In an international community increasingly worried about worsening effects of climate change, geothermal can play an important role in reducing air emissions. Experts generally agree that effects of climate change pose significant environmental dangers, including flood risks, drought, glacial melting, forest fires, rising sea levels, loss of biodiversity, and potential health dangers. Geothermal involves no combustion, and most geothermal plants being developed will produce nearly zero air emissions. So, using geothermal helps to offset energy-related carbon dioxide, which accounted for 82% of greenhouse gas (GHG) emissions in the U.S. in 2011.
Using geothermal also eliminates the mining, processing, and transporting required for electricity generation from fossil fuel resources; and, it has among the smallest surface land footprint per kilowatt (kW) of any power generation technology.
Geothermal power plants are designed and constructed to minimize the potential effects on wildlife and vegetation in compliance with a host of state and federal regulations. A thorough environmental review is required before construction of a generating facility can begin. Subsequent monitoring and mitigation of any environmental impacts continues throughout the life of the plant.
5.1. How effectively does geothermal help in improving air quality and decreasing greenhouse gas emissions?
At geothermal power plants, billows seen rising from cooling towers are composed of water vapor or steam, not burned fuel or smoke emissions, and are caused by the evaporative cooling system. A binary or flash/binary geothermal plant produces nearly zero air emissions; air emission levels at dry steam plants are considered to be slightly higher, because even without human intervention, geothermal systems already contain naturally-occurring dissolved gases. The exact relationship between human-caused and naturally occurring geothermal emissions at geothermal power plant sites is difficult to characterize, and varies based on the site’s unique resource chemistry, the resource temperature, type of power plant, and a number of other factors. In summary, though, geothermal technology is considered environmentally benign and any emissions are negligible when compared with using technologies that involve combustion of fossil fuels (Figure 15).
Figure 15. Comparison of Coal, Natural Gas, and Geothermal CO2 Emissions
GEA estimated, “When comparing the CO2 emissions data obtained from the Environmental Protection Agency (EPA) and Energy Information Administration (EIA) for coal and natural gas power plants, the average rate of carbon dioxide emissions for coal-fired power plants and natural gas power plants are 2200 lbs CO2/MWh and 861 lbs CO2/MWh, respectively. Geothermal systems, on the other hand, produce significantly less emissions, approximately 197 lbs CO2/MWh.” Most new geothermal plants being built in the U.S. use binary technology, which produce zero or near-zero emissions (see section 1, Technology Basics).
GEA estimated the externality benefits of producing electricity using geothermal resources as opposed to fossil fuels are $0.01 for natural gas and $0.035 for coal per kWh. Geothermal provides approximately $278 million in externality benefits per year to the entire U.S. by avoiding fossil fuel emissions.
Nevada’s 300 MW of geothermal power can save 4.5 million barrels of oil (the equivalent fuel used by 100,000 cars) and avoid emissions of 2.25 million tons of CO2 annually.
Lake County, California, located downwind of The Geysers geothermal complex, has met all federal and state ambient air quality standards since the 1980s. An abatement system at the complex actually improves air quality by processing natural hydrogen sulfide, which would ordinarily be released into the atmosphere by hot springs and fumaroles, and reducing the amount released by 99.9%.
Reducing power plant emissions has substantial benefits to public health and associated costs. Clean Air Task Force estimated, in 2010, that the healthcare cost for illness and premature death associated with coal plant impacts in the U.S. alone exceeds $100 billion/year
5.2. How much land does geothermal energy use?
GEA “Promoting Geothermal Energy”
This included 13,200 deaths, 9,700 hospital admissions, 20,400 heart attacks, and over 1.6 million lost work days directly resulting from national power plant impacts.
Geothermal development activities result in lower long-term land disturbance than other technologies (Figure 16). In its 2008 Programmatic Environmental Impact Statement, the BLM estimated that the total surface disturbance for a geothermal power plant ranges from 53 to 367 acres. This covers all activities such as exploration, drilling, and construction, and reflects variability in actual area of land disturbance based on site conditions and the size and type of geothermal plant. Much of this land is reclaimed after the exploration, drilling, and construction phases of development, so the long-term land use is much lower.
Figure 16: Thirty-Year Land Use Comparison
Figure 17 breaks out land use throughout development, assuming plant sizes of a range between 30 MW and 50 MW.
Figure 17. Typical Disturbances by Phase of Geothermal Resource Development
Figure 17 Source: Bureau of Land Management
Geothermal plants are constructed to blend in with their environmental surroundings, minimizing the land use footprint and often allowing for activities such as farming, skiing, and hunting on the same lands in compliance with the BLM’s multiple use strategy. Pipelines, for example, which connect the geothermal resource base to the power plant, can be elevated so that small animals can roam freely and native vegetation can flourish. Natural color paint is a BLM requirement for power plants and piping on public land: for example, Ormat’s Mammoth Geothermal Power Plant on the eastern slope of the Sierra Mountains in California blends in with the high-desert terrain. Surface features such as geysers or fumaroles are not used during geothermal development, though some deterioration may occur if located near a facility, so sometimes special efforts are made to prevent this, especially if the features are of cultural significance.
