Geothermal Power: Pollution Or Progress?

Geothermal energy is a renewable and sustainable source of energy that has the potential to become a major global energy source. It is derived from the heat found inside the Earth's surface, which is used to operate electricity-generating steam turbines. However, despite its benefits, geothermal energy is underutilized compared to other renewable sources like wind and solar. This is partly due to its high upfront costs but also because of concerns about its environmental impact. So, can geothermal power pollute?

Water contamination

Most geothermal facilities have closed-loop water systems, where extracted water is pumped directly back into the geothermal reservoir after use, contained within steel well casings cemented to the surrounding rock. This prevents water contamination and land subsidence. In the United States, there have been no reported cases of water contamination from geothermal sites.

However, not all water removed from the reservoir is re-injected, as some is lost as steam. To maintain a constant volume of water in the reservoir, outside water must be used. The amount of water needed depends on the plant's size and technology. For example, the Geysers geothermal site in California uses non-potable treated wastewater for this purpose.

Geothermal plants also use water for cooling and re-injection. All US geothermal power facilities employ wet-recirculating technology with cooling towers. Depending on the cooling technology, plants can require between 1,700 and 4,000 gallons of water per megawatt-hour. Geothermal plants can use either geothermal fluid or freshwater for cooling, with geothermal fluid reducing the overall water impact.

Air pollution

However, it is important to note that the amount and impact of these emissions depend on the type of geothermal power plant and cooling technology used. Closed-loop systems, where gases and fluids are injected back into the ground, have minimal air emissions. On the other hand, open-loop systems release gases and waste steam into the atmosphere, resulting in greater environmental impacts.

Geothermal power plants emit 97% less sulfur compounds and about 99% less carbon dioxide than fossil fuel plants of similar size. Additionally, scrubbers are used to remove hydrogen sulfide from the geothermal reservoirs, and most plants recycle geothermal steam and water by injecting them back into the earth, further reducing emissions.

Furthermore, binary-cycle geothermal plants, which operate in a closed cycle, release essentially zero emissions. According to Argonne National Laboratories, binary power plants have some of the lowest emissions of any electricity-generating technology, with greenhouse gas emissions of approximately 5.7 gCO2eq/kWh.

Land use

The amount of land required by a geothermal plant depends on the properties of the resource reservoir, the amount of power capacity, the type of energy conversion system, the type of cooling system, the arrangement of wells and piping systems, and the substation and auxiliary building needs. The Geysers, the largest geothermal plant in the world, has a capacity of approximately 1,517 megawatts and takes up approximately 78 square kilometres of land, or about 13 acres per megawatt.

Geothermal power plants tend to have a lower profile and smaller land footprint compared to many other energy-generation technologies, and they do not require fuel storage, transportation, or combustion. They are more compact than comparable-capacity coal, wind, and solar photovoltaic power stations.

However, geothermal power stations still take up space and can leave a mark on the environment. The extraction of geothermal fluids has been linked to seismic instability and land subsidence. In the process of extracting geothermal energy, superheated fluids are drawn from deep beneath the Earth's surface, leaving an empty pocket. Over time, the land above this pocket may subside to fill the space, affecting both man-made structures and the environment.

To minimise the impact of land subsidence, most geothermal facilities re-inject wastewater back into geothermal reservoirs after the water's heat has been captured. The Geysers geothermal site in California, for example, injects non-potable treated wastewater into its geothermal reservoir.

Noise pollution

To mitigate noise impacts, specialised equipment is used at geothermal projects, and employees are trained on health risks associated with prolonged exposure to excessive noise. This training includes information on how to wear personal protective earwear and reduce noise while working.

Prior to construction, noise surveys are conducted to measure the existing background noise at the site. This helps determine the allowable noise levels and operational hours for the project. An acoustic engineer is also responsible for monitoring sound impacts throughout the project.

Mitigation measures are implemented to reduce noise impacts, such as restricting site traffic to specific hours, coordinating the arrival and departure of oversized vehicles with local traffic schedules, and setting up the drill rig to shield the motor and air compressors from nearby receptors. Additionally, the drill rig floor and V-door are reinforced with metal plates lined with wood or rubber to muffle the noise from rattling pipes.

Earthquakes

Geothermal power plants can cause earthquakes, a phenomenon that has been observed in several locations. In 2017, a magnitude 5.5 earthquake hit Pohang, South Korea, injuring dozens and leaving more than 1,700 residents homeless. This quake has been attributed to the development of a geothermal energy project. The Pohang earthquake is the largest ever linked to an enhanced geothermal system, which involves creating new pathways for the Earth's heat to reach the surface and generate power.

Geophysicist William Ellsworth explains that the process of pumping the Earth under high pressure is known to cause earthquakes. In the case of Pohang, the injection of fluids at high pressure activated a previously unknown fault, reducing the forces that would usually prevent it from moving. This led to a series of small earthquakes, which grew in magnitude over time.

A similar situation occurred at the Salton Sea Geothermal Field in southern California, where a strong correlation has been found between seismic activity and operations for geothermal power production. The rate of earthquakes in the region increased as geothermal operations expanded, with both climbing steadily after 2001.

The Geysers, a long-term geothermal project in northern California, has also been associated with seismic activity. The USGS has recorded about 4,000 quakes above magnitude 1.0 every year in the area, which are believed to result from steam withdrawal or injection. Residents of nearby Anderson Springs often experience tremors as small as magnitude 2.0.

While the drilling itself does not cause earthquakes, the removal of steam and water can create new instability along fault or fracture lines. This instability can lead to minor tremors and, in some cases, larger quakes.

To minimise the risk of earthquakes, geothermal projects should be sited an appropriate distance away from major fault lines. Constant monitoring and transparent communication with local communities are also crucial when a geothermal system is located near populated areas.

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