Technology Solutions: Thermal energy from the sea seeing renewed

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Technology M Solutions

Islands off the west coast of Scotland will be remembered for more than just their whiskey, now that wave power technology has moved ashore. The world’s first commercial wave power station began operating on the Island of Islay last November. If all goes as planned, enough power will be generated on the island for about 400 homes from a device called the Land-Installed Marine-Powered Energy Transformer (Limpet), which was codeveloped by Queen’s University Belfast and U.K.-based Wavegen. Many are watching to see how well Limpet fares, as wave energy has the potential to be a major contributor to renewable energy and the reduction of greenhouse gas emissions. Power from Limpet, which is rated at 500 kW, is being sold to Scottish Power and Scottish and Southern Energy for 5.95 pence/kWh (≈8.9¢/kWh in U.S. currency) under a 15-year power purchase agreement, according to David Langston, Wavegen’s business development manager. In comparison, the average price paid by the United Kingdom’s national grid is 2–2.5 pence/kWh (3–3.7¢/kWh). The price of wave power could come down as the technology improves, but “In general terms, the cost of the power is subject to the cost of local labor and concrete, which turns out to be a significant element for Limpet,” says Langston. Limpet is a shoreline device, consisting of an oscillating water column (OWC) made up of three 6 m2 concrete tubes set at an incline. “The bottom end is open to the water, and the top end is closed off and connected to a Wells turbine,” explains Langston. “The slow up and down movement of the ocean causes the water inside the tube to oscillate up and down. The compression of the air above the OWC forces air out of 80 A

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Wavegen

Wave power goes commercial

Limpet is built into the shoreline to better protect it from severe weather and keep it from being an eyesore.

the turbine, and when the water level goes back down, the air gets pulled back in,” he says. What makes Limpet possible is the Wells turbine, invented by Alan Wells, which rotates only in one direction even though air passing through it moves in two directions, says Langston. “All of this is achieved without any fancy mechanism, just a simple device with fixed blades,” he adds. One of the biggest downfalls for wave power technologies has been their inability to withstand storms. Limpet’s predecessor, the Osprey, was destroyed in a 1995 storm. To avoid the same fate, Limpet is made of solid concrete and is built into the shoreline, getting support from an existing cliff edge. Other demonstration wave power plants have also been damaged by storms, although they have provided information regarding how to (or not to) design and construct commercial

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 3, 2001

plants. As part of the European Commission’s JOULE program, a 400-kW OWC pilot plant was built on the island of Pico in the Azores (Portugal). Tests at the plant were delayed several times because of bad weather but eventually began in August 1999, according to António Falcão of the Technical University of Lisbon. “Full rotational speed and power were reached in October [1999],” he says. A few days later, however, another flood occurred, and the decision was made to relocate the electrical and control equipment to a different floor of the plant. The equipment was reinstalled at a level higher in October 2000, and the plant is expected to be operational in early 2001. Limpet is the first to go commercial, but a surge of wave power technologies under development may not be too far behind. U.K.-based Ocean Power Delivery, Ltd. (OPD), also has © 2001 American Chemical Society

plans for the west coast of Scotland. In contrast to Limpet, which sits in the shoreline, OPD is developing a semisubmerged offshore wave energy converter called Pelamis. Offshore devices have an advantage over nearor on-shore devices, in that they are capable of harnessing more energy per meter of wave front. In deep water (40 m or more), waves can deliver about 70 kW/m, whereas at the shoreline, they bring in only about 20 kW/m. However, shoreline devices do not need expensive cabling and are easier to connect to the grid. OPD was awarded a contract under the Scottish Renewables Obligation Third Order to install a pair of 375-kW Pelamis devices in Machir Bay, Islay. According to the company, the prototype devices are scheduled to be installed in early 2002 and should provide enough power for 150–200 homes. Several devices can be linked together through a single seabed cable. By 2010, OPD hopes to install as many as 900 of the devices in a large array format or “wave farm”, which could dramatically cut the cost of wave power and provide enough power to supply a small city. Several other companies also have plans to develop wave power arrays rather than individual devices in the next couple of years. “It affects the economics. If we build more, the power cost comes down,” says Wavegen’s Langston. In other parts of the world, wave power is also gaining interest. Australia’s Energetech has developed a shoreline device that includes an OWC and a unique turbine that adjusts the blade pitch depending on the conditions and energy content. Energetech’s system is also different from other shoreline OWC devices in that it uses a parabolic wave focuser to converge all of the energy of a wave crest onto a single point, allowing the energy to be extracted more readily. “The parabolic wall allows significantly more energy to be available at a minimal extra capital cost, and the new turbine greatly increases the efficiency,” says Tom Denniss, Energetech’s founder. Energetech is implementing its technology in Port Kembla, about 80 km south of Sydney, Australia. According to Denniss, the plant should start operating toward the end of 2001. Although Energetech is based in Australia, the company is

