Hot dry rock geothermal power is a specific type of geothermal power1. Geothermal power generation depends on capturing the heat produced naturally by the earth and transforming it into a more useful form of energy2. In the case of hot rock geothermal power generation, that more useful form of energy is steam. To approach generating the steam that is needed, boreholes are drilled into the earth to gain access to hot rocks below. Water is then fed into these boreholes and comes into direct contact with the hot rocks. Contact between water and the hot rocks produces high pressure steam. This steam is returned to the surface to be utilized in a steam turbine as one of the cleanest forms of power production known to date3.
This form of power generation has the potential to be very beneficial for several different reasons. The miniscule quantity of adverse impacts it has on the environment is one benefit that cannot be over emphasized. When a hot dry rock geothermal power plant is operating at a steady state there are virtually no emissions. No adverse elements are returned to the environment aside from a small amount of waste heat.4 The emissions of dry rock geothermal power are negligible when compared to, for example, the thousands of tons of sulfur dioxide and millions of tons of carbon dioxide released into the environment by coal power plants.5 Another advantage of this form of power production is that it is a very sustainable process. Hot water used in the process of power generation can be re-introduced into the boreholes to produce more steam.6 These qualities allow hot dry rock geothermal power generation to easily adhere to the demands of a world yearning for a greener future. Lastly, there is the matter of location convenience. A hot dry rock geothermal power plant can be located anywhere that it is possible to access hot rock within the earth by drilling. This allows for large freedom of choice with regards to location and may even allow for more power production in areas where it is inconvenient for other methods to be implemented.7 An example schematic of a HDR plant is shown to the right.
Hot dry rock geothermal process
In order to harvest the heat found deep within the Earth's crust, high pressure cold water is pumped down several kilometers (usually between 3 and 7 kilometers) into hot, porous rock. Once enough water has been pumped down to create a significantly large thermal reservoir, steam or hot water returns to the surface and is harnessed either directly or indirectly. Once the steam has entered the power plant, the rest of the power generation cycle is very similar to one that can be found in coal or nuclear power plant: the steam passes through a series of turbines, is condensed back to liquid water, and is pumped back into the cycle (in this case, that means that it is pumped back underground). The turbines spin shafts that are attached to the generators that make the actual electricity that is sent to homes or businesses.
In order to push the water down far enough into the Earth, extremely high pressures are used. These pressures are tyipcally on the order of tens of megapascals, with some sources reporting operating pressures at up to 40 MPa for a drilling depths of about 5 km8. During the drilling of the holes that carry the water, enough holes must be drilled to accomodate the the water flow down and the steam flow up. Typically between 10 and 30 holes are drilled for flow each way.
The technology behind hot dry rock geothermal power was developed at Los Alamos National Lab between 1970 & 1996; this technology resulted in a test plant being built on the Los Alamos Fenton Hill site. This site was used to conduct tests on the theory behind the HDR technology and to specifically examine the stability of these manmade geothermal wells. The Fenton Hill test ended successfully and the technology behind it was incorporated into a larger program encompassing hydrothermal environments. That program is lead by Princeton Economic Research, Inc. (PERI). PERI is working with the US geothermal industry to apply technology developed from the Los Alamos HDR effort to issues facing commercial geothermal production which is currently comprised entirely of natural hydrothermal resources. PERI is also formulating longer-term plans and designing programs that should eventually lead commercial utilization of HDR resources. There is continuing field research in HDR power occurring in both northern France and on the island of Honshu in Japan. There is also a very large commercial HDR plant currently in construction in Australia. All of these new plants benefit greatly from the advances found by the researchers at Los Alamos and the Fenton Hill site.9 A photograph of the Fenton Hill site is shown to the right.
HDR geothermal systems carry an inherent risk of causing earthquakes. This risk is easily alleviated through proper placement of the plant, away from tectonic activity. The most well known incident of this occurred in 2006 at a new HDR installation in Basel Switzerland. An earthquake rated 3.4 on the Richter scale occurred only eight days after they began injecting water10. Although Basel is well known for its seismic activity, home to what is generally thought to be the most significant seismic event in central European history, the great Basel earthquake of 1356, it is clear that the HDR plant had a hand in this quake as its epicenter was found to be exactly at the bottom of the injection borehole11.
Aside from this minor issue, HDR plants are a safe way to generate large amounts of green power in almost any location. Once the reservoir is filled, a well designed HDR plant will output fifty times less CO2, NOX, and sulfur than a traditional fossil fuel burning plant; this coupled with their almost infinite supply of “fuel” (heat from the earth) makes them one of the most environmentally friendly methods of power generation.