Tapping into Ocean Power

The oceans of the world are a vast unexploited source of clean, reliable and predictable renewable energy. Could this energy help replace fossil fuels and be a solution to climate change?

Humans have been trying to harness ocean energy for centuries, beginning with a French engineer named Pierre-Simon Girard in 1799. The constant pounding of the waves and the ebb and flow of tidal currents, as well as other properties of the ocean, if harnessed, could produce 20,000 to 80,000 terawatt hours of electricity, according to the International Energy Agency. That is more than the world’s current energy consumption of almost 20,000 TWh

The Electric Power Research Institute estimates that the waves breaking along the U.S. coastline alone could generate 2,640 TWh each year. But since shipping, fishing, naval operations or environmental concerns take precedence in certain areas, the amount of power that is “recoverable” is estimated at 1,170 TWh a year, almost a third of the amount of electricity the U.S. consumes each year.

Energy is inherent in the movement of ocean waves, in the difference in temperature between warm surface waters and cooler deep waters, in the disparity in salinity between fresh water and salt, and in marine currents and tides. The International Energy Agency estimates that wave power could potentially produce 8,000 to 80,000 TWh yearly; ocean thermal energy could produce 10,000 TWh; osmotic power (from salinity differences) could produce 2,000 TWh and tides and marine currents could produce 1,100 TWh. Ocean thermal energy, osmotic energy, marine currents and some types of wave energy could produce base load power, electricity that is consistent and reliable.

The areas with the most wave energy potential are the Pacific Northwest and Alaska in the United States, and the U.K. and Scotland. Tidally driven waves running along the coasts of China, Korea and parts of Europe hold the most promise for dynamic tidal power (see below), while tropical oceans along the equator are the best places to exploit ocean thermal energy.

Researchers have developed a variety of designs and devices to utilize ocean energy. Here are some examples.

Wave Energy

Point absorber buoy  – A floating buoy is anchored to a stabilizing base on the seafloor. As the buoy bobs up and down with the waves, it moves a generator in the shaft that connects the buoy to the base, creating electricity. The electricity is pumped to a collection device that moves the electricity to the grid onshore.

Surface attenuator –  This device has multiple arms that float on the surface. The undulation of the waves creates a flexing motion at the joints connected to hydraulic pumps that drive a generator, producing electricity. The electricity is carried to shore via a cable under the sea.

Oscillating water column  – A concrete structure is constructed with an enclosed chamber and an opening below sea level. As waves enter through the opening, the water level in the chamber rises, forcing the air above it through a turbine connected to an upper opening in the chamber. The airflow drives the turbine that generates electricity. When the wave ebbs, the air is sucked back into the lower chamber, turning the turbine in the opposite direction and generating electricity.

Overtopping device  – This structure can be onshore or float off shore. It captures waves that break over it into a storage reservoir. The waves then go through a turbine in a chute that generates electricity as the waves pass through it back to the sea.

Wave carpet – A flexible membrane extends along the ocean floor and moves up and down as waves run over it, pushing seawater to a discharge pipe through vertical pumps connected to the underside of the membrane. The pressurized water from the pipe drives a turbine on shore that can generate electricity. The carpet captures 90 percent of the waves’ energy (compared to solar panels that only capture 20 percent of sun’s energy).

Oscillating Wave Surge Converter  – The device has one end fixed to a structure or the sea floor and another that moves as it is pushed by the waves. The movement forces a piston to move in and out, driving pressurized water through piping to a turbine onshore that produces electricity.

Ocean thermal energy

Seventy percent of the sun’s energy coming to Earth lands on the ocean and most is captured in the surface layers of the ocean as heat. The difference in temperature between warm surface waters and the colder deep water, which is usually at least 20˚C, can generate electricity.

Both warm and cold seawater are pumped into heat exchangers that separate the different fluids. Ammonia, which has a low boiling point of room temperature, is fed into one heat exchanger. The warm seawater in an adjacent heat exchanger boils the ammonia to create vapor. The pressurized vapor goes through a pipe to run a turbine connected to a generator that produces electricity. After the ammonia vapor leaves the turbine, it comes down through a pipe into a chamber surrounded by tubes of cold seawater. The ammonia vapor is cooled and becomes a liquid again so it can continue the cycle.

