3.2. Wave energy
3.2.1. Background
Wave energy has long been considered one of the most promising renewable technologies. Not only is the energy resource vast, but it is more dependable than most renewable energy resources—wave power at a given site is available up to 90 percent of the time, while solar and wind availability tend to be available just 20–30 percent of the time [16]. There are a more than 1000 different patented proposals for wave energy devices [17], and several have demonstrated the potential for commercially viable electricity generation [18].
3.2.2. Current status
After several disappointing experiments dashed high expectations for wave power in the oil crisis era of the 1970s, interest waned. But interest has increased in wave energy with the introduction of several new technologies that dramatically increase the efficiency and feasibility of wave power, and a shift in focus toward smaller plants, making the initial capital costs less prohibitive. Unlike OTEC, wave power is already commercial, with recent advances continually coming from companies investing in wave energy devices around the world.
The first commercial wave plant in the world, Limpet 500, was installed on the island of Islay, Scotland, in 2000, and has been providing power to the grid for the UK since late November 2000 [19]. The Limpet 500 is a 0.5 MW capacity plant designed by Wavegen for siting on exposed shores, utilizing an oscillating water column design. Wavegen has also created a near-shore device, OSPREY 2000 (Ocean Swell Powered Renewable EnergY), a 2 MW station designed for 15 m deep water up to 1 km from shore, and the WOSP 3500, a combined OSPREY and offshore windmill unit, rated at a total of 3.5 MW (2 MW OSPREY plus 1.5 MW wind) [20].
Also on the island of Islay, Ocean Power Delivery Ltd. of EDINBURGH, Scotland is installing a small offshore wave power device, which will power up to 200 homes. Installation should be finished in 2002. The plant will produce 2.5 million kW h electricity/yr. With support from the Scottish Renewable Obligation of 1999, OPDL eventually plans to install up to 900 devices, with a total capacity of 700 MW, producing more than 2.5 billion kW h/yr [21].
In the United States, the Monitor, a hybrid system designed by Demi-Tek that combines tide, wave and wind power, has been working just off Asbury Park, New Jersey since August 1990. The Monitor produces enough electricity to light the city's boardwalk and convention hall. In addition, the Monitor was deployed to help reduce wave action and protect beaches from erosion. It is anchored to the ocean floor by cables similar to those used for offshore oil drilling, and electricity is brought to shore by an undersea cable [22].
3.2.3. Potential
The greatest potential for wave energy exists where the strongest winds are found—at the temperate latitudes between 40° and 60° north and south, on the eastern boundaries of oceans. One of the richest nations in terms of potential for wave energy is the UK, with the north of Scotland having particularly high potential. The Science and Technology Committee of the British Parliament reports that, based on estimates from the Department of Trade and Industrys Energy Technology Support Unit, in the UK alone, wave energy devices could practicably contribute more than 50 TW h/yr [5]. In the US, a reasonable potential for wave energy development may exist off the Pacific northwest coast [21]. Worldwide, wave energy could potentially provide up to 2 TW of electricity, according to the World Energy Council [23], approximately 1/5 of current global energy demand.
The economics of wave energy power, though not yet competitive with fossil fuels, are promising, and the situation is improving with more advanced technology. Costs have dropped rapidly in the last several years, and now companies are aiming for less than 10 cents/kW h, to as low as 5 cents/kW h, for the latest designs. This price would allow wave plants to compete favorably with conventional power plants [24].
3.2.4. Environmental impacts
Small-scale wave energy plants are likely to have minimal environmental impacts. However, some of the very large-scale projects that have been proposed have the potential for harming ocean ecosystems. Covering very large areas of the surface of the ocean with wave energy devices would harm marine life and could have more widespread effects, by altering the way the ocean interacts with the atmosphere.
Wave power plants act as wave breakers, calming the sea. While this is often a desired effect in many harbors (in fact wave energy devices could be combined with wave break devices), the result may be to slow the mixing of the upper layers of the sea which could adversely impact marine life and fisheries. Demersal fish will probably not be directly affected; however, changes in surface productivity linked to reduced mixing could potentially reduce food supply to benthic populations. Changes in waves and currents would most directly impact species that spend their lives nearer the surface. Many fish species depend in part on currents to transport larvae, so wave energy devices that alter the currents between spawning grounds and feeding grounds could be harmful to fish populations [25].
