Hybridised energy storage systems for automotive powertrain applications

Abstract

This study explores whether the requirements of the Energy Storage System (ESS) in electrified automotive powertrains can be more effectively met by Hybridised Energy Storage Systems (HESS) than by batteries alone, and aims to quantify the benefits. Given their complementary characteristics, this study focuses upon combinations of Lithium-ion Batteries (LIBs) and Supercapacitors (SCs). In order to develop a comprehensive understanding of HESS operation and capabilities, a literature review is conducted and experiments and modelling tools are designed to address knowledge gaps. The electrochemical and thermal performance of SCs and LIBs is examined in detail, and the amassed knowledge is used to generate modelling tools. Following a review of HESS topologies, a model of a passive HESS is developed through combination of the SC and LIB models. The developed models are validated using experimental data for automotive drive-cycles. The degradation of a HESS is assessed using experimental analysis. The main findings of this study are split into three chapters: Supercapacitors This chapter explores the electrochemical and thermal characteristics of SCs and demonstrates how they vary with the operating conditions relevant for latter comparison with LIBs. Particular focus has been placed upon the attributes of SCs which affect their performance in a HESS, namely capacity and resistance variation with operating conditions, heat generation during operation, low temperature performance and performance degradation following extended use. An understanding of these factors is used to develop a model capable of predicting SC performance over an array of operating conditions, enabling latter combination with a lithium ion battery model to predict HESS behaviour in both cell-level and high voltage systems. An emphasis was placed upon the development of a high fidelity thermal model as the academic literature provided a limited understanding of thermal management and the effect of thermal gradients within SCs. The development of a high fidelity thermal model led to novel findings with respect to heat generation with SCs during operation. Lithium-ion Batteries An understanding of the performance and characteristics of LIBs relative to SCs is necessary for the development of an understanding of a HESS. This chapter briefly explains why LIBs are the dominant form of energy storage in electrified passenger vehicles, and subsequently discusses how LIBs function. The relationship between each component within the LIB and its performance is reviewed, and common internal design trade-offs such as material selection and thicknesses are explained. LIB performance variation as a function of operating conditions is subsequently presented and described analytically. This knowledge is used to generate a model to predict battery performance over a range of operating conditions, with a particular focus placed upon performance over an automotive drive-cycle. The developed model enables estimation of internal temperature during use. The causes of long term performance degradation are examined and a summary of degradation methods and their causes is provided. Hybridised Energy Storage Systems This chapter investigates whether it is possible to combine supercapacitors suitable for a micro hybrid electric vehicle (mHEV) with high-energy batteries suitable for use in a battery electric vehicle (BEV) to create a Hybridised Energy Storage System (HESS) suitable for use in a hybrid electric vehicle (HEV). A passive HESS topology is investigated due to its low cost and complexity and therefore high reliability. A low cost HESS is found to be capable of meeting the electrical demands of a HEV during a drive cycle. The operating principles of HESSs are discussed and factors limiting system performance are explored. The performance of the HESS is found to be significantly less temperature dependent than battery-only systems, however the heat generated suggests a requirement for thermal management. A correctly sized HESS is shown to generate less heat than a specialised HPB ESS. The HESS degrades at a similar rate to a specialised HPB. In a HESS, battery resistance rises faster than supercapacitor resistance; as a result, the supercapacitor provides a greater current contribution over time, therefore the energy throughput, temperature rise and rate of degradation of the batteries is reduced.