Earth (”Rock”) Borewall Outer “Grout 2” Pipe Pipe Surface Split @ Center of Pipe Inner “Grout 1” Grout Boundary Earth (”Rock”) Borewall Outer “Grout 2” Grout Boundary Inner “Grout 1” Pipe Surface Concentric Pipe (a) Two Pipe Configuration (b) Four Pipe Configuration (2) GHEX Simulation Model - Concentric-pipe Bore Configuration (1) GHEX Simulation Model - Multi-pipe Bore Configuration Fluid Bore Cell C Grout 1 D 0 D 1 C Grout 2 R Grout 1 R Grout 2 R Earth-1 C Earth-1 R Earth-N-1 C Earth-N-1 R Earth-N C Earth-N R Earth-2 C Earth-2 Borewall 3 2 1 7 5 4 8 9 10 11 6 Advanced Testing Method for Ground Thermal Conductivity Xiaobing Liu (Oak Ridge National Laboratory), Rick Clemenzi (Geothermal Design Center, Inc.), Liu Su (University of Tennessee) Introduction Effective ground thermal conductivity (EGTC) is a critical parameter for designing geothermal (ground source) heat pump (GHP) systems. EGTC at a given location is usually measured with a thermal response test (TRT). Current Industry Standards (ANSI/CSA C448 Series-16) for TRT requires: • Minimum 36–48 hours • Stable power supply throughout the entire TRT (variation less than ±1.5% of the average, peaks less than ±10% of the average) Results 1. Min. TRT duration for determining EGTC • 21%–46% shorter in all cases except case 3 (which had varying heat input) and the determined EGTCs are within ±5% of those determined with full ~48 hours of TRT data • TRT time could be further reduced with earlier cutting time 2. Prediction of GHEXs fluid temperature • Root Mean Square Error (RMSE) of the predicted average fluid temperature was less than 0.1°F compared with measured values when heat input is stable and continuous • Predicted average fluid temperature matched measured values very well even when the heat input was interrupted 3. EGTC determined with the new R-C model • 12 datasets (36–48 hours) of TRTs were used to determine EGTCs with both the conventional line-source (LS) and the new R-C model • EGTCs determined with the new R-C model are very close to those determined with the conventional line source method (with less than 10% variance) except in cases 2 and 5 (cases 1-6 were all within a 2 block area making the R-C model likely more accurate) • Difference between the reported grout thermal conductivity values and those determined with the R-C model are significant in most cases. It is found that some reported grout thermal conductivity values were not correct (e.g., in case 4, due to difficulties of grouting, grout was pumped into the borehole from the ground surface instead of from bottom up as required by industry standard) Conclusions • A new method for analyzing TRT data is developed, which uses a R-C model for vertical bore GHEX developed by Geothermal Design Center and a new algorithm developed by ORNL for determining the minimum TRT duration • This new method can reduce testing time by 40%–60% compared with the current practice while retaining same level accuracy (±5%). It can reasonably estimate EGTC value even with varying and disrupted heat inputs • This method can also be used to estimate other parameters of the GHEX, including grout conductivity and the heat capacity of the ground and the grout, to help verify whether the GHEX was installed to the design specification Proposed Future Work • Determine the causes for the spikes of EGTC values determined with data at the later time of some TRTs • Further develop the method so that it can be used to reliably verify installation quality of GHEX Acknowledgments This study is sponsored by the Small Business Voucher Pilot program of the US Department of Energy. A Summary of 12 TRT Cases and GTC Values Determined by the New R-C Model EGTC determined with the “floating window” method showed spikes at the later time of some TRTs while heat input was stable, which may indicate heterogeneous thermal properties of the ground in radial direction. Methods Current practice uses the line source model (Ingersoll and Plass 1948, Ingersoll et al. 1950) to determine EGTC value based on TRT results. The early 10-18 hours of data are usually discarded. Green Dots indicate EGTCs determined with the new algorithm Approximation of the geometry of a borehole with three different loops Notes: Kgrd in case 3 was not be able to be determined with the conventional