ABSTRACT
Renewable energy can be used to decrease global dependence on natural resources, and tidal power can be the primary form of renewable power utilized. Built upon steam turbine knowledge, tidal turbines draw on innovative technology and design to operate on both the inflow and outflow of water through them. Two case studies, Annapolis Royal and La Rance, prove that tidal power plants are capable of producing reliable and efficient power. Problems, such as initial cost and power transportation hinder future implementation of tidal power plants. This paper emphasizes the possibilities of utilizing the power of the oceans by pollution free, tidal Power generation. Tidal power utilizes twice the daily variation in sea level caused primarily by the gravitational effect of the Moon and, to a lesser extent by the Sun on the world's oceans. The Earth's rotation is also a factor in the production of tides.
INTRODUCTION
The sources for 90% of the electric energy generated today are non-renewable. Natural resource emissions are over 120 times greater than that of renewable emissions. The depletion of the finite resources, environmental pollution, global warming became more apparent near the end of the 20th century. World energy consumption is expected to rise 60 per cent by 2020. In order to meet that demand, while limiting production of green house gases, renewable energy sources considered as an alternative to traditional forms of energy production.
Renewable sources of energy are necessary because the Earth will eventually run out of the resources to create non-renewable energy. There are three types of renewable energy sources: solar, wind, and waterpower. Both solar and wind power are drastically affected by weather variations, while tidal power varies little when the weather changes power. Over the last fifty years, engineers have begun to look at tidal and wave power on a larger, industrial scale. However, until the last few years, wave power and tidal power were both seen as uneconomic. Although some pilot projects showed that energy could be generated, they also showed that, even if cost of the energy generated was not considered, there was a real problem making equipment which could withstand the extremely harsh marine environment.
Tidal energy is an essentially renewable resource which has none of the typical environmental impacts of other traditional sources of electricity such as fossil fuels or nuclear power. Changing the tidal flow in a coastal region could, however, result in a wide variety of impacts on aquatic life, most of which are poorly understood. Tidal power works because of the Moon’s constant rotation around the Earth. This is very convenient because scientist’s can predict the electricity production on a daily basis. .
The tides produce the electricity for tidal power by flowing in and out of turbines. A hydrostatic head or adequate water height difference on either side of the turbine. The simple idea of utilizing hydrostatic head to power turbines will be the crux of our article.
Using the Energy of the Ocean:
There are three basic ways to tap the ocean for its energy.
We can use the ocean's waves,
we can use the ocean's high and low tides, or
We can use temperature differences in the water.
Let’s take a look at each,
1. Wave energy
Kinetic energy (movement) exists in the moving waves of the ocean. That energy can be used to power a turbine. In this simple example, to the right, the wave rises into a chamber. The rising water forces the air out of the chamber. The moving air spins a turbine which can turn a generator. When the wave goes down, air flows through the turbine and back into the chamber through doors that are normally closed. This is only one type of wave-energy system. Others actually use the up and down motion of the wave to power a piston that moves up and down inside a cylinder. That piston can also turn a generator. Most wave-energy systems are very small. But, they can be used to power a warning buoy or a small light house.
2. Tidal Energy
Another form of ocean energy is called tidal energy. When a tide comes into the shore, they can be trapped in reservoirs behind dams. Then when the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant.
In order for this to work well, you need large increases in tides. An increase of at least 16 feet between low tide to high tide is needed. There are only a few places where this tide change occurs around the earth. Some power plants are already operating using this idea. One plant in France makes enough energy from tides to power 240,000 homes.
3. Ocean Thermal Energy
The final ocean energy idea uses temperature differences in the ocean. If you ever went swimming in the ocean and dove deep below the surface, you would have noticed that the water gets colder the deeper you go. It's warmer on the surface because sunlight warms the water. But below the surface, the ocean gets very cold. That's why scuba divers wear wet suits trapped their body heat to keep them warm. Power plants can be built that use this difference in temperature to make energy. A difference of at least 38 degrees Fahrenheit is needed between the warmer surface water and the colder deep ocean water. Using this type of energy source is called Ocean Thermal Energy Conversion or OTEC. It is being used in both Japan and in Hawaii in some demonstration projects.
WAVE ENERGY
Wave Power I - sea-based devices
A recent review has shown that there are new types of wave power devices which can produce electricity economically. The “Salter” Duck is the device which can produce electricity for lower cost. `The “Salter” Duck was developed in the 1970s by Professor Stephen Salter at the University of Edinburgh in Scotland and generates electricity by bobbing up and down with the waves. Although it can produce energy extremely efficiently it was effectively killed off in the mid 1980s when a European Union report miscalculated the cost of the electricity it produced by a factor of 10. In the last few years, the error has been realized, and interest in the Duck is becoming intense.
The “Clam” is another device which, like the “Salter” Duck can make energy from sea swell. The Clam is an arrangement of six airbags mounted around a hollow circular spine. As waves impact on the structure air is forced between the six bags via the hollow spine which is equipped with self-rectifying turbines. Even allowing for cabling to shore, it is calculated that the Clam can produce energy for around $US0.06kW/hr.
