Engagement

Activity

1. 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.
• Experts estimated that only 2% (60 gigawatts) can potentially be recovered from tides for electricity generation.
  • Can you guess why this is the case?
2. Currently, only in places with large tidal range (greater than 5 meters) can tidal power be extracted economically. Visit the Oceanographic Products and Services Division website to find whether any stations along the U.S. northeast coast have a tidal range of 5 meters or greater.
• Click here to plot yesterday's water levels along the U.S. northeast coast.
• To get yesterday's "Water Level" data:
1. Either click any station's red dot or use the "pull-down" menu
2. Click on the "Water Level Plot" button (lower left)
3. Hit the "SUBMIT" button
   • After you've hit "SUBMIT," you may get a "warning message" that says "Any information that you submit is insecure..." Hit "OK" to continue.
• You may not have time to check all thirteen available stations. If this is true, you may want to check every other station or, perhaps, every third station. Keep in mind, it is important to sample over a fairly wide geographic range of stations.
3. Assess whether of the stations you researched any meet the 5-meter tidal range requirement.
• If any do, categorize them as "OK SO FAR."
• If possible, print the "water level plots" for these "OK SO FAR" stations. (You will use them in Step 6. If you cannot print the data, write down the times of high and low tide for these stations.)
• Locate the "OK SO FAR" stations on an atlas.
4. In order to harvest tidal power, a structure must be built across a tidal bay or estuary. Because building these is expensive, the best sites are those where a bay has a narrow opening (thus reducing the length of dam).
• Do any of your "OK SO FAR" stations have narrow openings where such a structure could be built? (Please continue even if the answer is "no.")
• If yes, how might such structures affect local habitats of wildlife? Salmon migration?
• If yes, how might sediment that is trapped upstream of the structure affect an estuary?
  • Considering these potential impacts, are any of your stations still "OK SO FAR"? (Please continue even if the answer is "no.")
5. At certain points along the dam, gates and turbines are installed. After the dam is completed, when there is an adequate difference in the elevation of the water on the either side, the gates are opened. This causes water to flow through the turbines, turning an electric generator to produce electricity. (Click here to see a "side view" of this.)
• To demonstrate how a turbine works, ask a student volunteer to blow on a pinwheel. Be sure that he or she blows toward its circular face (not on the pinwheel's "sides").
  • At first he or she should blow lightly and note the rate the pinwheel turns. Then he or she should blow harder. How much faster does the pinwheel turn? Which "blowing rate" would produce more energy?
    • Note that sea water has a much higher (832 times greater) density than air. Thus, currents running at velocities of 5 - 8 knots (9.25 km/hr - 16.7 km/hr) have the same energy potential as a windmill site with windspeeds of 390 km/hr!
• At the La Rance tidal power station (see "Engagement" section), they have installed turbines that can spin on both the incoming and outgoing tides.
  • Have a student alternatively blow on opposite sides of the pinwheel's circular face.
    • Does it turn correctly in both cases?
    • Examine the design of the pinwheel's "blades"-- do they look the same on opposite sides?
    • Given the results of this experiment, what can the students conclude about the design of La Rance's new turbines?
6. Look at the water level plots (or high / low tide data) produced in Step 3.
• Would tidal energy plants at these locations be able to generate power 24 hours per day?
• In terms of timing, do you think that the supply of electricity from any tidal power plant would match the customer demand?
7. A "tidal fence" has been prosed as a alternate design to the tidal turbine-dam design.
   • One advantage of a tidal fence is that all the electrical equipment (generators and transformers) can be kept high above the water.
     • Scroll down to see the designs for the "tidal turbine-dam" and "tidal fence."
     • Can you name the advantages or disadvantages of one design over the other?

Explanation

• Over the past forty years, there has been constant interest in harnessing tidal power. Initially, this interest focused on estuaries, where large volumes of water pass through narrow channels generating high current velocities. But, increasingly the environmental costs of such a design became clear. Reduced tidal range would destroy much of the habitat used by wading birds, fish (such as salmon) would be unable to travel upstream to breed, and sediment trapped behind the barrage could quickly reduce the volume of the estuary. By the early 1990s, interest in estuarine-derived tidal power had declined, and scientists and engineers began to look at the potential of coastal currents.
• Tidal fences effectively block channels and if deployed across the mouth of an estuary they can be very environmentally destructive. However, in the 1990s their deployment in channels between small islands or in straights between the mainland and island has increasingly been considered as a viable option.
• According to experts, there are many sites around the world where tidal turbine dams would be effective. They believe the best sites could generate more than 10 megawatts of energy per square kilometer. The European Union has already identified 106 sites which would be suitable for the turbines, 42 of them around the United Kingdom.
• The more expensive that conventional sources of energy (coal, gas) become, the greater the interest in harvesting energy from the ocean will become. The hope is that future improvements in technology will make building "ocean power plants" feasible.