Osmotic power

Osmotic power technology is based on the natural phenomenon that water with a low concentration of salt seeks a higher concentration of salt. When waters of different salinity meet at the mouth of a river and combine to reach a uniform salinity, they release energy. Osmotic power stations replicate this occurrence using osmosis—separating a tank of fresh water from a tank of salt water with a semi-permeable membrane that allows fresh water to pass through, but not the salt water. Fresh water is drawn into the saltwater side, equalizing the salinity, and raising the water pressure in the saltwater tank. That pressure is used to drive a turbine that produces electricity.

Tides and Currents

Tidal barrage  – A dam-like structure is built by a bay, lagoon or river. At high tide, the dam holds the water out. When sluice gates open to let the water flow in, turbines at the sluice gates generate electricity as the water moves through. When the lagoon fills up, the process is reversed, generating more electricity.

Dynamic tidal power – A dam 30 or more kilometers long is built perpendicular to the coast and fitted with many turbines. A barrier parallel to the coastline at the far end of the dam helps intensify water pressure on both sides of the dam. The pressurized water is driven through the turbines in the dam. A 40-kilometer dam could hold 2,000 turbines of 5 MW each, producing 10 GW of electricity, enough to power millions of homes.

Tidal current turbine  – A tall turbine (much like a wind turbine) anchored to a base, is placed on the sea floor. The tidal currents move the rotors, generating electricity. When the tide goes out, the rotors reverse direction and continue to generate electricity. Electricity is sent to the grid on shore via a cable.

Ocean power has significant potential as a renewable energy resource, yet it is decades behind other forms of renewable energy because it faces numerous challenges.

Devices of steel or structures of concrete must stand up to the constant pounding of waves and the corrosion of salt water.

“Anything on the ocean is difficult. It’s an unforgiving environment. Part of the problem is that the ocean is rougher because the energy is denser,” said Klaus Lackner, senior scientist at the Earth Institute’s Lenfest Center for Sustainable Energy and director of the Center for Negative Carbon Emissions and professor of sustainable engineering at Arizona State University. Although Lackner does not work directly with ocean energy, he has given a lot of thought to its potential as a solution to climate change.

If the ocean power industry is to be scaled up, devices must not utilize hard-to-obtain materials or be too complex; they must be easy to maintain and economical. According to the Northwest National Marine Renewable Energy Center, which researches wave energy technology, “The key technology challenges are associated with not only electrical generation and output, but mechanical systems, mooring and anchoring, survivability and reliability, predictability (wave forecasting), and integration of the generated power into the existing electrical grid.”

Globally, there are hundreds of different designs for wave energy conversion, but few have been tested.

“We are not sure what the best type of wave energy converter is yet,” said Ted Brekken, associate professor in energy systems at Oregon State University and co-director of the Wallace Energy Systems and Renewables Facility. “It’s complicated, because some may be able to capture more energy, but others may be cheaper or more robust. The ability to effectively convert energy is not the only thing that’s important. We have to look at the cost/benefit.”

In terms of cost, the ocean energy industry is affected by the overall energy market. “Right now, wave energy is three to five times more expensive than wind,” said Brekken. “With fracking and natural gas being relatively inexpensive, it’s difficult for all other energy forms to compete.”

There are also environmental impacts to consider. Chemicals used in anti-corrosion coatings or grease for the machinery can leak into ocean waters. The moving parts of devices can harm resident or migratory wildlife; and even static installations can alter breeding and feeding behavior if animals try to avoid the devices. Entanglement in cables or moorings is also a threat to wildlife. The electromagnetic fields generated around cables can cause changes in behavior, migration or reproduction. Noise from devices can also interfere with behavior and affect hearing or nerves, as sound travels easily in water. Lighting on devices, needed to guide ships or planes, might attract creatures, impacting normal behavior.

Brekken and his colleagues studied the environmental impacts of wave energy converting devices such as leaks of oil, electromagnetic fields, the physical device itself and mooring lines. And because anything that sits in the water gets colonized, they looked at the impact of “biofouling” on the behavior of local marine life. “There was no single thing that we found that had a deeply concerning environmental impact,” he said. “The effects were pretty typical of other marine vessel issues, like boats and navigational buoys. So the strategies to deal with the impacts are similar…such as bringing things to dry dock to clean them off, or burying or shielding lines to deal with the electromagnetic fields.”

