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Tidal Barrage & Tidal TurbinesGenerating Electricity from the Tide | Turbines Used in Barrier Tidal Power Stations | Trends in Generation Technologies | Tidal Fences | Tidal Turbines | Tidal Lagoons | Tidal Power in Australia | Tidal Power Around the World | Planned Projects | Prototype Tidal Generator Designs | Further Information | ReferencesGenerating Electricity from the TideThe generation of electricity from tides is very similar to hydroelectric generation, except that tidal water both ebbs and flows and this must be taken into account in the development of the generators. The simplest generating system for tidal plants, known as an ebb generating system, involves a dam, known as a barrage, across an estuary. Sluice gates on the barrage (see Figure 1) allow the tidal basin to fill on the incoming high tides (flood tides) and to exit through the turbine system on the outgoing tide (known as the ebb tide). Alternatively, flood generating systems, which generate power from the incoming tide are possible, but are less favoured than ebb generating systems. 
Figure 1 Ebb generating system with a bulb turbine
(Adapted from Energy Authority of NSW Tidal Power Fact Sheet).
Turbines Used in Barrier Tidal Power StationsSeveral different turbine configurations are possible. For example, the La Rance tidal plant near St Malo on the Brittany coast in France uses a bulb turbine (Figure 2). In systems with a bulb turbine, the turbine is completely immersed, making maintenance difficult, as the water must be prevented from flowing past the turbine to gain access to it. Rim turbines (Figure 3), such as the Straflo turbine used at Annapolis Royal in Nova Scotia, reduce these problems as the generator is mounted in the barrage, at right angles to the turbine blades. Unfortunately, it is difficult to regulate the performance of these turbines and they are unsuitable for use in pumping. Tubular turbines have been proposed for use in the Severn tidal project in the United Kingdom. In this configuration (Figure 4), the blades are connected to a long shaft and orientated at an angle so that the generator is sitting on top of the barrage and is accessible for maintenance checks. 
Figure 2 Bulb Turbine
(Copyright Boyle, 1996).
Figure 3 Rim Turbine
(Copyright Boyle, 1996).
Figure 4 Tubular Turbine
(Copyright Boyle, 1996). Trends in Generation TechnologiesIt has been over 30 years since the world's largest tidal power station was constructed on the La Rance Estuary in France. At 240 MW, it is far larger than the 20 MW station at Annapolis Royal, Canada, which was completed in 1984 and the smaller (less than 500 kW) systems in the Bay of Kislaya and Jangxia Creek, China completed around the time of the La Rance project. Tidal FencesTidal fences are composed of a number of individual vertical axis turbines which are mounted within the fence structure, known as a caisson. They can be thought of as giant turnstyles which completely block a channel, forcing all of the water through them as shown in Figure 5. 
Figure 5 Artists impression of a tidal fence in operation
(courtesy of Blue Energy Canada Unlike tidal barrage power stations, tidal fences can also be used in unconfined basins, such as in the channel between the mainland and a nearby off shore island, or between two islands. As a result, tidal fences have much less impact on the environment, as they do not require flooding of the basin and are significantly cheaper to install. Tidal fences also have the advantage of being able to generate electricity once the initial modules are installed, rather than after complete installation as in the case of barrage technologies. Tidal fences are not free of environmental and social concerns, as a caisson structure is still required which can disrupt the movement of large marine animals and shipping. A 2.2GW tidal fence using the Davis turbine, was being planned for the San Bernadino Strait in the Philippines to be constructed by the Blue Energy company. The project, estimated to cost $US 2.8 Billion is unfortunately on hold due to political instability. 
Figure 6 The Philippine Dalupiri 2.2 GW Blue Energy Project
(Copyright ©1997-2004 Blue Energy Canada Inc.) Tidal TurbinesProposed shortly after the oil crisis of the 1970s, tidal turbines have only become reality in the last five years, when a 15kW 'proof of concept' turbine was operated on Loch Linnhe, Scotland. Resembling a horizontal axis wind turbine (see Figures 7 and 8), tidal turbines offer significant advantages over barrage and fence tidal systems, including reduced environmental effects. 
