Projects in Scotland, Norway, France, and San Francisco are moving the world's fleets of cargo and passenger ships toward zero-emission power using hydrogen fuel cells.

The San Francisco project seems furthest along. A shipyard in Almeda, California, is building a 70-foot, 84-passenger catamaran ferry that's scheduled to be launched for testing this fall. Hydrogen fuel cells will power its two electric motors.

Fuel cells emit only warm, moist air. However, refining the hydrogen, compressing or liquifying it for storage, keeping it compressed or liquified, and transporting it can all create carbon emissions, depending on the processes used.

Those emissions can be reduced (see below). And ships today usually run on highly polluting heavy fuel oil, also called bunker fuel. When burned, it emits sulfur, particulate matter, and greenhouse gases.

"If shipping was a country, it would be the sixth-largest polluter in the world," Nerijus Poskus of the shipping technology company Flexport told NPR. "About 3% of global emissions are released by ocean freight shipping."

Here are snapshots of the other three hydrogen vessel projects:

  • Scotland's Orkney Islands generate more renewable energy from the wind, tides, and waves than they can use. So employing some of that excess energy to produce hydrogen from water essentially stores the energy for use later, when the wind isn't blowing, the tide is slack, or the sea is calm. Tapping renewable energy and keeping the hydrogen local minimize carbon emissions from manufacturing, storage, and transportation. The islands, which depend on a ferry system for everyday getting around, also plan to use hydrogen to fuel a car and passenger ferry. The vessel is scheduled to enter service in 2021.
  • The French and Norwegian vessels will also get their hydrogen by means of renewable energy. The French are working on a utility boat for the Rhône River at Lyon, and the Norwegians on an 80-car, 299-passenger ferry for the popular Hjelmeland-Skipavik-Nesvik route over the Jøsenfjorden. Both are scheduled to start operation in 2021.