Does Geothermal Have Pollution?
Geothermal energy is a renewable energy source that taps into the natural heat inside the earth to generate electricity and provide heating and cooling (https://www.eia.gov/energyexplained/geothermal/). It utilizes the constant temperature of the earth’s interior, which ranges from 4500 to 7000 degrees Celsius just 10km below the earth’s surface, and relies on conduction or convection of this heat towards the earth’s crust (https://www.twi-global.com/technical-knowledge/faqs/geothermal-energy). Wells can be drilled into underground reservoirs to pump hot water or steam to the surface, which can then be used to drive turbines that generate electricity. Alternatively, geothermal heat pumps can tap into shallow ground temperatures for residential and commercial heating and cooling.
Types of Geothermal Systems
There are three main types of geothermal systems utilized for energy production:
Hydrothermal systems rely on naturally occurring pockets of steam or hot water that exist below the Earth’s surface. Wells are drilled into hydrothermal reservoirs to bring the hot water or steam to the surface. The steam can be used to drive turbines and generate electricity, while the hot water can be used directly for heating purposes.
Enhanced Geothermal Systems (EGS) are human-made reservoirs created in hot dry rock through hydraulic fracturing. Water is pumped down injection wells at high pressures, fractures the rock, and creates an artificial geothermal reservoir. The water circulates through the fractures, heats up, and is brought back up production wells as hot water or steam for energy production.
Direct use systems take advantage of shallow ground temperatures for direct heat applications like space heating. These systems circulate water or a working fluid through pipes buried just below the Earth’s surface to provide heating and cooling for buildings, greenhouses, fish farms and other facilities.
Sources:
https://www.energy.gov/energysaver/geothermal-heat-pumps
Geothermal Emissions
Geothermal power plants do release emissions, but at much lower levels compared to fossil fuel plants. The primary emissions from geothermal plants are hydrogen sulfide (H2S), carbon dioxide (CO2), ammonia (NH3), methane (CH4), and trace amounts of mercury. According to the EIA, “Geothermal plants emit on average about 122 kilograms of CO2 per megawatt hour (MWh) of electricity generated, compared to around 1,001 kg CO2/MWh from a typical coal-fired plant.” 1
Hydrogen sulfide emissions occur from geothermal reservoirs and can cause an unpleasant rotten egg smell. Binary cycle plants are designed to reinject geothermal fluids back into the reservoir to minimize emissions. Scrubbers and condensers can also capture over 97% of H2S. 2
Trace amounts of mercury may also be released through geothermal steam, but strict emission regulations and control technology limit mercury pollution. Overall, geothermal emissions are vastly lower compared to fossil fuels when factoring in greenhouse gases.
Water Pollution
Geothermal power plants can contaminate groundwater when water is pumped back into the ground after extracting the heat (U.S. Fish & Wildlife Service). The geothermal fluids brought to the surface contain contaminants like sulfur, silica, and salts. Reinjecting these geothermal fluids underground risks polluting aquifers and underground sources of drinking water (UCSUSA). Additionally, reinjected water can mobilize naturally occurring heavy metals like mercury and arsenic already present in the subsurface environment. These contaminated waters may eventually make their way into rivers, lakes, and streams.
However, most modern geothermal plants are designed with safeguards to prevent groundwater contamination. Plants mainly reinject geothermal fluids into isolated deep wells that don’t interact with drinking water sources (EIA). Proper well construction along with monitoring of fluid volumes and pressures can mitigate risks of aquifer contamination.
Land Subsidence
Land subsidence is one of the negative environmental effects that can be caused by geothermal energy production. It occurs when fluid extraction from underground geothermal reservoirs causes compaction and settling of the overlying ground.[1] As geothermal fluids are pumped out from deep below the earth’s surface, pore spaces in the reservoir rocks can collapse, leading to gradual sinking of the ground above.[2]
This effect has been observed in some geothermal fields where there has been excessive withdrawal of geothermal fluids. For example, parts of the Wairakei geothermal field in New Zealand have sunk by up to 6 feet due to subsidence caused by geothermal production over several decades.[3] However, with careful reservoir management and limits on fluid extraction, subsidence can be minimized. Monitoring ground deformation and limiting the drop in reservoir pressure can help reduce subsidence risks.
Overall, while geothermal energy can trigger subsidence in some cases, careful field management and extraction limits can control this effect. Subsidence is not inherent to all geothermal projects. With proper precautions, geothermal can provide renewable energy while avoiding significant ground deformation.
[1] https://iopscience.iop.org/article/10.1088/1755-1315/42/1/012022
[2] https://pangea.stanford.edu/ERE/pdf/IGAstandard/NZGW/2001/Bloomer.pdf
[3] https://ugspub.nr.utah.gov/publications/open_file_reports/ofr-601.pdf
Noise Pollution
Geothermal power plants can generate noise pollution, especially during construction when drilling and site development occurs. According to the Renewable Energy Fact Sheet, “Geothermal plant construction is loud, but must remain below federal noise pollution limits.” [1]
Once operational, geothermal plants are generally quieter than fossil fuel plants. However, noise can still be an issue for some geothermal facilities that vent steam directly into the atmosphere. As one study found, “The geothermal fluid is directly vented vertically from the wellhead and can generate sound power levels at source of 140 [decibels].” [2] Proper sound insulation and noise mitigation techniques are important to minimize the noise impact on surrounding areas.
