Environmental Impact of Geothermal Power Plants

By:Ronalyn S. Petalino

Certain environmental impacts associated with the development of geothermal sites and the operation of plants are inevitable. However, under normal conditions they are generally confined to the immediate vicinity of the plant and are of lesser impact than those of other electric power generation technologies, particularly those using carbon based fossil fuels and nuclear fuels.

Overview

There have now been more than one hundred years of experience in developing geothermal fields, and in building, operating, upgrading, and even decommissioning geothermal plants of various types.

Most countries have laws that regulate the construction and operation of power plants with the intent of preserving the natural environment and safeguarding the health and well-being of people as well as the flora and fauna of the region. The United States has federal, state, and local regulations that cover a broad range of possible environmental impacts.

Regulations

Clean Air Act National Environmental Policy Act National Pollutant Discharge Elimination System Permitting Program Safe Drinking Water Act Resource Conservation and Recovery Act Toxic Substance Control Act Noise Control Act Endangered Species Act Archeological Resources Protection Act

The following laws and regulations must be adequatelyaddressed before any geothermal project can be

completed:

Hazardous Waste and Materials Regulations Occupational Health and Safety Act, and Indian Religious Freedom Act.

Although there are no uniform international standards regarding the environmental impact of geothermal plants, it is common for most countries to require that plants meet appropriate environmental regulations.

Gaseous emissions to the atmosphere Water pollution Solids emissions to the surface and the atmosphere Noise pollution Land usage Land subsidence Induced seismicity Induced landslides

General impacts of electricity generation

Water usage Disturbance of natural hydrothermal manifestations Disturbance of wildlife habitat and vegetation Alteration of natural vistas Catastrophic events

Of these some are of serious concern for geothermal plants. Abatement technology is available and usually deployed to mitigate the most potentially harmful impacts. Compared with other types of power plants, geothermal plants hold significant advantages for many of these impacts.

Environmentally Friendly - Geothermal energy is a renewable energy source that's highly environmentally friendly. Little disruption is made to the environment as a result of the various processes that are used to harness this energy source in order to provide electricity. Few chemicals and pollutants feature in geothermal electricity production.

Environmental advantages of geothermal plants

There is great concern worldwide about atmospheric emissions of carbon dioxide, CO2, owing to its heat-trapping properties and the fear of its effect on the global climate. Geothermal power plants have very low gaseous emissions, albeit most of which is CO2, on a per MWh-generated basis, when compared with all other power generation technologies that emit CO2 as a normal part of operation.

Geothermal binary plants normally emit no gases at all. Using the same basis of comparison, geothermal plants 23 Environmental Impact of Geothermal Power Plants 485 use much less land than any other type of power plant. With one notable exception, geothermal fluids used in power plants are fairly innocuous chemically and pose little hazard in terms of solids pollution. Reinjection of waste brines from geothermal plants avoids contamination of surface and groundwater aquifers. Thus, taken in broad scope, geothermal power plants are one of the most, if not the most, environmentally benign sources of electrical power.

Gaseous emissions from dry- and flash-steam geothermal plants stem from the no condensable gases (NCG) that are carried in the geofluid in dissolved form. Unless the NCG are removed upstream of the turbine (which is currently not done in commercial plants), the NCG will accumulate in the condenser, thereby raising the backpressure on the turbine.

Carbon dioxide and hydrogen sulfide, H2S, are the most common and prominent NCG in geothermal steam, but gases such as methane, hydrogen, sulfur dioxide, or ammonia can also be found, usually in very low concentrations

Gaseous emissions

Currently it is not required to capture or treat CO2, but H2S is strictly regulated in the United States owing to its offensive odor at very low concentrations, 30 parts per billion, and to its toxicity at higher levels.

In an attempt to control global warming, rules and regulations are being discussed around the world that would penalize plants that emit carbon into the atmosphere. One approach would place a “cap” on carbon emissions for power plants; another would institute a “carbon tax” on emissions.

Table 23.1 shows acomparison of gaseous emissions from typical geothermal plants with other types ofpower plants [8]. It is worth noting that the NOx and SO2 emissions at The Geysers onlyresult from the method used to treat H2S in the NCG, namely, a combustion process thatoxidizes the H2S in a few of the units. Most geothermal steam plants do not rely on combustionfor H2S abatement and therefore emit no NOx at all.

