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Modelling and monitoring thermal response of the ground in borehole fields
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-5093-9070
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This Ph.D. dissertation aimed at developing tools for the evaluation of ground response in borehole fields connected in parallel through modelling and monitoring studies.

A total heat flow and a uniform borehole wall temperature condition equal in all boreholes have often been accepted when mathematically modelling the response of vertical ground heat exchangers connected in parallel. The first objective of this thesis was the development of a numerical model in which the ground controls the temperature response at the borehole wall, instead of imposing heat flow or temperature conditions at this interface. The unavoidable fluid-to- borehole wall thermal resistance and the variation of the heat flux distribution along the borehole depth violates the assumption of uniform temperature at the borehole wall. This aspect, which is often disregarded, was taken into account in this model. The results obtained from the numerical simulations are believed to come closer to reality and can be used as a reference for other approaches.

When bore field sizing, the worst ground load conditions are usually assumed to occur after several years of operation, but these may occur during the first year of operation. The second objective of this work was the development of a general sizing methodology that calculates the total required bore field length for each arbitrary month during the lifetime of the installations. The methodology was also used to investigate to what extent the borehole spacing can be reduced without increasing the total required bore field length when the ground load condition is thermally quasibalanced.

The ground source heat pump community still lacks detailed and accurately measured long-term data for validation of modelling tools. In order to partially contribute to filling this gap, the last part of this Ph.D. study focused on state of the art monitoring activities. The main goal of this part is to provide a comprehensive description of the ground thermal loads and response measurements at a large bore field, that is being monitored from the beginning of its operation. Unique data sets, showing the thermal loads and ground thermal response during extraction and injection, along with measurement error analyses are reported in the thesis. 

Abstract [sv]

I denna doktorsavhandling presenteras nya verktyg för utvärdering av berggrundens respons i geotermiska borrhålslager där borrhålen är kopplade parallellt. Avhandlingen beskriver både modellering och fältstudier.

Ett givet totalt värmeflöde och likformiga väggtemperaturer i alla borrhålen har ofta varit accepterade randvillkor vid matematisk modellering av den termiska responsen av parallellkopplade borrhålsvärmeväxlare i grupper av vertikala borrhål. Det första målet i arbetet med denna avhandling var att utveckla en numerisk modell i vilken berggrunden tillåts bestämma temperaturresponsen vid borrhålsväggen, istället för att som randvillkor ange ett visst värmeflöde eller en viss temperatur. Det ofrånkomliga värmemotståndet mellan flödet i slangen/värmeväxlaren och bergväggen och variationen i värmeflöde längs borrhålet innebär att dessa förenklade antaganden inte stämmer. Denna aspekt, som ofta förbisetts, har inkluderats i den här presenterade modellen. Resultaten som erhållits med denna numeriska modell kan förväntas vara närmare verkligheten än tidigare modeller och kan användas som referens för andra beräkningar.

Vid dimensionering av borrhålslager antas vanligen de värsta berggrundstemperaturerna inträffa efter flera års drift, men i avhandlingen visas att de värsta förhållandena kan uppstå under det första driftåret. Det andra målet med arbetet var att utveckla en generell dimensioneringsmetodik som kan användas för att beräkna den totala nödvändiga borrhålslängden för varje månad under installationens hela livslängd. Metodiken användes också för att undersöka i vilken mån borrhålsavståndet kan minskas utan att den totala borrhålslängden behöver ökas, under förutsättningen att borrhålslagret är ungefär balanserat.