Figure 7 An artists impression of an axial flow, seabed mounted marine current turbine for shallow sea tidal power generation
(picture courtesy of Tidal Stream). Tidal turbines utilise tidal currents that are moving with velocities of between 2 and 3 m/s (4 to 6 knots) to generate between 4 and 13 kW/m2. Fast moving current (>3 m/s) can cause undue stress on the blades in a similar way that very strong gale force winds can damage traditional wind turbine generators, whilst lower velocities are uneconomic. 
Figure 8 Artists impression of the 4MW Pentland Firth design at a depth of 60 m and a 20 m rotor for power generation.
(picture courtesy of Tidal Stream). Figure 9 is a photo of Marine Current Turbines (MCT’s) existing ‘SeaFlow’ 300 kW prototype turbine, which is the world’s first offshore tidal turbine and was installed off Lynmouth, Devon in May 2003. 
Figure 9 A hybrid mage of the Seaflow turbine installed at Lynmouth, May 2003 (photo above, artist's impression below, courtesy of MCT)
Recently a new "open-centre" tidal turbine designed by OpenHydro has been installed at the European Marine Energy Centre (EMEC) in Orkney, Scotland (See Figure 10). The open-centre turbine has a slow-moving rotor and lubricant-free construction with just one moving part and no seals. It is a self-contained rotor with a solid state permanent magnet generator encapsulated within the outer rim, minimising maintenance requirements (OpenHydro, 2007). 
Figure 10 OpenHydro’s turbine research facility at
the EMEC in Orkney,
Scotland.
OpenHydro’s technology will be used to establish a tidal energy demonstration project in the Bay of Fundy, which when completed, will be the largest in-stream tidal generating unit integrated into an electricity grid in the world. Following successful completion of this installation, Nova Scotia Power plan to develop large utility scale tidal farms in the Bay of Fundy. OpenHydro was selected by Nova Scotia Power following a global procurement process that considered over 20 technology suppliers world-wide (OpenHydro, 2007). James Ives, the CEO of OpenHydro stated that the company was delighted with the selection: “This project further validates OpenHydro’s future programme for farms of tidal turbines, silently and invisibly generating renewable energy under the world's oceans” (OpenHydro, 2007).
Another experimental tidal turbine has reportedly been installed in Kvalsundet, south of Hammerfest, Norway and became operational in November 2003. The installed tidal power turbine is said to generate a maximum of 300 kW at a maximum speed of the current of 2.5 m/s (Hammerfest STRØM AS, 2002)
Tidal LagoonsOffshore tidal power generation ("tidal lagoons") is a new approach to tidal power conversion that resolves the environmental and economic problems of the familiar "tidal barrage" technology. Tidal lagoons use a rubble mound impoundment structure and low-head hydroelectric generating equipment situated up to a mile or more offshore in a high tidal range area (see Figure 11). Shallow tidal flats provide the most economical sites. Multi-cell impoundment structures provide higher load factors (about 62%) and have the flexibility to shape the power output curve in order to dispatch power in response to demand price signals. 
Figure 11 A tidal lagoon proposed for the Swansea Bay area in Wales
(courtesy of the BBC). Tidal Power in AustraliaTidal power has been proposed in the Kimberley region of Western Australia since the 1960s, when a study of the Derby region identified a tidal resource of over 3000 MW. In recent years a proposal to construct a 50 MW tidal plant in the Derby region (situated at the head of two adjacent inlets off the King Sound) has been developed by Derby Hydro Power. This project received a $1 million grant through the Australian Greenhouse Office's Renewable Energy Commercialisation Program to further develop the project. The inlets would be connected via an artificial channel. By damming each inlet, differences in water levels in each basin could be controlled which would enable flow via the connecting channel (see Figure 12). Power take-off would be achieved from a bank of turbines housed in a structure built in this channel. 
