Anan Suleiman

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Overview of Geothermal Energy

Anan Suleiman Department of Electrical Engineering

Columbia University in the City of New York New York, United States [email protected]

Abstract—As economies expand, populations increase, and energy-intensive technologies spread, our demand for energy is growing greater. The limited energy resources coupled with climate change and rising environmental concerns have driven us to explore alternative, sustainable, and renewable sources of energy in the past few decades. Geothermal resources are abundant, widely available, and renewable and maybe harnessed at scales ranging from central-station power plants to residential heating systems. This paper attempts to describe the resources and discuss the technology employed to convert geothermal energy into electricity. Systems used for geothermal energy production are outlined and the path for future development is described. Some drawbacks to the development of geothermal energy are presented. Economic factors surrounding the industry are also described.

Keywords— Renewable sources of energy, geothermal energy.

I. INTRODUCTION Geothermal energy is the heat from the earth; it is

the thermal energy contained in the rock and fluid in the earth’s crust. Solar heating that results from solar radiation only penetrates about 10 meters underground, after which both pressure and temperature increase with depth. The main source of the underground heat is the radioactive decay of different isotopes in the core and mantle of the Earth. Other sources include residual heat from the formation of the Earth billions of years ago, gravitational forces pulling more dense materials toward the center of the earth and hence creating

heat, and latent heat from the solidifying of molten rock in the core of the Earth [1].

Many countries already tap geothermal energy as an alternative, reliable, affordable, and sustainable source of energy. More than 10,715 megawatts (MW) of electricity are produced by large, utility-scale geothermal capacity in 24 countries around the world. These plants provide reliable base-load power for well over 60 million people, mostly in developing countries. Additionally, about 28 gigawatts (GW) of direct geothermal heating capacity is installed for district and space heating, industrial processes, desalination and agricultural applications [2].

Developing geothermal energy has many advantages. Geothermal power is environmentally friendly, and it reduces dependence on fossil fuels, and the global warming and public health risks that result from their use. The most visible byproduct of geothermal power plant operations is steam plumes. Geothermal power generation offers essentially zero fuel cost plus unquestioned status as a greenhouse gas mitigation option since releases of conventional air pollutants, greenhouse gases, and other chemicals generally are low to nonexistent.

Most importantly, unlike variable-output renewable energy sources such as wind and solar energy, geothermal plants are dispatchable and capable of base-load operation: energy production is not affected by daily or seasonal resource supply fluctuations. These characteristics avoid many of the grid integration challenges associated with variable-output sources like wind and solar energy [3].

To explore the advancement of geothermal energy technology, a brief general review of geothermal energy is first presented, followed by

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examining different types of geothermal power plants, and the future of geothermal energy.

II. GEOTHERMAL ENERGY SOURCE REVIEW Geothermal energy originates from the Earth’s

core. Surrounding the Earth’s core is the mantle, thought to be partly rock and partly magma. The outermost layer of the Earth, the insulating crust, is not one continuous sheet of rock, like the shell of an egg, but is broken into pieces called plates. These plates drift apart and push against each other in a process called plate tectonics. This process can cause the crust to become faulted (cracked), fractured, or thinned, allowing plumes of magma to rise up into the crust. This magma can reach the surface and form volcanoes, but most of it remains underground where it can lie under regions as large as a mountain range. The magma can take from 5000 to more than 1 million years to cool as its heat is transferred to surrounding rocks. In areas where there is underground water, magma can fill rock fractures and porous rocks. The water is then heated and can circulate back to the surface to create hot springs, mud pots, geysers and fumaroles, or it can become trapped underground, forming deep geothermal reservoirs. Geothermal energy is called a renewable energy source because the water is replenished by rainfall, and the heat is continuously produced within the Earth by the slow decay of radioactive particles that occurs naturally in all rocks [9].

