Amsterdam is a city of cyclists: As of 2017, 68% of commutes to and from work or school were done via bicycle. Every day, people bike a total of 1.25 million miles. As they cycle, they're generating a lot of energy–an estimated 19.5 million watt-hours, which is enough to power thousands of people's homes. What if that energy could be harnessed?

That's the idea behind S-Park, a new bike rack system proposed by designers Guillaume Roukhomovsky and Blaž Verhnjak as part of the city's Clean Energy Challenge. The system includes a front wheel that can be popped into any bike frame; as the rider bikes around the city, the wheel stores the kinetic energy produced by the wheel's circular motion in batteries. Then, when the cyclist returns home to their neighborhood, they park their bike in a communal bike frame that's connected to the grid. The energy that the batteries stored from during their commute flows into that area's electrical grid. The wheel can stay on the bike for a long period of time, acting just like any other bike wheel.