Dpurdy's List: Tidal Energy

• Advantages      

  Once you've built it, tidal power is free.       It produces no greenhouse gases or other waste.         It needs no fuel.       It produces electricity reliably.       Not expensive to maintain.       Tides are totally predictable.       Offshore turbines and vertical-axis turbines are   not ruinously expensive to build and do not have a large environmental   impact.

   
   

   

    

  ---

   

    Disadvantages

   
   
   
  • A barrage across an estuary is very expensive to build,   and affects a very wide area - the environment is changed for many   miles upstream and downstream. Many birds rely on the tide uncovering   the mud flats so that they can feed. Fish can't migrate, unless "fish   ladders" are installed.
     
     
   
  • Only provides power for around 10 hours each   day, when the tide is actually moving in or out.
     
     
   
  • There are few suitable sites for tidal barrages
   
   
   
   
   
   
       

  ---

      Is it renewable?

  Tidal energy   is renewable.   The tides will continue to ebb and flow, and the energy is there for the   taking.

• Tides are caused by the gravitational pull of the moon and sun, and the rotation   of the Earth. Near shore, water levels can vary up to 40 feet due to tides.  

    

  Dam of the Tidal Power Plant on the Estuary of the Rance River, Bretagne, France

    

  Source: Stock photography (copyrighted)

    

   

  Tidal power is more predictable than wind energy and solar power. A large enough tidal range — 10 feet — is needed to produce tidal energy  economically.
• The United States has no tidal plants and only a few sites where tidal energy could be   produced economically. France, England, Canada, and Russia have much more potential to use this type of energy.
• Tidal turbines are basically wind turbines in the water that can be located anywhere there is strong   tidal flow. Because water is about 800 times denser than air, tidal turbines have to be much sturdier than wind turbines. Tidal turbines are heavier and more   expensive to build but capture more energy.
• Waves are caused by the wind blowing over the surface of the ocean. There is tremendous energy in the ocean waves. It's estimated that the total potential off the coasts of the United States is 252 billion kilowatthours a year, about 6% of the United States' electricity consumption in 2010.  The west coasts of the United States and Europe and the coasts of Japan and New Zealand are good   sites for harnessing wave energy.
• Many more ways to capture wave energy are currently under development. Some of these devices being developed are placed underwater, anchored to the ocean floor, while others ride on top of the waves. The world's first commercial wave farm using one such technology opened in 2008 at the Aguçadora Wave Park in Portugal.

3 more annotations...

• Power generation using wave energy is at a much earlier stage of
   development. Wave energy offers more predictable outputs than wind, but in early 2003
   there was only around one megawatt of generating capacity installed worldwide, all of it essentially with demonstration prototypes. Proposed projects are likely to take this to about 6 MW over the next few years. The wave industry is characterised by a wide variety of novel devices
•   

  Can Already Cost Under $.06 per Kilowatt Hour

   

  * Pneumatic devices, such as the oscillating water column (OWC), use wave motion to compress and decompress air, and drive a turbine.

   

  * Float-based devices utilise a buoyant float moving with the waves, reacting against a sea bed anchor in order to harness energy.

   

  * Spillover devices utilise wave height to replenish a reservoir of seawater, which runs a turbine.

   

  * Raft-type devices use the relative motion of adjacent rafts or pontoons to harness wave energy.

   

  * Moving-body devices articulate in the water, inducing motion, which may be used to drive a hydraulic motor.
• Tidal stream devices extract energy from the diurnal flow of tidal currents (caused by the gravitational pull of the moon). Unlike wind and wave power, tidal streams offer entirely predictable output. However, as the lunar cycle is of around 25 hours’ duration, the timing of peak outputs differs by around an hour each day and tidal energy cannot be guaranteed at times of peak demand.
• However, several large grid-connected demonstration projects are expected to enter the water in the near future. Tidal stream is thus a few years behind wave energy.
• Marine Current Turbines is about to field test a submerged 300 kW tidal turbine off Devon in the United Kingdom
• The manufacturers of all these devices expect to deliver
   energy at a cost of 10-14 US cents per kWh, falling to below 6 US cents as
   experience grows and technologies mature

4 more annotations...

