Electric cars and renewable energy can reinforce each other, creating a bigger impact than either would have on its own. That's what Renault hopes to prove by combining the technologies in one "Smart Island" off the coast of France. Similar to a previous project for the Portuguese island of Porto Santo, Renault plans to transition the island of Belle-Île-en-Mer to renewable energy.

This project will not only reduce the island's carbon footprint, but also make it more energy independent, Renault noted in a statement. Islands make excellent test cases for renewable energy because importing fuel or electricity from the mainland can be difficult and expensive.

Beginning in 2019, island residents and visitors will have access to a fleet of electric vehicles through a car-sharing service. Renault Zoe hatchbacks and Kangoo ZE vans will use newly-installed charging stations. Those stations will get power from solar panels installed on the roofs of large buildings. Power from the solar panels will be routed between buildings and cars as needed, according to Renault. So panels on the roof of a school, for example, will power classroom lights during the week, and charging stations on weekends and holidays.

Renault will also supply so-called "second-life" batteries to store energy harvested by the solar panels. These are batteries from electric cars that have degraded too much for continued automotive use, but still have enough usable capacity for stationary energy storage. They'll store excess energy when the sun is shining, and discharge that energy when it isn't. That means buildings and electric cars alike can use renewable energy more often.

Energy storage helps create a symbiotic relationship between electric cars and renewables. The technology that makes electric cars possible—batteries—also makes solar or wind installations more effective. At the same time, charging cars from renewable sources lowers their overall carbon emissions. That's why Renault and other automakers are pursuing second-life batteries, as well as other ways to integrate electric cars with the power grid.

"It will be possible to carry over the Belle-Île-en-Mer system not just to other islands but also to cities and suburban areas," Gilles Normand, senior vice president of Renault's electric-vehicle division, said in a statement. As electric-car sales increase, look for systems like this to proliferate as more secondhand batteries become available.