Power of Geothermal Energy
Payel Chatterjee*
Geothermal energy is the heat energy stored inside earth. It has the potential to solve energy crisis in many countries where geothermal resources are available. This article briefly describes the technologies and resources required for geothermal power generation and the worldwide scenario of geothermal power production.
Introduction
Energy sources are the lifelines of modern civilization. Access to energy is one of the key pillars for socio economic development but conventional energy sources have some serious impacts on our environment. Greenhouse gases produced by fossil fuels are the fundamental cause of global climate change. So, the world needs a significant transition in its energy sources to avoid climate change and to ensure everyone has access to energy irrespective of social classes.
Figure 1. Global primary energy consumption measure in TWh/year [1].
Geothermal energy: An alternative energy source
The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is a renewable energy source stored inside the earth’s core as heat energy. It can be directly used as heat source using heat pumps or to generate electricity in geothermal power plant.
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The source of this energy is the earth’s internal energy. This heat has been radiating from the earth’s core for 4.5 billion years. A portion is generated by the continuous decay of radioactive isotopes.
The Earth’s temperature increases with depth and at the center of earth which is approximately 6371 km deep, temperature is same as the sun's surface temperature i.e., 5500°C. Earth’s heat flows from its interior toward the surface. Scientists estimated that about 42 million megawatts (MW) of power flow from the Earth’s interior. This constant flow of heat ensures an inexhaustible supply of energy for billions of years.
Figure 2. Temperatures in earth [2].
Resources
Earth has three layers—core, mantle, and crust. Core has two layers. Solid iron core at the center and mixture of iron and hot melted rock, called magma, at the outer core area. The middle layer is called mantle which is a mixture of rock and magma. The shell of the Earth is called crust.
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The Earth’s crust is broken into 7 major tectonic plates. Magma is less dense than rocks, so it moves upward towards the surface and carries heat from the core of the earth. Volcanic eruptions occur when hot magma comes outside the surface through the crack in the plates. However, most magma remains below earth’s crust and heats the surrounding rocks and water. Sometimes this water comes up to the surface through cracks as hot springs or geysers. Geothermal reservoir is created when this rising hot water and steam gets trapped in permeable rocks under a layer of impermeable rocks. These reservoirs are the sources of geothermal energy.
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Different kind of techniques, such as geological, hydrogeological, geophysical, and geochemical techniques, are used to determine the location of the potential geothermal reservoirs. Different types of surveys such as, thermal survey, seismic survey, gravity survey, and magnetic survey, are carried out to determine important characteristics of deep geological structures.
Figure 3: Layers of Earth [3].
Figure 4: Schematic representation of an ideal geothermal System [9].
Ring of Fire
The thermally active areas are found near the boundaries of the tectonic plates where volcanos and earthquakes prone areas are concentrated. These areas are suitable for generation of geothermal energy. Most of the geothermal activity in the world occurs in the area called the Ring of Fire which is approximately 40,000 Km long. Ring of fire traces the meeting points of many tectonic plates, including the Eurasian, North American, Juan de Fuca, Cocos, Caribbean, Nazca, Antarctic, Indian, Australian, Philippine, and other smaller plates, which all encircle the large Pacific Plate. 70% of earth’s volcanic eruptions and 90% of its earthquakes occurs in this region.
Figure 5. Pacific Ring of Fire [2]
Figure 6. World resource map of hydrothermal reservoir [6].
Technology
From ancient times geothermal energy has been used by different civilizations across the world. The Maoris in New Zealand and Native Americans used water from hot springs for cooking and medicinal purposes for thousands of years. The people of Pompeii extracted hot water from the earth to heat their buildings. Romans used geothermal waters for treating eye and skin disease. The Japanese have enjoyed geothermal spas for centuries.
Power Generation Technology
Geothermal power plant technology was first introduced by Prince Piero Ginori Conti in 1904 at Laderello, Italy. There are three major technologies which are commercially used to produce electricity from geothermal energy. These are known as dry steam, flash steam and binary cycle systems. These technologies depends on the temperature and pressure of the geothermal reservoir. Operation of geothermal power plant is independent of weather unlike solar, wind and hydro power plants.
Figure 7. First geothermal power plant [7].
Figure 8. Schematic of geothermal power plant [8].
Dry steam plant
The first geothermal power generation plant was built using dry steam technique. It uses steam from the geothermal reservoir as it comes from well and sends it directly through turbine units to produce electricity. No water is used in this system, hence the nomenclature. 22% of geothermal plants installed worldwide use dry steam technology.
Figure 9. Schematic of dry steam system [9].
