Waves hold a lot of energy. The challenge is to turn that motion into steady electricity. When speaking with Mikael Sidenmark, inventor of the InfinityWEC technology at Ocean Harvesting, the focus quickly moves to the PTO, the system that converts motion into electricity. This is where much of the development is taking place within the INFINITY project.
From the start, the work has been shaped by developing a PTO that is capable of high annual energy production in real sea conditions, which is all about the capability to provide a force that controls the buoy motion to match the phase optimally with every waves. Material selection, manufacturing and logistics are critical at early stage to develop an affordable solution that is also scalable for rapid rollout of large arrays with hundreds of WEC units. Main tasks for Mikael in the INFINITY project are concept development, supply chain, full-scale system design and to develop and build a scale prototype for testing of the PTO at VGA in Italy. The full-scale design will then be further optimized and refined based on the test result, simulations with the new model predictive control algorithm developed in the project, circularity by design principles, environmental impact assessment and LCOE modelling.
– Our main focus when developing the InfinityWEC wave energy converter is on improving the material efficiency, the cost and CO2 foot print of the materials being used per installed MW and produced MWh, says Mikael Sidenmark, inventor of the InfinityWEC technology at Ocean Harvesting.
A system under constant movement
The PTO is constantly moving with the cyclic wave motion with changing size and frequency from wave to wave. Model predictive control (MPC) is used to optimize the PTO force through every wave cycle by predicting the system dynamics, to achieve optimal power output considering both efficiency and lifetime degradation of the system. The PTO uses a combination of a hydrostatic pre-tension system providing a constant spring force from the hydrostatic water pressure outside the hull at 80-meter depth, and torque control of electric motors connected to ball screw actuators. Ball screws are very efficient at converting linear motion into high-speed rotation, which is then used to generate electricity. But they are also sensitive to side forces, bending moments and are limited in the stroke length and velocity they can handle. The power take-off design is the result of a close collaboration with the Japanese ball screw manufacturer NSK to create the conditions required for reliable operation and long life.
Another key function is keeping the mooring line under tension even when the ball screws runs out of stroke in large waves. If the line becomes slack in a deep through, it is not possible to control the buoy motion and it will move too fast in the next wave rise, which would risk over speeding ball screws and cause snap loads when the mooring line gets tensed again. To avoid this, a hydraulic mooring tensioner has been added below the PTO. It is implemented with a valve system in a hydraulic cylinder, that contracts rapidly when the mooring force is lost to keep the mooring line in tension. And then returns slowly to the set position while the PTO applies the force necessary to control the buoy motion.
– It is not only about capturing energy from waves, but about ensuring reliable operation and long life in the cyclic load conditions with every wave, as well as survival through extreme events in storm conditions, says Mikael Sidenmark.
From cyclic wave motion to stable power output
An electric flywheel energy storage system has been implemented to provide the reactive power required for optimal control with MPC when using direct motor/generator torque. Access to reactive power on board the power take-off enables MPC to reach its full potential in terms of annual energy production and is also used for smoothing the power output to a nearly constant level in any given sea state. Energy storage in the PTO also enables significant downsizing of downstream power electronics which is beneficial for the LCOE.
– Smoothing the short-term variations within a sea state with highly irregular wave sizes is a critical issue for grid integration and compatibility, that does not yet have enough attention in wave power, says Mikael Sidenmark.
Getting closer to real use
So far, the work has mainly been done through concept development and simulation driven design and performance assessment with regular MPC. The next step is to test the PTO together with MPC algorithm running on a real time control system, which has not been easy for wave power in the past due to very heavy computational burden of simulating hydrodynamics of a wave energy converter. A new much more time efficient moment-based MPC will be implemented for InfinityWEC in a joint effort by Irish COER and Italian Politecnico Di Torino to overcome this.
The next step after the INFINITY project is to test the PTO as part of a full wave energy system at sea in scale 1:3. In the longer term, wave energy is expected to become an important complement other renewables by producing power at different times than wind and solar, and also has the potential to be highly competitive in terms of LCOE and the use of materials per MW installed capacity.
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