Power generation and its possible environmental effects-2
Land use
The various development activities and installations need land surface, temporarily or steadily. After constriction completions substantial areas become free again, leaving some restrictions e.g. for agricultural use. Compared to other power generation technologies the geothermal option has the advantage that the production takes place at the resource, whereas for other technologies like coal-fired or nuclear power plants the –often extended– mines are located far away. Figure 5 shows a comparison of the geothermal land requirement with other power generation technologies.
Figure 5. Comparison of land use for different power generation technologies. From DiPippo(1991), modified.
Noise
Here are mainly the drilling activities relevant; power plant operation creates little noise. Drilling with compressed air generates noise around 120 dB (muted: around 85 dB), production tests with steam venting 70 – 110 dB (muted), drilling with mud 80 dB. Noise levels decrease with distance by about 6 dB/km; high-frequency noise (e.g. steam venting) is perceived as being more disturbing than lower-frequency noise (e.g. drilling Diesel). In environmental impact considerations the geothermal noise must be judged in comparison with noise from other sources: intensive city traffic (80 – 90 dB), large-space office (50 – 60 dB), quiet residential area (50 dB); the pain limit is for noise in the frequency range 2000 – 4000 Hz lies at 120 dB.
Emissions into the atmosphere
Geothermal steam usually contains non-condensable gases (mainly CO2 and H2S, besides some NH3 as well as trace amounts of Hg, B, CH4, N2, H2 and Rn) in concentrations of 2.5 to 47 g per kg steam (Barbier 1997; see Table 2). The most significant continuous gas emissions are from gas ejectors of the power plants, but discharge can occur during well drilling, bleeding, clean-outs and testing as well as from line valves and waste drilling mud degassing, which is usually insignificant in comparison. Drilling rig auxiliaries such as Diesel generators and compressors also produce gaseous emissions and exhaust smoke.
Table 2: Steam composition at various geothermal fields, after Barbier (1997).
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Among the non-condensable gases the CO2 emissions are the highest (although relatively modest when compared to other power generation technologies, see Figure 1). The invisible and smell-less CO2 can cause human health damage due to pH changes in blood: 5 % CO2 concentration in air: shortness of breath and vertigo, 10%: unconsciousness and suffocation. Limits set by authorities are: exposure to around 10’000 ppm during 10 minutes. It should be mentioned here that natural CO2 emissions, especially in volcanic and geothermal areas, can exceed emissions caused by from geothermal development (natural values up to 1’500 g CO2/m2,day; Dr. Aldo Baldacci/ENEL, Pisa/Italy, personal communication). The problematic compound emitted by geothermal power plants is H2S. Fortunately the smell threshold of the “rotten egg smelling” H2S is much lower than concentrations with health effects: 0.3 ppm versus 10 ppm, burning eyes; or 700 ppm, suffocation. Limits set by authorities are some tens of ppm during 10 minutes. Reported emissions from geothermal power plants amount to a few grams per produced kWh electricity: Wairakei/New Zealand, see Hunt (2002). There are various retention possibilities, like the Stratford, BIOX, AMIS methods, but with different economic consequences. Geothermal power plants have no NOx emissions. Binary plants have, thank to the closed fluid circuit design, practically no emissions of non-condensible gases (the substantial CO2 emission of the binary plant at Nghawa/New Zealand makes an exception here). Working fluid discharges up to 4’000 ppm have been however reported (Hunt, 2002). Introduction and implementation of emission limits by authorities for working fluids is indispensable, in view of problems like the disposal of refrigerators.
Reference :
Rybach, L. (2003): Geothermal energy: sustainability and the environment. Geothermics
Tags: emissions into the atmosphere, Environmental effects, noise, power generation