Flash Steam Plant
Nowadays flash steam technologies are commonly used in geothermal power generation plants. Here water at temperatures greater than 150°C is used and pumped under high pressure to the equipment at the surface. The sudden decrease in pressure converts or “flash” some of the hot water into steam. This steam is then used to power the turbine units to produce electricity. The remaining hot water and the water condensed from the steam is pumped back into the reservoir. There are two types of flash steam technology- single flash steam and double flash steam. In double flash steam plant, a second flash drum is used at lower pressure to increase the quantity of steam. This increases the output power upto 15-20%.
Figure 10. Schematic of flash steam system [9].
Binary Cycle plant
In the binary cycle system, the water or steam from the geothermal reservoir never comes in contact with the turbine units unlike dry steam and flash steam systems. The geothermal water is passed through a heat exchanger and used to heat another secondary fluid (isobutene or pentane) which has a lower boiling point than water. Geothermal water and the secondary fluid are each confined in separate circulating systems and never come in contact with each other. This secondary fluid is vaporized and expanded through a turbine to generate electricity. This fluid is condensed and recycled for another cycle. Binary cycle power plants can achieve higher efficiencies than flash steam plants and they allow the utilisation of lower temperature resources. Corrosion problems are also avoided. However, binary cycle plants are more expensive and large pumps are required which consume a significant percentage of the output power of the plant.
Figure 11. Schematic of binary cycle system [9].
Geothermal power generation worldwide
The success of the first geothermal power plant at Larderello, Italy indicated the huge potential of geothermal energy that has to develop from then on. After the Second World War many countries were attracted by geothermal energy. It is economically competitive with other forms of energy. It did not have to be imported and in some cases, it was the only energy source available locally. By 1942 the installed geothermoelectric capacity had reached 127,650 kW. The utilization of geothermal energy in developing countries has exhibited an interesting trend over the years. In the five years between 1975 and 1979 the geothermal electric capacity installed in these countries increased from 75 to 462 MW. By the end of the next five-year period this figure had reached 1495 MW. In the next sixteen years, from 1984 to 2000, there was a further increase of almost 150%. According to a GEA 2016 report, the production of geothermal energy is forecasted to reach 18.4 GW by 2021.
Figure 12. Geothermal power generation by different countries over the years [10].
Figure 13. Geothermal power generation capacity by countries in 2016 [11].
Outlook
Environmental: Geothermal energy has some negative environmental impacts. Geothermal fluids usually contain gases like Carbon Dioxide, Hydrogen Sulphide, Ammonia, Methane, and dissolved chemicals. The waste water from geothermal plants also have a higher temperature. Therefore, geothermal fluid is a potential thermal pollutant. Water with high concentrations of chemicals such as boron, fluoride or arsenic should be treated and re-injected into the reservoir after using it. Extraction of large quantities of geothermal water may cause gradual sinking of the land surface. It should be monitored systematically and reinjection of geothermal waste water can prevent this. Appropriate monitoring and proper treatment of wastes are required to avoid deposition of radioactive elements in the pipes of power plants. Induced seismicity is also a major risk of geothermal exploration. Systematic strategies are required to properly monitor geothermal projects. Optimizing induced seismicity and maintaining the hazard and risk within acceptable limits, is the primary challenge.
Resource and technology: Estimation of geothermal resources can be made at a very rough level based on the current subsurface technologies. High uncertainties remain in key parameters such as temperatures, permeability and volume. The probability of finding and developing sustainable geothermal system is low because favourable geological conditions are rare. High resolution 3D seismic surveys are required to estimate deep geothermal resources.
Economy: The average cost of geothermal power generation depends on many geological factors which is impossible to assess without drilling. Research on deep geothermal drilling induces massive costs. There is still a need to further analyse the geological data properly to reduce the cost of geothermal power production. An improved understanding of the reservoir creation process and its dependence on geological conditions is crucial. At present, the initial cost for geothermal power plant in US is around $2500 per installed kW. Operating and maintenance costs range from $0.01 to $0.03 per kWh.
According to world energy council, sustainability of energy depends on three factors; energy security, energy equity (access and affordability), and environmental sustainability. The future of geothermal energy depends on how well the technology and the associated regulatory framework can satisfy these factors. An enormous amount of thermal energy is present in underground. Geothermal energy can assume an important role in creating energy balance if it is exploited correctly.
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* Payel Chatterjee is a PhD scholar in physics, in Norwegian University of Science and Technology, Norway.
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References
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8.https://inlportal.inl.gov/portal/server.pt/community/geothermal/422/what_is_geothermal_energy
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10. S. Hirschberg et al., Energy from the Earth, 2015
11. Geothermal power production report, GEA, March 2016 http://geoenergy.org/reports/2016/2016%20Annual%20US%20Global%20Geothermal%20Power%20Production.pdf
12. https://www.nationalgeographic.com/science/earth/ring-of-fire/
13. http://www.geothermal.nau.edu/about/generation.shtml
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