TIDAL ENERGY

In coastal areas with large tides, flowing tidal waters contain large amounts of potential energy. The principal of harnessing the energy of the tides dates back to eleventh century England when tides were used to turn waterwheels, producing mechanical power. More recently, rising and falling tides have been used to generate electricity, in much the same manner as hydroelectric power plants. Tidal energy is an essentially renewable resource which has none of the typical environmental impacts of other traditional sources of electricity such as fossil fuels or nuclear power although changing the tidal flow in a coastal region could result in a wide variety of impacts on aquatic life, consideration of which must be at the forefront of any investigation into harnessing this source of power.

The daily rise and fall of ocean levels relative to coastlines, are a result of the gravitational force of the moon and sun as well as the revolution of the earth. The moon and the sun both exert a gravitational force of attraction on the earth however the moon exerts a larger gravitational force on the earth because, although it is much smaller in mass, it is a great deal closer than the sun. This force of attraction causes the oceans, which make up 71% of the earth's surface, to bulge along an axis pointing towards the moon. Tides are produced by the rotation of the earth beneath this bulge in its watery coating, resulting in the rhythmic rise and fall of coastal ocean levels.

The gravitational attraction of the sun also affects the tides in a similar manner as the moon, but to a lesser degree. As well as bulging towards the moon, the oceans also bulge slightly towards the sun. When the earth, moon and sun are positioned in a straight line (a full or new moon), the gravitational attractions are combined, resulting in very large spring tides. At half moon, the sun and moon are at right angles, resulting in lower tides called neap tides. Coastal areas experience two high and two low tides over a period of slightly greater than 24 hours. The friction of the bulging oceans acting on the spinning earth results in a very gradual slowing down of the earth's rotation although this will not have any significant effect for billions of years. Therefore, for human purposes, tidal energy can be considered a sustainable and renewable source of energy.

In order to produce practical amounts of electricity, a difference between high and low tides of at least five metres is required and there are about 40 sites around the world with this magnitude of tidal range. Certain coastal regions experience higher tides than others and this is a result of the amplification of tides caused by local geographical features such as bays and inlets.

The higher the tides, the more electricity can be generated from a given site, and the lower the cost of electricity produced. Worldwide, approximately 3000 gigawatts (1 gigawatt = 1 GW = 1 billion watts) of energy is continuously available from the action of tides although there are insufficient suitable sites to harness this fully.

The technology required to convert tidal energy into electricity is very similar to the technology used in traditional hydro-electric power plants. The first requirement is a dam or "barrage" across a tidal bay or estuary and since building dams is an expensive process the best tidal sites are those where a bay has a narrow opening, thus reducing the length of dam which is required. At certain points along the dam, gates and turbines are installed. When there is an adequate difference in the elevation of the water on the different sides of the barrage, the gates are opened. This hydrostatic head which is created causes water to flow through the turbines, turning an electric generator to produce electricity.

Electricity can be generated by water flowing both into and out of a bay. As there are two high and two low tides each day, electrical generation from tidal power plants is characterized by periods of maximum generation every twelve hours, with no electricity generation at the six hour mark in between. Alternatively, the turbines can be used as pumps to pump extra water into the basin behind the barrage during periods of low electricity demand. This water can then be released when demand on the system its greatest, thus allowing the tidal plant to function with some of the characteristics of a pumped storage hydro-electric facility.

The demand for electricity on an electrical grid varies with the time of day and the supply of electricity from a tidal power plant will never match the demand on a system. But tidal power, although variable, is reliable and predictable and can make a valuable contribution to an electrical system which has a variety of sources. Tidal electricity can be used to displace electricity which would otherwise be generated by fossil fuel-fired power plants, thus reducing emissions of greenhouse gasses and creation of acid rain.

Current operational plants - although the technology required to harness tidal energy is well established, tidal power is expensive, and there is only one major tidal generating station in operation. This is a 240 megawatt station at the mouth of the La Rance river estuary on the northern coast of France (a large coal or nuclear power plant generates about 1,000 MW of electricity). The La Rance generating station has been in operation since 1966 and has been a very reliable source of electricity for France. La Rance was supposed to be one of many tidal power plants in France, until their nuclear program was greatly expanded in the late 1960's. Elsewhere there is a 20 MW experimental facility at Annapolis Royal in Nova Scotia, and a 0.4 MW tidal power plant near Murmansk in Russia.

Studies have been undertaken to examine the potential of several other tidal power sites worldwide. It has been estimated that a barrage across the Severn River in western England could supply as much as 10% of our country's electricity needs (12 GW). Similarly, several sites in the Bay of Fundy, Cook Inlet in Alaska, and the White Sea in Russia have been found to have the potential to generate large amounts of electricity.

One of the main barriers to the increased use of tidal energy is the cost of building tidal generating stations. Operating and maintenance costs of tidal power plants are very low because the fuel - sea-water, is free; but the overall cost of electricity generated is still very high.

Changing tidal flows by damming a bay or estuary could result in negative impacts on aquatic and shoreline ecosystems, as well as navigation and recreation. The few studies that have been undertaken to date to identify the environmental impacts of a tidal power scheme have determined that each specific site is different and the impacts depend greatly upon local geography.