SHIPS that harvest energy from the waves and store it in batteries could one day generate electricity from the world's oceans more cheaply than today's wave-power devices.
The ships would sail to a suitable location, drop anchor and start generating electricity from wave energy. Once their batteries were fully charged they would return to shore and feed the electricity into the grid.
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.
The 50-metre-long ships would harvest wave energy via buoys attached to their sides by pivoting arms. While the hull remains relatively stable, the buoys would bob up and down on the waves, causing the arms to pivot back and forth and drive a generator producing up to 1 megawatt of electrical power. 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.
Unlike conventional wave-power devices, the power-generating mechanism will not have to withstand severe storms, as the ships could be kept in port during bad weather. Fixed wave-power generators must be built to cope with extremely high waves, which adds substantially to their cost, Sharon says. Costs could be cut further by retrofitting existing vessels, which could either have their own engines or be towed out to sea and back.
Sharon used 3D printing to produce a prototype, and tested it in a wave tank. He calculates that the system should generate electricity at a cost of $0.15 per kilowatt-hour. This would make it cheaper than energy from existing wave systems, which costs between $0.30 and $0.65 per kWh. Offshore wind energy costs from $0.15 to $0.24 per kWh, and solar power around $0.30 per kWh.
Mark Jacobson, director of the atmosphere and energy programme at Stanford University in Palo Alto, California, calls the idea "very creative". He notes that unlike electricity from many other renewable sources, power from the batteries could be held back and used at times of peak demand.