Technical, Environmental and Economic Assessment of Medium Deep Borehole Thermal Energy Storage Systems

In Germany, the reduction of greenhouse gas emissions has been stagnating for a couple of years now. As a consequence, the climate targets for 2020 are at a risk of being missed. The energy transformation has strongly focused on the electricity sector while mostly disregarding the heating sector. Solar thermal energy and industrial waste heat offer a considerable potential for the replacement of fossil fuels in the heating sector. However, their utilization is hampered by the asynchronous seasonal fluctuation of heat demand and heat supply. Thermal energy storage technologies are required, which are able to absorb large amounts of heat in summer, store it for several months and release it during winter with minimal losses. Borehole thermal energy storage (BTES) is such a technology for seasonally storing heat on a district scale. A dense array of multiple borehole heat exchangers (BHE) exploits the natural subsurface as a heat storage medium. Conventional BTES systems usually do not exceed a depth of 200 m. Consequently, their operation implies a large thermal impact on shallow geologic formations. This, in combination with comparatively strict groundwater regulations in Germany, impedes the construction of such shallow systems. The unprecedented, still unrealized concept of medium deep borehole thermal energy storage (MD-BTES) is expected to remedy these shortcomings. MD-BTES systems consist of much less, but appreciably deeper BHEs (up to 1000 m). Consequently, they require significantly less ground surface. Therefore, they are particularly advantageous in densely populated urban areas, which are characterized by large heat demands and scarcely available space. More importantly, a large portion of the thermal energy is stored into deeper geologic formations, reducing the thermal impact on shallow aquifer systems. However, the magnitude of this reduction has not been quantified yet. Furthermore, the general applicability of MD-BTES systems, as well as their economic and environmental implications remain unclear. As part of this thesis, a large number of numerical simulations was analyzed in a parameter study to investigate the influence of various design and operation variables on the performance of MD-BTES systems. In total, 200 different MD-BTES geometries were compared. Moreover, the influence of subsurface conditions, operating temperatures and the interconnection scheme of BHEs was studied. The results demonstrate the excellent suitability of MD-BTES systems for large scale seasonal heat storage. With a proper dimensioning and in convenient geological and hydrogeological framework conditions, these systems can reach storage efficiencies of 80% or more, while maintaining relatively high supply temperatures of 30 °C. Further numerical simulations provide evidence for a significant mitigation of the thermal impact on shallow groundwater resources by the application of MD-BTES systems instead of their shallow counterparts. In order to resolve the economic and environmental questions connected to MD-BTES, a MATLAB based assessment tool was developed. It is used for a comprehensive economic and environmental life cycle assessment study on the integration of MD-BTES into district heating concepts. The results reveal the dependency of the economic and environmental impacts on the assumed financial and economic boundary conditions. However, they also demonstrate the high economic competitiveness of MD-BTES in combination with solar thermal collector fields, when supposing a likely increase of energy prices in the future. Furthermore, the combination of a solar thermal collector field, an MD-BTES system and a small combined heat and power plant undercuts the emissions of system combinations without any seasonal storage by 32% and more, when assuming a probable decrease in the emission factor of the electricity grid mix

Human Practice. Digital Ecologies. Our Future. : 14. Internationale Tagung Wirtschaftsinformatik (WI 2019) : Tagungsband

