DOE announced on September 15 its selection of seven projects to research, develop, and demonstrate cutting-edge geothermal energy technologies involving low-temperature fluids, geothermal fluids recovered from oil and gas wells, and highly pressurized geothermal fluids. Today's geothermal power plants draw on underground reservoirs of water or steam that are heated by "hot spots" in the Earth's crust. While traditional geothermal power plants require reservoirs at temperatures greater than 360°F, hot enough to easily generate steam, the latest generation of power plants is using binary-cycle technology to draw on lower-temperature resources. This technology uses a "working fluid" that vaporizes at a lower temperature than water. By passing the geothermal fluid through a heat exchanger, the working fluid is vaporized, and its vapor is used to drive a turbine, which spins a generator to produce power. The vapor is then condensed and reused in a closed cycle. Because the geothermal fluid is kept apart from the working fluid, the two fluids form two separate cycles, hence the name "binary-cycle" technology.
Three of the selected projects will develop new advances to binary-cycle technology, including a scale-resistant heat exchanger that could increase power production by 40%, as well as systems that use carbon dioxide and ammonia-water mixtures as working fluids. A fourth project will aim to extract more energy from the geothermal fluid by developing a generator that can run on the waste heat from an existing geothermal power plant. The geothermal fluid will then be used as the heat source for a heating system, a greenhouse, and a fish farm. This "cascading" use of the geothermal resource is meant to improve the economics of tapping nearly 1,500 potential low- to moderate-temperature well sites in towns and medium-sized cities throughout the West.
The three remaining projects seek to tap unconventional sources of geothermal energy. In one case, ElectraTherm, Inc. will aim to draw power from the hot geothermal fluids that oil and gas wells often generate as a byproduct. The company plans to develop a low-cost, modular, mobile power plant that can be moved from well site to well site, reducing the need to generate on-site power from diesel generators. The other two projects will aim to draw power from geothermal fluids that exist at high pressures under certain geological conditions, particularly along the Gulf Coast. Such "geopressured" fluids often contain high concentrations of dissolved natural gas. To demonstrate viable energy production from such geopressured resources, Louisiana Geothermal will produce power from a resource in Cameron Parish, Louisiana, while NRG Energy Inc. will aim to demonstrate the cost-effective recovery of heat, kinetic energy, and natural gas from geopressured resources.