How Ocean Power Works


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water crashing along the coast
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Water crashing along the coast is a clear indication of the ocean's awesome power and the energy potential it holds. See more green science pictures.

Just off Hout Bay, near Cape Town, South Africa, waves can grow to 30 feet (9 meters). Watch these mountains of water crash down on the reef- and rock-lined coast, and you'll have no doubts about the ocean's awesome power. But it doesn't take a visit to a surfing hot spot to appreciate the energy potential of our planet's watery realm. Tides can exchange millions of gallons of water in just a few hours. And currents, like underwater highways, can travel thousands of miles, carrying marine life, debris and nutrients.

Humans have been interested in harnessing the energy of the ocean for centuries. In 1799, a Frenchman teamed up with his son to design a giant lever attached to a ship. As the ship rocked on the sea, it would move the lever up and down, creating a reciprocating motion that could be used to drive pumps, mills and saws located on the shore. The idea sank when the steam engine emerged as the preferred method to perform mechanical work. Nearly a hundred years later, another Frenchman proposed a different technique: Use heat energy stored in Earth's oceans to generate electricity. A few plants were built to test the idea, but they were not cost-competitive with conventional power technologies.

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The first real success came in 1966, when a tidal power plant opened in Bretagne, France, on the Rance River. Today, the station produces 240 megawatts of power -- better than a typical wind farm, but less than a coal plant. Only two other tidal plants have had comparable commercial impact. The first is a 20-megawatt station in Nova Scotia, on the Bay of Fundy. The second is a 0.5-megawatt station located in Russia on the White Sea.

Based on such limited success, you might think that ocean power is doomed to languish in the shadow of other energy alternatives. But the tide, if you'll pardon the pun, is turning. Researchers all over the world are experimenting with scores of technologies to convert the energy of waves, tides and currents into electricity. And a few companies believe they can be competitive power generators within the decade.

This article will explain how ocean power works and will examine some of the most promising technologies currently being developed. Let's start with systems designed to capture the mechanical forms of ocean energy.

Ocean Power Systems: Waves, Tides and Currents

Bay of Fundy in Novia Scotia
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The Bay of Fundy in Novia Scotia is home to a 20-megawatt tidal station.

Two of the most noticeable features of the ocean are waves and tides. Winds drive waves, which travel for long distances as a series of crests and troughs. Watch any object floating at the ocean surface as it encounters a wave, and you'll notice that it rises up with the crest and falls with the trough. It's possible to convert this oscillating motion into electricity in a number of ways. How Wave Energy Works covers these various systems in great detail.

The gravitational pull of the moon, not wind, is responsible for tides. All coastal areas experience two high and two low tides over a period of about 24 hours, but only about 40 sites are suitable for electricity generation. That's because the difference between high and low tides must be at least 16 feet (4.88 meters) to create enough water flow to make electricity efficiently. The Bay of Fundy is a classic example. This relatively narrow inlet features the highest tides in the world -- 50 feet (15.24 meters) -- and a short tidal cycle. In about six hours, 110 billion tons (100 billion metric tons) of seawater flows in and out of the bay [source: Bay of Fundy Tourism].

One way to harness the kinetic energy of all that moving water involves building a dam, known as a barrage, on a smaller arm of the bay. Sluice gates along the barrage open when the tides produce an adequate difference in the level of the water on opposite sides of the dam. This allows water to flow across turbines that look just like those used in a traditional hydroelectric power plant. The turbines turn a generator, which produces electricity.

Another way to take advantage of ocean tides is to tap into tidal currents, which run close to the shore at water depths of about 65 to 100 feet (20 to 30 meters). To do this, power companies use turbines resembling those seen on terrestrial wind farms, except they are oriented so that the rotors are underwater. The rotors, each about 66 feet (20 meters) in diameter, are also spaced more closely than those on wind farms. As tidal currents surge past the turbines, the rotors spin, turning a generator.

