Active Mechanical Motion Rectifier (AMMR) for Wave Energy Converters
Background:
Power take-off (PTO), the intermediate machinery that transmits wave capture structure motion to the motion of the generator, is long viewed as the most critical subsystem of a wave energy converter (WEC).
Previous studies on PTO design primarily focus on enhancing transmission efficiency under fixed damping control, without considering the overall conversion efficiency after applying advanced control.
The mechanical motion rectifier, a successful PTO design for passive damping controlled WEC, is thus not suitable for executing advanced control algorithms. A control co-designed alternative that is able to enhance overall conversion efficiency is needed.
Innovation:
An active mechanical motion rectifier (AMMR) is developed, where the one-way clutches used for rectification is replaced with actively controllable electromagnetic clutches. The new design makes it possible to simultaneously rectify the generator motion and apply arbitrary control force.
The AMMR PTO is designed, fabricated, and tested in different scale versions. The models of the PTO are constructed and experimentally calibrated. The functions of the prototypes have been experimentally validated.
A power computation method based on steady-state solution is proposed to enable fast evaluation of power capture potential for different WEC configurations using the AMMR PTO.
A closed-form electrical power expression of a WEC using the AMMR PTO is derived based on an equivalent circuit modelling of the wave-to-wire system and considering unstructured open-loop control.
A tailored dynamic programming method is developed to find the optimal switching and torque control of the rectifier PTO and the generator. This method can be adapted for real-time implementation in a model predictive control framework.
Key results:
Case studies of a flap-type oscillating surge wave energy converter show the optimally controlled AMMR-based PTO outperforms the previous MMR PTO in terms of power capture by 2 to 3 folds depending on the wave periods.
Compared to conventional rigidly connected PTOs, the AMMR PTO increases the electrical power potential by 10-30% around the resonance wave period. If the power flow is restricted to be unidirectional, the AMMR PTO can increase power by 10-120% depending on the wave periods.
The proposed multistep DP method increases power by 12% in irregular wave simulation compared to a rigidly connected PTO. If the generator is connected to passive loads, the optimally controlled AMMR PTO is shown to increase power by 50% in irregular wave hardware-in-loop simulation.