Energy can be harnessed from the tides in two ways: using the change in height of the tides (potential); and using the flow of the water (kinetic).
Tidal power is very sensitive to speed. The power output varies as the cube of the speed. In other words, if the water flows twice as fast, it makes eight times the power.
Also, tidal turbines do not have to spin as fast as windmills to generate power, because water is roughly 800 times more dense than air.
The in-stream tidal turbines tested at FORCE use the flow of the water as the source of power. Each of the turbines is designed to use the flow of the passing water to turn an impellor, just like a windmill. Each turbine is different in how it manages this but, in the end, each uses the rotation of its turbine to turn an electrical generator.
Designed to operate in the open flow of water in the Minas Passage, the turbines have to operate in a range of water speeds, from zero to a maximum of 10 knots. As water speeds can vary from the surface to the sea floor, test conditions for each turbine will differ slightly and depend on both the site location and the depth at which they are positioned.
In-Stream Devices make use of the kinetic energy of moving water to power turbines, in a similar way as windmills use moving air. This method is gaining in popularity because it’s removable, it can be scaled up gradually (from one device, to an array, to a larger farm), and has lower potential costs and ecological impact (compared to barrages).
Barrages make use of the potential energy in the difference in height – or head – between high and low tides. They are essentially dams across the full width of a tidal estuary – or the mouth of a river that has a free-flowing connection to the ocean. Barrages have very high costs, a worldwide shortage of viable sites and associated environmental concerns.
Tidal Lagoons are similar to barrages but can be constructed as self-contained structures not extending fully across an estuary. Some suggest this may reduce both costs and overall impacts. They can be configured to generate continuously, which is not the case with barrages.
An in-stream tidal turbine, also called a tidal current turbine, works a lot like an underwater windmill. In-stream technology is designed to use the flow of the tides to turn an impellor, just like a windmill uses the flow of air to turn its blades. Each turbine technology deals with this challenge differently, but each uses the rotation of a turbine to turn an electrical generator.
OpenHydro, for example, houses its impellors in a shroud or duct, to accelerate the flow of water over the blades, and improve the efficiency of the units. Marine Current Turbines uses two reversing pitch propellers, just like a conventional wind turbine, and uses the design of their blades to maximize efficiency.
The turbines are designed to operate in the open flow of water. In the Minas Passage, they must operate in a range of speeds from zero to 8 knots, depending on where they are sited and how deep they are positioned. Water speed is fastest at the surface and slowest near the sea floor. Tidal power output is very sensitive to water speed, just as windmills are to wind speed. For example, if the water speed doubles, the turbine will produce eight times more power!
FORCE is designed to accommodate a number of turbines throughout the demonstration site. These turbine “berths” are supported by four 34.5kV subsea power cables (each 2 to 3 km in length) designed to transfer power to the shore and on to the Nova Scotia electricity grid.
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.
In 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:
In-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.
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:
Before in-stream technology development grows to larger, commercial farms, more science is needed to ensure the technology is both safe and viable.