Geothermal

History of Geothermal Power Systems

Geothermal energy has been used by humans for many centuries in applications such as space and water heating, cooking, and medicinal bathing. The first geothermal power generation plant was constructed in 1904 in Larderello, Italy. This had a capacity of 250 kW, and used geothermal steam to generate electricity. The second geothermal power station built was in the 1950’s at Wairakei, New Zealand (see Figure 1). This was followed by the Geysers in California in the 1960’s (Brown, 1996) (see Figure 2).

 

There is currently an estimated 12,000 MW of direct use and over 8,000 MW of generating capacity in geothermal resources worldwide. To put geothermal generation into perspective, this generating capacity is about 0.4% of the World total installed generating capacity. In 2003, there was 8,402 MW of installed geothermal electricity generation capacity worldwide. This total is stabilising after a growth period due to the over exploitation of the Californian fields in the United States, where output is decreasing, balancing with an increase in geothermal capacity from other countries. The US is the largest producer of geothermal electricity, followed by the Philippines, Mexico, Indonesia, Italy, Japan and New Zealand (International Geothermal Association, 1998a) (See Table 1 and Figure 3).

Geothermal resources are also used extensively in non-electrical, direct heat applications, such as space heating and in a range of agricultural and industrial processes. Nearly 10,000 MW of thermal energy is sourced worldwide from geothermal plants in a number of countries, including Japan, China, and Iceland (Wright, 1998).

Geothermal resources are not strictly renewable, in the sense that geothermal activity occurs on a much larger time scale than human lifetimes. However, they are renewable if the rate of extraction of the energy is less than the rate of replenishment of the resource. The natural recharge rate of geothermal reserves varies from a few to over 1,000 MW of thermal energy (Wright, 1998). However, in current practice, all installations are exceeding the recharge rates of the resources. The geothermal resources are therefore being used in a non-sustainable manner, and are effectively being ‘mined’ as fossil fuels are.

Electricity Generation Technologies

High temperature geothermal resources can be used for electricity production. There is currently over 8.4 GW of installed geothermal electricity generation capacity worldwide (see Table 1). There are a number of energy conversion technologies that use the geothermal resource. These include dry steam, flash steam and binary cycle systems.

Geothermal electricity can be used for base load power, as well as for peak load demand as required. In many parts of world, geothermal electricity is competitive with conventional energy sources.

Dry Steam Power Plant

The dry steam power plant (see Figure 4) is suitable where the geothermal steam is not mixed with water. Production wells are drilled down to the aquifer and the superheated, pressurised steam (180°C - 350°C) is brought to the surface at high speeds, and passed through a steam turbine to generate electricity. In simple power plants, the low pressure steam output from the turbine is vented to the atmosphere, but more commonly, the steam is passed through a condenser to convert it to water. This improves the efficiency of the turbine and avoids the environmental problems associated with the direct release of steam into the atmosphere. The wastewater is then reinjected into the field via reinjection wells.

The waste heat is vented through cooling towers, in common with conventional fossil fuel plants. As with conventional power stations, the energy conversion efficiencies are low, around 30% (Brown, 1996). However, as the water is heated by geothermal activity (as opposed to combustion of fuels) this is not as crucial an issue compared to conventional fossil fuel generation plants. The efficiency and economics of dry steam plants are affected by the presence of non-condensable gases such as carbon dioxide and hydrogen sulphide. The pressure of these gases reduces the efficiency of the turbines, and in addition, the removal of the gases on environmental grounds adds to the cost of operation.

Dry steam power plants are the simplest and most economical technology, and therefore are widespread. The technology is well developed and commercially available, with units typically in the 35 MW –120 MW range (World Energy Council 1994). The United States and Italy have the largest dry steam geothermal resources, but these resources are also found in Indonesia, Japan and Mexico. The Geysers field in California is a dry steam field. It is the largest geothermal power source in the world, with an installed capacity of about 1,100 MW.

Flash Steam Power Plant

Single flash steam technology is used (see Figure 5) where the hydrothermal resource is in a liquid form and is the most common type of geothermal power plant. The fluid is sprayed into a flash tank, which is held at a much lower pressure than the fluid, causing it to vapourise (or flash) rapidly to steam. The steam is then passed through a turbine coupled to a generator as for dry steam plants. To prevent the geothermal fluid flashing inside the well, the well is kept under high pressure.

