Geothermal energy: GRS develops first monitoring system for naturally occurring radionuclides in thermal waters
A test phase running over several months has just been successfully completed at the geothermal power plant in Bruchsal. The power plant is operated by Geothermiegesellschaft Bruchsal GmbH - a joint subsidiary of EnBW Energie Baden-Württemberg AG and Stadtwerke Bruchsal GmbH. The data obtained with the so-far unique monitoring system allow more accurate information on the geological properties and the behaviour of the geothermal reservoir located at a depth of about 2500 meters. A better understanding of these properties can help to operate geothermal power plants even more safely and economically.
The new system is unique because of the conditions under which measurements are made: At a temperature of approximately 120 degrees Celsius and a pressure of around 20 bar, the concentration of the naturally occurring radioactive noble gas radon-222 in the thermal water is determined continuously by two different detectors located shortly behind the output well. Measuring systems that operate reliably under such extreme conditions are not yet commercially available.
By measuring in the direct vicinity of the borehole, the researchers want to obtain the most accurate data about the radon content of the thermal water in the deep underground. "The pressure and the temperature of the water change when it passes through the system. This leads to chemical and physical processes that change its original properties. In principle, the same thing happens when a bottle of water is opened and the CO2 is emitted", environmental scientist Sebastian Feige of GRS explains. He heads the ANEMONA project, in which since 2014 GRS - sponsored by the Federal Ministry of Economics and Technology - has been working together with EnBW and the Geoscientific Center of the University of Göttingen to develop monitoring systems for geothermal power plants. The researchers use a special feature of the Bruchsal power station for the measurements: the so-called gas bridge. By means of a bypass, the gas bubbles contained in the thermal water are guided around the plant and back into the deep underground. "Otherwise there will be losses in the heat transfer to the power station - just as radiators will not get warm at home when they contain air. At the same time, we can better control the pressure in the plant and avoid the formation of deposits ", says Dr. Thomas Kölbel, geothermal energy expert at EnBW research.
To find out more about what the geothermal reservoir at a few thousand meters' depth is like, Feige and his colleagues go even one step further. In a spectroscopic analysis, three further measuring points are used to determine the amount by which additional naturally occurring radioactive substances contribute to the overall activity. This method is based on the fact that the gamma radiation emitted by each of these substances has a specific energy. "From the quantitative ratios of these natural radionuclides, we may conclude, for example, the size of the reservoir and the permeability of the rock", Feige enthuses. In addition and as a control variable for the spectroscopic measurement, a total pulse measurement is also carried out with a separate device. Although its results do not allow for a differentiation according to individual radionuclides, the underlying technology is easy to use and can thus easily be integrated into the plant monitoring system. The data collected with the new monitoring systems are available to the experts of the GRS in real-time via an Internet connection.
The knowledge gained from the composition of the naturally occurring radionuclides in thermal water through the behaviour and geology of the reservoir also allows conclusions to be drawn about the formation of so-called scales. These are crustal deposits of minerals in pipelines at various points in the installation. These scales also contain part of the naturally occurring radionuclides that are extracted with the thermal water, for example certain radium and lead isotopes. It is because of these substances, which are also referred to as NORM (naturally occurring radioactive material), that operators of geothermal power plants - as well as other branches of industry - must observe the provisions of radiation protection law. "Operating procedures can ensure that the radiation protection of workers is observed during maintenance or the expansion of components. In addition, residues such as scales must also be disposed of in Germany in such a way that any unauthorized radiation exposure of the population is excluded ", says Feige. A better understanding of the processes leading to the generation of such residues is therefore an advantage for operators, e.g. to plan for scales reduction.
However, the analysis of the thermal water in the plant is not yet quite enough for the project partners. As a next step, they have planned to collect samples at a depth of around 2,500 meters. Special containers will make it possible to maintain the ambient pressure of around 250 bar even at the surface. This way, possible changes in the chemical composition of the water are to be avoided as far as possible. In the laboratories of GRS Braunschweig and the Geoscientific Center in Göttingen, the samples are then to be thoroughly examined and will hopefully give insights into the chemical inventory of the geothermal system. Here, the researchers at GRS put their main focus on the suitability of measuring sensors in the expected temperature and pressure range. The aim is to improve the quality of the input data for geochemical modelling and the understanding of the underlying processes. The project is expected to be completed in spring 2018. The results will then be published in a final report.