The tides go in and out as they are pulled by the moon's gravitational field, and we can capture energy from this massive movement of water (kinetic energy of the water) with tidal power stations.

Tidal power stations are built to stretch over places that are affected by the tides such as deltas or estuaries. A barrage is set across a beach or river. When high tide comes in, water flows through a turbine to create electricity. The water level in the river mouth is now high so a gate is lowered , trapping the water behind it and preventing it from just going out with the tide. When low tide comes, the gate is raised and the water flows out, transferring its energy through turbines. This way, electricity is created with a two-way turbine.

History

Tidal mills were invented in the early 1900s. They didn't have two-way turbines then - they only used one tidal direction. They chose to capture the tides as they were receding, because there was the most possible outcome from there. When the tide came in, the floodgate lowered, trapping the water above it. When low tide came, the gate was lifted up, and the water turned a water wheel. The wheel could then power the mill.

Advantages

Running costs are very low - there are no fuel costs, just maintainance costs

Does not produce greenhouse gases or acid rain

Disadvantages

Tidal power stations are very expensive to build - capital outlay is high

They create electricity only in time with the tides (not when necessarily needed). The energy from them has to be stored for them to be efficient.

Many fish, like salmon, swim up these estuaries where the barrages are and have already been killed by the turbines.

Building the barrage can destroy plants and the homes and food supply of many birds, fish and other animals (change the ecosystem).

They also can trap pollutants up stream which damages the environment