As an alternative and renewable energy source, the shallow geothermal energy evolving as one of the most popular energy source due to its easy accessibility and availability worldwide, and the ground source heat pump (GSHP) systems are the most frequent applications for extracting the energy from the shallow subsurface. As the heat extraction capacity of the GSHP system applications arises, the design of the borehole heat exchangers (BHE), which is the connected part of the system in the ground, become more important. The backfilling materials of BHEs, particularly, the grout material must provide a suitable thermal contact between the ground and the heat carrier fluid in the high density polyethylene (HDPE) pipes and ensure durability to the induced thermal stresses due to the heat loading. In addition, for the heating purposes of buildings, BHEs immerged in groundwater may be operated below the freezing point of water with anti-freeze mixture in the pipe, leading to freezing-induced ice pressure which may damage the grout.In order to propose a proper grouting for BHEs, the thermo-hydro-mechanical behavior of the grout and its interferences with the adjacent ground conditions must be evaluated in the near field, and the thermal interactions of each BHE in a multi-BHEs field in the long-term operations must also be considered at a further field.Primarily, we have evaluated the performance of various grouting materials, through thermal, hydraulic and mechanical laboratory characterizations. In particular, we have proposed a homemade grout material, with the addition of graphite powder to improve the thermal properties of grout material. In parallel, the characteristics of two different widely used commercial grouting materials (i.e. calcite-based and silica-sand based materials) have been also investigated. In the subsequent study, the heat flow rate per meter of a BHE and the borehole resistance of borehole heat exchangers are assessed experimentally in a 1×1×1 m3 sandbox under, successively, dry sand and fully water-saturated sand conditions. During the operations, the monitored temperatures in the sandbox are in good agreement with analytical predictions. This study demonstrated that the homemade admixture prepared with 5 % natural flake graphite can be considered as an appropriate grout for BHEs regarding to its rheological and thermo-physical properties. Thermally-enhanced grouting can be of significant interest in a high thermal conductivity ground (such as saturated sand) because it minimizes the thermal resistance of the BHE.After characterizing and testing the efficiency of various grout materials, the thermal stresses occurred in BHEs due to heat injection or extraction has been investigated with the analytical solution of hollow cylinder model that is adapted for time-dependent heat loading, the geometry of a BHE, and the thermo-mechanical properties of surrounding ground conditions. Firstly, the hollow cylinder model has been solved for the considered boundary conditions in 2D plane stress. Secondly, the temperature differences at the inner and outer circles of the cylinder is evaluated with the heat line source models for continuous and discontinuous loadings to observe the impact of the heat loading schedule. The developed analytical solution for thermal stress investigation is validated with numerical models. It is demonstrated that the analytical solutions agree well with numerical results for two types of BHE configurations (co-axial and single U-shaped pipes). Furthermore, the calculated maximum stresses are compared with the tensile strength of grout materials obtained from Brazilian tests. It is predicted that thermal contraction of the grout, partially constrained by the surrounding rock, generates tensile stresses that may lead to cracking in the BHE. According to the results, the stiffness of rock has primary role on the developed tensile stresses, and the relationship between the thermal conductivity of the ground and of the grout induces a proportional impact on the magnitude of thermal stresses.Another major concern is the freeze-resistance of the grout materials, when the system is operated for heating purposes. Firstly, we conducted an experimental setup in a small-scale sandbox to understand the behavior of the grout material by evaluating the permeability change during freeze-thaw cycles of a BHE. According to the results, the permeability of grout materials did not change after 10 freeze-thaw cycles due to the thermal transfer with the adjacent soil partially reducing the impact of freezing in the grout material. Therefore, in order to test the freeze-resistance of a BHE, we have investigated the freezing impact of pore water pressure and thermal stress with analytical models and experimental setups on BHEs. For the theoretical approach, an analytical solution has been developed by using the hollow cylinder model that accounts for both the HDPE pipe and the grout material. Firstly, the freezing pore water pressure is adapted to the generalized Hooke’s law equations in 2D plane stress, and secondly the model is solved for the considered boundary conditions. In order to validate the developed model, the experimental setup is conducted in agreement with the geometry of the considered analytical model and the BHE probes are prepared with three different grout materials having large difference in the thermal and hydraulic characteristics (i.e. silica-sand based, calcite based and the homemade enhanced thermally with natural flake graphite powder). According to the experiments for 50 h of freezing operation, the calcite based grout and the homemade grout, having lower permeability and relatively higher porosity, are fractured. In contrast, the silica-sand based grout having higher permeability did not exhibit any damage. Compared with the theoretically obtained results, the observations from the experiments are consistent with the calculated stress results. The effective tangential stress induced by the freezing pore water pressure causes the crack development and agrees with the crack patterns. As a conclusion, the porosity and the permeability play a significant role on the grout failure.In a multi-BHEs field, the thermal interaction between each BHE may have a significant influence on the