Researchers at Australian National University (ANU) and the publicly traded Australian company, Geodynamics Limited are exploring the viability of hot-dry rock (HDR) geothermal power generation.
HDR geothermal power utilizes the hot temperatures (up to 570 C) of underground granite rock layers that are mildly radioactive (the heat source) and are trapped beneath insulating layers of low thermal conductivity sediments. As ANU's hot rock site explains:
Heat is extracted by pumping water through an engineered heat exchanger connecting two or more wells. This heat exchanger is a volume of hot dry rock with enhanced permeability. It is fabricated by hydraulic stimulation. This involves pumping high pressure water into the pre-existing fracture system that is present in all rocks to varying degrees. The high pressure water opens the stressed natural fractures ... The result is a million-fold permanent increase in permeability along the fracture systems and a heat exchanger that can be used to extract energy.
Water is then injected into a borehole and circulated through the "heat exchanger". The water is heated through contact with the rock and is then returned to the surface through another borehole where it is used to heat another liquid with a lower boiling point within in a closed loop system. This liquid is flashed into steam which is used to spin turbines and generate electricity. The water is then re-injected into the first borehole to be reheated and used again. The HDR plant thus involves two closed-loop systems, the subsurface water loop and the power plant loop that generally contains organic fluids such as refrigerants and iso-pentane.
Australia contains a significant HDR resource. According to Geodynamics, "One cubic kilometre of hot granite at 250 degrees centigrade has the stored energy equivalent of 40 million barrels of oil." The image below shows the estimated temperature at a depth of 5km across Australia. Blue hues indicate relatively low temperatures at this depth while reds represent areas where the temperature is estimated to be particularly high. Notice that the old crust of regions in Western Australia is relatively cool whereas parts of Central Australia are predicted to be >300 C at 5km depth (temperatures suitable for HDR geothermal).
Two HDR pilot sites are being explored in Australia. The first, in an area south of Muswellbrook in the Hunter Valley of New South Wales is, a joint project between ANU and Pacific Power and research at this site began back in 1999. The results of the project stimulated commercial interest in Australia’s hot dry rock resources to such an extent that a new company, Geodynamics Limited, was successfully floated on the Australian Stock Exchange in 2002. This company has now acquired the Hunter Valley geothermal tenement from Pacific Power in addition to an adjoining site and two sites in the Cooper Basin in north-east South Australia. The Cooper Basin site is the second pilot site. Australian National University (and its Department of Geology) has been a major shareholder in Geodynamics since its first seed capital raising in early 2001 and is still involved in research into HDR's potential.
Geodynamics estimates that the total potential thermal HDR resource at their Cooper Basin tenaments "are equivalent to 50 billion barrels of oil. This consists of a 1 kilometre thick slab of granite covering 1000 square kilometres, all above 5km depth, and with an average temperature of 270 C." For comparison, Australia's current total oil reserves are 2.9 billion barrels, and the US oil reserves are 20 billion barrels.
However, Geodymanics recognizes that:
several challenges have to be overcome to realise this potential. First of all, the development of an underground heat exchanger poses several local challenges. It has to be established whether horizontal underground heat exchangers will develop as predicted by the known tectonic stress conditions. Once a horizontal heat exchanger has been developed successfully, the flow rate through the underground heat exchangers also has to meet certain criteria. The energy required to pump the water through the underground heat exchangers also needs to be low enough for the project to be sustainable.
Geodymanics is making significant progress though. They report that Stage One of the their Business Plan is now in full progress and that the Habanero #1 well in the Cooper Basin was spudded on 15 February 2003. When it is completed, it will be the deepest well onshore Australia. Geodynamics has raised a further AUD$4 million in May 2003, the purpose of which was to enable the deepening of the Habanero 1 and 2 wells from 4,400m to 4,900m. Gross funds for the Stage One program are therefore now AUD$20.5 million.
Additionally, Geodymanics has developed relationships with HDR experts in other countries worldwide and they estimate that these relationships have brought the company access to the equivalent of US$500 million of HDR R&D results generated internationally.
I couldn't pass up blogging about this development out of ANU where I studied last year. I had heard about HDR's potential before but didn't realize that ANU had its hands in it. As THE major Australian research university, I'm not surprised.
HDR geothermal seems to have some serious potential, at least in Australia. It would be interesting to see what kind of potential their is for this technologies development in the US. North America is a much younger continent, geologically speaking than Australia and so may lack the necessary radiogenic granite deposits. This is still a technology in development but it seems that the technical kinks should be worked out in time. They technologies it uses are largely well developed technologies already used in the oil and gas exploration and traditional geothermal fields. Regardless, this is a power generation technology worth watching.
[A hat tip to Treehugger]