Renewable energy (2)
Hydroelectric power and geothermal power
generation energy conversions involved, pumped storage systems explained, geothermal energy extraction explained
- advantages and disadvantages of these
Doc Brown's GCSE level Physics exam study revision notes
The technology of hydroelectric power plants for generating electricity
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
Rainwater/snow, then stream/river water etc. is collected upstream from the
surroundings and stored behind the dam to create a reservoir of water -
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
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
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
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
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.
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
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
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
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
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
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
A reliable source of heat energy.
Running costs are low.
There is limited impact on the
Disadvantages of geothermal energy
Relatively undeveloped technology in most
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
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