Ocean and Tidal Energy Technologies

Summary

Every continent on the planet is surrounded by a cleaner, safer, more efficient energy resource. As conventional energy supplies are depleted, means are being developed, and in some cases are in operation, to convert the energy found in waves, tidal currents, ocean and river currents, ocean thermal gradients, and offshore wind into usable electric power for utility-scale grids, independent power producers, and the public sector.

Definition and Basic Principles

The tides were the earliest source of obtaining power from the ocean. The requirements were simple: a dam to contain a head of water that was brought in by high tide and a water wheel to turn as the water was let out, thus generating the power. The obtaining of power from crashing waves was another dream of oceanographers, and it has finally been realized in a variety of modest projects that generate power in a number of ingenious ways. Another dream of oceanographers has been deriving power from fast-moving currents in estuaries on in the ocean itself. Several so-called in-stream devices have been developed, but the ultimate goal of obtaining power from Florida's famous Gulf Stream, the fastest-moving ocean current in the world, is still elusive although study is under way. Another interesting project was the famous ocean thermal energy conversion (OTEC) project to obtain power from the temperature difference between warm- and cold-ocean waters. This project has had some success in Hawaii, and is being looked at further by several countries including island nations. And a spectacular newcomer for obtaining power from the ocean is the development of offshore wind turbines. These giant windmills, whirling high on stilts, are enjoying great success in the coastal waters of Europe, although development in the United States has been slow.

Background and History

Power has been generated from the tides in Europe since at least the Middle Ages, and tide mills were common in Great Britain, France, Ireland, and along the east coast of the United States until the middle of the nineteenth century. By 2011, the world had four power stations generating electricity from the tides. The largest was located on the Rance River estuary in France and rated at 240 megawatts. Others were on the Annapolis River in Nova Scotia, Canada (rated at up to 20 megawatts), the Kislaya Guba, Russia, plant (rated at 1 to 2 megawatts), and the Xiamen, China, plant (rated at up to 3 megawatts). As of 2020, 535 megawatts of ocean energy capacity had been installed worldwide, according to Energy Post. The generation of power from the waves has been a more recent development. Small-scale installations are generating power in Scotland, Spain, and at other locations, and projects involving various new methods are under way.

How It Works

The model for a successful tidal barrage plant is on the Rance River estuary in Brittany, France. Here the tide range is about forty feet. As the rising tide passes through the circular openings in the barrage, which is located near the mouth of the estuary, rotors spaced at regular intervals in the circular openings generate power as the water level rises. After six hours, the tide will have turned and the flow of water is in the opposite direction. Then the rotors are turned 180 degrees so that the plant can generate power as the tide flows out to sea.

Wave Power. As many as a thousand ingenious devices have been designed to obtain electric power from waves. Several promising devices are in the developmental stage, but no single design has been judged the best. One type is the point absorber, a bottom-mounted or floating structure that can absorb wave energy coming from all directions. A second type is the terminator, which reflects or absorbs all the wave energy coming at it. Another type is the linear absorber, which is oriented parallel to the direction of the oncoming waves. It is composed of interlocking sections, and the pitching and yawing of these sections, because of the waves, pressurizes a hydraulic fluid that turns a turbine. A fourth device is an overtopping device, which, much like a hydroelectric dam, produces electricity by emptying a reservoir of seawater and thereby turning turbines. Factors that must be considered are the corrosive and occasionally violent marine environment; biofouling, which begins the moment any device is placed in the ocean; and competing uses and environmental degradation of potential sites. Advantages include consistent, predictable generation and the higher energy density of wave energy as compared to solar power.

Power from Tidal and Ocean Currents. The dream of obtaining power from the ocean's fast-moving currents may finally be nearing realization. Tidal generators are being tested in several estuaries and ocean channels, using either bottom-mounted rotors or rotors suspended from floating barges. Power generation is nearly continuous, with the rotors turning for both flood and ebb currents. The greatest challenge will be harnessing power from the ocean's famous Gulf Stream. The Southeast National Renewable Energy Center at Florida Atlantic University in Boca Raton, Florida, is building a rotor device that will be tested in this current as it flows between Florida and the Bahamas. Water depths approach 2,500 feet there, so attaching the rotors to the seabed will be a challenge. In addition, there will be the usual problems of biofouling, corrosion, and getting the power to shore. The rotors will constitute a potential hazard for passing ships, submarines, and large marine mammals, such as whales, so these concerns will also have to be addressed.

