Uncertain electric vehicle charging flexibility, its value on spot markets, and the impact of user behaviour

Simultaneous charging of electric vehicles (EVs) increases peak demand, potentially causing higher electricity prices and increased procurement costs for charging, making EVs less economically appealing. Smart charging addresses this challenge by utilising EVs as flexible assets, adjusting their charging behaviour in response to both power system conditions and user requirements. In our paper, we take the perspective of an energy provider using smart charging algorithms to reduce their electricity procurement costs (EPC) by charging the EVs when the electricity prices are lower. However, EV usage uncertainties introduce variability in the flexibility EVs provide and subsequently impact the energy providers’ EPC when trading in electricity markets. Our paper considers uncertainties arising due to variable driving patterns and charging preferences. Within the charging preferences, we specifically focus on two charging preferences such as a minimum state of charge (SOCmin) requirement – the percentage of the battery up to which EV needs to be charged immediately at full power when connected to the charging point; and the frequency of EV connection to the charging point – how often EV users connect their EV to the charging point. We develop a flexibility model that quantifies the flexibility in terms of energy and power as a function of time. To calculate the energy provider’s EPC, we develop a scenario-based robust optimisation model, minimising the energy provider’s EPC while trading in German day-ahead and intraday markets. As expected, an increase in SOCmin requirements and a decrease in frequency of EV connections results in reduced EV flexibility and subsequently increases the EPC. However, our cost sensitivity analysis reveals that even with an 80 % SOCmin, EPC can be reduced by up to 33.5 % and 36.9 % for the years 2022 and 2023, respectively, compared to fully uncontrolled charging. When EVs offer full flexibility (0 % SOCmin), the cost reduction is only slightly higher, at around 43.6 % and 49.6 % for the years 2022 and 2023, respectively. Flexible EV charging, even with low flexibility, thus possesses high economic value, allowing energy providers to achieve substantial monetary gains with minimal impact on user convenience.