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Tidal Energy Harvesting

By Prof. A. R. Ghode, Mr.Kukkar Paresh K. Amrutvahini College of Engineering, Sangamner

Abstract Tidal Energy or achieved by capturing the energy contained in moving water mass due to . Two types of tidal energy can be extracted: kinetic energy of current between ebbing and surging tides and potential energy of currents between high and low tides. The f ormal method – generating energy f rom tidal current – is considered much more f easible today than building ocean-based dams or barrages, and many coastal sites worldwide are being examined f or their suitability to produce tidal energy. Tidal power is reliable predictable (unlike wind energy and solar power).

1.0 INTRODUCTION As a brief introduction, I would like to explain my interests in studying Tidal Power as a means f or generating reliable, carbon-f ree electricity.

It’s a plain f act that we are in a world where almost 80 percent of the demanding energy is f urnished by sources such as natural gas, coal, or oil, which are quickly being depleted as well as being environmentally unf riendly. We have also developed some destructive processes such as the nuclear power plants, which would also be a sword of Damocles of all human beings. Luckily, we have already realized the importance of making an enormous change in our way of lif e and our way of using the energy, so looking f or renewable resources to substitute current ones is much urgent f or us. Tidal power is classif ied as a source, because tides are caused by the orbital mechanics of the solar system and are considered inexhaustible within a human timef rame. Energy f rom tidal power is also a f orm of pollution f ree energy, which has a lot of potential. Though these potentials have not been f ully realized yet, we can’t deny the advantage of such kind of a renewable energy. This paper gives some basic introductions of tidal power and the basic principle of how tidal generator works, and it also f ocuses on the development of tidal power energy of the world.

1.1 History(Eling Mill) The Eling Mill, located in the south of England, is an excellent demonstration of how tidal mills may have worked over a thousand years ago. Eling Mill is a tidal powered f lour mill and has been f or many centuries. The mill was included in the domes day survey in1086, which took an inventory of owned what throughout the country on England. Originally the Mill was owned by the King because Eling was a royal manor, but King John sold the mill in the early 13th century. Historically, a mill was built on the site of a f ormer mill in 1419 by Thomas Millington. Most of the grain that was milled at the site was not locally produced. Of tentimes, grain f rom several hundred miles around the coast was brought to the Mill by ship. At maximum output the mill would have produced f our tons of f lour each day

The Mill that currently occupies the site was reconstructed in the 1770s af ter several f loods damaged the millhouse and the dam. It has two separate wheels, each with its own machinery which allows two dif f erent milling operations to occur in the same mill. In 1382, the Mill was purchased by the Bishop of Winchester and was given to Winchester College as a means to f und the college. Winchester College owned the Mill f or over f ive hundred years, until 1975 when the New Forest District Council purchased it. Although the Eling Mill has been rebuilt quite a f ew times, it has basically operated in the same manner f or over 900 years. In the late 1800s, large steam-powered roller mills were built throughout the country to mill imported grain, usually f rom Canada. Many of the tidal mills were f orced to close, and only several of these historic sites remain today. The local governing body, the New Forest District Council, restored the mill and reopened it in 1980. Eling is unique in that it remains a f unctional mill that produces f lour.

1.2 Need of Tidal Energy Aside f rom my f ascination with the Norf olk Tides minor league baseball team, I knew nothing about this renewable source of energy and wanted to discover the science behind it, and the potential of harnessing the crash of the ocean. Both old school and cutting edge, tidal power is always of f ered as an alternative energy source, but is largely ignored in f avor of wind and solar power. However, renewable energies are like Slim-f ast cookies: using less polluting technologies to produce our power does not mean we can over- consume. As a replacement to traditional f ossil f uels, tidal power can make a signif icant contribution on a local and regional scale to the power grid of several countries. Although there are signif icant environmental impacts f rom large-scale tidal schemes, there are also existing environmental impacts f rom coal power plants, oil ref ineries, etc. so I believe we should educate ourselves about the costs and benef its of a wide variety of energy sources. Dif f erent localities will be impacted in unique ways, so what is true f or one community may not be true f or another. We should not let indecision over environmental concerns stagnate possibilities f or cleaner energy, but let it urge us to look f or the best solution available to us at the current level of technology and society. Cooperative and democratic governing structures will enable clear communication between the various stakeholders: f rom citizens to cabinet members, we will be more f lexible and willing to work together not only to create new power schemes, but also to allocate resources more equitably.