Ocean Thermal Energy Conversion(OTEC). An OTEC plant has operated successfully in Hawaii and is being considered by several European countries, despite its high cost. This plant derives power from the differential between 40-degree-Fahrenheit deep water and 80-degree-Fahrenheit surface water. The 40-degree-Fahrenheit water, drawn from thousands of feet down, is used to condense ammonia, which is then brought in contact with 80-degree-Fahrenheit surface water, causing it to vaporize explosively, driving a turbine. One problem is the disposal of the 40-degree-Fahrenheit water after it has been warmed in the vaporization process. It cannot be returned to the ocean, where it would kill reefs and tropical fish, so the solution in Hawaii was to pipe it through the soil where it fooled cool-weather crops such strawberries and asparagus into growing in a tropical climate. Another approach, which is planned for an offshore OTEC structure, is to mix warm and cold water to a temperature of 61 degrees Fahrenheit and return it to the ocean at a depth of 330 feet.

Wind Power. The latest method for obtaining energy from the ocean is giant offshore turbines. According to the Global Wind Energy Council (GWEC), China was the world leader in installing these turbines as of 2021 and was generating 27.7 GW. These huge turbines function much like windmills on land, except that they are firmly anchored to the seafloor, often in plain sight of coastal residents. Environmental and aesthetic concerns have long held up the installation of such turbines along the east coast of the United States. As of 2021, five turbines at the Block Island Wind Farm off the Rhode Island coast generated about 30 MW, while the Coastal Virginia Offshore Wind pilot project comprised two turbines in federal waters generating 12 MW. The first US commercial-scale offshore wind project, Vineyard Wind, began construction off the Massachusetts coast in November 2021.

Applications and Products

The number of tidal barrage power plants in the world is extremely limited because of the large tidal range required—a minimum of a ten-foot rise between low tide and high tide. Few coasts in the world have that great a rise in a body of water narrow enough to be dammed, and even fewer have it in an area that is sufficiently populated to provide a market for the power generated. One problem faced by all tidal barrage plants is that the power generated is intermittent. The time suitable for power generation shifts steadily as the Moon orbits around the Earth. This means that the supply of power and the demand for power will not always coincide. For several nights customers will have ample power to cook their supper and enjoy evening activities, but the next few nights they will have no power at all.

Power Generation from Waves. One installation that has been generating power successfully since 2000 is the Limpet, the world's first commercial wave-power station, on the rockbound coast of the Island of Islay in Scotland. The plant is of the oscillating water column design with a fortress-like exterior fronting the waves just at sea level along the rocky shore. When a breaking wave enters the long, concrete tube, with its opening just below water level, it drives air in and out of a pressure chamber through a specially designed air turbine, generating electricity. The Limpet produces 0.5 megawatts of power, which is fed into the island's power grid. The design makes the Limpet easy to build and install, and its low visible profile does not intrude on the coastal landscape or the ocean views. According to the International Energy Agency's Ocean Energy Systems (OES), as of 2021 installations were operational off the coasts of Australia (200 kW), France (150 kW), India (900 kW), Italy (128 kW), New Zealand (500 kW), South Korea (500 kW), and Spain (1,096 kW).

Power from Currents. Besides the proposed giant rotors in the Gulf Stream, which would be an open-ocean device, a number of projects have been designed for obtaining power from tidal currents in estuaries and other constricted passages using bottom- or surface-mounted turbines. These turbines do not require construction of an expensive barrage. The flow of water simply turns the turbine as the tide comes in, and, if the turbine is reversed 180 degrees, it can also generate power as the tide goes out. An experimental array of three turbines was installed in New York City's East River in 2020 and began testing in 2021. Similar arrays have been installed in the United Kingdom, Italy, Korea, Canada, China, and France. By 2021, more than 8,000 megawatts of tidal current capacity had been installed worldwide, according to the OES. The turbines strongly resemble torpedoes, with the rotor at one end, and they stand on a stout pedestal firmly attached to a channel bed or they can be suspended from floating barges. Power is transmitted to shore by means of cables.