Forskarsamhället saknar fortfarande detaljerade och noggrant uppmätta långtidsdata som kan användas för att validera olika modelleringsverktyg. I avsikt att i någon mån bidra till att fylla detta tomrum, innehåller den sista delen av avhandlingen beskrivningar av god teknik för utvärdering av borrhålslager.  Huvudmålet för denna del av avhandlingen är att ge en utförlig beskrivning av värmelasterna och responsmätningarna i ett stort borrhålslager som studerats sedan det först togs i bruk. Unika data rörande termiska laster och berggrundens termiska respons under värmeuttag och värmelagring, samt felanalys, presenteras i denna avhandling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 124
Series
TRITA-ITM-AVL ; 2018:1
Keywords [en]
ground temperature response, borehole heat exchanger, modelling, monitoring, sizing
Keywords [sv]
bergtemperaturrespons, borrhålsvärmeväxlare, modellering, övervakning, dimensionering
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-222007ISBN: 978-91-7729-667-6 (electronic)OAI: oai:DiVA.org:kth-222007DiVA, id: diva2:1178493
Public defence
2018-02-23, Sal D2, KTH, Lindstedtsvägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council for Environment, Agricultural Sciences and Spatial PlanningSwedish Energy Agency
Note

QC 20180130

Available from: 2018-01-30 Created: 2018-01-29 Last updated: 2018-02-07Bibliographically approved
List of papers
1. A novel numerical approach for imposing a temperature boundary condition at the borehole wall in borehole fields
Open this publication in new window or tab >>A novel numerical approach for imposing a temperature boundary condition at the borehole wall in borehole fields
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2015 (English)In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 56, p. 35-44Article in journal (Refereed) Published
Abstract [en]

The design of a borehole field should be based on a long-term simulation of its thermal response for the intended energy loads. A well-known method to evaluate the response is based on a pre-calculated dimensionless function, the g-function. When calculating g-functions, there are two commonly used approaches for treating the boundary condition at the borehole wall: a constant heat flux at every instant of time, or a uniform temperature at a constant total heat flow to the borehole field. This paper is focused on a new approach to model the thermal process of borehole fields; in particular with a precise representation of a uniform temperature boundary condition at the borehole wall. The main purpose of this model is to be used as a research tool to either generate g-functions for particular cases or handle situations that cannot be addressed by others methods. First, the almost constant temperature along the borehole heat exchanger in operation requires a boundary condition of essentially isothermal boreholes along the depth. In a common case, the borehole heat exchangers are connected in parallel, thus all boreholes should have the same temperature. Also, the total heat flow to the borehole field should be constant over time. For this purpose, a numerical model in which the boreholes are filled with a hypothetical highly conductive material has been built, reproducing the isothermal condition. By thermally interconnecting the boreholes, the equal temperature condition is satisfied. Finally, the specified total heat flow is fed into one spot at the highly conductive material. The model is validated by generating g-functions of some simple borehole field configurations. The g-functions present, in general, a good agreement with the existing solutions for a similar boundary condition. Moreover, the model is also tested against real experimental data from a 2. ×. 3 borehole field at an office building. The simulated daily fluid temperatures are compared with measured daily fluid temperatures for the sixth year of operation. The simulated values present, in general, a good agreement with the measured data. The results show that there are no significant differences with regard to the boundary conditions at the borehole wall, which for this specific case is due to the fact that the system is thermally balanced.

Keywords
Borehole, G-Function, Long-term performance, Uniform borehole temperature, Boundary conditions, Conductive materials, Enthalpy, Heat exchangers, Heat flux, Heat transfer, Isotherms, Office buildings, Walls (structural partitions), Borehole heat exchangers, Borehole temperature, Constant temperature, G function, Isothermal conditions, Long term performance, Long term simulation, Temperature conditions, Boreholes, borehole geophysics, borehole logging, design, heat flow, numerical model, temperature, Calluna vulgaris
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-167700 (URN)10.1016/j.geothermics.2015.03.003 (DOI)000358095700003 ()2-s2.0-84925424683 (Scopus ID)
Note

QC 20150602

Available from: 2015-06-02 Created: 2015-05-22 Last updated: 2018-01-29Bibliographically approved
2. Numerical Modelling of Ground Thermal Response with Borehole Heat Exchangers Connected in Parallel
Open this publication in new window or tab >>Numerical Modelling of Ground Thermal Response with Borehole Heat Exchangers Connected in Parallel
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

