FACT SHEET 10: TIDAL ENERGY

FACT SHEET 10:
                                             TIDAL ENERGY

Ocean tides can be used to produce electricity. Tides occur twice a day and
are caused by the gravitational effect of the Moon, and to a lesser extent
the Sun, on the world's oceans. Tidal energy has been used since the 11th
Century in Britain and France for milling grains. It is a form of
hydroelectricity but the height of the ‘dammed’ water is generally less than
10 metres.

Tidal Energy - Site Selection Criteria
The height of tides varies greatly with location.

 Figure 1.
 Global tidal ranges around the world
 (from Davies 1964) © Western Power

A place which might be suitable to produce electricity from tidal energy
would generally have a tidal range greater than 7 metres, a narrow
entrance to the inlet and plenty of water moving through it at each tide.
There should also be a nearby demand for electricity, otherwise the energy
that is produced has to be stored in some way or transported to where it is
needed, which increases the cost. Environmental impacts need to be
assessed.

Generating Electricity from the Tide
Turbines can make electricity when the water turns their blades. The
simplest electricity generation system using tides is known as an ebb
generating system. It uses a dam, known as a barrage, across an estuary.
Sluice gates on the barrage are opened to allow the tide to flow into the
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estuary on the incoming high tides. They are closed to prevent the water
flowing back on the outgoing tide (known as the ebb tide) except through
the turbine system.

 Figure 2.
 Ebb generating system with a bulb turbine
 (Adapted from Energy Authority of NSW Tidal Power Fact Sheet)

Two way generation systems, which generate electricity on both the
incoming and outgoing tides, are also possible.

Benefits of and constraints to
Tidal Power Generation
Tidal power generation has some advantages. These include reduced
greenhouse gas emissions by utilising tidal power in place of fossil fuels.
Traffic or rail bridges can be built on tidal barrages and can improve
transport across estuaries and the lakes created by permanently flooding
estuaries can be used for aquaculture or recreation.
There are also some significant disadvantages. Intermittent supply, cost
and environmental problems make tidal power, particularly barrage
systems, less attractive than some other forms of renewable energy.

    TIDAL CHANGES
The construction of a tidal barrage in an estuary will change the tidal range
in the estuary basin. This change will also have an effect on the
sedimentation and movement of water within the basin. In addition,
navigation and recreation can be affected as a result of a sea depth
changes due to increased sedimentation within the basin. Impounding
waters at high tide levels could result in flooding of the shoreline, which
could have an effect on the local marine food chain and coastal processes.

    ECOLOGICAL CHANGES
As very few tidal barrages have been built, very little is understood about
the full impacts of tidal power systems on the local environment. What has
been concluded is that the effect due to a tidal barrage is highly dependent
upon the local geography and marine ecosystem.

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INTERMITTENT ENERGY SUPPLY
The maximum amount of energy can be extracted from the tides by waiting
until the difference between the water levels in the basin behind the
barrage and the ocean is maximised before the water is allowed to move
through the turbines. This approach produces short bursts of very high
power and a large electricity supply grid (e.g. that of a large city) is required
to absorb this power. A more uniform output can be obtained by using
some of the energy to pump water into nearby raised storage. This can be
released later allowing the turbines to operate for longer periods. Another
method used to produce a more uniform output is to generate electricity
with a two way system during both the rise and fall of the tide.

 Figure 3.
 Power output of two way single basin tidal power station with pumping.

Continuous power generation is possible by using two basins (Figure 4).
One basin is topped up at each high tide and the other is emptied at each
low tide. A turbine is placed between the two basins and can generate
electricity continuously. The output while continuous will still have
significant daily and monthly variations.

 Figure 4.
 Schematic diagram of two basin tidal power station

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COST
Obtaining electricity from tidal power stations is an expensive undertaking.
The 8000 MW proposed Severn Estuary barrage system in the UK is
estimated to cost US$15 billion and the proposed San Bernadino Strait,
2,200 MW tidal fence in the Philippines will cost US$3 billion (SMEC 2003).

Tidal Power in Australia
The North West of Australia has some of the highest tides in the world with
up to 10 metres. 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 3,000 MW.
In recent years a proposal to construct a 50 MW tidal plant near Derby was
developed by Derby Hydro Power. This project received a substantial grant
from the Australian Greenhouse Office's to further develop the project. To
make use of the energy generated a 500 kilometre transmission line was
needed to take the electricity to Broome and Fitzroy Crossing as well as a
number of remote Aboriginal communities (SMEC 2003). The tidal power
proposition faced significant challenges in terms of the initial construction
cost and perceived impacts on the environment. Consequently, a natural
gas powered system was adopted for most of the regional power needs.

Tidal Power Around the World
   CURRENT COMMERCIAL SITES
There are currently two large commercial scale barrages in operation
around the world. One is a 240MW bulb turbine at the mouth of the La
Rance estuary in Brittany France, where the average tidal range is 8
metres. It was built in 1966 and meets the needs of the equivalent of a city
of 300,000 people. (SMEC 2003).
A 16MW plant at Annapolis Royal, Nova Scotia, Canada on the Bay of
Fundy coastline meets the needs of 4500 houses. The plant generates 30
million kWh of electricity per year from a tidal change in the range of 10.8
metres (SMEC 2003).
Other plants are at Kislaya Bay, on the White Sea in Russia (0.4MW, 2.4m
tide) and Jiangxia, China (3.2MW, 7.1m tide)
(http://www.energy.org.uk/EFTidal.htm).

   POTENTIAL SITES
Potential sites for producing electricity from tidal energy using barrages
include the Severn Estuary project in England which could provide 10% of
that country’s needs (8000MW, 8.8m tide)(SMEC 2003).
A 48MW tidal power plant at Doctors Creek in Derby, Western Australia
(population 5000) has been proposed since about 1990. To make use of
the energy generated a 500 kilometre transmission line would have to be
built to take the electricity to Broome and Fitzroy Crossing as well as a
number of remote Aboriginal communities. (SMEC 2003).

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Located between the islands of Samar and Dalupiri on the southern side of
the San Bernadino Strait in the Philippines a tidal power plant could be
constructed that does not involve barrage structures. Instead a four-
kilometre tidal fence consisting of series of Davis turbines would be
secured to the sea-bed across the strait. As many as 274 turbines of
between 7MW and 14MW capacity each would generate 2200MW (SMEC
2003).

Further Information
For further information on “What Causes Tides” and the types of turbines
used in tidal power stations visit the RE-Files

Tidal Electric Company
Tidal Power Fact Sheet (Ontario Energy Educators)
Research Report by the Fujita Corp. Japan
Blue Energy Company, Canada
Snowy Mountains Hydro-electric Scheme (SMEC)
Talk Energy

Acknowledgements
This information was developed by Katrina Lyon and Mark Rayner, with
assistance from Philip Jennings (Murdoch University) in June 1999. It was
reworked by Christine Creagh (2004, Murdoch University) and edited by
Philip Jennings (Murdoch University) and Mary Dale (Australian Institute of
Energy).

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