Mother Earth - a source of sustainable energy?
Dr Neil Williams
Executive Director, Australian Geological Survey Organisation
INTRODUCTION
Modern society has an insatiable thirst for energy. Holland and Petersen (1995) estimate that the average consumption of energy worldwide is around 175 MJ per day per capita. By contrast, they note that in the world's most developed countries, such as the United States of America, consumption rates are much higher, averaging around 950 MJ per day per capita. Australia's consumption is about 710 MJ per day per capita (Bush et al, 1997).
Our thirst for energy is quenched largely by non-renewable resources from Mother Earth, the most important being the fossil fuels coal, oil, and gas. The World Resources Institute (World Resources, 1994) reports that in 1990-1991 petroleum dominated the world's energy mix, making up 37.1% of all commercial energy consumption globally. Solid fossil fuels (hard coals, soft coals or lignite, and peat) accounted for another 29.2%, with gaseous fossil fuels (mostly natural gas) accounting for a further 23.7%. For the 1995-96 financial year, Australia's total energy consumption comprised 36.8% petroleum, 39.9% solid fossil fuels, and 17.6% natural gas (Bush et al, 1997).
The World Resources Institute (World Resources, 1994) also reports that globally the most important sources of primary electricity (ie, electricity produced from any source other than fossil fuel-fired thermal generating plants) are large-scale hydroelectric plants and nuclear-fuelled thermal plants. Primary electrical production accounts for only 9.5% of the world's energy supplies. Nuclear power provides about two thirds of primary electricity, with hydroelectric power accounting for almost all the rest.
It is clear from these statistics that renewable energy resources comprise only a small proportion of energy supplies, both domestically and globally. The most important renewable energy sources are wind, solar power, and geothermal.
ENERGY SUPPLY CHALLENGES
With society's massive dependence on non-renewable energy resources (the fossil and nuclear fuels) there hasbeen concern that we would soon exhaust these resources. This concern peaked in the 1970s when public opinion was strongly influenced by the view that non-renewable energy resources consisted of fixed pools or stocks, and that with continuing production and consumption it would only be a matter of time before these stocks were exhausted. With the wisdom of hindsight we know that the worry was unfounded as it overlooked the dynamic nature of non-renewable resources (Tilton and Skinner, 1987). Non-renewable resources are not small, fixed stocks, slowly but inevitably being depleted. Rather they are constantly changing fractions of stocks that are astronomically large. These fractions are continually changing in response to economic and technological developments, sometimes declining, often enlarging, but never reaching a point where it can be said "this resource is used up forever". From the dynamic viewpoint there will always be material left for exploitation provided we are prepared to pay its price of recovery (Tilton and Skinner 1987).
The present outlook for fossil fuels availability is very positive. The World Resources Institute's 1994 report offers the following comment on this outlook:
"The World Energy Council, in its 16th Survey of Energy Resources, found that "fears of imminent [resource] exhaustion that were widely held 20 years ago are now considered to have been unfounded. The concepts of exhaustion, or even scarcity, fail to appear anywhere in this survey". Estimates of proven recoverable reserves of petroleum rose 11.4% between the end of 1987 and the end of 1990, while those of natural gas liquid rose 5.3% and those of natural gas rose 17.9%. These increases can be attributed to both the reevaluation of existing reserves and the discovery of new reserves. Given current economic conditions and technologies, proven reserves alone could supply petroleum needs for 40 years, natural gas for 50 years, and coal for well over 200 years; there is also the expectation that new fossil fuel reserves will be discovered in the coming years" (World Resources, 1994).
Today concern about our reliance on fossil fuels has shifted from resource exhaustion to environmental impact. Since the start of the industrial revolution in the nineteenth century, when society's dependence on fossils fuels began, there has been a significant increase in the carbon dioxide concentration of the Earth's atmosphere. If present trends were to continue, it is estimated that by the year 2030 the atmosphere's carbon dioxide concentration would reach 600 ppm, that is, double the pre-industrial level (Boyden, 1992). As carbon dioxide is a greenhouse gas, society is becoming increasingly concerned that such a concentration increase would lead to global warming and climate change. Consequently the issue is now high on the international political agenda and there is growing pressure for society to reduce carbon dioxide emissions.