• Where Does Tidal Energy Come From?

    

  The gravitational force of the Earth stops the ocean from floating off into space, just like everything else. The Moon also has a very weak gravitational effect on the Earth, which is not normally noticable as far as falling teacups are concerned, but the ocean, which can flow around the globe to even out differences, is noticably affected.

• Tidal energy is one of the oldest forms of energy used by humans. Indeed, tide mills, in use on the Spanish, French and British coasts, date back to 787 A.D.. Tide mills consisted of a storage pond, filled by the incoming (flood) tide through a sluice and emptied during the outgoing (ebb) tide through a water wheel. The tides turned waterwheels, producing mechanical power to mill grain. We even have one remaining in New York- which worked well into the 20th century.
• Tidal power is non-polluting, reliable and predictable.Tidal barrages, undersea tidal turbines - like wind turbines but driven by the sea - and a variety of machines harnessing undersea currents are under development. Unlike wind and waves, tidal currents are entirely predictable.
• A tidal range of at least 7 m is required for economical operation and for sufficient head of water for the turbines
• Currently, 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 (1 megawatt = 1 MW = 1 million watts) at the mouth of the La Rance river estuary on the northern coast of France
• Tidal energy is a renewable source of electricity which 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. Changing tidal flows by damming a bay or estuary could, however, result in negative impacts on aquatic and shoreline ecosystems, as well as navigation and recreation.
• There is a high capital cost for a tidal energy project, with possibly a 10-year construction period.
• 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.
• Tidal electricity can be used to displace electricity which would otherwise be generated by fossil fuel (coal, oil, natural gas) fired power plants, thus reducing emissions of greenhouse and acid gasses.

6 more annotations...

• 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.

• 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.
• 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
• Natural current driven tidal generators can be built into the structure of existing bridges. These generators will involve virtually no aesthetic problems.
• Tidal energy use may not be a big player in our energy future, but it can make a contribution.

3 more annotations...

•      

  Method	Cents/kW-h	Limitations and Externalities
  Wind Currently supplies approximately 1.4% of the global electricity demand. Wind is considered to be about 30% reliable.	4.0 - 6.0 Cents/kW-h	Wind is currently the only cost-effective alternative energy method, but has a number of problems. Wind farms are highly subject to lightning strikes, have high mechanical fatigue failure, are limited in size by hub stress, do not function well, if at all, under conditions of heavy rain, icing conditions or very cold climates, and are noisy and cannot be insulated for sound reduction due to their size and subsequent loss of wind velocity and power.
  Geothermal Currently supplies approximately 0.23% of the global electricity demand. Geothermal is considered 90-95% reliable.	4.5 - 30 Cents/kW-h	New low temperature conversion of heat to electricity is likely to make geothermal substantially more plausible (more shallow drilling possible) and less expensive. Generally, the bigger the plant, the less the cost and cost also depends upon the depth to be drilled and the temperature at the depth. The higher the temperature, the lower the cost per kwh. Cost may also be affect by where the drilling is to take place as concerns distance from the grid and another factor may be the permeability of the rock.
  Hydro Currently supplies around 19.9% of the global electricity demand. Hydro is considered to be 60% reliable.	5.1 - 11.3 Cents/kW-h	Hydro is currently the only source of renewable energy making substantive contributions to global energy demand. Hydro plants, however, can (obviously) only be built in a limited number of places, and can significantly damage aquatic ecosystems.
  Solar Currently supplies approximately 0.8% of the global electricity demand.	15 - 30 Cents/kW-h	Solar power has been expensive, but soon is expected to drop to as low as 3.5 cents/kW-h. Once the silicon shortage is remedied through alternative materials, a solar energy revolution is expected.
• Tide
  
  2 - 5 Cents/kW-h Blue Energy's tidal fence, engineered and ready for implementation, would provide a land bridge (road) while also generating electricity. Environmental impact is low. Tides are highly predictable.

• "But if we could harness 0.1 percent of the energy in the ocean, we could support the energy needs of 15 billion people.