investigating the wave power market from a global perspective. When ES&T contacted Denniss, he was in Vancouver, checking out the possibility of building a wave power plant on Vancouver Island. “We also have a likely plant in Bilbao, Spain, to possibly start construction [in 2001],” he says. Several other prototype devices have been tested or are under development in various countries around the world, including Japan, Norway, Sweden, the Netherlands, Denmark, China, India, and Indonesia. In the United States, however, wave energy activity is relatively limited. Accord-

ing to the U.S. National Renewable Energy Laboratory (NREL), “The greatest wave energy potential in the United States is on the coasts of Washington, Oregon, and California.” In the United States, ocean thermal energy conversion (OTEC) is more widely pursued than wave energy conversion, primarily because ocean thermal energy is fairly constant, whereas wave energy is intermittent. NREL researchers worked on the preliminary design of an OTEC power plant in Hawaii (see sidebar); however, NREL is currently not involved in ocean energy research. —BRITT E. ERICKSON

Thermal energy from the sea seeing renewed interest Most of the worldwide research and development of ocean thermal energy conversion (OTEC) to date has taken place at the Natural Energy Laboratory of Hawaii Authority (NELHA), where a 250-kW pilot plant, the largest such plant ever put into operation, was tested in 1999. The plant is not currently producing electricity, but the pipes that were laid for its operation are still pumping seawater for aquaculture and desalinization activities. The technology uses the heat energy stored in tropical oceans to generate electricity. The key is to have a temperature difference of about 20 °C between the surface and deep water layers. In one design, a closed-cycle system, OTEC is essentially refrigeration technology expanded to a larger size, says Thomas Daniel, NELHA’s scientific director. Heat transferred from the surface layer causes a working fluid, typically ammonia because it boils at a low temperature, to turn to vapor. The expanding vapor drives a turbine attached to a generator, which produces electricity. Cold seawater passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid that is then recycled through the system. Another design is based on an open-cycle system, in which the warm surface water is used as the working fluid. The water vaporizes as a result of low pressure in the evaporator, and the expanding vapor drives a turbine to produce electricity. A cold-water-fed condenser converts the vapor back into liquid. Hybrid systems use portions of both open- and closed-cycle systems. The potential of this technology is so enormous, according to the U.S. National Renewable Energy Laboratory that “if less than 0.1% of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.” A major hurdle to OTEC’s commercialization, however, is its cost. OTEC does not become competitive with traditional energy sources until oil prices reach $40 per barrel, says Desikan Bharathan, an NREL engineer. But there are already some niche markets for the byproducts resulting from the use of the cold, deep seawater, Daniel says. For one, the water can be used for airconditioning, which has saved NELHA about $4000 per month in cooling costs. Likewise, it can be used for industrial cooling processes; to chill soils, allowing for the growth of more temperate plants; and, because the cold seawater is rich in dissolved inorganic nutrients and relatively free of pathogens, it can be used for aquaculture operations. Salt from the seawater gets left behind in the evaporator, so potable water can also be collected from the condensate produced through the process. Some U.S. military bases have also been considering OTEC—in particular, a facility at Diego Garcia, an island in the Indian Ocean—for energy and air-conditioning purposes, according to Bharathan and Daniel. In addition, India was expected to begin operation of a 1-MW OTEC pilot plant on a floating ship off its southern coast in midJanuary. —KRIS CHRISTEN

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