Geothermal heat from low-temperature fields located in sedimentary basins of geologically stable platforms has long been used for district urban heating, industrial processing, domestic water and space heating, leisure, and balneotherapy (thermal bath therapy) applications. Useful heat and power produced globally totals about 2 exajoules (EJ)/year. Higher temperature fields located in geodynamically active regions with temperatures above 250°C utilize high-temperature natural steam or a steam/brine mixture under pressure. Where this resource can be reached by drilling to depths of less than 2 km, electricity generation using steam turbines can usually be achieved directly at competitive prices.
The installed geothermal generation capacity of over 8.9 GWe (gigawatts electrical) in 24 countries produced 56.8 TWh (0.3%) of global electricity in 2004 and is growing at about 20% per year.13 Proven resources of over 10 GW are yet to be utilized; from these resources, over 1,000 TWh/year of electricity could be produced, to possibly reach about 2% of total global electricity generation by 2030.6 Binary power plants (using low-boiling-point heat-transfer fluids and heat exchangers) and organic Rankin cycle systems (using low-boiling-point heat-transfer fluids) are more recent developments to suit lower-enthalpy (temperature and heat concentration) fields, but often at additional generation costs. Plant capacity factors range from 40% to 95% for power plants suitable for use in providing the baseload (rather than for occasional use to meet peak power demands).
Fields of natural steam are rare. Most are a mixture of steam and hot water requiring single or double flash pressure reduction systems to separate out the hot water, which can then be used in binary plants or for direct use of the heat.Sustainability concerns relating to land subsidence, heat extraction rates exceeding natural replenishment, chemical pollution of waterways (e.g., if traces of arsenic are present), excessive warming of river water used for cooling, and associated release of gaseous CO2 emissions from well drilling have resulted in some geothermal power plant permit applications being declined.
These concerns could be partly overcome by re-injection of the fluids after heat removal to maintain a more constant pressure
in the reservoir in the long term and hence prolong the life of the field. Capital costs have declined by about 50% from the 1980s level of $3,000–5,000/kW for all plant types (with binary cycle plants being the more costly). Power generation costs vary with quality of resource (high-quality being >250°C and low-quality being <150°C), access to shallow or deep resources, size of field, permit conditions as granted in a resource use consent, and practical applications for any excess heat. Power generation costs of large, high-enthalpy geothermal fields running at baseload can be competitive with those of coalfired and gas-fired plants varying in the range of $30–80/MWh. Operating costs could increase if the high level of CO2 emissions released from some geothermal bores entail a carbon charge or, indeed, if governments eventually regulate to require carbon
dioxide capture and storage.
The major difference between traditional electricity and heat generation and the application of geothermal resources is the potentially severe corrosion of metals caused by the geothermal fluids. Chemicals often present that account for most corrosion include oxygen, hydrogen sulfide, ammonia, chloride, sulfates, and hydrogen ions. Development of better materials
for use in downhole equipment for geothermal resource exploitation has continued for more than two decades. Areas of relevance include tubular lining materials, highly alloyed metals, high-temperature cements, high-temperature elastomers,
drilling tools, and downwell pumps. Nonmetallic materials such as polymers, concrete– polymer composites, and refractory cements could also be used in the development of geothermal fields. Testing of materials including carbon and stainless steels, titanium, copper, and many alloys needs to be conducted first in the laboratory in brine and with steam temperatures up to 260°C and for exposure times of at least two years. Such testing can then be followed by field exposures to ensure that good durability is maintained in the corrosive environment. The American Society for Testing and Materials has been actively involved for over 25 years in this area.
Tests conducted in Japan14 for corrosion and scaling of turbine materials assessed six different coatings over a 12-month
period. Two types, Cr3C2-NiCr and Cr3C2-FeCrAlY, were selected as anti-abrasion coatings, and Cr3C2-NiCr, ZrB2, CoNiCrAlY+TiN, and CoNiCrAlY+Al2O3?TiO2 were selected as anti-scale-deposition coatings. The anti-abrasion coatings were subjected to corrosion and fatigue tests, whereas the anti-scale-deposition coatings were subjected to only the corrosion tests. Two types of rotor materials, three kinds of blade materials, and one casing material were tested over one year of exposure to geothermal steam. Only one coating experienced scaling was observed on it.
Deeper drilling up to 8 km to reach molten rock magma resources might become cost-effective in the future, partly depending on materials scientists being able to develop harder wearing drill bits. Deeper and more reliable drilling technology could also help to develop widely abundant “hot dry rocks,” where the bedrock is artificially fractured, water is injected, and
the heat can then be extracted as steam. Pilot schemes exist but tend not to be cost-effective at this stage, and sites are limited to rock substrates that are sufficiently porous for water to flow through them. Several advanced energy conversion technologies are becoming available to enhance the use of geothermal heat, including combined-cycle systems for steam resources, trilateral cycles forbinary total flow resources, remote detection of hot zones during exploration, absorption/regeneration cycles (e.g., ground heat pumps), and improved power generation technologies.15 Enhanced geothermal systems have a long-term potential of 88 GW in the United States alone with 36 GW projected to be developed by 2050 if research and development goals are met.16 Cascading multipurpose heat systems that fully utilize the heat by having a range of varying applications as the temperature declines are under further development. The location of the geothermal resource, however,
can limit the total heat use if nearby demands for heat for such purposes as food and fiber processing, industry, grain drying, prawn farming, or greenhouse heating do not exist.
It is expected that improvements in characterizing underground reservoirs, low-cost drilling techniques, more efficient
conversion systems, and utilization of deeper reservoirs will increase the uptake of geothermal resources, as will the market
value for extractable co-products such as silica, zinc, manganese, and lithium. At the small scale, the demand for shallow-well, ground-to-air heat pumps for space heating (and cooling) in buildings is anticipated to increase, especially if the drilling, installation, and hardware costs can be reduced. Several technologies for shallow geothermal heat extraction are
available, including direct flow from a well drilled in the aquifer, horizontal or vertical grids and loops, or even “intelligent” thermal foundations that can automatically absorb or release heat depending on the internal temperature of the building. Costs vary widely according to the choice. Greater consumer usage of these ground heat pumps has already resulted from lower capital costs due to greater mass production and improved heat exchanger loop systems, but such systems remain a costly option. Drilling the bores to install the horizontal or vertical pipes that are used to deposit or extract the heat in the ground can reach up to one-half of the total investment costs. Indeed, improved drilling techniques and bit materials are
MRS BULLETIN • VOLUME 33 • APRIL 2008 • http://www.mrs.org/bulletin