Feature ArticleAutonomous PowerBuoys Generate Power for Ocean Applications
By Dr. Philip Hart
Chief Technology Officer
Ocean Power Technologies Inc.
Pennington, New Jersey
Converting the energy from waves into electricity is certainly not a new idea. Take a cursory look at history and you can quickly trace back the patent portfolio to at least 1890. With more than 100 years of inventions and development, one would think that the main technical issues were long since addressed, and the oceans should be replete with various incarnations of systems powering the requirements of the world’s populations. It turns out that the technical issues are complex, and we are only now at the point where wave power can be economically extracted for some applications.
The PowerBuoy, developed by Pennington, New Jersey-based Ocean Power Technologies Inc. (OPT), is one of the recent wave energy converter technologies, and it has matured into a commercial product offering.
The LEAP PowerBuoy being deployed by U.S. Coast Guard buoy tender.
OPT’s PowerBuoy is based on a dual-absorber approach, where two hulls are arranged to respond dynamically to prevailing wave forces. The first hull is a vertical spar structure, with a heave plate at its lower extremity. The heave plate entrains a body of water in the vertical extent, adding virtual mass to the overall system if the spar hull tries to move axially. This manifests itself as increased inertia against the heave motion induced by wave action. The spar hull therefore responds slowly to a wave’s excitation forces and ideally would remain stationary in the ocean.
The second hull is a circular float structure that wraps around the spar toward its upper section. The float rides on bearing surfaces that constrain the travel of the float to, in essence, purely vertical reciprocal motion. The float is a significantly buoyant structure and responds readily to the forcing functions of the wave, in contrast to the spar. The characteristic difference in motion between the two hulls represents mechanical energy, which can then be captured and transformed into electrical energy.
Mechanical energy capture is achieved by coupling the two hulls via a bridge structure that spans the float and holds a push rod, which acts onto the power take-off (PTO) housed in the spar. The PTO applies this linear motion onto rotary generators, turning them to make electricity.
To accomplish long-term deployments of these wave energy converters, the PowerBuoy’s control systems monitor the prevailing wave conditions and autonomously configure the device to match them, maximizing survivability. By altering the control settings on the PTO, it is possible to maximize the product of force on the float and the distances moved over a wider range of waves, within the constraints of the structural design, and, thus, maximize energy capture across variable wave conditions.
Depending on the PTO technology chosen, tuning the response of the overall system at a rate of 10 hertz or more is required in order to get peak energy extraction in a wide spectrum of sea conditions. Mechanical control systems used in some wave energy converters have obvious limitations in response times and granularity. OPT’s control systems overcome these limitations by using an electronic approach, thereby offering a continuous spectrum of values rather than a quantized approach. This enables operation at very high speeds, maximizing flexibility, response and power-capture efficiency. The changes in configuration of the device are accomplished without operator intervention, under the supervision of the onboard control system.
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Dr. Philip Hart joined Ocean Power Technologies Inc. in 2009 as chief technology officer, leading the company’s engineering and technology development activities. He previously led engineering and business development at Global Marine Systems Ltd. (Chelmsford, England). Hart holds advanced degrees in subsea engineering from Cranfield University.