Langues:
Geothermal energy comes from the natural heat of the Earth, primarily due to the decay of the naturally radioactive isotopes of uranium, thorium and potassium. Because of the internal heat, the Earth’s surface heat flow averages 82 mW/m2, which amounts to a total heat of about 42 million megawatts. On average, the temperature of the Earth increases with depth, about 25–30˚C/km above the surface ambient temperature, and the temperature of the earth at 10 km would be over 300˚C. At the base of the continental crust, temperatures are believed to range from 200 to 1000°C, and the heat is transferred towards the surface mostly by conduction. In some areas, the heat flows more easily to the surface, due among others, to intrusion of molten magma from depth, to high surface heat flow due to a thin crust, or to ascent of groundwater that has been heated.
The thermal energy of the Earth is immense, but only a fraction of it can be utilized in areas where geological conditions permit a carrier (such as water), to ‘transfer’ the heat from deep hot zones to or near the surface, thus creating geothermal resources. Geothermal production wells are commonly more than 2 km deep, but rarely much more than 3 km, and most geothermal exploration and use occur where the temperature is higher than the average of 300°C, where drilling is shallower and less costly.
Geothermal resources are usually classified in various categories depending on the mean annual ambient temperature these can provide. These can be dry vapor or hot water with, like for hurricanes, an upflow zone at the centre, an outflow zone or plume of heated water moving laterally away from the centre of the system, and a downflow zone where recharge is taking place. The hot water has indeed a lower density than the surrounding cold groundwater and therefore it flows up towards the surface along fractures and other permeable structures to appear at the earth’s surface as hot springs, fumaroles, geysers or travertine deposits. For hot dry rock resources, new experimental technologies are being tested, including, like for shale gas extraction, hydraulic fracturing under pressure, followed by cold water circulating down one well and producing hot water from a second well in a closed system.
In general, resources above 150˚C are exploited for electric power generation and resources below 150˚C are usually used in direct heating and cooling. Ambient temperatures in the 5–30˚C range can be used with heat pumps, which provide both heating and cooling.
The magnitude of low-temperature geothermal resources in the world is about 140 EJ/yr of heat or one third of the present world energy consumption. The expected geothermal electricity potential ranges from 35 to a maximum of 140 GWe. The potential may be 5-10 times higher, based on enhanced geothermal systems (EGS) technology, and the most likely value for the technical potential of geothermal resources suitable for electricity generation is 210 GWe. It is considered possible to produce up to 8.3% of the total world electricity with geothermal resources, supplying 17% of the world population. Thirty nine countries (located mostly in Africa, Central/South America and the Pacific) could potentially produce 100% of their electricity using geothermal resources.
The main advantage of geothermal heating and power generation systems is that they are available 24 hours per day, 365 days a year and are only shut down for maintenance. Power generation systems typically have capacity factors of 95% (i.e. they operate at nearly full capacity year round), whereas direct-use systems have a capacity factor around 25 to 30%, owing to heating not being required year round. Heat pump systems have operating capacities of around 10–20% in the heating mode and they double this if the cooling mode is also included.
Exploitable geothermal systems can be found in a number of geological environments. They can be broadly divided into two groups depending on whether they are related to young volcanoes and magmatic activity. High-temperature fields (>180°C) are the fields where volcanic activity takes place mainly along tectonic plates boundaries and used for conventional power production as the crust is highly fractured and thus permeable to water, and other sources of heat.
Most of the plate boundaries are below sea level, but in cases where the volcanic activity has been intensive enough to build islands, or where active plate boundaries transect continents with high-temperature geothermal fields are scattered along the boundaries, such as the ‘ring of fire’ that surrounds the Pacific Ocean, Iceland and ‘hot spots’ such as Hawaii and Yellowstone.
Low-temperature fields (<180°C) are geothermal resources that, with some exceptions, can also be associated with volcanic activity. Warm springs can occur in most rock types of all ages, but are most frequent in mountainous regions, where warm springs appear along faults in valleys. Areas of young tectonic activity are commonly rich in this type.
