Outside the main stadium at the London Olympics, 20 circular slabs lie underfoot, ready to capture the kinetic energy from bustling spectators when they run, walk, and jump. With every step, the recycled-rubber tiles will light up nearby LED lamps for 30 seconds. The millions of steps expected to pound the pavement between the stadium and a new mall could make it worthwhile. With three days of battery storage, the system can provide electricity as crowds ebb and flow. PaveGen, which makes the tiles, hopes to eventually lay its slabs where they make major impacts, such as off...