Environmental Impacts of Geothermal Energy Generation and Utilization
The extent to which geothermal exploitation affects the environment is proportional to the scale of its exploitation.
Geothermal Power - how it is done
Unlike most energy sources that require heat to be manufactured by humans, geothermal energy utilizes the natural heat of the earth and its tectonic processes. In order to access this energy, wells are drilled into areas below the crust where there are aquifers of already heated water, or steam. As pressure increases the deeper into the earth, water is unable to turn into steam as it is heated because there is so much pressure. As a result, "superheated water" is produced at very deep depths of the earth.
As wells are drilled into the rock that houses this water, its steam is converted into mechanical energy for utilization. There are three main types of geothermal plants: direct dry steam, flash, and binary plants. In a direct dry steam plants steam goes directly to a turbine which drives the generator that produces electricity. Flash plants utilize man-made changes in pressure to vaporize the hydrothermal fluid. The fluid is placed in a tank that is held at a much lower pressure than the fluid causing it to vaporize, and this vapor then moves the turbine which then generates the electrical power. In a binary cycle plant such as Ormat's Olkaria III plant in Naivasha, Kenya, hot water and steam rises through the production well. This is a deep hole that is drilled for the insertion of a pipe that conveys the high-temperature water and steam that is to be pumped from the ground. The hot water and steam are directed to a heat exchanger alongside the working fluid. Fluids that have much lower boiling points than water are used as working fluids.
The vapor produced by the heat exchanger is directed to the turbines which cause it to rotate at high speeds creating energy. The energy produced by the turbine is then transformed in the generator from where it goes to the power lines. After the steam has been used, it is directed to a condenser where it is turned back into water and re-injected back into the ground through the injection well. The water that rose with the steam also gets returned via the re-injection well.
Binary plants operate by passing hot water through a heat exchanger with another fluid that has a much lower boiling point (isobutene, pentafluoropropane), the fluid called binary or working fluid then vaporizes which powers the generators and moves the turbines of the plant. Binary plants are more common than steam plants because they can operate at colder temperatures which make finding suitable locations for energy extraction much easier than for steam plants.
Advantages of geothermal energy
Geothermal energy is seen as a viable form of energy because it is clean and it is renewable. Another advantage of geothermal energy is that it can be extracted without burning a fossil fuel such as coal, gas or oil. Geothermal fields produce only about one sixth of the carbon dioxide that a natural gas fueled power plant produces. Binary geothermal plants release essentially no emissions. Unlike solar and wind energy, geothermal energy is available constantly. Lastly, geothermal energy is relatively inexpensive; savings from direct use can be as much as 80 percent over fossil fuels.
Environmental impact of a geothermal power plant
The utilization of energy from geothermal wells releases greenhouse gases trapped in the earth core such as carbon dioxide, hydrogen sulfide, methane, and ammonia. These emissions are lower than those associated with the use of fossil fuels for which the adoption of geothermal energy sources is considered to have the potential to mitigate global warming and have a favorable impact on the environment. While the environmental effect of geothermal energy generation may be favorable if compared to other sources of energy generation; however it is not insignificant and can cause substantial environmental and human heath deleterious effects. In 1975 Robert Axtmann studied the environmental effects of the Wairakei geothermal plant in New Zealand. He concluded that the Wairakei plant discharged approximately 6.5 times as much heat, 5.5 times as much water vapor, and 0.5 times as much sulfur per unit of power produced as would a coal plant at the time in New Zealand.
The data also showed contamination of the Waikato River with hydrogen sulfide, carbon dioxide, arsenic, and mercury at concentrations that had adverse but not calamitous effects. It was noted that the plant had been designed in the 1960s when knowledge of the environmental impact of geothermal energy extraction and the hazards of climate change were less understood. It was suggested that the use of techniques which were in development at the time such as reinjection of the hot waste water for liquid-dominated fields would reduce the environmental impact substantially. Ground subsidence was acknowledged as a potential problem but was not observed. More important, the study identified several environmental characteristics unique to geothermal power: first, pollutant formation may be independent of the power production rate; second, effluent pathways may change abruptly generating hazards that have not been anticipated; third, pre-operational testing and random bore holes contribute negatively to the overall impact; and four, waste water may be discharged at temperatures high enough so that utilization of the waste heat becomes both practical and imperative.
