<p>The sales of Ground Source Heat Pumps in Sweden and many other countries are having a rapid growth in the last decade. Today, there are approximately 360 000 systems installed in Sweden, with a growing rate of about 30 000 installations per year. The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a Borehole Heat Exchanger (BHE), a closed loop in a vertical borehole. The fluid transports the heat from the ground to a certain heating and/or cooling application. A fluid with one degree higher or lower temperature coming out from the borehole may represent a 2-3% change in the COP of a heat pump system. It is therefore of great relevance to design cost effective and easy to install borehole heat exchangers. U-pipe BHEs consisting of two equal cylindrical pipes connected together at the borehole bottom have dominated the market for several years in spite of their relatively poor thermal performance and, still, there exist many uncertainties about how to optimize them. Although more efficient BHEs have been discussed for many years, the introduction of new designs has been practically lacking. However, the interest for innovation within this field is increasing nowadays and more effective methods for injecting or extracting heat into/from the ground (better BHEs) with smaller temperature differences between the heat secondary fluid and the surrounding bedrock must be suggested for introduction into the market.</p><p>This report presents the analysis of several groundwater filled borehole heat exchangers, including standard and alternative U-pipe configurations (e.g. with spacers, grooves), as well as two coaxial designs. The study embraces measurements of borehole deviation, ground water flow, undisturbed ground temperature profile, secondary fluid and groundwater temperature variations in time, theoretical analyses with a FEM software, Distributed Thermal Response Test (DTRT), and pressure drop. Significant attention is devoted to distributed temperature measurements using optic fiber cables along the BHEs during heat extraction and heat injection from and to the ground.</p> / QC 20100517 / EFFSYS2 / Efficient Use of Energy Wells for Heat Pumps

The borehole heat exchangers of today suffer from poor thermal and hydrodynamic performance. The purpose of this thesis is to improve the performance of ground source heat pump systems and thermal energy storages by increasing the energy efficiency of the borehole heat exchangers. For this reason, the annular coaxial borehole heat exchanger (CBHE) has been analyzed. This type of heat exchanger is interesting in terms of both thermal and hydrodynamic performance. A model has been set up in the program Comsol Multiphysics in order to investigate the heat transfer characteristics along the borehole. A literature survey that summarizes the analytical calculation methods developed in earlier Swedish research is presented in the report. Different geometries with or without insulation of the central pipe have been analyzed and the effective borehole resistance for each geometry has been calculated based on the simulation results. The model has been validated against a recently performed thermal response test, and shows very good correlation with reality. The results from the simulations show that by using the annular CBHE an increase of 2-3 °C in the evaporator of the heat pump can be achieved. Calculations show that the pump work (head loss) can be reduced to 1/6 of the corresponding case with a single U-pipe. There arises a vertical temperature gradient in the bedrock when recharging and extracting heat with the annular CBHE. This means that the annular CBHE acts like a counter-flow heat exchanger which is thermally optimal. In total, the simulation result shows that the annular CBHE geometry in this thesis can increase a system's seasonal performance factor (SPF) with 10-19 % in comparison with a U-pipe BHE. This is equivalent to 10-19 % lower electrical power consumption every year.

