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Explainer: what is geothermal energy?

Geothermal means, literally, “earth heat”. The temperature of the earth increases as we drill deeper towards its core. We can use that heat for energy by circulating water through hot subterranean reservoirs…

With improvements in enhanced geothermal systems technology the earth’s heat could become a major electricity generator. Flickr/xavierbt

Geothermal means, literally, “earth heat”. The temperature of the earth increases as we drill deeper towards its core. We can use that heat for energy by circulating water through hot subterranean reservoirs, bringing the hot water or steam to the surface. We can then convert the energy in the hot fluid to mechanical and electrical power at the surface using a heat engine.

The private investment in Australian geothermal power development slowed down after a very fast start ten years ago. This should pick up again as new developments bring commercial viability to enhanced geothermal systems. These have the potential to grow exponentially, promising abundant power for the world.

Conventional geothermal systems

In 2010, the world generated 20 terawatt hours (TWh) of geothermal electricity. All of this came from conventional resources. These are naturally occurring hot water or steam flows heated by magma and circulating through permeable rock. They are associated with volcanic systems and limited to regions with active or young volcanoes.

The use of conventional resources is not new. In 1904, a plant was built in Larderello, Italy, to generate electricity from geothermal steam. Since then, geothermal power installations have spread at a steady rate but have been limited to resources that are relatively easy to access.

Enhanced geothermal systems

There is another form of geothermal heat that is abundant and generally available around the globe. It is produced by the radioactive decay of potassium, uranium, and thorium isotopes found in some granite types.

In granites in the Cooper Basin, in South Australia, there are isotope concentrations at trace level. Heat is generated at a very low rate. At such a low rate, it would take almost one million years for a mass of granite to increase its temperature by 100°C. Through that time the rock has to remain well-insulated at great depth.

To access this deep heat, at least two wells must be drilled. Cold water is injected from one well, it is heated by the rock and is extracted from the other well. The two wells are typically 800 to 1000m apart. In such a system, the water will travel from one well to the other only if the rock between the wells is sufficiently permeable.

Natural rock permeability is not high enough but can be enhanced by well-established oil and gas engineering techniques. These systems are called Enhanced Geothermal Systems (EGS).

Commercial EGS development

Electricity from enhanced geothermal systems is not cost-competitive now because deep wells are expensive. To generate enough electricity to pay for the wells and for the surface equipment, the hot water has to be brought to the surface at rates close to 100kg/s. One of the best EGS flow rates in the world was achieved by the Australian company Geodynamics, and it was only a third of this target. Although this is a significant achievement, it is not enough to make EGS electricity commercially competitive.

It’s possible to compensate for these low flow rates if you can improve the efficiency of converting the energy into power. But improvements in this area have yet to see electricity production become cost-competitive.

EGS flow rates must be tripled before EGS electricity is commercially feasible. It was reported last December that a promising technique for geothermal wells might have been developed by the US company AltaRock. If the reported results are substantiated by further testing, it will provide new breath for the EGS geothermal development around the world and in Australia.

Other Australian geothermal resources

The division between conventional and enhanced geothermal systems is artificial and the actual global geothermal resource covers a continuous spectrum extending from naturally occurring hot springs to enhanced geothermal systems.

Recent results show that although Australia is not a volcanic country, it has areas where magma gets close to the surface to produce hot water in sedimentary rock. The Australian geothermal industry calls these hot sedimentary aquifers. They are expected to have natural permeabilities higher than EGS but maybe not as high as conventional geothermal systems.

Around the world, there has not been much work in this type of resource but there are new developments in Australia as well as other places like Turkey and South America directed at such resources.

Environmental impacts

In accessing geothermal energy, the technologies involved can have slight effects on the immediate environment. As seen by the Paralana experience, slight tremors may be felt on the surface but these do not constitute a significant risk. Geothermal energy does not use any more water than other renewable thermal power energy applications. Also the fracture stimulation used in the EGS projects does not pose a risk for surface aquifers because EGS reservoirs are very deep and the wells are sealed in steel casings.

In some volcanically-sourced conventional geothermal resources, the dissolved gases released to the atmosphere may affect the immediate environment. This is probably the most serious hazard associated with geothermal energy but it is limited only to some conventional resources (not relevant to Australia) and is easy to control.

The future for geothermal

The fundamentals for geothermal energy are strong. It is abundant – the resource underneath the Great Artesian basin is estimated to be large enough to deliver the current Australian annual energy consumption for 6000 years. It is one of the few renewable power sources that can completely replace coal as a baseload electricity generator. It also has a very low environmental impact.