Comparing Geothermal Pollution
Though geothermal energy is considered a clean renewable energy source, it is not completely free from environmental impacts. When compared to conventional fossil fuel energy sources like coal and natural gas, geothermal power produces significantly lower emissions of greenhouse gases like carbon dioxide ([1]). According to the U.S. Department of Energy, geothermal plants emit approximately 1/6th of the carbon dioxide of a natural gas power plant and very little nitrous oxide or sulfur dioxide ([2]).
In addition, geothermal plants do not directly produce air pollutants like smog-forming nitrogen oxides, particulate matter, or toxic mercury emissions associated with burning fossil fuels. The steam and hot water used for geothermal is replenished naturally under the earth’s surface, so geothermal is considered a renewable energy source. Unlike coal, oil, and natural gas which are finite resources, geothermal energy is constantly renewed and available.
However, geothermal systems are not completely emission-free. Hot water pumped from underground reservoirs can contain dissolved chemicals and gases that must be disposed of or re-injected underground. Hydrogen sulfide emissions are a concern, though scrubber systems are often used to capture sulfur compounds. Water consumption for geothermal plants is also higher compared to some other renewable sources like wind or solar.
Overall, while geothermal systems have some environmental impacts, they produce dramatically fewer emissions compared to conventional fossil fuel power plants. With proper management and mitigation strategies, geothermal energy can provide a much cleaner renewable energy alternative.
Mitigating Geothermal Pollution
There are several ways to reduce the potential pollution caused by geothermal energy systems.
For geothermal plants that produce electricity, using air cooling towers instead of water cooling towers eliminates the risk of thermal pollution in waterways. Closed-loop geothermal systems also mitigate water pollution risks. Installing zero-liquid discharge systems at plants can capture brine and condense it for disposal rather than discharging it into the environment.
To reduce emissions, hydrogen sulfide abatement systems can remove over 99% of hydrogen sulfide before geothermal gases reach the plant and surrounding environment. Low-sulfur geothermal fields can also be selected. According to the U.S. Department of Energy, “advanced scrubbing systems can reduce chemical emissions to near-zero levels.”
Careful geothermal reservoir management through modeling, monitoring, and strategic reinjection of geothermal fluids helps minimize land subsidence. Operating at optimal pressures and flow rates, and using downhole pumps to reduce water drawdown, also lessens subsidence risks.
Noise pollution can be mitigated by selecting quieter cooling tower fans, improving sound insulation, and using noise barriers. Geothermal plants can also be located an appropriate distance from residential areas.
With careful site selection, advanced emissions controls, zero-liquid discharge systems, and other mitigation measures, geothermal power’s environmental impact can be significantly reduced.
Future Outlook
Research into enhanced geothermal systems (EGS) could help reduce pollution from geothermal energy in the future. EGS technology creates reservoirs in hot, dry rock through hydraulic fracturing, allowing geothermal power plants to be built in areas that don’t have natural hydrothermal resources (1). This expands the potential locations for geothermal plants and also allows them to be built away from groundwater resources, reducing the potential for water pollution.
Advances in EGS could also lead to closed-loop geothermal systems that inject water back underground after use, eliminating surface water discharge. New technologies like supersaturated turbine systems may allow geothermal plants to produce more energy from the same amount of steam, reducing the need for high-pressure pumping that can cause subsidence (2).
With increased adoption, economies of scale could further drive down geothermal costs and pollution. The U.S. Department of Energy has set goals for EGS to provide 100 GW of energy by 2050, which would require significant expansion of geothermal capacity while lowering emissions and water use (1, 2).
Sources:
(1) https://science.howstuffworks.com/environmental/energy/future-geothermal-energy.htm
(2) https://energy.mit.edu/research/future-geothermal-energy/
Conclusion
In summary, geothermal energy does have some potential pollution risks, though they are relatively minor compared to fossil fuel power plants. The main concerns with geothermal are air emissions, water usage and contamination, land subsidence, and noise pollution. However, with proper mitigation strategies and advanced technologies, these impacts can be minimized and controlled.
Overall, geothermal energy is considered one of the cleanest and most sustainable energy sources available today. The emissions are negligible compared to coal and natural gas. Water can be re-injected back into the reservoirs, and technologies like binary power plants dramatically reduce water usage. Subsidence is highly site specific and can be avoided with proper management. And noise pollution is only an issue for those living very close to a geothermal plant.
In conclusion, geothermal energy has the potential for some pollution, but not nearly to the same extent as traditional fossil fuels. With responsible development and management, geothermal can provide clean, renewable power with minimal environmental impact.