The ground source resources are becoming more and more popular and now the ground source heat pumps are frequently used for heating and cooling different types of buildings. This thesis aims at giving a contribution in the development of the thermal modelling of borehole heat storage systems. Furthermore, its objective is to investigate the possibility of implementing of a GSHP (ground source heat pump) with vertical boreholes, in order to deliver the heating and cooling demand for a passive house and to emphasize some certain advantages of this equipment even in the case of a small building (e.g. residential house). A case study is presented to a suitable modelling tool for the estimation of the thermal behaviour of these systems GSHP by combining the outcome from different modelling programs. In order to do that, a very efficient residential solar house (EFden House – a passive residential single-family house, which was projected and built in Bucharest with academic purposes) is being analysed. The numerical results are produced using the software DesignBuilder, EED (Earth Energy Designer) and a sizing method for the length of the boreholes (ASHRAE method). The idea of using 2 different modelling programs and another sizing method for the borehole heat exchanger design (ASHRAE method) is to make sure that all the calculations and results are valid and reliable when analysing such a system theoretically (in the first phases of implementing a project), before performing a geotechnical study or a thermal response test in order to assess the feasibility of such a project beforehand. The results highlight that the length of the borehole, which is the main design parameter and also a good index in estimating the cost of the system, is directly influenced by the other fundamental variables like thermal conductivity of the grout, of the soil and the heat carrier fluid. Also, some correlations between these parameters and the COP (coefficient of performance) of the system were made. The idea of sizing the length of boreholes using two different methods shows the reliability of the modelling tool. The results showed a difference of only 2.5%.  Moreover, the length of borehole is very important as it was calculated that can trigger a difference in electricity consumption of the GSHP up to 28%. It also showed the fact that the design of the whole system can be done beforehand just using modelling tools, without performing tests in-situ. The method aims at being considered as an efficient tool to estimate the length of the borehole of a GSHP system using several modelling tools. / <p>The presentation was made via Skype due to the programme being online based</p>

In the context of energy transition, geothermics play an important role for the heating and cooling supply of both residential and commercial buildings. Thereby, the increasingly and intensive utilisation of shallow geothermal resources bears the risk of over-exploitation and thus poses a future challenge to ensure the sustainability and safety of such systems. Particularly, the well-established technology of borehole heat exchanger-coupled ground source heat pumps is applied for the thermal exploitation of the shallow subsurface. Due to the complexity of the involved physical processes, numerical modelling proves to be a powerful tool to enhance process understanding as well as to aid the planning and design processes. Simulations can also support the management of thermal subsurface resources, planning and decision-making on city and regional scales. In this work, the so-called dual-continuum approach was adopted and enhanced to develop a coupled numerical model considering flow and heat transport processes in both the subsurface and borehole heat exchangers as well as the heat pumps’ performance characteristics, and including the relevant phenomena influencing the underlying processes. Beside the temperature fields, the efficiency and thus the consumption of electrical energy by the heat pump is computed, allowing for the quantification of operational costs and equivalent carbon-dioxide emissions. The model is validated and applied to a number of numerical studies. First, a comprehensive sensitivity analysis on the efficiency and sustainability of such systems is performed. Second, a method for the quantification of technically extractable shallow geothermal energy is proposed. This procedure is demonstrated by means of a case study for the city of Cologne, Germany and its implications are discussed. / Im Rahmen der Energiewende nimmt die Geothermie eine besondere Rolle in der thermische Gebäudeversorgung ein. Die zunehmende, intensive Nutzung oberflächennaher geothermischer Ressourcen erhöht die Gefahr der übermäßigen thermischen Ausbeutung des Untergrundes und stellt damit eine wachsende Herausforderung für die Nachhaltigkeit und Sicherheit solcher Systeme dar. Zur Erschließung oberflächennaher geothermischer Energie wird insbesondere die etablierte Technologie Erdwärmesonden-gekoppelter Wärmepumpen eingesetzt. Aufgrund der daran beteiligten komplexen physikalischen Prozesse erweisen sich numerische Modelle als leistungsfähiges Werkzeug zur Erweiterung des Prozessverständnisses und Unterstützung des Planungs- und Auslegungsprozesses. Zudem können Simulationen zum Management thermischer Ressourcen im Untergrund sowie zur Planung und politischen Entscheidungsfindung auf städtischen und regionalen Maßstäben beitragen. Im Rahmen dieser Arbeit wurde, basierend auf dem sogenannten ”dual-continuum approach” und unter Berücksichtigung des Einflusses der Wärmepumpe, ein erweitertes gekoppeltes numerisches Modell zur Abbildung der in Erdwärmesonden und dem Untergrund stattfindenden Strömungs- und Wärmetransportprozesse entwickelt. Das Modell ist in der Lage, alle relevanten Einflussfaktoren zu berücksichtigen. Neben den Temperaturfeldern im Untergrund und der Erdwärmesonde werden die Effizienz und damit der Stromverbrauch der Wärmepumpe simuliert. Damit können sowohl die Betriebskosten als auch der äquivalente CO 2 -Ausstoß abgeschätzt werden. Das Modell wurde validiert und in einer Reihe numerischer Studien eingesetzt. Zuerst wurde eine umfassende Sensitivitätsanalyse zur Effizienz und Nachhaltigkeit entsprechender Anlagen durchgeführt. Weiterhin wird ein Verfahren zur Quantifizierung des technisch nutzbaren, oberflächennahen geothermischen Potentials vorgestellt und anhand einer Fallstudie für die Stadt Köln demonstriert, gefolgt von einer Diskussion der Ergebnisse.