Wind Farms. The major components for a wind turbine are a tower, a rotor with hub and blades, a gear box, and a generator. Offshore systems are larger than those on land because of the greater cost to install and service them. Most are rated at three to five megawatts, compared with one and one-half to three megawatts for those on land. They are usually fixed in water depths of fifteen to seventy-five feet and require a foundation driven deep into the seabed to support the weight of the tower and the rotor. Frequently the turbines are arranged in arrays to reduce maintenance and cabling costs. The power generated is sent to shore through a high-voltage cable buried in the seabed. A service area is always required, with boats and a hoist crane for repairs and maintenance.

Wind farms, both onshore and offshore, are becoming more common worldwide. GWEC that by the end of 2021, 837 GW of total wind power capacity had been installed worldwide, primarily in China, the United States, Germany, India, Spain, and the United Kingdom. In the United States, the leading states for installed wind power capacity as of 2020 were Texas, Iowa, Oklahoma, Kansas, and Illinois, according to the American Clean Power Association. GWEC figures showed that offshore wind farms were most common in the United Kingdom, China, and Germany.

Careers and Course Work

College-level students seeking careers in marine renewable energy are advised to take a general oceanography course to familiarize themselves with the characteristics of the environment in which they will be working. In addition, they should supplement this study with basic courses in math and physics so that they can understand the technology involved in the design of the various marine-energy projects. For those students planning to go on to graduate study, engineering courses are highly recommended, especially for those students interested in designing and building ocean energy generators. The jobs available in the United States for students seeking opportunities in the field of marine-renewable energy remain somewhat limited because most of the American marine energy projects operating are just getting under way or are still in developmental stages. Additional wind farm projects are planned but the permit process for them is a lengthy one. Several coastal wave energy projects are operating, as well as a few tidal stream projects in rivers and estuaries with strong tidal flows, but many are only in developmental stages. Once they mature into full-time operations, job opportunities in these areas should improve.

Social Context and Future Prospects

For many years the United States has been totally dependent on traditional energy sources, such as oil, natural gas, and coal, as well as water power and nuclear energy. Attention is turning to the generation of electricity from marine renewable energy sources. Tidal stream and wind farm installations have shown the most promise. The potential for them to make a significant contribution to US energy needs is great because nearly half the states have access to the oceans along their borders. Although the United States has ample supplies of coal, natural gas, and oil, the burning of these fossil fuels harms the environment. Attention to marine-energy sources cannot help but increase in the coming years. The deployment of an offshore wind farm for Rhode Island is an encouraging sign. Such wind farms are already making a significant contribution to energy needs in Europe, and coastal conditions along the Atlantic and Gulf coasts of the United States are equally favorable. The cost of these projects is large, but they will provide many jobs for the installation and maintenance of the equipment, as well as for the manufacture of the turbines.

Bibliography

Annual Report 2021, Ocean Energy Systems, International Energy Agency, 4 Mar. 2022, www.ocean-energy-systems.org/publications/oes-annual-reports/document/oes-annual-report-2021/. Accessed 8 June 2022.

Bedard, Roger, et al. “An Overview of Ocean Renewable Energy Technologies.” Oceanography 23, no. 2 (June, 2010): 22–31.

Boon, John D. Secrets of the Tide: Tide and Tidal Current Application and Prediction, Storm Surges and Sea Level Trends. Chichester, England: Horwood, 2004.

Carter, R. W. G. Coastal Environments: An Introduction to the Physical, Ecological, and Cultural Systems of Coastlines. San Diego: Academic Press, 1988.

"Global Wind Report 2021." Global wind Energy Council, 2022, gwec.net/global-wind-report-2021/ Accessed 8 June 2022.

Segar, Douglas A. Introduction to Ocean Sciences. 2d ed. New York: W. W. Norton, 2007.

Sverdrup, Keith A., and E. Virginia Armbrust. An Introduction to the World's Oceans. 10th ed. New York: McGraw-Hill, 2009.

Thresher, R., and W. Musial. “Ocean Renewable Energy's Potential Role in Supplying Future Electrical Needs.” Oceanography 23, no. 2 (June, 2010): 16–21.

Ulanski, Stan. Gulf Stream: Tiny Plankton, Giant Bluefin, and the Amazing Story of the Powerful River in the Atlantic. Chapel Hill: University of North Carolina Press, 2010.