Tidal Energy

Tidal Energy Coastal areas with huge and flowing tidal waters carry vast potential energy. 11th Century England was the first to harness this energy, using water wheels to produce mechanical power. These days the rise and fall of tides have become the basis to produce electrical power similar to the principles of hydroelectric power generation. Origin The daily rise and fall in the level of ocean water relative to the coastline is referred to as tide. Tides originate from the motions of the earth, moon and sun. The gravitational pull of the Moon and Sun along with the revolution of the Earth result in tides. (The magnitude of the gravitational attraction of an object is dependant upon the mass of an object and its distance.) The moon exerts a larger gravitational force on the earth, though it is much smaller in mass, because it is a lot closer than the sun. This force of attraction causes the oceans, which make up 71 percent 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 similarly, 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 i.e on the occasion of a full or new moon, the gravitational attractions are combined, resulting in very large spring tides. At half moon, the sun and moon are positioned at right angles, resulting in lower neap tides. Coastal areas experience two high and two low tides over a period of 24 hours and slightly above. The presence of geographical features such as bays and inlets result in higher tides. To produce enough amounts of power (electricity) that can be put to practical use, a difference of at least five meters between high and low tides is a must. There are about 40 suitable sites around the world with this kind of tidal range. The higher the tides, the greater is the amount of electricity that can be generated from a given site. It is inversely proportional to the cost of electricity produced, making such sites also more economical. Approximately 3000 GW (1 Giga Watt = 1 GW = 1 billion watts) of energy are available from the tides, worldwide. However considering the limitations as mentioned above, only about 2% (= 60 GW) can potentially be exploited for electricity generation. Generating tidal energy The technology required to convert tidal energy into electricity is comparable to technology used in traditional hydroelectric power plants. The first requirement is a dam across a tidal bay or estuary. However building a dam is expensive and the best sites are those where a bay has a narrow opening, thus reducing the length of dam required. Gates and turbines are installed. When there is adequate difference in the levels of the water on the different sides of the dam, the gates are opened. This causes water to flow through the turbines, turning the generator to produce electricity. Electricity is generated by water flowing both inwards and out of a bay. There are periods of maximum generation every twelve hours, with no electricity generation at the six-hour mark in between. The turbines may also be used as pumps to pump extra water into the basin behind the dam at times when the demand on electricity is low. This water can later be released when the demand on the system is very high, thus allowing the tidal plant to function like a "pumped storage" hydroelectric facility. Uses and economy The friction of the bulging oceans acting on the spinning earth results in a very gradual slowing down of the earth's rotation but this is not expected to impact us for billions of years. Therefore, for practical purposes, tidal energy can be considered a sustainable and renewable source of energy. It can prove to be a valuable source of renewable energy to an electrical system. The demand of electricity from a grid varies with the time of the day. 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 provides a good alternative to conventional methods of generating electricity, which would otherwise be generated by fossil fuel (coal, oil, natural gas) etc, thus reducing emissions of greenhouse and acid gases. Dam across La Rance Although the technology is well in place, tidal power is an expensive affair. Operating and maintenance costs of tidal power plants are very low because the fuel, being seawater, is free. However the overall cost of electricity generated is still very high. There is only one major tidal generating station in operation - the 240-MW tidal plant (1 megawatt = 1 MW = 1 million watts) at the mouth of the La Rance river estuary on the northern French coast. In operation since 1966, this plant has been a very reliable source of electricity for France. Researchers are examining the potential of several other tidal power sites and some of the prospects include the Severn River Dam in western England, the Bay of Fundy in Canada, Cook Inlet in Alaska, and the White Sea in Russia. Environmental studies so far indicate that tidal energy does not result in the emission of gases responsible for global warming or acid rain associated with fossil fuel generated electricity. Use of tidal energy could also decrease the need for nuclear power, with its associated radiation risks. One concern is that the tidal flows caused by damming a bay or estuary could, result in negative impacts on the immediate environment. This is still unclear as very little is understood about how altering the tides can affect incredibly complex aquatic and shoreline ecosystems. However each specific site is different and the impacts depend greatly upon local geography. Local tides changed only slightly due to the La Rance dam, and the environmental impact has been negligible. It has been estimated that in the Bay of Fundy, tidal power plants could decrease local tides by 15 cm. The negative environmental impacts of tidal barrages are probably much smaller than those of other sources of electricity. Tidal energy has the power to generate significant amounts of electricity at suitable sites around the world. But the potential is yet to be fully explored.

