Power Take-Off Simulation for Scale Model Testing of Wave Energy Converters

Scott Beatty,Bryce Bocking,Jens Kofoed,Bradley Buckham,Francesco Ferri

doi:10.3390/en10070973

Abstract

Small scale testing in controlled environments is a key stage in the development of potential wave energy conversion technology. Furthermore, it is well known that the physical design and operational quality of the power-take off (PTO) used on the small scale model can have vast effects on the tank testing results. Passive mechanical elements such as friction brakes and air dampers or oil filled dashpots are fraught with nonlinear behaviors such as static friction, temperature dependency, and backlash, the effects of which propagate into the wave energy converter (WEC) power production data, causing very high uncertainty in the extrapolation of the tank test results to the meaningful full ocean scale. The lack of quality in PTO simulators is an identified barrier to the development of WECs worldwide. A solution to this problem is to use actively controlled actuators for PTO simulation on small scale model wave energy converters. This can be done using force (or torque)-controlled feedback systems with suitable instrumentation, enabling the PTO to exert any desired time and/or state dependent reaction force. In this paper, two working experimental PTO simulators on two different wave energy converters are described. The first implementation is on a 1:25 scale self-reacting point absorber wave energy converter with optimum reactive control. The real-time control system, described in detail, is implemented in LabVIEW. The second implementation is on a 1:20 scale single body point absorber under model-predictive control, implemented with a real-time controller in MATLAB/Simulink. Details on the physical hardware, software, and feedback control methods, as well as results, are described for each PTO. Lastly, both sets of real-time control code are to be web-hosted, free for download, modified and used by other researchers and WEC developers.

Highlights

Similarity of Froude number is widely relied upon as a scaling law for the estimation of full scale wave energy converter (WEC) dynamics from model scale WEC observations
Because the WEC dynamics are highly dependent on the power-take off (PTO) force, the choice of PTO at the experimental scale is critical to ensuring dynamic similarity
The objectives of this work are to demonstrate the utility of actively controlled PTO simulators by presenting two separate applications based on feedback controlled linear motors, and to present the methodology used in each application for other researchers to implement in their own work

Summary

Introduction

Similarity of Froude number is widely relied upon as a scaling law for the estimation of full scale wave energy converter (WEC) dynamics from model scale WEC observations. Previous experimental studies of scale model WEC devices have used a variety of systems to emulate the reaction forces exerted by a PTO device. Because these small scale systems are not intended to convert the scale model mechanical power into the final power commodity (electricity, fresh water, etc.), these systems are referred to as PTO simulators. Given a geometric scale ratio of α between experimental model and full scale WECs, under Froude scaling, the power absorption scales with α3.5 net With such sensitivity in the scaling law, power losses from forces that are not dynamically similar, such as friction and mechanical backlash, must be minimized because they will obfuscate the power absorption results [1]

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Conclusion