Tidal Power Plants.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
TIDAL POWER
Tidal energy differs from all other energy
sources in that the energy is extracted from the potential and kinetic energies
of the earth-moon-sun system. The well known ocean tides result from this
interaction, producing variations in ocean water levels along the shores of all
continents. As the water level fluctuates twice daily through this range, it
alternately fills and empties natural basins along the shoreline, suggesting
that the currents flowing in and out of these basins could be used to drive
water turbines connected to generators and thus to produce electricity. The
higher the tides, the more electricity can be generated from a given site, and
the lower the cost of electricity produced. The technology employing this energy
source is very similar to that of low head hydropower.
POTENTIAL
World-wide, approximately 3000 GW of energy is continuously available
from the action of tides. Experts estimated that only 2% (60 GW), what is about
50 times less than the world’s potential of hydroelectric power capacity, can
potentially be recovered from tides for electricity generation. Currently, only
in places with large tidal range (greater than 5 meters) can tidal power be
extracted economically.
In some places of the world tidal energy is
quite attractive. For coastal areas, usually at the entrances to large
estuaries, resonance can occur, leading to far greater than average tidal ranges
which could relatively conveniently be blocked off. Such circumstances are found
e.g. in Canada, with a mean tidal range of 10,8 metres or in the Severn Estuary
in Britain with a mean range of 8,8 metres, making large scale projects at both
these locations economical.
DEVELOPMENT
Over the past forty years, there has been constant interest in harnessing
tidal power. Initially, this interest focused on estuaries, where large volumes
of water pass through narrow channels generating high current velocities.
Engineers felt that blocking estuaries with a barrage and forcing water through
turbines would be an effective way to generate electricity. From an engineering
point of view they were right. But, increasingly the environmental costs of such
a design became clear.
There are three commercial-scale tidal power plants (barrages) in
operation: a 240 MW plant which was completed on the estuary of the La Rance
River near St. Malo, France in 1967, a 1MW plant on the White Sea in
Russia completed in 1969 and a 16 MW plant in Nova Scotia, Canada. The
environmental problems have prevented further development of the barrage
technology.
Tidal power plant in La Rance.
 Tidal power plant at La
Rance River has turbines that can also serve as pumps; thus, the installation
can function as a pumped hydro storage facility to even out the loads on a large
electricity generating and distribution system. In this way water pumped into
the basin during times of low power demand increases the head on the turbines at
other times. Tidal range there is up to 13,4 meters. The dam’s width is 760
meters. At high tide, the dam traps Atlantic waters in the bay. At low tide, the
water flows back to the sea. En route, it passes through 24 turbines connected
to generators that produce 240 megawatts of power. This provides enough
electricity for a city of 300.000. In 1997, they began installing turbines that
can spin on both the incoming and outgoing tides.
TECHNOLOGY
Tidal power is a proven technology: it has been used for centuries in
small mill-type applications where natural conditions make it possible. Tidal
energy can be converted into electrical energy in several ways. Conventional
systems such as barrages (or low dams) store water in inlets from high tides for
release through hydraulic turbines during lower tides. The newest technology
which converts tidal or coastal currents to power seems to be very promising
because it is less environmentally destructive.
The usual technique (referred to as “barrage” technology) is to dam a
tidally-effected estuary or inlet, allowing the tidal flow to build up on the
ocean side of the dam and then generating power during the few hour high tide
period. In this way it is functioning in La Rance. After the water level reaches
maximum high tide, gate valves are closed and the water is impounded and awaits
low tide when it is released and produces power. The gates can be opened or
closed in sequence with the tides permitting water flow only when there is
sufficient head to power the turbines. The basic technology of power production
is similar to that for low head hydro power plants what means that the head
drives the water through the turbine generators. The main difference, apart from
the salt water environment, is that the turbines in tidal barrages have to deal
with regularly varying heads of water. The turbines are designed so that the
flow of water both into and out of the basin produces electricity. Because of
the intermittent nature of this flow, the effective duty factor of such an
installation is less than 100%. A tidal power station produces only about one
third as much electrical energy as would a hydroelectric power plant of the same
peak capacity operating continuously. Tidal barrages are effectively fences
which completely block a estuary channel.