Wave Power II- Shore based systems
Where the shoreline has suitable topography, cliff-mounted oscillating water column (OWC) generators can be installed. OWC systems have a number of advantages over the Clam and the Duck, not the least of which is the fact that generators and all cabling are shore-based, making maintenance much cheaper. The OWC works on a simple principle. As an incoming wave causes the water level in the unit's main chamber to rise (see diagram), air is forced up a funnel which houses a Well's counter-rotating turbine. As the wave retreats, air is sucked down into the main chamber again. The Well's turbine has been developed to spin in the same direction, whichever way air is flowing, in order to maximize efficiency. Although most previous OWC systems have had vertical water columns that in LIMPET is angled at 45° - which wave tank test show to be more efficient.
OWC schematic
OWC machines have already been tested at a number of sites, including Japan and Norway. A commercial-scale (500 kW) installation is due to be commissioned on the Scottish Island of Islay in September 2000. The Islay OWC (known as LIMPET) is a joint venture between Queens University, WAVEGEN, Instituto Superior Técnico (Portugal), the European Union and Charles Brand Engineering. It is the direct successor of an experimental 75 kW turbine (built by researchers from the Queen's University of Belfast) which operated on the island between 1991 and 1999. Another LIMPET is currently being developed (at pilot-plant scale) on the Azores.
Construction of OWCs
One of the great problems with shoreline-based OWCs is their construction, which must necessarily take place on rocky shores exposed to wind and waves. In the case of the prototype Islay OWC system it was relatively easy to build a temporary dam on the shoreline to protect the unit. However, LIMPET is a much larger system, with a lip 20m wide. It was therefore ultimately decided to build the unit back from the coastline and remove a bund to make the system fully operational.
However, both OWC-systems and ocean-wave systems suffer from trying to harness violent forces. The first Norwegian OWC was ripped off a cliff-face during a storm, the Islay station is completely submerged under storm conditions. Thus, researchers are looking at other ways of generating electricity from the ocean, and are increasingly turning to tidally-generated coastal currents
TIDAL ENERGY
Tidal energy works from the power of changing tides. Tidal changes in sea level can be used to generate electricity, by building a dam across a costal bay or estuary with large differences between low and high tides. The high tides allow immense amounts of water to rush into the bay. The gates of the dam then shut when water level is at its maximum height. Holes in the bottom of the dam let water (at great speed and pressure) to rush past turbines. The flow of water generates enough power to turn the turbines which creates electricity. The entire process repeats with each high tide.
Two current technologies which are used to harness the kinetic energy of tidal flow:
1) Drag Devices Water wheels:
insufficient compared to other modes of generation
blade speed can not exceed that of the current
2) Lift Devices Turbines:
wind mill technology applied to liquid environment
more efficient then drag devices
refined propeller achieves speeds several times faster then the current
GENERATION OF ELECTRICITY USING TIDAL POWER
1.ABSTRACT
Tidal power is the only form of energy which derives directly from the relative motions of the Earth–Moon system, and to a lesser extent from the Earth–Sun system. The tidal forces produced by the Moon and Sun, in combination with Earth's rotation, are responsible for the generation of the tides. Other sources of energy originate directly or indirectly from the Sun, including fossil fuels, conventional hydroelectric, wind, biofuels, wave power and solar. Nuclear is derived using radioactive material from the Earth, geothermal power uses the Earth's internal heat which comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[4]
Variation of tides over a day
Tidal energy is generated by the relative motion of the water which interact via gravitational forces. Periodic changes of water levels, and associated tidal currents, are due to the gravitational attraction by the Sun and Moon. The magnitude of the tide at a location is the result of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth rotation, and the local shape of the sea floor and coastlines.
Because the Earth's tides are caused by the tidal forces due to gravitational interaction with the Moon and Sun, and the Earth's rotation, tidal power is practically inexhaustible and classified as a renewable energy source.
A tidal generator uses this phenomenon to generate electricity. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal electricity generation.
Tidal movement causes a continual loss of mechanical energy in the Earth–Moon system due to pumping of water through the natural restrictions around coastlines, and due to viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since formation. During the last 620 million years the period of rotation has increased from 21.9 hours to the 24 hour.we see now; in this period the Earth has lost 17% of its rotational energy. While tidal power may take additional energy from the system, increasing the rate of slowdown, the effect would be noticeable over millions of years only, thus being negligible.
2. Categories of tidal power
Tidal power can be classified into three main types:
• Tidal stream systems make use of the kinetic energy of moving water to power turbines, in a similar way to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.
• Barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages are essentially dams across the full width of a tidal estuary, and suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.
• Dynamic tidal power exploits a combination of potential and kinetic energy: by constructing long dams of 30–50 km in length from the coast straight out into the sea or ocean, without enclosing an area. Both the obstruction of the tidal flow by the dam – as well as the tidal phase differences introduced by the presence of the dam (which is not negligible in length as compared to the tidal wavelength) – lead to hydraulic head differences along the dam. Turbines in the dam are used to convert power (6–15 GW per dam). In shallow coastal seas featuring strong coast-parallel oscillating tidal currents (common in the UK, China and Korea), a significant water level differential (2–3 meter) will appear between both sides of the dam.
