GEOTHERMAL ENERGY: A NEW SOURCE OF CLEAN ENERGY

Introduction

The world is trying to phase out the use of fossil fuels and radioactive substances to generate electricity, due to the amount of carbon emissions from the former, and radioactive waste from the latter. These carbon emissions are one of the major causes of ozone layer depletion, which in turn leads to global warming. Many countries are therefore leaning towards cleaner and renewable energy sources like solar, hydro, wind, geothermal etc. The Paris climate accord which had been signed by 195 countries has the goal of reducing global temperature increase in this century well below 2 degrees centigrade above preindustrial levels[1][5]. The agreement is set to be implemented starting from the year 2020[3][4]. This means all countries party to the agreement must start implementing and using clean energy, so as to be able to meet up with the set target by 2020.

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One of the clean sources of energy countries are looking to turn to is geothermal energy. The heat generated and stored by the earth is called geothermal energy. The origin of geothermal energy includes the heat from the original formation of the planet during the Big Bang, and also from radioactive decay of materials[2]. Geothermal energy is renewable, which means it is inexhaustible. It is also a source of clean energy, meaning greenhouse gases that are harmful to human and causes environmental damage are not emitted. Geothermal energy is used in many countries to generate electricity, and can also be used directly for heating purposes. China is the largest consumer of geothermal energy [7], though the United States leads the world in geothermal energy production [8].

HOW GEOTHERMAL ENERGY IS CONVERTED TO ELECTRICITY

Generating electricity through geothermal energy is a new industry, which first manifested in 1904 in Italy. Wells, sometimes about 1.6km deep or more, are drilled into underground reservoirs to extract steam and very hot water that is used to power turbines linked to electricity generation. There are different kinds of power plants that are used to harness geothermal energy. Some geothermal power plants utilize steam to directly turn the turbines, while others use the steam to heat a liquid that is used to turn the turbines.

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TYPES OF GEOTHERMAL POWER PLANTS

Reservoirs that are liquid dominated are frequently found with temperatures above 200°c, and are found near recently formed volcanoes around the Pacific ocean. They are also found in rift zones and hotspots. The most common type of power plant used to generate electricity from these sources are flash plants.
• FLASH PLANT: These power plants use flash steam to generate electricity. Flash steaming is the process whereby hot water with very high pressure is vaporized into steam inside a flash tank by lowering the pressure. The steam is then used to turn turbines, connected to a generator leading to the production of electricity. Flash plants are the most popular and most common type of power plants in the world. The largest liquid system as of 2013, is in Cerro Prieto in Mexico, which generates about 750MW from temperatures close to 350°c.
Reservoirs with temperatures between 120-200°c require pumping [35]. The energy of this reservoir can be tapped using a binary cycle power plant.
• BINARY CYCLE PLANT: Binary cycle power plant enables reservoirs with low temperatures to be utilized. Companies pump up hot water from the hot water reservoir below through the production well. The water is passed through a heat exchanger where water vaporizes an organic fluid known as the binary fluid (normally a pentane hydrocarbon or a butane) that drives a turbine. Binary plants originated form the U.S.S.R in the late 1960s, and they have no emissions[6][35].
• ENHANCED GEOTHERMAL PLANTS: In this type of plant, water is injected into the wells to be heated and pumped back out. water, usually injected under high pressure. The method was copied from oil and gas extraction techniques. This system makes use of deeper wells and no organic fluids or chemicals are used, reducing the likelihood of damage to the environment [6].
Geothermal energy can also be used directly without conversion to electricity. Iceland is the world leader in direct geothermal energy application. About 92.5% of its homes are heated with geothermal energy, saving the country over $100million annually in avoiding oil imports[9]. The world’s biggest district heating system is located in Reykjavik, Iceland. It is used to heat up pathways to avoid accumulation of ice.

ADVANTAGES OF GEOTHERMAL ENERGY

Geothermal energy has many advantages which includes:

• It is a renewable source of energy. Geothermal energy is always extracted from the core of the earth, and will be available as long as the earth exists.
• It is environment friendly. Geothermal energy is a clean source of energy in all aspects of its production and application. There are zero carbon emissions and no greenhouse gases are emitted.
• It is abundant in supply. Geothermal energy is abundant, and not subject to the same issues experienced by other clean energy sources such as solar and wind.
• It is a pilot of innovation. Geothermal energy is a new source of clean energy and as such, there is currently a lot of research dedicated to it, which is bound to bring about innovations.

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REFERENCES

[1.](Dye, S. T. (2012). "Geoneutrinos and the radioactive power of the Earth". Reviews of Geophysics. 50 (3). arXiv:1111.6099  .Bibcode:2012RvGeo..50.3007D.doi:10.1029/2012RG000400.)
[2.](Gando, A.; Dwyer, D. A.; McKeown, R. D.; Zhang, C. (2011). "Partial radiogenic heat model for Earth revealed by geoneutrino measurements". Nature Geoscience. 4 (9): 647.Bibcode:2011NatGe...4..647K.doi:10.1038/ngeo1205.)
[3.](Turcotte, D. L.; Schubert, G. (2002),Geodynamics (2 ed.), Cambridge, England, UK: Cambridge University Press, pp. 136–137,ISBN 978-0-521-66624-4)
[4.](Lay, Thorne; Hernlund, John; Buffett, Bruce A. (2008), "Core–mantle boundary heat flow", Nature Geoscience, 1: 25,Bibcode:2008NatGe...1...25L,doi:10.1038/ngeo.2007.44)
[5.](Nemzer, J. "Geothermal heating and cooling". Archived from the original on 1998-01-11.)
[6.](Moore, J. N.; Simmons, S. F. (2013), "More Power from Below", Science,340 (6135): 933,Bibcode:2013Sci...340..933M,doi:10.1126/science.1235640,PMID 23704561)
[7.](Lund and, John W.; Boyd, Tonya L. (April 2015), "Direct Utilization of Geothermal Energy 2015 Worldwide Review" (PDF),Proceedings World Geothermal Congress 2015, retrieved 2015-04-27)
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[10](Pahl, Greg (2007), The Citizen-Powered Energy Handbook: Community Solutions to a Global Crisis, Vermont: Chelsea Green Publishing)
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