This third in the series of articles looking a geothermal power will seek to address the environmental impact of geothermal plants.
Typically, geothermal plants do not make use of fuel through the act of burning like fossil fuel plants do, and therefore do not produce air emissions like fossil fuel plants do. The fumes, which look like smoke, which are attributed to most open loop geothermal plant systems, and can be observed from the atmosphere surrounding these plants, are most likely to be water vapour. Nevertheless, there are four known important pollutants that are emitted generally by open loop geothermal plants. These pollutants include:
1) nitrogen oxide (NO)
2) sulphur dioxide (SO'2)
3) particulate matter (PM)
4) Carbon dioxide (CO2).
Of the four substances, carbon dioxide (CO2) is released into the atmosphere at the highest concentration with levels ranging from 0-40Kg/MW.
Studies have shown that geothermal plants do not emit sulphur dioxide directly into the atmosphere, as coal fired power plants do. Instead hydrogen sulphide is released as a gas into the environment, which is then ultimately converted into sulphur dioxide and sulphuric acid. Consequently any substantial amount of the sulphur dioxide that is found in the surrounding atmosphere of geothermal plants is simply a derivative of hydrogen sulphide.
There are several negative health effects that nitrogen oxide, sulphur dioxide, particulate matter and carbon dioxide can have on the general population. Nitrogen oxide has been known to cause varied symptoms of lung irritation including coughing, wheezing and persistent colds. Nitrogen oxide also contributes to smog.
Nitrogen oxide contamination can also impact on the quality of the water, making the water not very safe for human consumption, and in particular, school aged children. Sulphur dioxide causes more severe effects that can lead to wheezing, tightness of the chest, serious respiratory illnesses, and long lasting damage to the ecosystem. Particulate matter has been known to aggravate the symptoms of asthma, and cause bronchitis and certain systemic cancers such as lung cancer. It can lead to atmospheric deposition of the environment and impaired visibility for all types of aircrafts. Global warming is consequently caused by too much carbon dioxide in the atmosphere.
Apart from the four main pollutants mentioned above it has also been determined that mercury may be present in areas where geothermal power plants are in operation. In these cases, equipment is specifically designed to reduce the level of the substance in its surrounding environment and to effectively wipe out the emission levels by as much as 90% or more.
The method used to convert geothermal steam or hot water to electricity directly affects the amount of waste generated. Closed-loop systems produce almost none of the pollutants listed earlier, since gases or fluids removed from the geothermal well are not exposed to the atmosphere and are usually re-injected back into the ground after giving up their heat. Although this technology is more expensive than conventional open-loop systems, in some cases it may reduce scrubber and solid waste disposal costs enough to provide a significant economic advantage.
Open-loop systems, on the other hand, can generate large amounts of solid waste as well as noxious fumes. Metals, minerals, and gases leak out into the geothermal steam or hot water as it passes through the rocks. The large amounts of chemicals released when geothermal fields are utilised for commercial production can be hazardous or objectionable to people living and working nearby.
At The Geysers, one of the largest geothermal development in California in USA, steam vented at the surface contains hydrogen sulphide (H2S) accounting for the "rotten egg" smell in the area as well as ammonia, methane, and carbon dioxide. At hydrothermal plants carbon dioxide is expected to make up about 10% of the gases trapped in geo-pressured brines. For each kilowatt-hour of electricity generated, however, the amount of carbon dioxide emitted is still only about 5% of the amount emitted by a coal or oil fired power plants.
Scrubbers reduce air emissions but produce a watery sludge high in sulphur and vanadium; vanadium is a heavy metal that can be toxic in high concentrations. Additional sludge is generated when hydrothermal steam is condensed, causing the dissolved solids to precipitate out. This sludge is generally high in silica compounds, chlorides, arsenic, mercury, nickel, and other toxic heavy metals. One costly method of waste disposal involves drying it as thoroughly as possible and shipping it to licensed hazardous waste sites. Research under way at Brookhaven National Laboratory in New York points to the possibility of treating these wastes with microbes designed to recover commercially valuable metals while rendering the waste non-toxic.
Usually the best disposal method is to inject liquid wastes or re-dissolved solids back into a porous stratum of a geothermal well. This technique is especially important at geo-pressured power plants because of the sheer volume of wastes they produce each day. Waste must be injected well below fresh water aquifers to make certain that there is no communication between the usable water and waste-water strata. Leaks in the well casing at shallow depths must also be prevented. If waste is allowed to contaminate water tables, or is released into rivers or lakes instead of being injected into the geothermal field or processed and disposed of safely, these pollutants can damage aquatic life and make the water unsafe for drinking or irrigation, impacting on surrounding wild life and habitat.
There are other environmental effects to be considered. Extracting geothermal fluids can reduce the pressure in underground reservoirs and cause the land to sink. This is known as Subsidence. The largest subsidence on record is at Wair'kei in New Zealand, where the centre of the subsidence bowl is sinking at a rate of almost half a metre every year. In 2005 the ground was 14 metres lower than it was before the power station was built. As the ground sinks it also moves sideways and tilts towards the centre. This puts a strain on bores and pipelines, may damage buildings and roads, and can alter surface drainage patterns. Subsidence may usually be controlled or eliminated by the re-injection of the geothermal fluid in the region of the well, typically at a level deeper than the original tapping well, once the heat is extracted.
One of the major faults' with all geothermal plants is that they are almost always located directly above or very near a fault; by its nature geothermal occurs in technically active areas. Power production practices of re-injection, serving both to dispose of toxic geothermal fluids while at the same time helping to replenish and somewhat prolong reservoir use and combat subsidence, have been linked with induction of seismic activity. Studies conducted by scientists with the U.S. Geological Survey concluded that geothermal power production induces seismicity. One of the possible reasons for this is that the re-injection of the spent fluids, which is generally done at a deeper level than the original tapping well, lubricates the different fault line plates as well as altering the pressure upon them, causing them to slip. Another theory is that tapping the geothermal reservoirs depletes the pressure built up underground causing the plates to shift. Perhaps both of these factors work synergistically to cause minor quakes in the magnitude of 3.5 to 4.0 on the Richter scale. The Geysers in the California USA area has experienced quakes of these magnitudes that have been associated with geothermal production. Geothermal production areas in Mammoth Lakes, California have also experienced swarms of quakes. Studies are on-going attempting to further understand the correlation between commercial geothermal energy production and tectonic activity. One indicator has been notable land subsidence in the areas above geothermal reservoirs.
This is by no means an exhaustive look at the environmental impact of geothermal power production; there are still many studies being conducted on existing systems. It is also important to note that every geothermal system that exists is unique in its own right and may not only possess is own unique set of characteristics, but also its own unique set of unpredictable behaviours.
Editor-in-Chief's Note: Daniel Magnusson is a freelance contributor with MNI Alive.