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term marine current power. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.
2.1 Tidal stream generators
2.1.1 Types of tidal stream generators
Since tidal stream generators are an immature technology, no standard technology has yet emerged as the clear winner, but a large variety of designs are being experimented with, some very close to large scale deployment. Several prototypes have shown promise with many companies making bold claims, some of which are yet to be independently verified, but they have not operated commercially for extended periods to establish performances and rates of return on investments.
The European Marine Energy Centre[8] categorises them under four heads although a number of other approaches are also being tried.
Axial Turbines
Evopod - A semi-submerged floating approach tested in Strangford Lough.
These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. These include:
Kvalsund, south of Hammerfest, Norway.[9] Although still a prototype, a turbine with a reported capacity of 300 kW was connected to the grid on 13 November 2003.
A 300 kW Periodflow marine current propeller type turbine — Seaflow — was installed by Marine Current Turbines off the coast of Lynmouth, Devon, England, in 2003.[10] The 11m diameter turbine generator was fitted to a steel pile which was driven into the seabed. As a prototype, it was connected to a dump load, not to the grid.
Since April 2007 Verdant Power[11] has been running a prototype project in the East River between Queens and Roosevelt Island in New York City; it was the first major tidal-power project in the United States.[12] The strong currents pose challenges to the design: the blades of the 2006 and 2007 prototypes broke off, and new reinforced turbines were installed in September 2008.[13][14]
Following the Seaflow trial, a fullsize prototype, called SeaGen, was installed by Marine Current Turbines in Strangford Lough in Northern Ireland in April 2008. The turbine began to generate at full power of just over 1.2 MW in December 2008[15] and is reported to have fed 150 kW into the grid for the first time on 17 July 2008, and has now contributed more than a gigawatt hour to consumers in Northern Ireland.[16] It is currently the only commercial scale device to have been installed anywhere in the world.[17] SeaGen is made up of two axial flow rotors, each of which drive a generator. The turbines are capable of generating electricity on both the ebb and flood tides because the rotor blades can pitch through 180˚.[18]
OpenHydro, an Irish company exploiting the Open-Centre Turbine developed in the U.S., has a prototype being tested at the European Marine Energy Centre (EMEC), in Orkney, Scotland.
A prototype semi-submerged floating tethered tidal turbine called Evopod has been tested since June 2009 in Strangford Lough, Northern Ireland at 1/10th scale. The company developing it is called Ocean Flow Energy Ltd,[21] and they are based in the UK. The advanced hull form maintains optimum heading into the tidal stream and it is designed to operate in the peak flow of the water column.
Tenax Energy of Australia is proposing to put 450 turbines off the coast of the Australian city Darwin, in the Clarence Strait. The turbines feature a rotor section that is approximately 15 metres in diameter with a gravity base which is slighter larger than this to support the structure. The turbines will operate in deep water well below shipping channels. Each turbine is forecast to produce energy for between 300 and 400 homes.[22]
Vertical and horizontal axis crossflow turbines
Invented by Georges Darreius in 1923 and Patented in 1929, these turbines that can be deployed either vertically or horizontally.
The Gorlov turbine[23] is a variant of the Darrieus design featuring a helical design which is being commercially piloted on a large scale in S. Korea,[24] starting with a 1MW plant that started in May 2009[25] and expanding to 90MW by 2013. Neptune Renewable Energy has developed Proteus[26] which can be used to form an array in mainly estuarine conditions.
In late April 2008, Ocean Renewable Power Company, LLC (ORPC) [4] successfully completed the testing of its proprietary turbine-generator unit (TGU) prototype at ORPC's Cobscook Bay and Western Passage tidal sites near Eastport, Maine.[27] The TGU is the core of the OCGen technology and utilizes advanced design cross-flow (ADCF) turbines to drive a permanent magnet generator located between the turbines and mounted on the same shaft. ORPC has developed TGU designs that can be used for generating power from river, tidal and deep water ocean currents.
Trials in the Strait of Messina, Italy, started in 2001 of the Kobold concept.[28]
Oscillating devices
Oscillating devices do not have a rotating component, instead making use of aerofoil sections which are pushed sideways by the flow. Oscillating stream power extraction was proven with the omni- or bi-directional Wing'd Pump windmill.[29] During 2003 a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast.[30] The Stingray uses hydrofoils to create oscillation, which allows it to create hydraulic power. This hydraulic power is then used to power a hydraulic motor, which then turns a generator.[6]
Pulse Tidal operate an oscillating hydrofoil device in the Humber estuary.[31] Having secured funding from the EU, they are developing a commercial scale device to be commissioned 2012.[32]
The bioSTREAM tidal power conversion system, uses the biomimicry of swimming species, such as shark, tuna, and mackerel using their highly efficient Thunniform mode propulsion. It is produced by Australian company BioPower Systems.
