Tuning wave energy converters to produce more energy
By Anthony Capkun
September 10, 2012 – In the search for alternative energy, our oceans’ waves offer great potential. However, it’s a challenge to tune Wave Energy Converters (WECs) so that they are able to harvest the maximum energy from waves, which differ in terms of their size and force. This unpredictability leads to intermittent energy collection. WECs also need to withstand the harsh winds and storms to which they are subjected in the open sea.
Working with a team at the University of Exeter in the United Kingdom, Professor George Weiss of Tel Aviv University’s School of Electrical Engineering and Center for Renewable Energy has developed a control algorithm that, when used in conjunction with previously-developed wave prediction technology, helps WECs calculate the correct amount of force needed to collect the maximum energy possible, allowing the device to respond to each wave individually. The system, which was recently published in the journal Renewable Energy, doubles the energy previously collected by WECs.
Weiss explains WECs have two parts: a fixed or weighted lower part, possibly attached to the ocean floor, and an upper section that moves up and down based on the motion of the water. The device collects energy generated by the resistance force between the parts. Unlike wind turbines or solar panels, which collect as much or as little energy as comes their way, WECs need to adjust themselves to each oncoming wave to function properly, which requires knowledge of the characteristics of the incoming wave.
When there is zero resistance between the two parts of the WEC, the upper part moves freely with the waves, and no electricity is generated, Weiss explains. On the other hand, when there is so much resistance that it suppresses movement, the device turns rigid. At both of these extremes, no energy is produced. The ideal is a happy medium based on measurements of the incoming wave.
So Weiss and fellow researchers developed a control algorithm that is responsible for setting the correct resistance force for the WEC based on the predicted wave information. A processor attached to the WEC runs the algorithm five times per second to determine then implement an optimal mechanical response to the coming waves.
In the lab, the researchers have run simulations using wave data gathered from the ocean. Combining prediction technology with their new algorithm, energy collection was improved by 100%, double the amount of energy that WECs had collected previously.
The most important piece of information is the height of the wave, says Weiss, which the WEC needs to know in advance. “You would think that the longer the WEC knows the wave height in advance, the better, but in a surprising finding, it turns out that a one-second prediction horizon is enough,” he says, noting that a longer prediction time does not actually improve the energy harvest.