Civil Engineering Reference
In-Depth Information
12.1
Renewable Energy Types
12.1.1
Geothermal Energy
Geothermal energy generation which is using up the energy that is generated and
stored in earth is not a new technical advancement due to the fact that it has been
used for power generation since early twentieth century. But recently and not only
due to importance of energy conservation but also because of high electricity pro-
ducing potential of geothermal systems its utilization has been reconsidered by the
engineers.
Different technologies of utilization of the geothermal energy can be divided into
four major categories of conventional hydrothermal systems, low-temperature sys-
tems, enhanced geothermal systems, and direct-use systems.
In general conventional hydrothermal, low-temperature, and enhanced geother-
mal systems are all used to generate electricity, while the direct-use systems that
ground source heat pump systems are a major member of this category are used for
space heating and cooling systems, hot water production, industrial processes,
agricultural processes, etc. Based on 2008 geothermal technology market report
(July 2009) the utilization of geothermal direct use energy from year 1990 to 2007
has grown from 0.0048 to 0.0094 quadrillion Btu (almost twice as much). Also
based on 2010 geothermal energy international market update, electricity genera-
tion from geothermal resources in the USA has risen from 2,678 MW in 2007 to
3,086 MW in 2010.
To be able to generate electricity from a conventional hydrothermal system in
addition to earth heat two other resources of hot fl uid (steam or hot water) in an
underground reservoir and permeability of the earth structure shall be naturally
available in one place. A production (supply) well will be used as a path for bringing
the hot fl uid to the power plant and an injection (return) well will be used for return-
ing the low-heat-low-energy fl uid back to the reservoir. High energy fl uid which has
moved through the production well will be delivered to the power plant turbines.
Power plant turbines use this energy to run the generators that consequently gener-
ate electricity. Commonly there are three different types (dry steam, fl ash steam, and
binary cycle) of interactions between the turbine and the working fl uid. In a dry
steam power plant, the steam out of the production well will be directly introduced
to the turbine, while in fl ash steam power plant steam enters a fl ashing container
with much lower pressure to be converted to fl ash steam before entering the turbine.
In a binary cycle power plant fl uid from the production well introduces to a heat
exchanger in which the ground heat will be transferred to another closed circuit
fl uid with a much lower boiling point. This fl uid then is used to run the turbines
(Figs.
12.1
,
12.2
, and
12.3
)
The difference between a conventional hydrothermal system and an enhanced
geothermal system is that in the latter the main three elements of reservoir with the
fl uid, earth heat and permeability are not available naturally in one place, and usually
additional effort such as increasing the permeability of the earth structure is needed
to generate the proper environment for utilizing this system.
