Renewable energy (2) Hydroelectric power and geothermal power

 hydro-electricity power generation   energy conversions involved,  pumped storage systems explained, geothermal energy extraction explained -  advantages and disadvantages of these renewable energy resources

Doc Brown's GCSE level Physics exam study revision notes

The technology of hydroelectric power plants for generating electricity

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/snow, then stream/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 or snows regularly, but there maybe problems in an extreme drought!

Some dams are built on large rivers e.g. in China, but the rivers are still fed from precipitation in valleys upstream in the mountains.

The landscape of Norway is very mountainous, with lots of valleys and rivers and lots of melted snow - therefore most electricity in Norway is generated in hydroelectric power plants.

You do need the right sort of terrain for a hydroelectric power station e.g. a suitable valley and lots of precipitation of rain.

The sluice gates - valves, are used to control the water flow rate, so controlling the rate of electricity generation - this enables you to increase or decrease the power generation of a hydroelectric plant.

Energy store changes:

gravitational potential energy store (water held behind the dam)

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

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

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

At each stage there will be loss of thermal energy to the thermal energy stores of the water, dam wall, and generator-turbine construction materials.

This is due to friction - a little due to the axles of rotating parts of the turbine and generator (which will be lubricated with oil/grease), but most energy loss is due water bushing against ANY surface - the water is doing work against the surface of pipes and turbine blades.

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.

Except in times of extreme drought, hydroelectric power schemes are very reliable, the dam can hold plenty of water in reserve.

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.

In more recent times, on a 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 - though its not practical or economic to connect such hydroelectric schemes to a national power line network.

Disadvantages of hydroelectric power

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

Hydroelectric dm schemes have a big impact on the environment.

Disruption and loss of habitat for plants and animals, loss of vegetation and species (perhaps a village of humans goes too!).

Vegetation that rots under water, with lack of oxygen, produces the greenhouse gas methane - but I would expect this to be temporary?

There is 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 and other countries are often sited in a remote valley.

Some may regard the reservoir as unsightly and look even worse in times of drought when they partially dry up - in fact hydroelectric power does rely on regular rainfall.

Pumped storage systems - extra use of 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 i.e. 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 from conventional power stations to pump up and store water (kinetic to GPE energy stores) and release it when required the following day at peak demand times.

The full cycle of energy store 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.

The basic principles of a geothermal power plant

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 - where they can defrost pavements in winter from the geothermal heat supply!

Advantages of geothermal energy

Free source of heat energy, the thermal energy extracted from hot rocks is replenished from the thermal energy store deep underground.

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,

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