Enhanced geothermal systems (EGS)
Enhanced Geothermal Systems (EGS) are a new type of geothermal power technologies that do not require natural convective hydrothermal resources. Until recently, geothermal power systems have only exploited resources where naturally occurring heat, water and rock permeability is sufficient to allow energy extraction from production wells. However, the vast majority of geothermal energy within reach of conventional techniques is in dry and non-permeable rock. EGS technologies “enhance” and/or create geothermal resources in this hot dry rock (HDR) through hydraulic stimulation.
When natural cracks and pores will not allow for economic flow rates, the permeability can be enhanced by pumping high pressure cold water down an injection well into the rock. The injection increases the fluid pressure in the naturally fractured rock which mobilizes shear events, enhancing the permeability of the fracture system. This process, termed hydro-shearing, used in EGS is substantially different from hydraulic tensile fracturing used in the oil & gas industries.
Water travels through fractures in the rock, capturing the heat of the rock until it is forced out of a second borehole as very hot water, which is converted into electricity using either a steam turbine or a binary power plant system. All of the water, now cooled, is injected back into the ground to heat up again in a closed loop.
EGS / HDR technologies, like hydrothermal geothermal, are expected to be baseload resources which produce power 24 hours a day like a fossil plant. Distinct from hydrothermal, HDR / EGS may be feasible anywhere in the world, depending on the economic limits of drill depth. Good locations are over deep granite covered by a thick (3–5 km) layer of insulating sediments which slow heat loss. HDR wells are expected to have a useful life of 20 to 30 years before the outflow temperature drops about 10 degrees Celsius and the well becomes uneconomic. If left for 50 to 300 years the temperature will recover.
There are HDR and EGS systems currently being developed and tested in France, Australia, Japan, Germany, the U.S. and Switzerland. The largest EGS project in the world is a 25 megawatt demonstration plant currently being developed in the Cooper Basin, Australia. The Cooper Basin has the potential to generate 5,000–10,000 MW.
Commercial projects are currently either operational or under development in the UK, Australia, the United States, France and Germany.
The largest project in the world is being developed in Australia’s Cooper Basin by Geodynamics. The Cooper Basin project has the potential to develop 5–10 GW. Australia now has 33 firms either exploring for, drilling, or developing EGS projects. Australia’s industry has been greatly aided by a national Renewable Portfolio Standard of 25% renewables by 2025, a vibrant Green Energy Credit market, and supportive R&D collaboration between government, academia, and industry.
Germany’s 23 cent/kWh Feed-In Tariff (FIT) for geothermal energy has led to a surge in geothermal development, despite Germany’s relatively poor geothermal resource. The Landau partial EGS project is profitable today under the FIT. Drawing on the success of the Landau project EGS Energy are developing sites in the UK. The first of these will be built in collaboration with the Eden Project.
The AltaRock Energy effort is a demonstration project being conducted to prove out the company’s proprietary technology at the site of an existing geothermal project owned and operated by NCPA in The Geysers, and does not include power generation. However, any steam produced by the project will be supplied to NCPA’s flash turbines under a long-term contract.
The recently established Center for Geothermal Energy Excellence at the University of Queensland, has been awarded $18.3 million (AUS) for EGS research, a large portion of which will be used to develop CO2 EGS technologies.
Research conducted at Los Alamos National Laboratories and Lawrence Berkeley National Laboratories examined the use of supercritical CO2, instead of water, as the geothermal working fluid with favorable results. CO2 has numerous advantages for EGS:
- Greater power output
- Minimized parasitic losses from pumping and cooling
- Carbon sequestration
- Minimized water use
EGS Potential in the U.S.
A 2006 report by MIT, and funded by the U.S. Department of Energy, conducted the most comprehensive analysis to date on the potential and technical status of EGS. The 18-member panel, chaired by Professor Jefferson Tester of MIT, reached several significant conclusions:
- Resource Size: The report calculated the United States total EGS resources from 3–10 km of depth to be over 13,000 zettajoules, of which over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements — sufficient to provide all the world’s current energy needs for several millennia. The report found that total geothermal resources, including hydrothermal and geo-pressured resources, to equal 14,000 ZJ — or roughly 140,000 times the total U.S. annual primary energy use in 2005.
- Development Potential: With a modest R&D investment of $1 billion over 15 years (or the cost of one coal power plant), the report estimated that 100 GWe (gigawatts of electricity) or more could be installed by 2050 in the United States. The report further found that the “recoverable” resource (that accessible with today’s technology) to be between 1.2–12.2 TW for the conservative and moderate recovery scenarios respectively.
- Cost: The report found that EGS could be capable of producing electricity for as low as 3.9 cents/kWh. EGS costs were found to be sensitive to four main factors: 1) Temperature of the resource, 2) Fluid flow through the system measured in liters/second, 3) Drilling Costs, and 4) Power conversion efficiency.