These parameters have been taken into consideration in modern geothermal extraction technology for which it is recognized that exploitation of geothermal energy has an impact on the environment but lower than other available sources of energy. The environmental effects of geothermal energy production may then be analyzed from different perspectives which include:
- Environmental pollution such as air quality, water quality, underground contamination and chemical or thermal pollution
- Adjacent terrain changes such as land subsidence;
- Social impacts such as conflicts with cultural traditions and archaeological sites and social-economic disruptions
- Consequences of large scale industrial activity such as high noise levels, industrial accidents and the generation of industrial waste.
The extent to which geothermal exploitation affects the environment is proportional to the scale of its exploitation. In general the environmental effect is more significant in plants with geothermal direct-use applications and potentially greater in the case of conventional back-pressure or condensing power-plants. This is particularly relevant in regards to air quality. Although the consequences of air quality pollution may be high, the probability of such events is considered low for which it is deemed an acceptable risk. An analysis of the Argonne National Laboratory concluded that geothermal waters pose a large potential risk to water quality, if released into the environment, due to high concentrations of toxics including antimony, arsenic, lead, and mercury but that the risk of release can be virtually eliminated through proper design and engineering controls. None the less, the release of toxic substances, especially hydrogen sulfide remains of concern.
The use of geothermal energy sources requires the drilling deep holes (boreholes) and the insertion of pipes for pumping high-temperature fluids out from the ground. The rocks that contain high temperature fluids also contain minerals, which tend to form residues inside the pipes and production equipment. If the rocks contain radionuclides, such as radium, the mineral scale, production sludge, and waste water will contain Technologically-Enhanced, Naturally-Occurring Radioactive Materials (TENORM).
The primary radionuclides produced with the geothermal fluids are radium-226 and radium- 228. The study of the Argonne National Laboratory has indicated that effective removal of these contaminants may not be feasible in an industrial scale. Geothermal power plants usually re-inject the hot water that they remove from the ground back into wells but a small amount of water may evaporate and not be returned to the ground with the potential for ground collapse and sink holes. The withdrawal and re-injection of geothermal fluids also may trigger or increase the frequency of seismic events. However these are micro seismic events that can only be detected by means of instrumentation. The likelihood of a major seismic event is very small never having been documented.
Local communities, governments and local organizations have increased awareness of the effect of large scale industrial activity in their environments. The expectation is that there will be a complete disclosure of all the potential impact of the industrial activity. Because of the large number of variables involved in an operation such as geothermal energy extraction, consideration of all the possible consequences of the activity may not be possible which may lead to community opposition to geothermal energy projects. In particularly sensitive areas, the development of geothermal sources may not be feasible. For example, there is ongoing debate whether the development of geothermal energy is even an option in Wildlife Conservation areas in Kenya, in spite of the immense social needs of the community that would be addressed by such industrial activity. In addition, there is an increasing conflict of interest between the growth of the tourism industry, population growth and resort community development with their high demands for water and the needs of the geothermal plant. This applies specifically to the Long Valley Caldera Geothermal Area.
Large scale industrial activity
Large scale industrial activity has numerous environmental consequences even when all the variables have been addressed and the negative effects neutralized. Accidents are inevitable in this setting and geothermal energy extraction poses a small but definitive risk of groundwater contamination, especially during the drilling of wells and extraction of hot water or steam. This problem was identified in New Zealand as early as 1975. There are reliable technical solutions that minimize this risk such as re-injecting used water back into the ground through separate wells instead of discharging the used water into surface waters thus preventing underground minerals or pollutants from entering surface waters. Nonetheless, At Casa Diablo plant rare leaks through the heat exchanger have resulted in the presence of trace amount of isobutene in samples taken from wells, hot springs, and fumaroles throughout the caldera. This means that there is high horizontal connectivity that enables the migration of fluids throughout the relatively shallow hydrothermal system in the caldera's south rim. Installation of pipelines to transport geothermal fluids and construction of ancillary structures also affect animal and plant life and the landscape. Scenic views may be modified although some times with favorable results such as in Larderello, Italy, where the network of pipelines crisscrossing the countryside and the power-plant cooling towers have become an integral part of the panorama and become a famous tourist attraction.
The thermal energy present in the underground is enormous and broadly available. Geothermal energy production is an effective and reliable source of energy. Its most effective utilization is at the local or regional scale. The implementation of industrial resources for wider distribution of the obtained energy may have substantial environmental and physical consequences but if properly and continuously managed its environmental impact may be minimal. As such is an alternative to fossil fuels worth full consideration