A 2kW prototype relying on the use of two oscillating hydrofoils in a tandem configuration has been developed at Laval University and tested successfully near Quebec City, Canada, in 2009. A hydrodynamic efficiency of 40% has been achieved during the field tests.[33]
Venturi effect
Further information: Venturi effect
This uses a shroud to increase the flow rate through the turbine. These can be mounted horizontally or vertically.
The Australian company Tidal Energy Pty Ltd undertook successful commercial trials of highly efficient shrouded tidal turbines on the Gold Coast, Queensland in 2002. Tidal Energy has commenced a rollout of their shrouded turbine for a remote Australian community in northern Australia where there are some of the fastest flows ever recorded (11 m/s, 21 knots) – two small turbines will provide 3.5 MW. Another larger 5 meter diameter turbine, capable of 800 kW in 4 m/s of flow, is planned for deployment as a tidal powered desalination showcase near Brisbane Australia in October 2008. Another device, the Hydro Venturi, is to be tested in San Francisco Bay.[34]
2.1.2 Commercial plans
RWE's npower announced that it is in partnership with Marine Current Turbines to build a tidal farm of SeaGen turbines off the coast of Anglesey in Wales,[35] near the Skerries.[36]
In November 2007, British company Lunar Energy announced that, in conjunction with E.ON, they would be building the world's first deep-sea tidal energy farm off the coast of Pembrokshire in Wales. It will provide electricity for 5,000 homes. Eight underwater turbines, each 25 metres long and 15 metres high, are to be installed on the sea bottom off St David's peninsula. Construction is due to start in the summer of 2008 and the proposed tidal energy turbines, described as "a wind farm under the sea", should be operational by 2010.
British Columbia Tidal Energy Corp. plans to deploy at least three 1.2 MW turbines in the Campbell River or in the surrounding coastline of British Columbia by 2009.[37][dated info]
An organisation named Alderney Renewable Energy Ltd is planning to use tidal turbines to extract power from the notoriously strong tidal races around Alderney in the Channel Islands. It is estimated that up to 3 GW could be extracted. This would not only supply the island's needs but also leave a considerable surplus for export.[38]
Nova Scotia Power has selected OpenHydro's turbine for a tidal energy demonstration project in the Bay of Fundy, Nova Scotia, Canada and Alderney Renewable Energy Ltd for the supply of tidal turbines in the Channel Islands. Open Hydro
Pulse Tidal are designing a commercial device with seven other companies who are expert in their fields.[39] The consortium was awarded an €8 million EU grant to develop the first device, which will be deployed in 2012 and generate enough power for 1,000 homes. Pulse is in a good position to scale up production because the supply chain is already in place.
2.1.3 Energy calculations
Turbine power
Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "ξ" is known the equation below can be used to determine the power output of a turbine..
The energy available from these kinetic systems can be expressed as:[40]
where:
ξ = the turbine efficiency
P = the power generated (in watts)
ρ = the density of the water (seawater is 1025 kg/m³)
A = the sweep area of the turbine (in m²)
V = the velocity of the flow
Relative to an open turbine in free stream, depending on the geometry of the shroud shrouded turbines are capable of as much as 3 to 4 times the power of the same turbine rotor in open flow. .[40]
Resource assessment
While initial assessments of the available energy in a channel have focus on calculations using the kinetic energy flux model, the limitations of tidal power generation are significantly more complicated. For example, the maximum physical possible energy extraction from a strait is given by:[41][42]
where
ρ = the density of the water (seawater is 1025 kg/m³)
g = gravitational acceleration (9.81 m/s2)
ΔHmax = maximum differential water surface elevation across the channel
Qmax= maximum volumetric flow rate though the channel.
2.1.4 Potential sites
As with wind power, selection of location is critical for the tidal turbine. Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses. The following potential sites are under serious consideration:
• Pembrokeshire in Wales[43]
• River Severn between Wales and England[44]
• Cook Strait in New Zealand[45]
• Kaipara Harbour in New Zealand[46]
• Bay of Fundy[47] in Canada.
• East River[48][49] in the USA
• Golden Gate in the San Francisco Bay[50]
• Piscataqua River in New Hampshire[51]
• The Race of Alderney and The Swinge in the Channel Islands[38]
• The Sound of Islay, between Islay and Jura in Scotland[52]
• Pentland Firth between Caithness and the Orkney Islands, Scotland
• Humboldt County, California in the United States
2.1.5 Environmental Impacts
Very little direct research or observation of tidal stream systems exists. Most direct observations consist of releasing tagged fish upstream of the device(s) and direct observation of mortality or impact on the fish.
One study of the Roosevelt Island Tidal Energy (RITE, Verdant Power) project in the East River (New York City), utilized 24 split beam hydroacoustic sensors (scientific echosounder[53]) to detect and track the movement of fish both upstream and downstream of each of six turbines. The results suggested (1) very few fish using this portion of the river, (2) those fish which did use this area were not using the portion of the river which would subject them to blade strikes, and (3) no evidence of fish traveling through blade areas.
