Appendix C: CO2 as a working fluid for enhanced geothermal systems (EGS)


Enhanced geothermal systems (EGS) formerly known as hot fractured rocks (HFR) or hot dry rocks (HDR) are a new type of geothermal technology whereby underground reservoirs which are not naturally suitable for geothermal energy extraction 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 (Gurgenci, 2008).

In standard EGS, water or brine is circulated in a continuous loop through the reservoir, located three kilometres or more below the Earth’s surface where heat is generated by special high heat producing granites. The circulating fluid extracts heat from the granite raises it to the surface where it is transferred to a secondary fluid (typically isopentane) through to a turbine generator to generate electricity.

A new approach to this concept is currently being pursued whereby supercritical CO2 is circulated as the heat exchange fluid (or working fluid) instead of water or brine to recover the geothermal heat from the reservoir and either (a) transfer heat to a power cycle fluid or (b) generate power directly through a supercritical CO2 turbine before being sent back to the reservoir. Supercritical CO2 holds certain thermodynamic advantages over water in EGS applications and would achieve geologic storage of CO2 as an ancillary benefit. This new concept is expected to significantly increase the cycle efficiency and have a favourable effect on the financial viability of an EGS project (Gurgenci, 2008).

The process will leave significant volumes of CO2 sequestered underground, (geological storage). However, long term permanence (leakage) and MMV will be key issues.

Technology status

Commercial production of geothermal energy is currently limited to hydrothermal systems. EGS for power generation is still relatively novel technology and is not yet developed at a large scale. Attempts to develop the technology have all employed water as the heat transfer medium (considered as conventional EGS). Two systems are in operation in France and Germany generating 1.5 MW and 3 MW respectively. There are a number of conventional EGS projects being developed and tested in Europe, United States, Australia and Japan. Currently the largest project in the world is a proposed 25 MW demonstration plant in the Cooper Basin in Australia.

Utilisation of supercritical CO2 as the heat transfer medium in EGS is not yet a proven technology and is currently in the early stages of research and development. Testing the use of supercritical CO2 as the working fluid in geothermal systems is projected to commence in 2013.

Research status

The fundamental CO2 science and the deep crustal environment are not yet understood. A number of research projects to develop the use of CO2 as an EGS working fluid are underway. Two projects funded by the US Department of Energy (DOE) include:

  • Symmyx Technologies, California – currently studying the chemical interactions between geothermal rocks, supercritical carbon dioxide and water.
  • Argonne National Laboratory – studying the structural changes resulting from chemical interactions of supercritical CO2 and water binary fluids with rocks under environments directly relevant to EGS.

In 2008 The Queensland Government awarded the Centre for Geothermal Energy Excellence at the University of Queensland AU$15 million for EGS research (over five years), a large portion of which will be used to develop CO2 EGS technologies.

Project development

Currently there are two developers seeking financing for field demonstration of supercritical CO2 based EGS:

  1. GreenFire Energy and Enhanced Oil Resources Joint Venture plan to build a 2MW CO2 based EGS demonstration plant near the Arizona-New Mexico border. The drilling of wells to access hot rock is proposed to commence in 2010. The proposed location is projected to yield enough heat to generate 800 MW of power with potential to absorb much of the CO2 generated by six large coal-fired plants in the region.
  2. Geodynamics Ltd is one of about 16 companies active in geothermal power generation in Australia (and are the most advanced). Geodynamics Limited Innamincka ‘Deeps’ Joint Venture with Origin Energy are constructing a 1 MW EGS power plant at Habanero. Electricity generation is expected to occur by early 2012 following the successful completion of Habanero 4 and Habanero 5 (reservoirs), which will be the first Enhanced Geothermal Systems in Australia. Testing of use of supercritical CO2 as the working fluid in the EGS is projected to commence in 2013.In November 2009, Geodynamics was successful in securing AU$90 million in funding under the Federal Government’s Renewable Energy Demonstration Program to facilitate the delivery of the 25MW commercial-size demonstration plant. Geodynamics is due to make final investment decision on proposed $300 million, 25MW geothermal demonstration plant in the Cooper Basin by early 2013, after 12 months of successful operation of the Habanero closed loop. (This is two years later than previously stated). Geodynamics is targeting production of more than 500 MW by 2018, with capacity extending to 10,000 MW – The equivalent of 10 to 15 coal-fired power stations.

CO2 utilisation

Based on long term reservoir pressurisation/fluid loss studies, fluid losses during circulation may amount to approximately 5 per cent of injection (Duchane, 1993). These figures suggest that there is potential capability to continuously sequester CO2 by diffusion into the rock mass surrounding the reservoir.

