Note: This article contains technical engineering material and scientific analyses. I apologize to those readers encountering some terms for the first time.
Interest in all forms and sources of energy resources is growing in the United States and globally. The energy industry is expanding its activities to incorporate geothermal, nuclear, wind, solar, and other resources into their energy portfolios. In fact, most U.S. states now require that part of their electricity production come from renewable energy sources. The reader should note that this energy transition involves a particularly significant set of changes to the present pattern of energy use and application.
Improving access to all energy resources and services is considered by most to be a critical component for improving the quality of life. No discussion on this topic would be complete without an attempt to differentiate between the two major energy sources in use today: nonrenewable and renewable resources. Nonrenewable energy sources primarily include the fossil fuels, i.e., coal, petroleum, natural gas, propane, etc. These energy sources are called nonrenewable because they cannot be replenished in a short period of time. Renewable energy sources include geothermal, biomass, hydropower, tidal, solar, and wind because these supplies can be replenished within a short period of time or represent sources of potential power driven by the Sun or planetary forces. Thus, renewable energy is a sustainable energy source that is continually replenished by nature – the sun, wind, water, and plants. Renewable energy technologies turn these fuels into usable forms of energy – most often electricity, but also heating / cooling, or mechanical power.
It should be noted that although humans have found many different sources of energy to power their endeavors, fossil fuels remain the major source by a wide margin. Oil has been the major fuel supply for industrialized society since the middle of the 20th century and provides more of the energy used by humans than any other source. Coal is second on this list, followed closely by natural gas. Together they accounted for 82% of the world’s energy use in 2022.
On to geothermal energy, your author’s solution to society’s energy concerns. The primary application of geothermal energy is in the generation of electricity from the heat stored in the Earth’s crust. Other uses of geothermal energy for processing or space heating are termed direct heat.
Geothermal energy is replenished by a nuclear process that has been going on since the Earth’s formation and will effectively continue for many millennia. It is virtually limitless and is based on the fact that the Earth is hotter the deeper one drills below the surface. These resources are theoretically more than adequate to supply humanity’s energy needs, but only a very small fraction may be profitably exploited since drilling and exploration for deep resources is very expensive. However, these costs are decreasing at a near exponential rate, stamping this as the energy source of the future. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, existing subsidies, environmental issues, and the value of money (interest rates).
America’s first district heating system, employing geothermal energy, occurred in 1892 in Boise, Idaho and was copied in Klamath Falls, Oregon in 1900. Pacific Gas and Electric (PG&E) began operation of the first successful geothermal electric power plant in the U.S. at the Geysers in California in 1960. The plant’s life was approximately 30 years and generated over 10 megawatts (MW) of power. A binary cycle power plant was introduced to the U.S. in 1981; a binary cycle plant in Hot Springs, Alaska came online in 2006, generating electricity from a record low fluid temperature of 135°F.
The areas with the highest underground temperatures are in regions with active geological volcanoes. The Pacific Rim, often called the “Ring of Fire” for its many volcanoes, has many hotspots in the U.S., including those in Alaska, California, and Oregon. In addition, Nevada has hundreds of hotspots, covering much of the northern part of the state. A similar “ring” extends through southern Europe and across the middle of Asia connecting with some of the South Sea Islands.
These regions also exhibit seismic activity (a scientist’s term for earthquakes). Earthquakes and magma movement break up the covering rock, allowing water to circulate. As the water rises to the surface, natural hot springs and geysers occur such as Old Faithful at Yellowstone National Park (visited by your author). The water temperature in these systems can be more than 430°F. The majority of the geothermal resources located in the U.S. are in the following western states: California; New Mexico; Arizona; Utah; Nevada; Washington; Oregon; and, Idaho. Eighty percent of the 3,000 MW is located in California, where more than 40 geothermal plants provide nearly 5 percent of the state’s electricity.
Active hotspots are not the only places where geothermal energy can be found. There is a steady supply of milder heat – useful for direct heating purposes – virtually in any location on Earth at depths of anywhere from 10 to a few hundred feet below the surface. In addition, there is a vast amount of heat energy available from dry rock formulations much deeper below the surface (2 to 6 miles).
