When today’s energy developers look offshore, most of them are concerned with oil and natural gas reserves lying beneath the ocean floor and the winds blowing a hundred or more feet above the water’s surface. Now, though, a small but dedicated group of scientists and engineers is looking to the ocean’s wave and tidal energy, and several startup companies are testing designs that could make waves of their own should they live up to their creators’ expectations.
At first glance, some of these plans can look more like fanciful children’s drawings than serious power-generation equipment, as they seem to draw inspiration from shellfish and sea serpents. Their ability to produce real results will soon be put to the test.
The largest of such installations is beginning in Scotland’s Western Isles, off the country’s northwest coast, near the town of Lewis. That is where Edinburgh-based startup Aquamarine Energy has received approval for a 40-megawatt (MW) array of its Oyster 800 devices, which the company claims could power up to 30,000 homes. (See “Tidal Power Makes Waves in Scotland,” July 2013, www.ECmag.com.)
The Oyster draws its name from a hinged-flap design, and the system is already contributing electricity to the Scottish grid through a demonstration installation in Orkney. The device sits just at the ocean surface and is moored in near-shore locations, at depths between 30 and 45 feet. As wave motion pitches the flap back and forth, twin internal hydraulic pistons force water at high pressure through a connected pipeline to drive a conventional hydraulic turbine onshore.
Also based in Edinburgh, Pelamis Wave Power Ltd. has engineered a design resembling its namesake sea snake species Pelamis platarus. It consists of a series of five tube-shaped sections that are linked in sequence by universal joints and floats on the surface of the water. Hydraulic cylinders at the joints resist the resulting wave-induced motion and pump fluid into high-pressure accumulators to generate electricity, which is transported to shore using standard subsea cables. The company is developing several wave farms, including a 10-MW installation in the same general wave-rich region where Aquamarine is installing its Oyster machines.
A different design has caught the attention of a major player in more traditional energy technologies. The French manufacturer Alstom, which competes with the likes of GE and Siemens in supplying industrial steam and wind turbines to electric utilities and independent power producers, has begun investing in wave- and tide-energy startups. In 2011, the company acquired a 40 percent stake in Inverness, Scotland-based AWS Ocean Energy Ltd., adding its investment to one already made by Royal Dutch Shell through its Shell Technology Ventures venture-capital fund.
AWS is developing a full-size wave-energy converter, which consists of 12 flexible-membrane absorbers, called “cells,” that convert wave motion to pneumatic energy that is, itself, converted to electricity through connected turbine generator sets. Cables carry the electricity to an onshore substation. The 12-cell units will be grouped around a circular structure and are anticipated to generate up to 2.5 MW. These groups will be arrayed in farms capable of generating hundreds of megawatts in total.
The United States is represented in wave-energy development by Pennington, N.J.-based Ocean Power Technologies. Instead of snakes or shellfish, this company has created a device it calls a PowerBuoy, which looks much like the standard navigational buoys found in any working waterway. A surface-topping buoy-shaped “float” is attached to a stationary, tubular “spar” that reaches down more than 100 feet (a minimum water depth of 180 feet is required). The vertical motion of the float relative to the spar drives a mechanical system coupled to a generator to produce alternating-current electricity that is transmitted through cable to a central undersea substation. The output of up to 10 PowerBuoys can be gathered by a single substation, where an undersea cable transmits it to shore.
Tide-power developers face significantly larger engineering challenges than those encountered by wave-equipment makers. Tides may be more predictable than waves, but their strength can chew through steel. A highly publicized 2009 tidal-turbine installation in Canada’s Bay of Fundy, along Nova Scotia’s coast, failed after only 20 days. All 12 of the turbines’ rotor blades were destroyed by tidal currents that reached 10 knots (approximately 11.5 miles per hour).
Open Hydro, the turbines’ Irish manufacturer, was recently acquired by French naval defense manufacturer DCNS, and it is working with French utility Electricite de France on a test installation of a 2-MW turbine off the coast of Brittany. If the test is successful, an additional three turbines could be added to the site to create the first grid-connected tidal-turbine array.
Competing with OpenHydro is AWS Energy’s parent, Alstom, which is advancing development of a submerged blade-driven turbine initially created by another of its recent acquisitions, Bristol, England-based Tidal Generation Ltd. A 1-MW, commercial-scale demonstration unit is undergoing trials, with an eye toward a 2014–2016 deployment of a precommercial array that could be used as a model for future commercial customers.