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Could the moon be the solution to a future energy crisis? See more moon pictures.

2008 HowStuffWorks

How can the moon generate electricity?

If you discount all of the wars, conflicts, consumer enslavement and carbon dioxide emissions it's generated, oil has been pretty good to humanity. It's fostered our global development at warp speed pace. But oil is finite -- it's a nonrenewable resource, which means that we will eventually run out of it.

The sense that humanity is racing against the fuel-source clock makes alternative energy a front-and-center issue. Ideas like switchgrass ethanol, biodiesel and wind and solar power may soon actually power the globe. But each of these still has its own obstacles to large-scale energy production. So researchers continue to look around our planet for novel ways to power it.

Some believe we've already hit peak oil -- the point at which our supply, provided by oil rigs like this one in Azerbaijan, declines.

Wojtek Laski/Getty Images

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Some researchers are looking beyond our planet to the night sky. It turns out, there's a way that we can generate electricity from the moon -- thanks to the tides created by the gravitational pull the moon exerts on Earth's oceans. The Earth is tugged by the sun and moon. The sun dwarfs the moon in size, but the moon is much closer to Earth -- around 239,000 miles away, compared to the distance of 93 million miles between the sun and the Earth. Proximity trumps size when it comes to tidal movement here on Earth: The moon exerts more than twice as much gravitational force on Earth than the sun does [source: Office of Naval Research].

Think of the water that's found on Earth as a single, rubbery covering that encapsulates the planet. When the moon pulls this covering toward it, it stretches so that it thins out and widens on each side. These are the swollen, high tides. The wrapper is stretched thinnest on the top and bottom. This is where the low tide is found. The moon's pull is constant; it's the rotation of the Earth on its axis that causes different areas to experience high and low tides.

Because there are these predictable tides on Earth, some places around the planet are being powered by tidal movement. Find out how on the next page.

Tidal turbines: Imagine these windmills, outside San Diego, Calif., below the sea.

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Underwater Turbine Electricity Production

The moon's gravitational pull on water bodies creates tides. In turn, this movement creates kinetic energy that is carried by the water. Anything that moves has kinetic energy -- whether it's wind or a ball rolling down a hill. Kinetic energy can be captured by humans through windmills. Researchers are trying to tap into the power of the tides through a design similar to a windmill

Underwater (or tidal) turbines are a fairly straightforward concept, as far as cutting-edge energy technology goes. They are essentially windmills installed onto an ocean floor or river bed. The underwater current produced by the tides spins blades arranged like an airplane propeller. These turbines are attached to a gear box, which is connected to an electrical generator. This produces the electricity that is carried by cable to shore. Once it's plugged into an electrical grid, the electricity can be distributed [source: New Scientist].

Although underwater turbines are essentially the same thing as windmills, they have a few advantages over their above-ground cousins. Windmills require land, especially wind farms -- assemblages of dozens or hundreds of windmills. The future of land use (how land is developed and what it's used for) is becoming a major topic of discussion. With 6 billion people on the planet and counting, space is at a premium

-- not just for housing, but for crop production and more. Underwater turbines overcome this problem.

Another advantage of underwater energy capture comes from water's high density. Water is denser than air, which means that the same amount of energy can be produced by an underwater turbine as a windmill, but at slower speeds and over less area. What's more, while the amount of wind that passes over any given area of land can be unpredictable, the kinetic energy of tidal areas is dependable. The ebb and flow is so predictable, a given tidal region can be expressed in the amount of kilowatt hours of electricity it can produce per turbine.

Scientists have been examining the amount of energy found in a tidal pool in monthlong periods. There are two main measurements. Mean spring peak velocity is the highest velocity of tidal movement that can be found in an area during a single month. Mean neap peak cycle is the lowest point in velocity that a tidal area experiences in a month [source: Carbon Trust]. These two measurements can help approximate the greatest and least amounts of velocity found in any given tidal pool over the course of a month.

Besides the tides, there are other characteristics that affect the velocity of water. The surrounding terrain -- for example, whether the area is rocky or sandy -- determines how water moves. Whether a tidal area is narrow or wide can also impact velocity. A narrow channel can concentrate water's movement, causing it to speed up.

Tidal movement and water bodies' characteristics can be taken into account on paper, but it's not until real-world tests are undertaken that actual understanding of the impact of tidal turbines can begin. On the next page, find out about some projects around the world that are helping researchers to better understand underwater turbine power production.

Small-scale projects are underway to reveal the real impact underwater turbines will have on the ecology of tidal areas.

Jeffrey L. Rotman/Time Life Pictures/Getty Images

Diving in without testing the water?

