Chapter 7 – Geothermal Energy

Chapter 7 – Geothermal Energy

            Inside the earth crust, lots of molten rocks, metals and minerals are present. As one go towards the centre of earth the temperature increases continuously. There exists a temperature gradient of about 30°C/km towards the depth from earth crust. The high temperature is due to radioactive radiations emitted inside the earth crust.

7.1 Earth Structure

On the basis of temperature and structure entire earth can be divided into concentric shells.

S.N.	Particular		Depth from earth surface (km)	Structure	Other
1.	Crust		90 km	Limestone, coal, gold, petroleum etc.	Lithosphere
2	Rigid part (separating crust and mantle)		90 to 100 km	-
3	Upper Mantle		190 to 900 km	Iron and magnesium
4	Lower Mantle		900 to 2900 km	Less rigid and hotter (magma)	Asthenosphere (Capable of being deformed)
5		Outer core	2900 to 5200 km	liquid
6		Inner core	5200 to 6370 km	Nickel-iron alloy (solid)

7.2 Geothermal Resources and Sites

Geothermal resources are available in the following form:

i.                    Hydrothermal or hydro-geothermal energy (heat absorbed in ground water)

·         Dry steam

·         Wet steam

·         Hot water

ii.                  Geo-pressured resources

iii.                Hot dry rocks

iv.                Magma

Geothermal sites are available mainly at the boundaries of tectonic plates. These are also the regions of earthquakes and volcanoes.

7.3 Harnessing Geothermal Energy

7.3.1 Steam/hot water

Steam may be used to run turbine to produce electricity. In wet steam liquid is separated from steam and then the steam is used to run turbine. Hot waters at moderate temperatures may be used for space heating and for industrial purpose.

7.3.2 Geo-pressured Resources

This contains brines at moderate temperature (160°C) with dissolved methane gas. It is under high pressure of 1000 bar at a depth of 2 to 10 km from the earth crust. The brine is collected from a well and methane is separated at normal pressure just by gravity separation technique. Methane may be used as energy source and hot brine for electricity generation.

7.3.3 Hot dry rocks

To harness thermal energy from hot dry rocks, deep well is drilled up to the level of rocks. Water is pumped from one side, and hot water/steam is collected from other site at the surface. The entry and exit point at hot rock is connected inside the rock.

7.3.4 Magma

At present no technology is available for recovery of thermal energy from magma as its temperature is very large (650°C).

7.4 Advantages and Disadvantages of Geothermal Energy

Advantages

·         Non-polluting.

·         Greater amount of energy is available.

·         Available at all time.

·         Versatile use.

Disadvantages

·         Drilling operation is noisy.

·         Large area is needed.

·         Removal of large amount of resource from inside may results in instability.