Tides transfer energy to ocean energy converter (OEC) devices through two pathways: the kinetic energy of moving water as currents flow in and out, and the potential energy stored in the height difference between high and low tide. Different OEC designs are engineered to capture one or both of these energy forms, converting water movement into mechanical motion and then electricity.
What makes tidal energy particularly powerful is water’s density. Seawater is roughly 800 times denser than air, which means a slow-moving tidal current carries far more energy than wind at the same speed. Power from a fluid flow increases with the cube of its velocity, so when water speed doubles, the available power increases eightfold.
Kinetic Energy: Tidal Stream Devices
Tidal stream devices work like underwater wind turbines. They sit in the path of tidal currents and capture kinetic energy from the moving water. The two main designs differ in how they orient their blades relative to the flow.
Axial flow turbines have spinning blades that face directly into the current, just like a conventional wind turbine turned on its side. As tidal water pushes past the blades, it forces them to rotate, and that rotation drives a generator. These are the most common design in commercial tidal energy. The Orbital O2, currently one of the world’s most powerful tidal turbines, uses this approach with two rotors up to 24 meters in diameter, producing a combined 2.4 MW of rated power. It begins generating electricity at current speeds as low as 1 meter per second and reaches full output at 2.5 meters per second.
Cross-flow turbines take a different approach. Their blades spin perpendicular to the water’s direction, similar to an eggbeater. When mounted vertically, these turbines can capture energy regardless of which direction the current flows, making them useful in locations where the tide reverses direction or where flow patterns are complex. This eliminates the need to reorient the device as the tide shifts between flood and ebb.
Potential Energy: Barrages and Lagoons
Tidal range devices exploit the height difference between water levels on opposite sides of a barrier. Rather than relying on the speed of the current, they store water at a higher elevation and release it through turbines, converting gravitational potential energy into electricity.
A tidal barrage is essentially a dam built across a tidal estuary. Sluice gates open during the incoming tide, allowing water to fill a basin behind the barrage. When the tide turns and the sea level drops, there’s now a height difference (called “head”) between the trapped water and the open ocean. The gates close, and the stored water is released through turbines built into the barrage. Some installations improve output through a technique called over-topping, where extra water is pumped into the basin to increase the head and squeeze more potential energy from each cycle. In some locations, tidal ranges exceed 12 meters, creating substantial energy reserves with each tidal cycle.
Tidal lagoons work on the same principle but don’t span an entire estuary. Instead, they create an enclosed body of water using a breakwater along the coast. The lagoon fills and empties with the tide, and turbines embedded in the wall capture energy in both directions. Because they don’t block a whole estuary, lagoons can be sited more flexibly.
Lift Forces: Oscillating Hydrofoils
Not all OEC devices use spinning blades. Oscillating hydrofoil turbines extract energy through a back-and-forth flapping motion, more like a fish tail than a propeller. The hydrofoil is a wing-shaped surface mounted on an arm. As tidal water flows past it, the hydrofoil pitches to specific angles that generate lift, the same force that keeps an airplane airborne.
During the downstroke, the hydrofoil tilts to a negative pitch angle, creating a downward lift force that pushes it toward the seabed. At the bottom, it reverses pitch and begins the upstroke, now generating upward lift. This continuous up-and-down motion drives a hydraulic pump or mechanical linkage connected to a generator. The key engineering principle is that the hydrodynamic force always aligns with the direction the foil is moving, keeping the energy transfer efficient throughout the cycle. Adding a deflector upstream of the hydrofoil can boost efficiency by roughly 32% by increasing the lift forces during each stroke.
Accelerating Flow: Shrouded Turbines
Some tidal devices use a physical structure to speed up the water before it hits the turbine. A funnel-shaped duct or shroud surrounds the rotor, narrowing the channel the water must pass through. This exploits the Venturi effect: when a fluid is forced through a smaller opening, it accelerates. The tradeoff is lower pressure, but the speed increase is what matters for power generation.
Because turbine power scales with the cube of flow speed, even a modest acceleration produces outsized gains. A shroud that doubles the water velocity across the rotor would theoretically yield eight times more power than the same turbine without the shroud. This lets smaller, lighter turbines generate meaningful output in locations where natural current speeds alone would be marginal.
Converting Motion to Electricity
Regardless of how an OEC device captures tidal energy, it needs a power take-off (PTO) system to convert mechanical motion into usable electricity. Most PTO systems have three core components: a drive train (either mechanical gears or a hydraulic system), a power generator, and an electrical control system that conditions the output for the grid.
Mechanical drive trains use gearboxes to step up the slow rotation of a tidal turbine to the higher speeds a generator needs. Hydraulic systems, more common in oscillating devices, use the back-and-forth motion to pressurize fluid, which then drives a hydraulic motor connected to a generator. Some newer designs skip the gearbox entirely with direct-drive generators, where the rotor’s slow rotation is converted to electricity without intermediate mechanical steps. This reduces the number of moving parts and lowers maintenance demands, a significant advantage for devices submerged in saltwater.
How Much Energy Gets Captured
No turbine can extract all the kinetic energy from a flow. In open water, the theoretical maximum for any turbine is the Betz limit: about 59.3% of the available kinetic energy. In practice, most individual tidal turbines operate at power coefficients between 26% and 40%, meaning they convert roughly a quarter to two-fifths of the water’s kinetic energy into rotational energy at the rotor.
Tidal turbines can exceed the traditional Betz limit in certain real-world conditions. When turbines are placed in confined channels or arrays where blockage effects come into play, the theoretical ceiling shifts upward, potentially reaching a power coefficient of 0.798 under high-blockage, low-speed conditions. Arranging turbines in arrays also changes performance. Lateral (side-by-side) arrangements can boost output by up to 7%, while streamwise (one behind the other) arrangements can yield improvements up to 11% when blockage effects channel more water through the rotors.
For tidal range systems, the available power depends on a different set of variables: the area of the enclosed basin, the tidal amplitude, and the frequency of the tidal cycle. Larger basins with bigger tidal swings produce more energy per cycle. Friction losses in the channel connecting the basin to the sea reduce the actual output, and these losses scale with the length and cross-sectional area of the channel relative to the basin size. Engineers optimize these dimensions during design to maximize the fraction of available potential energy that reaches the turbines.