The area required to support a geothermal power plant, including the well field, substation, access roads, and auxiliary buildings depends on the power plant rating, the type of energy conversion system, the properties of the geothermal reservoir fluid, and the piping system chosen for collecting the geofluid from the production wells and disposing of the waste brine to the injection wells. The power plant must be built close to the production wells to avoid thermodynamic losses caused by long geofluid pipelines. Although a well field for a 2050 MW power plant can cover a considerable area,5 to 10 km2 or more, the well pads themselves typically cover only about 2% of the total area. Directional drilling allows multiple wells to be drilled from a single pad and minimizes the area needed for the well pads.

Land usage

Table 23.2, using data from [6] and elsewhere, presents a comparison of land usages for typical geothermal flash and binary plants with those of coal, nuclear, hydroelectric, solar thermal, photovoltaic, and wind plants [1]. Realistic capacity factors have been used in the calculations for each technology; furthermore, average power outputs, not rated values, were used for the solar plants.

The solids that could potentially be discharged into the environment from geothermalplants are confined to materials that are initially dissolved in the geofluid and which precipitate during the processes undergone within the power plant. Of all the plants now in operation around the world, only those at the Salton Sea field in Southern California pose a threat in this regard.

Solids discharge

The two methods for coping with these high-salinity brines, namely, flashcrystallizer/ reactor-clarifier (FCRC) and pH-modification (pH-mod) systems. By controlling the precipitation of the solids, these methods allow either for the solids to remain in solution long enough to pass through the plant and be reinjected back into the reservoir (pH-mod) or for the solids to precipitate in a manner and place where they can be removed from the geofluid and collected for proper disposal (FCRC). The latter approach cleans the brine and permits it to be reinjected without the possibility of solids precipitation within the reservoir where it could adversely affect the permeability of the formation. Thus, with proper design of the treatment system, the solids naturally occurring in the brine are not allowed to escape uncontrolled into the environment.

Water is needed at every stage of development of a geothermal project. This is no different from any other large power development project. However, the needs for geothermal projects are relatively easy to satisfy. Furthermore, water use can be managed in most cases to minimize environmental impacts. The two main areas of water usage are the drilling of wells and the discharge of waste heat if a water cooling tower is used.

Water usage

Well drilling. describes the drilling operations for geothermal wells. The water required during this phase of development cools the drill bit, removes rock chips, and provides structural integrity of the hole until casing can be set.

Cooling water for heat rejection. Whenever power is generated on a continuing basis, the rejection of heat into the environment is an inevitable consequence of the Second Law of thermodynamics. The customary method of discharging waste heat in 492 Geothermal Power Plants: Principles, Applications, Case Studies and Environmental Impact geothermal steam or flash plants is the use of water cooling towers.

It is not necessary to use any water for cooling purposes if a dry cooling system is adopted. Air-cooled condensers are widely used with binary plants where water may be in short supply since binary plants do not supply their own make-up water as do flash-steam and dry-steam plants While air-cooled condensers eliminate the need for fresh make-up water, they occupy large tracts of land, as mentioned earlier, owing to the poor heat transfer properties of air versus water. Additionally, there is a larger parasitic power requirement compared to water cooling towers owing to the large number of electric motor-driven fans.

There are several places where geofluids may get into the environment during field development or normal operations. Since these fluids may contain minerals and elements harmful to humans, flora, or fauna, the onus is on the plant designers to provide barriers to prevent these fluids from entering the biosphere. The amount of dissolved solids increases significantly with temperature, making high-temperature geofluids more risky than moderate- or low-temperature ones. Some of these dissolved minerals (e.g., boron and arsenic) could poison surface or ground waters and also harm vegetation or animals.

Water pollution

Liquid streams might endanger surface waters through run-off during well testing. Thus, fluids discharging during tests are directed to impermeable holding ponds. Also steam pipelines are fitted with traps to remove condensate and that liquid is sent by pipelines to holding ponds. Later the collected fluids are reinjected deep underground.

Despite all these design precautions, it is nevertheless prudent to have monitoring wells strategically located in the well field to rapidly detect any problems with subsurface leakage and permit prompt remediation. For those few developments where 100% reinjection of residual brines is still not practiced, it is essential to monitor all discharge streams to avoid exceeding allowable limits of contaminants.

Land subsidence Geothermal reservoir production at rates much

greater than recharge can lead to surface subsidence. This was observed, for example, beginning with the first few years of operation of the power plant at Wairakei when all the residual brine was allowed to flow to the adjacent Waikato River. The production wells at Wairakei were drilled through a relatively shallow cap rock (Huka Falls Formation) containing pumice breccias and mudstones.

