GEOTHERMAL
ENERGY
Where does this energy come from?
Geothermal energy is the heat energy
emitted by the mantle and core of the Earth, which then passes up to the
surface, mainly by conduction. This energy can be harnessed and then used to
heat water or help generate electricity [6].
Much of the energy emitted escapes through faults in the Earth’s crust as
a result of tectonic plate movement. These plate movements allow very hot
underground water (of about 200C) to rise near to the surface (see Fig 5). It is at these fault lines where
geothermal energy is most exploited, such as the west coast of the USA
around the San Andreas Fault (see Fig 6)
[7].
Fig 5 – Areas of
geothermal energy include geysers,
Fig 6 – With higher underground temperatures in darker
colours, it can be seen that the area of highest geothermal potential in the
Direct Geothermal Energy
This energy is not just limited to
plate boundaries. Lower energy sources can be found just tens of metres below
the surface in most places. Although it is not hot enough to turn a steam
turbine, it is useful to heat local buildings. Direct geothermal energy is
primarily used for this purpose, and is one of the oldest uses. Water from hot
springs on, or just below the surface of the ground, is pumped up to a heat
exchanger. This water is usually impure as it contains dissolved salts, making
it similar to brine. Consequently,
it is more corrosive and is unsafe to be directly pumped into the heating
system. The heat exchanger is therefore corrosion-resistant, and is usually
made of aluminium (or one of its alloys), which also has a high thermal
conductivity. At the heat exchanger, thermal energy is transferred to the
heating system. The brine is then pumped back into the springs below, where it
is reheated [8] [9]. Other uses include the heating of
roads to melt ice and snow in very cold climates, such as
Types of Plant
Geothermal energy can be converted
into electricity when very hot underground water is available. There are three types
of power plant, all of which use steam or hot water to drive a generator. The
used water is then returned back into the underground well, where it is
reheated, to be used again.
Dry steam power plants directly use steam from geysers. The
steam is gathered through pipes and is then used to turn a turbine, which
drives the generator. Using a condenser, the steam is then turned back into
water (see Fig 7). Each dry steam
plant has a power output of about 40MW, which is high in comparison to wind
turbines [7]
[11].
Fig 7 – Basic model of a dry steam plant (http://www.txucorp.com/responsibility/education/generation/images/geo_the.gif)
Flash steam plants use very hot pressurised water of
about 160C found just below the surface.
As this water is extracted above ground, the pressure drops to
atmospheric, and so the boiling point of the water lowers below the temperature
which it is at, turning it to steam. This steam is then used in a similar way
to that in the dry steam plant to drive the generator. The power output is just
less than the dry steam plants, at about 30MW. However, it is not as cost
effective, as the pipes drawing up the pressurised ‘brine’ need to
be corrosion-resistant [8] [12].
Binary cycle plants use the low boiling point property
that hydrocarbons possess, such as butane and propane. As moderately hot water
(with temperatures of 50C to 150C) passes through a heat exchanger, its thermal
energy is transferred to the hydrocarbon in a separate piping system. The
hydrocarbon evaporates with the heat influx and is able to turn the blades of
the turbine. Both fluids are then recycled (see Fig 8). These plants are more widely used, as low temperature
geothermal reservoirs are more common. Power output is fairly low at about
10MW, due to the lower thermal energy [13].
Fig 8 – Basic model of a binary cycle plant (http://www.daviddarling.info/images/binary-cycle_power_plant.jpg)
Disadvantages
Compared to wind energy, geothermal energy does not depend
upon the weather. There are also not many protrusions above the surface that
can be seen. However, unlike wind, it cannot be guaranteed that geothermal
energy is available to everyone, as the depths and temperatures of the
reservoirs may be too extreme. As some of the reservoirs are several kilometres
deep, it can be very expensive and time consuming to drill to these depths.