Power Plant Engineering

2.20.1 HOT SPRINGS
Earth tremors in the early Cenozoic period caused the magma to come close to the earth’s sur-
face in certain places and crust fissures to open up. The hot magma near the surface thus causes active
volcanoes and hot springs and geysers where water exists. It also causes steam to vent through the
fissures.
The hot magma near the surface solidifies into igne-
ous rock. The heat of the magma is conducted upward to
this igneous rock. Ground water that finds its way down to
this rock through fissures in it will be heated by the heat of
the rock or by mixing with hot gases and steam emanating
from the magma. The heated water will then rise convec-
tively upward and into a porous and permeable reservoir
above the igneous rock. A layer of impermeable solid rock
that traps the hot water in the reservoir caps this reservoir.
The solid rock, however, has fissures that act as vents of the
giant underground boiler. The vents show up at the surface
as geysers, fumaroles, or hot springs. The natural heat in the
earth has manifested itself for thousands of years in the form
of hot springs. A well taps steam from the fissure for use in
a geothermal power plant.
Geothermal power stations have been installed at a number of places around the world, where
geothermal steam is available.
The share of geothermal produced electricity in the year 2000 is 0.3% of the total electricity
produced in the world. In India, there are more than 300 hot water springs.
2.20.2 STEAM EJECTION
Hot water geothermal energy deposits are present in several locations around the earth.
Underground water collects heat from surrounding hot rocks. Such hot water reserves are with small
con-tent of steam. Rain water collected over the land areas of several hundreds of square kilometers
percolates through the ground to the depths of 1 to 6 km where it is heated by thermal conduction from
the surrounding hot rocks. The hot water moves upwards through the defects of restricted areas in the
rocks. The ‘defects’ are of fractures and highly permeable portions in the rock. The hot water moves
upwards to the surface with relatively little or no storage in between. If however a zone of geothermal
energy deposits is covered by a impermeable rock with a few fractures or defects, the energy deposits
will be stored under ground readily available for extraction.
The energy available in such deposits can be extracted by means of production wells drilled
through the impermeable rocks. In hydro-geothermal energy deposits, the geothermal fluid are in form
of geothermal brine, hot mineral water and steam. Steam deposits are very few in number.
Fig. 2.30. Hot Springs in Steamboat
Springs, Nevada.

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POWER PLANT ENGINEERING
However, largest geothermal energy reserve of petro-geothermal type and are called Hot Dry
Rock type that is without underground water. HDR deposits have largest geothermal energy potential
in the world.
Extraction of geothermal energy through these hot dry rocks requires injection of water into
artificially created fractured rock cavities in hot dry rock and extraction of hot water and steam by
means of production wells. Cold water is injected into the well by means of injection wells and hot
water and steam is extracted by the production wells. Water injected into the well acts as a heat collect-
ing and heat transporting medium. Cavity in hot dry rock acts like a boiler steam generator. Cold water
is injected and hot water/steam is obtained.