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Harnessing Earth’s trapped geothermal heat

 By Robin Wylie

How a little-known form of geothermal power generation could open up vast reserves of clean, renewable energy…

From its core to the surface, the earth is a gigantic storehouse of thermal energy. The upper 10 km of the earth’s crust contains an estimated 1.3 x 10^27 Joules — enough to meet the world’s current energy needs for approximately 200 million years.
Of course, only a small fraction of earth’s heat output can realistically be accessed by humans. But with some estimates suggesting that the earth’s technically exploitable geothermal electrical potential is up to 1,200 GW, there is still a giant bounty waiting to be tapped.
But most of it cannot be reached. The majority of geothermal power produced today exploits naturally occurring hydrothermal aquifers. However, the majority of the earth’s hydrothermally heated rocks do not host aquifers, meaning that, with conventional techniques, this heat is out of bounds.
But there is a geothermal technique which can harness geothermal energy in the absence of an aquifer. Known as an “Enhanced Geothermal System” (EGS), this technique creates artificial fractures in impermeable, hydrothermally heated rocks, by injecting cold fluid (usually water). The resulting steam is then used to produce electricity in the same manner as a conventional geothermal plant.

Unlike conventional geothermal, Enhanced Geothermal Systems allow geothermal energy to be harnessed in any setting. They are also more efficient, meaning that the capacity for electricity generation is greater for an EGS system than a conventional geothermal system of the same size.
Compared to conventional geothermal, which has a global capacity of approximately 20 GW, EGS is still a minnow, with only a few tens of megawatts of installed capacity as of 2015. Before the technology can grow to significant levels, questions will need to be answered regarding the potential for induced seismicity, which has been reported, albeit at safe levels, in several EGS drill sites.
Despite this, recent developments suggest that, should the technique be proven safe, EGS could become a significant player in the renewable power sector.

Enhanced Geothermal Energy resesarcher at Idaho National Laboratory, US (Idaho National Laboratory, Flickr)

From the ground up

A handful of EGS test sites have been operated successfully in the United States, Europe and Japan since the mid-1970s. However, it is only in the past decade that EGS has begun to be used to produce power on a commercial scale.
Europe has been the earliest to take up EGS on a significant scale. The first commercial EGS development, a 1.5 MW plant in Soultz-sous-Forets, northern France, came online in 2010, after 14 years of development.
Since then, two MW-scale EGS plants have also come online in Germany: a 3.8 MW plant in Landau, and a 4.8 MW plant in Insheim. The United Kingdom also has plans for two EGS plants, one 10 MW plant and another 3 MW plant, both in located in Cornwall.

The United States — which operated the world’s first experimental EGS plant in New Mexico, between 1974 and 1991 — currently has one commercial EGS plant in operation, a 1.7 MW plant in the Desert Peak Geothermal Field. The plant generates its power using 15 to 27 wells, repurposed from conventional geothermal production.
But the U.S. is looking to advance its EGS program. The country has plans to bring five other EGS plants online by 2020. These include two 5 MW developments, one at the Raft River geothermal site, Utah, and another at the Geysers geothermal field, California. An EGS exploration well has also been drilled at the Newberry Volcano Geothermal Test Facility, in central Oregon, which developers hope could eventually lead to a 35 MW binary EGS geothermal plant.

Australia is another early adopter of EGS. In 2015, a 40 MW EGS power plant was commissioned in 2015 at the Habanero geothermal site, in Southern Australia. A 400 MW EGS power plant is also expected to be constructed in the Olympic Dam region, also in Southern Australia, by 2020. This would make it by the largest EGS plant in the world.
These first few EGS projects will be an important test for the technology, including the issue of induced seismicity, and their success could inspire its use on a wider scale. But significant challenges still need to be overcome.

Operational EGS power plant in Landau, Germany (Claus Ableiter, Wikimedia)

The cost issue

Aside from environmental concerns, the significant barrier to EGS development is its high price. The levelized cost of electricity for EGS currently stands at $0.23-0.35 per kilowatt hour, compared to $0.06-0.08 per kilowatt hour for conventional geothermal.
But with the promise of a clean energy bounty beneath their feet, many countries are currently developing novel techniques to reduce EGS investment costs.
In Europe, for example, the Enhanced Geothermal Innovative Network for Europe (ENGINE) initiative is working towards enhancing geothermal drilling technology in order to reduce EGS drilling costs by 20-30 percent by 2020. And between 2008 and 2014, the United States Department of Energy (DOE) invested approximately $180 million in EGS technology development in the United States, exceeding its investment in conventional geothermal technology.

These initiatives could have a real impact on EGS prices. In Europe, EGS costs are expected to drop below $0.12 per kilowatt hour by 2030. The DOE is also aiming to reduce EGS costs to $0.06 per kilowatt hour in the U.S. by 2030.
Researchers also envision a bright future for EGS. A new study in Renewable and Sustainable Energy Reviews concluded that there is an 85 percent chance of global installed EGS capacity reaching 70 GW by 2050. And a report by the Massachusetts Institute of Technology states that with a modest R&D investment of $1 billion over 15 years (around the cost of one coal power plant) 100 GW or more of EGS capacity could be installed by 2050 in the United States alone.
If these forecasts are correct, and the technology can be applied responsibly, EGS could come to significantly boost the contribution of the earth’s internal heat to humanity’s clean energy future.

SEE MORE: Moving up the green energy charts by Amanda Saint

about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.