Summary
The recent New York Times article highlights the massive potential of Hot Dry Rock Geothermal energy (or Engineered Geothermal Systems -EGS), and raises public awareness of this unique and virtually untapped source of energy. Unfortunately, the article also may trigger undue fear and anxiety by focusing on one particularly flawed example and failing to explore the broader range of technical options for accessing this abundant and renewable energy resource. The public deserves a fair and balanced discussion of the technology and all of its implications.
Analysis
Hot Dry Rock (HDR) geothermal energy offers considerable advantages compared to other renewables such as Solar or Wind - including the potential for 95% availability and the ability to load-follow and peak.
HDR energy is harvested from the pressurized circulation of water through man-made (“engineered”) reservoirs created in hot crystalline basement rock which has been heated by conduction from the Earth’s mantle and by the decay of radioactive elements in the crust. A recent MIT study estimates the extractable portion of HDR energy to “…exceed 200,000 exajoules or 2,000 times the annual consumption of primary energy in the United States in 2005” - orders of magnitude greater than all of the US fossil fuel sources combined. (Ref. The Future of Geothermal Energy)
Hot Dry Rock reservoirs must be created in geologic regions that are free from known fault zones where the presence of open joints would frustrate the achievement of a "confined" reservoir system.
HDR Energy, LLC is working with the original research team from the HDR test site at Fenton Hill, NM who, for almost 40 years have been involved in studying the production of energy from rock deep in the earth that is hot but essentially dry. Rather than drilling into open zones of known faulting, our approach entails drilling into hard basement rock that is free of open fractures, and then creating an array of interconnected joints -- the earth heat exchanger -- by using hydraulic pressure. The very small movements of the rock as the joints are opened, creates numerous small tremors detectable by sensitive seismic devices. These small tremors define how the HDR reservoir is developing as pressurization continues.
At Fenton Hill, thousands of microearthquakes were detected as the rock was fractured, but none that were felt at the surface. These microseismic events indicated only minor shifting of the rock as existing joints were forced open only a small amount by the injected water, representing less than 0.1% of the rock volume.
Fenton Hill was the first and only HDR site in the world to successfully engineer a “contained” reservoir and achieve “closed-loop” circulation. While the thermal power produced was modest, it demonstrated that a geologically stable “engineered” reservoir/circulation system could serve as a sustainable base-load power generation resource.
It is also worth noting that the open-loop (water to steam) process described in the subject article, is not the only, or even the preferred way to extract energy from Hot Dry Rock. HDR Energy, LLC is focused on a binary plant approach that employs two independent re-circulating loops with no environmental releases.
Hot Dry Rock (HDR) geothermal energy represents over 99% of the earth's accessible geothermal resource. The Fenton Hill work demonstrated that geothermal energy can be extracted from HDR, and that it can be done with scientific and engineering diligence in a manner that avoids the unintended consequences experienced in Basel, or feared with the Altarock project in California.
While we are grateful to Mr. Glanz for creating a public disourse on this transformational energy resource, its unfortunate that his article failed to address the many positive attributes and potential of HDR energy, and instead focused disproportionately on what we believe is a manageable risk.
