Geothermal energy from hot dry rock: A renew able energy technology moving towards practical implementation

Abstract

The technology to extract useful amounts of energy from the large, ubiquitous, hot dry rock (HDR) resource has been under development for more than twenty years. Initial work during the 1970's and 1980's showed that it is possible to access and extract HDR energy using techniques originally conceived and tested by the Los Alamos National Laboratory. The process entails drilling a well deep enough to reach hot rock, injecting water at high enough pressure to open natural joints in the rock, and returning the water, heated by the rock, to the surface through one or more additional wellbores. After extraction of its thermal energy, the water is recirculated through the hot rock to mine more heat. In this closed-loop process, nothing is released to the environment except heat, and no long-term wastes accumulate. The practical potential of HDR technology was demonstrated in a series of flow tests conducted from 1992 to 1995 at the HDR pilot facility at Fenton Hill, NM. These tests showed that energy can be extracted from HDR in significant amounts over extended time periods. Extensive analyses and reservoir characterization studies carried out as part of the testing program provided a large amount of information about the dynamics of the heat extraction process and confirmed that HDR energy production facilities can be operated with minimal environmental effects. Operating strategies evaluated during the test period included the production of energy at steady-state levels for extended periods and on a variable-output schedule particularly suited to load-following electricity generation.