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School Physics notes: Energy 4. Renewable wave and tidal power generation

Renewable energy stores (3) Wave power and tidal barrage power

Wave power for generating electricity and tidal power electricity generation - The advantages and disadvantages of these renewable energy resources

Doc Brown's GCSE level Physics exam study revision notes

The technology of wave power

Wave power: One method of using wave movement is to use its kinetic energy of up and down oscillation to compress air in a funnel and tunnel the air through a turbine connected to a generator on the sea shore. It has not so far (as I know?) proved very successful.

kinetic energy store of the waves == mechanically ==> kinetic energy of the turbine and generator rotor ==> electrical energy as the rotor of the generator rotates in a magnetic field

Note that most of the energy in the waves originally comes from the kinetic energy store of the wind - stronger winds make bigger waves.

The initial cost is high, but bar storm damage, the running costs are low.

Free source of energy and available every day.

There is no pollution.

A successful scheme might be useful for an island with a small population.

There are several problems eg variable wave height giving variable power output.

Storm damage is a regular risk.

Spoiling the view.

Hazard to boats.

The seabed may be disturbed affecting the ecosystems.

You need lots of small wave-powered turbines on the coast to generate a significant power output.

Unreliable due to the variability of wind speed which affects the wave height and power.

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Tidal power - another approach to hydroelectricity generation

The technology of tidal barrage electricity generation plant

Tides are caused by the gravitational pull of the moon and sun on water, and the flow of tides involves the movement of huge quantities of water, and a rise and fall in height of water of several metres.

The Earth spins through the maximum height of water created by the combined gravitational pull of the Sun and moon (but don't forget the Earth and moon pull on the Sun too!).

This means the Earth rotates through two tidal “bulges” every lunar day, producing two high and two low tides approximately every 24 hours.

Using a tidal barrage is the most common way to utilise this tidal movement to generate electricity.

A tidal barrage scheme is another type of hydroelectric power plant that consists of building a long relatively low dam wall across a river estuary, and building generators and sluice gate water flow controls into its internal structure.

Note that the turbine blades can be designed to operate in both directions so you can extract energy as the tide comes in and when the tide flows out - but its usual to generate power when the trapped water is released to flow downstream.

A tidal barrage scheme is the most common way of utilising the power of the tides in a suitable location e.g. near the mouth of a river estuary.

TIDAL BARRAGE

The incoming tide is allowed to flow upstream through the open sluice gates, the collected water is held behind the dam barrier, so producing a gravitational potential energy store - after you have closed the sluice gates!

At high tide the sluice gates are closed, and, at that point, there is no difference in water height between each side of the barrage dam. The maximum height difference between the two sides of the dam is governed by the high water mark (highest tide level) and the low water mark (lowest level of tide).

(You can use the incoming tide to generate power too - depending on h.)

At the turn of high tide we now have a great store of gravitational potential energy (GPE) which can be released to generate power. As the tide recedes, the difference in water height (h on diagram) increases.

Eventually the difference in height (maximised at low tide), allows the water to flow back down sufficiently powerfully to drive the turbines,

Using the sluice gates ('valves') you can control the conversion of the water's GPE, mechanically into the kinetic energy store of the water which is converted to electricity via the turbines and generators in the wall of the barrage.

The incoming tide drives the turbines as does the controlled released of the huge amount of stored water (GPE) stored behind the barrage at high tide.

It is an advantage to store huge quantities of water that can be released at electricity peak demand times.

Energy store changes:

kinetic energy store of incoming tide (caused by the gravitational effect of the moon)

==> gravitational potential energy store (water held behind the barrage)

==> mechanically ==> kinetic energy store (flowing water)

==> mechanically ==> kinetic energy store (turbine and generator rotation)

==> mechanically ==> electrical energy (generator output via rotation of the rotor in a magnetic field)

The energy from the 'up and down' tidal movement seawater is free and there is no pollution and maintenance costs are relatively low.

Its a reliable source of energy  (whatever the weather!) - the tides rise and fall twice every day due to the combined gravitational attraction of the Sun and Moon.

Tides are reliable, twice a day, and times/heights can be accurately predicted, but there periods of time when the water levels are similar on both sides of the barrage, therefore little effective water flow and no electricity generation at different times of the day.

A tidal barrage power station can generate large amounts of electrical energy, unlike many other renewable energy schemes.

Hydroelectric tidal barrage schemes are very costly to build needing a large capital investment and take a long time to build, but there are no fuel costs and maintenance costs are relatively low.

There are problems with the water course become silted up with sand.

Environmental costs - tidal barrages have a big impact on the local ecology:

Wildlife habitats are affected disrupting local ecosystems on the estuary bed/seabed and other species from wading birds to fish stocks.

Some might find, as with wind farms, barrage schemes unsightly.

The height of the tides is variable and you need a good difference in height to have a large GPE store of water.

There are actually four times in the day (two high tides and two low tides) when the difference in heights of the water on either side of the barrage are two small to allow sufficient flow through the turbines to generate electricity.

The maximum height of tides varies through the year - lower neap tides and higher spring tides.

Disruption of leisure/commercial craft on the river and estuary, you need to add locks to allow boats to go up and down the river from the estuary.

Limited locations where you can build a hydroelectric tidal barrage scheme - not all estuaries are suitable.

Keywords, phrases and learning objectives on wave power and tidal power

Be able to describe and explain how a wave power plant operates to generate electricity.

Be able to describe and explain how a tidal barrage power plant operates to generate electricity.

Be able to discuss the advantages disadvantages of generating electricity from wave and tidal renewable energy resources.

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