Aquatic researchers have a pretty good handle on the movement of water in tidal areas, but some factors remain unknown. Some researchers fear that humans may push underwater turbine technology along quickly, without fully understanding the impact it can have [source: Roach]. What happens when vast numbers of underwater turbines are concentrated in tidal areas? While energy cannot be created or destroyed, it can be captured and transferred to other uses, like fulfilling our electrical needs. But remember that the kinetic energy captured in the ocean was serving the aquatic environment -- perhaps in ways we don't fully understand yet.

One reason some fear we may jump into tidal turbine energy production too quickly is that the technology is attractive. Underwater turbines produce no CO2 emissions. And the technology is benign: Turbine energy production is passive, simply capturing some of the kinetic energy found in the tidal movement and transforming it into electricity.

There's little data concerning the impact underwater turbines can have on marine ecosystems. A rapidly spinning blade could easily turn a little fishie into chum with a single swipe. Fish lovers will be happy to learn that, at present, underwater turbines spin slowly -- one set turns at 10 to 20 rotations per minute (rpm) [source: New Scientist]. Turbines that move a couple feet per second don't pose much of threat to fish. But what about next-generation turbines that may rotate at a faster rate?

The lack of understanding of the turbines' environmental impact goes both ways, too. Questions remain as to what kind of effect the aquatic environment will have on the technology. For example, will barnacles accumulate on the turbines or rotors, slowing or even stopping them?

To answer these questions, underwater turbine pilot projects have been created throughout the world. The first to produce electricity is found on the bottom of the Kvalsund Channel in Norway. This turbine features 33-foot-long blades that spin at a rate of 7 rpm and stands nearly 66 feet tall at its highest point beneath the water's surface. In September 2003, the turbine's generator was connected to the electrical grid of Hammerfest, a local village. The single turbine produces 700,000 kilowatt hours annually -- which provides power to an average of 35 homes in the area [source: Roach].

Another group is evaluating the impact of turbines on aquatic life. Verdant Power deployed five 35-kilowatt turbines with equipment to monitor the surrounding aquatic life. Fish are detected and tracked within 54 feet (18 meters) of the equipment and the data is recorded. So far, no fish has been struck by the turbines [source: Verdant].

Verdant is also involved in a project in New York's East River. Six 35-kilowatt tidal turbines were installed in a channel with currents that flow at a rate of up to 4 knots (about 4.6 miles per hour). The six turbines currently produce electricity that powers a grocery store and parking garage nearby. Verdant is planning to add more turbines to the channel, which should produce enough electricity to power 4,000 homes [source: Popular Mechanics].

The United Kingdom is also studying the potential for underwater turbine electricity production. The U.K. company Marine Current Turbines has planted a pair of turbines attached to a single pile, driven into the floor of the North Sea off the coast of Ireland. The turbines are massive; each blade is 60 feet long. As they spin, the turbines produce 1.2 megawatts of electricity [source: New Scientist].

For more information on energy and other related topics, visit the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great Links­So­urces
  • Kleiner, Kurt. "Underwater turbines set to generate record power." New Scientist. August 21, 2007. http://technology.newscientist.com/article/dn12519-underwater-turbines-set-to-generate-record-power.html
  • Roach, John. "Underwater windmill helps power Arctic village." National Geographic. October 9, 2003. http://news.nationalgeographic.com/news/2003/10/1009_031009_moonpower.html
  • Sofge, Erik. "Underwater wind turbines tap river energy." Popular Mechanics. April 2007. http://www.popularmechanics.com/science/earth/4213223.html
  • "Environmental monitoring: Overview." Verdant Power. 2007. http://verdantpower.com/what-environmonitor
  • "Ocean in motion: Tides - characteristics." Office of Naval Research. http://www.onr.navy.mil/Focus/ocean/motion/tides1.htm
  • "Pilot project." Florida Atlantic University Center of Excellence in Ocean Energy Technology. 20007. http://coet.fau.edu/?p=pilot
  • "Tidal stream resource and technology summary report." Carbon Trust. July 2005. http://www.carbontrust.co.uk/NR/rdonlyres/19E09EBC-5A44-4032-80BB-C6AFDAD4DC73/0/TidalStreamResourceandTechnologySummaryReport.pdf
  • "Tidal streams and tidal stream energy device design." Carbon Trust. 2008. http://www.carbontrust.co.uk/technology/technologyaccelerator/ME_guide3.htm
  • "World." CIA World Fact Book. March 20, 2008. https://www.cia.gov/library/publications/the-world-factbook/geos/xx.html