When geothermal reservoirs are located near the surface, they can be reached by drilling wells. Some wells are more than 2 miles deep. (There are also several 6 miles deep.) Exploratory wells are first drilled to search for energy reservoirs. Production wells are drilled once a reservoir has been located. The wells contain steel (usually) pipe conduits (casing) that provide an open passageway for steam and/or hot water at elevated pressures to rise to the surface of the Earth. This energy can then be employed for heating / cooling (with a heat pump) purposes or for generating electricity in a traditional power plant. Hot water and steam – at elevated temperatures of 300°F to 700°F – are extracted to the surface and used to generate electricity at power plants near the production wells. The usual way of extracting the energy from geothermal sources has cooler water seep into the Earth’s crust where it is heated up, and then rises to the surface. When heated water and / or steam is forced to the surface, it is a relatively simple matter to capture the steam and use if for the aforementioned heating purposes and / or to drive electric generators. It should also be noted that hot water at lower temperatures is generally not suitable for producing electricity. More recently, however, these resources have been employed to generate electricity; this is accomplished in the binary power plants mentioned earlier where the not-so-hot water has its energy transferred to another fluid with a lower boiling point (vaporizing at a lower temperature than water) creating a more “forceful” heating fluid prior to entering the power plant.
At present, geothermal wells are rarely more than 2 miles deep. Upper estimates of geothermal resources assume wells as deep as 7 miles. Drilling at this depth is now possible. Although drilling is obviously an expensive process, it continues to decrease. The challenges facing engineers are to drill wide holes at minimum cost and to disengage rocks.
Geothermal power plants are not dissimilar to other steam turbine thermal power plants. Here, heat from the Earth’s core is used to heat water or another working fluid. The working fluid is then used to turn a turbine, which in turn employs a generator to produce electricity. The fluid is then usually cooled and returned to the heat source in a closed loop system.
The reality today is that geothermal energy applications are not economically far removed from conventional energy applications, including producing electricity. The energy industry will soon approach a state where low temperature geothermal energy resources will be routinely used for nearly all heating / cooling purposes and high temperature resources will be generating electricity in power plants for an increasing number of applications.
Geothermal sources have a reputation for being “clean” reservoirs of energy, but geothermal development has numerous potential negative environmental impacts including air, water, and noise pollution; land subsidence; induced seismic (earthquakes) activity, and a number of non-condensable gases. In addition, the release of substantial amounts of water vapor has led to instances of local fogging around some geothermal sources. On the whole, however, the aforementioned environmental problems in geothermal development are not complex. The pollutants are not “exotic” and they are contained in the raw geothermal fluid and are natural constituents derived from rock and mineral components dissolved in water.
On the positive side, when compared to other sources of energy, geothermal energy uses and applications produce no smoke and only traces (if any) amounts of pollutants, use very little land, are almost always sealed — protecting aquifers and national sources of drinking water, operate with no intermittences (run day and night), and are inexhaustible (there is natural replenishment) while free for the taking. Finally, another often unrealized environmental benefit of geothermal applications involves injecting useless wastewater (as the source of water) within the geothermal energy reservoir. Quite a bit for this industry to hang its hat on.
For the near future, energy from near-surface geothermal resources employing heat pumps will continue to extract heat from shallow ground anywhere in the world to provide home heating and cooling. But some industrial and most utility applications require the higher temperatures of deep resources since the thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. It is here that mankind will find the long-awaited solution to society’s energy problem.
Note: The bulk of the material in this article was adapted from:
- K. Skipka and L. Theodore, “Energy Resources: Availability, Management, and Environmental Impacts,” CRC Press/Taylor & Francis Group. Boca Raton, FL, 2014.
- M. Reynolds, R.R. Dupont, W. Matystik, and L. Theodore, “Geothermal Energy: Principles and Applications,” CRC Press/Taylor & Francis Group, Boca Raton, FL, 2026.
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