Though little recognized by the general public, geothermal energy (“earth heat”) is the third largest source of renewable energy in the United States, behind hydropower and biomass. Hundreds of thousands of megawatts of geothermal power lie beneath the surface in the United States, practically inexhaustible and waiting for new exploration tools, better resource techniques, lower-cost drilling technologies, and improved methods of predicting the behavior of geothermal reservoirs. As new technologies are being developed, so is the interest in mining the earth's heat.

Accelerating oil and electricity prices have also encouraged the use of the earth's heat, which is generated both from drilling wells deep into the earth to produce electricity from heated water (hydrothermal heat) and from the the high temperatures of the earth itself closer to the surface, which is used to heat and cool buildings and homes. This method most often uses geothermal heat pumps that work with heat exchangers to transfer heat between warm and cool spaces.

Geothermal power plants have much in common with traditional power-generating stations, using many of the same components, including turbines, generators, transformers, and other standard power generating equipment. View these animations from the U.S. Department of Energy to learn more about how geothermal systems work.

Texas Geothermal Resources

Texas has thousands of wells that have high enough temperatures for the possible development of geopressured and geothermal resources. Geothermal energy can be generated anywhere in the state. However the biggest technical issue facing geothermal development is determining where and how deep the resources are and how companies can get to and utilize them.

In 2007 state lands were leased for the first time to research possible geothermal energy development. Ten percent of any income from energy produced on this land will go to the state’s Permanent School Fund.

Using Oil & Gas Wells

Accessing geothermal power by drilling for water (photo) or steam, is similar to drilling for oil and gas. Texas has an advantage in drilling for geothermal resources as the state has decades of experience with oil and gas extraction, with access to detailed subsurface analyses of heat resources, reservoirs and deep water availability.

Oil and gas wells co-produce additional hot fluids that require disposal. This fluid could be a bonanza for the geothermal industry, while sparing the oil and gas industries the expense of disposal of these co-produced fluids. Equally advantageous is the existence of large amounts of data on existing wells.

Using tapped out oil and gas wells could greatly reduce the costs involved in exploration and drilling even though retrieving geothermal resources to generate electricity is a significantly different process from that of oil and gas drilling and would involve redesign and redrilling.

Texas Geothermal Research

State Energy Conservation Office (SECO) has been working with the Southern Methodist University's Geothermal Laboratory and the University of Texas at Permian Basin (UTPB) to analyze existing oil and gas-well data to develop the expertise to determine how this information can be used for geothermal energy exploration. SMU's Geothermal Laboratory estimates that within the next ten years, Texas could have 2,000 to 10,000 MW in generating capacity from geothermal resources accessed through oil and gas wells.

In January 2007, the State Energy Conservation Office (SECO) published a ground-breaking study, Geopowering Texas: Conversion of Deep Gas Wells and Fields into Geothermal Energy Wells, by Dr. Richard Erdlac at UTPB. This data provides advance knowledge of the best areas to go for heat energy acquisition. The three-year exploration and resource research project focused on West Texas, identifying and assessing potential sites for converting depleted deep gas wells and fields into geothermal energy wells to generate renewable electrical power. This UTPB map shows five areas in Texas with either deep sedimentary basins or areas of high temperatures sufficiently close to the Earth's surface for possible use in electrical energy production.

The study proposes that the idea behind Texas geothermal production is the reuse of the wells drilled by the oil and gas industry that are either sufficiently deep to encounter hot water, or that could be deepened into these hot zones. In Pecos County, for example, temperatures of nearly 300°F are found in wells of around 18,000 feet.  One purpose of the study was to develop the needed subsurface databases and maps to enable the energy industry to expand their activities to acquiring the subsurface heat for generating renewable electrical power from sedimentary basins. Researchers estimate that electric power production potential from Texas oil and gas wells range from 400 MW to over 2,000 MW.

In early 2008, SECO will publish a second geothermal study performed by the Southern Methodist University (SMU) Geothermal Laboratory that will focus on East Texas (east of IH-35). The use of hot waste water from existing hydrocarbon wells is a feasible electrical energy solution. The project goal is to develop site specific locations for geothermal electrical power production in existing Texas hydrocarbon fields, with the ultimate goal to develop specific examples for proof-of-concept tests of geothermal electrical generation in Texas.

SECO has also contracted with Southern Methodist University's Geothermal Laboratory to create a geothermal outreach and networking group, in conjunction with the U. S. Department of Energy's GeoPowering the West program, with the intent to move Texas from a state of little geothermal knowledge and consumption, to one where geothermal becomes an important component of the renewable energy industry. A program goal is the development of additional geothermal projects in Texas. One new geothermal project being developed is geothermal energy generation from oil and gas waste fluids.