Erschienen bei: universi - Universitätsverlag Siegen. - ISBN: 978-3-96182-063-4Aus dem Inhalt: Track 1: Produktion & Cyber-Physische Systeme Requirements and a Meta Model for Exchanging Additive Manufacturing Capacities Service Systems, Smart Service Systems and Cyber- Physical Systems—What’s the difference? Towards a Unified Terminology Developing an Industrial IoT Platform – Trade-off between Horizontal and Vertical Approaches Machine Learning und Complex Event Processing: Effiziente Echtzeitauswertung am Beispiel Smart Factory Sensor retrofit for a coffee machine as condition monitoring and predictive maintenance use case Stakeholder-Analyse zum Einsatz IIoT-basierter Frischeinformationen in der Lebensmittelindustrie Towards a Framework for Predictive Maintenance Strategies in Mechanical Engineering - A Method-Oriented Literature Analysis Development of a matching platform for the requirement-oriented selection of cyber physical systems for SMEs Track 2: Logistic Analytics An Empirical Study of Customers’ Behavioral Intention to Use Ridepooling Services – An Extension of the Technology Acceptance Model Modeling Delay Propagation and Transmission in Railway Networks What is the impact of company specific adjustments on the acceptance and diffusion of logistic standards? Robust Route Planning in Intermodal Urban Traffic Track 3: Unternehmensmodellierung & Informationssystemgestaltung (Enterprise Modelling & Information Systems Design) Work System Modeling Method with Different Levels of Specificity and Rigor for Different Stakeholder Purposes Resolving Inconsistencies in Declarative Process Models based on Culpability Measurement Strategic Analysis in the Realm of Enterprise Modeling – On the Example of Blockchain-Based Initiatives for the Electricity Sector Zwischenbetriebliche Integration in der Möbelbranche: Konfigurationen und Einflussfaktoren Novices’ Quality Perceptions and the Acceptance of Process Modeling Grammars Entwicklung einer Definition für Social Business Objects (SBO) zur Modellierung von Unternehmensinformationen Designing a Reference Model for Digital Product Configurators Terminology for Evolving Design Artifacts Business Role-Object Specification: A Language for Behavior-aware Structural Modeling of Business Objects Generating Smart Glasses-based Information Systems with BPMN4SGA: A BPMN Extension for Smart Glasses Applications Using Blockchain in Peer-to-Peer Carsharing to Build Trust in the Sharing Economy Testing in Big Data: An Architecture Pattern for a Development Environment for Innovative, Integrated and Robust Applications Track 4: Lern- und Wissensmanagement (e-Learning and Knowledge Management) eGovernment Competences revisited – A Literature Review on necessary Competences in a Digitalized Public Sector Say Hello to Your New Automated Tutor – A Structured Literature Review on Pedagogical Conversational Agents Teaching the Digital Transformation of Business Processes: Design of a Simulation Game for Information Systems Education Conceptualizing Immersion for Individual Learning in Virtual Reality Designing a Flipped Classroom Course – a Process Model The Influence of Risk-Taking on Knowledge Exchange and Combination Gamified Feedback durch Avatare im Mobile Learning Alexa, Can You Help Me Solve That Problem? - Understanding the Value of Smart Personal Assistants as Tutors for Complex Problem Tasks Track 5: Data Science & Business Analytics Matching with Bundle Preferences: Tradeoff between Fairness and Truthfulness Applied image recognition: guidelines for using deep learning models in practice Yield Prognosis for the Agrarian Management of Vineyards using Deep Learning for Object Counting Reading Between the Lines of Qualitative Data – How to Detect Hidden Structure Based on Codes Online Auctions with Dual-Threshold Algorithms: An Experimental Study and Practical Evaluation Design Features of Non-Financial Reward Programs for Online Reviews: Evaluation based on Google Maps Data Topic Embeddings – A New Approach to Classify Very Short Documents Based on Predefined Topics Leveraging Unstructured Image Data for Product Quality Improvement Decision Support for Real Estate Investors: Improving Real Estate Valuation with 3D City Models and