The failure to achieve sufficient flow was hindering its commercial development but this problem may be solved in the near future. If that happens, geothermal power could become one of the leading renewable power technologies for the 2020s.

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27 Comments sorted by

  1. George Michaelson


    "Natural rock permeability is not high enough but can be enhanced by well-established oil and gas engineering techniques."

    and... those techniques involve creating fractures in the rock, a process we call ... come on ... there's a word in there ...

    1. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to George Michaelson

      Stimulation or fraccing (if you want to use the F word) of geothermal reservoirs should not be a concern because the geothermal fractures are generated 4-5 kilometers below the surface. This is well below of any surface aquifers and there is no danger of contamination.

      Such depths are necessary because otherwise you will not have a hot rock resource. The heat needs to be kept in to accumulate and depth is a good insulator.

  2. John Newlands

    tree changer

    It's hard to not say either the F word or the N word. The latter is puzzling because deep greens enthuse about dry rock geothermal yet the heat source is radioactive decay. Why not bring the same elements in a sealed box above ground and accelerate the process via controlled fission?

    I note elsewhere some are careful to say renewables AND geothermal. As a zone of hot rock cools a diesel powered drill is needed to drill more holes hopefully not too far from the surface plant. Reminiscent of slash and burn agriculture near a hut. I don't think we should begrudge the public money spent so far on unsuccessful geothermal in nonvolcanic areas since we needed to find out if it would work at scale. Now we know.

    1. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to John Newlands

      The radioactive decay is at a very very low rate (5 microwatts per cubic meter of rock). The only reason the rocks get hot is because they are well-insulated and the heat builds up over millions of years. Such rocks occur naturally on the surface as well but they are not hot because there is no insulation.

      Regarding the comment on public money, it was mostly private money that has been spent in Australia so far. Grants were announced but not all were claimed.

      Whether it works or not? I do not think we know yet but it looks like it will. This space was not sufficient to comment on the recent US experiment. You might want to check the link provided on that topic in the article.

    2. Mark Pollock


      In reply to Hal Gurgenci

      Didn't our government give $150 million or so to one of these failed experiments? Wasn't it the one with waterfront flannery on the board?

      How's it doing now?

      Don't get me wrong. Geothermal is a great idea. I thing there a very effective plant operating in Birdsville or some and the township of 200 or so get cheap power from it. As a replacement for coal base load in Australia it's another green fantasy indUlged in with other people's money.

    3. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to Mark Pollock

      The federal government promised $150m towards two plants in Innamincka (with Geodynamics) and Paralana (with Petratherm). You can read the details in my blog entry at that time:

      Only a small part of those grants have been claimed by the companies because the grants were contingent on new investment (more than $150m) from private equity. This did not happen, partly due to the GFC. So that money is still with the government.

      Most of the money spent on geothermal energy in Australia so far was private money. Geodynamics at some point was capitalised at close to half a billion dollars mark if my memory is correct.

    4. Mark Pollock


      In reply to Hal Gurgenci

      I am very pleased only a small part of our money has been wasted on this nonsense. I hope it stays that way.

      Geodynamics is currently trading at 8.1 cents down from a high of $1.60 and has current capitalisation of about $33 million. It floated at $320 million. A lot of people have done a lot of dough on this flying pig.

  3. John Campbell


    I believe there is another form of geothermal energy we perhaps all should be using.

    It simply amounts to pipes embedded in the ground at a depth greater than 2m(?).

    The idea is that you draw air through these pipes into your house. As the temp. under ground is relatively stable it is a cheap way of heating your house in winter and cooling it in summer.

    Does anyone have experience of such systems?

    1. Felix MacNeill

      Environmental Manager

      In reply to John Campbell

      I believe they're quite common and popular in the UK and parts of Europe, but I've not yet heard of much happening here in Oz...

    2. Dennis Alexander

      logged in via LinkedIn

      In reply to John Campbell

      Hi John,
      Such systems as you describe generally use a saltwater "coolant" and a heat exchanger and are commercially available for households. They are considerably cheaper to install at the construction stage than post-construction. Whether they could be scaled up to provide power is dubious. However, nuclear waste in Synroc form could provide an alternative heat source for dry geothermal applications and it recycles current "hot" waste rather thanmining and manufacturing any new fuel. All it really needs is for a useful sealed heatpump to be developed and a safe location for the facility. Because fission as usually understood is not being forced or controlled, it could be safer than conventional nuclear power stations and would make good use of the extended decay periods of nuclear waste.

    3. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to John Campbell

      Absolutely right. Geothermal Heat Pumps or Ground Source Heat Pumps are growing fast in Europe and North America. I expect them to pick up here too.

