4 CHAPTER FOUR
GEOTHERMAL ENERGY RESOURCES
4.1 Introduction
Geothermal energy is the thermal energy generated and stored in the earth. Geo stands for earth
and thermo stands for heat.
Temperatures in excess of 1000oC exist deep in the earth. The resulting thermal gradient, which
is approximately 25oC/Km, creates a heat flow to the surface of the earth, and this is in effect, the
source of geothermal energy. It is considered a renewable form of energy, since it is
continuously generated by the flow of heat from the earth’s core.
4.2 Classification of Geothermal energy systems
1. Low temperature hydrothermal energy (below 180oC)
2. High temperature hydrothermal energy (above 180oC)
3. Geothermal energy from hot dry rock
4. Geothermal energy from geopressurized zones
5. Geothermal energy from magma
4.3 Exploration of Geothermal Energy Resources
Geothermal energy can be categorized into:
i. Electricity generation through turbine plant
ii. Heating purposes through heat pump systems
For the power generation, exploration of the resources requires some vital information which can
be obtained from mining, oil exploration and geothermal survey. The most important parameter
is the temperature gradient and accurate measurements depend on leaving the drill hole un-
disturbed so that the temperate equilibrium is re-established after drilling. Deep drilled survey
wells commonly reach depth of 6 km and technology is also available for drilling up to 15km.
In each of the geothermal regions, it is possible to extract heat in principle by the following
approaches;
1. Natural hydrothermal circulation, in which water percolates to deep aquifer to be heated
to dry steam or vapour/liquid mixtures or hot water. Emission of each type can be
observed in nature. If pressure increases be steam generation at deep levels, speculation
geysers may occur.
2. Hot igneous systems, these are associated with heat from semi-molten magma that
solidifies to lava. The first power-plant using this resource was the 3 MW station at
Hawii completed in 1982.
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, 3. Dry rock fracturing poorly conducting dry rock e.g. granite stores heat for a million years
with subsequent increase in temperature, artificial fracturing from bore holes enables
water to be pumped over the rock to extract the heat.
4.4 Dry rock and hot aquifers analysis
Consider the cross-section of geothermal crust below:
Z0 Area A Surface Temperature T0
Depth
Z
z1 T1 Minimum Useful Temperature
T-Hot dry
z1 rock
Z2 T2 – Temperature at max depth
4.4.1 Dry Rock
Let r =be density of the rock
c r = Specific that capacity of the rock
A =Cross-sectional area
Assuming that the rock is uniform material and that here is no convection. There will be a
linear increase in temperature with depth. He depth z increase down wards from the
surface at z=0, at any depth z
dT
Temperature T To Z To Gz ………………… (1),
dz
Where G= gradient
Let the minimum temperature be T1 at depth z1
T1 T0
T1 T0 Gz1 , z1 ………………………. (2)
G
The useful heat content E at temperature T greater than T1 in an element of thickness
z at depth z is given by
E r Az cr T T1
E r Az cr Gz z1 ………………………. (3)
The total useful heat content of the rock to depth z, therefore becomes
46
GEOTHERMAL ENERGY RESOURCES
4.1 Introduction
Geothermal energy is the thermal energy generated and stored in the earth. Geo stands for earth
and thermo stands for heat.
Temperatures in excess of 1000oC exist deep in the earth. The resulting thermal gradient, which
is approximately 25oC/Km, creates a heat flow to the surface of the earth, and this is in effect, the
source of geothermal energy. It is considered a renewable form of energy, since it is
continuously generated by the flow of heat from the earth’s core.
4.2 Classification of Geothermal energy systems
1. Low temperature hydrothermal energy (below 180oC)
2. High temperature hydrothermal energy (above 180oC)
3. Geothermal energy from hot dry rock
4. Geothermal energy from geopressurized zones
5. Geothermal energy from magma
4.3 Exploration of Geothermal Energy Resources
Geothermal energy can be categorized into:
i. Electricity generation through turbine plant
ii. Heating purposes through heat pump systems
For the power generation, exploration of the resources requires some vital information which can
be obtained from mining, oil exploration and geothermal survey. The most important parameter
is the temperature gradient and accurate measurements depend on leaving the drill hole un-
disturbed so that the temperate equilibrium is re-established after drilling. Deep drilled survey
wells commonly reach depth of 6 km and technology is also available for drilling up to 15km.
In each of the geothermal regions, it is possible to extract heat in principle by the following
approaches;
1. Natural hydrothermal circulation, in which water percolates to deep aquifer to be heated
to dry steam or vapour/liquid mixtures or hot water. Emission of each type can be
observed in nature. If pressure increases be steam generation at deep levels, speculation
geysers may occur.
2. Hot igneous systems, these are associated with heat from semi-molten magma that
solidifies to lava. The first power-plant using this resource was the 3 MW station at
Hawii completed in 1982.
45
, 3. Dry rock fracturing poorly conducting dry rock e.g. granite stores heat for a million years
with subsequent increase in temperature, artificial fracturing from bore holes enables
water to be pumped over the rock to extract the heat.
4.4 Dry rock and hot aquifers analysis
Consider the cross-section of geothermal crust below:
Z0 Area A Surface Temperature T0
Depth
Z
z1 T1 Minimum Useful Temperature
T-Hot dry
z1 rock
Z2 T2 – Temperature at max depth
4.4.1 Dry Rock
Let r =be density of the rock
c r = Specific that capacity of the rock
A =Cross-sectional area
Assuming that the rock is uniform material and that here is no convection. There will be a
linear increase in temperature with depth. He depth z increase down wards from the
surface at z=0, at any depth z
dT
Temperature T To Z To Gz ………………… (1),
dz
Where G= gradient
Let the minimum temperature be T1 at depth z1
T1 T0
T1 T0 Gz1 , z1 ………………………. (2)
G
The useful heat content E at temperature T greater than T1 in an element of thickness
z at depth z is given by
E r Az cr T T1
E r Az cr Gz z1 ………………………. (3)
The total useful heat content of the rock to depth z, therefore becomes
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