Tidal Barrages

Tidal barrages have a unique feature in that the source they derive their energy from (to drive their turbines) can be constant. Because they rely on the movement of the tide which is controlled by the moon, they are not dependent on weather conditions. If the Moon fails to perform, there is little point worrying about green energy as life as we know will likely to have failed anyway!

The Moon circles the Earth and as it does so, it exerts a gravitational pull on the Earth's surface. This has no discernible effect on the land mass although the Moon does not rotate around the axis of the Earth, but rather the Earth and Moon rotate around an axis point closer to the Earth. 

As the Earth and Moon move around one another, gravity causes the oceans to be affected. Water is drawn to the Moon where it is nearest, causing a high tide. On the opposite side of the Earth, gravitational pull from the Moon is weaker and due to centrifugal forces, water is 'pushed' out to form another high tide bulge. Whilst all of this is going on, the oceans on the sides of the Earth are drawn away to where the high tides are, causing a low tide.

All of this movement helps to create the ocean currents and the effects of those are utilised in tidal stream energy.

With tidal barrages we therefore have a reliable flow of ocean water almost twice every day. Weather conditions, i.e. pressure and wind can effect the predicted tide height and these effects can cause tides to be far higher or lower than expected..

Land geography can cause tides to have a larger effect movement as well. If the geography of the coast line forms a 'V' shape, with the open top of the 'V' pointing out to sea, when the tide rises, the weight of water pushing up the 'V' channel will cause the water to rise much high at the point of the 'V'. If the coastline was straighter then this forcing effect would not occur and the rise would be less. 

Tidal barrages work because they are designed to trap large volumes of water, creating a reservoir as the tide rises. A large dam wall is created to make an artificial lagoon on the coastline. This wall, called the barrage has a sluice gate built into it which will allow water to come in and out as is required. When the tide rises, the sluice gate is opened and this lets sea water enter the lagoon.. Once the tide has finished rising and starts to recede, the sluice gate is opened to let the water back out and be channelled through a turbine to create electricity. 

Because the water is getting lower on the outside of the barrage (ocean side) there is a drop in the water levels and this allows the water to drive the turbine. Equally, once all of the water has been released from the lagoon and the tide is at its low position,  the tide turns to come back in, the cycle is reversed. As the tide gets higher and the level of water is higher outside, water flows in once again passing the turbines to create electricity.

So long as the tide is coming in and out, the tidal barrage will continue to create electricity. One drawback is that there is a period called slack water when the tide goes from receding to incoming. because of this there may be 2 hours or so where there is not a substantial enough flow to generate power. To counter that, two lagoons are constructed side by side. 

To start, one lagoon is filled on high tide. As the tide stops running, slack water, the full lagoon lets the water into the empty lagoon via a turbine. As the tide reaches its low mark, this lagoon is emptied back into the sea, again via a turbine. The timing of this is critical but allows the likelyhood of more continuous power generation. 

The problem with barrages is they are generally perceived to have to be very large to be economical though this is open to debate. Small schemes utilising ports and smaller inlets could become very useful in the future for meeting green power requirements and hopefully further investigation and trials will be forthcoming.

What is certainly a problem is the ecological impact of such schemes. Trapping sea currents tends to create problems with silt. Water inside the lagoons allows the sediment it contains to settle and could cause the whole system to become clogged. As tidal flows are interrupted there is also the problem of scrubbing. Scrubbing is where the tides are shifting sediment and by introducing barrages, it can alter how it works. This may effect erosion and cause silt dumping to the detriment of the environment.

With careful planning and design most of these problems could be alleviated. The picture above shows the Rance tidal barrage in France which has been in operation since 1963 and has some effect on the local environment as would any man made intrusion but it is carefully monitored and whereas some bio-life has disappeared, others have become established.

Cost is certainly an issue and environmental concerns remain high. Because of these two issues, tidal barrages have not been as widespread as they might of been. With the onset of tidal flow generation coming it is possible that such barrage ideas may be left aside further still.