Wave Energy

Winds are generated by the differential heating of the earth and as they pass over open bodies of water, they transfer some of their energy into the form of waves [1]. Extraction of energy from the waves is more efficient than direct collection of energy from the wind due to the fact that waves are a more concentrated form of wind energy [2]. However, the practicality and economics of harnessing wave energy is dependent on many factors, particularly geographic location, season, storm frequency, and storm location [3]. The concept of extracting energy from waves is not recent. The first patent taken out to capture, and utilize wave energy, dates back to the Napoleonic era when, in 1799, two Frenchmen proposed a machine whose purpose was to turn wave energy into mechanical energy using an artificial raft [2].

There are many different approaches to extracting energy from the waves. In the patent literature there are more than a thousand different proposals for the utilization of wave energy [4]. Some methods use the vertical rise and fall of successive waves in order to build up water, or air-pressure, to activate turbines. Others take advantage of the to-and-fro, or rolling wave motions of waves by vanes or cams that rotate turbines, while still others concentrate incoming waves in a converging channel allowing the build-up of a head of water that is used to operate a turbine [2]. At present, the most common wave energy devices are of relatively small scale to provide electricity for navigational buoys in harbors and in other waterways. Developed in the 1960's, these buoys are the only wave-activated devices to see common usage world-wide with some in operation for 20 years [5]. One of the more developed wave energy systems is Oscillating Water Chamber (OWC).

 

Currently, a commercial-scale 500-kilowatt powerplant called LIMPET (Land-Installed Marine-Powered Energy Transformer) produced by the company Wavgen on the island of Islay, Scotland is connected to the UK's national grid. Built into the shoreline and relying on existing cliff edge for support, this modular OWC forms a partially submerged shell into which seawater is free to enter and leave. As the water enters or leaves, the level of water in the chamber rises or falls in sympathy. A column of air, contained above the water level, is alternatively compressed and decompressed by this movement to generate an alternating stream of high velocity air in an exit blowhole. This air stream is allowed to flow to and from the atmosphere via a pneumatic turbine. Energy can be extracted from the system and used to generate electricity. The plant is expected to have a 60 year structural design life [6].