Figure 12 The proposed Derby tidal development options (as of 2001)
(courtesy of SEDO and Hydro Tasmania). The tidal system was compared with an alternative gas-fired power plant and in July 2000 it was decided not to proceed with the Derby tidal project. The committee that made this decision compared the two bids on financial and technical grounds as well as community benefits and environmental impacts (World Energy Council, 2001). No further progress has been made on this particular project. Tidal Power Around the WorldThere are currently a few large scale barrages in operation around the world, including the 240 MW bulb turbine at La Rance, Brittany (see Figures 13 and 14), France and a 20 MW plant at Annapolis Royal, Nova Scotia, Canada. The 240 MW experimental La Rance tidal power project in Brittany, France was commissioned in 1966. This plant (operated by Electricite de France) is equipped with 24 bulb-type turbine generators. The turbines measure 5.35m in diameter with generators rated at 10 MW. These machines are designed to generate energy on either the incoming or outgoing tide, to pump water into or out of the basin during periods of low tide, and to serve as orifices, passing water either into or out of the basin. The plant therefore can, and quite often does, operate as a single high-basin plant, generating energy on the outgoing tide. With the given versatility of its turbine generator equipment, the plant also operates equally well as a single low-basin plant, generating energy during the incoming tide. In addition it can operate as a single-basin double-effect plant, generating energy on both the incoming and outgoing tides (Wilmington Media Ltd, 2004). The Annapolis pilot tidal power plant (TPP) in Canada's Bay of Fundy on the Atlantic coast in the province of Nova Scotia features a rim-type turbine generator with a 7.6m diameter Straflo turbine and a generator with a 20 MW capacity. It is a modern version of the axial flow turbine with rim-type generator, patented by Leroy Harza in 1919. This single high-basin plant was inaugurated in 1984 and has been in successful operation ever since (Wilmington Media Ltd, 2004). By the end of 1984, there were eight TPPs in operation in China. Since 1984, four of those plants have been closed down. The Jiangxia experimental TPP is located in Zhejiang province, about 200km to the south of Hangzhou. The plant was built in the dry season within the left bank, behind cofferdams, and operates in double effect, generating energy on both the incoming and outgoing tides. The first 500 kW bulb unit was commissioned in May 1980, with the second, a 600 kW unit, in June 1984. By the end of 1985, five units were in operation. The third, fourth and fifth units each have a rated capacity of 700 kW. The installed capacity with five units amounts to 3200 kW. The sluice structure, originally built as part of a land reclamation project, has five openings 4.2m high by 3.3m wide, controlled by reinforced concrete gates. The highest basin level is restricted to 1.2m. Approximately 3.8 km2 of land was reclaimed in the basin above 1.2m, which was utilised to plant orange trees, sugar cane, cotton and rice. The inter-tidal zone of the basin with an area of 1.2km2 is used for oyster culture and clam fishery. The basin area at lowest low water is 0.8km2. The plant is still in operation, producing 6 GWh of energy per year (Wilmington Media Ltd, 2004). The Shashan TPP began as a single, high-basin plant. Starting out with a wooden turbine, the plant provided mechanical energy for the grinding of grain. In 1964, the wooden turbine was replaced by a steel runner with a matching 40 kW generator. The plant produced 0.1 GWh in 1984, which was used for irrigation. This plant has since been closed down. (Wilmington Media Ltd, 2004). The Haishan TPP is noteworthy as it is the only linked-basins plant in existence in the world, similar to that which was proposed for Derby in Australia. The plant features a high and a low-basin with the power plant in between, generating energy from water flowing from the high into the low-basin. The plant is located on Maoyan Island in Zhejiang province where