09-01-2010, 03:18 PM
Tidal Power
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Tidal Power
Introduction
If there is one thing we can safely predict and be sure of on this planet, it is the coming and going of the tide. This gives this form of renewable energy a distinct advantage over other sources that are not as predictable and reliable, such as wind or solar. The Department of Trade and Industry has stated that almost 10% of the United Kingdomâ„¢s electricity needs could be met by tidal power.
Why do the tides come and go It is all to do with the gravitational force of the Moon and Sun, and also the rotation of the Earth. This is displayed in the following diagram
.
Figure 1 Gravitational effect of the Sun and the Moon on tidal range
(Adapted from Boyle, 1996)
Sourced: (ACRE) Australian CRC for Renewable Energy LTD
The diagram shows how the gravitational attraction of the moon and sun affect the tides on Earth. The magnitude of this attraction depends on the mass of the object and its distance away. The moon has the greater effect on earth despite having less mass than the sun because it is so much closer. The gravitational force of the moon causes the oceans to bulge along an axis pointing directly at the moon. The rotation of the earth causes the rise and fall of the tides. When the sun and moon are in line their gravitational attraction on the earth combine and cause a spring tide. When they are as positioned in the first diagram above, 90 from each other, their gravitational attraction each pulls water in different directions, causing a neap tide.
The rotational period of the moon is around 4 weeks, while one rotation of the earth takes 24 hours; this results in a tidal cycle of around 12.5 hours. This tidal behaviour is easily predictable and this means that if harnessed, tidal energy could generate power for defined periods of time. These periods of generation could be used to offset generation from other forms such as fossil or nuclear which have environmental consequences. Although this means that supply will never match demand, offsetting harmful forms of generation is an important starting point for renewable energy.
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State of the art / Current Status
There are two options for getting energy from the tide, a tidal barrage or utilising tidal streams.
THE TIDAL BARRAGE
Introduction
This is where a dam or barrage is built across an estuary or bay that experiences an adequate tidal range. This tidal range has to be in excess of 5 metres for the barrage to be feasible (iclei.org). The purpose of this dam or barrage is to let water flow through it into the basin as the tide comes in. The barrage has gates in it that allow the water to pass through. The gates are closed when the tide has stopped coming in, trapping the water within the basin or estuary and creating a hydrostatic head. As the tide recedes outwith the barrage, gates in the barrage that contain turbines are opened, the hydrostatic head causes the water to come through these gates, driving the turbines and generating power. Power can be generated in both directions through the barrage but this can affect efficiency and the economics of the project.
This technology is similar to Hydropower, something that we have a lot of experience with in Scotland. There is potential for a project of this kind in Scotland, one place in particular which has been looked at is the Solway Firth in south west Scotland, where there is a tidal range of 5.5 metres [1-The Open University Renewable Energy Pack T251].
The construction of a barrage requires a very long civil engineering project. The barrage will have environmental and ecological impacts not only during construction but will change the area affected forever. Just what these impacts will be is very hard to measure as they are site specific, and each barrage is different.
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Current Technology
The following diagram is a simplified version of a tidal barrage.
Figure 2 Ebb generating system with a bulb turbine
(Adapted from Energy Authority of NSW Tidal Power Fact Sheet)
Sourced: (ACRE) Australian CRC for Renewable Energy LTD
There are different types of turbines that are available for use in a tidal barrage. A bulb turbine is one in which water flows around the turbine. If maintenance is required then the water must be stopped which causes a problem and is time consuming with possible loss of generation. When rim turbines are used, the generator is mounted at right angles to the to the turbine blades, making access easier. But this type of turbine is not suitable for pumping and it is difficult to regulate its performance. Tubular turbines have been proposed for the UKâ„¢s most promising site, The Severn Estuary, the blades of this turbine are connected to a long shaft and are orientated at an angle so that the generator is sitting on top of the barrage. The environmental and ecological effects of tidal barrages have halted any progress with this technology and there are only a few commercially operating plants in the world, one of these is the La Rance barrage in France, for more information see the La Rance Case Study.
Figure 3: Bulb Turbine (Copyright Boyle, 1996)
Sourced: (ACRE) Australian CRC for Renewable Energy LTD
Figure 4: Rim Turbine (Copyright Boyle, 1996)
Sourced: (ACRE) Australian CRC for Renewable Energy LTD
Figure 5: Tubular Turbine (Copyright Boyle, 1996)
Sourced: (ACRE) Australian CRC for Renewable Energy LTD
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Pumping
The turbines in the barrage can be used to pump extra water into the basin at periods of low demand. This usually coincides with cheap electricity prices, generally at night when demand is low. The company therefore buys the electricity to pump the extra water in, and then generates power at times of high demand when prices are high so as to make a profit. This has been used in Hydro Power, and in that context is known as pumped storage.
The power available from the turbine at any particular instant is given by:
Where,
Cd = Discharge Coefficient
A = Cross sectional area (m2)
G = gravity = 9.81
r = density (kg/m3)
The discharge coefficient accounts for the restrictive effect of the flow passage within the barrage on the passing water.
The equation above illustrates how important the difference between the water levels of the sea and the basin, (Z1-Z2), is when calculating the power produced.
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Economics
The capital required to start construction of a barrage has been the main stumbling block to its deployment. It is not an attractive proposition to an investor due to long payback periods. This problem could be solved by government funding or large organisations getting involved with tidal power. In terms of long term costs, once the construction of the barrage is complete, there are very small maintenance and running costs and the turbines only need replacing once around every 30 years. The life of the plant is indefinite and for its entire life it will receive free fuel from the tide.
