New material findings and pressure retention concepts for an efficient and procedurally safe use of deep geothermal brine

For geothermal power plants deep geothermal brines get extracted which often contain high amounts of dissolved gases and minerals. Thus, the used systems engineering of the thermal water cycle is exposed to corrosion, degassing and precipitation reactions. For this thesis measures were developed which make the use of deep geothermal brine more efficient and procedurally safer. For this reason, a literature analysis was done concerning the resistance of metal materials in thermal waters and polymer coatings were investigated under geothermal conditions. Furthermore, pressure retentions systems were developed which can suppress precipitation reactions and degassing. The evaluation of the published corrosion experiments exhibits that the PREN (Pitting Resistant Equivalent Number) gives information about the behavior of stainless steels and nickel-based alloys in different geothermal regions. Polymer coatings can replace high alloyed, expensive metallic materials since those are resistant, too and have partially a deposit-avoiding character. For this thesis four polymer coatings (PEEK, PFA, PTFE and Epoxy) were investigated in a laboratory experiment related to their chemical and thermal resistance as well as their tendency to avoid deposition formation. Precipitations in brine circuits can be avoided inter alia by installing a pressure retention system to suppress those reactions. But, the most of the existing pressure retention valves could be mounted just above ground and the few pressure retention systems sitting in the injection well were badly controllable, so far. Now, a novel type of pressure retention valve was developed for the sub-surface use in the injection well. The valve is continuously controllable from above ground and axially mountable in the pipe. By using a newly developed calculation method, the valve can be designed for the site-specific conditions like the required pressure level, flow rate range and casing diameter. The calculation model enables a valve design in which no degassing or cavitation will occur. The technical and economic efficiency of geothermal plants can be raised with this adjustable, downhole pressure retention valve. A further pressure retention concept with recuperation of electric energy was investigated, basing on turbines. Therefore, two types of turbines were analyzed regarding their pressure retention behavior and energy recovery potential.