Diagnosing Encrustation Problems in Open-loop Groundwater Heat Pump Systems in Melhus, Norway

Authors: Lars Aaberg STENVIK, Sondre GJENGEDAL, Bjørn Syver FRENGSTAD, Randi Kalskin RAMSTAD
Keywords: open-loop GWHP, well clogging, incrustations, iron, manganese, groundwater, geochemistry
Conference: World Geothermal Congress Session: Geothermal Heat Pumps
Year: 2020 Language: English
Abstract: Groundwater in unconsolidated sediments represents a local and renewable energy resource. Utilizing the groundwater as an energy source for open-loop ground source heat pumps offers the possibility of cost-effective heating and cooling of buildings. Still the method is not utilized to its full potential. However, in the town of Melhus, Norway the method is widely used, with currently nine plants operating. All the plants have experienced incrustation problems, with iron oxides as the major incrustation product. This paper includes groundwater chemical data from seven of these plants. The groundwater is anoxic and with a relatively low pH, leading to metal dissolution. When the water enters the system, mixing of different water qualities, contact with the atmosphere and CO2 degassing may occur. This will lead to oxygenation and a pH increase respectively, consequently triggering oxidation leading to insoluble oxides precipitating. The possible presence of iron related bacteria catalyze oxidation. These problems may be accompanied with mechanical incrustations, caused by sedimentation of suspended solids in the groundwater. With time, these precipitations will encrust and clog parts of the system, e.g. well screens and heat exchangers, leading to a decreased water extraction and heat transfer capacity. Results from field measurements and lab analysis of water samples in seven open loop systems in Melhus suggest that the water in the open-loop system is oversaturated with regards to iron oxides. High iron concentrations ( more than 3 mg/l) lead to oversaturation, despite a low dissolved oxygen content (≤ 0,05 mg/l). This matches data on incrustation type from video inspection of wells and mineralogical analysis of the incrustation material. However, data on oxygenation pathways, CO2 degassing, microbiology and suspended solids in the systems must be retrieved to fully diagnose the incrustation problems in Melhus.
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