Marine Energy

The tidal barrage is undoubtedly the most controversial methods of extracting energy from the sea, mainly because of environmental concerns.

However with the ever escalating prices of fossil fuels and the availability of them to the power supply industry, the government are again looking into tidal barrage as a means of generating electricity to the national grid.

The barrage is built across an estuary and in order for the turbines to operate efficiently there should be at least 5m tidal range i.e. the vertical difference in height between the highest flood tide and lowest ebb tides.

The River Severn Estuary has been the subject of many government studies, and always the two main sticking points have been the environment and capitol cost.

We shall investigate these two points in an environmental article at a later date but in the meantime we will examine the operating principles of a tidal barrage and, also look at three different types of turbines which could be employed to drive the power turbines.

Once the site for the tidal barrier has been selected a massive civil works contract lasting up to five years will commence.

A foundation for the barrier is laid then the barrier itself is built incorporating slots for the turbines and sluice gates along with a lock facility if required this will allow the passage of ships. Once built the barrages have the following key components;

• Sluice Gates – these allow the seawater to fill behind the barrage
• Turbines – there are currently three types, used to drive the power turbines, and preferably able to reverse and be used as a storage pump
• Power generators – these can be submerged or built above the turbines driven by a vertical or inclined shaft.
• Lock Gate – some of the dissenters are against barrages because it stops the passage of ships up the river. A lock gate can facilitate this.

The incoming tide fills the area behind the barrage through the sluice gates which are closed at high tide. The sluice gates remain closed and once the tide has receded beyond the barrier, the turbine gates are opened allowing the seawater through the turbines producing a large electrical output of power via the generators for up to ten hours. This can be extended by allowing the incoming tide to pass through the turbines as it fills up the area behind the barrage; however this has been found to be inefficient and uneconomical, so most designs incorporate a one-way power generation system.

However, depending on the type of turbine employed water can be pumped into the storage area at low demand using cheap electricity usually at night and then generate at high demand times charging premium rates.

The Severn barrage output has been estimated at 8,640 MW during outflow of stored sea and river water, flowing through 216 turbines each generating 40 MW, which occurs in five hour bursts which may not suit demand i.e. at night time when demand is low, also such a large influx to the grid would probably mean a grid upgrade.

We shall now have a look at the different types of turbines and their advantages and disadvantages;

Bulb Turbine

This is the type of turbine used at the La Rance river estuary on the coast of France, which has been generating 240MW since the sixties. The generator is contained within a bulb, the turbine and generator being completely submerged (see sketch). The turbine and generator are reversible either to generate power on the flood tide or act as a motor to pump seawater for storage in an integrated basin or to supplement the stored water behind the barrage, as discussed earlier.

The big disadvantage with the bulb turbine generator is that it has to be lifted clear of the water for generator maintenance.

Rim turbine – the Straflo

This type of turbine has been used in a few small output tidal barrages but larger versions have had trouble with water seals; however the rim turbine is used successfully in a lot of ‘run of river’ locations e.g. the Rhine.

The generator rotor is located in the rim of the turbine with the stator on the barrage structure (see sketch), This leads to a more compact but less flexible component which can only operate one way - on the ebbing tide. It also has the disadvantage of having to be removed for maintenance of the turbine; however the generator can be accessed when water inlet gate been closed and water drained off.

Tubular turbine

This turbine is the most recent innovation and has been selected as the preferred type for the river Severn barrage,

It is a three bladed turbine which is mounted at an angle in the aperture in the barrage. A drive shaft protrudes upwards at the same angle and drives a generator that is mounted on the concrete structure above the waterline.

Once again this allows for maintenance of the generator without withdrawal from the water, but cannot be reversed to operate on flood tide or used to pump storage.

Tidal Barrage and Different Turbines