Environmental challenges of geothermal plants

The thickness of the cap rock varies from 150200 m in the northern part of the field to only 3090 m in the western part. Tests have shown that the pumice breccias and mudstones exhibit compressibilities sufficiently high to account for the subsidence [22]. It is important to note that the greatest subsidence correlates with the thickest part of the cap rock (R. Glover, personal communication, May 16, 2007

Although reinjection does not guarantee the avoidance of subsidence, it can reduce the risk, provided it is carried out so as to maintain reservoir fluid pressure. Nowadays, geothermal developers normally incorporate reinjection into reservoir management programs right from the start both to minimize this risk and to prolong the life of the reservoir.

Induced seismicity Induced seismicity is a phenomenon in which a

change in fluid pressure within astressed rock formation leads to movement of the fractured rocks. The energy released is transmitted through the rock and may reach the surface with enough intensity to be heard or felt by persons in the area. This may happen, for example, when the reservoir for a hydroelectric station is first filled, when fossil fuels are extracted from oil and gas fields, or when fluids are injected underground at high pressure. The likelihood and the severity of the event depend on the local state of stress within the formation.

Figure 23.7 Wairakei drop structure (hot-water drainage channel) to the Wairakei Stream not far from the area ofmaximum subsidence. Photo courtesy of S. Tamanyu, Geological Survey of Japan; photo first published on the coverof Chishitsu News, No. 531, 1998 [WWW].

Induced landslides Many geothermal fields lie in rugged

volcanic terrain prone to natural landslides.Indeed, some fields have been developed atop ancient landslides. Landslides can be triggered by earthquakes, and, as we have discussed, while it is possible that geothermal production or injection could lead to induced seismicity, it is highly improbable that such activities could lead to an event large enough to cause a major earthquake.

Landslides have occurred at geothermal fields, but the cause is often unclear. The worst disaster happened at the Zunil field in Guatemala in January 1991 in which at least 23 people were killed . Figure 23.9 shows the devastation when a large portion of a steep slope above the field collapsed, spreading rock and moisture-laden debris a distance of 8001200 m onto a relatively flat plateau.

Noise pollution In the development stage of a geothermal power project, noise is generated during road construction, excavation for drilling sites, well drilling, and well testing. While these may be disturbing to nearby residents (if any), they are of limited duration. Furthermore, the most objectionable sounds can be significantly reduced with appropriate mufflers and other sound deadening materials.

During normal plant operations, various components are sources of noise; these include: transformers, generators, water cooling towers, motors, pumps and fans for circulating water and air associated with the heat rejection system, brine and steam flowing through pipes, etc. These are generally confined to the area within the plant fence boundary.

Disturbance of natural hydrothermal manifestations there have been numerous cases where hydrothermal

developments have compromised or totally destroyed natural hydrothermal manifestations such as geysers, hot springs, mud pots, etc. The drawdown, or lowering of the hydrostatic water level, from production wells disturbs the natural thermo hydraulic balance that gives rise to the manifestations. In particular, geysers are delicate phenomena that are subject to nature’s whims, even without humankind’s interference.

Disturbance of wildlife habitat, vegetation, and scenic vistas any power generation facility constructed where

none existed will alter the view of the landscape. Conventional power plants in developed, commercial, or industrial settings, while objectionable to many for other reasons, do not stand out as sharply as a geothermal plant in a flat agricultural region or on the flank of a volcano. Even so, with care and creativity geothermal plants can be designed to blend into the surroundings.

Catastrophic events Besides landslides, some other serious events that

might occur at a geothermal plant include well blowouts, phreatic explosions, ruptured steam pipes, turbine failures, fires, etc. Most of these accidents are similar to what can happen at any power generation facility and have been known to cause casualties. The ones that are unique to geothermal power plants involve well drilling and testing. In the early days of geothermal energy exploitation, well blowouts during drilling were a fairly common occurrence, but nowadays the use of fast-acting blowout preventers have practically eliminated this potentially life-threatening problem.

Thermal pollution Although thermal pollution is currently not a specifically regulated quantity, it does represent an environmental impact for all power plants that rely on a heat source for their motive force. As was discussed in Sect. 5.4.6, heat rejection from geothermal plants is higher per unit of electricity production than for fossil-fueled or nuclear plants.

Figure 23.13 shows a comparison between a geothermal single-flash plant and several alternative cycles. For example, using a reservoir fluid temperature of 220C, the flash plant rejects about three times as much heat per unit of useful electrical generation as an ideal Carnot cycle operating between the reservoir temperature and the assumed condensing temperature of 52C. The other practical cycles all reject far less heat per unit of generation than the flash plant.

Figure 23.13 Heat rejection per unit of electrical generation for various plants: GT5simple gas turbine;N5nuclear; C5coal; GTCC5gas turbine combined cycle