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. This
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.
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.
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.
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.
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
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.
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.
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