What can be done to lower carbon dioxide emissions? Strategies being considered include shifts away from solid and liquid to gaseous fossil fuels, improved energy efficiency including cogeneration, greater reliance on nuclear energy, and greater reliance on renewable energy resources. The remainder of this paper focuses on geothermal energy, a renewable energy resource that has received considerable attention in some parts of the world, but not in Australia.
GEOTHERMAL ENERGY
The Earth's surface receives most of its energy from the Sun, but a small amount, totalling some two orders ofmagnitude less than the solar energy flux, comes from the Earth's hot interior. This internal energy is known as geothermal energy. About 80% of the Earth's geothermal energy comes from the decay of various long-livedradioactive isotopes, the main ones being Th-232, U-238, K-40, and U-235. The remaining 20% is primordial heat associated with the Earth's formation (Morgan, 1989).
Geothermal energy is transferred to the Earth's surface dominantly by conduction (>90%) or by mass convection of magma (molten rock) and water. The Earth's geothermal gradient is typically in the range of 10o to 50o Celsius per kilometre (Morgan, 1989). In the upper 10 km of the Earth's crust there is about 1021 MJ of geothermal energy but, like all Earth resources, most of it is too deeply buried or too dispersed to be exploited (Guffanti, 1989).
As with the exploitation of any Earth resource, the challenge to using geothermal energy is to locate sufficiently high-grade concentrations of the resource to be economically attractive and/or to invent innovative technologies that facilitate economic exploitation. To date there have been four main approaches to the exploitation of geothermal energy. They are conventional geothermal electricity production, direct use applications, the hot-dry rock approach, and geothermal heat pumps.
Conventional Geothermal Electricity Production
The biggest geothermal energy developments to date all involve the use of geothermal energy to produce electricity. These developments are located in geographically restricted areas where there are naturally occurring high grade geothermal energy resources in the form of large volumes of very hot water (>100o Celsius) within about 1 to 2 kms of the Earth's surface (Ellis, 1975). Typically these areas are in regions of present or recent past volcanism where there are naturally high geothermal gradients and large active hydrothermal systems.
In developments in favourable areas, hot ground water or preferably steam is extracted through drill holes and used to drive conventional electricity-generating turbines. In 1990, world energy production from such areas exceeded 30,000 GWh, the largest producers being the United States (16,900 GWh), the Philippines (5,470 GWh), and Mexico (5,124 GWh). Other large producers include Japan, Italy, Iceland, and New Zealand (World Resources, 1994). In the United States the cost of electricity from typical geothermal systems ranges from $US0.05 to $US0.08/kWh (DOE, 1997).
Although geothermal electricity production is generally classified as a renewable energy production activity, experience over the last 60 years has shown that this is not quite correct (Holland and Petersen, 1995). Because the steam and water produced from geothermal reservoirs are usually discarded, the productivity of reservoirs typically falls with time, even at plants where there is reinjection. Another problem with geothermal electricity generation is chemical pollution. Hydrothermal waters in geothermally active areas often have high concentrations of many compounds and elements, some of which are quite dangerous, such as hydrogen sulphide, mercury, and arsenic. Chemical pollution can be lessened by reinjection of water and steam into underground reservoirs (Holland and Petersen, 1992), but this adds to energy production costs.
Because there has been little recent volcanic activity in Australia, the potential for geothermal electricity generation here is low.
Direct Use Geothermal Applications
Geothermal reservoirs of low to moderate temperature (20o to 150o Celsius) are more widespread than the higher temperature reservoirs suitable for electricity production. In favourable locations lower quality reservoirs are often used to provide direct heat for residential, industrial, and commercial uses (DOE, 1997). In 1996 direct-use geothermal applications around the world used nearly 5.8 billion MJ of geothermal energy per annum (DOE, 1997).