The majority of the geothermal fluid does not flash, and this fluid is reinjected into the reservoir or used in a local direct heat application. Alternatively, if the fluid remaining in the tank has a sufficiently high temperature, it can be passed into a second tank, where a pressure drop induces further flashing to steam. This steam, together with the exhaust from the principal turbine, is used to drive a second turbine, or the second stage of the principal turbine to generate additional electricity. Typically, a 20 - 25% increase in power output is achieved, with a 5% increase in plant costs (Brown, 1996).

Flash steam plant generators range in size from 5 MW to over 100 MW, with a standard size of 20 MW used in some countries, including the Philippines and Mexico (World Energy Council, 1994).

Binary Cycle Power Plant

Binary cycle power plants (see Figure 6) are used where the geothermal resource is insufficiently hot to efficiently produce steam, or where the resource contains too many chemical impurities to allow flashing. In addition, the fluid remaining in the tank of flash steam plants can be utilised in binary cycle plants (e.g. Kawerau, New Zealand).

In the binary cycle process, the geothermal fluid is passed through a heat exchanger. The secondary fluid, which has a lower boiling point than water (e.g. isobutane or pentane), is vapourised and expanded through a turbine to generate electricity. The working fluid is condensed and recycled for another cycle. All of the geothermal fluid is reinjected into the ground in a closed cycle system.

Binary cycle power plants can achieve higher efficiencies than flash steam plants, and they allow the utilisation of lower temperature resources. In addition, corrosion problems are avoided. However, binary cycle plants are more expensive, and large pumps are required which consume a significant percentage of the power output of the plants. The unit sizes are typically in the range of a few hundred kW to 5 MW, used in a modular arrangement.

Hot Dry Rock Technology

Project Feasibility and Assessment Processes

Only limited amounts of geothermal energy are used in Australia

There is a general sequence of exploration methods that will allow comprehensive research and data collection for a particular resource. The primary reason that there is such a recommended sequence is due the extremely high costs of many of the exploration methods. The following is an ideal sequence recommended by Barbier in chronological order;

Inventory and Survey of Surface Manifestations

Information gathering on the surface thermal manifestations, such as springs, vents etc. and their physical and chemical characteristics is of primary importance. This is one of the lowest cost exploratory techniques and is a useful starting point in any hydrothermal exploration program. The first part of the inventory gathering is the collation of existing published and/or available data on the area. The next element is the gathering of new samples and measurements (Barbier, 1997).

Hydrological Surveys

Hydrological surveys are used to determine whether an aquifer is confined or unconfined in its distribution. Correlating the hydrothermal resources with other geological features, such as fractures and faults allows the reconstruction of the flow of groundwater circulation. This step often requires large amounts of computer modeling to define down much of the interaction of complex variables (Barbier, 1997).

Geochemical Surveys

Sampling the hydrological chemical constituents and ratios of certain important chemicals (such as sodium, potassium, magnesium, calcium, silica, hydrogen, oxygen, tritium and radioisotopes etc.) can be used to estimate much information. For example the amount of dissolved silica in the fluid samples can indicate the temperature, as the solubility of many silicates are temperate dependant. These types of surveys can also determine the specifics of whether the resource is dominated by vapour or water, and can provide information about its homogeneity and direction of groundwater flow, as well as any fouling issues that may become a problem in exploiting the resource. These types of surveys can take place simultaneously with hydrological surveys and the collation of inventory information (Barbier, 1997).

Geophysical Surveys

Classical geophysical surveys are generally known as indirect methods of obtaining information about the hydrological resource. This is because classical geophysical techniques, such as seismic, gravity and magnetic surveys, collect data on the nature of the host rocks around and in the hydrological resource, rather than the hydrological resource itself. In addition to these classical geophysical techniques there are other methods that reveal variations of the properties of the host rocks due to the presence of hot and saline fluids. Examples of such techniques include electrical resistivity, electromagnetic, and thermal measurement methods. Thermal measurement methods include drilling small diameter wells to a shallow extent and electrical thermometers are used to determine the thermal gradient as the thermal measurement well drilling rig descends. These geophysical surveys tend to be expensive and are generally the last of the four areas of geological surveys before exploratory wells are drilled. This allows wells that are drilled to be useful in obtaining more data and avoid drilling in marginal areas or dry holes (Barbier, 1997).

