Power generation and its possible environmental effects
Geothermal power generation creates a much lower emission of greenhouse gases than most other technologies. In any comparison it is important to consider the entire production cycle is considered, i.e. all phases before, during and after power plant operation. Geothermal power plants have particularly low CO2 emissions compared to other technologies; when CO2 abatement is concerned, they are therefore more attractive options for power generation than coal, oil or gas sources (Figure 1).
Figure 1. Greenhouse gas emissions (CO2 equivalent) of different power generation technologies (from Hunt 2001).
Environmental effects during geothermal power generation cannot be excluded. These can differ according to the characteristics of site, reservoir and power plant type. Power plants of the binary type (=closed system in which the turbine is driven by steam of a working fluid, not directly by geothermal steam) have by far the smallest effects, except for waste heat. In the following, the common geothermal power plant types will be shortly presented, along with their general environmental effects. These will be subsequently discussed in detail. Mitigation/remediation
measures will also be presented. In general terms, the power plant types used for electricity generation depend on the resource characteristics. Various plant types are in use, depending on the characteristics of the produced
fluids (temperature, steam/water ratio, chemistry), see Table 1.
Table 1: Geothermal power plant types and their possible environmental effects (from Brown 1995).
Power plants of the direct flash type are in use only at The Geysers (US), Larderello (Italy) and Matsukawa (Japan), since they require a vapor-dominated reservoir with dry steam. They emit the used steam directly to the atmosphere retention techniques are available but costly. Much more common are water-dominated reservoirs from which a mixture of steam/hot water is produced. Depending on the steam/water ratio (= a function of pressure and temperature) the steam is separated (“flashed”) from the mixture once or twice before the turbine(s). The steam condensate and the cooled waster are usually reinjected into the subsurface; by these means the drop in
reservoir pressure due to production can be compensated, at least partially. At present thy binary power plant type is favored for new developments. With this type the produced fluid does not get in contact with components of the machinery (which would be problematic with highly mineralised or corrosive fluids). The geothermal heat of the fluid is transferred to a “secondary” working fluid with low boiling temperature and high vapor pressure (isobutane, isopentane, ammonia) in a heat exchanger, where the secondary fluid is vaporized. The vapor is decompressed in a low-pressure turbine and liquidized again in an air- or water-cooled condensator (=ORC, Organic Rankine Cycle). This power plant type can operate already with relatively low geothermal fluid temperatures (~100 °C) but which correspondingly low heat/power conversion efficiency. Due to the closed circuit of the binary plant there are practically no discharges to the environment. Leakages through which the working fluid (sometimes toxic and/or flammable) could escape are possible but manageable since the modular units are usually small (0.5
to a few MWe). The environmental effects can be quite different, since they depend on the power plant type, size and on the locally produced geothermal fluid. In general, the effects increase with increasing scale of geothermal development and, in particular, with increasing fluid production. The various environmental effects are successively discussed below.
Change of natural features
Natural features such as hot springs, mud pools, geysers, fumaroles and steaming ground are associated with most geothermal systems. Because of their unique nature, these are often tourist attractions or are used by local residents. Besides, these visible signs of geothermal activity are part of a country’s heritage and the thermal features may also hold significant cultural and spiritual importance to many indigenous peoples. Whereas many natural beauties in Natural Parks and other protected areas can be excluded from geothermal development, especially thermal springs (mainly their flow-rate) can be influenced by nearby geothermal fluid production wells. Of course, often the effects become visible only after a certain time following the start of geothermal operations.
Careful hydrogeologic studies, including numerical modelling can help to assess the reaction, also on the long term, of natural systems on geothermal production activities like fluid withdrawal. This should indicate the maximum permissible fluid production levels.
Landscape aesthetics
During the exploration and feasibility phases of geothermal development the environmental effects originate mainly from drilling activities and are thus only of temporary nature. Remaining installations like well-head equipments, transmission pipelines, power plant building, electric connections etc. are relatively small. All geothermal power plant types require cooling. The models used in early days were massive (Figure 2); nowadays much smaller and inconspicuous types are being built (Figure 3), while the cooling units of binary plants manifest themselves more long than high (Figure 4). Their visibility remains by all means; nevertheless pipelines and other equipment
can be made less conspicuous by appropriate painting.
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Figure 1. Old, tall cooling towers at Larderello/Italy.
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Figure 3. Modern, low cooling tower at Larderello, Italy; for a 20 MWe power plant (from Barbier, 1997).
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Figure 4. Air-cooled binary power pant in Kawerau/New Zealand (3.5 MWe, from ORMAT, 2001).
Reference :
Rybach, L. (2003): Geothermal energy: sustainability and the environment. Geothermics
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