Energy management of a multi-battery system for renewable-based high power EV charging

Abstract

Hybrid fast charging stations with battery storage and local renewable generation can facilitate low-carbon electric vehicle (EV) charging, while reducing the stress on the distribution grid. This paper proposes an energy management system (EMS) for a novel multi-battery design that directly connects its strings to other DC components through a busbar matrix without the need for interfacing power converters. The EMS has two primary control tasks: (i) allocating strings to other DC microgrid components, in this case a photovoltaic (PV) system, two EV fast chargers, and a grid-tie inverter, and (ii) managing the energy exchange with the local distribution grid. For the grid exchange, a basic droop control is compared with an enhanced control that introduces a deadband with dynamic limits, which are adjusted based on PV energy forecasts. The performance of the EMS is extensively assessed by Monte Carlo simulations generating a vast number of random scenarios based on empirical EV fast charging data. The tests are performed for various utilisation levels of the chargers (EVs/day) and actual PV measurements from different seasons in Denmark. The numerical analyses show that the enhanced control increases self-sufficiency by reducing grid exchange, and decreases the number of battery cycles. For a utilisation level of 20 EVs/day in summer, the enhanced control achieves a self-sufficiency of 87.3% compared with 73.8% in the base control. In winter, with significantly lower PV production, these values drop to 9.3% and 9.1%, respectively. The corresponding annual battery cycles are estimated at 399 for the enhanced and 456 for the base control. However, the enhanced control operates the battery closer to its charge limits, which may accelerate calendar ageing.