Current Status of Tidal Power Generation


This is the second part of the first portion of a multi-part series featuring papers on OE-related topics by students from the 2.65 class. The series is intended to raise interest and awareness of ocean energy research at MIT and in the world. This first section is on Tidal Power generation and is written by James Modisette, a graduate student in course 16. His sources and the full-text with citations are available upon request.

Kinetic Energy Turbines - Tidal Steam Generators
Some consider the phrase “fast moving currents” to be an overstatement as the fastest currents in the world are only about 10mph, but it is the density difference between water and air that makes tidal kinetic energy turbines possible. The density of water is about 850 times larger than the density of air, so it is easy to see that the energy in the tidal current, 1/2pv^2, can be comparable or better than that of wind power, which has already been harnessed and deemed economically feasible.

Using tidal stream generators to generate power is a fairly new technology. These generators fall into the second category of tidal power generators using the kinetic energy in fast moving tidal streams. Most of their development has been aided by the 35 year history of wind turbines. Stream generators can be broken down into several different design types. There are turbines, with either vertical or horizontal axes, with or without shrouds, and then oscillating devices. There are different benefits with each design. The vertical axis turbine has the potential to have fewer moving parts underwater and shrouds can be used to increase the flow rate.

Aided by wind turbine technology, horizontal turbines currently dominate the market. Due to this, horizontal turbines at times can be hard to distinguish from their wind turbine cousins, as shown in figure 3. The prominent difference is that they are not as large or as sleek as wind turbines. Bulking up the hub and blades is necessary to withstand larger stress loads produced by tidal currents.

Figure 3: Diagram of horizontal tidal stream turbines. These generators use the kinetic energy in tidal streams to drive rotors. The rotational motion passes through a gearbox and then drives a generator. a) Verdant turbine being tested in the East River between Manhattan and Roosevelt Island b) SeaGen installed in the Strangford Lough off Northern Ireland

Another design option is the oscillating device shown in figure 4. These devices are newer and have been designed to have a minimum of underwater moving parts. Instead of rotating parts they feature fins or wings that are at an angle of attack relative to the tidal stream. The angle of attack causes lift to be generated which can be used to force a hydraulic cylinder to extend or retract. As the fin reaches the top/bottom of its swing the angle of attack changes so that the cycle can continue.

Figure 4: Representation of an oscillating style stream generator. The
oscillating device uses the kinetic energy in tidal streams to create lift
on a wing which is converted to linear motion to drive a piston.
Although oscillating devices have some inherent benefits, the technology is too recent to have broken into the market in the same way as rotor driven turbines.

Be sure to keep following this series, the next portion of this paper will feature current installations of the technologies which have been discussed..