Barrage vs In-stream

Tidal power technology is evolving.

The earliest methods of harnessing the tides existed centuries ago in Rome, Britain, and France. In the 17th century, Nova Scotia had a number of tidal mills – although they did not produce electricity.

Barrage Technology

annapolistidalstationIn 1984, North America’s first and only tidal generating station was built in Annapolis Royal, Nova Scotia. Still in operation, the plant produces about 20 megawatts when running. The Annapolis plant is very similar to a conventional hydro-electric dam, called a barrage.

Barrages capture water in a holding area, making use of the difference in water height from one side of the barrage to the other. Water is held then released through a large turbine (or turbines) as it flows out with the ebb tide (watch Nova Scotia Power’s video on the Annapolis Tidal Station).

While barrages use known technology (concrete, steel, conventional hydroelectric turbines), they have a number of challenges: high costs, they cannot be scaled-up gradually, and, as with any dam, a number of environmental concerns including effects on sediment, coastal erosion, and fish mortality.

Research conducted in the 1980s during and after construction of the Annapolis plant indicated a concerning rate of fish mortality related to the project.

A barrage:

  • may provide little alternative to fish passing through the turbines, which makes fish strikes more likely
  • may create a substantial pressure drop as fish pass through that is also harmful to fish
  • may also cause major changes to sediment distributions, which affect fish, birds, and other animals that live in, or depend on, the intertidal zone.

In-stream Technology

Hammerfest StromIn-stream devices are in some ways a response to barrage technology. Because they do not use dams, concrete walls or enclosures, they have the potential to leave a much smaller environmental footprint.

An in-stream turbine makes use of  the power of moving water to produce electricity.

Their advantages include: removability, scalability (they can be installed one device at a time, rather than built all-at-once), and have lower potential costs and ecological impact. While some early research indicates fish may avoid in-stream turbines, FORCE believes much more research is needed in the Minas Passage to understand how marine life may interact with in-stream technology.

In-stream devices:

  • do not force marine life and migrating fish to pass through them
  • have smaller effects on water flow and sedimentation (which can lead to problems with erosion and land drainage)

Before in-stream technology development grows to larger, commercial farms, more science is needed to ensure the technology is both safe and viable.