line source method due to large variations of heat input, but it was determined with the R-C model. EGTCs calculated with two different approaches (“Floating Window” and “Progressive Tracking”) during a TRT The above case study shows that, EGTC can be evaluated within less than 24 hours during a TRT and the result is about the same as that determined with 36-48 hours of TRT data. A new algorithm to determine whether a TRT has collected sufficient data to determine EGTC with acceptable accuracy These rigid requirements make EGTC testing expensive so that it is under used commercially and rarely for residential GHP systems. The lack of accurate EGTC data often leads to either oversized, more expensive ground heat exchangers (GHEXs) or to undersized, poorly performing GHEXs. Requiring rigid power also forgoes any opportunity of using varying heat sources including a building’s operating heat pumps or alternate sources of heat. The objective of this study was to develop a new method that determines EGTC with comparable or better accuracy, but with a shorter test time and without a highly stable power supply for an affordable EGTC testing for all GHP projects including post installation. A further objective was to, for the first time, provide a means to analyze the installed grout thermal conductivity of a GHEX. This simplified R-C model is used to determine EGTC value through a parameter estimation approach Improved heat transfer modeling for vertical bore ground heat exchangers Better algorithm for analyzing TRT data Account for transient heat transfer both inside and outside a vertical bore Dynamically determine minimum test duration 60 65 70 75 80 85 90 95 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Fluid Temperature [F] Time Lapse since the Beginning of the TRT [Hour] Measured In Measured Out Measure Avg Predicted AWT Temp In Temp Out 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Measured In Measured Out Measure Avg Predicted AWT Temp In Temp Out 50 55 60 65 70 75 80 85 90 95 Fluid Temperature [F] Time Lapse since the Beginning of the TRT [Hour] 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 Ground Thermal Conductivity [Btu/h-ft-F] Time Lapse since the Beginning of the TRT [Hour] Kgnd Floang Window Progressive Tracking Case # Loop Design Depth Grout Type Ground Temp. Mea. Kgrd (LS) Mea. Kgrd (RC) Difference Rep. Kgrt Mea. Kgrt (RC) Difference ft F Btu/h-ft-°F Btu/h-ft-°F Btu/h-ft-°F Btu/h-ft-°F 1 Co-axial (2.5” SDR-21) 214 Barotherm Max 62.8 2.05 1.93 -6% 1.79 0.80 -55% 2 Single U-tube (1” DR11) 319 Barotherm Gold 63.7 2.22 1.93 -13% 1.00 0.99 -2% 3 Double U-tube (1” PEX) 260 Barotherm Gold 63.2 NA* 1.93 NA* 0.88 1.73 96% 4 Co-axial 120 Barotherm Max 63.0 1.85 1.92 4% 1.6 0.42 -74% 5 Co-axial 150 Barotherm Gold 63.6 1.61 1.93 20% 0.88 0.78 -11% 6 Double U-tube (1” DR11) 260 Barotherm Gold 63.4 2.08 1.93 -7% 0.88 1.98 125% 7 Single U-tube (1.25” DR11) 600 PowerTECx 57.8 1.20 1.20 0% 1.20 1.18 -2% 8 Single U-tube (1.25” DR11) 402 Thermally enhanced 67.4 1.55 1.55 0% 1.00 1.32 32% 9 Single U-tube (1.25” DR11) 415 Std bentonite 64.8 1.64 1.64 0% 0.41 1.01 146% 10 Single U-tube (1.25” DR11) 300 Thermally enhanced 65.4 1.75 1.75 0% 1.00 1.29 29% 11 Single U-tube (1.25” DR11) 450 Std bentonite 56.4 1.88 1.97 5% 0.41 1.09 166% 12 Single U-tube (1.25” DR11) 450 Std bentonite 56.9 2.06 1.97 -4% 0.41 0.97 137% Final report of this project is available at http://info.ornl.gov/sites/publications/files/Pub74344.pdf With fixed number of TRT data (“Floating Window”) With fixed cutting time (“Progressive Tracking”) A resistor-capacitor circuit (R-C) model for the transient heat transfer process within a vertical bore and throughout the surrounding ground formation 0 2 4 6 8 10 12 14 16 18 -5 -4 -3 -2 -1 0 1 2 3 4 5 Tavg - Tground [K] Ln(Time) Slope = m k= q 4�m Circulation Pump T1 T2 Heater Flowmeter Thermal Response Test Apparatus
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Earth (”Rock”)Borewall
Outer “Grout 2”Pipe
Pipe SurfaceSplit @ Center of Pipe
Inner “Grout 1”Grout Boundary
Earth (”Rock”)Borewall
Outer “Grout 2”Grout Boundary
Inner “Grout 1”Pipe Surface
Concentric Pipe
(a) Two Pipe Con�guration (b) Four Pipe Con�guration
(2) GHEX Simulation Model - Concentric-pipe Bore Con�guration
(1) GHEX Simulation Model - Multi-pipe Bore Con�guration