III. GENERATING ELECTRICITY: GEOTHERMAL POWER PLANTS

The natural hot water and steam from the earth are used in geothermal power plants to turn turbine generators to produce electricity. Unlike fossil fuel power plants, no fuel is burned. Geothermal power plants give off water vapor, but have no smoky emissions. Geothermal power plants are unique in that they integrate fuel supply and power conversion technologies. The main components of a geothermal power plant include the reservoir, wells, surface piping, turbine-generator train, and condenser and heat rejection system, along with the controls and electrical components required for plants operations and grid interconnection. Generally, belowground fluid production systems are derived from the oil

and gas industry, and above ground conversion systems are based on traditional steam-electric power generation [3]. Different geothermal power plants are examined below. Flash Steam Power Plants Figure 1.0 below presents a schematic of a flash steam power plant. In a single flash steam technology, hot pressurized fluid from production wells is passed into a flash tank (flash tanks are large vessels allowing a portion of the liquid to expand to steam), which is held at a much lower pressure than the fluid, causing the liquid to vaporize (or flash) rapidly to steam [4,5]. The force of the steam is employed to spin the turbine generator. The production well is kept under high pressure in order to prevent the geothermal fluid from flashing inside the well. To conserve the water and maintain reservoir pressure, the hydrothermal resource (which is in a liquid form) is combined with the condensate and re-injected underground. Flash steam plant generators range from 10 MW to 55 MW; a standardized size of 20 MW is used in several countries.

Figure 1.0: Schematic of flash steam power plant [3]. Dry Steam Power Plants Dry steam power plants utilize naturally occurring resources of pressurized steam; a few geothermal reservoirs produce mostly steam and very little water. Production wells are drilled down to the aquifer, and the super heated, pressurized steam that is drawn from the production well is piped directly through a turbine to generate electricity [6]. In a dry

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steam power plant, the low-pressure steam output from the turbine is routinely run through a condenser, where the steam is turned back into liquid form to facilitate reinjection into the reservoir. Heat is rejected from the condenser via either dry or wet cooling towers [3]. The Geysers dry steam reservoir in northern California is the largest and most known dry steam field in the world, and fuels nearly half of current U.S. geothermal capacity. Figure 2.0 below presents a schematic of a dry steam power plant.

Figure 2.0: Schematic of dry steam power plant [3]. Binary Cycle Power Plants In a binary cycle power plant, the hydrothermal fluid from a production well is kept under pressure and passed through a heat exchanger. As it is passed through one side of a heat exchanger, its heat is transferred to a second (binary) liquid, called a working fluid, in an adjacent separate pipe loop. The working fluid; which is usually an organic compound (isobutene or pentane) that has a lower boiling point than water, is vaporized and expanded through a specially designed turbine to generate electricity. It is then condensed back to a liquid and returned to the heat exchanger. All of the hydrothermal fluid is reinjected into the ground in a closed-cycle system. The water and working fluid are confined to distinct, closed loops during the whole process. Figure 3.0 below presents a schematic of a binary cycle power plant. Binary cycle power plants can achieve higher efficiencies than flash steam power plants, and since the working

fluid boils and flashes at a lower temperature than does water, it allows the utilization of lower temperature resources [3]. This increases the number of geothermal reservoirs in the world with electricity-generating potential. Additionally, binary cycle power plants have virtually no emissions.

Figure 3.0: Schematic of binary cycle power plant [3]. Binary cycles also are the most likely option for harnessing hot dry rock resources, as well as geopressured resources found in conjunction with oil and gas deposits at sites where existing infrastructure yields favorable economics [3].

Hybrid Power Plants In a hybrid power plant, geothermal technologies are integrated with fossil and renewable generation options. Fuel consumption and emissions of coal, natural gas, and biomass steam-electric plants can be reduced if hydrothermal fluid is used for preheating water or organic working fluids. On the other hand, we may provide supplemental heat to a flash steam geothermal plant or a binary cycle geothermal plant by concentrating solar thermal fields, gas turbines, and other combustors. This provides operation flexibility, reduces the risk of premature reservoir depletion, and augments capacity during peak periods. Co-located geothermal and fossil generation plants also may share well systems to support CO2 capture and storage. These concepts, which leverage the attributes of individual generating options, may result in widespread exploitation of hot dry rock resources [3].

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IV. THE FUTURE OF GEOTHERMAL ENERGY Enhanced Geothermal Systems (EGS) Hot dry rock resources are the most abundant and widely distributed geothermal resource, but transforming their heat energy into electricity poses substantial engineering challenges. They are found in areas offering sufficient heat for power generation but lacking an in situ water-steam supply. By employing enhanced geothermal system technology; an artificial reservoir is created by drilling holes into a hot rock formation and pumping high-pressure water through an injection well. This hydraulic fracturing approach, which is commonly used for boosting production from oil and gas fields, is used to increase the formation’s permeability and allow for enhanced fluid flow and heat transfer. Hence, surface water is pumped into the fractured zone, where it is heated and then delivered to the surface via a production well [3, 7]. Enhanced geothermal systems extract heart from hot dry rock resources in a form suitable for electricity production. Binary cycle power plants of flash steam power plants may be used with enhanced geothermal systems, depending on the temperature of geothermal fluid extracted from the artificial reservoir created by hydraulic simulation [3]. Figure 4.0 below presents how EGS technology is employed to create an artificial reservoir to access heat energy stored in hot rock formations lacking a natural water source.

