Electrical Vehicles can be connected to the grid, and used to balance the (local) power distribution network. This is a feasible solution for an energy system with a higher penetration rate of intermittent renewable energy generation. Take for example the Netherlands: 6M electric vehicles with an average capacity of 20 kWh, with 20% connected to the network at any given time, providing 50% of the charging rate yields about 6M * 20 * 50% * 30% = 18 GW of instantaneous power, allbeit for a small time-frame. Downsides are the high costs: battery wear needs to be compensated, next to grid investments for connecting these cars. Also, the efficiency is probably not as low as with directly connected battery systems.
Applications: short-term storage, arbitrage, distributed/off-grid storage, frequency or voltage regulation, power quality, reserve grid capacity, UPS