Electrical power generation from ocean waves is possible from ships that harvests wave energy through daily trips to offshore locations and that return back to port for the delivery of energy to the grid.

Energy is stored locally on the vessel during harvesting phase and placed on the grid during periods of high demand (normally mid-day).

Unlike conventional wave-power devices, the ships would not need undersea cables to link to the electricity grid, says  Andre Sharon  at Boston University and the Fraunhofer Center for Manufacturing Innovation, also in Boston.

These cables typically cost more than $500,000 per kilometre and account for a significant fraction of the cost of conventional wave-generated electricity.

This idea of mobile wave energy harvesting is a simple one: you put wave energy harvesters on boats, send those boats out to sea, they sit out there for about a day riding the swell and charging their batteries, and then they come back in to shore and offload their electrical cargo.

There's no complex and expensive undersea construction, no need for cables to run back to shore (at $1 million per mile), you can move the generators around easily, and best of all, one can retrofit ships to do this relatively cheaply.

Each 150 foot ship (like the concept pictured above) would be able to harvest about one megawatt of energy per hour, which is enough to power about a thousand homes.

It would store up 20 megawatts in giant batteries over 20 hours at sea under average conditions, and then head back to port and pump all of that power into the grid. Once the transfer is complete, the generator can go back out again for another load.

The batteries are planned to have a capacity of 20 megawatt-hours, so the ships would have to stay at sea for at least 20 hours for a full charge. Sharon presented the concept at the  Clean Technology 2011  Conference and Expo in Boston.

Testing of scale models suggests that power produced this way could be as cheap as $0.15 per kilowatt-hour, which is half the cost of solar and between half and a third the cost of conventional (stationary) wave harvesters.

The residential consumers in the U.S. pay on average anywhere from $0.09 to $0.16 per kilowatt-hour, so this could definitely be a viable method for coastal areas.