Tidal Lagoons: Another Technique for Capturing Marine Renewable Energy

The power of the tides can be harnessed either by placing turbines in the tidal stream or by taking advantage of the tidal range.  The height difference between the high and low tides can be captured by the placement of a tidal barrage – effectively a dam like those used in rivers – across the mouth of an estuary or bay.  Alternatively, the tidal range can be captured within a lagoon by building an encircling wall within a tidal bay or inlet.  Key examples of tidal barrages include one developed at La Rance in Brittany (northern France) in 1966, or the placement of turbines in a causeway at the Annapolis Royal Generating Station in the Bay of Fundy in 1984.  Tidal barrages have not been constructed widely around the world as the development isolates the river mouth and estuary from free exchange with the ocean, effectively destroying many of the environmental functions of the free flowing water. Tidal lagoons are thought to cause much less environmental damage as they are created of built structures rather than manipulating a natural area, and do not cut off exchange between the estuary and the ocean.

The first ever tidal lagoon is proposed for Swansea Bay in the Severn Estuary in the Bristol Channel, in the south of Wales (United Kingdom.) The Severn Estuary has the second largest tidal range in the world (after the Bay of Fundy) with a range of 8.5 meters. The proposed lagoon will have an installed capacity of 320 MW of reliable electricity for 14 hours every day, equivalent to 90% of Swansea Bay’s annual domestic electricity use (155,000 houses). The lagoon proposal also provides recreational and cultural amenities, increased coastal defenses and flood control, as well as mariculture opportunities, as part of the causeway that will be built to enclose the lagoon.

Tidal lagoons are effectively large circular enclosures in the open water of an embayment; they operate like a giant retention basins.  During high tide, sleuth gates are opened to allow water to flow into the basin, passing through turbines to generate electricity. When the tide is low, the sleuth gates are opened again to allow water to flow out past the turbines, emptying the basin, and again generating electricity. The ‘bulb’ turbines mounted in the wall of a tidal lagoon operate bi-directionally, resulting in significant energy production. The larger the tidal range, the more head will be produced at high tide, the greater the flow of water past the turbines, and the more energy will be generated.

While the effects of tidal lagoons on marine wildlife, habitats, and ecosystem processes are thought to be considerably less than those of tidal barrages, the extent of potential environmental risk is not really known. The Swansea Bay Tidal Lagoon Project was granted a license to operate in 2015, following a formal environmental  impact assessment process, required under UK law. A number of environmental concerns were identified for which mitigation may need to be agreed prior to construction and operation of the lagoon. These include impacts on hydrodynamic and sediment processes, disruption to migratory fish routes, degradation of water quality, and impacts on marine mammals and birds during construction and operation.

An independent review into the feasibility and practicality of tidal lagoons is being undertaken on behalf of the UK Government, due to report in Autumn 2016. Construction on the Swansea Bay tidal lagoon could begin in 2017.  Additional proposals are pending for tidal lagoons in wales and elsewhere in the UK, and in other nations.