Modelling and control design of hybrid energy storage systems for electric vehicles based on Na-ion and Li-ion cells.

The automotive industry faces a significant challenge due to the increasingly stringent emissions regulations. Meeting these goals requires the development of technologies capable of achieving zero carbon emissions while remaining economically viable. From Hydrogen fuel propulsion to battery electric vehicles (BEVs), different paths are being explored to address this problem. However, factors such as resource-intensive production of Lithium-ion batteries and early battery degradation, hinder the widespread adoption of BEVs. The purpose of this thesis is to reduce stress on the Lithium-ion battery (LIB) by exploiting the capability of the Sodium-ion cells to sustain higher C-rates (ratio between the actual current and the nominal capacity) without damage, thereby extending the life of the Lithium battery. To achieve this, the first step involved modelling a hybrid energy storage system based on the two above mentioned cell chemistries for a subcompact crossover sport utility vehicle. Secondly, two different approaches to define the battery sizing are investigated. The first approach, termed Constant Mass, keeps the mass of the hybrid battery pack constant, while varying the mass of the constitutive LIB and Sodium-ion battery (NaIB). The second approach, named Constant Energy, maintains the energy level of the hybrid pack while varying the energy contributions of the constitutive LIB and NaIB. For both approaches, three different battery sizes were studied. The next step involved regulating the power flows within the system. Two distinct deterministic rule-based energy management strategies, Load Follower and Range Extender, were designed. In the Load Follower strategy, the contribution of the LIB varies inversely with the power request derivative, the higher is the power derivative the lower is the LIB’s contribution. In the Range Extender strategy instead, the contribution of the LIB is limited at a decided value that depends on the state of charge of the NaIB. The lower is the state of charge the higher is the contribution of the LIB. In conclusion, the analysis of the results shows that a battery configuration with 70% of energy provided by LIB and 30% by NaIB, in a constant energy setup, combined with Range Extender energy management, offers the best balance between C-rate reduction and range penalization. This solution allows for a reduction of up to 31.6% in LIB C-rate and a 5.9% decrease in range compared to a vehicle equipped solely with Lithium-ion cells. These findings pave the way for a promising solution to enhance electric vehicles by integrating NaIBs, which not only extend the lifespan of LIBs but also have lower material criticality, leading to a more durable and environmentally sustainable energy storage system. This thesis also laid the foundation for future prototyping of the energy storage system to have the opportunity to verify the validity of the model and the control strategy with Hardware-In-the-Loop tests.