Sensors could enhance wave energy generation efficiency

A team at Exeter University has developed a software and hardware solution that is currently being tested.

Wave energy generation technologies are relatively immature compared with solar and wind, and are not yet commercially competitive without subsidy.

Confident that wave power still has the potential to be an important player in the UK’s renewable energy mix, Dr Markus Mueller and colleagues from Exeter looked at ways to enhance the efficiency.

‘The sea is not stable, so you always have different conditions, every wave has a different amplitude, frequency and so on — this affects the wave energy converter,’ Mueller told The Engineer.

The first task was to make this instability slightly more predictable. Mueller and colleagues deployed simple sensor buoys — not dissimilar to the ones used in tsunami monitoring — to gather measurements of wave elevation and longitudinal displacement. These data are sent via a 1hz radio link to the generating devices themselves placed nearby.

To start with, the team is using point absorbers — floating devices partly tethered to the ocean floor that move up and down with the passing waves.

The dataset received from the sensors is processed on board using the team’s proprietary algorithms and the mechanical part of the generator adapts accordingly.

‘When you know what the next wave coming in is like — in terms of amplitude and frequency — you will be able to adapt the forces and essentially push the buoy more into the wave so you can get more out of the next wave, in the end optimising the energy outtake,’ Mueller said.

The team is currently performing comparative trials of its system, where, during a fixed time period, some of the generators are using the predictive enhancement and others are not. Early results suggest that the system may be able to achieve a two-fold increase in conversion efficiency.

The next stage of the work will be to use a more sophisticated sensor network on both the surface and ocean floor that employs acoustic doppler current profiling (ADCP) technology to precisely determine wave elevation.

This added statistical power could be used in conjunction with more complex wave-generating devices, such as Pelamis’s line absorber, which has multiple degrees of freedom.

‘The control we’re proposing doesn’t really limit anything in terms of the mathematics we’re doing. You can set your targets, for example, if you want to harvest as much energy as you can, it will be more risky for the buoy — the operator has to decide the trade-off,’ Mueller said.