24-03-2011, 11:11 AM
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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
