Wave energy is, in effect, a stored
and concentrated form of solar energy, since the winds that produce waves
are caused by pressure differences in the atmosphere arising from solar
radiation. Waves transmit this energy thousands of miles with minimal loss.
Wave size is a function of the wind's speed, how long it blows, and fetch,
which is the distance over which it blows.
The total power released by waves breaking along the world's coastlines has been estimated to be 2-3 million megawatts, equivalent to several thousand large power plants. Although this vast amount of energy is spread over thousands of miles of coasts, in favorable locations, the energy density can average 65 megawatts per mile of coastline. Favorable wave energy sites are generally western coastlines facing the open ocean such as those in North America and Northern Europe. In a 1991 study done for Pacific Gas and Electric, the electric utility company serving Northern California, of their indigenous renewable resources, wave energy's development potential was estimated to be second only to that of solar energy.
Designing a mechanical device to capture wave energy poses challenging engineering problems. The device must be capable of gathering useful energy from a relatively calm sea with wave heights of a few feet. It must also be able to survive sea conditions where wave heights can exceed 50 feet. In this hostile, salt-laden environment, simplicity and reliability become leading design criteria.The rule of thumb has been, the fewer moving parts the better.
These criteria have caused the Oscillating Water Column (OWC) approach to harvesting wave energy to emerge as a leading candidate. Briefly, the OWC operates similiar to a blow hole. It is a chamber of air closed above the surface and open underwater. As the water rises and falls outside the chamber, the water column inside the chamber oscillates, blowing out and drawing in the trapped air, either directly through a turbine, or rectified through one-way valves. The compressible air acts as a buffer, and the OWC's design and operation are simple.
One of the main problems the OWC has faced has been where to put it. On shore installations have proved prohibitively expensive. Additionally, waves have generally lost a great deal of their energy by the time they reach the shore. Floating OWCs face two problems: survival and efficiency. Any point absorbing wave energy collector, i.e. a small floating or buoy-like device, has a difficult time surviving everything the ocean is capable of doing to it. Efficiency suffers if the device itself moves because an OWC works when the water moves relative to the collector. Efforts have been made to design the collector to heave out of phase with the waves in order to create that differential, but that really only works well when the wave frequency matches the OWC's natural resonating frequency. That only occurs some of the time, and most of the time, the ocean is a cacophony of mutiple wave frequencies overlaid.
The PSP is intrinsically an OWC device. The oscillating water columns and the movement of pressurized air among its perimeter cylinders are its means of attenuating the waves and decoupling the platform from the wave induced forces. It is also a stable platform capable of hosting an OWC device.
The PSP was designed for specific uses such as floating breakwaters, harbors, airports, etc. This "dual use" capability has always been considered to be important to the economics of the PSP. If the cost of the PSP is supported by one or more of these uses, wave energy can be economically harvested from the air movement generated by the PSP's own oscillating water columns.
* NIMBY - Not In My Back Yard; BANANA - Build Absolutely Nothing Anywhere Near Anything