Introduction to Geothermal Energy
Geothermal is a general term describing the heat generated and contained within the earth. Over 90% of the total volume of the earth has a temperature exceeding 1000°F, and only a small amount of this heat gets close enough to the earth’s surface to be utilized by conventional technology and be considered an energy resource. When it does, the elevated heat manifests itself in uncommon geologic occurrences like lava flows and volcanic eruptions, steam vents or geysers, hot springs, or elevated geothermal gradients creating hot rock. In normal geologic situations, the majority of the heat slowly dissipates into the atmosphere by unseen heat transfer processes known as conduction, convection, and radiation.
At the surface of the earth, heat can also be gained from the sun during daylight hours. The sun, especially during the summer months, can heat to depths of 100 feet. When ground source heat pumps are used for heating buildings, the energy may come from either solar or geothermal sources. Below a depth of several tens of feet, any heat recovered from the earth will usually be geothermal in origin. Geothermal heat comes from two main sources: the original heat of the earth generated at its formation about 4.5 billion years ago and the more recent decay of the radioactive isotopes of potassium, uranium, and thorium.
Geothermal resources are found on all continents and have been used for a wide variety of purposes, ranging from balneology (the science of soaking in hot springs or hot mud baths) to industrial or direct use processes such as space heating, from process heat for drying things like fish or lumber to electrical power generation. Industrial uses require temperatures ranging from 150°F to around 300°F. For large-scale electrical power generation, (measured in megawatts or millions of watts) temperatures in the neighborhood of 300°F to 650°F are needed. In Alaska with its cold climate and abundant cold water resources it is possible to use much lower geothermal temperatures for small-scale electrical power generation.
Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels. As a result, geothermal power has the potential to help mitigate global warming if widely deployed in place of fossil fuels.
How Geothermal Energy Works
Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, from volcanic activity, and from solar energy absorbed at the surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.
Hot springs located along large faults in the earth are relatively widespread. These faults can extend well into the crust of the earth. If the fractures remain open under the great amount of pressure, these faults might allow water to percolate more than 2-3 miles deep. If this happens, the water will become hot by virtue of the significant depth reached (a typical geothermal gradient has a temperature of about 270°F at 3 miles depth). If the fault maintains porosity and permeability, this heated water can be forced to the surface (or near surface) and become a geothermal spring.
The Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a very small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, and interest rates.
Geothermal energy is harnessed in one of several ways:
- Geothermal Electric Plants
The thermal efficiency of geothermal electric plants is low, around 10-23%, because most geothermal fluids do not reach the high temperatures of steam from boilers. The laws of thermodynamics limits the efficiency of heat engines in extracting useful energy. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses or district heating (such as at Chena Hot Springs Resort). The low system efficiency (as compared to other energy sources) does not affect the operation costs of a facility, but it does affect the return on the the capital used to build the plant. In order to produce more energy than the pumps consume, electricity generation requires relatively hot geothermal sources. Because geothermal is a stable energy source (as opposed to solar or wind energy), the capacity for energy capture is quite large; energy use of up to 96% of a resource's capacity has been demonstrated. The global average, however, is lower - found to be 73% in 2005.
- Direct Heating
In the geothermal industry, low temperature means temperatures of 300 °F (149 °C) or less. Low-temperature geothermal resources are typically used in direct-use applications, such as district heating, greenhouses, fisheries, mineral recovery, and industrial process heating. However, some low-temperature resources can generate electricity using binary cycle electricity generating technology.
Direct heating is far more efficient than electricity generation and places less demanding temperature requirements on the heat resource. Heat may come from co-generation via a geothermal electrical plant or from smaller wells or heat exchangers buried in shallow ground. As a result, geothermal heating is economic at many more sites than geothermal electricity generation. Where natural hot springs are available, the heated water can be piped directly into radiators. If the ground is hot but dry, earth tubes or downhole heat exchangers can collect the heat. But even in areas where the ground is colder than room temperature, heat can still be extracted with a geothermal heat pump more cost-effectively and cleanly than by conventional furnaces. These devices draw on much shallower and colder resources than traditional geothermal techniques, and they frequently combine a variety of functions, including air conditioning, seasonal energy storage, solar energy collection, and electric heating. Geothermal heat pumps can be used for space heating essentially anywhere.
