A new cycloidal wave energy converter could be resilient enough to cope with severe ocean storms claims its inventor
Many attempts to deploy systems that can convert the energy from ocean waves into electric power have been beset with complications. Even if such systems have proved resilient enough to withstand the loads imposed by the wind, waves and currents in the open ocean, they are often still a financially uncompetitive means of generating power when compared with traditional fossil-fuel energy plants.
Now, an engineer from the US Air Force Academy in Colorado Springs has developed a wave energy converter that he believes will provide a cost-effective way to generate power from the oceans and resilient enough to endure the harsh conditions.
At first glance, Dr Stefan Siegel’s cycloidal wave energy converter appears similar to the sorts of cycloidal turbines that have previously been used in vertical-axis wind turbines. In such a turbine, blades are mounted in parallel to, and at a specific distance from, a vertical main shaft. But while the geometry of the turbine and the wave energy converter might look identical, the modes of operation are different.
To be useful as a wave energy converter, the cycloidal wave energy converter is synchronised to an incoming wave that ensures the maximum amount of energy can be extracted from it. This is achieved by means of a sensor that measures the incoming wave amplitude, the wave phase and the wave frequency and a feedback control system that uses that data to control the device, the pitch of the two wave energy converter blades and the main shaft power take-off from the system used to drive the generator.
’By controlling all these parameters simultaneously through the closed-loop control system, we can optimise the performance of the device to extract almost all the energy from an incoming wave,’ said Siegel.
The approach differs markedly from more conventional floating buoys that bob up and down at specific frequencies to extract energy from the ocean. While such mechanically resonant devices have the advantage that they are simpler than devices based on electronic feedback control systems, they are harder to tune over a range of waves with different wavelengths.
Through the use of the feedback control system, Siegel is confident his cycloidal wave energy converter will be able to efficiently extract energy from waves over a much wider range of wavelengths than such buoy-based devices. Indeed, simulation results have indicated that the converter has a bandwidth covering a factor of five in wavelength change while maintaining the same efficiency.
Siegel said the optimum size and shape of the converter’s blades depends on the wavelength of the waves that energy is to be extracted from. ’We have found from simulations that the optimal size of the diameter of the converter is about one third of the wavelength of the water wave,’ he added. ’In a typical deep-ocean setting, the wavelength would be 100-200m, so the diameter of the wave energy converter would be one third of that.’
As the cycloidal wave energy converter is entirely submerged, in a storm it will not be subject to the tremendous loads imposed on surface-bound devices that are exposed to wind and breaking waves. To survive a storm, the cycloidal converter will feather its blades and, if needed, submerge deeper where the effects of a storm are much smaller.
To model the efficiency of the wave energy device, Siegel has performed extensive potential flow simulations. In a potential flow simulation, which neglects the viscous or frictional effects that the water has on the surface of the wave energy converter, Siegel has demonstrated that the cycloidal wave energy converter can extract more than 99 per cent of the energy from a wave.
Obviously, however, in a real-life ocean setting, these viscous effects would lower the efficiency of the system. Nevertheless, Siegel is confident they will only account for losses of 25-30 per cent, meaning the wave energy converter will still deliver more than 70 per cent of the overall wave energy as shaft power.
The torque generated at the wave energy converter’s shaft is almost constant throughout the wave cycle. This means a generator could be attached directly to it, thus avoiding losses in the power take-off system.
While a single wave energy converter can almost entirely cancel an incoming wave, Siegel said that, in practice, many converters would be linked together into a cluster. As such, the reactive forces caused by waves at the shaft of each converter – which would cause the converters to move up and down – can be made to cancel each other out.
’We are looking at a cluster of three in series that will sit in the direction the wave is travelling. If you arrange them such that they are one half a wavelength apart, you can show that the reactive forces will cancel out if you control them appropriately,’ he said.
Unlike many other wave converters, this would allow the cluster to operate without any mooring lines. Siegel sees this as a distinct advantage for the new system, enabling the cost of deploying them to be lower, while enabling them to survive the severe ocean storms that have destroyed many other, more traditional wave energy systems.
The unit would connect to the grid through an electrical cable, but Siegel said the energy converter would not impose any loads onto the cable, which is a major difference to a mooring system that gets stressed the rougher the seas become. In addition, as the cable attaches far below the ocean surface to a feedback-stabilised frame, there would be little cyclical loading to fatigue the cable over time.
Depending on the type of environment in which the converter would be deployed, its shaft could be oriented vertically or horizontally. When used in the deep ocean, Siegel said horizontally would be favourable, while in shallow water the shaft would be better oriented vertically.
In April 2010, Siegel formed a company called Atargis Energy with the specific goal of commercialising the patented wave energy converter. Following the formation of the company, he built a 1/100 scale prototype of the two-bladed converter that he tested in a wave tank at the US Air Force Academy in Colorado Springs.
This year, armed with almost $500,000 in funding from the US Department of Energy, he plans to build 1/10 scale models of the converter, which will be taken to the Offshore Technology Research Center at Texas A&M University, where more testing of a single device, as well as a cluster of devices will take place.