Points of Interest Knowledge Discovery from CVs: A Topic Modeling Procedure Online Product Descriptions – Boost for your Sales? Entscheidungsunterstützung durch historienbasierte Dienstreihenfolgeplanung mit Pattern A Semi-Automated Approach for Generating Online Review Templates Machine Learning goes Measure Management: Leveraging Anomaly Detection and Parts Search to Improve Product-Cost Optimization Bedeutung von Predictive Analytics für den theoretischen Erkenntnisgewinn in der IS-Forschung Track 6: Digitale Transformation und Dienstleistungen Heuristic Theorizing in Software Development: Deriving Design Principles for Smart Glasses-based Systems Mirroring E-service for Brick and Mortar Retail: An Assessment and Survey Taxonomy of Digital Platforms: A Platform Architecture Perspective Value of Star Players in the Digital Age Local Shopping Platforms – Harnessing Locational Advantages for the Digital Transformation of Local Retail Outlets: A Content Analysis A Socio-Technical Approach to Manage Analytics-as-a-Service – Results of an Action Design Research Project Characterizing Approaches to Digital Transformation: Development of a Taxonomy of Digital Units Expectations vs. Reality – Benefits of Smart Services in the Field of Tension between Industry and Science Innovation Networks and Digital Innovation: How Organizations Use Innovation Networks in a Digitized Environment Characterising Social Reading Platforms— A Taxonomy-Based Approach to Structure the Field Less Complex than Expected – What Really Drives IT Consulting Value Modularity Canvas – A Framework for Visualizing Potentials of Service Modularity Towards a Conceptualization of Capabilities for Innovating Business Models in the Industrial Internet of Things A Taxonomy of Barriers to Digital Transformation Ambidexterity in Service Innovation Research: A Systematic Literature Review Design and success factors of an online solution for cross-pillar pension information Track 7: IT-Management und -Strategie A Frugal Support Structure for New Software Implementations in SMEs How to Structure a Company-wide Adoption of Big Data Analytics The Changing Roles of Innovation Actors and Organizational Antecedents in the Digital Age Bewertung des Kundennutzens von Chatbots für den Einsatz im Servicedesk Understanding the Benefits of Agile Software Development in Regulated Environments Are Employees Following the Rules? On the Effectiveness of IT Consumerization Policies Agile and Attached: The Impact of Agile Practices on Agile Team Members’ Affective Organisational Commitment The Complexity Trap – Limits of IT Flexibility for Supporting Organizational Agility in Decentralized Organizations Platform Openness: A Systematic Literature Review and Avenues for Future Research Competence, Fashion and the Case of Blockchain The Digital Platform Otto.de: A Case Study of Growth, Complexity, and Generativity Track 8: eHealth & alternde Gesellschaft Security and Privacy of Personal Health Records in Cloud Computing Environments – An Experimental Exploration of the Impact of Storage Solutions and Data Breaches Patientenintegration durch Pfadsysteme Digitalisierung in der Stressprävention – eine qualitative Interviewstudie zu Nutzenpotenzialen User Dynamics in Mental Health Forums – A Sentiment Analysis Perspective Intent and the Use of Wearables in the Workplace – A Model Development Understanding Patient Pathways in the Context of Integrated Health Care Services - Implications from a Scoping Review Understanding the Habitual Use of Wearable Activity Trackers On the Fit in Fitness Apps: Studying the Interaction of Motivational Affordances and Users’ Goal Orientations in Affecting the Benefits Gained Gamification in Health Behavior Change Support Systems - A Synthesis of Unintended Side Effects Investigating the Influence of Information Incongruity on Trust-Relations within Trilateral Healthcare Settings Track 9: Krisen- und Kontinuitätsmanagement Potentiale von IKT beim Ausfall kritischer Infrastrukturen: Erwartungen, Informationsgewinnung und Mediennutzung der Zivilbevölkerung in Deutschland Fake News Perception in Germany: A Representative Study of People’s Attitudes and Approaches to Counteract Disinformation Analyzing the Potential