The ocean represents a vast, largely untapped reservoir of renewable power, containing immense energy within its constant movements and thermal characteristics. Ocean energy, or marine energy, defines the technologies designed to convert this natural power into usable electricity. Unlike other variable renewables, the movements of the sea are highly predictable, giving these sources the potential to provide a steady, reliable flow of power. The process involves harvesting the kinetic energy from moving water, the potential energy from height differences, and the thermal energy from temperature gradients.
Harnessing Tidal Power
Tidal power generation relies on the predictable, twice-daily rise and fall of sea levels caused by the gravitational forces of the moon and sun. This interaction creates enormous volumes of water moving in and out of coastal areas, which can be captured and converted into electricity. The reliability of the tides is a significant advantage, as their timing can be predicted years in advance.
One method involves using a Tidal Barrage, a dam-like structure built across an estuary or bay to create a reservoir. Gates in the barrage allow water to flow in during high tide. Once the tide recedes, the trapped water is released through turbines to generate power from the height difference, capturing the water’s potential energy.
A separate approach utilizes Tidal Stream Generators, which are underwater turbines placed directly into fast-moving tidal currents. These devices capture the kinetic energy of the flowing water, operating much like submerged wind turbines. Tidal stream systems do not require a large dam structure and are positioned where natural constrictions, such as straits or inlets, accelerate the flow of water.
Converting Wave Motion into Electricity
Wave energy conversion captures the kinetic and potential energy contained in surface waves, generated by the transfer of wind energy to the ocean’s surface. The energy is concentrated in the up-and-down motion of the water and associated pressure changes. Wave Energy Converters (WECs) are engineered to move with this motion and transform it into mechanical energy that drives a generator.
A common type of WEC is the Point Absorber, a buoy-like device that floats on the surface and absorbs power from wave motion coming from all directions. The buoy moves relative to a fixed point, such as a sea-floor mooring or an internal mass. This relative motion is used to pump a fluid or directly drive a linear generator, allowing for deployment in deeper offshore waters.
Another design is the Attenuator, a long, multi-segmented floating structure oriented parallel to the oncoming waves. As a wave passes along the device’s length, the hinged joints connecting the segments flex and bend. This flexing motion is captured by hydraulic pumps or mechanical linkages, converting the wave’s kinetic energy into pressurized fluid or rotational motion for electricity generation.
Utilizing Ocean Temperature Differences (OTEC)
Ocean Thermal Energy Conversion (OTEC) harnesses the difference in temperature between the warm surface waters and the cold deep waters of the ocean. This gradient must be at least 20°C to be viable and is used to run a heat engine and generate continuous, baseload power. OTEC facilities are located in tropical and subtropical regions where this temperature differential naturally occurs.
The most common design is the Closed-Cycle OTEC system, which uses a working fluid, such as ammonia, that has a very low boiling point. Warm surface seawater is pumped through an evaporator, where it vaporizes the liquid ammonia into a high-pressure gas. This high-pressure vapor spins a turbine connected to an electrical generator.
After passing through the turbine, the ammonia vapor is routed to a condenser, cooled by frigid water pumped up from depths of 1,000 meters or more. This cold water causes the vapor to condense back into a liquid. The liquid working fluid is then pumped back to the evaporator to repeat the cycle continuously.
Capturing Energy from Ocean Currents
Energy can be captured from major, steady ocean currents, which are large, continuous flows of water distinct from tidal streams. Currents like the Gulf Stream represent a highly reliable energy source because they are constant, directional, and unaffected by daily weather patterns. This stability offers a significant advantage for grid planning and power delivery.
The technology used to capture this energy is similar to tidal stream generation, involving large, submerged turbine arrays. These turbines are anchored to the seabed or suspended in the water column within the current’s path. The kinetic energy of the moving water rotates the turbine blades, which drive a generator to produce electricity.
Because water is far denser than air, a relatively slow-moving ocean current possesses a substantial amount of energy. This allows underwater turbines to be smaller and rotate more slowly than their wind counterparts while still generating significant power.