Numerical Evaluation of Thermal Response Test

1
Numerical Evaluation of Thermal Response Test

• S. Signorelli1, S.Bassetti1, D. Pahud2, T. Kohl1
• 1GEOWATT AG, Zurich
• 2SUPSI, Lugano, Switzerland

2
Determination of Thermal Conductivity, ?

• Laboratory Measurements
• Steady state "Divided Bar"
• Transient line source
• Thermal Conductivity
• In-situ Measurements
• KTB Borehole probe
• Thermal Response Test TRT

3
Determination of ? by TRT
Tin
Tout

• Problems
• Experimental duration
• constant heating
• Insulation

• analytical calculation

4
Numerical Evaluation of TRT

• Theoretical Calculation of T(t) by numerical
  models
• Re-interpretation by line-source approach using
  typical TRT evaluation methods
• Comparison to input value
• Application to field test

5
Simulation of Borehole Heat Exchanger Systems
(BHE)

• Vertical Discretization
• Generally ?z 25 m
• Except
• Near Surface ?z 1 - 10 m
• Bottom Hole ?z 1 - 10 cm

• Horizontal Discretization
• Strong refinement around probe

6
Simulation of BHE systems

• Thermal / hydraulic properties
• Hydraulic conductivity
• Thermal conductivity
• Thermal capacity

• Boundary Conditions
• Temperature at Surface
• Basal heat flow
• Power
• Operation cycles

7
Testing the Line Source Approach

• Thermal conductivity evaluated by line source
  approach

• fixed at tE 200 hrs
• Influence of the borehole progressively reduces
  for later starting points of evaluation.

8
Duration of Measurement

• necessary test duration for 10 deviation of the
  line source model

• Shading 95 confidence interval, when 0.1 K
  white noise is superimposed on the synthetic
  thermal response.

9
Effect of Borehole Length
10
Stratified Subsurface

• TRT Heat injection
• BHE Operation Heat injection
• Layers may be activated differently
• Possible influence of stratification

2 4
4 2
11
Groundwater flow

• FEModel ? synthetic temperature data ? analytic
  interpretation
• Borehole length 150 m
• Heating Power 9 kW
• Total test duration 200 h
• Matrix therm. conductivity 3 Wm-1K-1
• Aquifer thickness 10 m

10 m
V variabel
12
Groundwater flow

• Influence of flow velocity
• 0.1 m/d lt vD lt 18 m/d

13
Measurement at SUVA Building Gisikon

• Data

• Conventional Line-Source Interpretation

14
Measurement at SUVA Building Gisikon

• Numerical Fitting Procedure

• Influence of outside temperature on model result

15
Conclusions

• TRT often used for in-situ therm.conductivity
  determination
• Verification of Numerical Procedure
• Quantification of error at conventional
  line-source-model
• Comparison for real field test highlights large
  deviations and influence of missing insulation