It's a Magnus effect sail. It's the force that makes a spinning ball curve. They've made the ball huge (5m diameter) and stretched it into a cylinder. So a crosswind meets the spinning tower and an orthogonal force, forward, is produced. Less fluttery than a sail, but for it to work you need a reliable crosswind. They've therefore been installed on ferries which run north-south in the Baltic Sea.

I don't think it really outputs power, it operates more like a sail. It looks like a cylinder on the front of the ship. It says it can function in winds at or below 80mph.