Getting the measure of waves

Wave energy converters could be made more efficient with a device that measures the size of each wave approaching the converter.

Scientists at Edinburgh University are investigating the feasibility of a ribbon-shaped sensors to provide real-time, 3D mapping of the wave field.

'Our idea is to have a floating array of sensors that would comprise ribbons you could assemble in various geometries. The array is split into two problems,' said Brian Sellar, the Edinburgh researcher heading the project.

'The first question is: how well does any sort of ribbon with flotation aids on it track the waves?

'The second is: what sensors are available to put on to these ribbons to try and output wave elevation, wavelength, periods [the length of time between two successive wave peaks] and things like that?' added Sellar.

Using this floating sensor technology, the ribbon would take the shape of the water's surface as a continuous line rather than as a small number of fixed points, which would be the case if measuring height using wave-riding buoys.

If an incoming wave is small the resistance of the energy converter needs to be low, and vice versa. Existing converters can only approximately vary according to the wave sizes, for example, hourly or every 10 minutes, rather than on a wave-by-wave basis, which is the Edinburgh team's goal.

'The reason we are looking at it is because wave energy converters need to be controlled to optimise power output or to improve survivability. If there is going to be a huge wave coming in it would be good to know about it so mitigating measures can be taken,' said Sellar.

In 2D flume tests at Edinburgh's hydraulics laboratory, the researchers used a seven metre fibre-optic sensor ribbon based on ShapeTape technology. This was developed by Canadian firm Measureand and was previously used to monitor body shape during sports research.

The technology consists of a box that sends light down the ribbon and receives light back at the same time. Wave height is indicated by measuring the light lost, and the greater the amount lost, the higher the curvature of the wave.

'Having done some proving trials at small scale in 2D, it looks very good,' said project investigator Dr Tom Bruce.

'Using conventional wave gauges and this device, we were showing very good agreement. But we get more precision with the ribbon than we could ever get with the gauges because it was giving us a very large number of measuring points.'

In the 2D setting, waves were sent in one direction down a channel measuring 20m long x 0.7m deep x 0.4m wide. Floats made out of a special, low-density modelling foam using a computer-controlled 3D milling machine, were attached to the ribbon.

Typical wave gauges are two metal rods placed next to each other, poking into the water. As the water rises and falls between the rods, it affects the resistance so that when there is water between the rod (high wave), there is more conductance, and when there is a low wave, conductance is less. This information is then sent to a computer and plotted on to a graph.

In the lab, three to six gauges tend to be sufficient, but Bruce said this was not enough to give as accurate a picture of wave shape as the ribbon, which takes 30 measurements a second.

Having proved the potential of the ribbons in a 2D setting, the forthcoming feasibility study will look at developing a lattice of 3D ribbons.

'We do not have the geometries of the lattice as yet. At the moment the specification of components is 50m ribbons. How we arrange them is part of the project,' said Sellar.

'We know that fibre-optics on their own will be limited in 3D, so we are going to incorporate extra inertial sensors on to the ribbons — things like triaxial MEMS inertial sensors, accelerometers, magnometers and gyroscopes.'

The scientists have noted that developing a device that is adaptable to an unpredictable environment such as the sea will have some challenges, such as twisting the lattice in 3D and mooring.

'Interaction is going to be a major challenge. How one part of the lattice affects the forces and motions at the other part,' said Sellar. 'Also, it is easy to moor sensors and flotation sensors and devices in the laboratory wave tanks, but it is difficult to do this in real conditions.'

Organisations such as the Maritime Institute of Ireland will help the scientists trial their device in real-time conditions at the Galway Bay test site.