it serves an isolated community of 760 families. The plant was designed for two 75 kW units of which only one was installed and commissioned in 1975. This unit operated continuously and the energy was used partly to pump fresh water for domestic and irrigation use into the community reservoir. The plant has since been upgraded to an installed capacity of 0.25 MW, producing 0.34 GWh per year (Wilmington Media Ltd, 2004). On January 6, 2006, the most recent Chinese TPP came into operation in Daishan County of eastern China's Zhejiang province. The 40 kW tidal power station was developed by Harbin Engineering University and assisted by the Daishan Technology Bureau (Power Engineering International, 2006). The Russian Federation have also constructed experimental TPPs since the 1930’s. A small pilot plant with a capacity of 400 kW was constructed at Kislogubsk near Murmansk and commissioned in 1968. The success of this installation led to a number of design studies for much larger tidal plants at sites in the north and east of the country: Lumbov (67 MW) and Mezen Bay (15 000 MW) in the White Sea, Penzhinsk Bay (87 400 MW) and Tugur Bay (6 800 MW) in the Sea of Okhotsk. Eventually the Tugur station emerged as the only feasible major scheme (World Energy Council, 2001). A pre-feasibility study of the Tugur tidal power station in the Khabarovsk Region assessed its generation volume to be around 16,200 million kWh per year. It seems unlikely that there will be any demand for these projects in the Russian Far East until at least 2020 and their development will only be possible within the context of international cooperation with neighbouring countries interested in importing power from Russia (Minakov, 2005). 
Figure 13 La Rance Tidal Power Station during and after construction
(Image courtesy of Popular Mechanics).
Figure 14 La Rance Tidal Power Station today
(Image courtesy of Science Museum ). Planned ProjectsIn late 2004 the Chinese Government endorsed a 300 MW Tidal Lagoon Cooperation Agreement signed in New York. The Chinese government has expressed its support of Tidal Electric's ambitious 300 MW offshore tidal lagoon in the waters near the mouth of the Yalu River. At 300 MW, the project would be the largest tidal power project in the world, topping the capacity of the 240 MW French tidal power plant in La Rance. Under construction in Korea is a single stream style generator at Ansan City’s Shiswa Lake, which will have a capacity of 252 MW. The system will comprise 12 units of 21 MW generators and an annual power generation is projected at 552 million kWh when completed in 2008. Designed by the Korea Ocean Research & Development Institute, the project is funded by the Korea Water Resource Corporation. Costs are estimated at US $ 320 million with a price per kWh of US $0.09. The system relies on a tidal differential of 5.6 m. If successful, this project will surpass La Rance (France) as the largest tidal power plant in the world. Korea is also planning a tidal current power plant in Uldol-muk Strait, a restriction in the strait where maximum water speed exceeds 6.5 m/s. The experimental plant will utilize helical or “Gorlov” turbines developed by GCK. The 1 kW system is anticipated to be operating in 2007 (IEEE Power Engineering Society, 2005). EDF Energy, one of the largest energy companies in the UK, has increased its investment in Marine Current Turbines Ltd (MCT) by an additional £2 million. The financial injection by EDF Energy will back the commercial development of MCT’s 1MW SeaGen tidal current device capable of providing clean and sustainable electricity to approximately 800 homes. The partnership will see electricity generated from the power of tidal currents fed into people’s homes for the first time. The prototype is set to be installed in Northern Ireland’s Strangford Lough and connected to the local grid in 2006. EDF Energy is keen to develop the new technology to gauge its potential future commercial application as a tidal farm with up to 30 turbines. (Marine Current Turbines, 2005).
Prospective Sites for Tidal Energy Projects
Table 1 Potential Tidal Power Projects (courtesy of World Energy Council).
