Report Tidal Power Generation Systems

Transcription

1 The American University in Cairo Engineering Department ENGR 318 Spring 2001 Report Tidal Power Generation Systems Submitted to: Prof. Dr. Mahmoud Gilany By: Sherif Masoud Maher Amer Mohamed Samir

2 Introduction The World electrical energy market is at $800-billion-a-year (US) and rising. It has been estimated that " there are 2 billion people who still lack electricity today, and the world demand in developing countries is doubling every eight years" (World Watch Institute, May 1997.) In order to meet that demand, while limiting production of green house gases, renewable energy sources must be developed. The sea has long been seen as a source of energy. In the middle ages ( AD) farmers used to trap seawater in millponds and use it to power water mills as the tide dropped. 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, particularly in Europe, 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 that could withstand the extremely harsh marine environment. In the late 1990s, it has become clear that technology has advanced to the point where reliable and cheap electricity from the oceans is becoming a real possibility. Many countries are seriously considering taking advantage of the tidal power systems. Definition of Tidal Power Tidal power is the power inherent in tides at sea or oceans, that is the power of motion of water actuated by tides. Tides are defined as the increase and decrease in water levels due to the motion of water from one place to the other. This motion of water is actuated by large amounts of energy due to the movement of the Sun, Moon and Earth relative to each other and also to their rotational movement. Thus there is a renewable source of energy in the tidal motion of water at seas and oceans. This source of energy could be used to generate other types of energy that could be useful in industrial applications. The generation of electricity using tidal power is basically the transformation of tidal power found in tidal motion of water in seas and oceans into electrical energy. 3

3 This is done using a very basic idea involving the use of a barrage or small dam built at the entrance of a bay where tides are known to reach very high levels of variation. This barrage will trap tidal water behind it creating a difference in water level, which will in turn create potential energy. This potential energy will then be used in creating kinetic energy as doors in the barrage are opened and the water rush from the high level to the lower level. This kinetic energy will be converted into rotational kinetic energy that will rotate turbines giving electrical energy. Fig. 1 shows the process in very simple terms. Tidal motion Potential energy Kinetic energy Electrical energy Barrage Opening of barrage doors Using turbines Fig 1. A diagram showing transformation of tidal energy to electric energy. Physical Concepts of the Tidal Phenomena Tidal movements in seas are due to the increase of water levels at certain areas in the globe and the decrease of water levels at other areas. This is basically due to two factors: 1- The gravitational forces between the Sun, Moon and Earth. 2- The rotation of the moon and earth. As there are gravitational forces between the Moon and the Earth, seas or oceans water is pulled away from earth toward the moon at the area where the moon and the earth are in front of each other. At the opposite side of the earth the water is being 4

4 pushed away from the earth due to centrifugal forces. Thus as shown in Fig. 2 there are two areas where the water levels are high and other areas where the water level is low. Thus, the tidal motion of water is created. This is called the lunar tide. The same concepts that apply for the moon apply for the sun, yet, the sun has a smaller effect on the water levels but when that can only contribute or lessen the effect of the moons gravitational power. This is described by "spring tides" where the lunar tide and solar tide are aligned and contribute to each other and by "neap tides" where the lunar and solar tides are at right angles of each other and lessen each other. This is shown in Fig 2. Fig. 2. The spring and neap tides. How Tidal Power Generation Systems Work In very simple terms a barrage is built at the entrance of a gulf and the water levels vary on both sides of the small dam. Passages are made inside the dam and water flows through these passages and turbines rotate due to this flow of water under head of water. Thus, electricity is created using the turbines. A general diagram of the system is shown in Fig 3. What follows will be a description of a general tidal power station with its components. Also, many systems of power generation will be described. 5

5 Fig.3. General scheme of the tidal power station. The components of a tidal power station are: 1- A barrage: a barrage is a small wall built at the entrance of a gulf in order to trap water behind it. It will either trap it by keeping it from going into the gulf when water levels at the sea are high or it will keep water from going into the sea when water level at the sea is low. 2- Turbines: they are the components responsible for converting potential energy into kinetic energy. They are located in the passageways that the water flows through when gates of barrage are opened. There are many types of turbines used in tidal power stations: Bulb turbines: as shown in Fig. 4 these are difficult to maintain as water flows around them and the generator is in water. Rim turbines: as shown in Fig. 5 these are better maintained than the bulb turbines but are hard to regulate as generator is fixed in barrage. Tabular turbines: as shown in figure 6 these turbines are fixed to long shafts and thus solve both problems that bulb and rim turbines have as they are easier to maintain and control. Fig.4. A Bulb turbine. 6

6 Fig.5. Rim turbines. Fig.6. Tabular turbines. 3- Sluices: sluice gates are the ones responsible for the flow of water through the barrage they could be seen in Fig Embankments: they are caissons made out of concrete to prevent water from flowing at certain parts of the dam and to help maintenance work and electrical wiring to be connected or used to move equipment or cars over it. These embankments are shown in Fig 7. 7

