Reconfigurable Batteries in Electric Vehicle Fast Chargers: Towards Renewable-Powered Mobility

The world is currently experiencing a paradigm shift as electric vehicles (EVs) are gradually replacing combustion engine vehicles as the means of road transportation. Driven by global ambitions to phase out fossil fuels, the linkage to the electric power system offers the opportunity for a future of renewable-powered mobility. Fast chargers are indispensable elements in this transformation, as they provide short recharge times at key locations like city centers and highways. However, fast chargers pose various technical challenges to the electricity network due to their intermittent load profiles with high peak demand, and their deployment is only feasible where grid infrastructure is sufficiently robust. One key solution is to equip fast chargers with battery storage, which mitigates the impact on the electricity network and facilitates local integration of renewable energy sources, such as photovoltaic (PV) systems.
This thesis investigates the feasibility and control aspects of a renewable-powered EV fast charging station, in which all conversion stages between the different system components are eliminated. The concept is based on a battery string technology that controls the power to or from other DC units by dynamically changing its cell configuration. Hence, the charging system can be designed without power converters, promising lower investment costs and higher efficiency. The thesis studies the proposed concept on two different levels.
The first part focuses on the technology-specific aspects of the reconfigurable string design, and its capability to function as a fully controllable storage unit. The thesis introduces a method for deriving the efficiency characteristic and the operating area of the technology, presenting a framework for optimizing the design and the component selection. A dynamic simulation model demonstrates that the control approach of dynamic cell reconfiguration is generally capable of following varying EV requests throughout the entire charging process. However, cell switching leads to variations of the charging current that can exceed the tolerance band defined in charging standards. The simulation results are confirmed through field tests with a full-scale prototype on the Danish island of Bornholm. Based on the findings from simulations and field tests, the thesis discusses future improvements in the design and control of reconfigurable battery strings.
The second part centers around the operation of EV charging stations employing this battery technology, with a particular focus on the prototype installed on Bornholm. The core of the charging station is formed by three reconfigurable strings that are individually connected—via a busbar matrix—to the other system components: two EV fast charging connectors, a PV system, and a grid-connected inverter. The key tasks of the management system are to (i) assign the battery strings to the other system components, and (ii) manage the energy exchange with the AC grid. The thesis breaks new ground by presenting heuristic and optimal control methods for this novel design concept. Based on an electro-thermal model of the prototype, the performance of the control strategies is assessed through various simulation scenarios, using actual PV production and EV charging data from the demonstration site. It is found that the self-sufficiency of the charging system can be effectively increased by including PV forecasts in the decision making process. In this context, the work critically reflects on the mathematical definition of self-sufficiency commonly applied in the literature, and proposes an alternative equation that captures distinctive factors introduced by storage units. Finally, the focus is extended to frequency regulation as an additional revenue stream for periods of lower charger utilization level. To this end, the thesis provides concrete recommendations for how to manage the battery energy level during service provision, and what share of the power capacity can be offered on the market, depending on the storage specifications. Hence, battery-buffered fast chargers could not only become a key element in electric transportation, but also contribute to balancing power systems that largely rely on volatile renewable generation.