The Earth

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Geothermal Energy All these reckonings of the history of underground heat, the details of which I am sure you do not wish me to put before you at present, are founded on the very sure assumption that the material of our present solid earth all round its surface was at one time a white-hot liquid. ~ Lord Kelvin (1824-1907) Geothermal energy is heat energy from magma (molten rock) deep within the earth, brought to the surface naturally by geysers and springs, by drilling into hot reservoirs or magma, or by exploiting the thermal mass of soil and ground water to drive a heat pump. Geothermal energy has been used since ancient times when the Romans, Greeks, and Japanese - among others - used naturally heated mineral water for bathing, cooking, heating, and medical applications. Today, hot springs and spas1 are still some of the most attractive spots for recreation and therapeutic bathing. In addition, geothermal energy is used for power generation, space heating, and in greenhouses, swimming pools, and steam processing for industrial applications. Geothermal energy can be considered renewable as long as the rate of heat extraction from a reservoir does not exceed the rate it is recharged by the earth’s heat, which, depending on its temperature, may take tens to hundreds of years. Although only 10% of the earth’s surface are at temperatures hot enough to be used as a practical energy source, the earth’s geothermal resources are large enough to provide many times the energy needs of the entire population.2 There are areas where a substantial amount of geothermal energy diffuses through the crust and reaches the surface of the earth. Low- to moderate-temperature geothermal reservoirs are present on most continents; the largest resources in the world are believed to be in Iceland. High-temperature geothermal resources are predominately found in volcanic ranges and island chains. Other especially attractive regions for geothermal energy are in Central America, Indonesia, East Africa, and the Philippines. Worldwide, geothermal energy provides about 8,000 megawatts of electricity and another 8,000 megawatts of direct-use thermal energy. The largest geothermal electricity generation plant in the world is located in the Geyser area in Northern California, generating about 1,000 MW – about 1/3 of the total US geothermal output and enough electricity to supply a city of over a million homes (Figure 9-1).3 In the past few decades, researchers have found new ways to trap natural 1 Named after town of Spa in Belgium, famed for healing hot mineral springs since 14th century. 2 Norton, G. A., and Goat, C. G., “Geothermal Energy – Clean Power from the Earth’s Heat,” US Geological Survey, USGS, Circular 1249, 2003. 3 Kutscher, C.F., “The Status and Future of Geothermal Electric Power,” National Renewable Energy laboratory ( CHAPTER 9 206 thermal gradients in dry rocks. When this technology matures, geothermal energy promises to become an important source of renewable energy available virtually anywhere on the planet. The Earth The earth is the third planet from the sun and has an average radius of 6,400 kilometers. Its internal structure includes an inner core, an outer core, lower and upper mantles, and a crust (Figure 9-2). The inner core – the center of the earth – is a sphere that is 2,400 kilometers in diameter. Because of the extreme pressures in the center, the earth’s inner core remains solid and becomes molten only in the outer core, allowing rocks to “flow” and form the convection cells within the mantles. Near the top of the mantle, rock cools sufficiently by conduction, causing it to solidify and form the earth’s crust. Heat transfer causes warming of the rocks and fluid, providing what we call the geothermal energy. The inner core consists mainly of iron, nickel, uranium, and various heavy metals. The ultimate source of geothermal energy is the radioactive decay of the isotopes of these materials and the original heat produced from the formation of the earth due to gravitational collapse. The outer core is 2,300 kilometers thick and composed of liquid molten metals, iron, nickel, and lighter elements. The outer core along with earth’s rotation is believed to be responsible for the earth’s magnetic field. The temperature of the outer core is relatively uniform and varies from 5000oC to about 4,000oC at the outer edges. Surrounding the core are the lower and upper mantles. The combined mantles constitute 80% of the earth’s volume and about 67% of its mass. The lower mantle is 2,300 kilometers thick and is made of highly viscous fluids. The upper mantle is solid, has a thickness of about 600 kilometers, and may contain pockets of high-temperature volcanic reservoirs. The top layer is the earth’s crust, which is not a continuous sheet of rock, but is broken into solid plates that float independently on top of the hot mantle. The crust is only about 40 kilometers thick and is comprised of all the earth’s water and landmasses. The temperature drops rapidly in the upper mantle and crust at a rate of 20-30oC/km until it reaches surface temperatures. Magmas reside at the Figure 9-1 Dry steam plants at the Geysers Source: National Renewable Laboratory. Figure 9-2 Earth’s internal structure. Crust The EarthMantleOuter coreInner core 207 Chapter 9 - Geothermal Energy depths of 5-10 kilometers beneath the crust and range in temperature from about 650 to 1,300oC, depending on chemical composition. Moving toward the surface, the temperature decreases; heat is transferred to nonporous, hot, dry rocks, which become increasingly porous and permeable near the surface. The capacity of a porous rock to transmit water is called permeability. As the rock becomes saturated, it will form reservoirs of water, steam, or both, which will eventually find their way to the earth’s surface through faults and cracks, making geothermal energy available for exploitation.4 Question: Considering that our best drills can barely cut through a very small depth of the earth’s surface, how can we know anything about the earth’s interior structure? Answer: We know that the average density of the earth (as measured by dividing its volume by mass determined from its gravitational force field) is higher than the surface density, so it can be concluded that the earth’s interior must be denser than its surface. Furthermore, detailed analyses of the seismic waves following an earthquake give us information about the internal structure. The speed of propagation of the waves is a function of the density of materials through which they travel.


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