Tidal Energy
Tidal stream systems are an example of first category. Often they resemble underwater wind turbines and in effect function in the same way, except tidal currents turn the turbines rather than wind. Whilst the installation of Tidal Stream Systems are a considerable civil engineering undertaking they are considered to have less of an environmental impact than the other alternative.
Tidal Barrages
Tidal Barrages consist of a kind dam built across a tidal estuary. As the tide comes in, water is allowed to flow in to the reservoir created by the dam, but at high tide a gate is closed, and as the tide falls again the water within the reservoir is trapped. At low tide, another gate is opened allowing the water to flow out of the reservoir through turbines. Barrage tidal energy power stations a huge civil engineering projects, requiring vast amounts of energy and resources to construct, and with a lasting impact on the local environment, and yet, once in place they have the potential to go on generating cheap and carbon neutral energy for generations.