Renewable energy (2) Hydroelectric power and geothermal power

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 hydro-electricity power generation

 energy conversions involved

 pumped storage systems explained

 geothermal energy extraction explained

 advantages and disadvantages of these energy resources



The technology of hydroelectric power

Hydroelectric power - hydroelectricity (ii): The potential energy of a head of water (deep water) can be released by allowing the falling water to flow down through turbines connected to electrical generators.

You need to build a dam to flood a valley and set the turbines and generators deep down in the dam's lower structure.

Rainwater or river water etc. is collected upstream from the surroundings and stored behind the dam to create a reservoir of water - a huge store of gravitational potential energy.

The dam will hold back water from any river or stream running into the valley and the water supply fairly constant as long as it rains regularly, there maybe problems in a drought!

gravitational potential energy store (water held behind the dame)

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

mechanically changed ==> electrical energy (generator output by rotation in a magnetic field)

Advantages of hydroelectric power

Non polluting - no waste gases like from burning fossil fuels, no nuclear waste.

Its a free source of energy and reliable except if there were very long periods of drought - very unlikely in the UK!

Relatively low running costs and no fuel costs.

Large scale hydroelectric power stations can respond immediately to a high electricity demand at peak times - you simply increase the flow of water - something you can't do with wind or solar power.

Disadvantages of hydroelectric power

Requires a big capital investment, so costly to construct, but relatively low running costs and the water comes free of charge!

Disruption and loss of habitat for plants and animals, loss of species (perhaps a village of humans goes too!), loss of agricultural land, which happens if a valley is flooded to build a large dam with several generators built into it. This is why hydroelectric schemes in the UK are often sited in a remote valley.

In more recent times more local small scale electricity generation schemes are being developed eg in remote areas using an Archimedes screw driven by river water to drive a turbine.

Some may regard the reservoir as unsightly and look even worse in times of drought when they partially dry up.

Pumped storage systems - hydroelectricity

A pumped storage system is way of storing extra energy (GPE) by linking to the National Grid in 'both directions'. Normally a hydroelectric power station works in one direction ie supplies the National Grid with electricity. In a pumped storage system, any excess electricity in the National Grid is used to run the generators and turbines in reverse, that is to pump water from a lower reservoir to the upper reservoir.

At peak demand times, the extra stored water is released to generate additional electricity. So where does the excess electricity come from? Conventional fossil fuel or nuclear fuelled power stations operate most efficiently, and therefore most economically by running at a fairly high and constant level of power production ie it is inconvenient and inefficient to alternate between high and low rates of power production.

However, through the night, power demand is at its lowest and so excess electricity is being generated. So, quite simply, the pumped storage system uses the surplus electricity at night to pump (kinetic) up and store water (GPE) and release it when required the following day at peak demand times.

The full cycle of energy changes would be:

electrical energy ==> kinetic energy ==> gravitational potential energy ==> kinetic energy ==> electrical energy

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Geothermal energy resources

The technology of geothermal energy

You should know that in some volcanic areas hot water and steam rise to the surface.

The heat comes from rocks relatively near the Earth's surface. The slow decay of radioactive atoms like uranium and other radioactive elements is believed to be the original source of the energy from deep inside the Earth.

Know and understand that the steam can be tapped and used to drive turbines and this is known as geothermal energy.

The rising hot water and steam is used to drive a turbine which in turns a generator, again free energy and no pollution.

thermal energy store in the rocks ==> thermal energy store of water mechanically ==> kinetic energy of the turbine and generator rotor mechanically ==> electrical energy out of the generator as the rotor turns in a magnetic field

Apart from electricity generation, you can also use geothermal energy from hot water/steam to heat homes and factories directly e.g. in Iceland.

Advantages of geothermal energy

Free source of heat energy.

A reliable source of heat energy.

Running costs are low.

There is limited impact on the environment.

Disadvantages of geothermal energy

Relatively undeveloped technology in most countries.

Quite costly to build relative to power output, but nothing like the cost of a nuclear reactor, and maintenance costs are low.

Many countries do not have suitable volcanic regions.


  • Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

    • demonstrating a model water turbine linked to a generator,


Energy resources, and transfers, work done and electrical power supply revision notes index

Energy resources, and transfers, work done and electrical power supply revision notes index

Types of energy store - a comparison with examples explained, mechanical work done and power calculations

Conservation of energy, energy transfers, efficiency - calculations and Sankey diagrams gcse physics notes

Energy resources & uses, general survey & trends, comparing sources of renewables, non-renewables & biofuels

Renewable energy (1) Wind power and solar power, advantages and disadvantages gcse physics revision notes

Renewable energy (2) Hydroelectric power and geothermal power, advantages and disadvantages physics notes

Renewable energy (3) Wave power and tidal barrage power, advantages and disadvantages gcse physics notes

Comparison of methods of generating electricity, 'National Grid' power supply, mention of small scale supplies

Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning gcse physics notes

See also The Usefulness of Electricity gcse physics electricity revision notes


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