July 20: Wave rotor tests the water
A tidal and wave turbine with a vertical axis, the Wave Rotor developed by Ecofys, is about to be tested off a pier in the Westerschelde, a tidal stream in the south of the Netherlands.
The pier is conveniently located near Netherlands' only active nuclear plant at Borssele. So any power produced can easily be fed into the grid. Off the pier a construction is made that can lift and lower the full scale prototype turbine into the Westerschelde.
The Wave rotor consists of three vertical blades, five meters long, along a central axis sticking vertically into the water. At the moment of our visit the tops of the blades are visible just above the waterline. While welding work on the construction is ongoing, the machine cannot yet be put into motion as the forces on the construction might be damaging.

“This is a unique device”, developer Peter Scheijgrond from Ecofys says. Directly followed by: “Like all other designs are too.” The features that make his design unique are that it works both for wave and tidal streams converting both in a rotation of the central vertical axis, and that the generator and all electronics are housed well above the water making them less vulnerable to sea water. On top of that, being circular in design, the device has no preference on the direction of the waves and currents.
The rotor blades have a wing profile. This causes them to rotate in response to both waves and currents. On top of that, the blades are connected to the vertical axis by symmetrical short wings. These so-called Wells turbine blades provide an extra motion to the system. In all, theoretically 40 percent of the motion of the water could be captured. Field tests, which are about to begin in august 2009, should make clear if that target will be met.
The axis length is 5 meters, as is the diametre of the rotor. With tidal streams up to 2 meters per second, as they occur on this site, Scheijgrond has calculated a maximal output of 30 kilowatts.
The proof of principle has been delivered with smaller prototypes. The first one, a quarter of the current size, has been tested at the wave test site at Heiligso operated by the Danish Folkecenteret. In 2002, it was one of the first devices to be tested there. A half scale prototype (2.4 metres diameter) has been tested in 2007 off the coast near Brest, France.
The current field test is scheduled for a year. During that time its efficiency will be measured (a certified current and wave measuring system has been installed next to the wave rotor). Other points of interest are its durability, corrosion effects, occuring defects, ease of maintenance and the amount of vibrations. The three blades have been coated with different antifouling paints in order to establish which paint best keeps the blades clean.

The next step would be to place the device in the stronger currents of the Oosterschelde. Water flowing out to the North Sea here reaches double the current speed (up to 4 meters per second) compared to the Westerschelde. Scheijgrond explains that means an eight-fold increase in power. Four of such devices there could deliver almost a megawatt of electrical power.
The costs involved for such a megawatt system are estimated on 8 to 10 million euro (the current prototype has costed 1.4 million euro, 70 percent of which was subsidised by Dutch and European governments). The cost of power production has been calculated at 0,37 euro per kilowatthour for an operational time of 15 years.
Alas, Scheijgrond explains, the Dutch feed-in tariff for tidal power has been set at only 17 cents per kilowatthour. This is too low to make the wave rotor commercially viable, even if the field tests are finished succesfully.
More info on the Ecofys website.