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A novel numerical approach for imposing a temperature boundary condition at the borehole wall in borehole fields
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.ORCID iD: 0000-0002-5093-9070
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.ORCID iD: 0000-0002-3490-1777
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
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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.

Place, publisher, year, edition, pages
2015. Vol. 56, p. 35-44
Keyword [en]
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: urn:nbn:se:kth:diva-167700DOI: 10.1016/j.geothermics.2015.03.003ISI: 000358095700003Scopus ID: 2-s2.0-84925424683OAI: oai:DiVA.org:kth-167700DiVA, id: diva2:816041
Note

QC 20150602

Available from: 2015-06-02 Created: 2015-05-22 Last updated: 2018-01-29Bibliographically 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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