Deep geothermal energy is the energy produced within the earth’s crust between 2 and 10 kilometers down. At these depths, the temperatures can range from 200-700 °C [1, 2, 3]. This heat is absorbed by layers of rock within the crust and the natural fluids which are trapped in the rock’s fractures and pores [1]. Typically, the fluid is liquid and gaseous water with some dissolved salts [1]. In comparison to the amount and distribution of oil and gas deposits, the amount of hot rock and water is substantial [1].

Historically, this deep energy has only been utilized when the hot water has found its way to the surface, as in a hot spring or geyser, or is otherwise easily accessible, as in areas on the edge of tectonic plates or areas with high levels of volcanism [1, 2]. For example, it was first used to produce electricity in 1904 in the steam fields found in Larderello, Italy [1]. More modern efforts to harness the energy have clustered in Iceland, New Zealand, Japan, the Philippines, and Indonesia, all areas on tectonic boundaries [1, 2]. However, with recent technological advances it has become feasible to be proactive and drill down directly to where these energy resources lie rather than waiting for them to emerge. As the entire planet is covered by rock and contains mostly water, using this energy as an alternative to fossil fuels has great potential. Indeed, development projects have started or are in the process of starting all over Europe, the United States, and Australia [1, 3].

The remainder of this section will give an overview of two methods currently used for accessing deep geothermal energy as well as describing its advantages and disadvantages.

Deep Geothermal Reservoirs

The most common method for using deep geothermal energy involves drilling down to a reservoir of hot water and extracting it. As it is brought up to the surface, the sudden reduction in pressure vaporizes the liquid water to steam which is then used to power a turbine electrical generator in a power plant above. [2]


As expected, this method requires three things: heat from the earth, porous rock, and a preexisting reservoir of hot water. [2] As depicted in the following diagram, the reservoir is formed when surface water makes it way down to the porous rock and becomes trapped by a cap of impermeable rock and heated. [2]


reserviorpic.jpg
Image from http://www.geo-energy.org/aboutGE/basics.asp


The latter two requirements for this method are limitations on the its productivity. Both the rock permeability and rate of water replenishment need to be high enough to ensure long-term economical use and these two characteristic are not able to be found together in all areas. [1]

Enhanced Geothermal Systems (EDS)

Enhanced Geothermal Systems are also referred to as Engineered Geothermal Syatems or Hot Dry Rock Geothermal Energy Systems. [1, 3] The basic mechanics of EDS are identical to the method using preexisting reservoirs: hot water and steam are extracted and used to power turbine electrical generators. It is different, however, in that the porous rock and water are artificially put into the system allowing it to be used on a much broader scale.

As shown in the following diagram, an injection well and a network of fractures are drilled into the dry hot rock. Water is injected into the system and extracted through the production well forming a closed loop heating/cooling system. [1, 3]



EDSimage.JPG
Image from The Future of Geothermal Energy – Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Report from the Massachusetts Institute of Technology, 2006

The output of EDS systems are expected to cool 10°C every 20-30 years which will recover if the rock is allowed to reheat [3]. However, this is not an insurmountable obstacle given that there are likely plenty of sources of hot dry rock to drill into. [1, 3] One report estimates that the EDS resources in the United States if fully exploited would be “sufficient to provide all the world’s current energy needs for several millennia.” [3]

Advantages and Disadvantages of Deep Geothermal Energy

The advantages of deep geothermal energy are the same as with any geothermal system: there is no consumption of fossil fuels or greenhouse gas emissions and it is renewable and sustainable in the sense that its source is that of heat from the earth which is not likely to disappear in the foreseeable future. Deep geothermal systems do have several distinct disadvantages, however. First, they are somewhat limited to locations where drilling is feasible and where a water reservoir is found, depending on the method used. Second, drilling may release harmful minerals or gases from underground which would have to be treated and disposed of. [1, 4, 5]. Third, drilling and water injection may cause ground subsistence or even earthquakes as occurred in Basel, Switzerland in 2006 only eights days after water was injected into the well. [3]. Despite these potential drawbacks, though, deep geothermal energy is a good candidate for a replacement for fossil fuels.


Sources:

[1] The Future of Geothermal Energy – Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Report from the Massachusetts Institute of Technology, 2006 available at http://geothermal.inel.gov (last visited 3/25/2009)

[2] Geothermal Energy Association, http://www.geo-energy.org/aboutGE/basics.asp (last visited 3/25/09)

[3] Wikipedia Entry – Hot Dry Rock Geothermal Energy, http://en.wikipedia.org/wiki/Hot_dry_rock_geothermal_energy (last visited 3/20/09)

[4] Nash, James, Advantages and Disadvantages of Geothermal Energy, http://www.articlexplosion.com/articledetail.php?artid=119588&catid=265 (last visited 3/20/09)

[5] Advantage and Disadvantages of Geothermal Energy, http://www.greenlivinganswers.com/archives/178 (last visited 3/20/09)