Challenges in Geothermal Energy
Geothermal energy is not without its challenges. First, the geothermal resource must exist at a shallow enough depth to be economically accessible. Second, most available geothermal technology requires the resource to provide very hot water. In the case of Chena Hot Springs, they have found a way to circumvent the challenge of low temperatures; however, this is an atypical situation where the developers of the resource were able to find alternative solutions to a typical geothermal challenge. Geothermal energy, while a well-known alternative energy resource, is not widely accessible since geothermal resources exist primarily along fault lines and where volcanic activity is frequent. Most places in the United States, with the exception of states located along the Pacific rim, do not have these resources.
For geothermal energy to be technically and economically feasible, a number of conditions must be met. These conditions include: (1) an anomalous thermal gradient or accessible heat in a near-surface region, (2) sufficient porosity and permeability within the section of ‘hot rock’ so that fluids can move freely and transfer heat, and (3) some form of conduit that allows a hot fluid to flow to the surface in sufficient quantities. There the energy can be converted into a usable form. Clearly, the higher the near-surface temperature and the higher the permeability and flow rates, the more feasible the resource becomes. Unfortunately, out of the thousands of natural springs in Alaska, only a few have sufficient temperature and flow rates necessary to produce electricity. In some limited cases where high near-surface heat exists, these fluid flow and heat transfer systems can be enhanced by drilling and fracture technology if geologic conditions are right (see EGS).
Alaska Specific Challenges
Relatively little is known about the most effective methods for implementing an enhanced geothermal system. Many variables such as temperatures, the temperature gradient, the type and characteristics of rocks present, and the existing stresses on the rocks, need to be considered in planning an enhanced geothermal project. Within Alaska there must be some areas where the overall conditions are more favorable for such a project than other areas. Each area is unique, and all variables need to be assessed to determine the feasibility of an enhanced geothermal system. Development of enhanced geothermal systems will continue to be mostly experimental in the next years. The EGS concept bears close watching because enhanced geothermal systems could be part of Alaska’s future.
In making electricity from geothermal steam or hot water, two basic types of equipment convert the heat energy into electrical energy. If the geothermal fluid temperatures are greater than about 350°F, a conventional low-pressure steam turbine is utilized. As the steam passes through a series of blades known as a rotor, the pressure is reduced. The steam expands, thus spinning the rotor. The rotor is attached by a straight shaft to a generator that spins and makes electrical power. In a few rare places in the world, geothermal production wells flow steam with no water. The steam is transported directly from the well to the turbine. In most cases a mixture of steam and hot water is produced by the well, and the water must be removed with a separator so that only pure dry steam enters the turbine. The geothermal liquid and the condensed steam are sent to an injection well, where they are returned to the reservoir to be utilized again and again. Essentially, a geothermal power plant ‘mines’ heat from a geothermal reservoir.
If the geothermal fluid temperatures are less than about 350°F, a different type of turbine is needed. Instead of steam passing through the turbine, a lower-boiling-point liquid, a working fluid such as isobutene, isopentane, or a refrigerant, is heated in a heat exchanger by the geothermal water. It becomes a vapor and is then sent through a turbine. No water (either as liquid or steam) passes through the turbine in this instance. Once through the turbine, the vapor is condensed and pumped back through the heat exchanger again and again. The geothermal fluid in this case is also returned to the reservoir to mine more heat.
|TECHNOLOGY SNAPSHOT: GEOTHERMAL|
|Installed Capacity (Worldwide)||approximately 10,715+ MW|
|Installed Capacity (Alaska)||680 kW installed|
|Resource Distribution||Dispersed resources exist across southeast Alaska, the Interior, and the Aleutians.|
|Number of communities impacted||Limited|
|Environmental Impact||Minimal, small plant footprints, little or no CO2 emissions, reduced surface flow of thermal springs|
|Economic Status||Payback of 5 to 8 years expected for the Chena Hot Springs Project, project economics vary widely depending upon size of project and sales price of electricity|
|Case Studies||Chena Hot Springs Resort, Pilgrim Hot Springs|
|Geothermal Working Group||Recommendations, References|
|News||DOE Geothermal News|
Most of the earth is not near volcanoes or major active faults so it lacks open space or fractures that can heat the fluids necessary for a shallow geothermal system. The geothermal industry has long known that developable heat exists within drillable depths in most areas of the globe, yet a technically economically feasible way to transfer that heat to the surface in economic quantities has been elusive. If this methodology can be developed, a tremendous energy resource can be tapped. One interesting aspect of this research effort is the use of techniques developed by the oil and gas industry to fracture rocks far below the surface, Huge volumes of fluid are pumped at high pressure into the deep strata. The theory is that once the rocks are broken and permeability is established, it is possible to pump cold water down one hole into hot rocks and recover it from a second hole located thousands of feet away. If all goes according to plan, the water will mine heat from the fracture surfaces between the two holes. It will become hot enough to utilize for direct use and/or electrical power generation. This concept is called enhanced geothermal system, or EGS.