These mechanical-energy systems are just one solution. Up next, we'll see how heat stored in the ocean can produce electricity.

Ocean Power Systems: Thermal Energy

Most people think of solar collectors as the typical silicon-based photovoltaic cells so commonly seen on residential and commercial buildings. But the largest solar collector is the ocean itself. Each day, the sun provides the equivalent of 250 billion barrels of oil in the form of thermal energy to Earth's oceans [source: Energy Efficiency and Renewable Energy, "Ocean Power" Lesson Plan]. Converting all of that potential energy into electricity requires a process scientists call ocean thermal energy conversion, or OTEC for short.

There are three types of OTEC systems. Each takes advantage of the temperature differential that occurs between warm surface water and colder, deeper ocean water, but they do it in slightly different ways.

Closed cycle systems contain a fluid that has a low boiling point. Ammonia, for example, has a boiling point of -28.01 degrees Fahrenheit (-33.34 degrees Celsius). When it's exposed to warm seawater in the system's heat exchanger, it immediately boils away into ammonia gas. The expanding vapor passes over a turbine, causing it to turn. Then it travels to a second heat exchanger into which cold seawater has been pumped. When the hot ammonia vapor encounters the cold water, it condenses back into a liquid and is ready for another cycle.

Get Your Juice -- with a Side of Water
One advantage of open or hybrid OTEC systems is that, on the way to making electricity, they also produce fresh water -- another resource in short supply in certain areas of the world. A single 2-megawatt OTEC plant could, in theory, produce 14,118.3 cubic feet (4,300 cubic meters) of desalinated water each day [source: Energy Efficiency and Renewable Energy].

Open cycle systems operate on a different principle. They start with warm surface water, which is placed into a vacuum chamber. As the vacuum pump removes air to create a low-pressure environment, the warm seawater boils. The resulting steam is almost pure water, and like the steam produced in a coal-fired power plant, it can be used to drive a turbine. Cold seawater, pumped from the ocean depths, cools the steam and changes it back into water.

Hybrid systems, as the name suggests, combine the best of their open and closed cousins. First, warm seawater changes into steam in a low-pressure container. The steam then vaporizes a low-boiling-point fluid in a closed-cycle loop that drives a turbine.

Currently, OTEC systems produce relatively small amounts of electricity. But the potential is enormous. Some experts believe that OTEC could produce billions of watts of power in the coming decades. As we'll see in the next section, these are the kinds of numbers that have driven energy companies to invest heavily in all types of ocean-power technologies.

Environmental Benefits of Ocean Power

Oceans cover more than 70 percent of Earth's surface, so clearly they represent an enormous energy resource. For many years, power companies have interested themselves primarily in the nonrenewable energy sources -- oil and natural gas -- trapped beneath seafloor sediments. But recently, as prices of fossil fuels continue to climb, the ocean's renewable energy sources have become more attractive. In the near term, power generation based on waves, tides and currents will likely contribute only a small sliver of the energy pie. In the longer term, that sliver should grow into a sizeable wedge.

From R&D to Reality

Concepts and patents abound when it comes to ocean power. But a few companies are getting closer to making their ocean-powered technologies commercially feasible. Here are some of the power players:

Verdant Power is testing its underwater turbines in New York City's East River. By 2010, the company plans to have 30 turbines in place to provide one megawatt of energy. Potentially, the East River site could produce up to 1,000 megawatts.

Pelamis Wave Power offers a wave energy converter that looks like a giant snake. Floating tubes, each 427 feet (130 meters) in diameter and weighing 750 tons (680 metric tons), bob on the heaving ocean, converting wave energy into electricity by pumping high-pressure fluid through motors. Pelamis has placed three of its tubes off the coast of Portugal, with the goal of producing 2.25 megawatts of energy.

