Rocking on with hot rocks geothermal energy
Key text
This topic is sponsored by the Australian Geothermal Energy Association and the Australian Government Department of Resources, Energy and Tourism
Geothermal energy is a clean, green resource with the potential to power Australia for centuries.
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You will get more from this topic if you have mastered the basics of The structure of the Earth – these links will take you to an annotated list of sites with helpful background information. |
The world is getting hotter. This is because of the increasing concentration of greenhouse gases in the atmosphere, due mainly to our excessive burning of fossil fuels. We burn them for the energy that is needed increasingly in our daily life – to drive to school, cool ourselves on hot summer days, blow-dry our hair and listen to our music. The resulting greenhouse gases trap radiation from the sun, preventing it from escaping back into space, causing the planet’s temperature to rise. But not all of the planet’s heat comes from the Sun; some of it is within the Earth; and rather than causing global warming it could help to wean us off fossil fuels.
This heat, geothermal energy, lies in abundance beneath our feet. If the energy stored in hot rocks inside the Earth could be tapped and used instead of fossil fuels, it could help to reduce the threat of climate change.
There seems to be enough geothermal energy to keep us all ‘rocking’. The Earth’s total heat content has been calculated at 12.6 x 1024 megajoules, which would meet the world’s current energy needs for several billion years. Unfortunately, most of this is inaccessible; but tapping just a fraction of it would make a substantial contribution in terms of reducing greenhouse gas emissions. Geothermal enthusiasts talk about an ‘almost limitless’ supply of energy. The challenge is tapping into it in a cost-effective way.
What is geothermal energy?
The deeper you go into the Earth the hotter it gets (Box 1: Layers of the Earth). Part of the heat is left over from the creation of the Earth, which started off as a hot cloud of gas and dust and has been cooling over time. The outer layers of the Earth have cooled most quickly, forming the crust; heat from the core continues to radiate outwards along what is known as a geothermal gradient. Heat flows from hot to cold – the inner core of the Earth is hotter than 5,000°C, while the surface is generally less than 30°C and outer space is close to absolute zero.
Geothermal energy has an even more important source: radioactive decay. Radioactive elements break down into more stable atoms by emitting radiation and nuclear particles. Naturally occurring uranium, thorium and potassium decay over very long periods of time. The radiation they emit as they decay heats the rocks in which they sit, adding to the geothermal resource. This creation of new heat by radioactive decay and the continuous flow of heat towards the Earth’s surface are reasons geothermal energy is considered a renewable resource.
Using geothermal energy
Geothermal resources vary in character and in the ways in which they can be used. Globally the main accessible resources are in hydrothermal and hot rock systems. High temperature geothermal systems can be used for electricity generation, cooler systems can be used for direct-use applications, and shallow ground source heat pumps can be used in a wide variety of geographical locations.
Hydrothermal systems contain naturally occurring groundwater that has been heated by either heat-producing rocks or by magma that has intruded into the Earth’s crust.
In Australia most hydrothermal systems are hot sedimentary aquifers which have underground water in porous or fractured rocks heated from below by hot radiogenic granite and capped on top by impermeable rocks. Heat is extracted by bringing the water or steam to the surface via holes drilled through the cap rock. The only operating geothermal power station in Australia, at Birdsville in Queensland, is an example of a hot sedimentary aquifer system.
(Image: Ergon Energy)
Volcanic or magma based hydrothermal systems occur most commonly at the boundaries of tectonic plates. Countries on the Pacific Ring of Fire, including Indonesia, New Zealand, Japan and the western United States, have substantial geothermal resources of this type. The only known significant, recent volcanism in Australia occurred over 4,500 years ago in south-eastern Australia, from near Mount Gambier in South Australia to the east of Melbourne in Victoria.
A video describing how energy is extracted from hot rock systems.
(Geodynamics limited, Australia)
Hot rock systems such as the one being developed in the Cooper Basin in South Australia, do not contain naturally circulating water. In Australia they generally consist of rocks with higher than normal amounts of heat-generating radioactive elements, insulated by a ‘blanket’ of sedimentary rock. To transfer the heat to the surface, water needs to be sent down to the hot rocks via drill holes. The hot rocks, often granites, also need to be fractured to create a reservoir in which the water can be heated. This can be done by pumping water into the hot rocks under very high pressure using a technique called hydrofracturing; it creates a network of tiny cracks that provides sufficient space for a water-heating reservoir. The heated water is then returned to the surface for use via one or more production or extraction wells. Such systems are sometimes called engineered or enhanced geothermal systems (EGS), hot dry rock or hot fractured rock.
(Image courtesy of Geodynamics Limited)
Australia’s resources
Hot rocks are the most abundant geothermal resource in Australia and hold promise as a major contributor to Australia’s future energy supplies. But Australia’s geothermal resources are only now starting to be understood. Data on rock temperatures down to a depth of about five kilometres are available from nearly 6,000 bore holes drilled during exploration for petroleum and minerals. These bore holes are distributed unevenly across the continent and many are shallower than five kilometres, so the geothermal resource is better known in some areas than others. Data on heat flow including thermal gradient and thermal conductivity, which are better suited to assessing the suitability of a resource for energy production, are much rarer. Their collection involves both field survey and the testing, in laboratories, of the thermal properties of rock samples.
Despite the need for more data, it is clear that Australia has massive geothermal resources. This is partly because of the presence of large quantities of heat-producing granites in the upper crust enriched by higher than normal concentrations of radioactive elements. The Cooper and Eromanga basins in South Australia and Queensland contain geothermal resources that reach temperatures of up to 250°C at 4.5 kilometres below the surface, putting them among the world’s hottest rocks at this depth (excluding volcanic systems).
