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.