The sales of Ground Source Heat Pumps in Sweden and many other countries are having a rapid growth in the last decade. Today, there are approximately 360 000 systems installed in Sweden, with a growing rate of about 30 000 installations per year. The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a Borehole Heat Exchanger (BHE), a closed loop in a vertical borehole. The fluid transports the heat from the ground to a certain heating and/or cooling application. A fluid with one degree higher or lower temperature coming out from the borehole may represent a 2-3% change in the COP of a heat pump system. It is therefore of great relevance to design cost effective and easy to install borehole heat exchangers. U-pipe BHEs consisting of two equal cylindrical pipes connected together at the borehole bottom have dominated the market for several years in spite of their relatively poor thermal performance and, still, there exist many uncertainties about how to optimize them. Although more efficient BHEs have been discussed for many years, the introduction of new designs has been practically lacking. However, the interest for innovation within this field is increasing nowadays and more effective methods for injecting or extracting heat into/from the ground (better BHEs) with smaller temperature differences between the heat secondary fluid and the surrounding bedrock must be suggested for introduction into the market. This report presents the analysis of several groundwater filled borehole heat exchangers, including standard and alternative U-pipe configurations (e.g. with spacers, grooves), as well as two coaxial designs. The study embraces measurements of borehole deviation, ground water flow, undisturbed ground temperature profile, secondary fluid and groundwater temperature variations in time, theoretical analyses with a FEM software, Distributed Thermal Response Test (DTRT), and pressure drop. Significant attention is devoted to distributed temperature measurements using optic fiber cables along the BHEs during heat extraction and heat injection from and to the ground. / <p>QC 20100517</p> / EFFSYS2 / Efficient Use of Energy Wells for Heat Pumps

Borehole heat exchangers are widely used in heat pumps of residential buildings and industrialsystems. It is known as one of the most energy ecient technologies which provides heatingand cooling by using sustainable geothermal energy. The life time of borehole heat exchangerslasts more than 50 years which is longer than combustion boilers. Therefore, designing abore eld with accurate sizing is important for its future applications. Due to the large volumeof the ground, the transient heat transfer process of the bore eld lasts for a long time span. Because of this, only a few of the heat transfer models for borehole ground heat exchangersare validated by experiments. Besides, experimental validation in a real scale borehole can bedicult because of the uncertainty of the composition and thermal properties of the ground. A solution to faster experimental validation is to scale down the size of the borehole andground. This report presents the construction process of a lab-scale model simulating a 4x4 bore eldof 300 m depth vertical boreholes. The process of experimental construction is describedin detail, including ground set up, conductivity test, construction of hydraulic system anddata acquisition system. The pressure drop of hydraulics system is around 2.8 bar under the a flow rate of 200 ml/min and corresponding pump speed is around 2900 to 3100 rpm. The property of the sand has been investigated through a series of conductivity tests, which shows an average thermal conductivity of 1.75 W / (m • K) and average thermal diffusivity of 8.14x10-7 m2/s. Numerical simulation (via COMSOL) is carried out for preliminary validation. Comparison of experimental and simulation results shows discrepancies and one possible reason can be: the actual heat injection rate in experiment is lower than simulation due to heat losses of hydraulic system; uncertainty of ground (saturated sand) conductivity and thermal diffusivity. / Borrhålsvärmeväxlare används ofta i värmepumpar i bostadshus och industrisystem. Det är känt som en av de mest energieffektiva teknikerna som tillhandahåller värme och kyla genom att använda hållbar geotermisk energi. Livslängden för borrhålsvärmeväxlare