SAPPORO, Japan - Ocean winds whip across the beaches, hillsides and sprawling plains of Hokkaido. There’s enough wind energy here for Japan’s northernmost island to power itself and export clean electricity to the rest of the country. But Hokkaido can’t harness all of that power unless it has a way to store energy when breezes are blowing and use it later when the gusts die down.Subscribe to The Post Most newsletter for the most important and interesting stories from The Washington Post. So, the