Exploratory Wells

The last stage of the assessment process is surveying by drilling exploratory wells. These wells are of a larger diameter than the thermal measurement wells, so as to use logging instruments to take data from top to bottom of the well. The angle of drilling generally follows fluid filled fractures. These wells can verify or contradict previous interpretations of the hydrothermal resource and are crucial to get the best results, as the costs of drilling and casing wells is very expensive and costs are continually rising (Barbier, 1997).

Geothermal Energy in Australia and New Zealand

Only limited amounts of geothermal energy are used in Australia, in stark contrast to New Zealand, which produces 7% of its total energy requirements from geothermal sources. In Australia there is a large hydrothermal potential found in the Great Artesian Basin from central South Australia through most of western Queensland to the Gulf of Carpentaria. However there are a small number of notable hydrothermal projects that have been developed in Australia.

Hydrothermal

Birdsville in Queensland maintains a 1221m deep well with 98°C water, flowing at 27 l/s, which had been producing for 75 years as the towns water source. The geothermal power station was constructed in the early 1990s to produce 3-phase power with a Rankine-cycle engine using Freon, but has had design problems with corresponding lower than expected performance. Recently the site has been upgraded with a grant of $95,300 through the Queensland Sustainable Energy Innovation Fund. The upgrade allowed an expansion of its capacity to 80kW of net power, and to convert from Freon to a new closed-loop working fluid, isopentane, that has no ozone depleting potential. The geothermal power station reduces diesel consumption by around 160,000 litres a year, mitigating the emission of 430 tonnes of carbon dioxide, and with its virtually noise free operation it provides the entire towns electricity supply at night and in winter (Burns, K.L., Weber, C., Perry, J. and H. J Harrington, 2000).

Mulka Station in South Australia has used a hot Artesian bore to produce a maximum 20kW of power for use on the Cattle Station since 1987. The bore is approximately 500m from the homestead and was drilled in 1904 to a depth of 1300m, flowing with 86°C water at 500 kPa. The bore was refurbished by the South Australian Department of Mines and Energy in 1985, and an 80 mm diameter fibreglass casing restricted the flow rate to 10 L/s. Mulka station’s geothermal power plant is currently not in operation due to being decommissioned for having a Freon-based working fluid (Burns, K.L., Weber, C., Perry, J. and H. J Harrington, 2000).

Portland in Victoria has sued hot water (58°C) extracted from a bore (1400m deep) at a rate of 90L/s, to heat more than 19,000 square metres of buildings for more than 15 years, and also heat 2000m³ of swimming pool. It is the only spaceheating project in Australia that uses a geothermal resource, as opposed to heap pumps. The total capacity of the Portland facility, operated by the Glenelg Shire is 10.4 MWt (Burns, K.L., Weber, C., Perry, J. and H. J Harrington, 2000). 

Traralgon, also in Victoria used hot water from a two 600m bores at a temperature of 68°C. This was used as process water in paper manufacturing in the 1950’s until dewatering associated with an expansion of brown coal mining in the area forced it to be decommissioned (Burns, K.L., Weber, C., Perry, J. and H. J Harrington, 2000).

In Australia the vast majority of geothermal use continues with ground and water source heat pumps being used for air conditioning, with an estimated 2000 installations in place.

Hot Dry Rock

The geological profile of Australia is such that there is an especially large potential for HDR technologies to be used for energy production in the eastern states of Australia. This is being made explicit through the interest in Geothermal Exploration Licences (GELs) and the number being granted. There are some major geothermal investments in South Australia’s Cooper Basin and the Hunter Valley in New South Wales. Companies such as Geodynamics, Scopenergy, Petratherm, Eden Energy, Green Rock Energy, Geothermal Resources, Pacific Hydro and others, are surveying and installing testing and generating capacity of their respective exploration licences.

