Lithium-ion 18650 cylindrical Battery Thermal Management: Multi-level CFD Simulations utilising Phase Change Materials for Enhanced Performance

This thesis undertakes a comprehensive exploration of Battery Thermal Management Systems (BTMS), with a specific emphasis on improving cylindrical 18650 batteries' performance, through the integration of passive cooling techniques via multi-level Computational Fluid Dynamics (CFD) simulations. The research was meticulously designed to enhance the thermal performance and lifespan of lithium-ion (Li-ion) batteries, pivotal components of Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs). The investigation commenced with an extensive literature review delving into Thermal Energy Storage (TES), focusing on Latent Heat Storage (LHS) and Phase Change Materials (PCMs). The study encompassed types, designs, and applications, including their integration with Electrical Energy Storage (EES) devices such as Li-ion batteries. The primary goal was to implement thermal renewable energy systems in EVs and HEVs, with a particular interest in passive cooling methods and their impact on battery performance from the cellular to the module level. Given the high energy density of Li-ion batteries, their safe operation under diverse temperature conditions poses a challenge, necessitating an effective BTMS. The literature review scrutinised thermal and electrochemical battery modelling, heat transfer mechanisms, BTMS developments, and CFD analysis methods. Different BTMS technologies were evaluated based on criteria like cost, efficiency, safety, and adaptability to various cooling and heating techniques. A specific focus on PCMs within the BTMS domain explored various types, including pure, composite, and hybrid-based systems, analysing the behaviour of cells and modules upon PCM integration. This multidimensional exploration aimed to contribute essential insights for advancing BTMS, which is crucial for EVs' widespread adoption and sustainability.

In terms of LHS, a proposed horizontal concentric double-pipe heat exchanger with N-eicosane PCM for Latent Heat Storage (LHS) was studied. It focused on the impact of fin type and orientation on charging (melting PCM) and discharging (solidifying PCM). Various cases were considered including fin orientation (longitudinal and transversal) and fin types (corrugated and flat), which were simulated for optimal thermal and heat transfer performance when compared with and without the addition of the fins. The results showed that natural convection enhanced melting while conduction dominated during discharging. Fins significantly reduced charging and discharging times, influenced by orientation and type. Higher fin surface area (corrugated) and transversal position exhibit a 27 % improvement in heat transfer. For melting, transversal corrugated fins outperform, achieving over 88 % reduction in melting time. The transversal corrugated fin design excelled in both processes, with the shortest overall processing time. Notably, longitudinal flat-finned arrangements were 1.2x faster, and transversal flat-finned arrangements were 8.7x faster than the unfinned case.

Additionally, a numerical study examined EV battery cell performance using a Latent Heat (LH) jacket for passive cooling. The battery cell was coupled with PCM and assessed through continuous cycles. Validation against literature values for a Panasonic 18650PF Li-ion cell displayed less than 1 % deviation. Thermal and electrical parameters were analysed under various climatic conditions. Passive cooling, especially with a 3 mm jacketed PCM, resulted in a significant 340 % thermal performance improvement at ambient weather (25 °C). Higher temperatures (40 °C and 55 °C) displayed improvements of up to 275 % and 440 %, respectively. At lower temperatures (-20 °C and 0 °C), passive cooling maintains stability with improvements of 162 % and 160 %, respectively. The study concluded that efficient passive cooling enhances EES safety and performance in EVs and HEVs.

Moreover, the efficacy of passive cooling with PCMs in BTMS was further explored. The study focused on a single cylindrical Panasonic 18650 battery cell with a circumferential LH jacket under real-world drive cycles when compared without. The challenge was understanding the diverse driving behaviours’ impact on battery performance and thermal stability with passive cooling. Using conjugated thermo-chemical and electrical models based on simulated real-world scenarios, the study indicated that significant battery performance enhancement was achieved with LH jackets, improving over 50 % in most cycles. Particularly, in aggressive drive cycles, the battery life extended from 2.2x to 2.4x. LH jackets maintain thermal stability and result in more than 45 % thermal performance improvement and over 200 % life extension across all cycles. This approach enhanced understanding and improved BTMS in real-world scenarios, especially in passive cooling through LH jackets.

Furthermore, another study investigated passive cooling’s impact on BTMS by analysing various module configurations under ideal continuous cycling and real-world drive cycles. Using numerical CFD, the study focused on circumferential PCM jackets for each cell in 3 dimensional (3D) modules consisting of 24 battery cells. Modules, arranged in different configurations, were simulated with and without LH/PCM jackets. Evaluation criteria included battery module temperature, PCM liquid fraction, state of charge (SOC), and passive zone potentials. Findings emphasised passive cooling, which stabilised the system and cell temperatures under normal and harsh conditions. The results indicated a stable and uniform temperature, with an average 20 °C module temperature reduction and less than 5 °C difference. There was over 200 % improvement in battery module life across all assessed drive cycles, regardless of module arrangements.