Note: This article appeared in the April 2013 issue of Power Engineering magazine.
By Tim Reinhardt, physical scientist, DOE's Geothermal Technologies Office
Investments by the U.S. Department of Energy (DOE) are helping to produce geothermal power at increasingly lower temperatures. While traditional geothermal energy relies on geologically rare hot spots in the subsurface, new technologies are making inroads with economics that could bring valuable returns on geothermal investment in the near-term.
Geothermal resources are reservoirs of hot fluid (brine) that exist below the Earth's surface. Wells can be drilled into these underground reservoirs to tap steam and very hot brine that can then be brought to the surface to generate electricity. In the U.S., most conventional geothermal reservoirs are located in the western states, which enjoy a predominance of geological anomalies that allow for relatively shallow and hot geothermal reservoirs to be accessed economically, creating electricity via turbine-generators (TG). Traditionally, these hot zones have been the main focus of geothermal development, as these resources can be directly used in a TG set via dry-steam or flash-steam technologies.
The development of binary technology, however, has allowed for an expansion of the temperature range of resources capable of producing electricity. In these systems, low to moderately heated geothermal fluid (below 300°F) and a secondary (hence, "binary") fluid, with a lower boiling point than water, pass through a heat exchanger, where the heat causes the secondary fluid to flash to vapor, driving the TG.
The DOE's Geothermal Technologies Office (GTO) develops and deploys a portfolio of innovative technologies for domestic power generation. Within this office is the low temperature and co-produced resources subprogram that conducts research and demonstration projects in partnership with industry that will lead to advanced geothermal energy use and electricity production from these lower temperature fluids. But, to truly capitalize on lower temperature resources, the "value" of produced fluid must be increased.
There are a number of ways to improve the value of geothermal fluid. One is through "cascaded" technologies, in which the geothermal fluid cascades from the highest available temperature for creating electricity down through direct use applications that require successively less heat, including spas, industrial processes and snow melting – each with distinct temperature ranges. The GTO is helping to develop this process through a number of projects, including one in partnership with the Surprise Valley Electrification Corp (SVEC). The SVEC project will produce over 3 MW of power from geothermal fluid which will then be used for aquaculture and irrigation purposes.
Another improvement is the harvesting of valuable materials from geothermal fluid. In addition to using the heat to create power, another project partner, Simbol Materials, is developing techniques to profitably extract strategic minerals from brines via new "geothermal mining" technologies. The project is validating improved lithium extraction techniques to transform mined materials into saleable products cost-effectively.
The best way of increasing the value of brines is by improving the efficiency and reducing the cost of power generation technologies. The DOE's Pacific Northwest National Laboratory is in the process of developing microporous metal-organic solids as the primary heat carrier and heat transfer medium that could increase power generation at binary plants by 15 percent.
Finally, by using water produced in oil and gas and mining processes, geothermal developers can leverage existing infrastructure and greatly reduce the cost of development, conversely increasing the value of the fluid stream. ElectraTherm, another industry partner, is demonstrating this waste heat-to-power technology at Florida Canyon Mine in Imlay, Nev. by using existing groundwork for a fraction of the cost of new development. This project developed an optimized solution for power generation using co-produced geothermal fluids that will produce a constant 75kW of electricity.