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. Groundwater 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 (see How Does It Form? ). 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 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.