PhD Defence Kevin Redosado Leon | Graphene-Coated Paraffin Composites: Numerical Investigation of Effective Thermal Properties

Graphene-Coated Paraffin Composites: Numerical Investigation of Effective Thermal Properties

Kevin Redosado Leon is a PhD student in the department Multiscale Modeling and Simulation. (Co)Promotors are prof.dr.ir. B.J. Geurts and prof.dr. A. Lyulin from the faculty of Electrical Engineering, Mathematics and Computer Science, University of Twente.

This thesis presents a numerical study of the thermal performance of graphene-coated paraffin composites embedded in air, with the aim of improving the effectiveness and responsiveness of thermal energy storage systems. It examines how microscale features such as sphere size, coating thickness, and volume fraction influence the overall heat conduction and thermal behaviour of the material. All simulations are performed using OpenFOAM within periodic representative volume elements, allowing direct modelling of structured composite domains.

 Chapter 1 introduces the broader motivation for this work, situating it in the context of the energy transition and the need for reliable thermal storage solutions. Paraffin is identified as a promising phase change material due to its high energy storage density, though its very low thermal conductivity limits its practical use. The addition of graphene coatings is proposed as a way to improve heat transfer while preserving storage capacity.

 Chapter 2 establishes the computational framework for simulating uncoated paraffin spheres in air. It evaluates the mesh resolution needed to capture heat conduction accurately in unit cells containing 1 or 2 spheres. The study finds that a minimum of 8 mesh cells across the sphere diameter is required to achieve second-order convergence in the simulations. Configurations with narrow gaps or slight overlaps between spheres are shown to be numerically stable and accurate when this resolution is applied. These findings serve as a baseline for the more complex models introduced in the following chapters.

 Chapter 3 adds a high-conductivity graphene coating around the paraffin sphere and focuses on a single coated inclusion in a periodic air matrix. The goal is to determine how much resolution is needed to model the layered structure accurately. It is shown that at least 16 cells are needed across the paraffin core and 2 cells across the graphene coating to capture the steep temperature gradients. The presence of the coating significantly enhances the effective thermal conductivity of the composite, especially when the volume fraction of the inclusion is high. This chapter sets the resolution criteria that are applied throughout the rest of the work.

Chapter 4 explores the dynamic thermal behaviour of the coated inclusion. A steady temperature difference is applied across the unit cell, and the simulation tracks how heat propagates over time. From these results, the overall heat capacity, conductivity, and thermal diffusivity of the composite are calculated. A key contribution of this chapter is the definition and analysis of the thermal reaction time, a measure of how quickly the system responds to heating. The results show that as more paraffin is added, the heat capacity increases steadily, but the thermal diffusivity decreases because the gain in heat capacity outweighs the improvement in conductivity. This leads to a slower thermal response in materials with higher paraffin content. The chapter identifies a clear design trade-off between high storage capacity and fast thermal response and offers practical guidance for selecting mesh and time step sizes to resolve transient behaviour correctly.

 Chapter 5 extends the analysis to domains containing multiple coated spheres. A seeding method is employed to generate increasingly large representative volume elements in a controlled and repeatable way. The objective is to identify the thermodynamic limit, defined as the point at which the effective thermal conductivity becomes independent of the domain size. As the number of spheres increases, the results converge toward a stable value of thermal conductivity, indicating that a representative macroscopic behaviour has been reached. At high volume fractions, where spheres are closer together, the simulations reveal the emergence of conductive paths through the network of graphene shells. This observation represents an early insight into percolation phenomena, although a formal study of percolation is not pursued in this work. All simulations strictly follow the resolution criteria established in previous chapters to ensure consistency and numerical accuracy.

Chapter 6 concludes that graphene-coated paraffin composites can achieve both high energy storage and improved thermal response, provided that geometry and resolution are carefully controlled. The developed simulation framework offers a reliable tool for evaluating such materials, with future work potentially addressing melting, interface resistance, and experimental validation.

This thesis contributes to the understanding and design of multiscale thermal energy storage materials by linking detailed microstructural features to macroscopic behaviour through systematic numerical analysis. The findings support the development of more efficient and responsive composites and offer a foundation for future design and optimisation.