The word “geothermal” comes from the Greek words geo (earth) and thermal (heat). It means the heat of earth. The energy potential beneath our feet, in the form of geothermal energy, is vast.
This tremendous resource amounts to 50,000 times the energy of all oil and gas resources in the world. Geothermal energy has attracted worldwide attention as an alternate source of energy for the last few decades. Presently this non-conventional energy constitutes about 1% of the total global electricity output but the scenario is changing very fast due to its eco-friendly pollution free and renewable nature. About onethird of the total available energy is spent on space heating, bathing, fish and green house farming and industrial uses.
The USA, Iceland, China and New Zealand make maximum use of geothermal energy for electricity generation or as heat energy.
Together, geothermal power plants and direct-use technologies are a winning combination for meeting our country’s energy needs while protecting the environment. Whether geothermal energy is used for producing electricity or providing heat, it’s a clean alternative for the nation. And geothermal resources are domestic resources. Keeping the wealth at home translates to more jobs and our national economic and employment picture improve.
Geothermal Energy Explained
The Earth’s crust is a bountiful source of energy. Nearly everyone is familiar with the Earth’s fossil fuels oil, gas, and coal but fossil fuels are only part of the story. Heat, also called thermal energy, is by far the more abundant resource. The Earth’s core, 4000 miles (6437 kilometers) below the surface, can reach temperatures of more than 9000°F (4982°C).
The heat geothermal energy constantly flows outward from the core, heating the surrounding area. Nearby rock melts at high temperatures and pressure, transforming into magma.
Magma can some times well up to the surface as lava, but most of the time it remains below the Earth’s crust heating nearby rock. Water seeps into the Earth and collects in fractured or porous hot rock, forming reservoirs of steam and hot water. If those reservoirs are tapped for their fluids, they can provide heat for many uses, including electricity production.
Before the Earth’s heat can be used for purposes such as generating electricity or heating buildings, conduits between the geothermal reservoir of hot water or steam and the Earth’s surface must be provided. This is done by drilling production and injection wells, which are often thousands of feet deep, into the reservoir. Drilling of exploratory wells also helps collect data to define the size and productivity of the geothermal reservoir. Construction of wells is clearly essential, but it is also expensive, accounting for 15 to 30 percent of the total cost of a geothermal power project.
To drill almost any well, a drill bit is mounted on the end of a long metal pipe called the drill string, which is rotated from the surface by machinery called a drill rig. New 30-foot lengths of pipe are added to the top of the drill string as the bore hole gets deeper. To cool and lubricate the drill bit and to carry away the chips of rock cut by it, a viscous fluid called drilling mud is pumped down the drill string.
The mud passes through holes in the drill bit and then flows back up the hole in the space between the bore hole wall and the drill string.
Drilling costs are greatly affected by how quickly the drill bit can penetrate the hard, abrasive, fractured rocks of a geothermal location, and by how long it can last before the drill string needs to be taken out of the hole to replace the bit. If both penetration rate and bit life were doubled, drilling costs would drop an average of 15 percent.
- Roller-cone bits
- Polycrystalline diamond compact (PDC) bits
For virtually all drilling in either geothermal or oil and gas wells roller-cone bits and polycrystalline diamond compact (PDC) bits. Roller cone bits have toothed cones that roll on the bottom of the hole as the bit rotates, each tooth crushing the small area of rock beneath it.
The PDC bit uses thin layers of synthetic diamond bonded to tungsten carbide-cobalt studs or blades. The diamond layer gives the cutter extreme resistance to abrasive wear in the shearing action of cutting. PDC bits are especially well suited to drilling through hot rock because they have no moving parts, so high-temperature seals, bearings, and lubricants are not an issue. They have gained this tremendous market acceptance because they have consistently drilled faster and lasted longer than roller cone bits. For geothermal drilling, however, PDC bits do not work reliably well in rock that is more than moderately hard.
These efforts will lead to enhance performance, extending full application of PDC bits, with its attendant cost savings, to the hot, hard rocks of geothermal reservoirs.
Bore hole Measurements
Measurements in the borehole are used both to evaluate the reservoir once the well is drilled and to provide data during drilling that will make the process faster, cheaper, and safer. To function effectively for geothermal drilling, this instrumentation must be adapted for slim hole drilling and high-temperature conditions. Sandia has developed tools that meet these temperature and size requirements, including a promising new selfcontained, battery-powered, memory-storage system. Several of these tools have been used extensively in the field and are available for application or have been commercialized; others are in the late stages of testing.
Baker Hughes has signed a licensing agreement with DOE for use of down hole instrumentation, and Board Long year, a supplier of drilling equipment for geothermal and mineral exploration, recently commercialized core tube data logging equipment.
Electricity production using geothermal energy is based on conventional steam turbine and generator equipment, where expanding steam powers the turbine/ generator to produce electricity. Geothermal energy is tapped by drilling wells into the reservoirs and piping the hot water or steam into a power plant for electricity production.
