New energy resources for the 21st century
Geothermal energy, or heat from the earth, is an excellent resource. It is sustainable, works 24/7, and has a minimal carbon footprint. One should be clear when discussing geothermal energy about which type is being discussed: direct use, electrical generation, heat pumps, or enhanced geothermal systems (EGS).
Historically, Colorado has been considered to have geothermal resources suitable only for direct-use applications. Until very recently, Colorado’s geothermal potential for generating electrical power has been assigned little promise. This appears to be based more on a lack of study, rather than on sound science.
Prior to the 21st Century, the CGS published numerous reports on various aspects of the State’s geothermal energy resources. With today’s improved technology, we are taking another look at our geothermal resources and are in the process of issuing revised and updated maps. It is exciting to see a number of companies actively looking at the potential for generating electricity from geothermal in several parts of the state.
What is it?
Geothermal is, literally, earth-heat (Greek: geo – therme). Heat is a form of energy. Geothermal becomes an energy resource when we can use this heat to our advantage. Most of the Earth’s heat is deep inside the Earth, beyond the reach of technology to extract the heat. At relatively shallow depths, depending on the temperature, the heat may be economically extracted and used.
At shallow depths the Earth may be used as a heat reservoir. In terms of magnitude, the Earth receives more than a thousand times more energy from the Sun than is loses from its internal reserves. However, the solar energy is lost back to space on a daily and seasonal basis. Except for small microclimates around hot springs and active volcanoes, the Sun controls the temperature of Earth’s surface and this temperature generally decreases with latitude from the equator to the poles and with elevation. Soil and rocks are poor conductors of heat and below a couple of meters (about 7 feet) below the surface, the annual and seasonal variations in surface temperature are damped out and the temperature is steady at approximately the mean annual ground surface temperature. Although this temperature is defined by the solar energy balance, it is soil and rock properties that make this zone good for use as a heat reservoir. Ground-Source Heat Pumps (or Geoexchange Heat Pumps) use this zone for heat storage and retrieval.
Where subsurface temperatures are significantly hotter than the surface temperatures heat may be extracted for surface use. This situation would occur where the geothermal gradient increases the temperature above the surface temperature. This difference may be a few degrees, or even a few degrees above winter surface temperature for some direct use applications, to a few hundred degrees Celsius for geothermal electricity generation.
A further requirement for “elevated” subsurface temperatures to be a resource is that there must a mechanism by which the heat can be brought to the surface. For some resources the mechanism may be natural, such as hot springs or artesian (naturally flowing) wells. Other sites may require the drilling of a well and pumping. At many sites high subsurface temperatures are found but the rocks lack sufficient permeability (pathways for fluid flow). These rocks may require artificial fracturing or down-hole heat-exchangers to extract the heat. Research on new technologies to extract heat from potential geothermal reservoirs is continuing.
How does it form?
The interior of the Earth is hot. Volcanoes are a dramatic reminder that there is heat in the Earth. Miners who work in deep mines know that the deeper the level, the higher the temperatures. Oil-well drillers also know that the drill pipes are hot when they are pulled from a deep well and the oil is hot as it rises to the surface. What is the source of this heat?
An early theory was that all of this heat was primordial, or remained from the formation of the Earth. However, with the discovery of radioactivity an additional source of heat was found. Unstable isotopes of uranium (235, 238U), thorium (232Th), and potassium (40K) exist in sufficient quantities in most rocks to supply a significant fraction of the heat that is lost from the modern Earth. The total, present-day rate of heat loss from the Earth is estimated to be 46 TW (terawatts or million billion watts), or the equivalent of approximately 69,000 average-sized US coal-fired power plants (average power generation capacity 667 MW).
The Earth is simmering in geologic time. These heat sources are not sufficiently concentrated enough to form a volcano or a geothermal resource directly, but are like a burner on low on a range top. Given enough time, they can bring soup to a simmer. The result is movement of the tectonic plates, a solid crust on the simmering pot of the Earth, broken into pieces that move relative to each other along their boundaries. Most geologic interactions, volcanoes, earthquakes, mountain building, occur close to these boundaries, although there are some important exceptions. Isolated volcanic centers, such as Yellowstone and the Hawaiian Islands pierce the plates far from their edges, sedimentary basins continue to develop long after they have an association with a plate boundary.
Plate tectonics is an important process for geothermal resources in a number of ways. Most volcanoes are associated with plate boundaries and high-temperature geothermal resources are usually found close to active volcanoes. Mountain belts are generally formed in association with plate tectonics. Sometimes the association is obvious, such as the collision of the Indian subcontinent with Asian to form the Himalaya. Sometimes the association is less clear such as the origin of the current elevation of the Colorado Rocky Mountains. However, topographic variations and young faults often allow water to circulate deep in the earth (a few km or a couple of miles) and rise to the surface as a hot spring/thermal resource. Finally, mineral resources can become concentrated in association with melting and recycling of the crust during the plate tectonic cycle. The minerals include those that contain the heat-producing isotopes and some rocks in the continental crust, particularly granitic rocks, contain significantly more heat production than other rocks. These high heat production rocks can produce local warm spots in the crust.
