10.9: Energy from Tides

The tides are a potential source of inexhaustible and clean energy. Tidal energy dissipates as heat by friction between water masses and the seafloor. Some of the energy can be captured by electricity-generating turbines installed in areas with very fast tidal currents. However, very few locations have maximum tidal currents fast enough to drive turbines efficiently. Even where tidal currents are swift, they generally need to be enhanced by channeling the tidal flow into the turbines through narrow openings. The best locations for capturing tidal energy are bays or estuaries with a large tidal range and swift tidal currents, and where a dam can be built across the entrance. The dam is constructed with a number of narrow openings that house the turbines. Water entering the bay is channeled through the turbines as the tide enters the bay, and again as the tide flows back out of the bay.

For a tidal power-generating plant of this type to be feasible, the tidal range needs to exceed about 5 m, and it must be possible to build a dam that isolates an area into which such a tide flows from the open ocean. The most promising locations are few and widely distributed (Fig. 10-21). A large tidal power-generating plant has operated successfully at La Rance in France since the 1960s (Fig. 10-22). There has been continuing interest in constructing other plants, notably in the Bay of Fundy, Nova Scotia. Such projects face several problems, two of which are especially important.

Figure 10-21. There are relatively few locations with a tidal range sufficiently large (more than about 5 m) that large scale tidal power-generating plants are considered to be feasible.

Figure 10-22. La Rance tidal power-generating plant in France.

First, a tidal power-generating plant cannot generate energy throughout the 24 hr in a day. Power cannot be generated during the period of slack water when tidal currents are low. Judicious operation can retain water outside or behind a dam to some extent and thus lengthen the period during which power can be generated, but generating power uniformly over a 24-h period is apparently not possible.

The second problem is that building a dam across a bay substantially alters the ecology and other aspects of the bay. For example, the Bay of Fundy sustains rich and diverse marine communities and fisheries and has a famous tidal bore. Building a dam across the mouth of the bay would drastically alter or destroy both the fishery and the tidal bore. Fishers who live around the bay would lose both their local catch and access to the open ocean for their boats. Tourism would decline with the loss of the tidal bore. The benefits and losses associated with tidal power-generating stations that use dams to close off arms of the sea will be difficult to balance. A decision to build or not build such a plant at the Bay of Fundy (or anywhere else) will be controversial and will generate opposing views from reasonable people.

As an alternative to tidal power plants that rely on dams, submerged turbines can be deployed, preferably in a location where the tide wave is restricted and becomes a reversing current rather than rotary one. Some such tidal power plants have been tested and deployed, but they generate limited amounts of power and do not generate power consistently at times of day when it is needed. Therefore, as with several renewable energy options, efforts are being made to produce power storage capabilities to use with such installations.  

While tidal power potentially available from the oceans is very large, its real potential for the foreseeable future is, like wave energy, believed to be small, and the technology is most suited to use in specific local situations.