7 Fig 7. Embankments. The following is a list of different methods of obtaining power from tidal power stations: 1- Ebb method: 1st- Water starts to ebb or go toward the sea. 2nd- The gates are left closed keeping the water trapped in basin to increase its level. 3rd- Then water is released out toward the sea rotating turbines creating electrical energy. 2- Flood method: 1st-Water is let into the basin when it is empty. 2 nd -As the turbines are rotated the electrical energy is created. 3- Ebb plus pumping method: 1st-The turbines are operated as pumps pumping the water into the basin at the flood period. 2 nd -The water level in the basin is increased creating greater head. 3 rd -At the ebb phase the water is let out of the basin creating energy for longer time than usual due to the increased head. 4- Two way power generation: 1st-Starting with the basin full the gates are opened letting water flow out generating energy. 2nd- The turbines are reversed as the flow will be reversed. 3rd- The gates are closed when the flood period or cycle starts. 8

8 4th- Water starts to build up behind the dam. 5th- When a sufficient head is achieved the gates are opened to start flood generation cycle as the water flows into the basin. 5- Two basin generation method: 1st-Two basins are built one called a high-level basin and the other is the low-level basin. 2nd- The turbines are placed in the wall dividing the two basins. 3rd- The high level basin is filled at high tide or flood period. 4th- Then the low-level basin is filled through the turbines from the high level basin. 5th- The low level basin is emptied at low tide ebb period. Advantages of Tidal Power Generation There are many advantages of generating power from the tide; some of them are listed below: Tidal power is a renewable and sustainable energy resource. It reduces dependence upon fossil fuels. It produces no liquid or solid pollution. It has little visual impact. Construction of large-scale offshore devices results in new areas of sheltered water, attractive for fish, sea birds, seals and seaweed. Tidal power exists on a worldwide scale from deep ocean waters. It offers short time scale between investing in the modular construction and benefiting from the revenue Tidally driven coastal currents provide an energy density four times greater than air, meaning that a 15-m diameter turbine will generate as much energy as a 60mdiameter windmill. Tidal currents are both predictable and reliable, a feature which gives them an advantage over both wind and solar systems. Power outputs can be accurately calculated far in advance, allowing for easy integration with existing electricity grids. 9

9 The tidal turbine offers significant environmental advantages over wind and solar systems; the majority of the assembly is hidden below the waterline, and all cabling is along the seabed. Seawater is 832 times as dense as air; therefore the kinetic energy available from a 5-knot ocean current is equivalent to a wind velocity of 270 km/h. Disadvantages and Constraints to Tidal Power Generation Unfortunately, there are also disadvantages and limitations to generating tidal power. Some of these are: At the present time both tide and wave energy are suffering from orientation problems, in the sense that neither method is strictly economical (except in few locations throughout the world) on a large scale in comparison with conventional power sources. Tidal power systems do not generate electricity at a steady rate and thus not necessarily at times of peak demand, so unless a way can be found of storing energy efficiently - and any storage devices currently available incur a considerable loss - they would not help in reducing the overall need for fossil power stations, but only allow them to run at a lower rating for a certain amount of the time. Tidal fences could present some difficulty to migrating fish. Examples of Tidal Power Stations Worldwide The most major tidal power station in operation today is a 240-megawatt at the mouth of the La Rance river estuary on the northern coast of France (a large coal or nuclear power plant generates about 1,000 MW of electricity). The La Rance generating station has been in operation since 1966 and has been a very reliable source of electricity for France. La Rance was supposed to be one of many tidal power plants in France, until their nuclear program was greatly expanded in the late 1960's. Elsewhere there is a 20 MW experimental facility at Annapolis Royal in Nova Scotia, and a 0.4 MW tidal power plant near Murmansk in Russia. This compares to 28 GW and 7-10 GW thought possible in the Bay of Fundy and the Severn Estuary (Great Britain) respectively. It has been estimated that this could supply as much as 10% of the country's electricity needs. 10

10 Tidal power has been proposed in the Kimberley region of Western Australia since the 1960s, when a study of the Derby region identified a tidal resource of over 3000 MW. In recent years a proposal to construct a 50 MW tidal plant in the Derby region has been developed Derby Hydro Power. Studies have been undertaken to examine the potential of several other tidal power sites worldwide. Similarly, several sites in the Bay of Fundy, Cook Inlet in Alaska, and the White Sea in Russia have been found to have the potential to generate large amounts of electricity. Conclusion Tidal power has the potential to generate significant amounts of electricity at certain sites around the world. Although our entire electricity needs could never be met by tidal power alone, it can be a valuable source of renewable energy to an electrical system. The negative environmental impacts of tidal barrages are probably much smaller than those of other sources of electricity, but are not well understood at this time. The technology required for tidal power is well developed, and the main barrier to increased use of the tides is that of construction costs. The future costs of other sources of electricity, and concern over their environmental impacts, will ultimately determine whether humankind extensively harnesses the gravitational power of the moon. 11

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