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Geothermal Energy : Combined Heat and Power plant case

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Author : Aroune Ghadbane, Rome, Italy

Geothermal energy allows the recovery of the heat contained in the subsoil in order to
produce electricity or for direct use, and it refers to the part of underground’s heat that can be
used and recovered by men. Geothermal energy has a very high capacity factor which is a
parameter that quantifies the amount of time a certain power source can be exploited. Unlike
other non-conventional energy sources, geothermal energy is available at all times in a
continuous way. It can be recovered either through drilling to access the hot water and steam
reservoirs deep underground, the shallow reservoirs that are close to the earth’s surface.
Guadeloupe for instance is a French territory that plays the role of a pioneer of geothermal
power generation in the Caribbean. In Guadeloupe, more precisely on the island of
Basse-Terre, there exists volcanic activity suitable for the development of high-temperature
geothermal energy. Apart from the Bouillante area, other zones of interest have been
identified such as the Vieux-Habitants which has been the subject of more in-depth studies
for several years.

Electricity from geothermal energy is particularly interesting for Guadeloupe because :

● It’s an area with volcanic activity and thus it enjoys a high geothermal potential.
● Its production can be fully controlled, while other renewable energies are more
dependent on weather conditions: solar, wind.
● Its costs do not depend on fluctuations in the world market;
● Geothermal energy does not emit greenhouse gases.

There exist three types of geothermal power plants:

Dry steam power plants use steam from reservoirs directly to drive generator turbines.
Flash steam power plants use high-pressure water from the underground and convert it to
steam. The third part of power plants transforms the heat from underground hot water to
another liquid with a lower boiling point, transforming to steam and driving the generator
turbine.

The first step in exploiting geothermal energy is reservoir assessment and a good
understanding of reservoir data such as thickness, length, width, temperature of the reservoir,
depth, porosity, permeability, rock heat capacity, etc. The amount of heat the reservoir
contains can be approximated using special equations and by dividing it into smaller
sublayers and calculating their temperatures through calculating the geothermal gradient.
Then the productivity index should be calculated using theim’s equation to estimate the
production rate of the well. A numerical simulation is also important to predict the
performance of the wells in the future, locate the production and injection well in a way that
they do not influence each other, find out the optimum number of the wells needed, and for
the sizing of the production and injection pumps.

Heat power combined power plan

The heat combined geothermal power (CHP)plant operates following the cascading working
principle where the heat that is a byproduct of electrical generation is used directly either for
industries that require a heat source, district heating, swimming pool, or aquaculture pools
heating. As shown in the diagram below, the liquid coming from both the separator and the
condenser mix resulting in a fluid with a certain temperature high enough to be transferred to
a secondary distribution system through a heat exchanger and transported to the dwelling.

Environmental impact

Like every human activity, a geothermal power plant leaves an impact on the environment,
potential impacts are of low gravity when compared to conventional energy generation
methods, however, it leaves an impact on the environmental matrix such as visual pollution
affecting the fauna and flora, land use. Acoustic effects because of the huge amount of noise
produced when drilling and operation, besides it can also result in slight chemical effect by
releasing gases into the atmosphere and fluid disposal. Geothermal fluids either steam or hot
liquid often contain several gases mainly H2S, NH3, CH4 that are released into the
atmosphere.

Example of electrical and thermal output of a CHP in Guadeloupe region

Considering a reservoir with the data below, a CHP can produce 6.34 MW as an electrical
output, considering an optimum separator temperature of 115 C. The byproduct heat that can
be used in district heating was approximated using thermodynamics formulas relating the
fluids coming from both separator and condenser to find the temperature of the resultant
geothermal fluid and then using it to calculate the amount of heat that can be delivered which
is around 35698,8 KWth, knowing that the heating requirements of a single house are 20,47
KW and neglecting the primary and secondary distribution system losses, the system can
meet the heating needs of around 1743 dwellings. The current installed power that is fossil
fuel-based in Guadeloupe is around 465,10 MW which means that this sample single flash
heat and power plant can reduce this value by 1,36% which amounts to around 2.5295 tonnes
of fuel consumption, besides providing uninterrupted steady heat supply to around 1743
houses.

Thickness :362 [m]
Width : 9563 [m]
Length :9504 [m]
Température :202.9 [C]
Porosity : 17 %
Depth : 2030 [m]
Permeability : 602 [mD]
Rock density : 2309 [kg/m3]
Rock heat capacity : 844 [J/kgK]
Geothermal gradient : 0,07989597467 [C/m]
Approximation of heat stored in the reservoir [2,39015E+16 KJ]
Number of production wells = 2
Number of Injection wells = 4

 

References
[1]C., L., A., A., P., & V. (n.d.). Geothermal Energy Use, Country Update for France.
European Geothermal Congress 2019.
[2]Published by Statista Research Department, & 5, J. (2021, July 05). Geothermal electricity
produced France 2019. Retrieved from
https://www.statista.com/statistics/782651/geothermal-electric-production-primary-la-france/
[3] 0 Datasets. (n.d.). Retrieved from https://www.geothermies.fr/viewer/
[4]U.S. Energy Information Administration – EIA – Independent Statistics and Analysis.
(n.d.). Retrieved from
https://www.eia.gov/energyexplained/geothermal/geothermal-power-plants.php
[5]Steel Pipe Dimensions – Ansi Schedule 40 [34wm5j8owml7]. (n.d.). Retrieved from
https://idoc.pub/documents/steel-pipe-dimensions-ansi-schedule-40-34wm5j8owml7
[6] Direct utilization of geothermal energy, suitable applications and opportunities for canada

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