The geological characteristics of many of the Australian hot rock sites, such as horizontal layers of granite, should make it relatively easy to create reservoirs of an optimal shape and connectivity for heat extraction. According to some estimates, Australia has enough geothermal energy within reach to supply the country’s energy needs (at current rates) for 20,000 years or more. This figure ignores the fact that geothermal energy is renewable – it is not a resource that will be depleted, unlike fossil fuels.
Direct uses
Geothermal resources can be tapped directly for their energy. The heat can be used, for example, to dry harvested crops and food. In Iceland, geothermal energy is used to heat greenhouses that produce year-round fruit and vegetables in an otherwise inhospitable environment. Geothermal heat can be used in a similar way to warm swimming pools, spas and buildings. Geothermal heat pumps are already being used to heat buildings around Australia including the Geoscience Australia building in Canberra and Antarctic Tasmania in Hobart. Given the prolonged droughts occurring across parts of Australia, the use of geothermal heat to desalinate seawater could also become important.
Indirect uses – electricity generation
In Australia, however, perhaps the most exciting potential application of geothermal energy is its indirect use to generate electricity. Most electricity today is generated using heat. In a coal-fired power station, for example, coal is burnt to heat water, which turns to steam, which is used to spin turbines, which generates electricity (Box 2: Electricity generation).
Geothermal heat can be converted into electrical energy in much the same way using dry steam, flash, or binary generation systems (Box 3: Geothermal electricity generation systems). Some countries are already producing electricity from their geothermal resources: the United States, for example, is generating about 16,000 gigawatt-hours per year, equivalent to around 10 million barrels of oil. Electricity was first generated in this way at Lardarello, Italy, in 1903.
Benefits of geothermal energy
Abundant and renewable: Geothermal energy has a great deal of appeal as an energy source for Australia’s future. It is abundant and it is renewable. While the heat of a hot rock reservoir tapped for its energy can be depleted, it will eventually be replaced. If managed carefully, by controlling the flow rate and/or rotating between a number of reservoirs, geothermal resources can be used sustainably.
Environmentally friendly: In almost every aspect of its development, geothermal energy is environmentally benign. Relative to its capacity for electricity generation and availability, it has a small footprint. Once the plant is established, geothermal energy production generates few greenhouse gas emissions or other forms of pollution. Its widespread use could make a substantial contribution to efforts to combat climate change.
Baseload and price: A drawback of many renewable energy options, such as solar and wind energy, is that, without storage systems, they are only available when the sun shines or the wind blows. Geothermal energy, in contrast, is available 24 hours a day and can therefore provide baseload power. With the introduction of carbon emission reduction schemes, geothermal’s low emission status should also make it price competitive compared to major greenhouse gas polluters such as coal-fired power stations. There are no on-going fuel costs, and it is a renewable energy source.
Issues
Given its benefits, why isn’t geothermal energy already being used commercially in Australia?
Technical challenges: One reason is that the techniques for tapping hot rock resources are only now being developed. Work at the Cooper Basin Innamincka site has shown encouraging results, but how the properties of hot rock reservoirs might change over time is unknown; such changes might also differ between locations. The combination of the high pressure and temperature of Australian hot rocks might also cause problems with drilling and well design.
Start-up costs: Given the lack of existing technologies and the costs of drilling, the start-up costs of hot rock geothermal operations are very high. To some extent these are offset by assistance from federal and state governments in Australia, but geothermal ventures still require investors willing to risk their money in technology that is developing.
Infrastructure: As for renewables including wind and solar, distance from population centres can be a problem. Some of the best-known geothermal resources are a long way from cities or even decent roads. Apart from the problems this remoteness creates for personnel, it also means that very long power transmission lines might need to be built, which would decrease efficiencies and increase costs. However, rather than erect long powerlines, some industries (for example, large computer data centres) may locate adjacent to large geothermal resources. Also, having the generation centre some distance from populated areas should reduce concerns over competing land use or other community objections to development.
In many of the geothermal resource-rich areas of Australia, the availability of water for hot rock operations might also be a considerable challenge. Once there, the water will be recycled through the system and losses are expected to be minimal.
Contaminants: In some systems, the hot geothermal fluids contain dissolved minerals and gases. Some of these might have commercial value, but there might also be a risk of groundwater contamination and the release of greenhouse gases to the atmosphere. Practises used in the petroleum extraction industry to minimise potential contamination will be adapted and adopted by geothermal operators. The very small amounts of radioactive substances that occur naturally in rocks are believed to be too low to be of concern. For example the level of uranium in high heat-producing granite is 100 to 1000 times lower than in a naturally occurring uranium ore body and about the same as Australian coal.
Careful management of the fluids and gases from a geothermal system is required. The use of closed systems like the one being developed at Innamincka overcomes many of these issues.
Discusses the magnitude and management of seismic activity triggered by the creation of geothermal reservoirs.
(Geoscience Australia)
Seismic effects: The hydrofracturing process employed in the creation of hot rock reservoirs can induce seismic activity – or mini earthquakes. Hydrofracturing experiments in the Cooper Basin, for example, have induced more than 27,000 small earthquakes, although few could be felt at the surface and none were sufficiently strong to cause any damage to nearby infrastructure. Geologists are learning more about the potential risks associated with hydrofracturing and developing strategies to minimise them.
The future
There are good reasons to be excited by the prospects for geothermal energy in Australia. Although current usage is miniscule, at least 48 companies are working on geothermal exploration and several expect to have working energy production operations within five years. Should the technology work, and prove cost-effective, geothermal energy could be set for a rapid expansion. If a sizeable industry develops it will help Australia to keep rocking on.
Boxes
1. Layers of the Earth
2. Electricity generation
3. Geothermal electricity generation systems
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Posted July 2009, edited August 2012.