Geothermal Energy: Pollution Or Clean Power Source?

Geothermal energy is one of the cleanest forms of energy used for electricity generation today. However, it is not entirely free of environmental concerns. The environmental impacts of geothermal energy depend on how it is used and converted into useful energy. Geothermal power plants emit low levels of greenhouse gases, such as carbon dioxide, methane, nitrous oxide, and sulfur dioxide. Open-loop systems release these gases and other pollutants into the atmosphere, while closed-loop systems inject them back into the ground, minimising air emissions. Water quality and consumption are also impacted by geothermal plants, which require large amounts of water for cooling and re-injection. Overall, the environmental impacts of geothermal energy are generally considered to be lower than those of fossil fuel power plants, but there are still some pollution and health risks associated with its use.

Geothermal energy and air pollution

Geothermal energy is heat contained below the Earth's surface. The only type of geothermal energy that has been widely developed is hydrothermal energy, which is generated by trapped hot water or steam. Geothermal power plants use this steam or hot water to turn a turbine connected to a generator, producing electricity.

Air and water pollution are two leading environmental issues associated with geothermal energy technologies. In particular, air pollution is largely dependent on the type of system used by the geothermal power plant. Closed-loop systems directly convert geothermal steam or hot water into electricity, and the gases or fluids removed from the well are injected back into the ground, minimising air emissions. In contrast, open-loop systems expel waste steam and gases into the atmosphere, including hydrogen sulfide, carbon dioxide, ammonia, methane, and boron.

Hydrogen sulfide, which has a distinctive "rotten egg" smell, is the most common emission from open-loop systems. Once in the atmosphere, it changes into sulfur dioxide (SO2), contributing to the formation of small acidic particulates that can be absorbed by the bloodstream and cause heart and lung disease. Sulfur dioxide also causes acid rain, which damages crops, forests, and soils, and acidifies lakes and streams. However, it is important to note that SO2 emissions from geothermal plants are significantly lower per megawatt-hour than those from coal plants, which are the nation's largest SO2 source.

Geothermal power plants emit 97% less acid rain-causing sulfur compounds and about 99% less carbon dioxide than fossil fuel power plants of similar size. Additionally, geothermal power plants do not burn fuel to generate electricity, further reducing their environmental impact.

Water contamination

To address these issues, most geothermal facilities have closed-loop water systems, where extracted water is pumped directly back into the geothermal reservoir after use. This prevents water contamination and land subsidence. In these systems, the water is contained within steel well casings cemented to the surrounding rock, ensuring that it is isolated from the environment, including groundwater. The safe and non-polluting use of geothermal fluids has been carefully developed and rigorously tested, with repeated examinations to ensure no leakage occurs.

However, it is important to distinguish between open-loop and closed-loop systems when considering water emissions. In open-loop systems, gases removed from the well are released into the atmosphere, leading to the emission of hydrogen sulfide, carbon dioxide, ammonia, methane, and boron. These gases contribute to air pollution and can have further indirect effects on water quality.

While there have been no reported cases of water contamination from geothermal sites in the United States, studies from other parts of the world, such as Turkey, have found heavy metal contamination in drainage channels and groundwater near geothermal fields. This contamination has been linked to the use of geothermal fluid in greenhouse cultivation, spa tourism, and agricultural activities.

Overall, the potential impact of geothermal energy on water contamination depends on the specific technology and cooling systems used. Geothermal power plants must carefully manage their water usage and emissions to minimize any potential negative effects on the environment.

Health hazards

The use of geothermal energy can cause pollution and poses several health hazards. Geothermal power plants emit gases such as hydrogen sulfide, carbon dioxide, ammonia, methane, and boron. While these emissions are significantly lower than those of fossil fuel power plants, they can still contribute to air pollution and cause health issues. For example, hydrogen sulfide, which has a distinctive "rotten egg" smell, can cause heart and lung disease when inhaled. Additionally, once in the atmosphere, hydrogen sulfide changes into sulfur dioxide (SO2), which causes acid rain and damages crops, forests, and soils.