Studies have indicated potential for geological storage of 24 tonne per day of CO2 per MWe of EGS (1tonne/s of CO2 per 1000MWe of EGS). This is equivalent to achieving geologic storage of the CO2 emitted from 3,000MWe of coal-fired power generation.

Although the above estimate is reported as being very rough, it suggests a very large potential for CO2 reuse and storage using EGS. Geodynamics target production of more than 500 MW by 2018 would potentially sequester 4.4Mt/y.


The U.S. Department of Energy recently awarded US$338 million in federal stimulus funds for research in geothermal energy.

Potential markets

EGS/HDR technologies using supercritical CO2 are expected to be a cost effective way to use CO2 from existing coal-fired power stations to generate new base load power, 24 hours per day.

Size of market

Australia is estimated to have 22000 EJ or 5000 times it annual energy consumption stored in EGS resources (K L Burns, 2000).

According to an estimate by Electricity Suppliers Association of Australia, EGS may provide up to 5 GW or 10 per cent of present Australian electricity generation 2030.

According to an MIT report the estimated US EGS resource base is more than 13 million EJ with an estimated extractable portion of over 200,000 EJ.

There is no detailed information available on the EGS potential of Europe or in developing countries.

Market drivers

The long term forecast price of EGS electricity would make it competitive in a most carbon constrained electricity markets around the world.

Level of investment required (to advance the technology)

A report by the Massachusetts Institute of technology states that with a modest R&D investment of $1 billion over 15 years (or the cost of one coal power plant), it is estimated that 100 GWe or more could be installed by 2050 in the United States (Kubik (ed.) et al 2006)

Potential for revenue generation

The revenue generation of EGS using CO2 as a transmission fluid will be dependent on a number of factors and will largely be affected by the individual locations and quality of the individual sites. The main drivers affecting the profit potential include:

  • the geothermal potential of the site (e.g. how much heat can be extracted through EGS);
  • the prevailing price and demand of other sources of energy (e.g. natural gas and crude oil);
  • the locality of a suitable CO2 source (e.g. co-location of a CO2 source will reduce costs associated with CO2 capture, transport and storage); and
  • whether a carbon trading scheme is in place.

Price sensitivity

The price of the technology will be affected by a number of factors including:

  • the prevailing price and demand of other sources of energy (e.g. natural gas and crude oil);
  • the forecast future energy demand; and
  • carbon price (if applicable in location).

Commercial benefit

The main commercial benefit of the technology is the potential to tap into the energy market to meet the high forecast growth in energy demands. This is further emphasised by the pledges and targets made by over 60 of the world’s major governments to increase the use of energy from renewable sources. The particular use of CO2 rather than water in this technology also has a number of commercial benefits such as the advantages of CO2 as a working fluid over water and the availability of CO2 sources globally.


There are a number of benefits associated with the use of supercritical CO2 instead of water for EGS. These include:

  • CO2 storage-potential to sequester 1 tonne per second of CO2 for each GW of electricity generated (site specific);
  • minimised water usage;
  • favourable thermodynamic properties resulting in much larger flow rates, reduction in circulating pumping power requirements, increased efficiency and greater power output;
  • minimised losses during heat transfer due to (potential) elimination of binary cooling;
  • reduction or elimination of scaling problems (such as silica dissolution and precipitation in water based systems);
  • HDR reservoirs with temperatures > 375°C (the critical temperature for water) could be developed without problems associated with silica dissolution; and
  • carbon credits gained from sequestering the CO2 would offset some of the costs of drilling deep EGS wells.

The series of potential advantages that supercritical CO2 offers may help to expedite commercial exploitation of some geothermal resources.


Enhanced geothermal systems for power generation are still a relatively novel technology and being proven. There are a number of significant issues that need to be resolved for successful development of the technology. These include:

  • the geochemistry of supercritical CO2;
  • dealing with reservoir water;
  • long term effects in terms of reservoir connectivity;
  • the source of CO2;
  • the long term retention of CO2, including seismic triggers and events resulting in CO2 leakage to the surfaces;
  • the lifetime of HDR geothermal systems may be difficult to prove; and
  • the design and optimisation of turbines and air-cooled heat exchanger systems to operate with supercritical CO2.

Potential barriers to implementation include

  • proximity of the CO2 source and access to at an acceptable cost;
  • proximity of the EGS to the electricity grid;
  • long-term responsibility for the resultant reservoir, including the liability for future CO2 leakage; and
  • the Geothermal industry has expressed concern regarding:
    • the high cost of CCS which may threaten the long-term viability of the use of CO2 for EGS.
    • the future availability of CO2 if CCS is only a transitionary technology.