published_or_final_version / Applied Geosciences / Master / Master of Science

Detta examensarbete utfördes på uppdrag av Umeå Kommun. Uppgiften bestod av att utvärdera dagens användning av en borrhålsbrunn samt undersöka vad som är det optimala användningsområdet för borrhålsbrunnen. Borrhålsbrunnen används i dagsläget för att förvärma samt kyla utomhusluft in till kontorsbyggnaden Kubens ventilationsaggregat. Ventilationens förvärmning består av två delar, en markkanal och ett geoenergibatteri. Viktiga parametrar hos förvärmningen analyserades med hjälp av mätvärden för temperatur och flöde. Resultaten jämfördes därefter med alternativet att använda borrhålsbrunnen tillsammans med en värmepump. Då borrhålsbrunnens kapacitet inte var tillräcklig för att klara byggnadens hela uppvärmningsbehov, undersöktes det hur en värmepump skulle kunna köras i kombination med fjärrvärme. Två driftstrategier, Bas och Kapatoppar, undersöktes. Bas leverar en basproduktion under hela vinterperioden medan Kapatoppar startar vid -6°C för att sänka effekttoppar. Utvärderingen av förvärmningen visade att geoenergibatteriet är mer kompatibelt med ventilationsaggregat av VAV-typ (Variable Air Volume) än vad markkanalen är. Däremot är inte förvärmning i kombination med roterande värmeväxlare ett bra koncept för byggnaden som den används idag. På grund av att förvärmningen är placerad före den roterande värmeväxlaren så är bara 15-20 % av förvärmningseffekterna energibesparande. Detta kombinerat med en optimerad drift av aggregatet sett till tidsstyrning och behovsstyrning av flöde gör att förvärmningen har en liten påverkan både på byggnadens maximala effektbehov och totala energibehov. Att köra värmepump i kombination med fjärrvärme för uppvärmning var ekonomiskt lönsamt främst på grund av att värmepumpen kunde kapa byggnadens effektoppar. Kostnaden för storleken på den abonnerade effekten uppgår idag till drygt 40 % av den totala fjärrvärmekostnaden. Dagens relation mellan el- och fjärrvärmepriser bidrog naturligtvis också till att värmepumpsalternativet var lönsamt. För de undersökta förutsättningarna så blev paybacktiden för en värmepump med den lönsammaste driftstrategin, Bas, 4,4 år.

Since the ancient years, human beings were using holes and caves to protect themselves from weather conditions making it the first known form of exploiting ground’s heat, known as Geothermal Energy. Nowadays, geothermal energy is mainly used for electricity production, space heating and cooling, Ground Coupled Heat Pump (GCHP) applications, and many other purposes depending on the morphology of the ground and its temperature. This study presents results of investigations into the evaluation of the thermal properties of the ground in Cyprus. The main objectives were i) to determine the thermal characteristics of the ground in Cyprus, ii) investigate how they affect the sizing and positioning of Ground Heat Exchangers (GHE) and iii) present the results for various ground depths, including a temperature map of the island, as a guide for engineers and specifiers of GCHPs. It was concluded that there is a potential for the efficient exploitation of the thermal properties of the ground in Cyprus for geothermal applications leading to significant savings in power and money as well. Six new boreholes were drilled and two existing ones were used for the investigation and determination of i) the temperature of the ground at various depths, ii) its thermal conductivity, iii) its specific heat and iv) its density. The thermal conductivity was determined by carrying out experiments using the line source method and was found to vary in the range between 1.35 and 2.1 W/mK. It was also observed that the thermal conductivity is strongly affected by the degree of saturation of the ground. The temperature of the undisturbed ground in the 8 borehole locations was recorded monthly for a period of 1 year. The investigations showed that the surface zone reaches a depth of 0.25 m and the shallow zone 7 to 8 m. The undisturbed ground temperature in the deep zone was measured to be in the range of 18.3 °C to 23.6 °C and is strongly dependent on the soil type. Since the ground temperature is a vital parameter in ground thermal applications, the temperature of the ground in locations that no information is available was predicted using Artificial Neural Networks and the temperature map of the island at depths of 20 m, 50 m and 100 m was generated. Data obtained at the location of each borehole were used for the training of the network. Data for the sizing of GHEs based on the ground properties of Cyprus were presented in an easily accessible form so that they can be used as a guide for preliminary system sizing calculations. With the aid of Computational Fluid Dynamics (CFD) software the capacity of the GHEs in each location and the optimum distance between them was estimated. Additionally, the long term temperature variation of the ground was investigated. For the first time since a limited study in the 1970’s, a research focusing on the determination and presentation of the thermal properties of the ground in Cyprus has been carried out. Additionally, the use of Artificial Neural Networks (ANNs) is an innovative approach for the prediction of data at locations where no information is available. The publication of this information not only contributes to knowledge locally but also internationally as it enables comparison with other countries with similar climatic conditions to be carried out.

