Electrical Generation

Historically, Colorado has been considered to have geothermal resources suitable only for direct-use applications. Until very recently, Colorado’s geothermal potential for generating electricity has been assigned little promise. This appears to be based more on a lack of study, rather than on sound science.

Colorado has the second largest heat flow anomaly in North America, as shown on the Geothermal Map of North America. Moreover, drilling by AMAX at Mt. Princeton shows heat-flow values that are substantially higher than the published values. Several of the state’s oil and gas basins reveal formation temperatures above 100°C (212°F) at relatively shallow depths. Indeed, 20 wells in the San Juan Basin have temperatures of 121°C (250°F) or more between 7,000-9,000 feet deep. One well has a temperature of 150°C (302°F) at a depth of only 7,400 feet.

TheWestern Governors’ Geothermal Task Forceidentified Colorado as having the potential for 20MW of power generation within a decade. The state is in the process of updating our geothermal database and evaluating this potential energy source, in response to Colorado’s renewable energy portfolio standard for utilities.

The 2008 MIT report on the potential for Enhanced Geothermal Systems (EGS) showed Colorado as the number-one-ranked state in the nation for potential commercial development of this, as yet unproven, resource.

Geothermal resources are in the form of thermal energy and find extensive applications directly in this form. However, to transport the energy away from its source, the energy must be converted into a more portable form, electricity. Electricity is usually generated by using mechanical energy to cause a coil to rotate in a magnetic field. The thermal energy must first then be converted into mechanical energy. As with most conventional generating systems, heat is used to make “steam,” which drives a turbine, which in turn rotates a generator.

Steam can be generated directly from the resource if the resource is sufficiently hot, as shown below. These systems typically require temperatures in excess of 175°C (350°F). Most resources at these temperatures are associated with young volcanic systems and the geothermal fluids can include undesirable gases, such as carbon dioxide, hydrogen sulfide, and sulfur dioxide. Often these gases already leak to the surface through volcanic fissures and vents. Geothermal development may increase their release if the geothermal fluids are exposed to the atmosphere. Pollution control techniques can control the release of these gases.

For lower temperatures (below about 175°C, 350°F), the geothermal heat is used to vaporize a fluid with a lower boiling temperature than water. Vapor (“steam”) from this secondary, or working fluid is used to drive a turbine and rotate a generator. These systems are called binary systems and their basic components are shown below. The geothermal fluid (water or brine) is pumped to the surface, through a heat exchanger and returned to its original reservoir through a reinjection well. Heat is transferred to the secondary working fluid in the heat exchanger. Vaporized working fluid droves a turbine, is cooled, and the pumped back through the heat exchanger. In addition to being able to operate with lower temperature resources, an advantage of the binary system is that the geothermal fluid is contained in a closed system and is not exposed to the atmosphere. It remains under pressure which prevents boiling and release of gasses. Pollution is eliminated and scaling (deposition if dissolved mineralsin the pipes) is significantly reduced. The working fluid is also contained in a closed loop, isolated from the geothermal fluid, and there is no release of fluid from this part of the system.

The efficiency of a turbine driven by a condensing fluid depends on the difference in temperature of the vapor entering the turbine and the temperature on the outflow side of the turbine. Thus, not only is the temperature of the geothermal fluid important, but efficient cooling of the fluid after it passes through the turbine is important. The most effective cooling system for a large flow of fluid is evaporative cooling. This system either directly mixes a relatively small volume of cold water with the geothermal fluid or sprays cold water onto pipes filled with the hot geothermal fluid. Some cooling occurs by heating the cold water, but primary cooling occurs by evaporation of the water (latent heat of vaporization). The evaporation is usually contained in a very large concrete cylinder, gently tapered toward the top and open at the bottom so that the heat and water vapor rises creating an updraft through the cylinder causing further cooling by the updraft of air. These concrete cylinders are the familiar white tower that discharge white clouds of steam associated with most conventional power plants.

For less demanding cooling needs, and where water is not available for evaporative cooling, the geothermal fluid may be cooled by air. Air is forced through coils or radiators through which the fluid from the turbine is circulated by large fans. One or more fans per megawatt of electricity generated are required and the fans must be placed so that there is an unrestricted flow of air into and out or the fans. For larger systems they are commonly placed horizontally about 7.5 m (25 feet) above ground level). Advantages of air cooling include a low profile compared with evaporative cooling towers and no water use or plumes of steam rising above the cooling system. Disadvantages include noise, variable cooling temperature, and a relatively large footprint. Large fans beat the air as they rotate. The maximum noise is emitted in the direction that the fan blows, which is generally up. By erecting sound barriers the noise from the fans can be mitigated. The cooling temperature of air cooling is strongly coupled to air temperature and varies throughout the day and throughout the seasons. Thus, the efficiency of an air-cooled power plant fluctuates down and up as the air temperature rises and falls, respectively.

The image to the right shows the Soda Lake II binary geothermal power plant in Nevada. There is a man in the center right of the image for scale. In the center of the image are two binary power plants, each capable of generating six and a half megawatts of power. Visible equipment includes heat exchangers, turbines, and generators. The only large part of the system is the air cooling which is mounted on the elevated platform above and just behind the generating units. Each short cylinder on the platform contains a cooling fan, and these extend off the image to the left and right. The power plant is still small, but it would be a fraction the size without the air cooling fans.

Where water is relatively abundant, but may not be consumed, jacketed water cooling may be used. This system has only been deployed in very small systems at present, but could be scaled up to much larger systems if sufficient water flows were available. To an everyday analogy, air cooling is like most motorcycle engines where air flows past the cylinder head to cool the engine. Jacketed water cooling is like most automobile engines which have a jacket around the cylinder head through which the engine coolant is circulated to cool the engine and the coolant removes the heat to be lost elsewhere (the radiator). Jacketed water cooling cools the fluid from the turbine by removing the heat with liquid water in a jacket or heat exchanger, keeping the cooling water separated from the working fluid. In a binary power plant the working fluid is then recirculated for reheating. The warmed cooling water is usually reinjected back into the ground and replaced with new cooling water. The advantages of this system are that it is relatively quiet, small, and maintains a very low, constant temperature on the outflow side of the turbine. The only disadvantage is that it requires a large volume of cooling water: the volume of cooling water is approximately equal to the volume of geothermal fluid pumped from the production well(s). However, the use is non-consumptive and non-polluting water use, and the warmed water is immediately replaced into the ground. The only consideration is the change in temperature of the water.

Geothermal resources and the nature of their locations vary considerably and geothermal power plants are generally custom built to meet their specific needs. Combinations of the technologies described above may be used in a single power plant. Compared with other conventional and alternative power generative technologies, when fuel requirements, access roads, and other requirements are considered over a 30–year period, geothermal power plants are considered to have the smallest footprint of any method of power generation per megawatt-hour of power generated. They also have the best record of power availability (capacity factor), with most power plants generating more than 95% of the time. Geothermal electricity has been produced in the US for more than 50 years without government subsidies at economically competitive process. Geothermal power is a clean, environmental-friendly, sustainable form of alternative energy.