Volcanic eruptions provide a vivid illustration of the vast amount of thermal energy stored within the earth, while geysers, hot springs, and related geothermal features demonstrate that this energy can be brought to the surface in a more benign manner over extended time periods. These latter phenomena have, in fact, been utilized as sources of heat since ancient ones. During the second half of this century, the use of natural geothermal fluids to generate electricity has rapidly expanded. Today, in excess of 5,000 megawatts of electric power are produced from geothermal energy sources around the world. The vast majority of geothermal energy is found, not in the form of hot fluids, but rather as hot dry rock (HDR) which exists almost everywhere beneath the surface of the earth. The object of this paper is to review and summarize the current state of development of HDR technology in the United States and around the world, including preliminary results of a long-term test now underway at the HDR heat mine in Fenton Hill, NM.
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