May 15, 2008

A new wave: Ocean power

By Sonal Patel

Wave power

Created when winds—which result from the planet’s heat differentials—pass on their energy to the oceans, waves are in essence a concentrated form of solar energy. According to an International Energy Agency estimate, wave energy could supply between 10% and 50% of the world’s yearly electricity demand of 15,000 TWh.

Several competing approaches have emerged to convert the kinetic energy of waves into electricity.

Pelamis. The floating snake-like Pelamis has already evolved from prototype to operating unit. Three of these devices collectively generate 2.5 MW at full capacity at the world’s first wave farm in Aquaçadora, northern Portugal. The device relies on vertical wave motion to move articulated joints in the body, which then pump high-pressure oil through generator-driving hydraulic motors. A 250-kW prototype module is 360 feet long and over 10 feet in diameter. Power-holding company Enersis has issued a letter of intent for an additional 20 MW of Pelamis equipment to expand the wave farm project, and plans are under way to use Pelamis technology to power an Orcadian wave farm in Scotland.

Buoy technologies. The buoy concept typically consists of modular buoy-arrays moored several miles offshore in choppy waters.

Finerva Renewables’ AquaBuOY converts the vertical component of waves’ kinetic energy into electricity by directing pressurized seawater through two-stroke hose pumps into a turbine-driven generator. The power is transmitted to shore via an undersea transmission line. Finerva is developing the first phase of a 2-MW commercial power project site using this technology in Fiqueria de Foz in Portugal. Construction of a 100-MW wave energy plant is planned if this project is successful.

Scottish company AWS Ocean Energy has devised an anchored underwater buoy generator system using Archimedes Wave Swing (AWS) technology (Figure 2). The buoys would drive generators as they bob with passing waves, and a pressurized gas cylinder inside the buoy would cause a float to oscillate based on the pressure differential of the water depth above as the wave passes. The AWS buoy was successful in its 2004 pilot test. The company plans to facilitate commercial development and deployment of the technology using a recent £2 million grant from the Scottish government.

2. One of the buoys. Buoys using Archimedes Wave Swing technology enjoyed a successful pilot test in 2004. The technology recently won a £2 million ($4 million) grant from the Scottish government. Courtesy: AWS Ocean Energy

Ocean Navitas’ technology, known as the Aegir Dynamo, functions by generating electrical current from wave motion in one phase via direct mechanical conversion and the use of a custom buoyancy vessel. Ocean Navitas has tested a 1-MWh buoy in the Orkneys, in Scotland, and plans to place a five-buoy array off the Welsh coast.

Iberdrola Renewables has begun testing an innovative U.S.-manufactured buoy called Power Take Off (PTO), which captures and processes wave energy for storage. The energy is later evacuated under optimum conditions. The company estimates that an array of 10 PTO buoys could produce 1.24 MW.

Breakwater and shoreline technologies. Voith Siemens Hydro’s Wavegen technology is integrated into a concrete power station built on a breakwater or coastal protection project. Breakwater turbines, each with an output of between 20 kW and 100 kW, are based on the oscillating water column (OWC) principle. Waves create oscillations on the water’s surface in a partially submerged hollow chamber that’s open at the bottom. The oscillations continuously compress and decompress an air column above the chamber. The difference in pressure converts the rotational energy to electricity via a turbine-driven generator (Figure 3). The world’s first breakwater power station is currently under construction in Mutriku on the Atlantic coast of Spain.

3. A breakwater turbine. Voith Siemens Hydro’s Wavegen technology employs the oscillating water column principle. Courtesy: Voith Siemens Hydro

Wavegen has also developed the Land Installed Marine Powered Energy Transformer (LIMPET), a shoreline energy-converter that uses an OWC to feed a pair of counter-rotating turbines, each driving a 250-kW generator. The current LIMPET model, Limpet-500, installed on a pilot plant on Scotland’s Islay island, is being performance optimized. If the pilot is successful, LIMPET will be used to build a series of commercial power generators.

Floating barge technologies. The Wave Dragon is a large floating barge with dynamic turbines that produces energy much as a low-head hydro power station does. By facing its outstretched collector arms toward oncoming waves, the Wave Dragon can concentrate the wave front toward the ramp at the front of the structure. This increases the wave height at the ramp—which acts like a beach, causing the waves to break over it and into the reservoir behind it (Figure 4). Electricity is generated when water runs through the turbines in the bottom of the structure. A 7-MW Wave Dragon device tested in Pembrokeshire in southwest Wales was commissioned in 2007 (Figure 5) and will be deployed this summer (see sidebar).

4. Wave Dragon. This floating barge’s design increases the wave height at the ramp, which, like a beach, causes waves to break over it and into the reservoir behind it. Courtesy: Wave Dragon Ltd.

5. Welsh wave power. This Wave Dragon device will generate enough power to supply 2,500 to 3,000 homes. Courtesy: Wave Dragon Ltd.