Types or geothermal power plants:
- Dry steam
- Flash steam
- Binary cycle
Dry steam power plants
The steam is piped directly from wells to the power plant, where it is directed into a turbine. The steam turns the turbine, which activates a generator. The steam is then condensed and injected back into the reservoir via a well.
Dry steam is the oldest type of plant first used in Italy in 1904 but it is still very effective.
The Geysers in northern California, the world’s largest single source of geothermal power, uses dry steam.
Flash steam power plants
Flash steam power plants tap into 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, the fluid pressure decreases and some of the hot water boils or “flashes” into steam.
The steam is then separated from the water once at the surface and is then used to power a turbine/generator unit. The remaining water and condensed steam are injected through a well and back into the reservoir.
Binary cycle power plant
Binary cycle power plants operate on water at lower temperatures of about 225° to 360°F (107° to 182°C). These plants use the heat from the geothermal 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 confined to separate geothermal temperatures required for direct use 70° to 302°F (21° to 150°C) are lower than those for electric power generation.
Hot water from geothermal resources can be used directly to provide heat for industrial processes, crop drying, or heating buildings. This iscalled direct use. The consumer of direct-use geothermal energy can count on savings of as much as 80 percent from traditional fuel costs, depending on the application and the industry.
Direct-use systems do require a larger capital investment compared to traditional systems, but have lower operating costs and no need for ongoing fuel purchases.
Geothermal Heat Pumps
Geothermal heat pumps, also known as GHPs, enable the ground to serve as an energy storage device.GHPs are similar to conventional air conditioners or refrigerators.GHPs discharge heat to the ground during the cooling season and extract useful heat from the ground during the heating season.
GHPs marketed today also provide hot water. There are over 500,000 GHPs in service today in the United States, including about 600 systems at schools and colleges.
The direct use of the geothermal resource, however, evolved into a modern system that today provides space and domestic water heating throughout the city of Boise to many homes, businesses, and government buildings. The hot water from a geothermal well can replace the traditional heat source often natural gas of a boiler, furnace, and hot water heater.
Geothermal water can also heat a working fluid that melts snow as it flows through piping installed underneath pavement. Generally, an individual home or building only needs one geothermal well for a heating system. In larger applications, like in Boise, a district heating system can be used to supply heat from a central location of one or more wells through a network of pipes to entire blocks of buildings. percent compared to the cost of natural gas heating.
The savings are much higher when compared to electric, propane, or fuel oil heating systems.
This number continues to rise as word spreads about the benefits of direct use in agriculture, such as lower operating costs and increased growth rates. These can be significant competitive advantages. Many crops like cucumbers, tomatoes, flowers, houseplants, tree seedlings, and cacti flourish in geothermally heated greenhouses.
Several fish farms and other aquaculture operations have found success using geothermal water as a habitat for their livestock, making it the fastestgrowing direct-use application in the country.
Geothermal direct use continues to show great commercial potential and competitive advantages for a variety of industries. Industrial applications include food dehydration, gold mining, laundries, milk pasteurizing, mushroom culture, and sewage digestion. Geothermal direct-use resources are especially well suited to vegetable dehydration operations, such as in the production of dried onions or garlic. The dry climates throughout much of the West also assist in the process.
The dehydration process begins with geothermal water flowing through a heat exchanger, which warms the air to temperatures ranging from 100° to 220°F (38° to 104°C).
Clean Energy from the Earth for the 21st Century
DOE funds research to reduce the cost of geothermal components, systems, and operations. Geothermal facilities use the natural heat in the earth’s interior to produce electricity or to satisfy other heat energy needs. The Program’s R&D activities closely align with its mission and goals.
With improved exploration methods, industry will locate and characterize new geothermal fields more accurately, reducing the high cost and risk of development. Better technology for drilling wells will make it possible to access deeper resources and reduce costs, thereby expanding the economic resource base.
Advances in energy conversion will establish air-cooled binary technology as a means of generating competitively priced electricity from more plentiful lower-temperature resources. These activities all contribute directly to reducing the cost of geothermal development and enabling the installation of more geothermal facilities. Geothermal electric generation projects are capital-intensive enterprises, with the major expenses being incurred before the plant begins to produce revenue. The high-cost components of a geothermal development project include: drilling exploration, production, and injection wells; and plant equipment and construction.
The primary risk in a geothermal project is confirmation of a viable reservoir, which usually requires extensive drilling and well testing.
- Improving technologies for exploration, detection of fractures and permeable zones, well sitting, and fluid injection
- Decreasing the cost of drilling and completing geothermal wells
- Reducing the capital, operation, and maintenance costs of geothermal power plants.
Reducing drilling costs will substantially cut the costs of geothermal development, thus helping the domestic geothermal industry to maintain its world-leader status and to expand its markets. Today, society uses only a small fraction of the geothermal energy resource base. The ultimate promise of geothermal energy is that a much larger fraction of the total resource base can be tapped.
New and improved drilling technologies can make this happen.