Work is currently being conducted by the Northwest National Marine Renewable Energy Center (NNMREC[54])to explore and establish tools and protocols for assessment of physical and biological conditions and monitor environmental changes associa er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
Renewable energy can be used to decrease global dependence on natural resources, and tidal power can be the primary form of renewable power utilized. Built upon steam turbine knowledge, tidal turbines draw on innovative technology and design to operate on both the inflow and outflow of water through them. Two case studies, Annapolis Royal and La Rance, prove that tidal power plants are capable of producing reliable and efficient power. Problems, such as initial cost and power transportation hinder future implementation of tidal power plants. This paper emphasizes the possibilities of utilizing the power of the oceans by pollution free, tidal Power generation. Tidal power utilizes twice the daily variation in sea level caused primarily by the gravitational effect of the Moon and, to a lesser extent by the Sun on the world's oceans. The Earth's rotation is also a factor in the production of tides.
INTRODUCTION
The sources for 90% of the electric energy generated today are non-renewable. Natural resource emissions are over 120 times greater than that of renewable emissions. The depletion of the finite resources, environmental pollution, global warming became more apparent near the end of the 20th century. World energy consumption is expected to rise 60 per cent by 2020. In order to meet that demand, while limiting production of green house gases, renewable energy sources considered as an alternative to traditional forms of energy production.
Renewable sources of energy are necessary because the Earth will eventually run out of the resources to create non-renewable energy. There are three types of renewable energy sources: solar, wind, and waterpower. Both solar and wind power are drastically affected by weather variations, while tidal power varies little when the weather changes power. Over the last fifty years, engineers have begun to look at tidal and wave power on a larger, industrial scale. However, until the last few years, wave power and tidal power were both seen as uneconomic. Although some pilot projects showed that energy could be generated, they also showed that, even if cost of the energy generated was not considered, there was a real problem making equipment which could withstand the extremely harsh marine environment.
Tidal energy is an essentially renewable resource which has none of the typical environmental impacts of other traditional sources of electricity such as fossil fuels or nuclear power. Changing the tidal flow in a coastal region could, however, result in a wide variety of impacts on aquatic life, most of which are poorly understood. Tidal power works because of the Moon’s constant rotation around the Earth. This is very convenient because scientist’s can predict the electricity production on a daily basis. .
The tides produce the electricity for tidal power by flowing in and out of turbines. A hydrostatic head or adequate water height difference on either side of the turbine. The simple idea of utilizing hydrostatic head to power turbines will be the crux of our article.
Using the Energy of the Ocean:
There are three basic ways to tap the ocean for its energy.
We can use the ocean's waves,
we can use the ocean's high and low tides, or
We can use temperature differences in the water.
Let’s take a look at each,
1. Wave energy
Kinetic energy (movement) exists in the moving waves of the ocean. That energy can be used to power a turbine. In this simple example, to the right, the wave rises into a chamber. The rising water forces the air out of the chamber. The moving air spins a turbine which can turn a generator. When the wave goes down, air flows through the turbine and back into the chamber through doors that are normally closed. This is only one type of wave-energy system. Others actually use the up and down motion of the wave to power a piston that moves up and down inside a cylinder. That piston can also turn a generator. Most wave-energy systems are very small. But, they can be used to power a warning buoy or a small light house.
2. Tidal Energy
Another form of ocean energy is called tidal energy. When a tide comes into the shore, they can be trapped in reservoirs behind dams. Then when the tide drops, the water behind the dam can be let out just like in a regular hydroelectric power plant.
In order for this to work well, you need large increases in tides. An increase of at least 16 feet between low tide to high tide is needed. There are only a few places where this tide change occurs around the earth. Some power plants are already operating using this idea. One plant in France makes enough energy from tides to power 240,000 homes.
3. Ocean Thermal Energy
The final ocean energy idea uses temperature differences in the ocean. If you ever went swimming in the ocean and dove deep below the surface, you would have noticed that the water gets colder the deeper you go. It's warmer on the surface because sunlight warms the water. But below the surface, the ocean gets very cold. That's why scuba divers wear wet suits trapped their body heat to keep them warm. Power plants can be built that use this difference in temperature to make energy. A difference of at least 38 degrees Fahrenheit is needed between the warmer surface water and the colder deep ocean water. Using this type of energy source is called Ocean Thermal Energy Conversion or OTEC. It is being used in both Japan and in Hawaii in some demonstration projects.
WAVE ENERGY
Wave Power I - sea-based devices
A recent review has shown that there are new types of wave power devices which can produce electricity economically. The “Salter” Duck is the device which can produce electricity for lower cost. `The “Salter” Duck was developed in the 1970s by Professor Stephen Salter at the University of Edinburgh in Scotland and generates electricity by bobbing up and down with the waves. Although it can produce energy extremely efficiently it was effectively killed off in the mid 1980s when a European Union report miscalculated the cost of the electricity it produced by a factor of 10. In the last few years, the error has been realized, and interest in the Duck is becoming intense.
The “Clam” is another device which, like the “Salter” Duck can make energy from sea swell. The Clam is an arrangement of six airbags mounted around a hollow circular spine. As waves impact on the structure air is forced between the six bags via the hollow spine which is equipped with self-rectifying turbines. Even allowing for cabling to shore, it is calculated that the Clam can produce energy for around $US0.06kW/hr.