Ecosystems can also be affected. By altering or removing energy from the physical environment, devices can change water flow, which may affect water quality, wave height, the delivery of nutrients and the natural transport of sediment that ensures coastal protection. These impacts could eventually disrupt food webs and the stability of the ecosystem.

Lackner is concerned about the impacts of altering local currents upon which entire ecosystems might be based. “By the time you pull some fraction of energy out of the waves, you will have an effect on the ocean currents,” he said. “Whole areas could be affected. Waves come from a long way away. If you intercept them halfway, say at Hawaii, waves at the shore will be smaller. … And with ocean thermal energy, you are bringing cold water to the surface, changing the conditions of the surface, which will have impacts on nutrients and carbon dioxide.”

“In some locations, ocean energy could be incredibly advantageous,” said Lackner. “But it’s a dumb idea to think we could run the world on it. We’d reach a point where…we’d be pushing beyond the limit of what is environmentally acceptable.”

Various attempts at ocean energy have been attempted and shelved. In Portugal, the world’s first commercial wave power plant closed in 2009 because it was too difficult to maintain. A small experimental osmotic power plant in Tofte, Norway, shut down in 2014 because it was not economically feasible. Ocean Power Technologies, a company developing wave energy, had planned a large-scale wave power project off the coast of Oregon in 2014 but could not raise enough money.

But there are also a few success stories.

The world’s largest tidal barrage has operated in Sihwa Lake, South Korea, since 2011. The 12.7-kilometer power plant produces 254MW of power.

The world’s first commercial-scale tidal turbine was installed in 2008 at Strangford Lough, a large sea inlet in Northern Ireland. The 1.2MW turbine installation has been producing electricity since.

The testing of the U.S.’s first wave power project began in 2016 at the Navy’s Wave Energy Test Site in Kaneohe Bay, Hawaii. It comprises two buoys of different designs a half-mile to a mile from the shore; one produces 18KW of electricity, while the other produces 4KW.

And there are plans for large new projects.

China has issued a five-year plan to develop ocean energy from tides, waves and temperature differences. A $40 billion dynamic tidal power project proposed by Dutch engineers is being planned along the coast between Xiamen and Shantou. The goal is to build a 60- to 100-kilometer-long dam containing 4,000 turbines to generate 15GW of electricity.

U.K.-based MeyGen is developing the world’s first multi-turbine tidal energy installation. Initially, four turbines will be deployed in a strait off the north of Scotland. If successful, there are plans for 100 more turbines over the next 10 years, enough to generate 398MW of electricity.

The U.S. has invested about $334 million into marine energy research over the last 10 years, but the U.K. and Europe have put in over $1 billion, according to the Marine Energy Council.

Brekken said that the U.S. Department of Energy has an active water power program that puts out requests for proposals, offers support in the development of technology and fosters collaborations between universities, but the dollar amount for ocean energy is much less than that for other renewable energy sources. The Department of Energy Office of Energy Efficiency and Renewable Energy also promotes innovation in the industry with its Wave Energy Prize, whose goal is to develop more efficient wave energy converting devices and lower their costs.

“We need enough support for the industry and for companies to survive long enough to get their products into the water,” said Brekken. “We need years of experience in the water.”

Lackner believes that we will not be able to take enough energy from the ocean to deal with climate change without critically impacting the environment and climate since oceans play a fundamental role in regulating the global climate. “If there is a place where you could make ocean energy work and it would contribute five to 10 percent to the overall decarbonization of society, that would be good,” he said. “This could be an optimal source of energy in some places, but it can’t be big enough to meet human consumption needs. It won’t be big enough to be a major player in dealing with climate change.”

Meanwhile the Ocean Energy Systems Technology Collaboration Programme, an intergovernmental collaboration between countries focused on developing ocean energy that is environmentally sound, is aiming to grow ocean energy capacity to meet 25 percent of the world’s needs by 2050, which would save almost one billion tons of CO2 emissions.

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