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Power Take Off modelling An important aspect of wave energy conversion is


accurate modelling of the Power Take Off (PTO). At the moment energy is converted into elec tricity in the PTO, the hydrodynamic behaviour of the struc- ture is changing. A few basic PTO types can be identified: - Grid connected PTO running at a fixed RPM, used in tidal current stream systems. Energy is delivered to the grid when the environmental conditions “try” to increase the generator RPM above the fixed RPM.


- PTO based on hydraulic cylinders delivering high pressures to smoothing accumulators. Hydraulic motors connected to these accumulators are used to drive an electric generator. This is often used in wave energy conversion systems.


- Linear PTOs based on direct-driven, standalone electric generators. The damping force produced in the PTO will have a linear relation to the PTO speed in case of a constant resistive load.


During scale model tests a flexible system is desired in order to simulate different PTO types and allow easy modification of settings. MARIN works with complete electric equivalents of PTO types using electric motors, feedback devices, digital controllers and dedicated software programs. These systems exert a realistic force on the structure as a function of motions and PTO characteristics.


Testing of the Green water concept including its Power Take Off


From an “electric point of view”, these simulators will not produce real energy that can be used to predict full-scale figures. Therefore, damping forces and a complete set of structure motions are measured to determine the dissipated mechanical energy, independent of converter efficiency. In this way measurement and control technology work closely together to make PTO modelling possible in model scale testing.


Modelling of the ‘Green water concept’ with diffraction theory and ComFLOW simulations


Schematic model of a hydraulic Power Take Off and actual PTO in a test modelling its hydraulic characteristics


report 17


92058


92046


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