With oceans covering 70% of Earth's surface, harnessing its energy producing potential has been a longtime pipe dream within the renewable energy sector. Despite the existence of patents for wave-powered devices dating back to the 18th century, the field of wave and tidal power is still in its relative infancy.

Whereas wave power uses the up and down motions of the waves to generate electricity, tidal power uses the action of the tides to turn underwater turbines. Although the energy available along suitable coastal areas around the world has been estimated to be greater than 2 TW  (by comparison, global energy use is around 16 TW ), its commercial development faces a number of practical challenges. These include converting constantly shifting wave motions  to a steady power load; constructing devices   that are able to withstand the force of the sea, saltwater corrosion and eventual storms; installing equipment requires special (and expensive) vessels; connecting generators  to the power grid requires building the necessary infrastructure; all these factors have so far compounded to the high costs of large-scale projects.

Fortunately, the changing economical and political climates have given way to a sea change (excuse the pun) in wave and tidal power development, spurring a a wave (sorry!) of government investment, venture capital, promising industry startups   and new technological approaches bringing commercially viable projects another step closer.

In terms of tidal power, Marine Current Turbines  boasts SeaGen  , the world's first commercial scale tidal stream turbine, which generates 1.2MW of power off the British coast. A 1.5 MW device is planned for Nova Scotia, Canada.  ScotRenewables   is developing a floating tidal turbine   that aims to bring down costs to the point of making tidal power competitive with offshore wind.

The world's first commercial wave farm is operating off the coast of Portugal and consists of 3 Pelamis machines  producing 750 kW of electricity each, with plans to expand capacity to a total of 21MW, while other projects in England and Scotland are still in development. The Pelamis device, inspired on the original and ill-fated Salter's Duck  , consists of connected cylindrical sections connected by hinged joints that move with the waves. The hydraulic rams in the joints pump high-pressure fluid through hydraulic motors, driving generators to produce electricity.

Other projects, using different floating buoy concepts, are currently being tested. These include the AquaBuOY  , developed by Finavera Renewables  , the PowerBuoy  , by Ocean Power Technologies   and AWS   Ocean Energy's underwater buoy  . A promising break for this type of devices is the Ocean Harvester  , that uses a counterweight to achieve a steady load on the generator and promises to achieve an "exceptionally high capacity factor"   .

Another exciting technology is the Wave Treader  , which combines wave and wind power (kind of like Captain Planet), by mounting the device on the foundation of an offshore wind turbine, generating a combined 500 kW. A working prototype is expected in 2010.

CETO  , an Australian technology, takes a radically different approach, by pumping seawater onshore under high pressure, generating electricity and freshwater, making it the most efficient and cost effective way to desalinate water using wave energy, a useful feature in a drought ridden country. A 50 MW commercial demonstration plant, enough to power a large city, is in the works   for 2011.

Aquamarine  's Oyster   also makes use of an onshore generator, but is designed to capture the energy found in amplified surge forces in nearshore waves. The first prototype will be deployed off the coast of Orkney, UK, in the summer of 2009 and will generate up to 600kW of power. A development agreement  between Aquamarine and Airtricity, the renewable energy development division of Scottish and Southern Energy, aims to 1 GW of marine power in UK and Ireland by 2020.

Given that there are over 50 types   of wave devices under development and over a hundred of ocean power startups, experts say   manufacturers will still have to agree on a few basic industry standards, such as machine design, in order for this sort of technology to become competitive.

celsias.com