of Graphical Building Information for Fire Emergency Responses: Findings from a Controlled Experiment Track 10: Human-Computer Interaction Towards a Taxonomy of Platforms for Conversational Agent Design Measuring Service Encounter Satisfaction with Customer Service Chatbots using Sentiment Analysis Self-Tracking and Gamification: Analyzing the Interplay of Motivations, Usage and Motivation Fulfillment Erfolgsfaktoren von Augmented-Reality-Applikationen: Analyse von Nutzerrezensionen mit dem Review-Mining-Verfahren Designing Dynamic Decision Support for Electronic Requirements Negotiations Who is Stressed by Using ICTs? A Qualitative Comparison Analysis with the Big Five Personality Traits to Understand Technostress Walking the Middle Path: How Medium Trade-Off Exposure Leads to Higher Consumer Satisfaction in Recommender Agents Theory-Based Affordances of Utilitarian, Hedonic and Dual-Purposed Technologies: A Literature Review Eliciting Customer Preferences for Shopping Companion Apps: A Service Quality Approach The Role of Early User Participation in Discovering Software – A Case Study from the Context of Smart Glasses The Fluidity of the Self-Concept as a Framework to Explain the Motivation to Play Video Games Heart over Heels? An Empirical Analysis of the Relationship between Emotions and Review Helpfulness for Experience and Credence Goods Track 11: Information Security and Information Privacy Unfolding Concerns about Augmented Reality Technologies: A Qualitative Analysis of User Perceptions To (Psychologically) Own Data is to Protect Data: How Psychological Ownership Determines Protective Behavior in a Work and Private Context Understanding Data Protection Regulations from a Data Management Perspective: A Capability-Based Approach to EU-GDPR On the Difficulties of Incentivizing Online Privacy through Transparency: A Qualitative Survey of the German Health Insurance Market What is Your Selfie Worth? A Field Study on Individuals’ Valuation of Personal Data Justification of Mass Surveillance: A Quantitative Study An Exploratory Study of Risk Perception for Data Disclosure to a Network of Firms Track 12: Umweltinformatik und nachhaltiges Wirtschaften Kommunikationsfäden im Nadelöhr – Fachliche Prozessmodellierung der Nachhaltigkeitskommunikation am Kapitalmarkt Potentiale und Herausforderungen der Materialflusskostenrechnung Computing Incentives for User-Based Relocation in Carsharing Sustainability’s Coming Home: Preliminary Design Principles for the Sustainable Smart District Substitution of hazardous chemical substances using Deep Learning and t-SNE A Hierarchy of DSMLs in Support of Product Life-Cycle Assessment A Survey of Smart Energy Services for Private Households Door-to-Door Mobility Integrators as Keystone Organizations of Smart Ecosystems: Resources and Value Co-Creation – A Literature Review Ein Entscheidungsunterstützungssystem zur ökonomischen Bewertung von Mieterstrom auf Basis der Clusteranalyse Discovering Blockchain for Sustainable Product-Service Systems to enhance the Circular Economy Digitale Rückverfolgbarkeit von Lebensmitteln: Eine verbraucherinformatische Studie Umweltbewusstsein durch audiovisuelles Content Marketing? Eine experimentelle Untersuchung zur Konsumentenbewertung nachhaltiger Smartphones Towards Predictive Energy Management in Information Systems: A Research Proposal A Web Browser-Based Application for Processing and Analyzing Material Flow Models using the MFCA Methodology Track 13: Digital Work - Social, mobile, smart On Conversational Agents in Information Systems Research: Analyzing the Past to Guide Future Work The Potential of Augmented Reality for Improving Occupational First Aid Prevent a Vicious Circle! The Role of Organizational IT-Capability in Attracting IT-affine Applicants Good, Bad, or Both? Conceptualization and Measurement of Ambivalent User Attitudes Towards AI A Case Study on Cross-Hierarchical Communication in Digital Work Environments ‘Show Me Your People Skills’ - Employing CEO Branding for Corporate Reputation Management in Social Media A Multiorganisational Study of the Drivers and Barriers of Enterprise Collaboration Systems-Enabled Change The More the Merrier? The Effect of Size of Core Team Subgroups