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.
Enhanced Geothermal Systems (EGS)
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 Energy in Alaska
Relatively little is known about the most effective methods for implementing an enhanced geothermal system. Many variables such as the 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.
Links and Resources
- Geothermal Technology: The U.S. Department of Energy's page on geothermal technology and its development.