Authors: J. Khan and G. Bhuyan, Powertech Labs, Canada
Tidal current energy represents a different approach to extracting energy from tides (or other marine currents). Rather than using a dam structure, the devices are placed directly “in-stream” and generate energy from the flow of water [16]. There are a number of different technologies for extracting energy from marine currents, including horizontal and vertical-axis turbines, as well as others such as venturis and oscillating foils.
Additionally, there is a variety of methods for fixing tidal current devices in place,
including seabed anchoring via a gravity base or driven piles, as well as floating or semifloating platforms fixed in place via mooring lines.
The energy available at a site is proportional to the cube of the current velocity at the site and to the cross-sectional area [1]. This means that, in general, the power that can be generated by a turbine is roughly proportional to its area, and that achieving high power outputs is dependent on having high flow velocities. For this reason, tidal current systems are best suited to areas where narrow channels or other features generate high velocity (2 to 3 m/s or more) flows. The velocity of a tidal current, and thus its power, varies throughout the day in a pattern similar to the height of the tide.
Horizontal-axis turbines
Horizontal-axis turbines are perhaps the most common means of extracting power from marine currents and are somewhat similar in design to those used for wind power.
Although there are a variety of approaches, including ducts, variable pitch blades and rim generators, all of these devices consist of a turbine with a horizontal-axis of rotation, aligned parallel to the current flow. These axial-flow turbines generally use a power take off mechanism involving a generator coupled to the turbine’s shaft, either directly or via a gearbox, to produce electricity.
While the low speed of rotation of the turbines can make the use of a gearbox attractive, the difficulty of accessing devices for maintenance, especially those fixed on the seabed, can make the use of a gearbox problematic. The varying speed of tidal flows means that variable-speed generators are used in many designs, which require frequency conversion in order to be connected to the power grid.
The horizontal-axis devices are further split into two categories: Ducted and non-ducted. Ducts can help steer and accelerate fluid flows through the device [2] and increase the effective power capture.
Vertical-axis turbines
Vertical-axis turbines have fallen out of use in the wind power industry [3]; however,
several ocean power companies are nevertheless developing designs for them.
There are several different designs in use, with some incorporating variable pitch blades (either controlled or freely moving) or shaped ducts to direct or restrict fluid flows. All of them possess some of the same advantages; vertical-axis turbines work well with fluid flows from any direction, and due to their shape, can have a larger cross-sectional turbine area in shallow water than is possible with horizontal axis turbines [4].
Several other tidal current systems are being investigated, which include non-standard mechanisms that do not use a conventional vertical or horizontal arrangement. These include venturi-based systems, oscillating hydrofoils and even magneto hydrodynamics.
[1] M. Gorlov. (2001) Tidal Energy, pp. 2956-2960. [Online]. Available:
http://www.gcktechnology.com
[3] Simon Meade. (2005, October) Lunar Energy - harnessing tidal power,” Hydrokinetic & Wave Energy Technologies - Technical & Environmental Issues Workshop, pp.2-3, 6-9. [Online]. Available: http://www.hydropower.id.doe.go/hydrokinetic wave/pdfs/day1/06 ducted turbine.pdf
[3] Danish Wind Industry Association. (2007, June) Wind Turbines: Horizontal or Vertical Axis Machines. [Online]. Available: http://www.windpower.org/en/tour/design/horver.htm
[4] Edinburgh Designs Ltd. (2006) Variable Pitch Foil Vertical Axis Tidal Turbine, pp. 8-10. [Online]. Available: http://www.dti.gov.uk.
