Ocean waves are a source of renewable energy. Wave Energy Conversion (WEC) is the process of capturing the mechanical energy contained within the ocean’s surface waves and transforming it into usable electrical power. This kinetic energy, created by wind blowing over the water’s surface, is dense and highly predictable, offering an advantage over other variable renewables like solar and wind power. Transforming the oscillating motion of the sea into a steady flow of electricity begins with the physical interaction of specialized devices with the waves.
The Fundamental Mechanics of Energy Capture
The intermediary link between the physical motion of the wave and the generation of electricity is the Power Take-Off (PTO) system. The PTO converts the slow, irregular mechanical motion captured by the wave device into a high-speed, controlled rotational movement suitable for driving an electrical generator. This conversion is challenging because the inconsistent input force from the waves requires the PTO to smooth and regulate the power flow.
One common PTO approach uses hydraulic systems. The wave’s motion drives a piston, pressurizing an internal fluid like oil. This high-pressure fluid is channeled to a hydraulic motor, which spins a rotary generator. Hydraulic accumulators store energy during peak wave compression, releasing it during troughs to maintain a consistent power output to the generator.
Another method employs pneumatic conversion, utilized in devices that trap air. The oscillating water column forces a bi-directional airflow, which drives a specialized air turbine connected to a generator. Direct mechanical or direct electrical drive systems simplify the process by using wave-induced motion to directly push or pull a component within a linear generator or turn a gearbox connected to a rotary generator.
Major Designs of Wave Energy Converters
Wave energy is captured by various converter designs, each engineered to extract energy from a specific characteristic of the wave motion. These devices are generally categorized by their orientation and size relative to the incoming waves. Understanding how each physically interacts with the water is crucial to grasping the overall conversion process.
Attenuators
Attenuators are long, multi-segmented floating structures oriented parallel to the direction of wave travel. Operation relies on the relative motion between hinged sections as a wave passes along the length, causing the hinges to flex up and down. This flexing movement is resisted by internal hydraulic rams or pistons located at each hinge point. The rams pump high-pressure fluid into an accumulator, which feeds a hydraulic motor to spin an electrical generator.
Point Absorbers
Point absorbers are buoy-like structures small compared to the wavelength of incoming waves, allowing them to capture energy from all directions. These devices primarily extract energy from the vertical rise and fall, or heaving motion, of the sea surface. The converter is a floating body tethered to a fixed reference point, such as the seabed or a submerged spar.
The kinetic energy is harvested from the relative movement between the floating component and its stationary reference point. This reciprocal vertical motion can drive a linear electrical generator, where a coil moves back and forth over a magnet array, or a piston that pressurizes hydraulic fluid. Point absorbers must be tuned to match the natural frequency of the waves to maximize energy capture.
Oscillating Water Columns (OWC)
Oscillating Water Columns are partially submerged, fixed or floating structures that utilize pneumatics to convert wave energy. The device consists of a hollow chamber open to the sea below the waterline, trapping a column of air above the internal water surface. As an incoming wave enters the chamber, the water level inside rises, acting as a piston that compresses the air above it.
When the wave recedes, the water level falls, creating a vacuum that draws air back into the chamber. This continuous, alternating flow of air is channeled through a duct containing a specialized bi-directional air turbine. This turbine is designed with symmetrical blades to ensure it rotates in the same direction regardless of whether the air is rushing in or out, providing the necessary continuous rotation to drive the connected electrical generator. The OWC effectively transforms the slow, powerful movement of the water into a high-velocity, consistent flow of air to generate power.
Transmitting Power to the Electrical Grid
Once the mechanical energy is converted into electrical energy within the wave device, the power must be efficiently transported to the onshore utility network. This final stage involves electrical conditioning and long-distance transmission, which is common to all offshore renewable energy projects. The generated electricity, often alternating current (AC), must first be processed to ensure a stable output.
Since wave power is inherently intermittent, power electronic interfaces on the WEC are used to condition the fluctuating current and voltage into a stable, grid-compliant signal. The electricity is then transmitted via specialized subsea power cables laid along the ocean floor back to the coast.
On reaching the shore, the subsea cable connects to an onshore substation. Here, transformers step up the voltage to the high levels required for long-distance transmission across the grid infrastructure. This final connection allows the harnessed wave power to be synchronized with the existing electrical network.