An additional SECO study, Texas Renewable Energy Resource Assessment, was performed by Virtus Energy Research Associates to evaluate Texas renewable energy resources in 1995, including geothermal. The report is currently being updated.

The map of Texas geothermal resources below shows that a largely untapped energy resource lies underneath our feet that can be developed for electrical power generation. Hydrothermal, geopressured and hot dry rock resources areas are identified (these terms are described below). The orange color represents the known potential for hydrothermal uses such as space heating, fish farming, desalinization and resort spas. The green color shows the known potential for geopressure uses such as heating, enhanced oil recovery and electricity. The blue areas show the known potential for hot dry rock uses such as heating and electricity.

The map also shows that there are at least five major regions within Texas that have a strong potential for geothermal electrical power production. These include East Texas, the Gulf Coast, the Delaware-Val Verde Basin region, the Trans-Pecos region, and the Aardvark Basin where it enters the Texas Panhandle. The region consisting of the Maverick Basin along the South Texas–Mexico border may represent an independent sixth area for consideration. These regions are based upon the existence of oil and gas wells with temperatures that can get above 212^oF. The locations and boundaries of the geothermal areas illustrated are approximate.

SMU's Geothermal Laboratory has provided a set of maps to illustrate the temperatures and fluid flow needed for successful geothermal electrical power production in Texas.

The following three descriptions refer to the resources identified by the map shown above.

Hydrothermal resources
Hydrothermal resources are composed of hot water and/or steam that is found in fractured or porous rock at shallow to moderate depths as a result of the intrusion of molten magma into the earth’s crust, or from the deep circulation of water through fault or fracture systems.

Geopressured resources
A geopressured resource consists of hot brine saturated with methane and found in large, deep aquifers that are under higher pressure due to water trapped in the burial process. The northern Gulf of Mexico is the largest region discovered that contains this type of resource.

Hot dry rock
Hot dry rock is a heated geological formation formed in the manner of hydrothermal resources, but containing no water due to the absence of aquifers or fractures required to transport water. This resource is huge in comparison to hydrothermal resources.

Future development of geothermal energy in Texas will fall along three lines: geoexchange systems (heat pumps), direct use activities, and electric power generation. Several different companies that install geoexchange systems are centered in Texas. Direct use application is less developed, and is used only locally at several places in Central Texas. Electrical power generation has not yet become commercially established. 

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Heat Pump (geoexchange system) A geothermal heat pump system consists of pipes buried in the shallow ground near the building. Fluid (mostly water) circulating in the pipes carries heat into a building in the winter and pulls heat out of the building in the summer, exchanging the heat with the cooler surroundings at either end of the loop. Like an air conditioner or furnace, a geothermal heat pump is an electrically powered heating and cooling system, but it moves heat from and to the earth to take advantage of the almost constant temperature occurring just a few feet underground, which is usually warmer than the air in winter and cooler than the air in summer. See this photo of piping being laid down for a geoexchange system. Geoexchange systems are more expensive to install, but much more efficient and cost- effective than competing fuel technologies, and can be developed anywhere in Texas for our homes, businesses and schools. Costs can be recouped in two to 10 years through energy savings.	Geothermal heat pump installation (loop water pumps and piping). Presently, many Texas homes, schools and other buildings are using geoexchange systems to take advantage of the cost savings in heating and cooling the inside of a building.
Direct Use Direct geothermal use taps hot water or heat from below the ground, near the earth's surface, to generate electricity for industrial heating needs, fish farming, food processing facilities, pasteurizing milk, spas and hot springs, nurseries, and residential and commercial heating. The hot water can also be piped directly under roads and sidewalks to melt snow, and even to heat a network of buildings in a community. Dehydration, the drying of onions and garlic, is the largest industrial use of geothermal energy. See this DOE article, Direct Use of Geothermal Energy. Texas Trans-Pecos region The key to the development of direct use geothermal energy in Texas is access to hot water. While there are some hot springs that have been identified in the Trans-Pecos region, an undeveloped source of hot water comes from oil and gas wells that were drilled by prospectors in the search for oil and gas.	Some of the manifestations of underground reservoirs are volcanoes, fumaroles, hot springs and geysers, such as Old Faithful, the most famous geyser in Yellowstone National Park. Direct use of geothermal energy for homes and commercial operations can achieve savings up to 80% lower than fossil fuels.
Geothermal Power Plant A geothermal power plant (photo) uses geothermal steam or heat to drive turbine-generators to produce electricity. Three different types make use of the various temperature ranges of geothermal resources: dry steam (photo), flash and binary (these terms are described below). The typical size range for a geothermal power plant is from 10 to 250 MW.  One MW (8,760,000 kWh) of electricity will meet the needs of approximately 1,000 households for one year. Meeting a 250 MW generating plant output from fossil fuels would require 1,288,994 bbls of oil, or 7,227,722 mcf of natural gas, or 280,769 tons of coal per year.  See this NREL web page describing steam plants and binary plants.	The Geysers in California supply power for several plants. These power plants emit only excess steam and very minor amounts of gases. Take a geothermal power plant virtual tour - an audio-visual description of a geothermal power plant.

