  In this paper a new aggregated model for electric vehicle (EV) fleets is
presented that considers their daily and weekly usage patterns. A
frequency-constrained stochastic unit commitment model is employed to optimally
schedule EV charging and discharging as well as the provision of frequency
response (FR) in an electricity system, while respecting the vehicles' energy
requirements and driving schedules. Through case studies we demonstrate that an
EV with vehicle to grid (V2G) capability can reduce system costs in a future GB
electricity grid by up to {\pounds}12,000 per year, and reduce CO2 emissions by
60 tonnes per year, mainly due to reduced curtailment of wind power. The paper
also quantifies the changes in the benefits of fleet V2G resulting from
variations in FR delivery time, the penetration of wind or uptake of
alternative flexibility providers. Finally , a battery degradation model
dependent on an EV's state of charge is proposed and implemented in the
stochastic scheduling problem. It enables significant degradation cost
reductions of 16% with only a 0.4% reduction of an EV's system value.
