Turning the rise and movement of seawater into electricity

Tidal power captures energy from changing sea levels or from the currents associated with tides. Two main approaches do this: barrages and lagoons exploit a difference in water level, while tidal-stream turbines extract energy from moving water. Both are renewable energy technologies, but they need different site conditions.

What drives the tides?

The Moon's gravity is the main influence, with the Sun's gravity also contributing. Earth's rotation and coastal geography shape the local pattern. Tides are distinct from waves, which are mainly driven by wind. A changing coastal water level is not necessarily tidal: wind and atmospheric pressure can also move the sea.

In a semidiurnal tidal regime, successive high waters are roughly 12 hours and 25 minutes apart. Other locations follow different patterns. A project therefore needs local water-level and current measurements, rather than a generic timetable.

How a barrage creates a generating cycle

A barrage across a bay or estuary controls the exchange between the sea and an enclosed basin. In a simple ebb-generation cycle, the rising tide fills the basin. Gates then hold the water while the sea outside falls, creating a difference in level, or head. Releasing water through turbines drives generators and produces electricity through hydroelectric conversion.

Two-way generating systems can produce electricity during both filling and emptying, but this capability is not universal. A tidal lagoon uses enclosing walls to create a similar basin. The useful resource is the head across the turbine and the flow through it, rather than the total depth of the sea.

Tidal-stream turbines work without impounding the water

A tidal-stream turbine extracts kinetic energy from the current. Suitable channels and coastal passages concentrate flowing water. Much like a wind turbine, its blades turn a rotor connected to a generator. Water's greater density also places substantial loads on the blades, supports and attachments.

Devices may be seabed-mounted or carried by floating platforms. When the current reverses, blade pitch, device orientation or another design feature can accommodate the new flow. Rotor reversal is not a requirement for every turbine. Different tidal technologies use different mechanical arrangements.

Predictable does not mean constant

Tidal timing and currents can be forecast. A tidal-stream turbine still loses output as the current weakens around slack water. A barrage also has less available power when the head falls. High water does not necessarily coincide with the strongest current. The relationship is specific to the location.

Grid operation can take advantage of the forecast production schedule. Energy storage may shift part of that output to another time, while coordinated operation with solar generation and other sources can help balance supply. Knowing when the tide will change does not make a plant deliver the same power every hour.

Measurements that determine a project's value

For a barrage or lagoon, assess basin area, head and flow rate. For a stream turbine, assess current speed, direction and turbulence. One observation of a strong current does not establish annual yield. The turbine's power curve, maintenance downtime, cable losses and flow interactions between devices all matter.

Rated power is expressed in kW or MW; generated energy is expressed in kWh or MWh. A 1 MW plant does not produce 1 MWh during every hour. Actual energy comes from its changing output over time. That yield and the operating costs underpin the economic assessment.

Controls and maintenance must suit the marine environment

A PLC can coordinate operating sequences and equipment commands. SCADA tracks production, alarms and downtime. An encoder or suitable speed feedback monitors rotor movement, while temperature sensors help monitor relevant bearings and equipment. Corrosion, biological growth on surfaces and maintenance access are practical design concerns offshore.

Sources: EIA: tidal power · EMEC: tidal devices · NOAA: causes of tides · NOAA: tides and currents · PNNL: resource and environment