Battery thermal management using composite phase change materials

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Zhao, Yanqi (2021). Battery thermal management using composite phase change materials. University of Birmingham. Ph.D.

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Abstract

This PhD study reports the research on an active cooling-based battery thermal management system (BTMS) using composite phase change materials (CPCMs). The hybrid BTMS has a great potential to replace current commercial BTMS as it could control not only the maximum battery pack temperature but also temperature uniformity. However, there are insufficient studies on the area and hence research gaps, particularly the following aspects needs to be overcome. Firstly, there has been inadequate studies of particle size effect on the thermal property of CPCMs. Secondly, many published papers did not include accurate descriptions of the battery thermal behaviour in their study. However, the heat generation rate can fluctuate wildly in the charge/discharge process. Thirdly, there have been very few studies compared the performance of different CPCMs in the BTMS. In this PhD study, three types of CPCMs, expanded graphite (EG)/paraffin, copper foam/paraffin and colloidal graphite/paraffin composites were investigated. Their surface morphologies, structures, phase change behaviour, thermal stability, and thermal conductivity were studied experimentally. Thermal behaviour and energy efficiency of the lithium-ion battery (LIB) was investigated experimentally, under different charge/discharge rates, and at different temperatures. An experimental set up and computational fluid dynamics (CFD) modelling using Ansys® fluent were carried out to examine the performance of the active cooling based BTMS using CPCM.
In the study of EG/paraffin composite, porous structure and the particle size of the EG are found to be the determining factors of the properties of EG/paraffin composites. The surface morphology, phase change temperature, thermal degradation temperature, cycling stability and thermal conductivity are affected by the porous structure and the particle size. In the investigation of copper foam/paraffin composites, the porosity of the copper foam can efficiently affect the thermal property of the CPCMs, with the latent heat inversely proportional to the thermal conductivity. In the study of colloidal graphite/paraffin composites, it was found that melamine is the excellent fire-retardant material for colloidal graphite/paraffin composite, which could vastly improve the thermal degradation temperature with a reasonable amount.
In the characterization of single cells, the temperature increase of the LIB under charge and discharge was accurately measured under an adiabatic condition. The heat generation rate was retrieved from temperature increase, and its variation was studied. It was also found that low current rates and high operation temperatures lead to higher specific energy, high specific power, and high round trip efficiency. Battery polarization has been identified as the main factor which caused the variation on the battery thermal behaviour and energy performance under different operating conditions.
In the BTMS study, two models with both active liquid cooling and passive CPCM cooling were then developed to examine the performance of BTMS. The first numerical model used a round tube with a relatively small volume occupancy and a small heat transfer area. An experimental investigation on the same device setup has successfully validated the accuracy of the model. Effect of inlet velocity of the heat transfer fluid (HTF) and the battery current rate has been investigated. High inlet velocity of HTF can reduce the maximum battery temperature, and CPCM can provide temperature uniformity to the battery pack. The second model used a tabular tube with a relatively large volume and a large heat transfer area. The simulation serves the objective to study the impacts of CPCM types and inlet velocity of HTF on both the maximum battery temperature and temperature distribution under different current rates. The results showed that liquid cooling is efficient in controlling battery temperature under continuous battery charging/discharging cycles. The EG/paraffin composite is better than the copper foam/paraffin composite in both reducing the maximum battery temperature and providing temperature uniformity to the battery pack. In a search for generalized results, dimensional analysis of the results is performed and presented as the Nusselt numbers and dimensionless temperature against the Fourier and Stefan numbers.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):