Fluid
Bore Cell
CGrout 1
D0
D1
CGrout 2
RGrout 1
RGrout 2 REarth-1
CEarth-1
REarth-N-1
CEarth-N-1
REarth-N
CEarth-N
REarth-2
CEarth-2
Bore
wal
l
3
21
7
5
4
8
9
10
11
6
Advanced Testing Method for Ground Thermal ConductivityXiaobing Liu (Oak Ridge National Laboratory), Rick Clemenzi (Geothermal Design Center, Inc.), Liu Su (University of Tennessee)
IntroductionEffective ground thermal conductivity (EGTC) is a critical parameter for designing geothermal (ground source) heat pump (GHP) systems. EGTC at a given location is usually measured with a thermal response test (TRT). Current Industry Standards (ANSI/CSA C448 Series-16) for TRT requires:
• Minimum 36–48 hours• Stable power supply throughout the entire TRT (variation less than ±1.5% of the average,
peaks less than ±10% of the average)
Results1. Min. TRT duration for determining EGTC
• 21%–46% shorter in all cases except case 3 (which had varying heat input) and the determined EGTCs are within ±5% of those determined with full ~48 hours of TRT data
• TRT time could be further reduced with earlier cutting time
2. Prediction of GHEXs fluid temperature • Root Mean Square Error (RMSE) of the
predicted average fluid temperature was less than 0.1°F compared with measured values when heat input is stable and continuous
• Predicted average fluid temperature matched measured values very well even when the heat input was interrupted
3. EGTC determined with the new R-C model• 12 datasets (36–48 hours) of TRTs were used to determine EGTCs with both the conventional line-source (LS) and the
new R-C model• EGTCs determined with the new R-C model are very close to those determined with the conventional line source
method (with less than 10% variance) except in cases 2 and 5 (cases 1-6 were all within a 2 block area making the R-C model likely more accurate)
• Difference between the reported grout thermal conductivity values and those determined with the R-C model are significant in most cases. It is found that some reported grout thermal conductivity values were not correct (e.g., in case 4, due to difficulties of grouting, grout was pumped into the borehole from the ground surface instead of from bottom up as required by industry standard)
Conclusions• A new method for analyzing TRT data is developed, which uses a R-C model for vertical bore GHEX developed by
Geothermal Design Center and a new algorithm developed by ORNL for determining the minimum TRT duration • This new method can reduce testing time by 40%–60% compared with the current practice while retaining same
level accuracy (±5%). It can reasonably estimate EGTC value even with varying and disrupted heat inputs• This method can also be used to estimate other parameters of the GHEX, including grout conductivity and the heat
capacity of the ground and the grout, to help verify whether the GHEX was installed to the design specification
Proposed Future Work• Determine the causes for the spikes of EGTC values determined with data at the later time of some TRTs • Further develop the method so that it can be used to reliably verify installation quality of GHEX
AcknowledgmentsThis study is sponsored by the Small Business Voucher Pilot program of the US Department of Energy.
A Summary of 12 TRT Cases and GTC Values Determined by the New R-C Model
EGTC determined with the “floating window” method showed spikes at the later time of some TRTs while heat input was stable, which may indicate heterogeneous thermal properties of the ground in radial direction.
Methods
Current practice uses the line source model (Ingersoll and Plass 1948, Ingersoll et al. 1950) to determine EGTC value based on TRT results. The early 10-18 hours of data are usually discarded.
Green Dots indicate EGTCs determined with the new algorithm
Approximation of the geometry of a borehole with three different loops
Notes: Kgrd in case 3 was not be able to be determined with the conventional line source method due to large variations of heat input, but it was determined with the R-C model.
EGTCs calculated with two different approaches (“Floating Window” and “Progressive Tracking”) during a TRT
The above case study shows that, EGTC can be evaluated within less than 24 hours during a TRT and the result is about the same as that determined with 36-48 hours of TRT data.