The dampening of waves may reduce erosion on the shoreline; whether this effect is beneficial or detrimental depends on the specific coastline [25]. While dampening of waves may have damaging ecological effects, and more research is needed to determine the extent of this impact, studies show that SHELTERING due to wave devices will have a hardly noticeable effect on the largest waves, so that the ecological role of very large waves as a disturbance that maintains high biodiversity will be unencumbered [17].
Wave energy promoters claim the devices could enhance marine life by providing structure, acting in much the same way as artificial reefs. This claim should be critically evaluated for specific projects, because the effects of artificial structures appear to be very site specific. In areas where hard substrate is clearly limiting to production, such structures may enhance marine life. Conversely, when other factors are limiting, artificial structures may simply draw organisms away from natural habitats and potentially increase their vulnerability to harvest [26], [27] and [28].
Wave energy is promising, holds a huge potential to reduce reliance on fossil fuels, and is considered to be relatively environmentally benign at this time. Further research into wave energy is recommended. For new wave plants, particularly of large capacity, siting should be carefully considered not only for the potential to generate power, but also for the ecosystem's reliance on and response to powerful waves, and wave plants should be avoided where calming of the waves would result in significant community changes or disrupt natural ecological processes.
3.3. Tidal
3.3.1. Background
Tidal power has the distinct advantage of being highly predictable, compared to solar, wind, and wave energy. The regularity of THE TIDES along with an immense energy potential helps make tidal energy development attractive. The first tidal barrages resemble dams built across the mouths of estuaries to harness the energy of the tidal flow. Unlike a hydroelectric dam, a tidal barrage must allow water to flow in both directions, although typically, the barrage only captures the energy of the water flowing out of the estuary from high to low tide. Tidal barrage technology is fairly well developed, and offers very large potential in some sites.
Tidal barrages have been found to be potentially damaging to the marine environment (see “Environmental impacts”). More recent innovations include tidal fences and tidal turbines, which take advantage of the currents set up by tidal flows. Tidal fences consist of turbines stretching entirely across a channel where tidal flow sets up relatively fast currents. The turbines are designed to allow the passage of fish, water and sediment through the channel [29]. Tidal turbines, also installed in channels with tidal currents, resemble underwater wind turbines and require current speeds of 2–3 m/s; at lower velocities, harnessing energy from the current is uneconomical, while higher velocities can damage the turbines.
3.3.2. Current status
The first and largest operational tidal barrage plant in the world, built in the early 1960s, is the La Rance plant on the Brittany coast of northern France. Taking advantage of the 2.4 m tidal height at the mouth of the La Rance estuary, the plant produces 240 MW of electricity. Other operation tidal plants exist at Kislaya in Russia, Jiangxia in China, and Annapolis in Canada [30].
No commercial tidal fence plants exist at this time, but the company Blue Energy Canada hopes to develop them in the near future. It is looking toward Southeast Asia for its first commercial tidal fence ventures, most notably a planned fence across the Dalupiri Passage in the Philippines. This site, with a peak tidal current of about 4 m/s, would allow for a 2200 MW peak power plant, with a base daily average of 1100 MW. As part of a larger proposed project, Build Own Operate Transfer (BOOT), the project could help the Philippines exceed its power needs and export electricity [29]. Tidal fence projects have also been proposed for sites beneath the Tacoma Narrows bridge in Washington and between Point San Pablo and East Brothers Island in San Francisco Bay [31] and [32]. Tidal turbines are not yet at the commercial development stage. The industry leader in tidal turbine research, Marine Current Turbines Ltd., plans to begin commercial development in 2004 after concluding a major research and development effort [33]. By 2010, the company states, 300 MW of power could be provided by underwater tidal turbines.
3.3.3. Potential
It is estimated that the United Kingdom could generate up to 50.2 TW h/yr with tidal power p