temperature for the Li-ion batteries.

The numerical results show good agreement with the experimental data, with maximum deviations of 3.8, 6, 2.3, 2, and 2.4% for the Air-Model, PCM-Model, PCM/Fin-Air-Model, HP-Model, and HP/PCM-Model, respectively.