The rate of increase in temperature with depth in the crust is called the geothermal gradient (see Colorado Geothermal Gradient Map). In most areas the geothermal gradient is in the range of 15 to 30°C/km (0.8 to 1.6°F per 100 feet). The average temperature in Denver is about 10°C (50°F), so with these gradients you would need to go down between 2.8 and 5.7 km (between about 5,600 and 11,150 feet) to reach a temperature of 95°C (202°F), the average temperature at which water boils at the elevation of Denver. [The boiling temperature of water decreases by about 1.1°C (2°F) for every 300 meters (1,000 feet) increase in elevation above sea level.] These depths are relatively deep to drill for such modest temperatures.
In some areas geothermal gradients are significantly higher than others. These areas are usually associated with plate boundaries, but can also be associated with high concentrations of heat producing radiogenic isotopes in the upper crust, thick sections of sedimentary rocks that conduct heat poorly, or hot spots, areas of mid-plate volcanism.
Water flow can increase the geothermal gradient at shallow depths: upward flow increases the gradient, downward flow decreases the gradient. Water flow may raise the gradient in addition to other mechanisms that increase the gradient. Where these increases in gradient occur are the locations of geothermal resources. Young volcanic activity is commonly associated with geothermal resources, but is not a requirement.
Where is it found?
The most economic geothermal resources are found where geothermal gradients are significantly higher than average. Gradients as high as 200°C/km (11°F per hundred feet) or higher are common in geothermal areas. These high gradients are usually associated with water flow brining heat toward the surface. Unfortunately a 200°C/km gradient may be caused by 50°C (122°F) water at a depth of 200 meters (663 feet) or 150°C (300°F) water at 700 meters (2,300 feet). The shallow warm water could be used for direct use; the deeper hot water could be used for electricity generation. Geothermal gradients are a good indication of the location of geothermal resources, but other exploration is required to determine the magnitude and potential uses of the resource.
Primary sources and controls of heat loss from the Earth that produce geothermal resources.
The outer shell of the Earth, the plates of the lithosphere, is relatively rigid and heat flows into the base of this layer from below this layer. In some areas temperatures are high enough to cause melting and additional heat is transferred with the upward flow of molten rock, or magma. Within the plates, heat is generated by the radioactive decay of isotopes of uranium, thorium and potassium. These radioactive isotopes are present in very small concentrations, typically less than 1 to no more than 20 parts per million. However, when present in a layer 10 km (6.25 miles) thick, the effect of even small concentrations becomes significant. Heat from the radiogenic decay of these isotopes roughly doubles the heat flow that enters the base of the plates on average, but concentrations of these isotopes are very variable making the surface heat flow very variable. In the near surface (the upper crust), heat flow may be changed dramatically by transport of heat by magma, often associated with volcanoes, and heat transfer by groundwater. Ground-water flow may be driven by heat from magmatic intrusions or by gravity-driven flow. Where water flows downward heat is transported downward; where water flows upward, heat is transported upward.
Ground source heat pumps do not use heat from the Earth and do not require a geothermal resource for operation. In theory they may be installed at any location. In practice they operate more efficiently where their ground loops are buried in materials with good thermal conducting properties, which are usually below the water table. They may be prohibitively expensive to install where the water table is deep and the near surface materials are dry. In addition, they operate more efficiently if the mean annual surface temperature is close to the average of the heating and cooling requirements of the system. If heating significantly exceeds cooling, or vice versa, the ground around the loops will cool down or heat up, respectively, and the system will become less efficient during the season of the dominant cycle. If the dominant cycle is heating, a system’s efficiency may be significantly increased by extracting heat from a low-temperature resources during the heating season, but a separate loop, or an alternating cooling system, such as evaporative cooling, must be used.
For direct use, low-temperature applications may be found at any location where natural groundwater circulation brings warmed water to the surface or wells are sufficiently deep for pumped water to be sufficiently hot for thermal use. Many low-temperature thermal springs (<35°C; <95°F) are the result of deep water circulation and have no association with anomalous deep heat source. For higher temperature natural springs there may or not be a deep heat source, but there generally is a requirement that the pathways to the surface for the thermal waters are kept open. Hot water typically deposits minerals as it cools, clogging the fractures and other permeability through which it rises to the surface. The most common mechanism by which fractures remain open is by fault movements, which generate small to moderate earthquakes. Higher temperature hot springs are commonly found in association with young (<20,000 years) faults and/or historical or modern earthquake activity. Higher temperature geothermal resources may also be found at many locations at depth of 2 to 4 km (6,500 to 13,000 feet) where the geothermal gradient is above average. Most of these locations are in sedimentary basins where boreholes often already exist from oil and gas exploration and production. At present, production from these resources is uneconomic, but they may be a resource for the future.
The highest temperature geothermal resources are generally found in areas with young (<20,000 years) volcanic activity. These areas are typically also associated with minor earthquake activity. Not only is there a relatively shallow heat source, but there are fracture that are kept open through which water circulates. Most of these resources are on plate boundaries or volcanic hot spots in mid plate regions. The Geo-Heat Center at the Oregon Institute of Technology identified 15 communities in Colorado that are within five miles of a geothermal resource with a temperature of 122°F or more, making them good candidates for community district heating or other geothermal applications.
Areas in Colorado that are prime for new exploration include the Rico Dome structure in southwest Colorado, Mount Princeton Hot Springs, Wuanita Hot Springs, and the San Luis Valley. These exploration targets represent potential sites with high heat flow. There are currently no geothermal electrical power generating facilities in Colorado.