Abstract
Increasingly stringent emission regulations and environmental concerns have propelled the development of electrification technology in the transport industry. The introduction of vehicle electrification both in the form of hybrid electric vehicles (HEV) and battery electric vehicles (BEV) is growing rapidly and soon will make up for most of the new vehicle sales in upcoming years. Nevertheless, the greatest hurdle to continuing this rapid development of electric vehicles is electrochemical energy storage which struggles to achieve profitable specific power, specific energy and cost targets.Electrochemical hybrid energy storage systems (HESS), which combine energy and power optimised battery technology, seem to be the most promising solution to improve the overall performance of energy storage. These systems are particularly well suited to high performance BEV applications due to their high-power demands, but they may also enable the implementation of new battery technologies on less performance focused vehicles.
This thesis establishes methodologies for evaluating and optimising electrochemical HESS technologies used in high performance BEVs to achieve the lightest design possible and improve driving performance. It was found that for an application with high power-to-energy ratio and the right battery technology, a HESS may enable up to 40% weight savings in cell mass. Moreover, an electrochemical HESS can improve the regenerative braking potential of a vehicle to achieve approximately 35% energy recuperation on a high power duty cycle and therefore enable further energy storage system downsizing compared to battery pack with lower peak charging capabilities. The impact of electric vehicle power and weight on lap time and energy was also another area studied using vehicle dynamic simulation. The results showed that interestingly electric vehicles performance may be hindered when increasing vehicle power excessively due to the required energy storage weight. However, by using a HESS with a very high power density power pack this can be improved significantly.
In depth analysis of the control strategies have presented novel concepts of energy transfer between battery packs, analysis of HESS topologies and development of effective BMS controllers for optimal control and communication with an overall power split controller. Some of the most important factors that impact the HESS effectiveness were also identified and evaluated. These include a minimum continuous C-rate from the energy cells depending on the duty cycle, minimum DCDC power level and ensuring the vehicle has a high regenerative braking capability. Dynamic simulations were also carried out for a HESS implementing future energy dense cells and showed that an energy density improvement from 250 Wh/kg to 500 Wh/kg enables over 40% overall weight saving despite reduced energy cell C-rate limits. Such future energy dense cell technologies would require almost four times less power density compared to the technology used in a (single cell) traditional pack arrangement.
| Date of Award | 23 Mar 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Richard Burke (Supervisor), Chris Brace (Supervisor) & Sam Akehurst (Supervisor) |