Wave Power II- Shore based systems
Where the shoreline has suitable topography, cliff-mounted oscillating water column (OWC) generators can be installed. OWC systems have a number of advantages over the Clam and the Duck, not the least of which is the fact that generators and all cabling are shore-based, making maintenance much cheaper. The OWC works on a simple principle. As an incoming wave causes the water level in the unit's main chamber to rise (see diagram), air is forced up a funnel which houses a Well's counter-rotating turbine. As the wave retreats, air is sucked down into the main chamber again. The Well's turbine has been developed to spin in the same direction, whichever way air is flowing, in order to maximize efficiency. Although most previous OWC systems have had vertical water columns that in LIMPET is angled at 45° - which wave tank test show to be more efficient.
OWC schematic
OWC machines have already been tested at a number of sites, including Japan and Norway. A commercial-scale (500 kW) installation is due to be commissioned on the Scottish Island of Islay in September 2000. The Islay OWC (known as LIMPET) is a joint venture between Queens University, WAVEGEN, Instituto Superior Técnico (Portugal), the European Union and Charles Brand Engineering. It is the direct successor of an experimental 75 kW turbine (built by researchers from the Queen's University of Belfast) which operated on the island between 1991 and 1999. Another LIMPET is currently being developed (at pilot-plant scale) on the Azores.
Construction of OWCs
One of the great problems with shoreline-based OWCs is their construction, which must necessarily take place on rocky shores exposed to wind and waves. In the case of the prototype Islay OWC system it was relatively easy to build a temporary dam on the shoreline to protect the unit. However, LIMPET is a much larger system, with a lip 20m wide. It was therefore ultimately decided to build the unit back from the coastline and remove a bund to make the system fully operational.
However, both OWC-systems and ocean-wave systems suffer from trying to harness violent forces. The first Norwegian OWC was ripped off a cliff-face during a storm, the Islay station is completely submerged under storm conditions. Thus, researchers are looking at other ways of generating electricity from the ocean, and are increasingly turning to tidally-generated coastal currents
TIDAL ENERGY
Tidal energy works from the power of changing tides. Tidal changes in sea level can be used to generate electricity, by building a dam across a costal bay or estuary with large differences between low and high tides. The high tides allow immense amounts of water to rush into the bay. The gates of the dam then shut when water level is at its maximum height. Holes in the bottom of the dam let water (at great speed and pressure) to rush past turbines. The flow of water generates enough power to turn the turbines which creates electricity. The entire process repeats with each high tide.
Two current technologies which are used to harness the kinetic energy of tidal flow:
1) Drag Devices Water wheels:
insufficient compared to other modes of generation
blade speed can not exceed that of the current
2) Lift Devices Turbines:
wind mill technology applied to liquid environment
more efficient then drag devices
refined propeller achieves speeds several times faster then the current
GENERATION OF ELECTRICITY USING TIDAL POWER
1.ABSTRACT
Tidal power is the only form of energy which derives directly from the relative motions of the Earth–Moon system, and to a lesser extent from the Earth–Sun system. The tidal forces produced by the Moon and Sun, in combination with Earth's rotation, are responsible for the generation of the tides. Other sources of energy originate directly or indirectly from the Sun, including fossil fuels, conventional hydroelectric, wind, biofuels, wave power and solar. Nuclear is derived using radioactive material from the Earth, geothermal power uses the Earth's internal heat which comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[4]
Variation of tides over a day
Tidal energy is generated by the relative motion of the water which interact via gravitational forces. Periodic changes of water levels, and associated tidal currents, are due to the gravitational attraction by the Sun and Moon. The magnitude of the tide at a location is the result of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth rotation, and the local shape of the sea floor and coastlines.
Because the Earth's tides are caused by the tidal forces due to gravitational interaction with the Moon and Sun, and the Earth's rotation, tidal power is practically inexhaustible and classified as a renewable energy source.
A tidal generator uses this phenomenon to generate electricity. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal electricity generation.
Tidal movement causes a continual loss of mechanical energy in the Earth–Moon system due to pumping of water through the natural restrictions around coastlines, and due to viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since formation. During the last 620 million years the period of rotation has increased from 21.9 hours to the 24 hour.we see now; in this period the Earth has lost 17% of its rotational energy. While tidal power may take additional energy from the system, increasing the rate of slowdown, the effect would be noticeable over millions of years only, thus being negligible.
2. Categories of tidal power
Tidal power can be classified into three main types:
• Tidal stream systems make use of the kinetic energy of moving water to power turbines, in a similar way to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.
• Barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages are essentially dams across the full width of a tidal estuary, and suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.
• Dynamic tidal power exploits a combination of potential and kinetic energy: by constructing long dams of 30–50 km in length from the coast straight out into the sea or ocean, without enclosing an area. Both the obstruction of the tidal flow by the dam – as well as the tidal phase differences introduced by the presence of the dam (which is not negligible in length as compared to the tidal wavelength) – lead to hydraulic head differences along the dam. Turbines in the dam are used to convert power (6–15 GW per dam). In shallow coastal seas featuring strong coast-parallel oscillating tidal currents (common in the UK, China and Korea), a significant water level differential (2–3 meter) will appear between both sides of the dam.