Geodynamics Limited holds GELs 97, 98 and 99 in the Cooper Basin (SA), and Exploration Licences (EL) 5560 and 5886 in the Hunter Valley (NSW), and have a pending application for the “Exploration for Permit Minerals” (EPM) licence 13583 for Western Queensland. Geodynamics received a $5 million Renewable Energy Development Initiative (REDI) grant from the Federal Government. In the Cooper Basin GELs Geodynamics are constructing a precommercial power plant of initially between 3 - 5 MW to be online in 2007 and plan to expand up to an approximate potential of over 200 MW of electrical capacity in the years to come. Geodynamics have progressed exploration the furthest so far in GEL 98, successfully drilling an exploration well last year to a 4421 m depth under the central Cooper Basin. The tested well was unexpectedly overpressured, which increases the possibility of exploiting the resource. The subsequent hydraulic stimulation proved extremely effective, being seven times larger than expected and being the largest such stimulation from a single well in the world. The second well at their GEL 97 site has also been successfully drilled and is preparing for stimulation to finally test the heat exchange capacity of the granite between the two sites.

Scopenergy Limited has also secured a REDI grant of $3,982, 855 to assist with financing a drilling program to further asses its resource in its licences (GEL 170, 171, 172, 173, 184 and 99). Scopenergy have a geographic advantage, as they have licences near transmission lines and the population centres of Melbourne and Adelaide. A planned “proof-of-concept” project on the Limestone Coast will potentially lead to an initial 50MW geothermal power plant, to exploit the potential 1000MW of geothermal power in the region.

Petratherm Limited is focusing on lower temperature HDR resources, still in excess of 220°C, except at a shallower depth (less than 3.5 km) and closer to existing infrastructure. Petratherm have obtained GEL156, and 157, near Leigh Creek, and GEL158 at Ferguson Hill, near Olympic Dam mine in South Australia. They have recorded a geothermal gradient of 81.5°C per km at GEL 156, which is amongst the highest shallow temperature gradients ever recorded in Australia. This particular site is only 130 kilometres from the electricity grid, and is 11 kilometres away from the Beverley Uranium Mine. A temperature gradient of 68°C per km was recorded at GEL157 in September 2005. These results falls within the Company’s target expectations to economically produce baseload electricity to the area. Further surveys are accurately mapping the area for drill testing (Petratherm, 2006).

Eden Energy, a subsidiary of Tasman Resources, holds GELs 166, 167, 168, 169, 175, 176, 177 and 185. It intends to exploit HDR for electricity production and hydrogen. Eden Energy wholly owns Brehon Energy, which holds technology and patents for cryogenic hydrogen storage and production and use of Hythane, a mixture of CNG and hydrogen. This technology was developed as part of the NASA space program and is at the point of commercialisation in China, India and USA (Tasman Resources, 2006).

Green Heat Resources (formerly Green Rock Energy) and Green Rock Geothermal (formerly Perilya Geothermal Energy) jointly hold GELs 128, 129, 161, 162, 163, 206, and 213 in central South Australia. The companies have recently been awarded a $68,000 PACE grant from the South Australian Government to fund drilling near Olympic Dam at the end of 2006. Additionally the company announced that exploration to date suggests homogeneous granite suitable for electricity generation and a geothermal resource of over 1000MWe, near to existing electricity grid infrastructure and a planned 400MWe plant to possibly supply the copper operations at Olympic Dam. Interest from both BHP Billiton and the South Australian Government in purchasing electricity from Green Heat Resources and Green Rock Geothermal are encouraging. The companies are nearing the pilot stage of drilling two deep wells, stimulating a fracture reservoir and test flow analysis (Green Rock Energy, 2006).

Geothermal Resources, owned by Havilah Resources currently holds GELs 181, 208, 209, 210, 214, 215, 216, 217 and 222 near the northern Otaway Basin and Lake From in South Australia. Geothermal Resources was recently awarded a $100,000 PACE grant by the government of South Australian for deep drilling to obtain heat flow measurements. The exploration to date has achieved the establishment of two deep wells 500m apart with a fracture network between them, reaching 210°C (Geothermal Resources, 2006).

Pacific Hydro, a large renewable energy company, has acquired 21 GELs in South Australia, totaling 8,894 square kilometres. GELs 188, 189, 190 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, and 205, as well as 236, 237 and 238 are testament to the growth in the geothermal prospects for Australia, and South Australia in particular (Department of Primary Industries and Resources, South Australia, 2006).

