Cycloidal Wave Energy Conversion at a Glance
The Cycloidal Turbine and Propeller
A Cycloidal Wave Energy Converter (CycWEC) is the adaptation of a Cycloidal Propeller to a different purpose, which is to extract energy from ocean waves. A cycloidal Propeller consists of one or more blades oriented parallel to a main shaft, and attached at a radius. While the geometry of a Cycloidal Propeller and a Cycloidal WEC looks identical, the purpose and mode of operation are entirely different.
To convert waves into useful electrical energy, the CycWEC must synchronize with ocean waves, perfectly cancelling the wave by producing an anti-wave 180 degrees out of phase. In doing so, the CycWEC extracts the ocean energy to drive a shaft on a generator, converting the wave power into electrical power.
Cycloidal Turbines are sometimes seen as a specialty wind turbines known as vertical axis wind turbines (VAWT). In the VAWT, the shaft is oriented vertically due to the orientation of their shaft, and the turbine bears some resemblance to an eggbeater. While they can operate as efficiently as the more common horizontal axis wind turbines, economic considerations favor the latter. In the marine industry, Cycloidal Propellers have been used for many years as propulsion devices for tug boats and ferries, and are commonly known as Voith Schneider Propellers.
Adaptation to Wave Energy Conversion
The CycWEC operates fully submerged in the ocean. For deep ocean waves the optimal CycWEC position is oriented such that the main shaft and hydrofoils are perpendicular to the incoming wave and parallel with the ocean surface. The hydrofoils are controlled by hydraulic actuation to adjust the blade pitch throughout a revolution of the main shaft.
In order to be useful as a wave energy converter, a Cycloidal WEC is synchronized to the phase of the incoming wave from which energy is to be extracted. This is achieved by an array of upwave surface elavation senors. Algorithms to esimtate wave directionality as well as instantaneous phase and amplitude of the incoming wave are utilized to control both the main shaft power take off as well as the pitch of the wave energy converter blades. When this system is properly tuned, by accurate synchronization and pitch scheduling, an incoming unidirectional, irregular wave can be entirely terminated, where more than 99% of the wave energy is extracted:
Note that in the above image which shows potential flow simulation results the surface elevation is exaggerated by a factor of 3 for plotting purposes, while the rest of the image is to scale.
While we have demonstrated more than 99% energy extraction in the inviscid limit, another important property of the cycloidal wave energy converter is that the output is shaft power of almost constant torque throughout the wave cycle. This allows for a more efficient direct attachment of a generator, avoiding most of the power take off losses that exist with other wave energy devices. The result is a very high efficiency of power conversion, which results in a very low cost of energy, as shown below:
The Atargis wave energy converter operates fully submerged. In a storm, it can be further submerged below the tremendously energetic storm-driven wind and surface waves. This allows Atargis to survive where other wave energy devices may be destroyed.
Atargis Energy has patented a number of different methods to install single or multiple CycWECs in the ocean. The selection of the installation method will greatly depend on the installation location and wave climate. Below we show two possible installations, for more options please refer to our patents and publications on this web site.
Intermediate Water Depths Close to Shore
In intermediate water depths, typically between 30m and 100m, the most economical installation for a Atargis CycWEC is most likely a monopile or jacket foundation:
This system utilizes well proven offshore construction methods and Atargis Energy plans to perform both small and full scale prototype testing in the near future using this installation approach. Depending on water depth, soil conditions, as well as local availability of a pile driving platform either a mono pile as shown, or a jacket type foundation may be more economical.
Deep Water Offshore
While a single wave energy converter can entirely cancel an incoming wave, it is advantageous to link several converters together into a cluster. By doing so, the reactive forces at the shaft of each converter can be made to cancel each other. This allows the cluster of wave energy converters to operate without any mooring lines reducing cost and improving storm survivability odds even further:
The arrows indicate the instantaneous forces acting onto the frame, which can be seen to cancel. The cluster of three WECs is nonetheless neutrally stable and will thus require a position control system to maintain its orientation and location in the presence of disturbances and drift forces.