The extreme tidal flow (~1 meter/sec) at Pigeon Creek, San Salvador, was measured with a current meter in "Blow-Outs" in the main channel.
Efforts to create tidal energy date back to the eleventh century where England used tides to turn waterwheels to generate mechanical power. The planet’s surface is composed of approximately 75% water and there is a lot of potential for energy production. Flowing tidal waters have a vast amount of potential energy; an estimate for potential energy worldwide can be as high as 3000 gigawatts (GW) of energy, which is continuously available from the action of the tides (Baird, 1993). To produce that amount of energy would end the United States dependence on fossil fuels, but do to constraints of the tidal energy process only about 2% or 60 GW can be converted to electricity. The constraints of tidal energy are due to the process in which tidal energy is created.
II. How is tidal energy made?
Tidal energy is made by creating a damn, also called a barrage, across the opening to a tidal basin or estuary (U.S. DOE, 2003). The tides rise and recede due to the gravitational pull that is exerted on the earth by the moon and sun. The high and low tides are created when the sun and moon’s gravitational pull are parallel to each other. When the high and low tides occur there is the greatest potential to turn potential energy into kinetic energy.
Kinetic energy is produced by allowing the gates, or sluice, on both sides of the barrage to open and allow the water to flow through a turbine, which then produces energy for electricity (figure 1). This event takes place twice daily to allow energy to be generated; this process is very similar to hydroelectric technology used throughout the world.
Available at http://www.acre.murdoch.edu.au/refiles/tidal/text.html under as figure 2.
Figure 1 Shows the water levels transferring and creating energy through a turbine (O’Mara, 1999).
There are various designs for turbines, which are designed to turn the water flowing through the barrage into electricity. There are three types of turbines in use today; the bulb, rim and tubular turbines, and each of the turbines are designed for different needs. There are two other ways of creating tidal energy that differ from the above process of using a damn and a turbine.
These two other processes use either a turbine or a damn, not both. One process uses a damn, which is called a fence, but instead of a turbine it uses giant turn styles to generate energy (figure 2). The final tidal process uses a turbine in open waters to generate energy; these turbines are placed in tidal currents to generate energy (figure 3). Open water turbines do not block passageways for aquatic organisms; this allows this process to be placed in other locations that are not suitable for the other two processes. Tidal turbines utilize tidal currents, which are moving with velocities of between 2 and 3 m/s to generate between 4 and 13 kW/m2 (O’Mara, 1999).
Available at http://www.acre.murdoch.edu.au/refiles/tidal/text.html as figure 6.
Figure 2 Shows a tidal fence that uses turn styles to generate energy (O’Mara, 199).
Available at http://www.acre.murdoch.edu.au/refiles/tidal/text.html as figure 7.
Also, Available at http://www.darvill.clara.net/altenerg/tidal.htm under offshore turbines
Figure 3 Demonstrates open water turbines (O’Mara, 1999).
III. What are the environmental impacts of using tidal energy?
Depending on the process chosen to generate tidal energy, there will be different environmental impacts. Also, not only does it matter what process you choose, but it matters what specific site you choose to install your technology at. Every site is different and the environmental impacts are different depending on the geography of the location. Knowing that there is a difference in environmental impacts depending on location and process chosen, there are some general environmental impacts that are associated with tidal energy technology.
Many scientists agree that there is a lack of knowledge when dealing with how changing of the tides will affect aquatic and shoreline ecosystems. There is a prediction that the building of tidal barrages in estuaries will affect tidal levels, as well as having an effect on sedimentation and turbidity in the water within the basins. The problem is that it is hard to predict how tidal barrages will affect tidal levels (O’Mara, 1999). By building barrages it can either lower or raise the tidal level, for example there is an estimated decrease of 15 cm to the tidal level if a barrage was built in the Bay of Fundy in Canada (Baird, 1993). On the contrary, if there would be an increase in tidal levels it would lead to flooding in the water basins. Stuard Baird and Dr. Hayhoe for Energy Educators of Ontario feel to increase “our knowledge about how tidal barrages affect ecosystems may be the study of the effects after such facilities have been built” (Baird, 1993).
Another environmental impact on aquatic ecosystems by building barrages is how will tidal stations affect plants and animals living in the basins. The barrages will restrict these aquatic organisms’ movements and no longer allow them to move with the tides. The only way to leave these areas is through the flow of water through a turbine, which will destroy most organisms.
Open water turbines have environmental impacts as well; figure 3 shows that there is no protection for organisms crossing these turbines. It looks like there will be a lot of organisms harmed by the blades of the turbine if there is no protection from them.
Listed above are the negative environmental impacts that are associated with tidal energy, but there are some very positive environmental impacts dealing with tidal power as well. There are no emissions or pollution associated with tidal energy, unlike the burning of fossil fuels. Tidal energy will help reduce our dependence on fossil fuels for energy and will provide a renewable resource for the future. Tidal energy will help reduce and even prevent many of the environmental problems that we face today.
