 |
|
Geothermal Energy and the Environment
The odor of hydrogen sulfide, often characterized as that of rotten eggs, is common near volcanic areas, such as Yellowstone National Park. The concentration of H2S emissions from geothermal plants is 0.03–6.4 g/kWh, varying significantly from field to field, depending on the amount of hydrogen sulfide contained in the geothermal fluid and the type of plant used to exploit the reservoir. While such emissions do not contribute to acid rain or global climate change, the odor they produce has created objections in many areas. Usually, these emission do not pose a significant impediment to development, but occasionally the have produced controversy. For example, at The Geysers, especially in the late 1970s and early 1980s, the odor of hydrogen sulfide threatened the development because it was interfering with the tourist economy downwind at the nearby resorts along Clear Lake. As this activity and the geothermal development are largely located in the same county, local politicians were forced to weigh one source of economic development against the other. Because the geothermal development itself is immovable, another solution was needed. Today, the removal of H2S from geothermal steam is mandatory in the United States, and the most common process is the Stretford process, which produces pure sulfur and is capable of reducing H2S emissions by more than 90%. More recent techniques include burning the hydrogen sulfide to produce sulfur dioxide, which can be dissolved, converted to sulfuric acid and sold to provide income. Land use is a particularly sensitive issue in the development of geothermal energy. On the credit side of the ledger, geothermal installations don’t require damming of rivers or harvesting of forests. Nor do they require mineshafts, tunnels, open pits, or waste heaps. Moreover, the long fuel chain of fossil and nuclear fuels is largely absent; all phases of this fuel chain are in one location. This trait concentrates whatever impacts that exist in one location. If, as at The Geysers, the terrain is steep and unstable, geothermal developments must take these conditions into account. If, on the other hand, the terrain is flat and agriculture, these conditions must be addressed. In every case of geothermal development, existing land use conditions are critical to everything from exploration to generation. Any environmental impacts cannot be spread out and diluted as they can with fossil and other fuels. Nonetheless, if one were to add up the actual land needs, they would find that geothermal power plants require relatively little land. An entire geothermal field uses only1–8 acres per MW versus 5–10 acres per MW for nuclear plants and 19 acres per MW for coal plants. This makes geothermal a land use bargain.
Figure 19 - Comparison of land requirements for baseload power generation. http://solstice.crest.org/geothermal/geothermal_brief_environmental_impacts.htmlWater contamination has in the past been a concern for geothermal developments, mostly with regard to water-dominated systems. In the Imperial Valley, the total dissolved solids (TDS) of much of the geothermal water exceed 300,000 mg/liter. If this were allowed to flow out onto the surface—as is permitted in other countries—the impact on the surrounding agricultural fields would be ruinous. The potential problem has been neatly solved, however, by the use of reinjection back into the reservoirs from which the fluids were derived. This has the added benefit of maintaining reservoir pressure and prolonging the life expectancy of the reservoir. Binary systems, as described above, use this procedure routinely.Reinjection is also used to slow or prevent ground subsidence that can result in some geological circumstances once water is withdrawn. Indeed, the possibility of subsidence was the subject of long delays in the development of the water-dominated Imperial Valley fields in California. The concern there was that the removal of geothermal water would result in compaction underground. Were such compaction to reach the surface as local authorities feared, it could have a devastating impact on the delicate balance of the irrigation and drainage system that makes the Imperial Valley so valuable for agriculture. To date, little if any geothermal subsidence has occurred in the Imperial Valley, but the concern was justified because of the experience at Wairakei, New Zealand. Here the ground has subsided as much as 42 feet. Monitoring has shown that a maximum subsidence rate of 18 inches/year occurred in a small region, outside the production area, with subsidence of at least 1 inch/year occurring all over the production field. Although Wairakei presents an extreme example, there is little that can be done to prevent or mitigate subsidence effects other to re-inject water at some distance from the production well to avoid the cooler rejected waste fluid from lowering the temperature of the production fluid. Another concern is noise, especially during exploration drilling and construction phases. Noise levels from these operations can range from 45 to 120 decibels (dBa). For comparison, noise levels in quiet suburban residences are on the order of 50 dBa, noise levels in noisy urban environments are typically 80–90 dBa, and the threshold of pain is 120 dBa at 2,000–4,000 Hz. Site workers can be protected by wearing ear mufflers. With best practices, noise levels can be kept to below 65 dBa, and construction noise should be practically indistinguishable from other background noises at distances of half a mile.
Figure 20 - Geothermal exploration and construction noise levels. Source: http://solstice.crest.org/geothermal/geothermal_brief_environmental_impacts.html |
 |
 |
 |
 |
 |
 |
 |