| Abstract |
Development of a cooling-system for research probes for geothermal boreholes Experimental determination of important characteristics and construction of a prototype Benedict Holbein Karlsruhe Institute of Technology KIT, Institute for Applied Computer Science, IAI, 76344 Eggenstein-Leopoldshafen, Herrmann-von-Helmholtz-Platz-1, Germany, E-Mail: benedict.holbein@kit.edu +49(0)721/608-25765, Web: www.iai.kit.edu/geothermie For an efficient, save an economic use of geothermal energy-sources, it´s necessary to have comprehensive information about the conditions in the earth´s crust, especially those of boreholes and their immediate environment. Achieved to the current situation, the capabilities to get this data are insufficient and connected with high costs. The modular geothermal probe ZWERG presents a solution for this problem. With its help it will be possible to capture comprehensive data of boreholes and the surrounding areas. Special modules such as cameras, measuring devices for various physical and chemical properties and sampling – modules and the associated electronic controllers permit the record, transfer and processing of information. The probe should be used in depths up to 5km which means extreme operating conditions as pressures around 600bar, corrosion promoting media and temperatures of 200°C. These circumstances make it difficult to realize the materials selection and dimension of the probe and to use electronic components. Even if there are some high temperature resistant electronics, they are expensive and don´t exist for each application. Therefore it´s preferred to use standard electronics which can only withstand up to 70°C for a long time. To allow long-term operations up to several weeks a cooling-system is needed. A concept for the heat management for the borehole-probe resulted from preliminary theoretical investigations. It contents multi-layer- and vacuum-insulation to reduce the heat-transfer from the environment and a cooling-system for the discharge of internal produced heat. During the circular-process which is realized by the cooling-system, the refrigerant absorbs the heat of the cooled chamber by vaporization and transfers it to the environment after a compression step. Even though it follows the principles of a known heat-pump, the individual components have to be developed especially for the conditions of the operations and the operating-environment. It is indispensable to simulate the cooling-process under realistic conditions in laboratory experiments, to create the base for a reliable design. The currently used refrigerant is acetone. It provides all basic properties for the use in the system, such as an evaporation temperature under 70°C and the capability of liquefaction at high temperatures and technical achievable pressures, but it is completely unknown as refrigerant. This solvent damages seals, makes the lubrication of the components like the compressor nearly impossible and brings a lot of additional difficulties with it. To learn more about the phase-change behavior and the fluid-properties of acetone in combination with the cooling-system and its components several test are necessary. These are also necessary for the construction of the compressor and heat-exchanger used in the system. The maximum diameter of 170mm of the probe which guarantees the use in 8 ½ inches boreholes in combination with the high-temperatures and pressures in the environment, poses some big challenges for their development. At present the construction of the outer heat-exchanger and the compressor is running and |