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term marine current power. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.
2.1 Tidal stream generators
2.1.1 Types of tidal stream generators
Since tidal stream generators are an immature technology, no standard technology has yet emerged as the clear winner, but a large variety of designs are being experimented with, some very close to large scale deployment. Several prototypes have shown promise with many companies making bold claims, some of which are yet to be independently verified, but they have not operated commercially for extended periods to establish performances and rates of return on investments.
The European Marine Energy Centre[8] categorises them under four heads although a number of other approaches are also being tried.
Axial Turbines
Evopod - A semi-submerged floating approach tested in Strangford Lough.
These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. These include:
Kvalsund, south of Hammerfest, Norway.[9] Although still a prototype, a turbine with a reported capacity of 300 kW was connected to the grid on 13 November 2003.
A 300 kW Periodflow marine current propeller type turbine — Seaflow — was installed by Marine Current Turbines off the coast of Lynmouth, Devon, England, in 2003.[10] The 11m diameter turbine generator was fitted to a steel pile which was driven into the seabed. As a prototype, it was connected to a dump load, not to the grid.
Since April 2007 Verdant Power[11] has been running a prototype project in the East River between Queens and Roosevelt Island in New York City; it was the first major tidal-power project in the United States.[12] The strong currents pose challenges to the design: the blades of the 2006 and 2007 prototypes broke off, and new reinforced turbines were installed in September 2008.[13][14]
Following the Seaflow trial, a fullsize prototype, called SeaGen, was installed by Marine Current Turbines in Strangford Lough in Northern Ireland in April 2008. The turbine began to generate at full power of just over 1.2 MW in December 2008[15] and is reported to have fed 150 kW into the grid for the first time on 17 July 2008, and has now contributed more than a gigawatt hour to consumers in Northern Ireland.[16] It is currently the only commercial scale device to have been installed anywhere in the world.[17] SeaGen is made up of two axial flow rotors, each of which drive a generator. The turbines are capable of generating electricity on both the ebb and flood tides because the rotor blades can pitch through 180˚.[18]
OpenHydro, an Irish company exploiting the Open-Centre Turbine developed in the U.S., has a prototype being tested at the European Marine Energy Centre (EMEC), in Orkney, Scotland.
A prototype semi-submerged floating tethered tidal turbine called Evopod has been tested since June 2009 in Strangford Lough, Northern Ireland at 1/10th scale. The company developing it is called Ocean Flow Energy Ltd,[21] and they are based in the UK. The advanced hull form maintains optimum heading into the tidal stream and it is designed to operate in the peak flow of the water column.
Tenax Energy of Australia is proposing to put 450 turbines off the coast of the Australian city Darwin, in the Clarence Strait. The turbines feature a rotor section that is approximately 15 metres in diameter with a gravity base which is slighter larger than this to support the structure. The turbines will operate in deep water well below shipping channels. Each turbine is forecast to produce energy for between 300 and 400 homes.[22]
Vertical and horizontal axis crossflow turbines
Invented by Georges Darreius in 1923 and Patented in 1929, these turbines that can be deployed either vertically or horizontally.
The Gorlov turbine[23] is a variant of the Darrieus design featuring a helical design which is being commercially piloted on a large scale in S. Korea,[24] starting with a 1MW plant that started in May 2009[25] and expanding to 90MW by 2013. Neptune Renewable Energy has developed Proteus[26] which can be used to form an array in mainly estuarine conditions.
In late April 2008, Ocean Renewable Power Company, LLC (ORPC) [4] successfully completed the testing of its proprietary turbine-generator unit (TGU) prototype at ORPC's Cobscook Bay and Western Passage tidal sites near Eastport, Maine.[27] The TGU is the core of the OCGen technology and utilizes advanced design cross-flow (ADCF) turbines to drive a permanent magnet generator located between the turbines and mounted on the same shaft. ORPC has developed TGU designs that can be used for generating power from river, tidal and deep water ocean currents.
Trials in the Strait of Messina, Italy, started in 2001 of the Kobold concept.[28]
Oscillating devices
Oscillating devices do not have a rotating component, instead making use of aerofoil sections which are pushed sideways by the flow. Oscillating stream power extraction was proven with the omni- or bi-directional Wing'd Pump windmill.[29] During 2003 a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast.[30] The Stingray uses hydrofoils to create oscillation, which allows it to create hydraulic power. This hydraulic power is then used to power a hydraulic motor, which then turns a generator.[6]
Pulse Tidal operate an oscillating hydrofoil device in the Humber estuary.[31] Having secured funding from the EU, they are developing a commercial scale device to be commissioned 2012.[32]
The bioSTREAM tidal power conversion system, uses the biomimicry of swimming species, such as shark, tuna, and mackerel using their highly efficient Thunniform mode propulsion. It is produced by Australian company BioPower Systems.
