Saturday, April 24, 2010

harnessing energy from moon

INTRODUCTION
'''Tidal power''', sometimes called '''tidal energy''', is a form of hydropower that exploits the movement of water caused by tidal currents or the rise and fall in sea levels due to the tides.
Although not yet widely used, tidal power has potential for future electricity generation and is more predictable than wind energy and solar power.
This is the only form of energy whose source is the moon. Some other energy sources, nuclear power and geothermal energy for instance, have the Earth as their source. The remainder, Fossil fuels, Wind energy, biofuels, solar energy, etc. have the Sun as their source, directly or indirectly.
The tidal power is generated by the Gravitational pull of the Moon on water. Due to these Gravitation gravitational forces the water level follows a periodic high and low. The height of the tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the local shape of the sea floor.
The tidal energy generator uses this phenomenon to generate energy. The higher the height of the tide the more promising it is to harness tidal energy. Here the diagram show when tides make up and take off.



CATEGORIES OF TIDAL POWER
Tidal power can be classified into two main types:
Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to wind mills use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact.
Barrages make use of the potential energy from the difference in height (or) Hydraulic head between high and low tides. Barrages suffer from the problems of very high civil infrastructure costs, few viable sites globally and environmental issues.
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean especially tidal currents using the tidal stream designs. Tidal stream turbines may be arrayed in high velocity areas where natural flows are concentrated
A factor in human settlement geography is water. Human settlements have often started around bays, rivers, and lakes. Future settlement may one day be concentrated around moving water, allowing communities to power themselves with non-polluting energy from moving water.
HOW IT WORKS
Specifically, the barrage (dam) blocks the incoming and out-going tides of a coastal basin. The barrage is equipped with sluices and turbines that will permit the retention of water entering at high tide and release it at low tide; normal turbines will produce electricity as the water flows out; reversible blade turbines, however, can produce electricity both as the water enters the basin and when it leaves. The basic difference between a hydraulic power plant on a river and a tidal power plant is this two-directional flow . If navigation to the upper part of the basin is necessary, a ship lock may be installed.




OPEN FLOW TURBINES
they are just as wind turbines, but underwater to which a generator is connected generates energy. the picture of turbine is shown below.

SHROUDED TIDAL TURBINES
An emerging tidal stream technology is the shrouded tidal turbine enclosed in a Venturi effect Venturi shaped shroud or duct producing a sub atmosphere of low pressure behind the turbine, allowing the turbine to operate at higher efficiency (than the Wind turbine design Axial momentum and the Betz limit of 59.3%) and typically 3–4 times higher power output than a turbine of the same size in free stream.
Considerable commercial interest has been shown in recent times in shrouded tidal stream turbines they can produce 3-4 times the power output of simialr sized open turbine.
While the shroud may not be practical in wind, as the next generation of tidal stream turbine design it is gaining more popularity and commercial use in Australia make use of the design and Lunar Energy use a double ended shroud. The tidal turbine is mono directional and the Lunar Energy turbine bi directional. Both constantly need to face upstream in order to operate. In the Tidal Energy Pty Ltd case this can be achieved by floated under a pontoon on a swing mooring, fixed to the seabed on a mono pile and yawed like a wind sock to continually face upstream. Lunar Energy use a wide angle diffuser to capture incoming flow that may not be inline with the long axis of the turbine. A shroud can also be built into a tidal fence or barrage increasing the performance of turbine


.

Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "Cp" is known the equation below can be used to determine the power output.
The energy available from these kinetic systems can be expressed as:
*P = Cp x 0.5 x ρ x A x V³
Where
Cp is the turbine coefficient of performance
P = the power generated (in Watts)
ρ = the density of the water (seawater is 1025 kg/m³)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)
Relative to an open turbine in free stream. Shrouded turbines are capable of higher efficiencies as much as 3–4 times the power of the same turbine in open flow.

The following equation can be used to calculate the revenue from a tidal stream turbine. By substituting variables such as size of the turbine, flow velocity and price into the equation it is possible to accurately predict an annual return.
Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y (x 3 for shrouded turbines)
Where:
Cp = the turbine coefficient of performance (say 20% for free stream or 60% for shrouded)
ρ = the density of the water (seawater is 1025 kg/m³ or 998 kg/m³ for fresh water)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)
Hr = the number of hours per day that the turbine would operate at maximum efficiency (12-22 hours for tidal and 24 for run of river)
LL = x .95 line losses (multiply by .95 )assuming a 5% loss in a cable run of 1000 meters. This may vary by manufacturer.
Gearbox and Generator Losses = x .95 (multiply by .95) assuming 5% for gearbox and generator losses
$ = the price per watthour that would be paid (prices vary with location)
Year = 350 days (allowing 15 days per year for maintenance if necessary)
Shrouded turbine produce approximately 3 times as much revenue.
For example, a tidal stream turbine with a sweep area of 1m² at a site with a 3 m/s flow velocity, operating at maximum output for 12 hours, and earning 10 cents per kilowatthour would earn
Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y
Annual Revenue = 0.20 x 0.5 x 1025 x V³ x 12 x 0.95 x 0.95 x 0.10/1000 x 350
Annual Revenue = $1,049.02 (or $3,147.06 for a shrouded turbine)
Keeping in mind this is only a 1m² sized turbine, in 3m/s flow velocity for only 12 hours per day. Many commercial turbines are 20-30 times or greater in size, in faster flow velocity, at 20 or more hours per day. A run of river turbine would operate for as long as the river flows, which is obviously 24 hours per day.
As the flow velocity doubles, the revenue increases by 8 times (as power is a function of the velocity cubed). The same turbine given in the example above, if installed in a 6 m/s velocity flow, would return $8,392 (or $25,176 for a shrouded turbine) for every square meter of sweep area of the turbine.

ENVIRONMENTAL IMPACT
The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the ecosystem. Many governments have been reluctant in recent times to grant approval for tidal barrages.
TURBIDITY
Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.
SALINITY
As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem.
SEDIMENT MOVEMENTS
Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.
FISH
Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15%
ENERGY EFFICIENCY
Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity, which is efficient compared to other energy resources such as solar power or fossil fuel power plants.

ADVANTAGES
•No pollution
•Renewable resource
•More efficient than wind because of the density of water
•Predictable source of energy vs. wind and solar
•Second generation has very few disadvantages
–Does not affect wildlife
–Does not affect silt deposits
–Less costly – both in building and maintenance
DISADVANTAGES
•Presently costly
–Expensive to build and maintain
–A 1085MW facility could cost as much as 1.2 billion dollars to construct and run
•Connection to the grid
•Technology is not fully developed
•Barrage style only produces energy for about 10 hours out of the day
•Barrage style has environmental affects
–Such as fish and plant migration
–Silt deposits
–Local tides change- affects still under study
CONCLUSION
A tidal power scheme is a long-term source of electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tonnes of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere.
If fossil fuel resource is likely to decline during the 21st century, as predicted by Hubbert peak theory, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

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