near-field investigation results in long-term operations. Therefore, in order to complete the near-field investigation, a far-field long-term operation study is required. However, existing analytical solutions for thermal analysis of ground source heat pump (GSHP) systems evaluate temperature change in the carrier-fluid and the surrounding ground in the production period of a single BHE only if a continuous heat load is assigned. In this study, we modified the Green’s function, which is the solution of heat conduction/advection/dispersion equation in porous media, for discontinuous heat extraction by analytically convoluting rectangular function or pulses in time domain both for single and multi-BHEs field. The adapted analytical models for discontinuous heat extraction are verified with numerical finite element code. The comparison results agree well with numerical results both for conduction and advection dominated heat transfer systems, and analytical solutions provide significantly shorter runtime compared to numerical simulations (approx. 1500 times shorter). Furthermore, we investigated the sustainability and recovery aspects of GSHP systems by using proposed analytical models under different hydro-geological conditions. According to the engineering guideline VDI 4640, a linear relationship between thermal conductivity of the ground and the sustainable heat extraction rate is demonstrated for multi-BHEs. In addition, we developed an MATLAB interface for users in which the analytical model can be used easily and more efficiently.In addition, in order to extend the case studies for a ground including several layers, we proposed a finite line source model for BHEs that takes into account conduction/advection/dispersion mechanism in multilayer porous media. Firstly, the anisotropy is added to the moving finite line source model, and we used an existing composite model approach for conductive multilayer ground. The comparison with the numerical model results demonstrates the suitability of the approach. The proposed model can provide a faster solution than classical numerical approaches and help to optimize the heat extraction rate in multilayer media. However, further investigations are required to validate the model with the field measurements. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

GSHP systems are common in Sweden but there are few evaluations quantifying the performance of the systems and highlighting problem that occurs during operations. The research project Annex 52 Long-term performance measurement of GSHP systems serving commercial, institutional and multifamily building part of IEA HPT TCP proves the need to systematically be able to evaluate GSHP systems. This thesis aims to expand the knowledge of how to evaluate GSHP systems and provide case studies for Annex 52. Three residential ground source heating systems used for heating has been evaluated and analyzed in this study. The evaluation has consisted of three parts. The first part analyzes the operation and stability of the GSHP systems. The second part consist of a detailed study of the performance of the GSHP systems. The seasonal performance factor has been calculated for different system boundaries based on the work done by SEPEMO. In addition, a method on how to evaluate the efficiency of the heat pumps based on the two temperature levels, source side temperature and the heat sink temperature, that the heat pump is operating at throughout a year has been developed within this thesis. This has included a method on how to normalize the temperatures based on the operation of the heat pump in order to quantify one temperature for each the two temperature levels. The third part consist of a comparison of the mean secondary fluid temperature between the calculated temperature using the software EED and the measured temperatures. This includes a comparison evaluation and sensitivity analysis on input parameters during the design of the borehole heat exchanger fields. This study has expanded the available reference cases of GSHP systems in Sweden. It also can be used as a guideline for those who will evaluate future GSHP systems. Designers of GSHP system will also benefit from the recommendations listed in this thesis regarding instrumentation and possible problems that may occur. The results show that the evaluation successfully managed to quantify the performance and operational issues that have occurred for each system. The method developed in this study was able to quantify the operation of the different systems based on the temperature levels and can be used for future GSHP evaluations of similar system type. / Bergvärmesystem är vanligt förekommande i Sverige men trots detta finns det få studier där prestandan har utvärderats och de vanligt förekommande problemen under drift har belysts. Forskningsprojektet Annex 52 Annex 52 Long-term performance measurement of GSHP systems serving commercial, institutional and multi-family building som är en del av IEA HPT TCP visar på behovet av att systematisk utvärdera bergvärmesystem. Detta examensarbete syftar till att utveckla och bidra till kunskap om hur bergvärmesystem kan utvärderas och att bidra med exempelstudier till Annex 52. Inom detta examensarbete har tre bergvärmesystem som betjänar flerbostadshus utvärderats och analyserats. Utvärderingen bestod av tre analyser. I den första analyserades driften av bergvärmesystemen och hur stabilt systemet har varit historiskt. Detta följdes av en detaljerad analys av olika nyckeltal för bergvärmesystemen. Årsverkningsgraden har beräknats för olika gränsdragningar vilka baseras på det tidigare arbetet utfört av SEPEMO. Inom detta examensarbete har även en metod tagits fram för att utvärdera verkningsgraderna för en värmepump baserat på de två temperaturnivåerna, köldbärarsidan och värmebärarsidan, som värmepumpen arbetar med under ett år. Till detta har en metod tagits fram om hur temperaturen kan normaliserats baserat på driften av värmepumparna för att kvantifiera en temperatur vardera för de två temperaturnivåerna. I den tredje utvärderingen jämfördes den beräknade medelfluidtemperaturen av köldbäraren i borrhålen med den uppmätta temperaturen. Till detta utfördes en känslighetsanalys av hur indata av dessa beräkningar påverkar resultaten.