25-05-2012, 05:45 PM
OCEAN ENERGY TECHNOLOGIES for RENEWABLE ENERGY GENERATION
Ocean Energy Technologies.doc (Size: 2.6 MB / Downloads: 47)
INTRODUCTION
The oil crisis in the 1970s emphasized the need to find other solutions to satisfy the growing global demand of energy.
The world's oceans may eventually provide us with energy to power our homes and businesses. Right now, there are very few ocean energy power plants and most are fairly small. But how can we get energy from the ocean that will also be cost competitive?
There are four 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; we can harness underwater currents; or we can use temperature differences in the water.
The most concrete example of this technology using ocean energy from the tides, located
in La Rance, France, was inaugurated in 1966 and so promoted technologies from the ocean for more development.
At the moment, the use of ocean energy remains mainly at the prototype stage. Yet some companies have set up commercial scale products in the last few years; currently, prototypes are no longer confined only to laboratories of universities. One of the recent achievements is the RITE Project on the East River – New York, NY. It is one proof of
the ability to create profitable and sustainable projects.
TIDAL ENERGY
The tides
The tides are cyclic variations in the level of seas and oceans. In effect, the tides represent
the planetary manifestation of the potential and kinetic energy fluxes present in the Earth–Moon–Sun system. This results in some regions of the world possessing substantially higher local tidal variation than others.
Use of the tides
There are two different means to harness tidal energy. The first is to exploit the cyclic
rise and fall of the sea level using barrages and the second is to harness local tidal currents, analogous to wind power also called ‘marine current turbine.’
Tidal barrage methods
Currently several places in the world are producing electricity from tides.
The biggest tidal barrage power plant is located in La Rance, France. It has been operating since 1966, generates 240MW. Thanks to a road crossing of the estuary, the financial profitability is guaranteed. Other operational barrage sites are in Nova Scotia
(20MW), near Murmansk, Russia (0.4MW) and the Eastern seaboard of China (3.2MW).
Principle of operation
An estuary or bay with a large natural tidal range is artificially enclosed with a barrier.
Electrical energy is produced by allowing water to flow from one side of the barrage to the other. To generate electricity, tides go through low-head turbines.
There are a variety of modes of operation. These can be broken down initially into single basin schemes or multiple basin schemes. The simplest of these are the single basin schemes.
Single basin barrage
It requires a single barrage across the estuary. That involves a combination of sluices
which when open can allow water to flow relatively freely through the barrage and gated turbines. These gates can be opened to allow water to flow through the turbines to
generate electricity.
Ebb generation mode
Ebb generation depends on the height of the tides. At high tide, water is retained behind the barrage by the sluices. At low tides, water flows reverse out through the turbine.
Double basin systems
Double basin systems allow for storage (adjusting the power output to demand of consumers). The main basin behaves like the Ebb generation mode.
Marine current turbine
Tidal stream generators harness energy from currents generally in the same way as wind turbines. The higher density of water, 832 times the density of air, means that a single generator can provide significant power at low tidal flow velocities (compared with wind speed). 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 (tide range is at a minimum).
Ocean current resources are likely limited to the Florida Current, which flows between
Florida and the Bahamas. Estimates of the energy present in the Florida Current date
back to the mid-1970s, when the use of this resource for electricity generation was first proposed. While these early studies indicate an energy flux potential of as much as
25,000 MW through a single cross-sectional area, the amount of energy that could be extracted is uncertain, primarily because of concerns that reducing the energy in this portion of the Gulf Stream could have negative environmental consequences. Early modeling suggested that an array of turbines totaling 10,000 MW of capacity would not reduce the current's speed by more than what has been observed as its natural variation, and thus might be feasible (Lissaman and Radkey 1979; Charlier and Justus 1993).
Further investigation is required to determine the magnitude of the technically available resource.
Since tidal stream generators are an untested technology, no commercial scale production facilities are yet routinely supplying power; no standard technology has yet emerged. A large variety of designs are being experimented with, some very close to large scale deployment. Several prototypes have shown promise with many companies, but they
have not operated commercially for extended periods to establish performances and rates
of return on investments.
Two kinds of footings exist for the deep water installations. Conventionally the fixed systems are useful for shallow water sites, moored systems for deep water. The European Marine Energy Centre categorizes them under three headings.
Axial turbines
Axial turbines 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. Although still a prototype, a turbine with a reported capacity of 300 kW was connected to the grid on 13 Nov. 2003.
• A 300 kW period flow marine current propeller type turbine — Seaflow — was installed by Marine Current Turbines off the coast of Lynmouth, Devon, England,
in 2003. The 11 meter 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 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. 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.
• Following the Seaflow trial, a full-size 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 1.2 MW in December, 2008, and was reported to have fed 150 kW into the grid for the first time on July 17, 2008.
It is currently the only commercial scale device to have been installed anywhere
in the world.