IV. What are the overall pros and con’s of tidal energy?
Tidal energy will have some negative environmental impacts on selected areas where the technology is implemented, but I believe the positives of no emissions, no pollution and no fossil fuels outweigh these negative impacts. Tidal energy is a renewable clean energy source that will improve our environment as a whole. Tidal energy will replace our dependence on fossil fuels along with reducing nuclear waste by utilizing tidal energy. Some estimated savings for switching to tidal energy; are 3 million barrels of oil, three hundred and thirty tons of coal and ninety-one tons of uranium saved each year (Brown, 1997).
Tidal energy can be produced 24 hours a day and can operate 365 days a year producing energy. Tidal currents are very predictable, regular and flows peak at different times and different sites so power can be phased into grids continuously (Maser, 2004). The “fuel”, water, for tidal energy is free and abundant. Tidal energy has a higher efficiency than coal and oil: coal/oil efficiency = 30%, tidal power efficiency = 80%. Therefore more energy can be produced from the same amount of inputs (Brown, 1997).
There are still problems that face tidal energy; tidal energy is still more expensive that fossil fuels per unit of energy output (coal/oil = $.06kWh; tidal $.10kWh) (O’Mara, 1999). For tidal energy to be possible there needs to be about a 7-meter differential between low tides and high tides to produce a sufficient amount of flow to produce energy when using the barrage process. Another economical problem is that tidal barrages have a very expensive upfront cost to be built across a basin, and can take up to ten years to build (POEMS, 2003). Also, the technology still needs improvements to generate more energy at a cheaper price before it will become more widely used. Finally, all environmental impacts are unknown, further research needs to be performed to know the extent of the environmental impacts associated with the production of tidal energy.
V. Future benefits of tidal energy
The future of tidal energy seems to be leaning towards open water turbines; which by not using a barrage, tidal energy does not have the initial upfront costs of building the damn and avoids some of the environmental impacts that are associated with barrages. Blue Energy Canada, Inc. has started using technology know as a vertical-axis tidal turbine to collect energy from ocean currents. This technology is supposed to get one hundred and ninety times the kWh per unit of fluid value of wind power (Maser, 2004). Also, Dr. Bahaj of the University of Southampton, reports for the Sustainable Energy Research Group that they estimate that tidal turbines have the potential, for the races of the Channel Islands site, to produce the same amount of electricity as three Sizewell B nuclear power stations (equal to 3GW) (Maser, 2004).
Also, a “2002 feasibility report on tidal current energy in British Columbia by Triton Consultants for BC Hydro stated, ‘Future energy costs are expected to reduce considerably as both existing and new technologies are developed over the nest few years. Assuming that maximum currents larger than 3.5 m/s can be exploited and present design developments continue, it is estimated that future tidal current energy costs between $.05 and $.07 per kWh are achievable’” (Maser, 2004).
VI. Conclusion on whether or not tidal energy would be a good source of alternative energy to focus on
Tidal energy should be focused on for the next alternative fuel source, it is a renewable energy source with many potential benefits. There are many sites world wide that can use the barrage and open water turbine processes. There is already a 240 MW barrage style tidal power station located at the mouth of the La Rance river estuary on the northern coast of France. The La Rance tidal power station has been generating electricity since 1966 and has become a very reliable source of electricity for France.
The open water turbines that use tidal currents to move their propellers show a lot of potential, and reduce some of the costs and environmental risks associated with tidal power using barrages. Even though there are some environmental impacts of concern for tidal energy these impacts are much smaller than the impacts and pollution seen from using fossil fuels and nuclear power. Tidal power will not be able to support all of our energy demands, but it will be a valuable source of renewable energy. If developed correctly tidal power can become the primary provider for our future energy requirements.
Baird, Stuard. “Energy Fact Sheet.” Tidal Energy. Energy Educators of Ontario 1993. Viewed 05/05/04. http://www.iclei.org/EFACTS/TIDAL.HTM.
Baker, Clive. "Tidal Power", Energy Policy, October 1991.
Brown, Phillip. “Tidal Energy.” University of Wisconsin. 1997. viewed 05/14/04. http://www.geology.wisc.edu/~pbrown/g410/tidal/tidal.html.
Howes, Ruth. “The Energy Sourcebook: A Guide to Technology, Resources and Policy”. American Institute of Physics, 1991.
Maser, Michael. “Tidal Energy.” Blue Energy Canada Inc. 2004. viewed 05/20/04. http://www.bluenergy.com/pdfsOceanBlueEnergy/TidalEnergyPrimer.pdf.
Natgerman, George, "Wave Power," in Encyclopedia of Energy Technology and the Environment, John Wiley & Sons, 1995.
O’Mara, Katrina. “Tidal Power.” The Australian Renewable Energy Website. Modified 08/05/99. Viewed O5/10/04. http://acre.murdoch.edu.au/ago/ocean/tidal.html.
POEMS. “Ocean Tidal Technical FAQ.” Practical Ocean Energy Management Systems, Inc. Updated 03/03. Viewed 05/15/04. http://www.poemsinc.org/FAQtidal.html.
U.S. DOE. Energy Efficiency and Renewable Energy. “Tidal Energy.” Updated 2/7/2003. Read 4/1/04. http://www.eere.energy.gov/RE/ocean_tidal.html.
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