Tidal Energy

Marine Energy

Over two-thirds of the Earth’s surface is covered by oceans, and the theoretical energy available from that source is greater than could ever be used. The diffuse nature of the resource, and its distance from where it is needed, reduce its potential usefulness; however, it remains a valuable prospect in many locations. The sources of energy that may be considered include tidal currents, wave energy, ocean thermal energy, and salt gradient energy. Ocean thermal energy could be a prolific source of energy, since it exploits the temperature difference between warm surface water and colder, deep water. However, to date only tidal energy has seen limited commercial development, although serious work in the area has been going on for only a little over a decade. This remains a potential energy source that requires a lot of research.

Tidal Energy

Until recently, tidal energy was obtained from one type of plant — a tidal barrage. This is essentially a large dam across a river estuary. As the tide comes in, the sluice gates are opened to allow the water to flow into the estuary. As the tide recedes, the flood control gates are closed to prevent the water from flowing back except through the turbine system. It is also possible to have a barrage that generates electricity on both incoming and outgoing tides.

Tidal power nova scotia
Tidal power generator in Annapolis Royal, Nova Scotia, on the Bay of Fundy.

The difficulty with tidal barrages is that they require high tides, so there are few locations (about 20) in the world which are suitable. Coupled with the high capital costs required to build the barrages, there are few operating tidal barrages in existence. One is located in Annapolis Royal, Nova Scotia on the Bay of Fundy.

Of course, once built, tidal energy is very cheap. However, the environmental impact of tidal barrages can be significant, particularly on marine life within the estuary. This is very difficult to predict, as each location is different and there have not been many studies done to compare the results. Possible changes include the water level (which could result in flooding). Flooding would affect the coastal vegetation which would have a direct impact on the aquatic and shoreline ecosystems. Local ecosystems could be affected in various other ways as well — water quality, safety of the fish who swim in the area of the barrage, birds could choose to migrate to other areas with better environmental conditions that meet their needs. These effects are not all bad, and may allow different species of plants and creatures to flourish in an area where they are not normally found. These issues are very delicate and need to be assessed independently for each area in question.

SeaGen Marine Current Turbine
SeaGen marine current turbine.

Tidal Stream Generators

A promising new technology is the tidal stream generator. It works like a windmill, except under water. Since water is much denser than air (about 800 times greater), the flow rate can be much lower than what is required for a windmill. The environmental effects should be greatly reduced over those of the barrages, although marine life would be affected by the blades. Another advantage is that the cost is much less than for a barrage.

There are many different designs of tidal stream generators proposed. In Canada, in 2006, Clean CurrentTM installed a demonstration turbine at Race Rocks Ecological Reserve in British Columbia. In 2009 Nova Scotia will be testing three different designs simultaneously at their new tidal energy research facility.

Sources:

Clean Current, www.cleancurrent.com/.
University of Strathclyde, Energy Systems Research Unit, www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/Tidal%20Power.htm.
Canadian Energy Research Institute, World Energy: The Past and Possible Futures, http://teachnuclear.ca/wp-content/uploads/2013/06/CNA_CERI07_EN.pdf.