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Numerical Modelling of Ground Thermal Response with Borehole Heat Exchangers Connected in Parallel
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-5093-9070
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-3490-1777
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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.  

Keyword [en]
borehole heat exchanger, numerical modeling, monitoring, fluid temperature prediction, boundary condition
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-222006OAI: oai:DiVA.org:kth-222006DiVA, id: diva2:1178484
Note

QC 20180130

Available from: 2018-01-29 Created: 2018-01-29 Last updated: 2018-01-30Bibliographically approved
In thesis
1. Modelling and monitoring thermal response of the ground in borehole fields
Open this publication in new window or tab >>Modelling and monitoring thermal response of the ground in borehole fields
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
Keyword
ground temperature response, borehole heat exchanger, modelling, monitoring, sizing, bergtemperaturrespons, borrhålsvärmeväxlare, modellering, övervakning, dimensionering
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-222007 (URN)978-91-7729-667-6 (ISBN)
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

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