Possible emissions from electricity and heat generation from geothermal energy by the use of F-gases in the energy conversion process by an Organic Rankine Cycle (ORC)

In case of low temperature heat sources Organic Rankine Cycle (ORC) is next to Kalina Cycle one of the few thermodynamic cycles suitable for power generation. Optimization strategies provide a better glide matching of the temperature profiles of heat source or sink to the ORC compared to the standard cycle. This leads to an increase in efficiencies in the range of 15 % to 25 %. In this context, selection of suitable working fluids, two-stage expansion, supercritical cycles or the usage of zeotropic mixtures as working fluids has to be mentioned. Due to the use of fluorinated hydrocarbons, the number of potential fluids as well as the efficiency increase significantly. However, an increase in emissions due to leakages during operation, filling and disposal is associated with fluorinated fluids compared to natural hydrocarbons. Such emissions cannot be completely avoided and according to information of manufacturers and operators they are annually in the range of 1 % to 3 % of the capacity. Based on legal regulations recording of the use levels of fluorinated hydrocarbons in ORC systems according to UStatG and EU Regulation 842/2006 is obligatory. The recording obligation exists regarding the national emission inventory based on the framework convention on climate change. To evaluate potential greenhouse gas emissions by geothermal power plants, in this study different scenarios depending on rate of emission and number of power plants are calculated. If a development in geothermal power generation as predicted takes place, the emissions until the year 2030 are to be classified as low. In case of the technical-ecological potential with 2120 power plants and a rate of emission of 3 % the emissions are between 0.24 Million t/a and 3.02 Million t/a depending on the considered scenario. A comparison to the greenhouse gases by fluorinated hydrocarbons in the year 2009 with 15.6 Million t/a shows that the emissions for this number of power plants are definitely relevant. Regarding actual discussion and positions of the EU-Commission an inclusion of the ORC in the ChemKlimaSchutzV and EU Regulation 842/2006 because of the use of fluorinated hydrocarbons seems to be possible. For a sustainable expansion of geothermal power generation the conflict of objectives, highlighted by this study, between efficiency increase and additional emissions has to be discussed in more detail. Therefore, reliable data for rates of emissions by ORC systems are important. Furthermore, the performed calculations will be extended by selecting working fluids according to the geothermal water temperature. In addition general approaches for the reduction of emissions and the increase in efficiency have to be intensified. In this context alternative thermodynamic cycles, steady improvement of ORC systems or the use of refrigerants with low global warming potential are examples.