A new algorithm to determine whether a TRT has collected sufficient data to determine EGTC with acceptable accuracy
These rigid requirements make EGTC testing expensive so that it is under used commercially and rarely for residential GHP systems. The lack of accurate EGTC data often leads to either oversized, more expensive ground heat exchangers (GHEXs) or to undersized, poorly performing GHEXs. Requiring rigid power also forgoes any opportunity of using varying heat sources including a building’s operating heat pumps or alternate sources of heat.
The objective of this study was to develop a new method that determines EGTC with comparable or better accuracy, but with a shorter test time and without a highly stable power supply for an affordable EGTC testing for all GHP projects including post installation. A further objective was to, for the first time, provide a means to analyze the installed grout thermal conductivity of a GHEX.
This simplified R-C model is used to determine EGTC value through a parameter estimation approach
Improved heat transfer modeling for vertical bore ground heat exchangers
Better algorithm for analyzing TRT data
Account for transient heat transfer both inside and outside a vertical bore
Dynamically determine minimum test duration
60
65
70
75
80
85
90
95
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Flui
d Te
mpe
ratu
re [F
]
Time Lapse since the Beginning of the TRT [Hour]
Measured In Measured Out Measure AvgPredicted AWTTemp In Temp Out
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Measured In Measured Out Measure AvgPredicted AWTTemp In Temp Out
50
55
60
65
70
75
80
85
90
95
Flui
d Te
mpe
ratu
re [F
]
Time Lapse since the Beginning of the TRT [Hour]
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Gro
und
Ther
mal
Con
duct
ivit
y [B
tu/h
-ft-
F]
Time Lapse since the Beginning of the TRT [Hour]
Kgnd
FloatingWindow
ProgressiveTracking
Case # Loop Design
DepthGrout Type
Ground Temp. Mea. Kgrd (LS) Mea. Kgrd (RC)Difference
Rep. Kgrt Mea. Kgrt (RC)Difference
ft F Btu/h-ft-°F Btu/h-ft-°F Btu/h-ft-°F Btu/h-ft-°F
1 Co-axial (2.5” SDR-21) 214 Barotherm Max 62.8 2.05 1.93 -6% 1.79 0.80 -55%
2 Single U-tube (1” DR11) 319 Barotherm Gold 63.7 2.22 1.93 -13% 1.00 0.99 -2%
3 Double U-tube (1” PEX) 260 Barotherm Gold 63.2 NA* 1.93 NA* 0.88 1.73 96%
4 Co-axial 120 Barotherm Max 63.0 1.85 1.92 4% 1.6 0.42 -74%
5 Co-axial 150 Barotherm Gold 63.6 1.61 1.93 20% 0.88 0.78 -11%
6 Double U-tube (1” DR11) 260 Barotherm Gold 63.4 2.08 1.93 -7% 0.88 1.98 125%
7 Single U-tube (1.25” DR11) 600 PowerTECx 57.8 1.20 1.20 0% 1.20 1.18 -2%
8 Single U-tube (1.25” DR11) 402 Thermally enhanced 67.4 1.55 1.55 0% 1.00 1.32 32%
9 Single U-tube (1.25” DR11) 415 Std bentonite 64.8 1.64 1.64 0% 0.41 1.01 146%
10 Single U-tube (1.25” DR11) 300 Thermally enhanced 65.4 1.75 1.75 0% 1.00 1.29 29%
11 Single U-tube (1.25” DR11) 450 Std bentonite 56.4 1.88 1.97 5% 0.41 1.09 166%
12 Single U-tube (1.25” DR11) 450 Std bentonite 56.9 2.06 1.97 -4% 0.41 0.97 137%
Final report of this project is available at http://info.ornl.gov/sites/publications/files/Pub74344.pdf
Withfixednumber ofTRTdata(“FloatingWindow”)
Withfixedcuttingtime(“ProgressiveTracking”)
A resistor-capacitor circuit (R-C) model for the transient heat transfer process within a vertical bore and throughout the surrounding ground formation
0
2
4
6
8
10
12
14
16
18
-5 -4 -3 -2 -1 0 1 2 3 4 5
Tavg
- Tg
roun
d [K
]
Ln(Time)
Slope = m
k =q
4�m
Circulation Pump
T1
T2
Heater
Flowmeter
Thermal Response Test Apparatus
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