Another health hazard associated with geothermal energy is the release of toxic substances during accidents or improper handling. For example, employees working in geothermal power plants have been injured or killed due to accidents involving hot water, steam, or toxic gases. In one incident, an employee died from asphyxia after being crushed by dirt when a trench he was working in caved in. In another incident, an employee suffered severe burns when a side hatch cover suddenly opened, spraying hot water. Additionally, the use of chemicals such as WD-40 and Graff-off in confined spaces can lead to dizziness, nausea, and breathing difficulties, as evidenced by a reported incident where an employee was diagnosed with pulmonary edema.

Furthermore, the brine or saltwater solutions used in some geothermal systems may pose hazards to workers. The exact nature of these hazards is not specified, but incidents involving these solutions can have serious consequences. For instance, an employee of Terra-Gen Operating Company, LLC suffered second- and third-degree burns after falling into a pool of heated geothermal water.

The environmental impacts of geothermal energy can also have indirect health effects. Land subsidence, caused by the removal of water from geothermal reservoirs, can damage infrastructure and contribute to ecological changes. Additionally, the siting of geothermal plants on geological "hot spots" can increase earthquake frequency and pose risks to nearby populations.

Overall, while geothermal energy has relatively low emissions compared to fossil fuel power plants, there are still health hazards associated with its use. These include direct risks to workers, as well as indirect risks to the general population through air and water pollution and ecological changes.

Solid waste disposal

While geothermal energy has a much lower environmental impact than fossil fuels, it is not without its drawbacks. Solid waste disposal is one area that requires careful consideration and management.

Firstly, it is important to note that many geothermal power plants do not generate significant amounts of solid waste. However, the solid byproducts that are produced often contain valuable minerals and metals, such as silica, sulfur, vanadium, arsenic, mercury, and other heavy metals. These byproducts can be recycled and reused in various industrial processes, reducing waste and promoting a more circular economy.

For example, at the Salton Sea geothermal power plants in California, the corrosive salts and heavy metals present in the mineralized geothermal brine are removed through a dewatering process. The remaining solids are rich in silica, which is an essential raw material in several industrial applications. Similarly, at The Geysers in northern California, valuable minerals and metals are recovered from the hydrogen sulfide treatment systems, with hydrogen sulfide being converted into elemental sulfur, which is then used in sulfuric acid production.

In terms of waste disposal, the safe disposal of hazardous waste is a critical consideration for geothermal energy facilities. This includes the management of toxic sludge produced by scrubbers, which capture and remove harmful gases such as hydrogen sulfide, sulfur dioxide, carbon dioxide, ammonia, methane, and boron from the air emissions of geothermal plants. The sludge contains captured materials, including sulfur, vanadium, silica compounds, chlorides, arsenic, mercury, nickel, and other heavy metals. Proper disposal of this toxic sludge at hazardous waste sites is essential to prevent environmental contamination.

Additionally, open-loop geothermal systems, which expel waste steam and gases directly into the atmosphere, can contribute to air pollution. These systems emit gases such as hydrogen sulfide, carbon dioxide, ammonia, methane, and boron. While open-loop systems have higher environmental impacts, closed-loop systems minimize air emissions by injecting gases and fluids removed from the well back into the ground.

To ensure the safe disposal of solid waste, federal, state, and local regulations mandate the use, concentrations, transportation, and disposal of hazardous materials. Comprehensive operations plans, including specific emergency protocols, are implemented to mitigate risks and reduce accidents at geothermal sites. These plans cover various scenarios, such as accidents, fires, injuries, spills, and discharges, with appropriate mitigation measures in place. Proper staff training is also crucial in minimizing accidents and improving emergency response capabilities.

Land subsidence

The decrease in ground surface level can have devastating effects on the construction of buildings, pipelines, canals, and rivers. It can also interrupt the balance of nearby ecosystems. In the context of geothermal energy, the removal of geothermal fluid from underground reservoirs can lead to land subsidence due to a drop in pressure, the presence of compressible rock formations, or the existence of high-permeability paths. This is more commonly observed in liquid-dominated fields, where reinjection practices are not implemented to maintain reservoir pressures.

To mitigate the risk of land subsidence, good management practices and survey methods, such as levelling and gravity measurements, can be employed. Additionally, the common practice of injecting spent geothermal fluids back into reservoirs helps to prevent subsidence from occurring.

The impact of land subsidence can be observed in the Wairakei field in New Zealand, where fluid withdrawal resulted in a maximum ground subsidence of 14 meters between 1950 and 1997. This caused structural damage to pipelines, roads, concrete drains, and transmission lines.

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