An American tidal power company Tidal Electric have proposed two offshore tidal schemes for Wales involving the construction of bounded tidal reservoirs (tidal lagoons) to trap high tides. The initial 60 MW project has been proposed for Swansea Bay in the UK, measuring 5km2 in area and about a mile offshore. WS Atkins has conducted a Feasibility Study on the proposed project and finds that the project is technically feasible, environmentally plausible, and economically viable. The larger project, dependant upon the success of the project at Swansea, would be built at Rhyl off the Wales coast and would have a generating capacity of 400MW. To provide more continuous output, the reservoir of the Rhyl scheme would be subdivided into segments with each being filled and emptied in turn. The reservoirs would be constructed from rocks (30 million tonnes for the Rhyl system), like a causeway, so they would not be as expensive as a conventional dam or tidal barrage. This would be the largest single renewable energy project in the UK - the Rhyl scheme would be nine miles long and two miles wide (The United Kingdom Parliament, 2001). Prototype Tidal Generator DesignsStingrayThe Stingray, developed by Engineering Business (EB), is designed to extract energy from water that flows due to tidal effects (see Figure 15). It consists of a hydroplane that has an attack angle relative to the approaching water stream varied by a simple mechanism. This causes the supporting arm to oscillate, which in turn forces hydraulic cylinders to extend and retract. This produces high-pressure oil that is used to drive a generator. EB has recently completed its programme to design, build, install offshore, test and decommission a full-scale demonstration of its Stingray tidal stream generator. The three-year programme delivered a mass of technical and commercial data, and has been completed on time and under budget. Unfortunately, given the timescales involved and investment required, EB cannot continue to sustain this project on a non-commercial basis and have decided to put the Stingray project on hold (Engineering Business Ltd, 2005). 
Figure 15 The Stingray before deployment for submerged testing
(courtesy of Engineering Business).
Gorlov Helical RotorGorlov's helical turbine is based on the Darrieus turbine, which was developed for windmills decades ago, but never proved to be practical because the straight airfoil design could break due to vibrations when turning in high speed winds. It could also work underwater to produce energy from currents, but had similar vibration issues. Gorlov’s helical turbine on the other hand has twisting blades in the shape of a helix, which solves the vibration problem and also is very efficient.
Figure 16 Above is the testing of the Gorlov Turbine in Massachusetts and below is the proposed design for South Korea(courtesy of The Museum of Science).
Axial-Flow Rotor Turbine Kinetic Hydropower SystemVerdant Power’s axial-flow rotor turbine kinetic hydropower systems generate electricity from the kinetic energy present in flowing water. The systems can operate in rivers, manmade channels, tidal waters, or ocean currents and do not rely upon the potential energy of a head of water. They also do not require the diversion of water through manmade channels, riverbeds, or pipes, although they might have applications in such conduits. The systems have a modular, self-contained turbine/generator unit that is designed for direct submersion in tailrace, tidal, and river currents without costly civil works. The units range from 25 kW to 250 kW depending on model size and water flow velocities. The models planned are 25 kW, 50 kW and 100 kW with the power conditioning and interface modules located onshore. These turbines consist of a concentric hub with radial blades, similar to that of a windmill. Mechanical power is applied directly through a speed increaser to an internal electric generator, or through a hydraulic pump that in turn drives an onshore electric generator (Verdant Power, 2003). 
Figure 17 Verdant’s 16 foot Axial-Flow Rotor Turbine installed in New York's East River
(courtesy of Verdant Power). The TidEl Tidal Stream GeneratorThe TidEl tidal stream generator is another reliable method of capturing energy from the sea's tides, which is economical to install, operate and decommission (see Figure 18). One reason is that the system requires no support structure, thus reducing costs and is maneuverable for maintenance. The TidEl tidal stream generator actually floats, but remains restrained and submerged to the seabed using a mooring. This positioning gives safety to the structure from damaging waves overhead with little impact of the systems performance as it is designed for typical tidal flows rather than storm conditions. The generator being free to move in line with the direction of the tide, is able to follow the tide backwards and forwards as it changes direction twice a day, without the need for additional controls to re-orientate it (SMD Hydrovision, 2004). 
Figure 18 SMD Hydrovision’s TidEl tidal stream generator(courtesy of SMD Hydrovision).
