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Context - Geothermal energy comes from the natural heat of the earth. It is used in some areas for electricity production and heating.

What is the place of geothermal energy in the energy mix?

This is a faithful summary of the leading report produced in 2013 by World Energy Council (WEC): " World Energy Resources: Geothermal" 

  • Source document:WEC (2013)
  • Summary & Details: GreenFacts

What is exactly geothermal energy?

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. 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 gets closer 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. These areas are usually found near the boundaries of tectonic plates or in mountainous regions. The exploitable geothermal systems can be broadly divided into high temperature fields (>180°C) where volcanic activity takes place and and low -temperature fields (<180°C).

How much does this potential geothermal energy represent?

Only a fraction of the thermal energy of the earth can be utilized in areas where geological conditions permit a carrier (water in the liquid or vapour phases) to ‘transfer’ the heat from deep hot zones to or near the surface. In general, resources that have a temperature above 150˚C are employed for electric power generation and resources below 150˚C in direct heating and cooling uses. Ambient temperatures in the 5–30˚C range can be used with heat pumps, which provide both heating and cooling.

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 need using geothermal resources.

How much energy is produced via geothermal sources?

By the end of 2008, the geothermal electricity generating capacity installed produced over 63 000 GWh/yr and direct heat utilization amounted about 120000 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 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.

Is geothermal energy economically competitive?

Financing is a critical factor in the economics of any project, the largest annual operating cost is the cost of setting up the infrastructure, 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. Development risks are high, and prediction of the quality of a resource requires capital investment in drilling and well tests.

Which are the main techniques used to exploit geothermal energy?

There are three main techniques:

  1. Electric Power Generation A vapour-dominated (dry steam) resource can be used directly to turn a turbine-generator set to produce electricity but is influenced by the drilling costs and resource.
  2. Direct Utilisation Geothermal energy resources for direct use projects in the low- to intermediate-temperature can meet up to 80–90% of the annual heating or cooling demand and exist in at least 80 countries at economic drilling depths.
  3. Geothermal Heat Pumps, which use the relatively constant temperature of the earth close to the surface to provide heating, cooling and domestic hot water for homes, schools and other buildings.

How to enhance the efficacy of geothermal systems?

Geothermal systems can be enhanced by a technique that is also used for the extraction of shale gas: hydraulic fracturing. 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. There are several such experimental projects under way. The practice of combined heat and power plants has also made low-temperature resources and deep drilling more economic.

Is geothermal energy production sustainable?

The energy removed from the resource is continuously replaced by more energy on time scales similar to those required for energy removal. The range of CO2 emissions from electricity production is variable, but much lower than that for fossil fuel plants.

Removal of hydrogen sulphide released from geothermal power plants is mandatory in the USA and Italy and, as the concentration of other gases is usually not harmful, they can be vented to the atmosphere. Depending on the geological conditions of different fields, geothermal fluids may also contain a variable quantity of chemicals but most are concentrated in the disposal water that is routinely re-injected into drill holes and thus not released into the environment.

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