In an article by Joy Green titled "Smart grid, Meet the Product-Service Model", Green writes about an MIT service model for personal urban mobility which is modeled after the Velib scheme in Paris. It's a self-service bike hire service system in which any person can pick up a bike for free outside metro stations and other places, and take it for a ride to their destination to leave there. It's purpose is to smooth peak demand during the day by allowing more Parisians to ride bicycles around town.
The MIT model mimics this scheme with the use of "self-service electric, stackable two-seater mini-cars". Six of these stackable mini-cars can fit in one parking space, thus eliminating ground cover needed to park the cars. The mini-cars would be available all around town similar to the bikes, and the use of a card would allow you to enter the car and drive it to your destination where it will "stack". As the cars are stacked, they would be connected to the electricity grid and depending on the energy demand, the grid would take up surplus energy or charge the car; in other words, these mini-cars provide mobile batteries in which to store energy thus providing a back up energy source to level off renewable energy resource variability.