varar mer än 50 år vilket är längre än förbränningspannor. Därför är det viktigt att utforma ett borrfalt med exakt dimensionering för dess framtida tillämpningar. På grund av den stora markvolymen varar den transienta värmeöverforingsprocessen i borrfältet under lång tid. På grund av detta är endast ett fåtal av värmeöverföringsmodellerna för borrhålsjordvärmeväxlare validerade genom experiment. Dessutom kan experimentell validering i ett borrhål i verklig skala vara svårt på grund av osäkerheten i markens sammansättning och termiska egenskaper. En lösning för snabbare experimentell validering är att skala ner storleken på borrhålet och marken. Denna rapport presenterar konstruktionsprocessen av en modell i labbskala som simulerar ett 4x4-borrfält med 300 m djupa vertikala borrhål. Processen for experimentell konstruktion beskrivs i detalj, inklusive markuppställning, konduktivitetstest, konstruktion av hydraulsystem och datainsamlingssystem. Tryckfallet for hydrauliksystemet är cirka 2,8 bar under en flödeshastighet pa 200 ml/min och motsvarande pumphastighet är runt 2900 till 3100 rpm. Sandens egenskaper har undersökts genom en serie konduktivitetstester, som visar en genomsnittlig värmeledningsformåga pa 1,75 W/(m • K) och en genomsnittlig termisk dffusivitet på 8.14x10 -7 m2/s.  Numerisk simulering (via COMSOL) utförs för preliminär validering. Jämförelse av experimentella och simuleringsresultat visar avvikelser och en möjlig orsak kan vara: den faktiska värmeinsprutningshastigheten i experimentet är lägre än simulering på grund av värmeförluster i hydraulsystemet; osäkerhet i markens (mättad sand) konduktivitet och termisk diffusivitet.

The UK government’s Carbon Plan-2011 aims for 80% carbon emission reduction by 2050, and the 2009 UK National Renewable Energy Action Plan has set a target of delivering 15% of total energy demand by renewable energy sources by 2020. Ground Source Heat Pump (GSHP) systems can play a critical role in reaching these goals within the building sector. Achieving such benefits relies on proper design, integration, installation, commissioning, and operation of these systems. This work seeks to provide evidence to improve the practices in design, installation and operations of large GSHP systems. This evidence has been based on collection and analysis of data from an operational large-scale GSHP system providing heating and cooling to a university building. The data set is of significance in that it is collected from a large-scale system incorporating fifty-six borehole heat exchangers and four heat pumps. The data has been collected at high frequency since the start of operation and for a period of three years. The borehole heat exchanger data is intended to form a reference data set for use by other workers in model validation studies. The ground thermal properties at the site have been estimated using a novel combination of numerical model and parameter estimation methods. The utility of the reference data set has been demonstrated through application in a validation study of a numerical borehole heat exchanger model. The system heat balances and power consumption data have firstly been analysed to derive a range of performance metrics such as Seasonal Performance Factors. Analysis has been carried out at the system and individual heat pump level. Annual performance has been found satisfactory overall. A series of analyses have been carried out to investigate the roles of circulating pump energy, control system operation and dynamic behaviour. Monitoring data from one of the heat pumps has also been analysed in further detail to make comparisons with manufacturer’s steady-state performance data and with consideration to variations in fluid properties. Some modest degradation from stated performance has been identified. The most significant operational factors accounting for degradation of overall system performance have been excessive pump energy demands and short cycling behaviour. Some faults in operation of the system during the monitoring period have also been identified. A series of recommendations are made as to ways to improve the design and operation of large-scale GSHP systems based on this evidence. These recommendations are chiefly concerned with better design for part-load operation, reduction in pump energy demands and more robust control systems.

