Electricity Generation from an Engineered Geothermal System using a Supercritical CO2 Thermosiphon
Professor Hal Gurgenci
Interim Director, Queensland Geothermal Energy Centre of Excellence
Tel: +61 7 336 35607
Hot Dry Rocks or Enhanced/Engineered Geothermal Systems (EGS) are defined as underground reservoirs not naturally suitable for geothermal energy extraction but can be made so through economically viable engineering procedures. The requirement for significant engineering work prior to heat extraction distinguishes EGS from conventional geothermal applications. Australia is estimated to have 22000 EJ or 5000 times its annual energy consumption stored in EGS resources
[1]. The State Government of Queensland has recently awarded $15m to the University of Queensland to carry out R&D towards electricity generation from such resources.
The EGS is already being targeted by a sizeable industry in Australia and, according to an estimate by Electricity Suppliers Association of Australia, may provide up to 5 GW or 10% of present Australian electricity generation 2030
[2].
In the standard EGS concept, a primary fluid, typically brine, is circulated through the geothermal reservoir in a continuous loop. The brine brings the heat to the surface, and transfers it to a secondary fluid, typically isopentane. This secondary fluid is then expanded through a turbine-generator to generate electricity.
The Queensland Geothermal Energy Centre of Excellence is pursuing a significant modification of this concept. In this modified approach, CO2 instead of brine is circulated through the subsurface reservoir at supercritical pressures and then expanded at the surface through a turbo-generator before being sent back to the reservoir.
The new approach is expected to significantly increase the cycle efficiency with an obvious favourable effect on the financial viability of an EGS project. On the other hand, a significant requirement is the availability of a steady stream of CO2 to make up for the losses through entrapment in the EGS reservoir and surrounding subterranean structures.
The presentation will address the technology and the infrastructure and resource implications associated with this proposal.
[1] K L Burns, C Weber, J Perry and H J Harrington (2000). “Status of the Geothermal Industry in Australia”. Proc. World Geothermal Congress 2000, Kyushu-Tohoku, Japan.
[2] The Australian Geothermal Energy Group's Submission To The House Of Representatives' Inquiry Into The Development Of The Non-Fossil Fuel Energy In Australia. http://www.aph.gov.au/house/committee/isr/renewables/submissions/sub136.pdf