 

PROS AND CONS OF TIDAL ENERGY USE

HYDRO ENERGY FROM THE MOON

Tidal energy use harnesses the water flow created primarily by the moon orbiting the Earth. As water is pulled toward the gravity of the moon, currents are created that can turn generator turbines.
Volumes have been written about tides and their effects on our planet. ThisWikipedia Tides article is a good primer on the subject. It is noteworthy that all tidal energy does not come from the moon. About a third of it comes from the gravitational influence of our sun.
The interplay of gravitational fields of the moon and the sun combined with the rotation of Earth, creates a twice a day ebb and flow of the tides of our world that varies in height and strength.
Those variations in height and strength are completely predictable. As we’ll see later, that predictability is an important aspect of tidal energy use.
Though renewable, practical tidal energy use will be limited. Tidal flows are global, but the key to using them economically is finding either natural high tidal flow areas, or large tidal basins that can be easily dammed to channel water through turbines. _________________________________________________________________
ENVIRONMENTAL FRIENDLINESS -
Tidal energy use involving dams creates many of the same environmental concerns as damming rivers. Tidal dams restrict fish migration and cause silt build up which affects tidal basin ecosystems in negative ways.
Systems that take advantage of natural narrow channels with high tidal flow rates have less negative environmental impact than dammed systems. But they are not without environmental problems.
Both systems use turbines that can cause fish kills. But these are being replaced by new, more fish friendly turbines. The art and science of environmentally friendly hydro engineering is well advanced and will certainly be applied to any tidal energy project.
But even with dams, the environmental impact of tidal energy projects may prove to be smaller than our use of any other energy resource. Economics will severely limit the number of tidal energy projects. _________________________________________________________________
COST -
Tidal energy projects involving tidal dams are more expensive per KW of installed power than similar size systems that use river dams. Tidal flow is intermittent. Twice a day tidal flows go through a flood stage, slow down, stop, reverse into an ebb tide, slow down, stop, and repeat the cycle.
This constant start and stop cycle creates intermittency problems similar to wind turbines and wave generators. Though a tidal dam might be identical to a river dam in every way including cost; the tidal dam will produce less than half the amount of electricity.
A typical average plant load factor for tidal energy generators is about 27%. Load factor defines the amount of actual power output expected from a given capacity. Installed generating capacity of 100 MW with a load factor of 27% would produce only 27 MW per hour when averaged over a given time, usually a year.
That makes tidal energy expensive.
This U.S. Department of Energy Tidal Energy Report concludes that tidal power costs are not competitive with fossil fuel plants.
But a private company, Blue Energy of Canada , believes that they can generate tidal electricity at rates that are highly competitive with existing conventional power generators at rates of less than $.05 per KWh.
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AVAILABILITY -
The key to reliable, economic power from tidal energy involves properly engineered, economical turbine generators placed at well researched sites with high tidal flow rates.
Because of intermittency and variable flow problems of tidal energy, it is a very limited resource. The DOE Tidal Energy link, above, states that there are only about 40 really good sites on Earth with high enough flows to be considered economically practical.
Few studies of tidal energy resources have been done, so information is sketchy at best.
The World Energy Council Ocean Current Report states that total electrical power available from tidal energy use is about 450 GW of installed capacity. The report is a bit confusing to read and appears to be mixing tidal information with other ocean current information. Still, it’s worth reading.
That 450 GW figure seems to be compatible with the data on the WEC Tidal Energy page.
If we apply the .27 average load factor for tidal energy use, we can expect it to deliver about 450GW x 24 hrs x 365 days x .27 LF = 1064 TWh (Terrawatt hours) annually, or a little over 6% of global electrical demand.
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AESTHETICS -
Tidal energy projects involving dams would involve about the same aesthetic concerns as other dams. But many of the systems that use natural tidal currents will be largely hidden from view.
Natural current driven tidal generators can be built into the structure of existing bridges. These generators will involve virtually no aesthetic problems.
And, the fact that tidal energy use will be extremely limited means that any aesthetic concerns will also be limited.
Tidal energy use may not be a big player in our energy future, but it can make a contribution.
Tidal intermittency is completely predictable. Power output from tidal generators is also completely predictable. That predictability makes tidal energy reliable and easy to integrate with the existing electrical power grid. All of that makes it valuable.
Though tidal energy use will provide only a small portion of electrical grid demand, it can be a reliable and important energy resource.