Utilisée depuis des milliers d’années sous ses manifestations naturelles par l’Homme, cette ressource fait l’objet d’une exploitation commerciale depuis seulement le XXe siècle, à destination du chauffage de bâtiments, de certains usages industriels ainsi que de la production d’électricité. La France compte parmi les pionniers concernant l’usage direct de la chaleur alors qu’aucune filière industrielle n’est véritablement effective pour la production d’électricité d’origine géothermique. Le projet sélectionné, intitulé FONGEOSEC, a pour objectif la conception et la réalisation d’un démonstrateur innovant préindustriel d’une centrale géothermique haute enthalpie exploité par cogénération d’électricité et de chaleur. Un travail de recherche et développement, conduit par un consortium composé de partenaires industriels et scientifiques, vise au lancement de la filière industrielle géothermique haute température en France. L’objectif général des travaux de thèse porte sur une meilleure compréhension globale des comportements hydrauliques, massiques et thermiques des formations profondes en réponse à une sollicitation anthropique de longue durée. Il s’agit d’identifier les paramètres clés régissant la réponse du complexe réservoir à la suite d’une exploitation géothermique. Un point particulier sera consacré à caractériser la part de chacun des modes de transport de chaleur en milieu poreux – conduction thermique, convection libre et forcée – dans l’établissement des performances thermiques de l’ouvrage considéré. Plusieurs dispositifs techniques d’exploitation seront proposés afin de réduire les incertitudes associées au système géothermique souterrain et garantir le succès du projet FONGEOSEC. L’impact des mécanismes thermo-convectifs au voisinage des forages d’exploitation géothermique de grande profondeur reste peu documenté, a fortiori dans le cas de dispositifs déviés adoptant une complétion particulière. L’outil retenu pour l’évaluation des performances du dispositif au contact de l’encaissant est la modélisation numérique distribuée. La variabilité des propriétés physiques de l’hydrosystème, de la conception et des modalités d’exploitation du dispositif sur le comportement hydraulique et thermique de l’exploitation est envisagée selon différentes approches développées à partir de modèles numériques 3D. / Used for thousands of years under its natural manifestations, this resource has been commercially exploited since the twentieth century, for the heating of buildings, certain industrial uses and the production of electricity. France is one of the pioneers in the direct use of heat, whereas no industrial cluster is truly effective for the production of geothermal electricity. The selected project, FONGEOSEC, aims to design and produce an innovative pre-industrial demonstrator of a high enthalpy geothermal power plant operated by cogeneration of electricity and heat. A research and development project, led by a consortium of industrial and scientific partners, aims to launch the high-temperature geothermal industrial sector in France. The general objective of this thesis is to improve the understanding of the hydraulic, mass and thermal behavior of deep porous formations in response to long-term anthropogenic stress. The aim is to identify the key parameters governing the response of the reservoir complex related to geothermal operation. A particular point will be devoted to characterize the part of each mode of transport of heat in porous medium – thermal conduction, free and forced convection – in the establishment of the thermal performances of the geothermal power plant. Several technical operating devices will be proposed to reduce the uncertainties associated with the underground geothermal system and guarantee the success of the FONGEOSEC project. The impact of thermo-convective mechanisms in the vicinity of deep geothermal borehole remains poorly documented, especially in the case of deviated wells with a complex inner geometry. The evaluation of the hydraulic and thermal performances of the device, based on 3D numerical modeling, is conducted according to different approaches.