Ocean Power Technologies takes a different approach to extract wave energy. Its patented PowerBuoy rises and falls on passing waves. This up-and-down motion is converted into electricity by way of a pistonlike structure that drives an electrical generator. Just off the northern coast of Spain, the company is in phase two of a test site that will produce 1.39 megawatts of energy.

Part of the appeal, of course, is that ocean power is a never-ending resource. The ebb and flow of the ocean has been ceaseless for billions of years. We can be reasonably assured that it will remain in motion far into the future. Not only that, ocean power is, for the most part, predictable. Tide charts list the high and low tides down to the minute, while maps show both the dimensions and speed of Earth's major currents. Oceanographers know, for example, that the Gulf Stream is about 56 miles (90 kilometers) wide at its core and travels at average speeds of 4.5 miles per hour (2 meters per second) [source: The Cooperative Institute for Marine and Atmospheric Studies].

Another key aspect of ocean power is related to the density of seawater. The density of air is about 1.25 kilograms per cubic meter. The density of seawater is about three times that, which means it can transmit more energy to turbines placed in the ocean [source: Physics Factbook]. Currents running no faster than 5 miles per hour (8 kilometers per hour) can turn a tidal turbine more than 30 revolutions per minute, which delivers better output than a wind turbine of comparable size operating in strong winds.

Finally, ocean power systems are easier on the eyes than wind and solar systems. They require less space and far fewer units than wind farms or solar arrays. Moreover, the equipment used to deliver ocean power is located offshore, either on the surface or below the surface, so the systems don't block views or interfere with aviation or radar. They also run silently, unlike wind turbines, which can produce aerodynamic noise, which some have described as a buzzing, whooshing, pulsing and even sizzling sound.

But don't think ocean power is free from controversy. In the next section, we'll explore some of the concerns surrounding wave, tidal and current energy systems.

Concerns about Ocean Power

manhattan bridge over east river
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Verdant Power is testing its underwater turbines in New York City's East River, which potentially could produce up to 1,000 megawatts.

Despite its enormous potential, ocean power has contributed very little to global electricity production. Like all renewable energy providers, ocean power companies must overcome several hurdles to rival the market share of fossil-fuel suppliers. One of the biggest hurdles is reliability. Marine environments can wreak havoc on mechanical systems, making them difficult to maintain. Verdant Power installed six tidal turbines in New York City's East River in 2006, only to find that strong tidal flows damaged all but two of the systems.

Issues like these make it difficult for ocean power to compete with fossil fuels on cost. Some estimates put the cost of wave energy between 9 and 16 cents per kilowatt-hour. For tidal energy, which benefits from advancements made in wind technologies, the cost is somewhat lower -- about 6 to 9 cents per kilowatt-hour [source: Ocean Renewable Energy Coalition]. But neither is nearly as cost-effective as coal, which costs just 3 cents per kilowatt-hour, or natural gas, which costs 4.7 cents per kilowatt-hour [source: Wald].

Then there are environmental issues. Placing large man-made structures in the oceans will clearly have some effect on marine life. Tidal barrages present perhaps the biggest challenges. The turbines in these structures can kill fish and impede their migration to spawning areas. Barrages also interfere with the normal flushing of silt and other dissolved pollutants. This affects water quality, which in turn affects bird and fish life. The table below summarizes some of the major concerns associated with ocean power.

Source of Ocean Power

Concerns

Waves

Variable intensity, limited survivability of equipment, navigation and sea-space concerns, release of lubricants

Tides

Slack intervals, high capital costs, limited to a handful of sites worldwide, major environmental impacts

Tidal Currents

Limited survivability of equipment, high operational costs, less widespread than waves

Ocean Currents

Limited number of sites, potential impact on ocean circulation patterns

Ocean Thermal Energy

High capital costs, limited to sites in tropical oceans, sites far from land must transmit electricity long distances


Of course, all of these issues must be weighed against the benefits of ocean power. The biggest is its ability to produce carbon-free energy. That alone may make ocean power one of the most important energy sources in the coming decades.

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Sources

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