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Chapter 4 Geothermal Energy g CO2eq/kWh for flash steam plants and less than 80 g CO2eq/kWh for projected EGS plants. The Bertani and Thain (2002) estimates are higher than these for several reasons. First, Bertani and Thain collected information from a very large fraction of global geothermal facilities (85% of world geothermal capac- ityin2001),whereasqualifyingLCAstudieswerefew.Someopen-loop facilities with high dissolved CO2 concentrations can emit CO2 at very high rates, though this is relevant for a minority of installed capacity only. For closed-loop geothermal systems with more common dissolved CO2 concentrations, most lifecycle GHG emissions are embodied in plant materials and emitted during construction. These were the cases exam- ined in the qualifying LCA literature displayed in Figure 4.6. Despite few available studies, it is tentatively observed that systems using flashed or dry geothermal steam appear to have lower GHG emissions than do systems combining EGS reservoir development with binary power con- version systems, though this difference is small relative to, for instance, coal-fired electricity generation GHG emissions (see Section 9.3.4.1). A key factor contributing to higher reported emissions for EGS/binary systems versus steam-driven geothermal systems is higher energy and materials requirements for EGS’ well-field development. Additional LCA studies to increase the number of estimates for all geothermal energy technologies are needed. Frick et al. (2010) compared LCA environmental indicators to those of European and German reference power mixes, the latter being com- posed of lignite coal (26%), nuclear power (26%), hard coal (24%), natural gas (12%), hydropower (4%), wind power (4%), crude oil (1%) and other fuels (3%), and observed that geothermal GHG emissions fall in a range between 8 and 12% of these reference mixes. At sites with above-average geological conditions, low-end GHG emissions from closed loop geothermal power systems can be less than 1% of corre- sponding emissions for coal technologies. For lifecycle GHG emissions of geothermal energy, Kaltschmitt (2000) published figures of 14.3 to 57.6 g CO2eq/kWhth for low-tempera- ture district heating systems, and 180 to 202 g CO2eq/kWhth for GHP, although the latter values depend significantly on the mix of electricity sources that power them. The LCA of intermediate- to low-temperature geothermal developments is dominated by larger initial material and energy inputs during the con- struction of the wells, power plant and pipelines. For hybrid electricity/ district heating applications, greater direct use of the heat generally pro- vides greater environmental benefits. In conclusion, the LCA assessments show that geothermal is similar to other RE and nuclear energy in total lifecycle GHG emissions (see 9.3.4.1), and it has significant environmental advantages relative to a reference electricity mix dominated by fossil fuel sources. 4.5.3 Local environmental impacts Environmental impact assessments for geothermal developments involve consideration of a range of local land and water use impacts during both construction and operation phases that are common to most energy projects (e.g., noise, vibration, dust, visual impacts, surface and ground water impacts, ecosystems, biodiversity) as well as specific geothermal impacts (e.g., effects on outstanding natural features such as springs, geysers and fumaroles). 4.5.3.1 Other gas and liquid emissions during operation Geothermal systems involve natural phenomena, and typically dis- charge gases mixed with steam from surface features, and minerals dissolved in water from hot springs. Apart from CO2, geothermal fluids can, depending on the site, contain a variety of other minor gases, such as hydrogen sulphide (H2S), hydrogen (H2), methane (CH4), ammonia (NH3) and nitrogen (N2). Mercury, arsenic, radon and boron may be present. The amounts depend on the geological, hydrological and ther- modynamic conditions of the geothermal field, and the type of fluid collection/ injection system and power plant utilized. Of the minor gases, H2S is toxic, but rarely of sufficient concentration to be harmful after venting to the atmosphere and dispersal. Removal of H2S released from geothermal power plants is practised in parts of the USA and Italy. Elsewhere, H2S monitoring is a standard practice to provide assurance that concentrations after venting and atmo- spheric dispersal are not harmful. CH4, which has warming potential, is present in small concentrations (typically a few percent of the CO2 concentration). Most hazardous chemicals in geothermal fluids are in aqueous phase. If present, boron and arsenic are likely to be harmful to ecosystems if released at the surface. In the past, surface disposal of separated water has occurred at a few fields. Today, this happens only in excep- tional circumstances, and geothermal brine is usually injected back into the reservoir to support reservoir pressures, as well as avoid adverse environmental effects. Surface disposal, if significantly in excess of natural hot spring flow rates, and if not strongly diluted, can have adverse effects on the ecology of rivers, lakes or marine environments. Shallow groundwater aquifers of potable quality are protected from contamination by injected fluids by using cemented casings, and imper- meable linings provide protection from temporary fluid disposal ponds. 419PDF Image | Geothermal Energy 4
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