2nd International FEFLOW® User Conference

Modeling Borehole Heat Exchanger

Systems H.-J.G. DIERSCH1), D. BAUER2),

W. RÜHAAK1), W. HEIDEMANN2) & P. SCHÄTZL1)

1) Groundwater Modelling Centre (GMC)

2) Institute of Thermodynamics and Thermal Engineering (ITW), University of Stuttgart

2FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Content

1. Introduction

2. Borehole Heat Exchanger (BHE)

3. Numerical Solution

4. Analytical Solution

5. Implementation

6. Applications

7. Conclusions

3FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Borehole Heat Exchanger

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4FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Heizzentrale

Kollektoren5670 m²

Erdsonden-Wärmespeicher63360 m³

2.Ausbau1.Ausbau

Pufferspeicher 2 x 100 m³

Nahwärmenetz3 Leiter Netz- -

ca. 300 Wohneinheiten,Schule, Ladenzentrum, ...

Motivation – SuN (Crailsheim)Buffer storage

Heat station

Solar collectorsapartments,

school building, shopping center, …

Local heat network3 conductors

1st stage

2nd stage

Borehole thermal energy store: BHE array

5FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

0

100

200

300

400

500

600

700

800

Jan Feb Mär Apr Mai Jun Jul Aug Sep Okt Nov Dez Jahr

Wär

mem

enge

[MW

h/a]

0

500

1000

1500

2000

2500

3000

3500

4000

Wär

mem

enge

[MW

h/a]

Solarenergie

zusätzliche Energie

solare Ein-strahlung (relativ)

Motivation – SeasonalHeat Storage

Solar energy

Ther

mal

pow

er

Ther

mal

pow

er

Additional energy

Solarization(relative)

6FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

-50

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12Monat des Jahres

Wär

mem

enge

[MW

h]

0

10

20

30

40

50

60

Tem

pera

tur [

°C]

Beladung EntladungWärmeverluste Temperatur

Crailsheim: Heat Balance of Borehole Thermal Energy Store

Ther

mal

ene

rgy

Tem

pera

ture

Month of year

Loading UnloadingThermal loss Temperature

7FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Tief

e

ohne Grundwasser

mit Grundwasser

Tief

e

Isothermendarstellung

Isothermendarstellung

Influence of groundwater flow on borehole thermal energy store

MotivationD

epth

Dep

th

Isotherms

without groundwater

with groundwater

Isotherms

8FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Types of BHE2U pipe

1U pipe

CXA (CXC) pipe

9FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

BHE Modeling ApproachesFully discretized vs. 1D-line element (BHE) approaches

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10FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

BHE Solution Strategies

Numerical: based on Al-Khoury et al. (2005, 2006) with significant extensions (generalized formulations for 2U, 1U, CXAand CXC type BHE, multiple grout points, improved relationships for thermal resistances, essentially non-iterative coupling method)

Analytical: based on Eskilson & Claesson (1988) with significant extensions (generalized formulations for 2U, 1U, CXAand CXC type BHE, improved relationships for thermal resistances, effective coupling to 3D FE-discretizations of porous matrices)

Restrictions: local steady-state conditions, appropriate for long-term predictions (robust and fast procedure)

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BHE Setting in FEFLOW

12FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

BHE Output in FEFLOW(1) Vertical temperature

profiles for fluid (inlet, outlet) and grout zones

(2) History of outlet temperature

(3) BHE selector

(4) Iteration behavior and control

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BHE Array Schematization

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BHE Model Equations

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Static Condensation

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Analytical Solution

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BHE vs. Fully Discretized Solutions

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Benchmarking

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TRNSYS (TRaNsient SYstemsSimulation)

TRNSYS is a tool designed to simulate the transient performance of thermal energy systemsCommercially available since 1975It was initially developed by the University of WisconsinWithin a joint project between ITW University Stuttgart and DHI-WASY GmbH an IFM extension for FEFLOW is under developmentQt-based and platform-independent (Windows + Linux) interfaceA new TRNSYS Type (331) is introducedModule is coupled with FEFLOW via the RPC protocol

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TRNSYS Coupling

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TRNSYS CouplingSeasonal solar thermal energy storage (at summer) and re-extraction (at winter) of BHE array consisting of 80 2U-pipe system (Crailsheim, Germany) controlled via FEFLOW-TRNSYS IFM interface (IfmTRNSYS)

22FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

3 m

Ø 30 m

Anbindung Heizzentrale

Verbindungsstelle

Sonde innerer KreisSonde äußerer Kreis

z

r

Simulation of Crailsheim Site

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aquifer 1

aquifer 2

Simulation of Crailsheim Site

24FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Loading: ½ yr

without groundwater

with groundwater

25FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Unloading: 1 yr

without groundwater

with groundwater

26FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Loading: 1½ yrs

without groundwater

with groundwater

27FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Unloading: 2 yrs

without groundwater

with groundwater

28FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Loading: 2½ yrs

without groundwater

with groundwater

29FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Unloading: 3 yrs

without groundwater

with groundwater

30FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Loading: 3½ yrs

without groundwater

with groundwater

31FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Unloading: 4 yrs

without groundwater

with groundwater

32FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Loading: 4½ yrs

without groundwater

with groundwater

33FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Unloading: 5 yrs

without groundwater

with groundwater

34FEFLOW 2009, September 14FEFLOW 2009, September 14--18, 2009, Potsdam/Berlin, Germany18, 2009, Potsdam/Berlin, Germany

Conclusions

BHE tools (analytical, numerical) available in FEFLOW (-> 5.4, 6.0).

Analytical method for long-term predictions.

Appropriate for analysis of single BHE and BHE arrays.

Improved modeling for complex borehole energy store systems (e.g., SuN) to analyze influences of groundwater flow.

Computation of arbitrary heating/cooling systems via FEFLOW-TRNSYS coupling.