Energy Under Our Feet

Diagram showing the different categories of geothermal resources. (From Muffler and Cataldi, 1978). The vertical axis is the degree of economic feasibility; the horizontal axis is the degree of geological assurance.

Geothermal energy is thermal energy generated and stored in the Earth. Comes from the heat within the earth. The word "geothermal" comes from the Greek words geo, meaning earth," and thermo, meaning "heat."  Because  of  the  internal  heat  generation.  The Earth’s  surface  heat  flow  averages  82  MW/m² which  amounts to a total heat loss of about 42 million megawatts.  The  estimated  total  thermal  energy  above  mean  surface  temperature to a depth of 10 km is 1.3 x 10²⁷ J, equivalent to burning 3.0 x 10¹⁷barrels of oil. Since the global energy consumptions for all types of energy, is equivalent to use of about 100 million barrels of oil per day, the Earth’s energy to a depth of 10 kilometers could theoretically supply all of  mankind’s  energy  needs  for  six  million  years  (Wright, 1998). On average, the temperature of the Earth increases about 30˚C/km  above  the  mean  surface  ambient  temperature. Thus, assuming a conductive gradient, the temperature of the earth at  10  km  would  be  over  300˚C.  However, most geothermal exploration and use occurs where the gradient is higher, and thus where the earth's core lies almost 4,000 miles beneath the earth's surface. The double-layered core is made up of very hot molten iron surrounding a solid iron center. Estimates of the temperature of the core range from 5,000 to 11,000 degrees Fahrenheit (F). Heat is continuously produced within the earth by the slow decay of radioactive

 the Earth's crust, mantle, and core. Top right: a section through the crust and the

 particles  that is natural in all rocks. Surrounding the earth's core is the mantle,thought to be partly rock and partly magma. The mantle is about 1,800 miles thick. 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 slabs of continents and ocean floor drift apart and push against each other at the rate of about one inch per year in a process called continental drift. Magma (molten rock) may come quite close to the surface where the crust has been thinned, faulted, or fractured by plate tectonics. When this near-surface heat is transferred to water, a usable form of geothermal energy is crea

ted.

   

                                                                            

DEFINITION AND CLASSIFICATION OF GEOTHERMAL RESOURCES                                                  

There is no standard international terminology in use throughout the geothermal community, which is unfortunate, as this would facilitate mutual comprehension. The following are some of the most common definitions and classifications in this discipline. According to Muffler and Cataldi (1978), when we speak generically about geothermal resources, what we are usually referring to is what should more accurately be called the accessible resource base; that is, all of the thermal energy stored between the Earth's surface and a specified depth in the crust, beneath a specified area and measured from local mean annual temperature. The accessible resource base includes the useful accessible resource base (= Resource ) — that part of the accessible resource base that could be extracted economically and legally at some specified time in the future (less than a hundred years). This category includes the identified economic resource (= Reserve ) — that part of the resources of a given area that can be extracted legally at a cost competitive with other commercial energy sources and that are known and characterised by drilling or by geochemical, geophysical and geological evidence. 

Diagram showing the different categories of geothermal resources. (From Muffler and Cataldi, 1978). The vertical axis is the degree of economic feasibility; the horizontal axis is the degree of geological assurance.

Schematic cross-section showing plate tectonic processe

Geothermal Electricity
                                                                             

Geothermal electricity is electricity generated from geothermal energy. Hydrothermal resources at high temperatures (300 to 700 degrees Fahrenheit) can be used to produce electricity. These high-temperature resources may come from either dry steam wells or hot water wells. We can use these resources by drilling wells into the Earth and piping the steam or hot water to the surface. Geothermal wells are one to two miles  

 deep.                                                                                                             

Types of Geothermal power plant
There are three types of geothermal power plants: dry steam, flash steam, and binary cycle.

Dry Steam
Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.

Dry steam power plant

Flash Steam
Flash steam power plants are the most common and use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Flash steam power plant

Binary Steam
Binary cycle power plants operate on water at lower temperatures of about 225°–360°F (107°–182°C). Binary cycle plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Currently, two types of geothermal resources can be used in binary cycle power plants to generate electricity: enhanced geothermal systems (EGS) and low-temperature or co-produced resources.

Binary cycle

Worldwide production                                                                                                             

 The International Geothermal Association (IGA) has reported that 10,715 megawatts (MW) of geothermal power in 24 countries is online, which is expected to generate 67,246 GWh of electricity in 2010.This represents a 20% increase in geothermal power online capacity since 2005. IGA projects this will grow to 18,500 MW by 2015, due to the large number of projects presently under consideration, often in areas previously assumed to have little exploitable resource. In 2010, the United States led the world in geothermal electricity production with 3,086 MW of installed capacity from 77 power plants. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The Philippines is the second highest producer, with 1,904 MW of capacity online. 

 

Country	Capacity (MW) 2007	Capacity (MW) 2010	percentage of national produc															tion
USA	2687	3086	0.3%
Philippines	1969.7	1904	27%
Indonesia	992	1197	3.7%
Mexico	95															3	958	3%
It
														aly
810.5	843	1.5%
New Zealand	471.6	628	10%
Iceland	421.2	575	30%
Japan	535.2	536	0.1%
El Salvador			20		4.4	20		4	2	5%
Keny		a	128.8	167	11.2%
Costa Rica	162.5	166	14%
Turkey	38	94	0.3%
Nicaragua	87.4	88	10%
Russia	79	82
Papua New Guinea	56	56
Guatemala	53	52
Portugal	23	29
China	27.8	24
France	14.7	16
Ethiopia	7.3	7.3
Germany	8.4	6.6
Austria	1.1	1.4
Australia	0.2	1.1
Thailand	0.3	0.3
TOTAL	9,731.9	10,709.7

                                              Worldwide production, after IGA

Economics
Geothermal power requires no fuel, it is therefore immune to fuel cost fluctuations. However, capital costs tend to be high. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet in Nevada can support 4.5 megawatt (MW) of electricity generation and costs about $10 million to drill, with a 20% failure rate.In total, electrical plant construction and well drilling cost about 2-5 million € per MW of electrical capacity, while the levelised energy cost is 0.04-0.10 € per kW·h.Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and levelized costs above $0.054 per kW·h in 2007.
Geothermal power is highly scalable: a small power plant can supply a rural village, though capital can be hig
Chevron Corporation is the world's largest private producer of geothermal electricity.The most developed geothermal field is the Geysers in California. In 2008, this field supported 15 plants, all owned by Calpine, with a total generating capacity of 725 MW.

Advantages

Renewability                                                                                           Geothermal power is considered to be renewable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 10³¹ joules (3·10¹⁵ TW·hr) About 20% of this is residual heat from planetary accretion, and the remainder is attributed to higher radioactive decay rates that existed in the past. Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it.

Sustainability                                                                                          Geothermal power is also considered to be sustainable thanks to its power to sustain the Earth’s intricate ecosystems. By using geothermal sources of energy present generations of humans will not endanger the capability of future generations to use their own resources to the same amount that those energy sources are presently used. Further, due to its low emissions geothermal energy is considered to have excellent potential for mitigation of global warming.