Groundwater heat pump systems. New insights on system design and on fluid flow in unconsolidated aquifers

Abstract

This PhD thesis concerns groundwater heat pump (GWHP) systems in Norway, which are systems that utilize groundwater as a source of thermal energy for heating and cooling applications in buildings. GWHP systems can typically enable consumers to save 70 % or more of their electric energy demand for heating applications compared to conventional electric heating. The main study site for this PhD project is in Melhus, just south of Trondheim, where ten such GWHP systems currently utilize groundwater from an unconsolidated aquifer beneath the town center, the so-called Melhus aquifer. Both the design and the operational strategy employed by the GWHP systems influence their efficiency and long-term operations. This thesis focus on, and demonstrate, that it is vitally important to understand how groundwater flows through both the aquifer and the GWHP system to ensure a reliable, sustainable, and efficient utilization of the groundwater resource.

Six scientific articles form the core of this thesis and addresses various topics of groundwater flow through GWHP systems. Five of the papers investigate problems due to clogging, which frequently trouble GWHP operations. All GWHP systems in Melhus are affected by clogging problems that disrupt the flow of water, and thereby the heating and cooling process. Clogging can occur due to a variety of reasons and is a severe challenge for the successful long-term operation. In some cases, clogging mechanisms have caused the entire GWHP system to malfunction. Routine surveillance of the operation and regular maintenance is thus important for GWHP systems. A surveillance method that measures the hydraulic and thermal performance of the GWHP system is presented and tested for this purpose. This method can detect where and when maintenance is needed and allow a GWHP-specialist to follow up and interpret the performance data and schedule for the correct maintenance measures. Also, one possible clogging trigger event has been analyzed specifically, because it is triggered by the design of the system layout and occurs when vacuum pressures develop in the pipes and in the wells. Vacuum pressures affect the solubility of gases in groundwater and can trigger exsolution of dissolved gas, which has the potential to catalyze chemical reactions that precipitate particles, such as iron or manganese hydroxides. These particles are frequently found to be causing the clogging itself.

The last paper focus on the groundwater flow properties of aquifers. Pumping of groundwater from an aquifer induce hydraulic strain in the soil. The magnitude of induced hydraulic strain is governed by the permeability of the pores within this soil. This phenomenon was studied by means of a novel and innovative 3D technique, which employs 3D-printing, CT scanning and laboratory work. It is demonstrated that the permeability of a single pore can be described by the Stokes equation. The pore permeability is a function of the pore geometry, where the pore shape, the specific porosity, the specific surface area, and the contraction ratio of the pore channel, have governing roles. This knowledge is valuable when interpreting hydrogeological data, GWHP performance data, and for numerical modeling of groundwater flow in aquifers in general.

Finally, the thesis highlights some practical aspects of GWHP design for Norwegian conditions. To ensure the best design of a GWHP systems it is crucial that the GWHP-specialist is involved at an early stage in new projects and that pre-investigations are performed on-site in advance of system construction. Regular maintenance and surveillance is crucial for a successful operation. Maintenance and operational control necessitate that the GWHP design and the groundwater pipe layout allow easy access for cleaning equipment to all parts of the groundwater system. For Norwegian conditions it is recommended that the GWHP operation utilize the full thermal potential of each liter of groundwater. The groundwater temperature should be lowered as much as possible, restricted to the lower limit of 2°C after heat exchange. This will enable the operation to rely on a minimal pumping rate which will contribute to a reducing the risk of clogging.