Every land is surrounded by copious amounts of tidal water. The periodic dif f erential water leveling created by lunar/earth interactions creates a predictable system capable of generating respectable amounts of kinetic energy through artif icial damming. Meanwhile, of f shore, strong underwater currents are potential sources f or copious amounts of energy through the use of ocean f loor turbines. Interestingly, the UK has major advantages in both systems over the US; the waters around the UK have substantially greater tidal dif f erences between low and high tide levels and a variety of cool water channels that house persistent ocean currents. Ironically, at the same time, the US embraces the idea of renewable energy through such systems much more f irmly than the UK. Not to be taken less seriously is the potential f or wave energy to one day be the leader in renewable energy systems in both countries. This document will explore the possibilities of all three energy systems while keeping in mind the relatively dif f erent general attitudes toward each nation’s waterways.

1.3 Objectives 1. Utilize an advanced ocean circulation numerical model (ROMS) to predict tidal currents. 2. Validate the velocities and water levels predicted by the model with available data. 3. Compute the tidal harmonic constituents f or the velocities and water levels. 4. Build a GIS database of the tidal constituents. 5. Develop GIS tools f or dissemination of the data. a. A f ilter based on depth requirements. b. Compute current velocity histograms based on the tidal constituents. c. Compute the available power density (W/m2) based on the velocity histograms. d. Compute the total available power within arrays based on turbine perf ormance parameters. 6. Develop a web based interf ace f or accessing the GIS database and using the GIS tools.

2.0 METHODOLOGY 2.1 Turbine Types 2.1.1 Waterwheel Turbines 2.1.1.1 Undershot Wheel Waterwheels were used f rom the invention of the tidal mill until the industrial revolution. The f irst turbine used was the basic undershot waterwheel. This is probably the oldest type of waterwheel dating back over two thousand years. It is mounted vertically on a horizontal axle and it has f lat boards located radially around a rim. It is turned by water f lowing under the wheel and striking the boards.

2.1.1.2 Overshot Wheel The second type of turbine used was an overshot waterwheel. The overshot wheel is much more ef f icient than the undershot wheel. Again, this turbine is mounted vertically on a horizontal axle, but the overshot wheel has buckets mounted around the rim.

Water f rom above f lows into the buckets, causing one side of the wheel to be heavier. Gravity then acts on the heavier side causing the wheel to turn.

2.1.1.3 Breast-shot Wheel The third type of turbine used was a breast-shot waterwheel. This type of wheel was developed in the late middle ages and combines the previous two waterwheels. It has buckets on a rim that f ace the opposite direction of the buckets on the overshot wheel. Water then f ills the buckets at the middle of the wheel. Again, gravity acting upon the water in the buckets causes the wheel to turn. 2.1.2 Tidal Steam Turbine These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. The turbines f eature a rotor section that is approximately 15 meters 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 f orecast to produce energy f or between 300 and 400 homes.

It can be f loated out to site, installed without cranes, jack-ups or divers, and then ballasted into operating position. At f ull scale the Triton 3 in 30-50m deep water has a 3MW capacity, and the Triton 6 in 60-80m water has a capacity of up to 10MW, depending on the f low. Both platf orms have man-access capability both in the operating position and in the f loat-out maintenance position. 2.1.3 Recent Turbine Developments Bulb turbines incorporated the generator-motor unit in the f low passage of the water. These turbines are used at the La Rance power station in France. The main drawback is that water f lows around the turbine, making maintenance dif f icult.

Rim turbines allow the generator to be mounted in the barrage, at right angles to the turbine blades. It is dif f icult to regulate the perf ormance of these turbines and it is unsuitable f or use in pumping.

Once the development of more tidal schemes occurs, additional types of turbines will be tested and implemented.

2.3 Construction In terms of construction, caissons, which are large units of concrete or steel that, are manuf actured at shore-based construction yards are delivered to water sites by barges and then positioned by cranes to allow f or the structures to correctly settle on the marine f loor. Overall, this is an extremely expensive process. Another method calls f or constructing diaphragm walls of reinf orced concrete within a temporary sand island. But the approach of f ers no signif icant cost advantages over caissons and studies f or the proposed Mersey Barrage in the United Kingdom indicate that the use of diaphragm walling could prolong construction time by about two years. (Johansson 519)

2.3.1 The tidal barrage is similar to a dam, which creates a tidal basin used f or electricity generation. The structure is extremely large, spanning the entire width and height of the estuary. The bottom of the barrage is located on the sea f loor and the top is above the highest level that the water can get at high tide 2.4 Working of Mill The basic schematic diagrams below show how the mill is run generally. When the tide comes in, one way gates are pushed open and the tide pond is f illed. At Eling, the tide pond is over 3 km long. At high tide, the mill cannot work because the water high in the tide pond is equal to that on the sea side of the mill, and no water f lows to cause the wheel to turn. Also, the wheel is under water, so there is too much drag f or the wheel to even turn. As the tide starts to f all, called the ebbing tide, the gates are f orced close, and the water is trapped in the tide pond at its high tide level. A waterwheel sluice is regulated the amount of water which is allowed to turn the blades of the wheel. When the water level f alls completely below the wheel, milling is started by opening the sluice several centimeters allowing water f rom the tide pond to turn the wheel. From this point, the tide then gets to its lowest height, and again starts to rise. Milling continues through this until the water rises to the bottom of the wheel. At Eling, the time of milling is approximately f ive hours. Because tides change twice a day, there are two milling periods each day of f ive hours each with the later milling period occurring twelve and a half hours af ter the f irst.