With bore fields for energy extraction it is often necessary to predict the temperature response to heat loads for many years ahead. Mathematical methods, both analytical and numerical, with different degrees of sophistication, are employed. Often the g-function concept is used, in which the borehole wall is assumed to have a uniform temperature and the heat injected is constant over time. Due to the unavoidable thermal resistance between borehole wall and the circulating fluid and with varying heat flux along the boreholes, the concept of uniform borehole wall temperature is violated, which distorts heat flow distribution between boreholes. This aspect has often been disregarded. This paper describes improvements applied to a previous numerical model approach. Improvements aim at taking into account the effect of thermal resistance between the fluid and the borehole wall. The model employs a highly conductive material (HCM) embedded in the boreholes and connected to an HCM bar above the ground surface. The small temperature difference occurring within the HCM allows the ground to naturally control the conditions at the wall of all boreholes and the heat flow distribution to the boreholes. The thermal resistance between the fluid and the borehole wall is taken into account in the model by inserting a thermally resistive layer at the borehole wall. Also, the borehole ends are given a hemispherical shape to reduce the fluctuations in the temperature gradients there. The improvements to the HCM model are reflected in a changed distribution of the heat flow to the different boreholes. Changes increase with the number of boreholes. The improvements to the HCM model are further illustrated by predicting fluid temperatures for measured variable daily loads of two monitored GCHP installations. Predictions deviate from measured values with a mean absolute error within 1.1 and 1.6 K.  

Keywords
borehole heat exchanger, numerical modeling, monitoring, fluid temperature prediction, boundary condition
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-222006 (URN)
Note

QC 20180130

Available from: 2018-01-29 Created: 2018-01-29 Last updated: 2018-01-30Bibliographically approved
3. A monthly based bore field sizing methodology with applications to optimum borehole spacing
Open this publication in new window or tab >>A monthly based bore field sizing methodology with applications to optimum borehole spacing
2016 (English)In: ASHRAE Transactions, ISSN 0001-2505, Vol. 122, no 1, p. 111-126, article id OR-16-009Article in journal (Refereed) Published
Abstract [en]

The required length of vertical ground heat exchangers(GHX) used in ground-coupled heat pump (GCHP) systems isdetermined so that the outlet temperature from the GHXremains within certain limits for the worst ground load condi-tions. These conditions may not necessarily occur after 10 or20 years of operation, as is usually assumed, but often occurduring the first year of operation.The primary objective of this paper is to develop a generalmethodology for the calculation of the total required bore fieldlength on a monthly basis during the first year of operationusing the framework of the ASHRAE bore field sizing method.Itisathreephaseprocess.Thefirstphaseconsistsofanalyzingandorderinggroundloadsaccordingtothefirstmonthofoper-ation.Next,afirstsetofrequiredlengthsisdeterminedbyusingthe analyzed ground load components and assuming atemperaturepenaltyTp=0.Then,aniterativeprocesstocalcu-latethetemperaturepenaltyattheendofeachmonthiscarriedout to obtain the final required length for the worst conditions.The methodology is exemplified in a particular case witha slight annual cooling thermal imbalance and with a highinfluence of the hourly peak in heating. For this particularcase, it is shown that the required bore field length occursduring the first year and that the starting month of operationhas a strong influence on the results.Finally,itisshownthatitispossibletoreducetheboreholespacing when the annual ground load is quasibalanced. In thecase studied here, the minimum length occurs for a borehole-to-borehole spacing of about 3.2 m (10.50 ft)

Place, publisher, year, edition, pages
ASHRAE, 2016
Keywords
borehole heat exchanger, sizing, borehole spacing
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-184184 (URN)2-s2.0-84974846125 (Scopus ID)
Note

