The technology

Tidal range makes use of the height difference between high and low tides and generates electricity by creating a difference in the height of the water either side of a structure. It then passes the water through turbines like a hydroelectric dam.

Tidal stream is still in the very early stages of commercial deployment. It makes use of the movement of the water from the naturally occurring tidal currents (imagine an underwater wind turbine).  

The difference between high and low tides in the Severn Estuary can be as much as 14 metres, or 45 feet. Due to this the feasibility study is focusing on tidal range technology as this is where the main opportunity for Severn tidal power lies. However, some tidal stream technologies are being assessed as part of the Severn Embryonic Technologies Scheme (SETS).

The shortlist

The five schemes that the study is conducting a high level impact analysis on, includes two tidal range technologies – barrages and lagoons. The embryonic schemes that are being supported through the Severn Embryonic Technologies Scheme (SETS) include one tidal range technology (the low-head barrage) and two tidal stream technologies (two different tidal fences).

Barrages and lagoons are based on the proven technology used in hydroelectric dams. Tidal stream technology is still in the early stages of development. The turbines proposed for the low-head barrage are a new type of turbine which have not yet been developed at scale.

Brief explanation of each type of technology

• Barrages work by building a wall across an estuary and effectively converting the wall into a hydroelectric dam. The water from the incoming tide is allowed to pass through sluices in the barrage, but is held behind the barrage as the tide goes out. When the water level on the seaward side is low enough, the water behind the barrage is released through a series of turbines, generating electricity. 
  
  
• Lagoons work on the same principle but capture areas of water rather than having a barrier across the estuary. Lagoons can be completely offshore, or a semi-circle with both ends connected to the land. 
  
  
• The low head barrage (proposed by RollsRoyce – Atkins) is similar in structure to a conventional barrage but with a new type of turbine (not yet developed at scale). The turbine is likely to draw upon technologies from both tidal stream and tidal ranage technologies. The turbine design would generate electricity on both the incoming and outgoing tides and operate at a lower depth and water level difference than conventional barrage turbines. 
  
  
• The tidal fence (proposed by the Severn tidal power fence consortium) works by installing around 500 – 800 tidal stream turbine in a line across the Estuary. The fence could be located between Brean Down and Lavernock Point (the same place as the Cardiff-Weston barrage) or between Minehead and Aberthaw. The study will look at tidal stream turbines currently under development as single devices at around MegaWatt (MW) scale and assess their feasibility to be scaled up and incorporated as several hundred devices in a fence.  
  
  
• The tidal fence (as proposed by VerdErg) is a radical new fence design and works in a completely different way to the other fence option. It still makes use of the tidal stream, but a large proportion of the tidal flow passes through a fence structure formed of vertical and horizontal tubes. When water passes between the vertical tubes a pressure difference is created and this causes water to flow at high speeding the horizontal connecting tubes. This then drives turbines to generate electricity on both incoming and outgoing tides.