Industry Market Trends

The Damage Done, Part 9 -- Geothermal Is Hot Stuff, but Is It Really Green?

Jan 31, 2012

Drawing of a geothermal systemA report from the Geothermal Energy Association (GEA) says, "The heat continuously flowing from the Earth'?s interior is estimated to be equivalent to 42 million megawatts of power" -- that is, enough to meet the power needs of all the homes on earth more than 100 times over. Geothermal power involves bringing steam or hot water up to the surface from as far as 3 km down in the earth to drive turbines and generate electricity.

(Drawing: Enhanced geothermal system, or EGS. 1: Reservoir; 2: Pump house; 3: Heat exchanger; 4: Turbine hall; 5: Production well; 6: Injection well; 7: Hot water to district heating; 8: Porous rock; 9: Well; 10: Solid bedrock. Credit: Siemens, via Wikimedia Commons.)

As renewable power goes, geothermal electrical generation somehow attains less prominence than other renewables, possibly because its physical manifestation involves drilling shafts in the ground rather than constructing sexy facilities like dams, wind towers, or solar arrays.

Geothermal plant in New ZealandNonetheless, geothermal deserves to be considered in our ongoing series about the "Damage Done" environmentally by electrical power production. Certainly, no important generation method should escape our scrutiny. But, as we shall see, geothermal is shaping up to be an important energy source -- and it stacks up quite well against other sources, when we start comparing external environmental effects. (See my previous articles on coal, natural gas, nuclear, hydro, wind, and solar.) (Photo: Geothermal power plant in New Zealand. Credit: Anne Beaumont, CC BY-SA 2.0.)

The Geothermal Energy Association's May 2010 report, "Geothermal Energy: International Market Update," says that as of that date 10,715 MW of geothermal electrical generation was on line, generating 67,246 GWh (gigawatt hours). Geothermal plants were operating in 24 countries, and 70 countries had projects under development or under active consideration. The association says the U.S. leads the world in geothermal electricity generation with 3,086 MW of installed capacity at 77 power plants. The U.S. is followed by the Philippines with 1,904 MW, Indonesia with 1,197 MW, Mexico with 958 MW, and Italy with 843 MW.

As you can see from this chart developed from Energy Information Administration (EIA) data, even by 2015, geothermal power is projected to account for a relatively small percentage of global renewable electrical generation. Even so, it is still expected to provide more power (112 billion kWH) than solar (at 87 billion kWh).

Global renewables projected for 2015

And this chart, from EIA's "Annual Energy Outlook 2011," seems to predict a small yet growing role for geothermal in the U.S. energy picture.

Renewables projected from 2008 to 2035

However, research suggests that geothermal has the potential to play a greater role than anticipated in U.S. power supply. The United States Geological Survey's (USGS) report, "Assessment of Moderate- and High-Temperature Geothermal Resources of the United States," says agency scientists have found that

[T]he electric power generation potential from identified geothermal systems is 9,057 Megawattselectric (MWe), distributed over 13 states. The mean estimated power production potential from undiscovered geothermal resources is 30,033 MWe. Additionally, another estimated 517,800 MWe could be generated through implementation of technology for creating geothermal reservoirs in regions characterized by high temperature, but low permeability, rock formations.

The Geothermal Energy Association asserts that the geothermal resources of "nine western states together have the potential to provide over 20 percent of national electricity needs," although geothermal provides less than one percent today.

A commissioner at the California Energy Commission recently told Felicity Carus of AOL Energy that the state is considering shifting to geothermal for its baseload power as nuclear and gas-fired plants get retired: "We're looking at 8,000 MW just in our back of the envelope assumptions of what we can do." California got 42 percent of its non-hydro renewable electricity from geothermal in 2011.

Map of US geothermal resources

Geothermal: 100 Percent Green and Clean?

So, given the potential for geothermal for electricity production, what are the environmental concerns?

In its "Guide to Geothermal Energy and the Environment," the Geothermal Energy Association naturally paints a rosy picture of geothermal's environmental impact. However, that doesn't mean they're wrong when they claim that

Because geothermal power plants do not burn fuel like fossil fuel plants, they release virtually no air emissions. A case study of a coal plant updated with scrubbers and other emissions control technologies emits 24 times more carbon dioxide, 10,837 times more sulfur dioxide, and 3,865 times more nitrous oxides per megawatt hour than a geothermal steam plant.

However, a geothermal plant's emissions are not necessarily zero. Exploiting geothermal resources involves drilling, installation of pipelines, and construction of power plants, all of which have environmental impacts. The fluids brought up from the earth can contain carbon dioxide, hydrogen sulfide, methane, and ammonia, as well as toxic chemicals such as mercury and arsenic. These materials do escape into the environment in some quantities.