ENVIRONMENT
The
barrage does not easily scale up to modern commercial levels of output capacity.
By increasing the size of the pond one increases the four major negative
environmental impacts of the barrage technology: navigation is blocked, fish
migration is impeded and fish are killed by passing through the turbines, the
location and nature of the intertidal zone are changed, and the tidal regime is
changed downstream. Reduced tidal range would destroy much of the habitat used
by wading birds, fish (such as salmon) would be unable to travel upstream to
breed, and sediment trapped behind the barrage could quickly reduce the volume
of the estuary. It seems that while there are few environmental impacts
associated with a smaller tidal facility, (i.e., no siltation, no negative
impacts to water tables, fisheries or fish migration), larger operations could
potentially limit fish and mammal passage and change tidal ranges, thereby
effecting salt water intrusion into local tributary streams and impacting salmon
spawning.
TIDAL TURBINES
By the early 1990s, interest in estuarine-derived tidal power had
declined, and scientists and engineers began to look at the potential of coastal
currents which can be harnessed by tidal turbines. Instead of using costly
barrages and low head turbines located in estuaries, it may be possible to
harness the kinetic energy of the tides in fast tidal currents or streams at
suitable sites, using relatively simple techniques - tidal turbines. As tides
ebb and flow, currents are often generated in coastal waters (quite often in
areas far-removed from bays and estuaries). In many places the shape of the
seabed forces water to flow through narrow channels, or around headlands (much
like the wind howls through narrow valleys and around hills). However, sea water
has a much higher density than air (832 times). Thus, currents running at
velocities of 5 - 8 knots (9,25 km/h – 16,7 km/h) have the same energy potential
as a windmill site with windspeeds of 390 km/hr! In addition, unlike the wind
rushing through a valley or over hilltops, tidally-generated coastal currents
are predictable. The tide comes in and out every twelve hours, resulting in
currents which reach peak velocity four times every day.
Tidal turbines are the chief competition to the tidal barrages but the
idea is as yet relatively underdeveloped. Looking like an underwater wind
turbine they offer a number of advantages over the tidal barrages. They are less
disruptive to wildlife, allow small boats to continue to use the area, and have
much lower material requirements than the dam. Tidal turbines function well
where coastal currents run at 2-3 m/s (slower currents tend to be uneconomic
while larger ones put a lot of stress on the equipment). In such currents a
turbine 20m in diameter will generate as much energy as a 60m diameter windmill.
The advantages of the tidal turbine is that it is neither seen, nor heard. The
whole assembly (apart from the transformer) is below the waterline.
There are many sites around the world where tidal turbines would be
effective. Coastal currents are strongest at the margins of the worlds larger
oceans. A review of likely tidal power sites in the late 1980s estimated the
energy resource was in excess of 330.000 MW. South East Asia is one area where
it is likely such currents could be exploited for energy. In particular, the
Chinese and Japanese coasts, and the large number of straits between the islands
of the Philippines are suitable for development of power generation from coastal
currents. In all of these regions underwater turbine farms can be developed. The
ideal site is close to shore, in water depths of about 30m where at the best
sites currents could generate more than 10 megawatts of energy per square
kilometre. The European Union has already identified 106 sites which would be
suitable for the turbines, 42 of them around the UK. The first tidal turbines
will be deployed off the Southwest coast of England. It will be 12-15 m in
diameter, and is expected to generate 300 kW (enough to power a small village).
It is estimated that the cost of energy from these early turbines will be
USD 0,10/kWh. This is more expensive than conventional sources of energy
(coal, gas), but significantly lower than what many island communities already
pay for energy. As the technology matures further, prices will probably continue
to drop.
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