The aim of the project was to study the potential of borehole systems in Portugal, once its need for acclimatization is increasing. At the same time endogenous and renewable energies are important and necessary to be implemented in the country. The borehole system is a renewable and reliable new technology for heating and/or cooling that has been used in several countries. The potential of borehole systems in Portugal was carried out by evaluating a number of criteria: heating/cooling demand, energy demand, climate, geology, hydrology and population density. The compilation of these criteria resulted in a number of different geographical areas well defined. For all of these areas the EED (Earth and Energy Designer) program was used to calculate the required borehole systems for three different types of buildings. It was concluded that Portugal has moderate to high potential for borehole system applications. The most favorable conditions were found in the north of the country. Borehole systems were increasingly less feasible from the centre to the south and from inland to the coast. The main reasons for this difference was the high cooling demand and the high ground temperatures of the southern areas. At the same time high acclimatization demands gives higher efficiencies for borehole system applications for all the areas of mainland Portugal. / <p>Validerat; 20101217 (root)</p>

This thesis investigates the differences in Bonner Sphere detector response for anisotropic neutron fields as a function of borehole orientation. Monte Carlo simulations using MCNPX were used to calculate the difference for a borehole oriented directly behind a unidirectional neutron field and one in which the borehole is normal to the neutron flux. The differences in detector response depend on the size of the Bonner Sphere and the energy of the incident flux, which could introduce significant error in the determination of the neutron field’s energy spectrum.

Ground heat storage systems can play an important role for the reduction of green house gases emissions by increasing the exploitation of renewable energy sources and “waste heat” with a consequent diminution of the use of fossil fuels. A ground heat storage consists in an array of vertical boreholes placed in such a way that promotes the mutual thermal interaction between the ground heat exchangers creating the necessary conditions required to effectively store and retrieve heat. Suitable modeling tools for the estimation of the thermal behavior of these systems are very important to build installations yielding economical performance compatible with what expected during the design phase. This thesis aims at giving a contribution in the development of the thermal modeling of borehole heat storage systems. The main objective is introducing in the modeling process a few features that are not usually considered in state of the art models, with the goal of improving the representation of the physical phenomena. These features are the mathematical description of the topology of the borehole heat exchangers network, and the modeling of borehole fields with arbitrarily oriented boreholes. The detailed modeling of the topology of the borehole heat exchangers is approached with a network model. The overall geothermal system is discretized into smaller systems called components. These are linked between each other in a network fashion to establish the logical relations required to describe a given boreholes connections arrangement. The method showed that the combination of a sufficient level of discretization of the system and of a network representation yields respectively the granularity and the flexibility required to describe any borehole field connections configuration. The modeling of non-vertical borehole fields is approached by developing a method for the calculation of g-functions for these configurations. The method is an extension of a recent work done by Cimmino on the computation of g-functions for vertical borehole fields. This modeling technique is based on describing boreholes as sets of stacked finite line sources and on the superposition principle. This approach requires the computation of response factors relative to couples of finite lines. A procedure for the fast computation of these response factors for the case of arbitrarily oriented lines is given. This yields computational performance that guarantees the practical feasibility of the methodology. The last part of the thesis deals with the modeling of the storage system from a broader perspective. The borehole field is considered as part of a larger system constituted by several interacting components (i.e. heat pump, building, etc.). Interactions play a key role in the resulting overall performance of these systems. The analysis of the mutual relations between building envelope and borehole field design is utilized as an example to highlight advantages and challenges of strategies yielding a more integrated design. / <p>QC 20150507</p>

In this thesis proposals for different designs of a borehole thermal energy storage (BTES) have been developed for the building DN-huset in Stockholm, Sweden. To build a BTES results in savings in energy costs by approximately 44 %, i.e. 2 million Swedish crowns annually. Furthermore, a BTES would reduce the annual environmental impact with roughly 75-157 tonnes of CO2 equivalents per year, depending on how the electricity consumption’s environmental impact is estimated. The payback period is about 11 years, including the warm-up period that is necessary before commissioning the BTES. The savings in environmental impact and operating costs are a result of energy being reused. During the summer heat is stored in the bedrock beneath the building for retrieval about half a year later in the winter, when there is a heating demand. In addition to developing proposals for different BTES designs the thesis also examines the influence of certain design parameters, conservative choices and operating conditions.

Thesis (M.Sc.(Earth Science Management and Practice))--University of Pretoria, 2007. / Abstract in English. Includes bibliographical references (leaves 95-99).

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 (Ground source heat pump), uses geothermal energy which is a form of green and sustainable energy.  Geothermal energy is also a continuous source of energy, unlike wind energy. The results from this thesis work will be applicable for both GSHP that are being used for space heating, and the ones which have a bottom organic Rankine cycle. The bottom organic Rankine cycle and continuous energy production of GSHP make it a potential source for electricity generation.  The GSHP is of various types, in regard to the configuration of the pipe and their setup in the ground and also based on their grouting. In this study only vertical GSHP and with a single u-tube and water filled grout will be analyzed. The GSHP performance is based on a number of parameters including, the depth of the heat exchanging unit in the ground, other key dimensions of the unit like diameter and outer wall thickness, the fluid flow, and the type of working fluid. Therefore it becomes necessary to study the effect of all of these parameters individually and their individual effect on the energy output and the performance of the BHE. One of the thesis objectives is to establish a sensitivity analysis of the BHE based on the above mention parameters and then further optimize the design with the heat enhancement devices. The major findings of this thesis work are how shank spacing (spacing between the inlet and the outlet pipe) affects the heat transfer in the BHE. The shank spacing seems to reduce the energy output of the GSHP, this is contrary to the high conductive solid grout, where the shank spacing doesn't affect the BHE so much. The diameter of the BHE in the water-filled grout has a completely opposite effect from the solid grout. Increasing the depth of the BHE after a certain length only increases the entropy of the system which reduces the energy output. The working fluid with a higher Prandtl number helps in higher energy output. The optimization results suggest that having a deeper borehole is not very energy efficient in spite of the greater thermal gradient available at a higher depth.