U.S. Department of the Interior “Final PEIS for Geothermal,” page ES-8 and Table 2-8
U.S. Department of Energy “Program Areas”
5.3. How do geothermal developers control noise levels?
During drilling, temporary noise shields can be constructed around portions of drilling rigs. Geothermal developers use standard construction equipment noise controls and mufflers, shield impact tools, and exhaust muffling equipment. Once the plant is built, noise from normal operation of power plants comes from cooling tower fans and is very low. Turbine-generator buildings, designed to accommodate cold temperatures, are typically well-insulated acoustically and thermally and are equipped with noise absorptive interior walls.
When noise issues arise, they can be dealt with effectively in ways that do not impact plant performance. Enel Green Power North America addressed high noise levels at its Stillwater, Nevada geothermal plant: “In response to unexpected high noise levels experienced during the start-up of the Stillwater Geothermal facility, Enel Green Power North America’s Nevada-based geothermal team worked diligently to design Acoustical Energy Dissipaters, or Silencers. The purpose of the Silencer is to significantly reduce the sound levels caused by the acoustical energy flowing in the discharge piping of the turbine, without affecting turbine performance and plant output . . . The final product not only addressed a technical issue, but also helped the Company effectively respond to community concern about noise levels from the plant.”
Enel Green Power application for GEA’s 2011 Honors awards
5.4. How much water do geothermal plants use?
Water is commonly used in electricity production across the spectrum of generating technologies. The amount of water used in geothermal processes varies based on the type of resource, type of plant, type of cooling system (wet/dry or hybrid cooling), and type of waste heat reinjection system.
Water is a critical component of geothermal systems. In a conventional system, it comes from the geothermal system source and is reinjected back into the reservoir to maintain reservoir pressure and prevent reservoir depletion. Rainwater and snowmelt feed underground thermal aquifers, naturally replenishing them. Geothermal resources are considered renewable on timescales of technological and societal systems, meaning that unlike fossil fuel reserves, they do not need geological times for regeneration when reinjection is done properly.
Reinjection keeps the mineral-rich saline water found in geothermal systems separate from groundwater and fresh water sources to avoid cross-contamination. Injection wells are encased by thick borehole pipe and are surrounded by cement. Once the water is returned to the geothermal reservoir, it is reheated by the Earth’s hot rocks and can be used over and over again to produce electricity.
For lifetime energy output, flash geothermal plants consume 0.01 gal/kWh; binary plants consume between 0.08 and 0.271 gal/kWh; and EGS projects consume between 0.3 and 0.73 gal/kWh (Figure 18; Table 4-3 of DOE “Water use”) , .
In 2011, Argonne National Lab found: “Average values of [life cycle water] consumption for coal, nuclear, and conventional natural gas power plant systems are higher than for geothermal scenarios. . . . With the exception of geothermal flash, which primarily relies on the geofluid in the reservoir for cooling, PV appears to be more water efficient, with consumption estimates of 0.07–0.19 gal/kWh. Overall, the geothermal technologies considered in this study appear to consume less water on average over the lifetime energy output than other power generation technologies.”
Geothermal energy can make use of wastewater that might otherwise damage surface waters (see section 3.2.3.) Additionally, studies have shown condensate at geothermal power plants could potentially be used to produce potable water, but no completed projects have thus far incorporated this. Section 3 includes a discussion of mineral recovery from geothermal water at power plant sites.
Figure 18. Aggregated Water Consumption for Electric Power Generation, Lifetime Energy Output
Figure 18 Source: Argonne National Laboratory
5.5. Does seismic activity affect geothermal applications (and vice versa)?
Reinjection to protect groundwater resources is a requirement for most geothermal applications under the EPA Underground Injection Control Program requirements, BLM, and state well construction requirements.
These numbers provided by Argonne are aggregated values from several sources including the Electric Power Research Institute, DOE, and The National Energy Technology Laboratory. Argonne notes in its report that some of the sources used modeling outputs rather than data from power plants.
Includes water consumed for drilling wells; assumes freshwater withdrawal. Flash systems use very little fresh water, while air-cooled binary plants use essentially no potable water.
U.S. Department of Energy “Water Use,” page 26
Geothermal Development Associates of Reno, Nevada worked on a design for a power plant in Djibouti, East Africa that would have produced potable water.
Seismicity is a natural geological phenomenon in geothermal areas. Geothermal operations can create low-magnitude events known as microearthquakes. These events typically cannot be detected without sensitive equipment.
The careful study and understanding of a geothermal reservoir’s seismic levels is included in a company’s preparation prior to development, and many geothermal companies monitor for induced seismicity throughout the life of a plant. According to BLM, “seismic risk is more likely to impact geothermal facilities than operation of geothermal facilities is to increase seismic risk.”
To address public concern, the U.S. DOE commissioned a risk assessment and a revised induced seismicity protocol in 2012. The authors met with the domestic and international scientific community, policymakers, and other stakeholders to gain their perspectives. They incorporated lessons learned from EGS projects around the world to better understand the issues. The protocol concluded that with proper study and technology development, induced seismicity will not only be mitigated, but will become a useful tool for reservoir management.
The reinjection of geothermal water practiced by most geothermal plants on line today (see section 5.4.) results in a near-zero net change in the resource. This is distinguishable from the practice of directly injecting high-pressure fluids into fault zones, which has been linked to micro-seismicity in some cases.
National Academy of Sciences “Induced Seismicity”
U.S. Department of the Interior “Final PEIS for Geothermal” pp. 4-18
U.S. Department of Energy “Protocol”
Updated April 3, 2014 by L. Blodgett
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