The most important type of geothermal resources not associated with young volcanic activity are characterised by deep basins filled with sedimentary rocks of high porosity and permeability. If these are properly isolated from surface ground water by impermeable strata, the water in the sediments is heated by the regional heat flow. The temperature of the thermal water depends on the depth of the individual aquifers and the geothermal gradient in the area concerned, but is commonly in the range of 50–100°C (in wells less than 3 km deep) in areas that have been exploited. Geothermal resources of this type are rarely seen on the surface, but are commonly detected during deep drilling for oil and gas.
By end 2008, the geothermal electricity generating capacity installed, produced over 63 000 GWh/yr, and direct heat utilization amounted about 120 000 GWh. The annual growth in energy output between 2008 and 2013, has been 3.8% for electricity production, and around 10% for direct use (including geothermal heat pumps). Energy produced by ground-source heat pumps alone has increased by 20% per annum over the same period. The low growth rate for electric power generation was primarily due to the low price for natural gas, the main competitor.
Financing is a critical factor in the economics of any project, and thus the potential for market penetration and development. For many new projects, the largest annual operating cost is the amortisation of the cost of capital, which can be as high as 75% of the annual operating expense for new geothermal district energy projects.
With increasing fossil fuel prices and limitations on the emission of greenhouse gases, development of geothermal energy has become more competitive as a renewable and ‘green’ energy resource. However, market development is highly dependent upon competition from other sources of electricity or from direct-use product supply (fish, vegetables, flowers, minerals, etc.). Remote areas, often off-grid, are excellent candidates for electrical energy. The availability of transmission lines can be critical and these are often lacking and expensive to construct over large distances. Direct-use projects must have a market and a transportation system to get economically the products to consumers. Unfortunately, geothermal resources that can be utilised are often remote, which may limit their development for commercial operations.
Development risks are high, and prediction of the quality of a resource requires capital investment in drilling and well tests.
There are three main techniques used :
The principle is simple: in the deep subsurface where temperatures are high enough for power generation (150–200°C) an extended fracture network is created and water from the deep wells and/or cold water from the surface is transported through this deep reservoir using injection and production wells, and then recovered as steam/hot water. The extracted heat can be used for district heating and/or for power generation.
Techniques need to be developed for creating, profiling, and operating the deep fracture system and some environmental issues, such as the chance of triggering seismicity and the availability of surface water, also need detailed investigation. There are several experimental projects under way. Other developments include the International Iceland Deep Drilling Project (IDDP) to improve the efficiency and economics of geothermal energy by harnessing deep unconventional geothermal resources. Recent advances in binary cycle technology, allows now lower-temperature fluids at around 100°C being utilised, thus increasing the number of potential locations.
The use of combined heat and power plants has also made low-temperature resources and deep drilling more economic. The geothermal fluid is utilised at progressively lower temperature, thus maximising the energy extracted. District heating using the spent water from a binary power plant can make such a marginal project, economic.
Geothermal energy is generally classified as a renewable resource as the energy removed from the resource is continuously replaced by more energy on time scales similar to those required for energy removal. The production system is able to sustain the production levels over long periods by using moderate production rates, which take into account the local resource characteristics (field size, natural recharge rate, etc.). The production of geothermal fluid/heat continuously creates a hydraulic/heat sink in the reservoir, and the regeneration of geothermal resources is a process which occurs over various time scales, depending on the type and size of the production system, the rate of extraction, and on the attributes of the resource. The range of CO2 emissions from high-temperature geothermal fields used for electricity production is variable, but much lower than that for fossil fuel plants.
Regarding their environmental impact, depending on the geological conditions of different fields, geothermal fluids contain a variable quantity of gases, largely nitrogen and carbon dioxide, with some hydrogen sulphide and smaller proportions of ammonia, mercury, radon and boron. Most of these chemicals are concentrated in the disposal water, which is routinely re-injected into drill holes and thus not released into the environment. Geothermal schemes are relatively benign, but they generally produce highly corrosive brine, which may need special treatment and discharge consents. Removal of hydrogen sulphide released from geothermal power plants is mandatory in the USA and Italy. The concentration of the other gases is usually not harmful and they can be vented to the atmosphere.
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