Groundwater in unconsolidated sediments represent a local and renewable energy resource. By use of a groundwater source heat pump system (open-loop system), 70% of the heating demand of buildings can be expected to be covered by “free” heat in the ground. The system also offers the possibility of cooling in warmer periods. Despite of this the method is still not much utilized in Norway, probably due to lack of knowledge about design, operation and maintenance of systems operating in challenging groundwater chemistry. Therefore, the research project ORMEL (Optimal utilization of groundwater for heating and cooling in Melhus and Elverum) was initiated in 2015 (until 2018). A sequel, ORMEL 2, followed from autumn 2018, with one of the R&D focuses being the challenging groundwater chemistry in Melhus.
Nine open-loop systems in the city centre of Melhus are extracting groundwater from a confined, anoxic aquifer with high concentrations of iron and manganese. In the open-loop system, iron and manganese is oxidized to form insoluble compounds incrusting well screens, pipes and heat exchangers. The incrustations are detected in all of the nine plants where they result in higher maintenance costs and lower energy savings for the systems. However, their genesis is still not fully uncovered, and may include oxygenation by air contact or mixing, bacteria, or CO2 degassing, possibly combined with mechanical incrustation. This hampers the selection of a proper remediation technique.
To investigate the groundwater chemistry and the cause of incrustations in the Melhus plants, a flow-through cell has been constructed. This is connected to the groundwater pipes through taps in the machinery rooms diverting water from the system through the cell. In the flow-through cell electric conductivity, dissolved oxygen, pH and redox potential is continuously monitored, thus facilitating easy, continuous measurement of time variations in the parameters without atmospheric interaction. In addition, field observations of bubbles, odors and sedimentation in the cell may be provide valuable information. Furthermore, shutting the valves and disconnecting the flow-through cell offers the possibility to investigate the kinetics and temperature dependence of CO2 degassing. Combining these field observations and data with alkalinity measurements and water samples analyzed for cations and anions, may uncover the redox processes and incrustation pathways in the open-loop systems.
In the presentation, results from flow-through cells measurements will be presented and analyzed. Also, alternative incrustation characterization approaches and possible remediation techniques will be discussed.