Letter A designates the sea hatches, or gates. B represents the Weir or Tumbling bay, which automatically maintains the head of water required to work the mill machinery. It can also act as a spillway f or f lood control. C corresponds to sluices that control the amount of water f rom the tide pond contracting the wheel.The picture below shows the Eling site, and each of the components f or the tidal scheme.

The Eling Tide Mill provides an excellent example of how tidal scheme technology had remained unchanged f or thousands of years. 2.4.1 From Milling to Electricity In order to create enough electricity to be economically f easible, the size and conf iguration of the structure has to be increased tremendously. Tidal Energy consists of generating kinetic energy f rom potential energy. If f alling water is f orced through ducts with rotators attached to them, the rotors will turn driving electric generators (Mc Gown 182). Generating electricity f rom tides is very similar to hydroelectric generation, except the tides f low in two directions rather than one. For tidal power, the most common generating system is the ebb generating system. In the scheme, a dam, or barrage is constructed across an estuary. The tidal basin is allowed to f ill when the sluice gates are opened and high tide is in. The gates are then closed when the tide turns trapping the water behind the gates. Once low tide is reached, the gates are opened the water f lows through the turbines located underneath the water generating electricity. The basic concept f or this type of scheme is extremely similar to that used at the Eling Mill. The schematic below shows the basic concept used in an ebb generation scheme.

In some cases, double ef f ect turbines are used, which are able to generate electricity when then basinis f illing. In this scheme, sluice gates located on either side of the turbine are opened, when the tidal basin is low, and the sea is at high tide level. Water will rush into the tidal basin, turning the turbines and generating electricity. This occurs until the water level on either side of the barrage is equal. Atthis point, the sluice gates are closed until the sea is at its low tide height. When this occurs, the gates are opened and water f lows f rom the basin to the sea, generating electricity a second time.

3. ADVANTAGES & DISADVANTAGES OF TIDAL ENERGY 3.1 Advantages • It is an inexhaustible source of energy. • Tidal energy is environment f riendly energy and doesn’t produce greenhouse gases. • As 71% of Earth’s surf ace is covered by water, there is scope to generate this energy on large scale. • We can predict the rise and f all of tides as they f ollow cyclic f ashion. • Ef f iciency of tidal power is f ar greater as compared to coal, solar or wind energy. Its ef f iciency is around 80%. • Although cost of construction of tidal power is high but maintenance costs are relatively low. • Tidal Energy doesn’t require any kind of f uel to run. • The lif e of tidal energy power plant is very long. • The energy density of tidal energy is relatively higher than other renewable energy sources.

3.2 Disadvantages • Cost of construction of tidal power plant is high. • There are very f ew ideal locations f or construction of plant and they too are localized to coastal regions only. • Intensity of sea waves is unpredictable and there can be damage to power generation units. 4.0 CONCLUSION The evolution of tidal energy f rom a primitive means of grinding grain, to a power stationgenerating over 8,000 megawatts is f ascinating. Today it is suf f icient to say that in most parts of America electricity is almost a basic human need. In the f uture we look to renewable energy f or a means of providing us with energy, as well as not damaging the air and water that we need f or survival. Tidal energy can of f er this, but only if we are willing to pay the price f or a clean environment.

REFERENCES • American Fisheries Society. “Tidal Power Development in Estuarine and Marine Environments”. http://www.f isheries.org/resource/page15.htm • Eling Tide Mill Trust Ltd. 2000, “Eling Tide Mill” (Online) Available: http://www.eling.aaugonline.net/sitem.html • Renewable Energy: Sources f or Fuel and Electricity. Island Press. Washington D.C. 1993. • “La Rance Tidal Barrage”. (Online) Available: http://www.esru.strath.ac.uk/EandE/Web_sites/01- 02/RE_inf o/tidal1.htm • United States. Cong. Senate. Committee on Environment and Public Works. The Ef f ects of the Proposed Tidal Hydroelectric Project in the Bay of Fundy. 98th Cong., 1st sess. S. Hrg 98-233. Washington: GPO, 1983. • White, Frank M. Fluid Mechanics. 4th ed. Boston: McGraw-Hill Inc., 1999.

We at engineeringcivil.com are thankful to Prof. A. R. Ghode and Mr.Kukkar Paresh K for submitting their paper on “Tidal Energy Harvesting”to us. We are sure this will be of great help to those seeking more information about how to harvest energy from Tidal Energy.