Survey of Energy Resources 2007
Geothermal - Environmental Issues
Geothermal fluids contain a variable quantity of gas, largely nitrogen and carbon dioxide, with some hydrogen sulphide and smaller proportions of ammonia, mercury, radon and boron. The amounts depend on the geological conditions of different fields. Most of the chemicals are concentrated in the disposal water, which is routinely re-injected into drillholes and thus not released into the environment. The level of concentration of the gases is usually not harmful and can be vented to the atmosphere. Removal of hydrogen sulphide released from geothermal power plants is a requirement in the USA and Italy.
The range in CO2 emissions from high-temperature geothermal fields used for electricity production is variable, but much lower than that for fossil fuels. The USA is the leading producer of electricity from geothermal fields with a generation of 18 000 GWh in 2004. Bloomfield, et al. (2003) compared the average values for all geothermal capacity in the USA, including binary power plants. Fig. 11-11 compares the CO2 emissions from geothermal power plants to those from fossil fuel plants. CO2 emission values for coal, oil and natural gas plants are calculated using data from the US DOE´s Energy Information Administration. The greenhouse gas emissions of the geothermal plants were reported as follows (in g/kWh): carbon dioxide 91, hydrogen sulphide 85, methane 750, and ammonia 599 (Bloomfield, et al., 2003).
Bertani and Thain (2002) reported on emissions from 85 geothermal plants currently operating in 11 countries and found a weighted average of CO2 emissions of 122 g/kWh, which compares fairly well with the value of 91 g/kWh reported for the USA plants by Bloomfield, et al. (2003). The survey of Bertani and Thain covered 85% of global geothermal power capacity in 2001.
The gas emissions from low-temperature geothermal resources are normally only a fraction of the emissions from the high-temperature fields used for electricity production. The gas content of low-temperature water is in many cases minute, as in Reykjavik, where the CO2 content is lower than that of the cold groundwater. In sedimentary basins, such as the Paris Basin, the gas content may be too high to be released. In such cases the geothermal fluid is kept at pressure within a closed circuit (the geothermal doublet) and re-injected into the reservoir without any de-gassing taking place. Conventional geothermal schemes in sedimentary basins commonly produce brines which are generally re-injected into the reservoir and thus never released into the environment, with consequently zero CO2 emissions.
Its geothermal district heating makes Reykjavik one of the cleanest capitals in the world. There is no smoke from chimneys and heating with polluting fossil fuels has been eliminated in Iceland. Almost 90% of all houses in the country are currently heated by geothermal water, with the remainder heated by electricity generated from hydro (83%) and geothermal (17%). Geothermal utilisation has reduced CO2 emissions by some 2 million tonnes annually compared to the burning of fossil fuels. The total release of CO2 in Iceland in 2004 was 2.8 million tonnes. The reduction has significantly improved Iceland's position globally in this respect and many countries could likewise reduce their emissions significantly through the use of geothermal energy.