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First Order Draft Contribution to Special Report Renewable Energy Sources (SRREN) 1 reduce the direct CO2 emission by at least 50% compared to other heating or cooling systems. Other 2 gas emissions from low-temperature geothermal resources are normally much less than the 3 emissions from the high-temperature fields conventionally used for electricity production. 4 Enhanced Geothermal Systems in the future are likely to be designed as closed-loop circulation 5 systems, with zero direct emissions. 6 Direct emissions of CO2 from geothermal direct uses (heating) are also negligible. In Reykjavik 7 (Iceland), the CO2 content of thermal groundwater used for district heating (0.05 mg/kWh) is lower 8 than that of the cold groundwater. In China (Beijing, Tianjin and Xianyang) it is less than 1 g 9 CO2/kWh. In the Paris Basin (a sedimentary basin), the geothermal fluid is kept under pressure 10 within a closed circuit (the geothermal ‘doublet’) and re-injected into the reservoir without any 11 degassing taking place. Conventional geothermal district heating schemes (such as Klamath Falls, 12 Oregon, US) commonly produce brines which are also re-injected into the reservoir and thus never 13 release CO2 into the environment. A similar closed loop arrangement with zero emissions generally 14 applies to pumped EGS or hybrid projects. 15 Most of the chemicals in geothermal fluids are concentrated in the water phase. Boron and arsenic 16 are the components most likely to be harmful to ecosystems if released in relatively large quantities 17 to natural waterways. Therefore, the water is routinely re-injected into wells and thus not released 18 into the environment. However, after separation and condensation, surplus steam condensate may 19 be suitable for stock drinking water or irrigation purposes instead of injection. The most likely 20 contaminates to be aware of will be boron, dissolved hydrogen sulphide, sulphuric acid, and added 21 biocides (to treat the cooling tower) or sodium hydroxide (to raise the pH). In some situations (e.g. 22 Wairakei, New Zealand) the steam condensate has been approved by environmental regulating 23 agencies for irrigation purposes, but each case will be chemically different and must be judged on 24 its merits. 25 4.5.2 Life-cycle assessment 26 As it is known, life-cycle assessment (LCA) analyses the whole life cycle of a product “from cradle 27 to grave”. For geothermal power plants all environmental impacts directly and indirectly related to 28 the construction, operation and deconstruction of the plant need to be considered in LCA, especially 29 referring to intermediate and low temperature geothermal plants due to the large effort to lock up 30 the reservoir relative to the usable energy. 31 Even though published results vary depending on assumptions made, for most existing geothermal 32 plants the global warming potential is small. Kaltschmitt et al. (2006) calculated CO2-equivalent 33 emissions of between 59 and 79 g/kWh for closed loop binary power plants. Pehnt (2006) 34 calculated a LCA CO2-equivalent of 41 g/kWh. Nill (2004) analysed the learning curve effects on 35 the life cycle and predicts a reduction in CO2-equivalent from binary plants from 80 g/kWh to 47 36 g/kWh between 2002 and 2020. Frick et al. (2009) compare two binary plants of the same capacity 37 (1.75 MWe) with resources at different depths and temperatures, and calculated a CO2-equivalent 38 between 23 and 63 g/kWh. They also presented other LCA indicators, which are compared to those 39 of the reference mix in Table 4.5, where it can be observed that the geothermal CO2-equivalent is 40 between 4 and 1% from the reference mix, such as for the finite energy resources. At a site with 41 above-average geological conditions, CO2-equivalent and the demand of finite energy resources can 42 reach below 1% of the environmental impacts of the reference mix. Do Not Cite or Quote 22 of 47 Chapter 4 SRREN_Draft1_Ch04_Version03 22-Dec-09PDF Image | Chapter 4 Geothermal Energy
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