Advanced shallow geothermal systems: Temperature induced cracking of backfill materials and system hydraulic conductivity

摘要

Shallow geothermal systems are one of the key technologies for a renewable and sustainablernenergy supply. The most common system is the borehole heat exchanger (BHE). These systems are capable tornprovide both, heating and cooling. A significant reduction of fossil thermal and electrical energy consumptionrncan be achieved by using this kind of renewable geothermal energy. Furthermore the underground can be used asrna thermal storage (UTES). For an efficient use of these advanced systems, fast thermal loading cycles are needed.rnHowever, legal constraints for the groundwater protection must be fulfilled as well. The fast heat extraction fromrnthe underground requires materials that are resistant to rapid temperature changes. These temperature changesrnmight include temperatures below zero degrees Celsius and consequently a potential freezing of the pore waterrnof the surrounding grouting material. The frost resistance of grouting materials is discussed controversy over thernpast decades. Recently a testing procedure was developed that quantifies the influences of freeze-thaw-cycles onrnthe hydraulic conductivity of the system BHE. The main component is a testing device that simulates the in-siturngeomechanical boundary conditions and quantifies the sealing capability of the grout. Due to the considerationrnof the in-situ direction of the freezing process, the results differ substantially from earlier investigations on frostrnresistance.With this procedure standardized and repeatable evaluations become feasible. This paper presents therntesting device itself including numerical and experimental proofs of concept. Besides the numerical simulationrnof the phase change processes inside a grout specimen, results from calibration and round robin test will berndiscussed. The thermo-hydraulic influences and implications on the modelling of the heat flow in BHEs arernanalysed. The experimental results enable a comparison of the frost resistance of commercial grouts used (e.g.rnin central Europe). Finally the options and needs for further investigations and the implementation for thernconstruction and operation of advanced geothermal systems are discussed.