In Australia there are some small direct-use developments in the south-eastern part of the continent where there are reservoirs of low to moderate temperature groundwater. A district heating system at Portland, in the Otway Basin of western Victoria, has been servicing a building area of about 19,000 metres2 for the last 12 years, while a geothermal well is providing hot water for paper manufacture at Traralgon in the Gippsland Basin of eastern Victoria (Anon, 1997a). There are significant resources of moderate and even high temperature geothermal reservoirs elsewhere in Australia like, for example, the Cooper Basin in western Queensland and northern South Australia (Anon 1997a), but they are unlikely to be developed in the near future because they are in remote areas distant from potential markets.
Hot Dry Rock Geothermal Energy
The hot dry rock approach to geothermal energy exploitation is aimed at increasing the range of areas where exploitation could occur by creating artificial geothermal fields. This is done by drilling deep drill holes into regions with higher than average geothermal gradients, fracturing the hot dry rocks at the bottom of the holes, and extracting energy through the injection and extraction of fluids from the surface.
The hot dry rock approach is very much in its infancy and the economic feasibility of the approach has yet to be demonstrated. However, because of the vast quantities of energy potentially available from hot dry rock reservoirs, some $400 million has been spent over the last 20 years in hot dry rock research (Anon, 1997a).
The world's largest and deepest (2.8 to 3.9 km deep) hot dry rock research project is located at Soultz-sous-Forets in the Alsace where the major hurdle to jump has been developing enough surfaces through underground fracturing to enable sufficient heat exchange to take place (Anon, 1997b). Over the last two years a fracture system has been produced hydraulically that connects two drill holes 450 metres apart. This has resulted in an estimated total heat-exchange area of 3 km2 which, at an outlet temperature of 140o Celsius and a flow rate of 25 kg/second, is equivalent to a thermal capacity of 10 MW. Research is continuing in the hope of creating a system that attains an economic capacity of 50 MW (Anon, 1997b; Wyborn, pers comm).
In Australia the hot dry rock approach is presently under investigation by Hot Rock Energy Pty Ltd, a company spun out of research by two former Australian Geological Survey Organisation (AGSO) geoscientists, Drs Doone Wyborn and Prame Chopra. They note that geological conditions in the Australian crust are more favourable than elsewhere for jumping the fracturing hurdle (AGSO, 1994), and they are presently evaluating a research site in the Hunter Valley, NSW (Anon, 1997a).
Geothermal Heat Pumps
Geothermal heat pumps (GHPs), do not have the same geographic and environmental restrictions as geothermal electricity generation and direct use applications and, unlike hot dry rock technologies, GHPs are currently economically attractive. The use of GHPs is growing, particularly in areas with continental climates where there are large temperature variations between summer and winter (DOE, 1997).
GHPs rely on the relatively constant temperature of the Earth beneath a depth of a few metres below the surface. In areas with continental climates the Earth is warmer than the overlying atmosphere in winter and cooler in the summer. GHPs take advantage of this situation by transferring heat stored in the Earth into a building in winter, and transferring it back into the ground in summer. The Earth is therefore a heat source in winter and a heat sink in summer. GHPs are technically simple, comprising an earth connection subsystem (usually a series of pipes or loops), a heat pump, and a distribution subsystem which is typically a conventional ductwork system that moves heated or cooled air around (DOE, 1997).
GHPs are being used more and more widely in the United States and Europe for both residential and commercial buildings, and a United States Environmental Protection Agency study has found that GHPs have the potential of significantly reduce fossil fuel consumption and corresponding green house gas emissions by as much as 44% compared with air-source heat pumps, and by as much as 72% compared with electric resistance heating and standard air conditioning equipment (GAO, 1994).
Despite conditions suitable for GHPs in much of Australia their use to date has been limited to small scale applications. However, this situation is set to change dramatically when the new Australian Geological Survey Organisation (AGSO) building is completed in Canberra in early 1998.