The economics of HDR projects are unclear in many of the aforementioned projects. However, the estimated break even electricity price for an HDR project proposed by Geodynamics Limited for the Cooper Basin was 6.2 c/kWh initially, at demonstration plant stage, falling to around 4.0 c/kWh at full-scale production. These figures are yet to be demonstrated, but there seems to be increasing reasons to consider further investment in HDR opportunities in Australia. (ERDC, 1994).

Overshadowing Australia, is New Zealand’s geothermal generating capacity (as of 2000) of over 400 MW. The current installations are:

New Zealand also has significant expansion plans for new geothermal power generation. New permits were approved for a 60 MW power station in the Wairakei-Tauhara geothermal field and a 70 MW plant at the Kawerau field, with more permits expected to be issued in the future.

Geothermal Energy in Asia

The Philippines, Indonesia and Thailand use geothermal energy for electricity production (see Table 1). China and Taiwan have direct use geothermal applications, and to a lesser extent electricity production.

The Philippines is the second largest producer of geothermal electricity in the world, with an installed capacity of 1,931 MW in 2003 (see Table 1). The geothermal resources are extensive in the Philippines due to its location on the edge of the Philippine and Eurasian plates. Their first geothermal plant commenced operation in 1979. The two largest fields are Makban (425.7 MW) and Tiwi (330 MW), which supply 16% of the electricity on the Philippines most populated island, Luzon (Unocal 1998). There is active development of new fields in the Philippines, which may soon become the largest producer of geothermal electricity in the world. Geothermal energy is also used directly for fish processing, salt production and drying coconuts and fruit (Geothermal Education Office 1997).

Indonesia currently produces about 807 MW of geothermal electricity (see Table 1 for 2003 values). The Indonesian islands are located on the boundary between the Eurasian and Australian plates, resulting in a very good geothermal resource. The first geothermal development was the dry steam resource at Kamojang in the 1920s, which now produces 140 MW of electricity. Currently, the largest field is Gunung Salak, with an installed capacity of 330 MW. The Indonesian government announced plans to increase production rapidly, with a goal of achieving 2,000 MW by 2009, and as much as 6,000 MW by 2020. Geothermal steam and hot water are also used directly for cooking and bathing.

The Future for Geothermal Energy Technologies

In the medium to longer term, technological developments will see the utilisation of the geothermal energy in hot dry rocks and geopressured reservoirs. Usable geothermal resources will no longer be limited to the shallow hydrothermal reservoirs. These resources represent a virtually limitless source of energy, and are the future of sustainable geothermal energy.

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

Geothermal Education Office

Geothermal Resources Council

US DOE Office of Geothermal Technologies

CADDET Geothermal Register

CREST

Chevron Thermal Energy

Hot Dry Rock research at UNSW (pdf)

Hot Dry Rock research at ANU

International Geothermal Association

References

Barbier, 1997. “Nature and Technology of Geothermal Energy: a Review. Renewable and Sustainable Energy Reviews. 1, No 1/2, ppl – 70.

Brown, G., 1996. "Geothermal energy", in Renewable energy- power for a sustainable future, ed. G. Boyle, Oxford University Press, Oxford.

Burns, K.L., Weber, C., Perry, J. and H. J Harrington, (2000). “Status of the Geothermal Industry in Australia.” (Online), Accessed May 17 2006.

Energy Research and Development Corporation (ERDC), 1994. "Hot Rock Feasibility Study, Report 243", The Australian Commonwealth Government, Canberra.

Geothermal Education Office, 1997. "Geothermal energy worldwide" (Online), Accessed May 17 2006.

Hinrichs, R.A., 1996, Energy, its use and the environment, 2nd edition, Saunders College Publishing, Fort Worth.

IGA (International Geothermal Association), 1998a. "Welcome to our page with United States data " (Online), Accessed May 17 2006.

IGA, 1998b. "Welcome to our page with data for Indonesia" (Online), Accessed May 17 2006.

Unocal, 1998. "Philippine geothermal" (Online), Accessed May 17 2006.

World Energy Council 1994, New renewable energy resources, Kogan Page, London.