The U.S. Department of Energy’s marine energy glossary defines technical terms related to marine energy. Visit Marine Energy Basics to learn more about the renewable energy source.
Wave
Attenuator: Wave energy capture device with principal axis oriented parallel to the direction of the incoming wave. Passing waves cause relative motion of the device which is then converted into energy.
Oscillating Water Column: Partially submerged structure that encloses a column of air above a column of water. Waves are funneled into the structure below the waterline, causing the water column to rise and fall. This alternatively pressurizes and depressurizes the air column, pushing or pulling it through a turbine. There are shore based and floating models.
Overtopping Device: Partially submerged structure. Waves that go over the top of the structure are funneled into a reservoir, water then runs through a turbine and back out to the sea. There are shore based and floating models.
Oscillating Wave Surge Converter: Any of several devices that capture wave energy directly through relative motion between a float/flap/membrane and a fixed reaction point. The float/flap/membrane oscillates along a given axis dependent on the device. Mechanical energy is extracted from the relative motion or body part realtive to its fixed reference.
Point Absorber: Wave energy capture device, with relatively small principal dimension compared to wavelength, that captures energy from a wave front greater than the physical dimension of the device. There are floating and submerged models.
Pressure Differential: A fully submerged point absorber wave energy capture device. A pressure differential is induced within the device as the wave passes, which then drives a fluid pump to create mechanical energy.
Current
Archimedes Screw: A helical surface surrounding a vertical cylindrical shaft designed designed to efficiently transfer water up a tube. Energy is generated as water flow moves up the spiral and rotates the device.
Axial Flow Turbine: Typically has two or three blades mounted on a horizontal shaft to form a rotor. Water current creates lift on the blades and make the rotor to turn. The rotor then drives a mechanical generator. There are shrouded and open rotor models.
Cross Flow Turbine: Typically has two or three blades mounted along a vertical shaft to form a rotor. Water currents lift the blades and cause the rotor to turn. The rotor then drives a mechanical generator. There are shrouded and open rotor models.
Kite: A tidal kite is comprised of a hydrodynamic wing, with a turbine attached, tethered by a cable to a fixed point that leverages flow to lift the wing. As the kite 'flies' loops through the water, the speed increases around the turbine, allowing more energy extraction for slower currents.
Ocean Current: Capturing energy from ocean currents
Reciprocating Device: Uses the flow of water to create the lift or drag of an oscillating part transverse to the flow direction. This behavior can be induced by a vortex, the Magnus effect, or by flow flutter.
- Oscillating Hydrofoil (Example of a Reciprocating Device): Similar to an airplane wing but in water: yaw control systems adjust their angle relative to the water stream, creating lift and drag forces that cause device oscillation. Mechanical energy from oscillation then feeds into a power conversion system.
Riverine: Capturing energy from river currents
Tidal: Capturing energy from tidal fluctuations
Ocean Thermal Energy Conversion (OTEC)
Closed-cycle: These systems use fluid with a low-boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger that vaporizes the low-boiling-point fluid. The expanding vapor turns the turbo-generator. Cold deep-seawater is pumped through a second heat exchanger which condenses the vapor back into a liquid. The resulting liquid is then recycled through the system.
Open-cycle: These systems use the tropical oceans' warm surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.
Hybrid: These systems combine the features of the closed-cycle and open-cycle systems. In a hybrid system, a vacuum chamber flash evaporates warm seawater into steam. The steam then vaporizes a low-boiling-point fluid that drives a turbine and produces electricity.