June 23: Whistling waves producing power
The same principle that makes fog horns blow at seas, is used to generate electricity by Voith Hydro Wavegen in Inverness. We visited their wave tank to see the principle at work.
The same principle that makes fog horns blow at seas, is used to generate electricity by Voith Hydro Wavegen in Inverness. We visited their wave tank to see the principle at work.
The wave tank is about six meters wide and 20 meters long - a bit like an undeep pool. Hydraulic walls at one end push against the water to create an irregular pattern of waves which then travel across the tank to end up on an artificial shore at the far end. Some meters in front of the carpet roll that simulates the coast, there are three hollow vessels in the water. They are open at the bottom, so that the water inside rises and falls with the passing waves. The top of the vessels is closed except for a blow hole. Place your hand above it as waves pass, and you'll feel a surpring strong short push of air as waves pass by underneath. The scale of the waves and the air chambers in comparison with reality is 1 to 40.

“At our test site at Islay, we sometimes listen to the air going in and out”, says Wavegen's Chief Executive Matthew Seed. “It feels like the ocean is breathing.”
The first device Wavegen has produced, based on the so-called Oscillating Water Column (OWC) technology, was named LIMPET (Land Installed Marine Powered Energy Transformer). It was installed on the island of Islay in 2000. The air that gets compressed by the water column is led from the air chamber through a 750 millimeter diameter Wells turbine, driving a 250 kilowatt generator. The nice thing about Wells turbines, using two propellors, is that in- and outgoing air stream both drive a generator. So that the maximal power from LIMPET doubles to 500 kilowatt.
Since 2000, the LIMPET site has been in operation, and it has clocked 40 thousand hours of generation time – a little less than 60 percent of the time.
Seed tells that Wavegen has acquired lots of experience with that first set-up. It has been in use as a testbed for prototypes for more recent projects. The availability originally was only 30 percent. “We learned a lot about operating in marine environment, about instrumentation and how best operate the plant by electrical controls. We also learned about the instrumentation and the mechanics, striving to keep the design as simple as possible.” The availability has steadily been increased up to over 90 percent for the latest designs.
Wavegen is now working on a 4 megawatt wave energy plant near Siadar on the Isle of Lewis, part of the Hebrides islands. It was commissioned by RWE energy company and approved by the Scottish government in Janaury 2009. It will consist of 36 turbogenerators of 110 kilowatts, housed in a 200 meter long breakwater some 350 meter offshore. It is to deliver power to 1,500 homes on the island. “The inhabitants support the project, not in the least because it provides them with a sheltered harbour for their boats”, explains Seed.

A key to the success of wave energy is obviously having large waves. A similar project in the north of Spain is expected to deliver 300 kilowatts from a 100 meter dam. In other words, the wave energy density at the Hebrides is six times bigger.
Hotspots for wave energy are west facing coastlines on higher latitudes (both on the northern and southern hemisphere) next to big open oceans. Examples being Scotland, Norway, Spain, Portugal, but also California, New Zealand and Peru.
Not much perspective for wave energy on the North Sea then. Not only are the waves generally too small for the current technology to win commercially viable power from, waves also tend to come from different directions, making the energy conversion less efficient.
Despite delivering megawatt-sized projects, Seed does not believe that the wave energy industry has not yet entered the learning curve in terms of reducing costs. “We haven't got enough equipment installed to have done benchmarking yet. And we haven't come to the scale of mass production.”
In 2005 Wavegen Ltd was acquired by Voith Hydro, a German company that is 35 percent owned by Siemens. It is now called Voith Hydro Wavegen Limited. Both the office and wave tank are located in Inverness, Scotland.