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In traditional geothermal electricity production, using near-surface high temperature water or steam, three methods are used to convert thermal energy into the mechanical energy of a spinning turbine. The following three descriptions refer to these methods.

Dry steam
Power plants using dry steam systems were the first type of geothermal power generation plants built. Dry steam is extremely hot water that is already in the form of steam and thus ready to drive a steam turbine. In this type of well, the very hot pressurized steam is produced at high speed. A dry steam power plant (photo) brings the steam to the surface of the earth to turn a turbine and generate electricity. At the end of the process, the water is returned to the thermal reservoir through an injection well to replenish this renewable energy resource. This technology is well developed and commercially available. See DOE's Dry steam power plants description.

Flash steam
Flash steam plants are the most common type of geothermal power generation plants in operation today. A flash steam power plant (photo) vaporizes water above 360° F by releasing it from the pressurized reservoir into a lower-pressure flash tank. The hot water is ‘flashed’ into steam through a pressure drop within the tank. The steam then drives a turbine to produce electricity. Flash steam power generators are in the size range of 10MW to 55MW, with 20MW being a standard size being used in many countries. See DOE's Flash steam power plants description.

Binary cycle
The use of two separate fluids for power generation gives the name ‘binary’ to this type of power plant. In this process, the geothermal fluid is brought to the surface and passed through a heat exchanger to heat another “working fluid” which is vaporized and used to turn the turbine/generator units. The geothermal water, and the working fluid are each confined in separate circulating systems or “closed loops” and never come in contact with each other. The secondary fluid is reused again and again as it runs through vaporization and condensing phases. The geothermal fluid is injected back into the ground in a closed-loop system. The binary cycle method has the greatest potential for expanded electricity generation because it allows producers to take advantage of lower-temperature fluids (225° F - 360° F). Additionally, this method produces no air emissions. See DOE's Binary-cycle power plants description.

What are hydrocarbons?
A hydrocarbon is a mix of hydrogen and carbon. The most common hydrocarbons are natural gas, oil, coal and methane. Petroleum (liquid geologically-extracted hydrocarbons) is a complex mixture of hydrocarbons. The oil reserves that are found in sedimentary rocks are the principal source of hydrocarbons for the energy industries. Our electric energy and heat sources (such as home heaters, which use either oil or natural gas) come from the energy produced when hydrocarbons are burned.

How can waste water from existing wells provide geothermal energy?
Once considered a nuisance, hot water encountered during petroleum production is attracting increasing interest due to the energy potential it offers. Wells currently drilled for petroleum extraction often reach the depths necessary to access moderate to high-temperature geothermal fluids. The hot water produced from these oil field wells is regarded as a waste product, but geothermal technologies can use this hot water to generate electricity to power on-site operations. In addition, existing transmission infrastructure can deliver excess power back to the grid.

Where can I find a quick history of geothermal energy in the United States?
Right here.

National Geothermal Reports

In it's 2008 Annual Energy Outlook, The U.S. Department of Energy's Energy Information Administration projected geothermal power production to increase 88.4% by 2030.

In 2007, an MIT-led panel, which included the Geothermal Lab of Southern Methodist University, released a comprehensive study, The Future of Geothermal Energy. The analysis showed that it is possible to supply 10% of the U.S. electric generation capacity from geothermal power plants by the middle of the century. It estimated that if 40 percent of the heat under the United States could be tapped, it would meet our nation's energy demand 56,000 times over. See the report summary.

The Geothermal Energy Association (GEA) has released a November, 2007 report, The State of Geothermal Technology, which says that the key to making use of the untapped geothermal energy resources that lie beneath our feet is to improve the technologies used to discover and tap into those resources. The report describes the techniques used to find and exploit geothermal reservoirs and fleshes out those techniques with specific case studies, providing a useful overview of the technologies employed by the geothermal energy industry and the challenges the industry faces. The report also calls for building a test facility to try out the concepts of so-called Enhanced Geothermal Systems, in which geothermal resources are modified to enhance their energy production.

GEA's May, 2007 report, Update on World Geothermal Development, examines the progress in international geothermal power development since 2005. It notes projects under development, major political and/or policy initiative related to development, and plans announced by either governments or in-country parties.

Another 2007 report, U.S. Geothermal Market Report estimates that there is a realistic potential to increase U.S. geothermal power production by a factor of six.

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