Geothermal aspects

View of the geothermal power plant Soultz-sous-Fôrets.
(Photo © T. Schäfer 2013)
Removed pipe section with mineral scalings.
(Photo © T. Schäfer 2013)

Earth’s crust offers a vast resource of heat that can be used and converted into energy both for electricity and heating/ cooling purposes. The utilization of this geothermal energy can make an important contribution to meet the targets of the envisaged energy turnaround. So-called “conventional” geothermal plants exploiting hot hydrothermal reservoirs have long been a fully commercial contributor to the energy provision in favorable geological settings such as Iceland or Tuscany/Italy. The concept of Enhanced Geothermal Systems (EGS), however, is a much younger approach to make the heat stored in Earth’s crust available for a stable supply of heat and power, independent of specific geological conditions. Such systems offer an enormous potential for a sustainable energy concept since they provide base-load energy and therefore constitute an important cornerstone in a future energy mix as counterpart to the increasing share of fluctuating energy sources being furthermore poor on CO2 emissions and practically inexhaustible.

Scale formation processes in the surface installations of geothermal power plants may have a negative effect on power plant performance. In addition, scales formed within the geothermal water circuit frequently accumulate natural radionuclides. Consequently, scale formation may lead to dose rates, which are of radiological concern, and scales, which may have to be disposed as radioactive waste. In order to minimize these problems and to foster geothermal power plant availability, it is of major interest to understand scale formation processes and to develop methods for their inhibition.  Geothermal brines In the Upper Rhine Valley are in general highly mineralized and become, upon cooling in the heat exchanger, supersaturated with respect to sulfate solid-solutions, e.g. (Ba,Sr)SO4, and other mineral phases. Some geothermal power plants very successfully tested the application of sulfate scaling inhibitors. Since the application of the sulfate inhibitor, sulfate minerals are no longer detectable in the scaling samples. Subsequent scalings are Pb-dominated and consist mainly of galena (PbS), elemental lead (Pb), arsenic (As) and antimony (Sb). As and Sb are likely present as a nanocrystalline intermetallic mixed compound ((Sb, As) or Pb3(Sb,As)2S3). The absence of barite-type minerals demonstrates the success of the application of the sulfate inhibitor. The precipitation of elemental Pb, As, and Sb, which are more noble than iron, may enhance the corrosion of mild steel pipes in the geothermal water circuit. Elution tests and oxidation of the scalings upon storage at atmospheric conditions demonstrate that proper disposal of the toxic heavy metal and metalloid containing scalings may be challenging.

Project leader: Thorsten Schäfer & Frank Heberling (KIT-INE)