Sea SnailTurbine installation costs are expensive due to the need for firm foundations and innovations such as Aberdeen's Robert Gordon University 'Sea Snail' device (see Figure 19) could prove cheaper to install due to its prefabricated nature. This 30 tonne platform device uses hydrofoils or 'sea wings' which harness the sea's own power to produce a downward directional thrust to anchor the device to the ocean floor. A turbine is then mounted onto this very stable platform. Robert Gordon University (RGU) has a 150kW prototype device deployed in the Eynhallow Sound, Orkney and should this prove successful hope to deploy a full scale 750kW device at a cost of £400,000 in the near future (Highlands and Islands Enterprise, 2005).
Figure 19 RGU’s 30 tonne Sea Snail prototype (courtesy of Robert Gordon University).
Further InformationRISE Resources - Information regarding available renewable energy resources.RISE Technologies - An extensive collection of information regarding renewable energy technologies.RISE Applications & System Design - Renewable energy application information and system designs.RISE System Displays - Case studies and information on installed renewable energy systems & performance data.Fore more prototype technologies visit this link from The University of Strathclyde. Research Report by the Fujita Corp. Japan Tidal Energy Fact Sheet - Australian Institute of Energy (PDF)
ReferencesEngineering Business Ltd, 2005. “Core Technologies – Stingray” (Online) http://www.engb.com/services_09a.php (Accessed 26 February 2007). Hammerfest STRØM AS, 2002. “The turbine is installed” (Online) http://www.e-tidevannsenergi.com/ (Accessed 26 February 2007). Highlands and Islands Enterprise, 2005. “Marine Energy – Tidal Power” (Online) http://www.hie.co.uk/aie/tidal_power.html (Accessed 26 February 2007). IEEE Power Engineering Society, 2005. “2005 Panel Session Harnessing the Untapped Energy Potential of the Oceans: Tidal, Wave, Currents and OTEC” (Online) http://www.ewh.ieee.org/cmte/ips/2005GM/oceans_2.pdf (Accessed 26 February 2007). Marine Current Turbines. 2005, “EDF Energy powers Marine Current Turbine’s First Commercial Prototype” (Online) http://www.marineturbines.com/home.htm (Accessed 26 February 2007). Minakov. V, 2005, “Transmission Line Project Linking the Russian Far East with the DPRK (Chongjin)” (Online) http://www.nautilus.org/aesnet/Minakov_Niigata_2004_Peport.pdf (Accessed 26 February 2007). Natural Resources Defence Council, 2005. “Alexander’s Marvellous Machine” (Online) http://www.nrdc.org/onearth/05spr/gorlov3.asp (Accessed 26 February 2007). OpenHydro, 2007. "Website" (Online) ) http://www.openhydro.com/home.html (Accessed 27 February 2007). Power Engineering International. 2006, “China Operated Tidal Power Project” (Online) http://pepei.pennnet.com/Articles/Article_Display.cfm?Section=ARTCL&Category=PRODJ&PUBLICATION_ID=6&ARTICLE_ID=244908 (Accessed 26 February 2007). SMD Hydrovision, 2004. “TidEl tidal stream generator” (Online) http://www.smdhydrovision.com/products/?id=27 (Accessed 26 February 2007). The United Kingdom Parliament. 2001, “Appendix 6 – Wave and Tidal Energy” (Online) http://www.parliament.the-stationery-office.co.uk/pa/cm200001/cmselect/cmsctech/291/291ap07.htm (Accessed 26 February 2007). Verdant Power, 2003. “Instream Energy Generating Technology” (Online) http://www.verdantpower.com/tech/lowimpact.html (Accessed 26 February 2007). Wilmington Media, International Water Power and Dam Construction. 2004, “Barriers against Tidal Power” (Online) http://www.waterpowermagazine.com/story.asp?storyCode=2022354 (Accessed 26 February 2007). World Energy Council. 2001, “2001 Survey of Energy Resources – Tidal Energy” (Online) http://www.worldenergy.org/wec-geis/publications/reports/ser/tide/tide.asp (Accessed 26 February 2007). |
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