Groundwater source heat pumps systems: Principles and challenges

Groundwater in unconsolidated sediments represents a local and renewable energy resource. Furthermore, by use of a groundwater based heat pump system (from here on the system), heating and cooling costs of buildings may decrease by 70% compared to conventional electrical heating and cooling. Yet the method is not much utilized in Norway, probably caused by lack of knowledge. In this regard, the ORMEL project (Optimal utilization of groundwater for heating and cooling in Melhus and Elverum) was initiated (2015-2018). Due to its success, a sequel, ORMEL 2, followed from autumn 2018. From a hydrogeological point of view, the system comprises three main components: 1) an extraction well, extracting water from the aquifer, i.e. the groundwater reservoir, 2) a heat pump system, where heat is transferred from/to the groundwater to/from the building, and 3) an injection well, where water is infiltrated back into the aquifer. However, the groundwater chemistry may cause problems for the system. One of the challenges is the dissolution and precipitation of iron and manganese. In the aquifer the groundwater is often reducing and with a relatively low pH, leading to metal dissolution. When the water enters the system, contact with the atmosphere and CO2 degassing may occur. This will lead to oxidation and a pH increase respectively, consequently triggering precipitations. The presence of bacteria and mixing of different water qualities will contribute as precipitation catalyzers. With time, these precipitations will encrust and thus clog parts of the system, e.g. well screens and heat pump system, leading to a decreased water extraction and heat transfer capacity. Unfortunately, there is a knowledge gap regarding remedies to the encrustation problems. Therefore, as part of ORMEL 2, the first author’s PhD will investigate the processes causing encrustations, and test and evaluate system design, maintenance and operation strategies to avoid the problem.