Monitoring of a heat pump system using deep borehole heat exchangers

Sammanfattning: Sweden has been one of the first countries in the world to use ground-source heat pumps (GSHP) to supply heating and cooling to its buildings. Today, it is the leading country in Europe and new installations tend to have larger capacities. The use of deeper borehole heat exchangers (BHE) is an opportunity to extract larger amounts of heat over small land areas. However, there are only few case studies on BHE deeper than 300 m, hence such systems may be optimized. This study focuses on a deep GSHP system recently installed in central Stockholm, composed of four 510 m deep BHE. The objectives were to get the system ready for monitoring and to analyze the first sets of data recorded. First, a review of all the sensors already installed, of the data needed, and of the different ways to extract it, has been led. Practically, an acquisition system has been set up and connected to new and existing sensors such as thermometers and flow-meters. Theoretically, a method to derive the thermodynamic cycles of the different heat pumps has been determined. It led to the determination of COPs for several days during late spring 2017. The system globally showed reasonable efficiency, with an overall performance factor (equivalent to SPF2) of 3.42 including the circulation pumps of the ground loop. However, it could certainly be improved in several ways, for example by avoiding short cycles or by finding an optimum flow in the secondary ground loop. Furthermore, these results should be juxtaposed with those that will be obtained during winter, when the heating demand will be the highest.