The following table using the GEA's data shows how emissions from geothermal generation compare to those from coal:


Emission Nitrogen oxide (NOx) Sulfur Dioxide (SO2) Particulate Matter (PM) Carbon Dioxide (CO2)
Geothermal (lb/MWh) 0.00 0.00 - 0.35 0.00 0.00 - 88.8
Coal (lb/MWh) 4.31 10.39 2.23 2,191.00


The GEA report says that "The primary pollutant some geothermal plants must sometimes abate is hydrogen sulfide, which is naturally present in many subsurface geothermal reservoirs." However, the association stresses that "With the use of advanced abatement equipment, ... emissions of hydrogen sulfide are regularly maintained below even California state standards." (California has the highest concentration of geothermal power plants in the world.)

Martin Pehnt of the Institute for Energy and Environmental Research in Germany conducted life cycle assessments for various renewable energy technologies. In Renewable Energy ("Dynamic life cycle assessment (LCA) of renewable energy technologies," Renewable Energy, 2006), Pehnt provides a comparison of the environmental effects of various renewable electrical generation technologies. The following chart shows some of the key measurements he developed. His analysis also compares renewables' environmental effects with those of a conventional energy mix (coal gas, nuclear, etc.) Geothermal appears in the far-right column.


Environmental Impact Conventional Energy Mix impact

Hydro impact

/kWh (large facility)
Wind (on-shore) impact

PV Solar impact

Geothermal impact

Non-Renewable Energy Demand in megajoules (MJ)






Iron Ore in grams (Proxy for non-energy resource consumption)






Global Warming (g CO2 equiv.)






Acidification (mg SO2 equiv., or sulfur dioxide equivalents)






Eutrophication (mg PO43- equiv., or phosphate equivalents)







According to Pehnt's analysis, all of the renewable technologies beat out the conventional energy mix on all metrics except iron-ore consumption, which Pehnt uses as a proxy for the use of physical resources in general. This is understandable, as all generation technologies require the construction of generation plants, and thus the use of physical resources. Geothermal power generation shows lower impacts than solar photovoltaic (PV) on all measurements, although it ranks below hydro and wind by most measures.

Writing in Energy Policy, Benjamin J. Sovacool of the Vermont Law School compares the carbon footprint of a number of power generation technologies. ("Valuing the greenhouse gas emissions from nuclear power: A critical survey," Energy Policy, 2008) Carbon footprint is usually expressed in carbon dioxide equivalents per kilowatt hour, written as gCO2eq/kWh. The following chart shows the carbon footprints from Sovacool's article:


Source/Technology Lifecycle CO2 Equivalents (gCO2eq/kWh)
Coal 960 to 1,050
Natural gas 443
Nuclear 66
Geothermal 38
Solar Photovoltaic (PV) 32
Hydroelectric 10 to 13
Wind 9


I note that the 38 gCO2eq/kWh for geothermal cited by Sovacool is very close to the 41 gCO2eq/kWh figure cited by Pehnt.

Besides the environmental effects already discussed here, the International Geothermal Association acknowledges that exploiting geothermal resources affects the geology of the area of operations:

Extraction of large quantities of fluids from geothermal reservoirs may give rise to subsidence phenomena, i.e., a gradual sinking of the land surface. This is an irreversible phenomenon, but by no means catastrophic, as it is a slow process distributed over vast areas. Over a number of years the lowering of the land surface could reach detectable levels, in some cases of the order of a few tens of centimeters and even meters, and should be monitored systematically, as it could damage the stability of the geothermal buildings and any private homes in the neighborhood. In many cases subsidence can be prevented or reduced by re-injecting the geothermal waste waters.

The organization also acknowledges the possibility of seismic effects:

The withdrawal and/or re-injection of geothermal fluids may trigger or increase the frequency of seismic events in certain areas. However these are microseismic events that can only be detected by means of instrumentation. Exploitation of geothermal resources is unlikely to trigger major seismic events, and so far has never been known to do so.

"The Damage Done" is an ongoing series about the environmental effects of energy technologies, both green and conventional. Here's where you can read the previous articles in the series:

The Damage Done, Part 1 - Is Green Energy Really Better for the Environment?

The Damage Done, Part 2 - How Do You Measure the Environmental Effects of Energy?

The Damage Done, Part 3 - Is Coal Really So Bad for the Environment?

The Damage Done, Part 4 - Natural Gas, Green or Dirty?

The Damage Done, Part 5 - Nuclear Power, the Green That Glows?

The Damage Done, Part 6 - How Green Is Hydro Power?

The Damage Done, Part 7 - Wind Power - Really Green, or Is It Just Spin?

The Damage Done, Part 8 - Does Solar Power Have Its Environmental Dark Side?

Geothermal plant in Iceland


(Photo: Geothermal power plant in Iceland. Credit: ThinkGeoEnergy, CC BY 2.0.)