Figure 4.0: An artificial reservoir is created using enhanced geothermal systems technology to access heat energy stored in hot rock formations lacking a natural water source.

Down-hole, Closed-loop Heat Exchange Systems Down-hole, closed-loop heat exchange systems generate electricity from hot dry rock resources by employing a binary cycle technique. A working fluid circulates from the surface, through a heat exchange installed in hot rock at the bottom of a well, where the working fluid is heated up, after which it travels back to the surface, where it delivers the heat to a second working fluid that drive the turbine, and hence generates electricity. This approach represents a promising alternative to EGS technology, since it is simpler and more controllable than using injection and production wells and creating an artificial reservoir to extract heat, bring it to the surface in the form of a hydrothermal fluid, and transform it into electricity. Hence, it could significantly lower development and production costs, and mitigate the risks associated with air emissions and materials degradation [3].

V. FURTHER REMARKS ON ENHANCED GEOTHERMAL SYSTEMS

Small-scale demonstrations in several countries have established the technical feasibility of EGS technology. However, there are risks and uncertainties associated with EGS technology that constitute major obstacles to commercial development, such as uncertainties regarding the resistance of rock formations to fracture, the resistance of engineered reservoirs to hydrothermal flow, the potential for thermal drawdown over time, and water losses [3].

A more effective technique might be using supercritical carbon dioxide (CO2) rather than water for hydraulic stimulation. Supercritical fluids have the unique ability to diffuse through solids like a gas while retaining the properties of a liquid. Studies suggest that injecting supercritical CO2 to fracture underground formations, create an artificial reservoir, and serve as a hydrothermal fluid may yield heat extraction rates from hot dry rock resources 50% greater than those achievable with water [3]. Hence, this approach promises not only to reduce the costs and improve the productivity of geothermal power plants, but it could also allow storage of CO2 captured from fossil generating

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facilities and hence help reduce greenhouse gas emissions associated with these facilities.

VI. DRAWBACKS OF GEOTHERMAL ENERGY The most significant barriers to near-term

development of all geothermal plants are the high costs and risks of exploration, resource verification, and well-field development [3].

VII. CONCLUSION Due to the steady heat flow from the inner parts

of the earth, geothermal resources can be regarded as renewable. A geothermal system can in many cases be recharged as a battery. To date, however, resource development has been generally restricted to geologically active areas where reservoirs of hot water and steam are found within permeable rock near the Earth’s surface and economical energy capture is possible using direct steam or flash steam technologies. These high temperature generation options are mature. Recent advances in binary cycle technologies are opening up access to the more abundant and widely distributed lower temperature resource base. Significant progress with EGS technology is required to exploit the vast quantities of energy located in hot dry rock formations that are ubiquitous but available deeper underground [3].

Problems are posed to the production of geothermal energy by depletion of the geothermal resources used and induced seismic activity. However, reinjection techniques can avoid reservoir depletion, and most production and injection wells reach depths generally shallower than those associated with seismic activity.

With nearly 100 years of commercial success, geothermal power plants have been proven as a reliable, cost-effective, and environmentally friendly source of electricity in the United States and around the world. Development activity is accelerating globally. Nevertheless, existing renewable portfolio standard requirements and incentives, recent federal funding commitments, the expectation that a new federal climate policy will assign some type of cost to CO2 emissions, and other market forces are driving significant growth in U.S. geothermal power

production [3]. On the whole the future looks bright for geothermal energy production.

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[5] Clean Energy Basics: Introduction to geothermal electricity production, NREL, www.nrel.gov/energy.

[6] Geothermal Energy Association, A guide to Geothermal Energy and the Environment, 2007. http://geo-energy.org/reports/environmental%20guide.pdf (Retrieved May 24, 2013)

[7] MIT, The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century. Cambridge, MA: 2006. http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf (Retrieved May 24, 2013)

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[10] Thomas Flynn, Division of Earth Sciences, The Nevada Geothermal Industry, Geo-Heat Bulletin Vol.17 No. 2. 1996.