A 2kW prototype relying on the use of two oscillating hydrofoils in a tandem configuration has been developed at Laval University and tested successfully near Quebec City, Canada, in 2009. A hydrodynamic efficiency of 40% has been achieved during the field tests.[33]
Venturi effect
Further information: Venturi effect
This uses a shroud to increase the flow rate through the turbine. These can be mounted horizontally or vertically.
The Australian company Tidal Energy Pty Ltd undertook successful commercial trials of highly efficient shrouded tidal turbines on the Gold Coast, Queensland in 2002. Tidal Energy has commenced a rollout of their shrouded turbine for a remote Australian community in northern Australia where there are some of the fastest flows ever recorded (11 m/s, 21 knots) – two small turbines will provide 3.5 MW. Another larger 5 meter diameter turbine, capable of 800 kW in 4 m/s of flow, is planned for deployment as a tidal powered desalination showcase near Brisbane Australia in October 2008. Another device, the Hydro Venturi, is to be tested in San Francisco Bay.[34]
2.1.2 Commercial plans
RWE's npower announced that it is in partnership with Marine Current Turbines to build a tidal farm of SeaGen turbines off the coast of Anglesey in Wales,[35] near the Skerries.[36]
In November 2007, British company Lunar Energy announced that, in conjunction with E.ON, they would be building the world's first deep-sea tidal energy farm off the coast of Pembrokshire in Wales. It will provide electricity for 5,000 homes. Eight underwater turbines, each 25 metres long and 15 metres high, are to be installed on the sea bottom off St David's peninsula. Construction is due to start in the summer of 2008 and the proposed tidal energy turbines, described as "a wind farm under the sea", should be operational by 2010.
British Columbia Tidal Energy Corp. plans to deploy at least three 1.2 MW turbines in the Campbell River or in the surrounding coastline of British Columbia by 2009.[37][dated info]
An organisation named Alderney Renewable Energy Ltd is planning to use tidal turbines to extract power from the notoriously strong tidal races around Alderney in the Channel Islands. It is estimated that up to 3 GW could be extracted. This would not only supply the island's needs but also leave a considerable surplus for export.[38]
Nova Scotia Power has selected OpenHydro's turbine for a tidal energy demonstration project in the Bay of Fundy, Nova Scotia, Canada and Alderney Renewable Energy Ltd for the supply of tidal turbines in the Channel Islands. Open Hydro
Pulse Tidal are designing a commercial device with seven other companies who are expert in their fields.[39] The consortium was awarded an €8 million EU grant to develop the first device, which will be deployed in 2012 and generate enough power for 1,000 homes. Pulse is in a good position to scale up production because the supply chain is already in place.
2.1.3 Energy calculations
Turbine power
Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "ξ" is known the equation below can be used to determine the power output of a turbine..
The energy available from these kinetic systems can be expressed as:[40]
where:
ξ = the turbine efficiency
P = the power generated (in watts)
ρ = the density of the water (seawater is 1025 kg/m³)
A = the sweep area of the turbine (in m²)
V = the velocity of the flow
Relative to an open turbine in free stream, depending on the geometry of the shroud shrouded turbines are capable of as much as 3 to 4 times the power of the same turbine rotor in open flow. .[40]
Resource assessment
While initial assessments of the available energy in a channel have focus on calculations using the kinetic energy flux model, the limitations of tidal power generation are significantly more complicated. For example, the maximum physical possible energy extraction from a strait is given by:[41][42]
where
ρ = the density of the water (seawater is 1025 kg/m³)
g = gravitational acceleration (9.81 m/s2)
ΔHmax = maximum differential water surface elevation across the channel
Qmax= maximum volumetric flow rate though the channel.
2.1.4 Potential sites
As with wind power, selection of location is critical for the tidal turbine. Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses. The following potential sites are under serious consideration:
• Pembrokeshire in Wales[43]
• River Severn between Wales and England[44]
• Cook Strait in New Zealand[45]
• Kaipara Harbour in New Zealand[46]
• Bay of Fundy[47] in Canada.
• East River[48][49] in the USA
• Golden Gate in the San Francisco Bay[50]
• Piscataqua River in New Hampshire[51]
• The Race of Alderney and The Swinge in the Channel Islands[38]
• The Sound of Islay, between Islay and Jura in Scotland[52]
• Pentland Firth between Caithness and the Orkney Islands, Scotland
• Humboldt County, California in the United States
2.1.5 Environmental Impacts
Very little direct research or observation of tidal stream systems exists. Most direct observations consist of releasing tagged fish upstream of the device(s) and direct observation of mortality or impact on the fish.
One study of the Roosevelt Island Tidal Energy (RITE, Verdant Power) project in the East River (New York City), utilized 24 split beam hydroacoustic sensors (scientific echosounder[53]) to detect and track the movement of fish both upstream and downstream of each of six turbines. The results suggested (1) very few fish using this portion of the river, (2) those fish which did use this area were not using the portion of the river which would subject them to blade strikes, and (3) no evidence of fish traveling through blade areas.
Work is currently being conducted by the Northwest National Marine Renewable Energy Center (NNMREC[54])to explore and establish tools and protocols for assessment of physical and biological conditions and monitor environmental changes associa er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
er than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed.[7] Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
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