Waves are generated by wind. The drag on the water surface builds them up. They continue long after the wind has dropped, so they are a more dependable source of energy than the wind itself. Waves on the Atlantic coast of Europe or the Pacific coasts of the US or Australia carry as much as 40 kW per meter. Many schemes for harvesting wave power have been suggested. Four have reached the prototype phase: buoyancy devices, overtopping devices, oscillating water-column devices, and flap-type absorbers. Buoyancy devices use the motion of waves to pump a working fluid through a turbine. Overtopping devices uses a vertical axis turbine with a floating ramp; waves ride up the ramp and spill into the turbine below. Oscillating water-column devices trap water and air within the structure, and as the waves pass under it, the air is alternately compressed and allowed to expand through a turbine. Flap-type absorbers use a flap, hinged on an axis parallel to the ocean floor and with the top edge sticking just above the water surface, to oscillate in rhythm with the incoming waves.
How much wave power is harvested at present? Very little. About a third of global energy now comes from renewables, but very little of that, about 0.02%, is ocean-based, and almost all of that is tidal, not wave. The total world installed capacity of wave power in 2020 was a mere 4 MW, contributing less than 0.0001% of global needs. Governments subsidize plans to harvest wave power to access a potentially large energy resource. This Case Study explores a possible sustainable development: that of building wave-energy capture systems that can make a significant contribution, meaning at least 10% of the nation’s needs.