Tidal Barrage!

No, the tides aren’t firing on me. A tidal barrage is a unique way to harness the power of the tides to produce electricity trapping water after a high tide and running it through a turbine as the tide changes. In 1984, Nova Scotia Power built such a system in Annapolis Royal. And unlike the Open Hydro experiment in 2009 that was destroyed after only a few cycles, this one is still running.

The history of this particular project is an interesting one. Like all large projects, there is always some environmental impact, but in this case, the impact mitigates the impacts of a previous project.

In the 1960s, the bridge across the Annapolis River at Annapolis Royal was in need of replacement. Nova Scotia at the time was finding that it was cheaper to build a causeway than a bridge, so that’s what they did here. And it had to be a very broad causeway because it had to stem the tides of the Bay of Fundy that at the time were causing the river level to change 25’ each tidal cycle (which is about 6. 25 hours). Because the river still flows, the causeway (which is essentially a dam) needed a control gate to let the water out. The Agriculture Ministry controlled the gate and essentially tried to keep the level stable.

You can imagine that there was a HUGE impact on the ecosystem of the former estuary, now river. It became a freshwater body instead of an estuary and of course the fish community and grass community that was based on salt water all died and were replaced with freshwater fish and grasses.

After the “energy crisis” of the 1970’s, Nova Scotia decided to experiment with tidal power. They took a current straight flow turbine design that was 2 meters in diameter and expanded it to 7 meters. They then had to design it for a saline and silty environment which means making it out of stainless steel and sealing the bearings so the silt wouldn’t get in and wear them out in a week. They also had to design it to take varying flow rates. Instead of trying to moderate the high flows which would have reduced the power output, they designed the generator into the turbine and are able to keep the speed of the turbine steady at 50 RPM by changing the power output. When there is high flow, the turbine is required to put out more power and as the flow drops, the turbine puts out less power.

A model of the turbine

A model of the turbine

The power company now allows about 800,000 cubic meters of water to flow up the river as the tide comes in. When high tide is reached, the control gates are closed. As the tide goes out and the water level on the seaward side of the causeway drops, the wicket gates at the turbine open and water starts flowing through the turbine. As the water on the seaward side drops the “head” (the difference between the water levels) increases and the power output increases. The turbine continues to run until the difference is less than 6’ (the difference required to effectively operate the turbine). The level change is also limited to 2’ (a far cry from the 25’ of old!) as people are now relying on its stability.

Operating Schedule for July 25, 2013

Operating Schedule for July 25, 2013

All in all, it’s a beautifully simple and effective form of production. Add to that the fact that it has been in operation since 1984 (with regular maintenance, of course) and you have a real success story.

With new run of the river turbines, the potential to produce real power seems within reach. As we saw from the 2009 experiment, it’s not always that simple, but considering that the tides in the Bay of Fundy represent the movement of 160 billion tons of water each tide cycle—4 times the estimated flow of all the rivers in the world—it seems worth pursuing.

For a little (very little, I’m afraid) more on this project, go to: http://www.nspower.ca/en/home/aboutnspower/makingelectricity/renewable/annapolis.aspx. You can also go to http://fundyforce.ca/ for more on the larger project near Parrsboro.

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Filed under Nova Scotia, July 2013

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