Geothermal Power Generation in India
Geothermal energy is the sensible heat energy of the earth's crust and the interior matter.
The thickness of earth's crust is not uniform. It varies from 5 to 35 km. The crust consists of six major and a few smaller 'plates' which are in motion relative to each other. The relative motion between the plates causes earth quakes near their boundaries, which constitute the seismic belt.
The surface of a plate away from its boundary is effectively insulated by it from the hot molten rock, called magma, present below it. The temperature of the crust in such places increases slowly with depth, the gradient being 200 to 30° C/km.
In the seismic belt the hot magma may penetrate the crust within a few kilometers from its surface in the form of volcano. High temperatures at relatively shallow depths below the surface may be found in such places.
Utilisation of geo-thermal energy involves extraction of thermal energy from the regions of high temperature below the earth's surface, normally in the form of hot water or steam.
Hot water or steam escapes through fissures in the rock or through holes drilled to intercept fissures. This hot spring may be in the form of geysers, fumarols etc.,
GEOTHERMAL SOURCES:
The geothermal sources can be classified as
1. Hydrothermal convective systems
Ex : Dry steam fields, Wet steam fields, Hot water springs
2. Geopressurised systems
3. Hot dry rock systems
4. Magma systems
5. Volcanos
Practically, only the hydrothermal systems are commercially practised at present. Hydrothermal system consists of high temperature water and/or steam inside a porous rock structure. It is said to be vapour - dominated if it contains only steam or vapour and liquid dominated it it containsly mostly hot water only.
GEOTHERMAL STATION
Figure shows the principle of working of a geothermal power station. For large scale use, bore holes are normally sunk to a depth of nearly 1000 m. Steam is released through them at a pressure of 30 bars and temperature of 200° to 300°C.
This hot stream of steam and liquid particles is passed to a steam separator where water is drained and pure steam is sent to the turbine. Turbine is coupled to electrical generator which produces electrical power. Alternatively, heat energy is the stream of hot water and steam may be transferred to a working fluid which operates the power cycle.