| Abstract |
For low temperature geothermal resources, as those existing in Germany, only binary power plants can be used for power generation. Different process technologies for such binary power plants are available. Besides pure working fluids (organic or non-organic), mixtures can be used as working fluid. Properties of working fluid mixtures are discussed, highlighting the ammonia-water mixture used in Kalina-cycles. The two main advantages of using mixtures instead of pure working fluids are nonisothermal evaporation and sorption effects due to changing concentrations of the components. Different process designs of Kalina-cycles are discussed. Depending on the design they make use of the different advantages of mixture properties. Some of these cycles were introduced by Kalex (2009) especially for low temperature and geothermal applications. Based on one of these power cycles, different optimisation strategies are discussed. Therefore, efficiency factors for geothermal plants with binary cycles are defined and the difference between the process and the overall plant efficiency of a geothermal binary power plant is highlighted. Furthermore, the difference between the net and the gross efficiencies is discussed. The analyses of different process parameters, changing operation conditions and process designs show that competing effects between these efficiencies have to be considered during the optimisation and the design of a binary power plant. It is shown that maximum efficiencies can be reached for each ammonia concentration of working fluid when adjusting live steam pressure and mass flow of the working fluid. But in addition, for an overall optimisation of a geothermal power plant, the coolingoff of brine and the effort for exploiting the brine resource from the reservoir has also to be taken into account. The paper outlines optimisation strategies and clarifies that different efficiencies have to be considered during an optimisation process. |