on Success of Open Source Projects The Impact of Anthropomorphic and Functional Chatbot Design Features in Enterprise Collaboration Systems on User Acceptance Digital Feedback for Digital Work? Affordances and Constraints of a Feedback App at InsurCorp The Effect of Marker-less Augmented Reality on Task and Learning Performance Antecedents for Cyberloafing – A Literature Review Internal Crowd Work as a Source of Empowerment - An Empirical Analysis of the Perception of Employees in a Crowdtesting Project Track 14: Geschäftsmodelle und digitales Unternehmertum Dividing the ICO Jungle: Extracting and Evaluating Design Archetypes Capturing Value from Data: Exploring Factors Influencing Revenue Model Design for Data-Driven Services Understanding the Role of Data for Innovating Business Models: A System Dynamics Perspective Business Model Innovation and Stakeholder: Exploring Mechanisms and Outcomes of Value Creation and Destruction Business Models for Internet of Things Platforms: Empirical Development of a Taxonomy and Archetypes Revitalizing established Industrial Companies: State of the Art and Success Principles of Digital Corporate Incubators When 1+1 is Greater than 2: Concurrence of Additional Digital and Established Business Models within Companies Special Track 1: Student Track Investigating Personalized Price Discrimination of Textile-, Electronics- and General Stores in German Online Retail From Facets to a Universal Definition – An Analysis of IoT Usage in Retail Is the Technostress Creators Inventory Still an Up-To-Date Measurement Instrument? Results of a Large-Scale Interview Study Application of Media Synchronicity Theory to Creative Tasks in Virtual Teams Using the Example of Design Thinking TrustyTweet: An Indicator-based Browser-Plugin to Assist Users in Dealing with Fake News on Twitter Application of Process Mining Techniques to Support Maintenance-Related Objectives How Voice Can Change Customer Satisfaction: A Comparative Analysis between E-Commerce and Voice Commerce Business Process Compliance and Blockchain: How Does the Ethereum Blockchain Address Challenges of Business Process Compliance? Improving Business Model Configuration through a Question-based Approach The Influence of Situational Factors and Gamification on Intrinsic Motivation and Learning Evaluation von ITSM-Tools für Integration und Management von Cloud-Diensten am Beispiel von ServiceNow How Software Promotes the Integration of Sustainability in Business Process Management Criteria Catalog for Industrial IoT Platforms from the Perspective of the Machine Tool Industry Special Track 3: Demos & Prototyping Privacy-friendly User Location Tracking with Smart Devices: The BeaT Prototype Application-oriented robotics in nursing homes Augmented Reality for Set-up Processe Mixed Reality for supporting Remote-Meetings Gamification zur Motivationssteigerung von Werkern bei der Betriebsdatenerfassung Automatically Extracting and Analyzing Customer Needs from Twitter: A “Needmining” Prototype GaNEsHA: Opportunities for Sustainable Transportation in Smart Cities TUCANA: A platform for using local processing power of edge devices for building data-driven services Demonstrator zur Beschreibung und Visualisierung einer kritischen Infrastruktur Entwicklung einer alltagsnahen persuasiven App zur Bewegungsmotivation für ältere Nutzerinnen und Nutzer A browser-based modeling tool for studying the learning of conceptual modeling based on a multi-modal data collection approach Exergames & Dementia: An interactive System for People with Dementia and their Care-Network Workshops Workshop Ethics and Morality in Business Informatics (Workshop Ethik und Moral in der Wirtschaftsinformatik – EMoWI’19) Model-Based Compliance in Information Systems - Foundations, Case Description and Data Set of the MobIS-Challenge for Students and Doctoral Candidates Report of the Workshop on Concepts and Methods of Identifying Digital Potentials in Information Management Control of Systemic Risks in Global Networks - A Grand Challenge to Information Systems Research Die Mitarbeiter von morgen - Kompetenzen künftiger Mitarbeiter im Bereich Business Analytics Digitaler Konsum: Herausforderungen und Chancen der Verbraucherinformati

