Geothermal energy is a clean and sustainable resource sourced from the Earth's heat, ranging from shallow ground to molten rock. Different types of geothermal reservoirs, classified by temperature, allow for various applications, including electricity generation through geothermal power plants that utilize steam or hot water. As technology improves, geothermal energy is becoming more cost-effective and is expected to play a significant role in future energy production.

1. GEOTHERMAL ENERGY

4. Geothermal Energy is clean and
sustainable. Resources of geothermal
energy range from the shallow ground
to hot water and hot rock found a few
miles beneath the Earth's surface,
and down even deeper to the
extremely high temperatures of
molten rock called magma.

5. Almost everywhere, the shallow ground or upper 10 feet of
the Earth's surface maintains a nearly constant
temperature between 50° and 60°F (10° and 16°C).
Geothermal heat pumps can tap into this resource to heat
and cool buildings. A geothermal heat pump system
consists of a heat pump, an air delivery system (ductwork),
and a heat exchanger-a system of pipes buried in the
shallow ground near the building. In the winter, the heat
pump removes heat from the heat exchanger and pumps it
into the indoor air delivery system. In the summer, the
process is reversed, and the heat pump moves heat from
the indoor air into the heat exchanger. The heat removed
from the indoor air during the summer can also be used to
provide a free source of hot water.

6. Hot dry rock resources occur at depths of 3 to 5
miles everywhere beneath the Earth's surface and
at lesser depths in certain areas. Access to these
resources involves injecting cold water down one
well, circulating it through hot fractured rock, and
drawing off the heated water from another well.
Currently, there are no commercial applications of
this technology. Existing technology also does not
yet allow recovery of heat directly from magma,
the very deep and most powerful resource of
geothermal energy.

7. Geothermal regions in the world

8. GEOTHERMAL RESERVOIR
A geothermal reservoir is a volume of rocks in
the subsurface which exploitation in terms of
heat can be economically profitable.
It should be noted that for producing the heat
from the subsurface is necessary the presence of
a transport fluid (usually water), and that
drilling to an enough depth to reach the
optimum operation temperatures is also
necessary. These factors and the technical and
other concerns entail costs which increase with
depth.

9. TYPES OF GEOTHERMAL RESERVOIRS
High temperature: These reservoirs provide enough heat to
make electricity from steam profitably. High temperature
reservoirs are generally more than 150° C, and are located in
areas of thin lithospheric thinness or active volcanism.
Middle temperature: Despite these reservoirs have a lower
temperature compared to the high temperature ones, they
allow extracting sufficient heat to produce electricity (but
with lower performances) using a volatile fluid. The
reservoirs usually reach temperatures between 100 and 150º
C, and are located in areas with favourable structural and
geological contexts and geothermal gradients higher than the
average. Their direct use may be in heating mode and their
main applications are in district heating systems and
industrial processes.

10. Low temperature: The temperature of these reservoirs is
between 100 and 30° C. They are located in areas with a
favourable geological context including deep aquifers; the
geothermal gradient is like the average in the region.
Their exploitation involves pumping hot groundwater from
the aquifer and re-injecting it after it has delivered the
heat and is cold again. These are used in direct
applications and for district heating systems and industrial
processes.
Very low temperature: The temperature of these
reservoirs is below 30° C. In these, the underground is
used as a heat exchanger, by means of a heat pump in a
closed circuit. Their applications are in domestic and
agricultural air conditioning systems. These kinds of
reservoirs may be anywhere, because their efficiency is
just determined by the underground thermal inertia in
normal (average) geothermal gradient conditions.

11. GEOTHERMAL POWER PLANTS
Geothermal power plants use hydrothermal resources that have
both water (hydro) and heat (thermal). Geothermal power plants
require high-temperature (300°F to 700°F) hydrothermal
resources that come from either dry steam wells or from hot
water wells. People use these resources by drilling wells into the
earth and then piping steam or hot water to the surface. The hot
water or steam powers a turbine that generates electricity. Some
geothermal wells are as much as two miles deep.
There are three geothermal power plant technologies being used
to convert hydrothermal fluids to electricity—dry steam, flash
steam and binary cycle. The type of conversion used (selected in
development) depends on the state of the fluid (steam or water)
and its temperature.

12. Dry Steam Power Plant
Dry steam plants use hydrothermal fluids that are
primarily steam. The steam travels directly to a turbine,
which drives a generator that produces electricity. The
steam eliminates the need to burn fossil fuels to run the
turbine (also eliminating the need to transport and store
fuels). These plants emit only excess steam and very
minor amounts of gases.
Dry steam power plants systems were the first type of
geothermal power generation plants built (they were
first used at Lardarello in Italy in 1904). Steam
technology is still effective today at currently in use at
The Geysers in northern California, the world's largest
single source of geothermal power.

14. FLASH STEAM POWER PLANT
Flash steam plants are the most common type of
geothermal power generation plants in operation today.
Fluid at temperatures greater than 360°F (182°C) is
pumped under high pressure into a tank at the surface
held at a much lower pressure, causing some of the fluid
to rapidly vaporize, or "flash." The vapor then drives a
turbine, which drives a generator. If any liquid remains
in the tank, it can be flashed again in a second tank to
extract even more energy.

16. Binary Cycle Power Plant
Binary cycle geothermal power generation plants differ from
Dry Steam and Flash Steam systems in that the water or
steam from the geothermal reservoir never comes in contact
with the turbine/generator units. Low to moderately heated
(below 400°F) geothermal fluid and a secondary (hence,
"binary") fluid with a much lower boiling point that water pass
through a heat exchanger. Heat from the geothermal fluid
causes the secondary fluid to flash to vapor, which then drives
the turbines and subsequently, the generators.
Binary cycle power plants are closed-loop systems, and
virtually nothing (except water vapor) is emitted to the
atmosphere. Because resources below 300°F represent the
most common geothermal resource, a significant proportion of
geothermal electricity in the future could come from binary-
cycle plants.

20. FUTURE
Geothermal is also getting cheaper, as the technology
improves. According to the Union of Concerned
Scientists, since 1980, the cost of operating geothermal
power plants has declined by as much as 50%. In some
markets, buying power from geothermal plants will soon
be as cheap as it is from its much dirtier fossil fuel
counterparts.
The reason that geothermal is expected to play an
important role in the future is that we're getting better
and better at doing this: we're now drilling geothermal
wells with increasing efficiency, allowing more energy to
be captured in each plant.

21. Countriwise usage of geothermal energy as per 2015

22. A Presentation by Sanjeet , Sagar , Rekhraj