      The article focus was on electricity generation only. Therefore, it did not mention geothermal heat pumps.

  4. Zvyozdochka

    logged in via Twitter

    For the size of the resource we've spent next-to-nothing trying to make EGS work.

    Geodynamics have suffered a significant number of setbacks on shoe-string budgets, but this month began commissioning their long promised 1MWe Habanero pilot plant.

    Make no mistake, the fossil-nuke boosters desperately want this company to fail.

  5. Steve Hindle

    logged in via email

    Thanks for this article Hal. I have often wondered why geothermal energy has been so slow to be developed considering how promising it sounds. This article has answered many questions, but here are a few more if you have the time.
    What sort of lifespan would a well have before localised cooling of the rock reduced its efficiency? Could the recent breakthroughs in accurate horizontal drilling have any applications or even be possible at extreme depths? What % of the water pumped down is lost into the permeable rock and never recovered?
    I am starting to think that only nuclear energy will be capable of supplying the vast amounts of reliable CO2 free base load power the world is going to require. Geothermal energy is probably my last hope before I go over to the dark side.

    1. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to Steve Hindle

      Thank you for your encouraging comments Steve. I will briefly try to answer your questions.

      How does a well last? There are many parameters of course but all numerical simulations indicate that
      an EGS well should last 20-25 years.

      Would accurate horizontal drilling help? It absolutely would. It is a lot harder however to do directional drilling in granite. I think what is going to happen first will be multiple horizontal fractured reservoirs (each generated using the present methods…

      Read more
  6. Paul Cm

    logged in via email

    Thanks for the explainer and follow up comments Hal, good luck with the research program.

  7. Leigh Burrell

    Trophy hunter at Trophy hunter

    Explainer: geothermal energy.

    An expensive, unproven con spruiked by stakeholder and Gaia worshipping loony Tim Flannery. Responsible for sending $90 million of our money down the tubes:

    There, fixed that for you.

  8. Andy Saunders


    Is the temperature of the Australian resource enough, and is the loss of heat in production sufficiently small? The main geothermal areas have been historically in magmatic areas where temperatures of 600F+ are available (in practice, whatever temperature the drilling/production system is capable of), at shallow depths of a few hundred meters. I recall insulated tubing being used in those areas, which would minimise heat losses (although I suspect the main reason for the insulation was protection…

    Read more
    1. Hal Gurgenci

      Professor of Mechanical Engineering at University of Queensland

      In reply to Andy Saunders

      There are a couple of questions in the post by Andy Saunders. I will try to answer them separately.

      The conventional geothermal is not necessarily shallow. Many new conventional geothermal developments drilled as deep as 2000 meters. As I said in the article, the gap between the conventional and EGS is closing up.

      Most geothermal wells (and certainly all EGS wells) are cased in steel and steel pipe is cemented outside to provide good sealing with the rock. This provides some insulation…

      Read more
    2. James Hill

      Industrial Designer

      In reply to Hal Gurgenci

      Now if some advantage were taken of the ice that can be generated in the desert locations in winter then the temperature range of the heat engines (Utilising the Carnot cycle of course) can be increased and more power developed, as in all heat engines from that original geothermal heat.
      Those medieval, earth sheltered "Ice Houses" would be easy to build, and as in ancient Persia, the ice lasts through the summer.
      It follows that solar heated, hot rocks could just as easily be stored underground to augment the deep geothermal sources.
      Only very marginally off topic.

  9. Shirley Birney

    logged in via email

    I haven't bothered yet to compare the geological specifics in various regions, however, the US EPA estimates the generation rate of geothermal energy production waste in California (the largest geothermal producer in the US) is 54,000 metric tons/year as of 2012.

    The table below shows the estimated average radioactivity in geothermal wastes, based on data from southern California geothermal power production facilities:

    Radiation Level [pCi/g]:

    Low: 10, average: 132, high: 254.


    Read more
  10. Terry Reynolds

    Financial and political strategist

    Good article and references Hal. I liked you putting forward the facts on Government grants to Geodynamics. So many Australians recklessly comment about Government programs usually from political bias and lack of fact checking, which is bloody irritating and does nothing for our great Country's advancement.

    I hope the valuable work you are doing will soon pay great dividends to the world, perhaps also generating power to convert ever abundant and widely spread sources of salt water into fresh water at nominal cost for mankind.

  11. Doug Hutcheson


    "does not pose a risk for surface aquifers because EGS reservoirs are very deep and the wells are sealed in steel casings" - and the Titanic was unsinkable. Having said that, EGS certainly holds more promise of public acceptance than nuclear, regardless of their relative merits. Certainly, an interesting field to keep one's eye on.