Technical, Environmental and Economic Assessment of Medium Deep Borehole Thermal Energy Storage Systems

In Germany, the reduction of greenhouse gas emissions has been stagnating for a couple of years now. As a consequence, the climate targets for 2020 are at a risk of being missed. The energy transformation has strongly focused on the electricity sector while mostly disregarding the heating sector. Solar thermal energy and industrial waste heat offer a considerable potential for the replacement of fossil fuels in the heating sector. However, their utilization is hampered by the asynchronous seasonal fluctuation of heat demand and heat supply. Thermal energy storage technologies are required, which are able to absorb large amounts of heat in summer, store it for several months and release it during winter with minimal losses. Borehole thermal energy storage (BTES) is such a technology for seasonally storing heat on a district scale. A dense array of multiple borehole heat exchangers (BHE) exploits the natural subsurface as a heat storage medium. Conventional BTES systems usually do not exceed a depth of 200 m. Consequently, their operation implies a large thermal impact on shallow geologic formations. This, in combination with comparatively strict groundwater regulations in Germany, impedes the construction of such shallow systems. The unprecedented, still unrealized concept of medium deep borehole thermal energy storage (MD-BTES) is expected to remedy these shortcomings. MD-BTES systems consist of much less, but appreciably deeper BHEs (up to 1000 m). Consequently, they require significantly less ground surface. Therefore, they are particularly advantageous in densely populated urban areas, which are characterized by large heat demands and scarcely available space. More importantly, a large portion of the thermal energy is stored into deeper geologic formations, reducing the thermal impact on shallow aquifer systems. However, the magnitude of this reduction has not been quantified yet. Furthermore, the general applicability of MD-BTES systems, as well as their economic and environmental implications remain unclear. As part of this thesis, a large number of numerical simulations was analyzed in a parameter study to investigate the influence of various design and operation variables on the performance of MD-BTES systems. In total, 200 different MD-BTES geometries were compared. Moreover, the influence of subsurface conditions, operating temperatures and the interconnection scheme of BHEs was studied. The results demonstrate the excellent suitability of MD-BTES systems for large scale seasonal heat storage. With a proper dimensioning and in convenient geological and hydrogeological framework conditions, these systems can reach storage efficiencies of 80% or more, while maintaining relatively high supply temperatures of 30 °C. Further numerical simulations provide evidence for a significant mitigation of the thermal impact on shallow groundwater resources by the application of MD-BTES systems instead of their shallow counterparts. In order to resolve the economic and environmental questions connected to MD-BTES, a MATLAB based assessment tool was developed. It is used for a comprehensive economic and environmental life cycle assessment study on the integration of MD-BTES into district heating concepts. The results reveal the dependency of the economic and environmental impacts on the assumed financial and economic boundary conditions. However, they also demonstrate the high economic competitiveness of MD-BTES in combination with solar thermal collector fields, when supposing a likely increase of energy prices in the future. Furthermore, the combination of a solar thermal collector field, an MD-BTES system and a small combined heat and power plant undercuts the emissions of system combinations without any seasonal storage by 32% and more, when assuming a probable decrease in the emission factor of the electricity grid mix

