Harnessing the power of the tides: Ton Van De Plas Msc

At 320MW installed capacity, with first power expected in 2018, Swansea Bay Tidal Lagoon will be the largest marine energy development in the world. Developed by Tidal Lagoon Power Limited, it will have an entirely predictable 495 GWh output each year of clean, green electricity and is estimated that it will power 155,000 homes for 120 years – that’s about 11% of Wales’ domestic use.

Tidal Lagoon Power plan to build at least five full-scale tidal lagoons in UK waters by 2023, generating 10% of the UK’s domestic electricity requirements. Ton van der Plas, senior engineer with Tidal Lagoon Power, explains the technical development of Swansea Bay Tidal Lagoon.

Tidal lagoon power is a renewable electricity technology which uses the rise and fall of the tides to generate electricity. Tidal range turbine technology has been in operation since the 1960s but all existing projects are barrage schemes across river estuaries. Given the significant forecast reduction in the UK’s capacity to meet demand for electricity from the end of 2015, we believe that now is the right time for the UK to consider harnessing one of its major natural resources, the sea.

The Severn Estuary holds the second highest tidal range in the world and Swansea Bay benefits from an average tidal range during spring tides of 8.5m. This tidal range offers significant potential for the extraction of renewable energy through the construction of tidal lagoons.

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A tidal lagoon generates electricity by releasing water through a series of turbines in combination with a pressure difference caused by the difference in water level between lagoon and sea. The main driver for this head difference is the motion of the tides. The Swansea Bay tidal lagoon is capable of generating in both ebb and flood conditions by making use of turbines that can work efficiently in both flow directions, thereby fully utilising the tidal range in Swansea Bay.

Although the Lagoon is the first of its kind, all component parts of the project have been proven elsewhere in the world, keeping technology challenges and risks low. The general sequence is as follows: with flood generation the lagoon is empty and the turbines and sluice gates are closed. The sea level rises (flood) and when it reaches about 4 to 5 metres above the lagoon water level, the turbines are opened. While the lagoon is filled through the turbines, electricity is generated. When the head difference drops down to about one metre, the power generation and flow through the turbines reduces quickly. In order to let as much water in for the next tidal cycle, sluices are opened to quickly fill the lagoon to a maximum level. When the sea level equals the lagoon level, the turbine and sluice gates are closed. This marks the start of ebb generation, where exactly the reverse sequence takes place.

Because the tidal water levels are entirely predictable, it is possible to compute the expected energy output for years to come. A large range of scenarios have been investigated for Swansea Bay Tidal Lagoon, each time trying to find the optimum operational cycle for a given number and specification of turbines and sluices. For the final design, energy output results were checked and confirmed by independent expert companies.Through an ongoing process of operational optimisation, we now expect power output for the Swansea Bay Tidal Lagoon to hit 495 GWh per annum, enough to power 155,000 homes.


The commercial viability of a tidal lagoon is primarily driven by the predicted energy output versus the construction cost. The energy output is directly related to the tidal range and the area enclosed by the lagoon, whereas construction costs are driven by the length and height of the wall and the cost of the turbines and sluice structures. An important ratio to quickly assess the viability of a tidal lagoon in a given location is the ratio of seawall length versus the enclosed area of the lagoon. Detached lagoons and elongated lagoons have a less efficient ratio than rounded shapes. The conceptual engineering for the Swansea Bay Tidal Lagoon project started in 2011 by considering various lagoon layouts ranging from detached (offshore) lagoons to shore-attached alternatives of various sizes and layout. The design iteration encompassed a total of 15 lagoon options for which the viability and key constraints were identified and assessed.

Key constraints in the development of the Swansea Bay tidal lagoon were the dredged access channels to Swansea Docks (via the River Tawe) and Neath Harbour (via the River Neath), and the Crymlyn Burrows SSSI to the east of the site. Coastal process modelling further showed that a detached lagoon would lead to a significant increase in flows between the coast and the rear wall of the lagoon, leading to significant erosion of the shoreline. The final design is a lagoon that is maximised in size within the limits of the two access channels on both sides. The size is further restricted by the depth to reduce the volume of material in the deeper bund wall sections.


The turbine and sluice structures are orientated in such a way that the currents do not lead to adverse sailing conditions for vessels entering or leaving Swansea Docks. To increase nautical safety an exclusion zone will be put in place around the turbines and sluices, which will be marked by dolphin piles and floating booms.

The total length of the lagoon wall is about 10km, with 9.5km of bund wall and 400m of concrete structures housing 16 turbines and 8 sluice gates. It encloses an area of about 11.5km2.

Importantly, Swansea Bay Tidal Lagoon represents the beginning of a whole new industry in South West Wales. With a minimum 50% of Welsh and 65% UK content, the project will see an investment of over £600m in three years in Wales, catalysing £900m additional output and £173m GVA. This will mean new jobs that place traditional skills alongside cutting-edge design
and engineering. The construction phase alone will generate about 1850 full time equivalent jobs in construction, followed by 60 much needed long-term jobs in operations and up to 90 jobs in leisure. Furthermore, this is energy infrastructure that will be at the heart of the community. It will stimulate regeneration, culture and education, leisure and tourism. Swansea Bay Tidal Lagoon will be a catalyst for social and economic growth, providing a diverse public amenity, a dramatic sporting and cultural venue and a magnet for tourism.

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Our plans include a Watersport Centre, fully equipped for disability sports, which will host a variety of national and international events, from triathlon to sailing, and an Offshore Visitor Centre that will work with local schools and colleges, as well as welcoming 100,000 tourists to the Bay each year.

We will also see the creation of new habitat, sea reefs and seabed sanctuaries where oysters, lobsters, mussels and fish can replenish themselves; the reintroduction of species through an onsite hatchery; and the protection of large chunks of coastline. With a massive 86% of local people in support of the lagoon, the people of Swansea, Neath Port Talbot and Wales want this iconic, game changing development to be given the go-ahead. A decision from the Planning Inspectorate is expected in early 2015.

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Visit www.tidallagoonswanseabay.com for more information.


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Ton van de Plas MSc, is Senior Marine Engineer for Tidal Lagoon Power. He has completed a number of large marine and coastal engineering projects worldwide, which include project manager for the Port of Rotterdam and consultant for Royal Haskoning-DHV.

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