Future Developments

Renewable energy technology is continuously evolving with the goal of reducing risk and lowering cost. The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy life-cycle cost of electricity of $0.03 per KWh.⁵⁸ To achieve the goal of lowering cost and risk, other types of nontraditional resources and experimental systems are being explored. Among these are hot dry rock resources, improved heat exchangers, and improved condenser efficiency.

Hot Dry Rock
Hot dry rock geothermal technology offers enormous potential for electricity production. These resources are much deeper than hydrothermal resources. Hot dry rock energy comes from relatively water-free hot rock found at a depth of about 4,000 meters or more beneath the Earth’s surface. One way to extract the energy is by circulating water through man-made fractures in the hot rock. Heat can then be extracted from the water at the surface for power generation, and the cooled water can then be recycled through the fractures to pick up more heat, creating a closed-looped system. Hot Dry Rock resources have yet to be commercially developed. One reason for this is that well costs increase exponentially with depth, and since Hot Dry Rock resources are much deeper than hydrothermal resources, they are much more expensive to develop. Figure 7 shows the projected capital cost for hot dry rock compared to traditional geothermal power technology from 1996 to 2030. The figure shows that the capital cost of hot dry rock will decrease by almost half in 30 years, but it will still be twice as expensive as other traditional geothermal technologies. If the technology can evolve to make hot dry rock resources commercially viable, hot dry rock resources are sufficiently large enough to supply a significant fraction of U.S. electric power needs for centuries.

Figure 7. Projected Capital Costs for Hot Dry Rock Compared to Traditional Geothermal
Power Technology, 1996–2030

Heat Exchanger Liners
The highly corrosive nature of geothermal plants poses a challenge to heat exchangers by reducing their thermal conductivity. Research is currently being conducted to replace the use of expensive heat exchanger materials, such as stainless steel and titanium, with new, less expensive polymer-base coated carbon steel. The polymer-base-coated carbon steel is proving to be as resistive to corrosion as the conventional, expensive materials.⁵⁹

Air-Cooled Condensers
Currently, the National Renewable Energy Laboratory (NREL) is investigating ways to improve the efficiency of air-cooled condensers that are commonly used in binary-cycle geothermal plants. Air-cooled condensers use large airflow rates to lower the temperature of the gas once it has passed through the system to produce condensation. The fluid is then collected and returned to the cycle to be vaporized. This cycle is important in binary-cycle geothermal plants because of the lack of make-up water. To increase the heat exchange efficiency, NREL is currently testing the use of perforated fins in the condensers, with all of the air flowing through the perforations, to increase the heat exchange and therefore, condensation. Initial tests have indicated a 30–40% increase in heat transfer. Such an increase in heat transfer technology could increase the efficiency of future binary-cycle geothermal plants.

As technological improvements continue to be discovered and more geothermal plants are brought online, geothermal generating capacity in the United States will continue to increase. Figure 8 shows projected geothermal power generation under these scenarios and projected generation from Annual Energy Outlook 2003. 60 Installed capacity is likely to increase via new installation, as well as technological improvement leading to increased yield. The U.S. DOE projects that U.S. geothermal generation will increase by over 160% from 2000 to 2025, from 14.1 to 36.9 billion kilowatt-hours per year.

Figure 8. DOE Annual Energy Outlook Projected Geothermal Generation 2000–2025
(Billion KWh) Projected Capital Costs for Hot Dry Rock Compared to Traditional
Geothermal Power Technology, 1996–2030

Source: EIA. Annual Energy Outlook 2003.

Endnotes
58. U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Geothermal Energy Program. http://www.eren.doe.gov/geothermal/geoelectprod.html, accessed Nov 5, 2002.
59. National Renewable Energy Laboratory. Center for Buildings and Thermal Systems. Geothermal Research and Development. http://www.nrel.gov/geothermal/georandd.html.
60. Department of Energy, Energy Information Administration, Annual Energy Outlook. http://www.eia.doe.gov/analysis/2002anal02.html, accessed Dec 8, 2003.