Wave Energy Converter

Overview

A wave energy converter (WEC) extracts power from ocean surface waves, a resource that runs 15-70 kW per meter of wave crest on exposed coasts and is denser and more predictable than wind. The machine described here is a two-body heaving point absorber, the most common architecture among utility-scale prototypes: a surface float bobs against a deep, nearly stationary spar, and the relative motion drives a hydraulic power take-off. Devices of this class rate 250 kW to 1 MW and moor in 50-100 m of water several kilometers offshore.

Hydrodynamics

The Surface Float is shaped and ballasted so its natural heave period sits near the dominant wave period of the site, typically 7-11 seconds; at resonance the float can absorb power from a wave front wider than its own diameter, the point-absorber effect. The Spar Reaction Body provides something to push against. Its Heave Plate, a large disc deep below the wave zone, carries so much added mass and drag that the spar barely moves as waves pass, so nearly all wave motion appears as relative stroke between the bodies. The float slides on seawater-lubricated Guide Bearing Pad pads, with Hydraulic End Stop buffers absorbing the impact if storm waves drive the stroke to its limit.

Power take-off

Wave motion is slow (periods of seconds), reversing, and irregular, while a generator wants thousands of constant rpm, and the hydraulic PTO bridges that gap. Each Hydraulic Ram is a double-acting cylinder stroked by the float at forces up to several meganewtons. The Rectifying Manifold rectifies the flow, sending oil to the high-pressure line on both up and down strokes, exactly as a diode bridge rectifies AC. Nitrogen-charged Gas Accumulator vessels at 200-350 bar store the energy of each crest and release it through the trough, smoothing power that varies by a factor of ten within a single wave group. The Hydraulic Motor, a variable-displacement axial-piston unit, swallows this smoothed flow and spins the Generator Set at near-constant speed, with its swash angle trimmed continuously to balance accumulator pressure. The Relief Valve and Low-Pressure Reservoir close the circuit and protect it in storms.

Control matters as much as hardware: the Control and Communications system reads the Motion Reference Unit and adjusts PTO damping wave by wave. Optimal damping roughly doubles annual energy versus a fixed setting, and in extreme seas the controller does the opposite, locking the PTO stiff or fully venting it so the device rides out the storm as a sealed buoy.

Electrical export

The Power Electronics convert the variable generator output to grid quality with a back-to-back IGBT Power Module converter: an active rectifier feeds the DC-Link Capacitor Bank, an inverter synthesizes 50/60 Hz, and the Grid LCL Filter strips switching harmonics before the Export Transformer steps up to 6.6-11 kV. A Braking Chopper absorbs generation during grid faults. Power and fiber leave through the Dynamic Umbilical, a dynamic cable hung in a lazy-wave arch formed by Cable Buoyancy Module modules so buoy motion never reaches the static seabed cable; Bend Stiffener cones protect the terminations, and a Wet-Mate Connector lets the buoy disconnect for tow-to-port service without disturbing the cable.

Station keeping and survival

The Mooring System is a three-leg catenary spread: Drag-Embedment Anchor drag-embedment anchors, heavy Mooring Chain whose sag provides compliant restoring force, and Synthetic Mooring Line synthetic sections that add elasticity and cut snatch loads. Each leg carries an instrumented Instrumented Shackle so fatigue consumption is tracked against the design life. The governing load case is the 100-year storm with significant wave heights of 12-15 m, which produces forces an order of magnitude above operating loads; surviving it while remaining cheap enough to pay for itself is the central engineering tension of the field.

Status of the technology

Wave energy remains pre-commercial. Devices from Pelamis, Oyster, and others reached full ocean scale in the 2000s-2010s and proved the conversion chain, but costs stayed high and both companies failed; current developers such as CorPower, Eco Wave Power, and Mocean target levelized costs competitive with early offshore wind by the 2030s. The persistent challenges are mooring and PTO fatigue across roughly 100 million wave cycles, biofouling and corrosion managed by the Sacrificial Anode Set and coatings, and marine operations costs, which drive design decisions like the Quick-Connect Coupling coupling that converts an offshore repair into a harbor job.