Projects are now operating in France, Germany, and Austria, where six small EGS projects are generating between 0.25 MW and 3.5 MW of electrical power from wells between 7000 feet and 16,000 feet deep and at temperatures from 300°F to 500°F. After the power is generated, additional heat is sometimes removed from the water for space heating as a part of some of the projects. These expensive, government-supported research projects have taken many years to develop. With this experience in hand, Germany has recently announced plans for over 100 future projects with outputs as high as 8.5 MW for some of them. In Australia numerous press releases tout much higher potential megawatt outputs, but no projects are yet on line.
Geothermal Potential and Projects in Alaska
Alaska's potential for geothermal energy is high, but largely undeveloped. Because of Alaska's volcanic activity and relatively young geography, there are many locations with accessible geothermal energy. Particularly in the Aleutians, some areas of the Interior, and Southeast Alaska, geothermic "hot" spots lie close to the surface and are easily accessed to draw up heat.
There are numerous sedimentary basins in Alaska, the most famous of which underlies the North Slope and hosts the Prudhoe Bay oilfield. Excellent porosity and permeability can be maintained in sedimentary rocks at depth, and if the geothermal gradient is sufficient, hot fluid can be produced from these formations. For example, the reservoir temperature at Prudhoe Bay at 7500 to 8000 foot depth is approximately 180°F to 200°F. Depending on the geothermal gradient of the basin and the relic permeability at depth, production of this hot water may become a viable small-scale energy source for oilfield operations, or even for communities in the immediate area. The high cost of drilling and permeability enhancement, along with relatively low geothermal temperatures, makes these resources difficult to economically develop on a stand-alone basis.
One case study of successful geothermal development can be found in Chena Hot Springs, near Fairbanks, Alaska. At Chena Hot Springs Resort, 500 gallons per minute of 163°F water is making around 200 kW of electricity, the amount of electricity used by a village of about 300 residents. The combination of high flow rates of hot water and low surface water temperatures in use allow Chena to be the lowest temperature geothermal power plant in the world. Chena uses a binary system to create their power. In this system, a secondary fluid with a lower boiling point than water is passed through a heat exchange with hot water from the geothermal source. The secondary fluid is instantly vaporized, which then spins a turbine and produces electricity. Chena Hot Springs Resort and their geothermal technology remain the only developed geothermal source in the state of Alaska, and have set a promising standard for further development in a state rich in geothermic resources.
Other case studies:
Unalaska – The Makushin geothermal resource near the community of Dutch Harbor on Unalaska (Aleutians) is the only proven high temperature geothermal system in Alaska that could be used for power generation. An exploratory drilling program, which took place in the early 1980s and was funded by the Department of Energy, made this determination. The City of Unalaska is planning further drilling in the area in the summer of 2009.
Akutan – The City of Akutan on the Aleutian Chain in 2010 was in the process of drilling two exploration wells in Hot Springs Valley to investigate providing power and heat to the city and a local fish processor.The first well produced water in excess of 360 degrees Fahrenheit. Though early, that result is encouraging for development of a large-scale geothermal power system.
Mt. Spurr – The Mt. Spurr volcano, across Cook Inlet from Anchorage, is a yet unproven resource, but its proximity to Anchorage makes it worth further assessment. Ormat, a geothermal developer and power plant manufacturer, purchased all but one lease section of the volcano for $3.5 million in a competitive bid process. In 2010, Ormat conducted several pre-drilling surveys such as aeromagnetic testing and ground-based geophysical surveys, as well as drilling two temperature gradient core holes, which yielded encouraging results. As of summer 2011, Ormat is drilling additional, deeper core holes in attempt to collect additional geological data about the actual geothermal source. They have projected a six-year best case scenario time line. If all goes well, Ormat hopes to build a plant by 2016.
Information on case studies from the Alaska Renewable Energy Project.
Links and Resources
Chena Hot Springs Resort - Geothermal Technology : A link to the Chena geothermal power plant page. An illustrated explanation of how they harness low-temperature geothermal energy.
Your Own Power : A link to a geothermal energy site supported by Chena Hot Springs. Gives information about geothermal energy at Chena Hot Springs Resort, as well as information about the development process and geothermal resources.
Geothermal Energy: A great resource for information about geothermal energy in Alaska, and other renewable energy resources.
Geothermal Technology: The U.S. Department of Energy's page on geothermal technology and its development.
Share this page on Facebook!