The economics of a tidal barrage are very complicated. The optimum design would be the one that produced the most power but also had the smallest barrage possible.
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Social Implications
The building of a tidal barrage can have many social consequences on the surrounding area. During the construction of the barrage, the amount of traffic and people in the area will increase dramatically and will last for a number of years. The La Rance tidal barrage in France took over 5 years to build. This will also bring revenue to the area from the tourism and hospitality industry that will accommodate all the different types of visitors that the barrage will bring. This will give a boost to the local economy.
The barrage can be used as a road or rail link, providing a time saving method of crossing the bay or estuary. There is also the possibility of incorporating wind turbines into the barrage to generate extra power. The barrage would affect shipping and navigation and provision would have to be made to allow ships to pass through.
The bay would become available for recreation; the waters would be calmer not immediately after the barrage but further in towards the land. This would be another tourist attraction and become a feature of the area.
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Environmental Aspects
Perhaps the largest disadvantages of tidal barrages are the environmental and ecological affects on the local area. This is very difficult to predict, each site is different and there are not many projects that are available for comparison. The change in water level and possible flooding would affect the vegetation around the coast, having an impact on the aquatic and shoreline ecosystems. The quality of the water in the basin or estuary would also be affected, the sediment levels would change, affecting the turbidity of the water and therefore affecting the animals that live in it and depend upon it such as fish and birds. Fish would undoubtedly be affected unless provision was made for them to pass through the barrage without being killed by turbines. All these changes would affect the types of birds that are in the area, as they will migrate to other areas with more favourable conditions for them.
These effects are not all bad, and may allow different species of plant and creature to flourish in an area where they are not normally found. But these issues are very delicate, and need to be independently assessed for the area in question.
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TIDAL STREAMS
Introduction
Tidal streams are fast flowing volumes of water caused by the motion of the tide. These usually occur in shallow a sea where a natural constriction exists which forces the water to speed up. The technology involved is very similar to wind energy, but there are some differences. Water is 800 times denser than air and has a much slower flowrate; this means that the turbine experiences much larger forces and moments. This results in turbines with much smaller diameters. The turbines must either be able to generate power on both ebbs of the tide or be able to withstand the structural strain. This technology is still in its infancy despite the potential for a reliable and predictable source; therefore it has not been included in the possible technologies discussed with relevance to the Renewables Obligation for Scotland.
The experiences from the development of wind power can be applied to the technology. Scotland has a definite potential for tidal stream energy to be converted to electricity, one area of focus is the Pentland Firth off the north coast.
Tidal stream technology has the advantage over tidal barrages when you compare environmental and ecological issues. This technology is less intrusive than on and offshore wind, and tidal barrages, any hazard to navigation or shipping would be no more than that experienced by current offshore installations. Tidal Stream systems often have to be installed in difficult coastal waters and the installation and maintenance methods are often complicated, but these hold they key for ensuring the success of the technology.
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Current Technology
Energy can be captured from tidal streams using two methods, Tidal fences and Tidal turbines.
Tidal Fences
These are effectively another form of tidal barrage. They therefore share some of the same environmental and social concerns, but also have the advantage of being able to have the electrical generators and transformers above the water. The flowing diagram shows an example of a tidal fence.
Tidal Turbines
This form of generation has many advantages over its other tidal energy rivals. The turbines are submerged in the water and are therefore out of sight. They donâ„¢t pose a problem for navigation and shipping and require the use of much less material in construction. They are also less harmful to the environment. They function best in areas where the water velocity is 2 - 2.5 m/s [2-Fujita Research]. Above this level the turbine experiences heavy structural loads and below this not enough generation takes place. The following diagram, figure X!!! is an impression of a tidal turbine farm.
One new technology that has been developed is the Stingray. This project has definite potential and planning is underway for a trial in the North of Scotland. For more information see the Stingray Case Study.
Impression of Tidal Turbine Farm
windenergy.co.uk/framestidal.htm
Economics
Tidal stream technology is still in its infancy and therefore there are no comparable projects at the present time. The cost of utilising tidal streams will be very site specific and depend on the technology used. The turbine or other generating plant equipment can be considered to have a similar cost to wind.
Once installed, electricity will be produced with no fuel costs and will be completely predictable. Maintenance costs will be the main costs during the life of the project.
Social Implications
Tidal Streams are common in remote areas. This means that careful consideration of the wishes of the local community is required to ensure the scheme can work to its potential. Being under water avoids aesthetic problems and shipping and navigation should not be affected provided it is taken into consideration when planning. The scheme can provide employment during construction and operation, which will add to the local economic prosperity. Also, these schemes are unique at present and would help to put the area on the map.
Environmental Aspects
The environmental effects of utilising tidal streams are in no way as severe as those for a tidal barrage. They will obviously affect the seabed where they are positioned and this might have an effect on the aquatic life in the area. This is again site specific and hard to predict; as long as proper environmental impact assessments are done then this can be avoided or minimised.
Conclusions
Tidal energy has potential to become a viable option for large scale, base load generation in Scotland. Tidal Streams are the most attractive method, having reduced environmental and ecological impacts and being cheaper and quicker installed. The proposed Stingray project is important to demonstrate the potential for the tidal energy industry in Scotland.