Finite element simulation of permeable fault influence on a medium deep borehole thermal energy storage system

Solutions for seasonal energy storage systems are essential for the reliable use of fluctuating renewable energy sources. As part of the research project SKEWS, a medium deep borehole thermal energy storage system with a depth of 750 m is under construction at Campus Lichtwiese in Darmstadt, Germany, to demonstrate this innovative technology. Prior to the design of SKEWS, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The geologic survey suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. A 3D finite-element numerical model was implemented to estimate the effect of the potentially higher hydraulic conductivity of the fault zone on the planned storage system. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone. The simulations reveal a limited but visible heat removal from the storage region with increasing groundwater flow in the fault zone. However, the section of the borehole thermal energy storage system affected by the fault is minor compared to the total depth of the system. This only constitutes a minor impairment of the storage efficiency of approximately 3%. In total, the amount of heat extracted varies between 320.2 GWh and 326.2 GWh for the different models. These findings can be helpful for the planning and assessment of future medium deep borehole thermal energy storage systems in fractured and faulted crystalline settings by providing data about the potential impact of faults or large fractures crosscutting the storage system

A Modelica Toolbox for the Simulation of Borehole Thermal Energy Storage Systems

Borehole thermal energy storage (BTES) systems facilitate the subsurface seasonal storage of thermal energy on district heating scales. These systems’ performances are strongly dependent on operational conditions like temperature levels or hydraulic circuitry. Preliminary numerical system simulations improve comprehension of the storage performance and its interdependencies with other system components, but require both accurate and computationally efficient models. This study presents a toolbox for the simulation of borehole thermal energy storage systems in Modelica. The storage model is divided into a borehole heat exchanger (BHE), a local, and a global sub-model. For each sub-model, different modeling approaches can be deployed. To assess the overall performance of the model, two studies are carried out: One compares the model results to those of 3D finite element method (FEM) models to investigate the model’s validity over a large range of parameters. In a second study, the accuracies of the implemented model variants are assessed by comparing their results to monitoring data from an existing BTES system. Both studies prove the validity of the modeling approaches under investigation. Although the differences in accuracy for the compared variants are small, the proper model choice can significantly reduce the computational effort

Optimized Layouts of Borehole Thermal Energy Storage Systems in 4th Generation Grids

Borehole thermal energy storage (BTES) systems are a viable option to meet the increasing cooling demand and to increase the sustainability of low-temperature district heating and cooling (DHC) grids. They are able to store the rejected heat of cooling cycles on a seasonal basis and deliver this heat during the heating season. However, their efficient practical implementation requires a thorough analysis from technical, economic and environmental points of view. In this comparative study, a dynamic exergoeconomic assessment is adopted to evaluate various options for integrating such a storage system into 4th generation DHC grids in heating dominated regions. For this purpose, different layouts are modeled and parameterized. Multi-objective optimization is conducted, varying the most important design variables in order to maximize exergetic efficiency and to minimize levelized cost of energy (LCOE). A comparison of the optimal designs of the different layouts reveals that passive cooling together with maximizing the heating temperature shift, accomplished by a heat pump, lead to optimal designs. Component-wise exergy and cost analysis of the most efficient designs highlights that heat pumps are responsible for the highest share in inefficiency while the installation of BTES has a high impact in the LCOE. BTES and buffer storage tanks have the lowest exergy destruction for all layouts and increasing the BTES volume results in more efficient DHC grids

Impact of operational temperature changes and freeze–thaw cycles on the hydraulic conductivity of borehole heat exchangers

A large share of the primary energy is consumed to provide space heating. Geothermal energy offers a regenerative alternative. For reasons of efficiency and environmental protection, it is important to ensure the system integrity of a borehole heat exchanger (BHE). Previous investigations have focused on the individual components of the BHE or on the grout and pipe systems’ integrity. This study focused on the analysis of the hydraulic system integrity of the complete subsoil–grout–pipe system as well as possible thermally induced changes. For this purpose, a pilot-scale experiment was built to test a 1-m section of a typical BHE under in situ pressure, hydraulic and temperature conditions. During the tests the hydraulic system permeability of the soil and the BHE was measured continuously and separately from each other. In addition, the temperature monitoring array was installed in a 50-cm cross-sectional area. Significant temperature-related fluctuations in the sealing performance could be observed. Hydraulic conductivity limits required by VDI 4640-2 (Thermal use of the underground—ground source heat pump systems, 2019) were exceeded without frost action. The succeeding application of freeze–thaw cycles further enhances the system permeability. The study shows that the thermally induced effects on the system integrity of the BHE are larger and more significant than the subsequent frost-induced effects. The hydrophobic character of the high-density polyethylene (PE-HD) pipes as well as its high coefficient of thermal expansion seem to be the main points of weakness in the system. Optimization research should focus on the interface connection between grout and pipe, whereby hydrophilic pipe materials such as stainless steel or aluminum should also be considered as well as manipulation of the pipe surface properties of PE-HD