PTO | BUOY | WAVE FARMS

InfinityWEC Solution

Performance

• 500 kW maximum continuous power output

• 1.5 – 2.5 GWh Annual Energy Production
  (depending on wave resource)

• 100 – 200 m installation depth

Prime Mover (buoy)

• 120-ton concrete and EPS in a honeycomb structure

• 12 m diameter, 4 m height, 3 MN net buoyancy

Power Take-off

• Actuation system: Ball screws + hydrostatic pre-tension

• 3.3 MN force capacity, Model Predictive Control

• 5 + 9 m stroke length (PTO + level cylinder)

• 30-ton weight (mostly steel)

Anchor

• Typical gravity base, 25-ton cage with 420-ton ballast

• Anchor type depend on sea bed conditions

Cost and tonCO₂eq estimates are based on similar fabricated structures with the materials being used (75% ballast, 18% concrete, 5% steel, 2% EPS). AEP depends on the wave energy resource at the location and is estimated to vary between 3-5 GWh / MW for a commercial installations. and can vary between 3-5 GWh /MW. Anchor not included since different types are used depending on site.

Power Take-Off

The PTO is based on a ball screw actuation system in combination with hydrostatic pre-tensioning, which is a highly efficient solution to provide instantly controllable force with sufficient capacity to fully control the buoy motion and stay within the stroke length of the PTO.

A Model Predictive Control algorithm (MPC) calculates the PTO force trajectory to control the phase of the buoy motion optimally with every wave. Power capture is amplified in small and medium waves, and reduced (de-tuned) in large waves to reduce extreme loads and limit the power production to the rated capacity.

The PTO technology is now being validated with a 1:4 scale prototype in a test-rig in the EUR 2.7 million EU-funded project INFINITY. Read more

Four ball screws converts linear motion into rotation of a generator connected to the bottom end of each ball screw shaft. The ball screw shafts are located in a lower PTO hull, connected to the anchor. The ball nuts are connected to the upper PTO hull and the mooring line to the buoy. The PTO is situated close to the anchor at approx. 80 meter depth. The differential pressure between the water outside and the air inside provides half of the available control force as a constant spring force. The ball screw assembly provides the other half as a bi-directional control force that either cancels or adds to the pre-tension spring force. The total control force is sufficient to fully control the buoy motion in all conditions and keep it submerged through the crest of large waves.

The PTO uses compact and efficient power electronics from EV. Geard axial-flux motors, SiC inverters and solid state transformer. High-speed flywheel batteries are used to provide reactive power necessary for phase control, and to smoothen the captured power to a constant output depending on the sea state.

Hydraulic Level Cylinder

Survival in extreme wave conditions is secured with a slack-catch feature in a hydraulic level cylinder below the PTO, which contracts rapidly when the PTO runs out of stroke in the downward motion, and then returns slowly to the set position in the next wave. The slack-catch feature is implemented with a valve system and internal reservoir in the cylinder, without use of hydraulic power pack or external accumulators and piping. The level cylinder is also used for adjusting the height of the PTO for tidal variations.

Avoiding slack in the mooring line between the buoy and PTO is critical to maintain motion control of the buoy and avoid snap loads in large waves.

Concrete Bouy

The EPS is assembled in a mould to form the cavities for the concrete structure. The highly flowable concrete is then poured into the mould and forms a jointless structure in a single curing process.

High performance concrete is 5x stronger than conventional concrete and reduces the thickness required in the honeycomb walls and the shell.

The InfinityWEC buoy is a unique design with a honeycomb structure in high performance concrete and EPS, enabling rapid local manufacturing with locally sourced raw materials.

The technology is being validated with a prototype in sea trials in the EUR 3 million EU-funded WECHULL+ project.

Read more

Compared to a buoy with a steel hull, the InfinityWEC buoy is equal in weight, but a quarter of the cost, a third of the CO₂ footprint and one tenth of the manufacturing time.

Enabling technology for rapid rollout of large wave farms with hundreds of WEC units.

InfinityWEC Wave Farms

A wave farm consists of multiple InfinityWEC´s arranged in star shaped 6 MW clusters á 12 WEC units. This topology is area efficient, provides good access to all WEC´s for service vessels, and the total length of the power cables are relatively short. In the wave farm below, three cable hubs are connected on a string cable to a central substation for the wave farm, where power is transformed to higher voltage and exported.  A total of 21 clusters arranged in seven strings provides a total power rating of 126 MW, with an electrical infrastructure very similar to an offshore wind farm.

The system is installable with a minimum of complicated offshore operations and no diving or heavy lifting are necessary offshore. An anchor cage is lowered to the seabed with a crane and filled with high density ballast from an installation vessel. The buoy and PTO are towed separately to the installation site. The PTO is first connected to the power cable retrieved from the cable hub with ROV, then lowered in a vertical position and connected to the anchor. The buoy is finally connected by pulling the upper mooring rope from the PTO through the center of the buoy and locking it with a pin from the top.