In the context of energy transition, geothermics play an important role for the heating and cooling supply of both residential and commercial buildings. Thereby, the increasingly and intensive utilisation of shallow geothermal resources bears the risk of over-exploitation and thus poses a future challenge to ensure the sustainability and safety of such systems. Particularly, the well-established technology of borehole heat exchanger-coupled ground source heat pumps is applied for the thermal exploitation of the shallow subsurface. Due to the complexity of the involved physical processes, numerical modelling proves to be a powerful tool to enhance process understanding as well as to aid the planning and design processes. Simulations can also support the management of thermal subsurface resources, planning and decision-making on city and regional scales. In this work, the so-called dual-continuum approach was adopted and enhanced to develop a coupled numerical model considering flow and heat transport processes in both the subsurface and borehole heat exchangers as well as the heat pumps’ performance characteristics, and including the relevant phenomena influencing the underlying processes. Beside the temperature fields, the efficiency and thus the consumption of electrical energy by the heat pump is computed, allowing for the quantification of operational costs and equivalent carbon-dioxide emissions. The model is validated and applied to a number of numerical studies. First, a comprehensive sensitivity analysis on the efficiency and sustainability of such systems is performed. Second, a method for the quantification of technically extractable shallow geothermal energy is proposed. This procedure is demonstrated by means of a case study for the city of Cologne, Germany and its implications are discussed. / Im Rahmen der Energiewende nimmt die Geothermie eine besondere Rolle in der thermische Gebäudeversorgung ein. Die zunehmende, intensive Nutzung oberflächennaher geothermischer Ressourcen erhöht die Gefahr der übermäßigen thermischen Ausbeutung des Untergrundes und stellt damit eine wachsende Herausforderung für die Nachhaltigkeit und Sicherheit solcher Systeme dar. Zur Erschließung oberflächennaher geothermischer Energie wird insbesondere die etablierte Technologie Erdwärmesonden-gekoppelter Wärmepumpen eingesetzt. Aufgrund der daran beteiligten komplexen physikalischen Prozesse erweisen sich numerische Modelle als leistungsfähiges Werkzeug zur Erweiterung des Prozessverständnisses und Unterstützung des Planungs- und Auslegungsprozesses. Zudem können Simulationen zum Management thermischer Ressourcen im Untergrund sowie zur Planung und politischen Entscheidungsfindung auf städtischen und regionalen Maßstäben beitragen. Im Rahmen dieser Arbeit wurde, basierend auf dem sogenannten ”dual-continuum approach” und unter Berücksichtigung des Einflusses der Wärmepumpe, ein erweitertes gekoppeltes numerisches Modell zur Abbildung der in Erdwärmesonden und dem Untergrund stattfindenden Strömungs- und Wärmetransportprozesse entwickelt. Das Modell ist in der Lage, alle relevanten Einflussfaktoren zu berücksichtigen. Neben den Temperaturfeldern im Untergrund und der Erdwärmesonde werden die Effizienz und damit der Stromverbrauch der Wärmepumpe simuliert. Damit können sowohl die Betriebskosten als auch der äquivalente CO 2 -Ausstoß abgeschätzt werden. Das Modell wurde validiert und in einer Reihe numerischer Studien eingesetzt. Zuerst wurde eine umfassende Sensitivitätsanalyse zur Effizienz und Nachhaltigkeit entsprechender Anlagen durchgeführt. Weiterhin wird ein Verfahren zur Quantifizierung des technisch nutzbaren, oberflächennahen geothermischen Potentials vorgestellt und anhand einer Fallstudie für die Stadt Köln demonstriert, gefolgt von einer Diskussion der Ergebnisse.

In recent years, deep geothermal energy has been widely exploited through closed-loop borehole heat exchanger system for building heating. In order to precisely evaluate the sustainable heat extraction capacity and the impact of different designs and operating parameters, two heat transfer models are implemented in the open-source scientific software OpenGeoSys (OGS), with respect to the Deep Borehole Heat Exchanger (DBHE) and Enhanced U-tube Borehole Heat Exchanger (EUBHE) system. Besides, three types of boundary conditions are implemented, including the constant inflow temperature, the constant heat extraction rate, and constant building thermal power that integrates the ground source heat pump (GSHP) module. By applying the two BHE models, the influence of different designs and operating parameters on the GSHP system is evaluated. The sustainable heat extraction capacity and efficiency of a deep EUBHE system are predicted. Moreover, its performance and efficiency are further compared against the 2-DBHE array system that has the same total borehole length. It is found that the soil thermal conductivity is the most important parameter in the design of DBHE and EUBHE systems. The sustainable specific heat extraction rate of the EUBHE system is 86.5 W/m higher than an array with 2 DBHEs. Under the building thermal load of 1.225 MW, the total electricity consumed by the EUBHE system is approximately 27 % less than the 2-DBHE array over 10 years. The average Coefficient of System Performance (CSP) value of the EUBHE system is 1.66 higher over 10 heating seasons. The two numerical models implemented in the OpenGeoSys software can be used to predict and optimize the thermal characteristics of the closed-loop DBHE and EUBHE systems in real projects.