QC 20160408

Available from: 2016-03-30 Created: 2016-03-30 Last updated: 2018-01-29Bibliographically approved
4. Monitoring of a borehole thermal energy storage in Sweden
Open this publication in new window or tab >>Monitoring of a borehole thermal energy storage in Sweden
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2016 (English)In: CLIMA 2016-proceedings of the 12th REHVA World Congress: volume 3 / [ed] Per Kvols Heiseberg, Aalborg University, Department of Civil Enginnering, 2016, Vol. 3Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents the description of the first stage of a project consisting on the monitoring of a newly installed borehole thermal energy storage (BTES) system that started to operate during the autumn of 2015. The BTES system is designed for approximately 4 GWh per year of heat injection and 3 GWh per year of heat extraction and will provide heating and cooling to a set of institutional facilities at Stockholm University, Sweden. The energy storage system consists of a set of 130 borehole heat exchangers, 230 meters deep. Strategic locations within the bore field have been selected to carry out the measurements. The monitoring system comprises temperature and energy flow meters. The temperature measurements are performed using a distributed temperature sensing set-up which allows to measure temperature along the depth of the boreholes, providing a large amount of data for the characterization of the thermal processes in the ground. During the upcoming years, the measured data will be utilized to evaluate and optimize the actual operational condition of the system, and to test the validity of assumptions made during the design phase. Moreover, the measured data will be utilized for validation of current bore field design methods and to have a better understanding of the thermal interaction between neighboring boreholes.

Place, publisher, year, edition, pages
Aalborg University, Department of Civil Enginnering: , 2016
Keywords
ground-coupled heat pump; multiple bore field; borehole heat exchanger; monitoring system
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-187482 (URN)87-91606-28-4 (vol3) (ISBN)87-91606-36-5 (set) (ISBN)
Conference
12th REHVA World Congress CLIMA, 22–25 May 2016
Funder
Swedish Energy Agency
Note

QC 20160603

Available from: 2016-06-02 Created: 2016-05-24 Last updated: 2018-01-29Bibliographically approved
5. First Measurements of a Monitoring project on a BTES system
Open this publication in new window or tab >>First Measurements of a Monitoring project on a BTES system
2017 (English)In: IGSHPA Technical/Research Conference and Expo, Denver, March 14-16, 2017, International Ground Source Heat Pump Association , 2017Conference paper, Published paper (Refereed)
Abstract [en]

Performance of Borehole Thermal Energy Storage (BTES) systems depends on the temperature of the secondary fluid, circulating through the ground-loop heat exchangers. Borehole systems are therefore designed in order to ensure that inlet and outlet temperatures of the secondary fluid are within given operational limits during the whole life-time of the system. Monitoring the operation of the bore fields is crucial for the validation of existing models utilized for their design. Measured data provides valuable information for researchers and practitioners working in the field. A first data-set from an ongoing monitoring project is presented in this article. The monitoring system comprises temperature sensors and power meters placed at strategic locations within the bore field. A distributed temperature sensing rig that employs fiber optic cables as linear sensors is utilized to measure temperature every meter along the depth of nine monitored boreholes, yielding data regarding both temporal and spatial variation of the temperature in the ground. The heat exchanged with the ground is also measured via power meters in all nine monitored boreholes as well as at the manifold level. The BTES system is located at the Stockholm University Campus, Sweden, and consists of 130 boreholes, 230 meters deep. After more than a year of planning and installation work, some selected measurements recorded in the BTES during the first months of operation are reported in this article.

Place, publisher, year, edition, pages
International Ground Source Heat Pump Association, 2017
Keywords
ground source heat pump, monitoring project, BTES
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-202532 (URN)
Conference
IGSHPA Technical/Research Conference and Expo, Denver, March 14-16, 2017
Note

QC 20170330

Available from: 2017-02-26 Created: 2017-02-26 Last updated: 2018-02-28Bibliographically approved

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