The New AGSO Building
The Australian Geological Survey Organisation (AGSO) is one of the seven Groups comprising the Commonwealth's Department of Primary Industries and Energy (DPIE). In early 1995 Parliament approved the construction in Canberra of a new purpose-built building for AGSO, with a budget of $105 million (at December 1994 prices). Because DPIE has carriage of the Commonwealth's energy policies, the design brief for the new building sought to demonstrate an appropriate, pragmatic response to Ecologically Sustainable Development (ESD) design principles. The building's architects, Eggleston Macdonald, created a design which showcases a number of significant energy conservation features including air conditioning which incorporates a large GHP system. The system was proposed by Bassett Consulting Engineers as part of Eggleston Macdonald's response to the AGSO design brief.
Construction of the AGSO building commenced in April 1996 and it will be opened in early 1998. The building includes both offices and laboratories and, together with an adjacent support building, has a total floor area of 40,000 metres2.
The AGSO GHP system developed by Bassett Consulting Engineers is an electrically powered system that will capitalise on the Earth's moderate temperature (around 17o Celsius) at a depth of 100 metres beneath the building site. When the AGSO building is fully operational water will be circulated through a "geothermal field" comprising loops in 350 bore holes (each 100 metres deep) in front of the AGSO building. In winter the circulating water will collect heat from the earth and carry it through the system into the building. In summer the system reverses and will extract heat from the building and transfer it back to the geothermal field.
Inside the building there are 200 water to air heat pumps, each serving up to four perimeter offices or eight interior offices. Each of these systems is independent of the others and can be switched off during office hours if they are not in use. They can also be switched on after hours if the area is occupied.
The decision to use a GHP system in the AGSO building was based on a life-cycle cost comparison of the system with conventional Variable Air Volume (VAV) air conditioning systems, and VAV systems with chilled water storage. The analysis found that the VAV system with chilled water storage had the highest net present cost, while the GHP system had the lowest - ~$AUS540,000 less than the VAV system and ~$AUS1,035,000 less than the VAV system with chilled water storage.
In addition to net present cost savings, other advantages of the GHP system to AGSO include:
- reduced annual energy consumption and therefore, reduced usage of non-renewable energy resources (~340MJ per m2 per annum for the GHP system versus ~400 MJ per m2 per annum for a VAV system and ~415MJ per m2 per annum for a VAV system with chilled water storage)
- reduced peak energy demand which reduces maximum-demand electricity charges (and ultimately the sizes of power station requirements).
- greater after hours flexibility as units can be left off in areas which are unoccupied
- less redundancy due to the configuration of the GHP system and the lower need for "standby" capacity
- less outside equipment and consequential corrosion
- reduced risk of major breakdown of costly central plant items
- increased plant warranties
- elimination of cooling tower water treatment and corrosion, thereby removing the risk of Legionnaires Disease.
CONCLUSIONS
Of the various proven and potential applications of geothermal energy, the one with the greatest promise for Australia in the immediate future is GHPs. AGSO's new building will showcase this application on a commercial scale building and it is anticipated that with growing awareness of the merits of GHPs, Australia will begin following the lead of the United States and Europe and increase its use of GHPs for space heating and air-conditioning. With the growing use of air-conditioning throughout society it is also anticipated that the use of GHPs will become increasingly widespread throughout climatically suitable parts of the world, a development that will represent a small but important step forward in the use of clean and renewable geothermal energy from Mother Earth.
In the longer term, a worthwhile challenge for society is the economic exploitation of the vast amount of geothermal energy in the Earth's crust for more energy intensive applications. The hot dry rock approach holds promise for extending the production of electricity using geothermal energy, but the challenge will be to demonstrate both the technical and economic feasibility of the approach.
ACKNOWLEDGMENTS
I would particularly like to thank Gordon Cheyne, Project Administrator for AGSO's new building for his assistance in the preparation of this paper; John Coffey, Director, Bassett Consulting Engineers, for providing important background information on the GHP system being installed in the AGSO building and for reviewing this paper; and Doone Wyborn, Hot Rock Energy Pty Ltd, who also reviewed the paper, as well as providing details of recent progress on the Soultz-sous-Forets hot dry rock research project.
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