HomeMy WebLinkAboutJuneau Airport Ground Source Heat Pump Project Thermal Conductivity Test and Data Analysis - Feb 2009 - REF Grant 2195359FORMATION THERMAL CONDUCTIVITY TEST
AND DATA ANALYSIS
Alaska Energy Engineering LLC
25200 Amalga Harbor Rd.
Juneau, Alaska 99801
Phone/Fax: 907-789-1226
Analysis for:
Juneau International Airport Test location:
Juneau, Alaska
Report Date: February 2, 2009
Alaska Energy Engineering LLC Test Performed by:
MAIN OFFICE:REGIONAL OFFICES: WEB SITE:
P.O. Box 150 BROOKINGS, SD ASHEVILLE, NC www.GRTI.com
BOWIE, TX 76230 (605) 692-9069 (828) 225-9166
(940) 872-2222 Fax: (605) 692-2604 Fax: (828) 281-4139
Fax: (940) 872-3678
GRTI
February 2, 2009 FTC Test and Data Analysis
Page 2 of 9
Executive Summary
A formation thermal conductivity test was performed at the Juneau International Airport site in
Juneau, Alaska. The vertical bore was completed on January 17, 2009 by Gregory Drilling, Inc.
GRTI’s test unit was attached to the vertical bore on the afternoon of January 20, 2009.
Geothermal Resource Technologies, Inc. analyzed the collected data using the “line source”
method.
This report provides a general overview of the test and procedures that were used to perform the
thermal conductivity test along with a plot of the data in real time and in a form used to calculate
the formation thermal conductivity. A limited amount of data was applicable for analysis for this
test. While short test intervals should not be relied upon to determine an accurate thermal
conductivity value, data of high quality may be used to give an indication where the true value
may lie. The following average formation thermal conductivity was found from the data analysis.
Calculated Formation Thermal Conductivity = 0.90 Btu/hr-ft-°F
Due to the necessity of a thermal diffusivity value in the design calculation process, an estimate
of the average thermal diffusivity was made for the encountered formation.
Formation Thermal Diffusivity 0.65 ft2/day
An estimate of the undisturbed formation temperature was determined from the initial
temperature data at startup.
Undisturbed Formation Temperature 44-45°F
A copy of the original collected data is available either in a hard copy or an electronic format upon
request.
GRTI
February 2, 2009 FTC Test and Data Analysis
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Test Procedures
The American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) has
published a set of recommended procedures for performing formation thermal conductivity tests
for geothermal applications. GRTI is committed to adhering to ASHRAE recommendations.
Some of these recommended procedures are listed below:
(1) Required Test Duration – A minimum test duration of 36 hours is recommended, with a
preference toward 48 hours.
(2) Power Quality – The standard deviation of the power should be less than or equal to 1.5% of
the average power, with maximum power variation of less than or equal to 10% of the
average power. The heat flux rate should be 51 Btu/hr (15 W) to 85 Btu/hr (25 W) per foot of
borehole depth to best simulate the expected peak loads on the u-bend.
(3) Undisturbed Formation Temperature Measurement – The undisturbed formation temperature
should be determined by recording the minimum loop temperature as the water returns from
the u-bend at test startup.
(4) Installation Procedures for Test Loops – The bore diameter is to be no larger than 6 inches,
with 4.5 inches being the target diameter. To ensure against bridging and voids, the bore
annulus is to be uniformly grouted from the bottom to the top using a tremie pipe.
(5) Time Between Loop Installation and Testing – A minimum delay of five days between loop
installation and test startup is recommended if the formation is expected to have a low
thermal conductivity or if low conductivity grouts (< 0.75 Btu/hr·ft·ºF) are used. A minimum
delay of three days is recommended for all other conditions.
GRTI’s testing procedures deviate slightly from those above with regard to item (5). While item
(5) bases the delay between installation and testing on the expected formation conductivity, GRTI
bases its delay on the type of drilling used in the installation. When air drilling is required, a five-
day delay is recommended to allow the bore to return to its undisturbed temperature. For mud
rotary drilling, a minimum waiting period of two days is sufficient.
For a complete list of recommended procedures, refer to the ASHRAE 2007 HVAC Applications
handbook, pages 32.12-32.13.
GRTI
February 2, 2009 FTC Test and Data Analysis
Page 4 of 9
Data Analysis
Geothermal Resource Technologies, Inc. uses the "line source" method of data analysis. The
line source equation used is not valid for early test times. Also, the line source method assumes
an infinitely thin line source of heat in a continuous medium. If a u-bend grouted in a borehole is
used to inject heat into the ground at a constant rate in order to determine the average formation
thermal conductivity, the test must be run long enough to allow the finite dimensions of the u-
bend pipes and the grout to become insignificant. Experience has shown that the amount of time
required to allow early test time error and finite borehole dimension effects to become
insignificant is approximately ten hours.
In order to analyze real data from a formation thermal conductivity test, the average temperature
of the water entering and exiting the u-bend heat exchanger is plotted versus the natural log of
time. Using the Method of Least Squares, the linear equation coefficients are then calculated
that produce a line that fits the data. This procedure is normally repeated for various time
intervals to ensure that variations in the power or other effects are not producing erroneous
results.
Through the analysis process, the collected raw data is converted to spreadsheet format
(Microsoft Excel ) for final analysis. A copy of this data can be obtained either in a hard copy or
electronic copy format at any time. If desired, please contact Geothermal Resource
Technologies, Inc. and provide a ship-to address or e-mail address at one of the following:
Phone: (605) 692-9069
Fax: (605) 692-264
E-mail: gstreich@grti.com
GRTI
February 2, 2009 FTC Test and Data Analysis
Page 5 of 9
Formation Thermal Conductivity Test Report
Date ...……………………………………………………………. January 20-21, 2009
Location ..………………………………………………………… Juneau, Alaska
Undisturbed Formation Temperature ...………………….…... Approx. 44-45°F
Borehole Data – As Provided by Alaska Energy Engineering LLC
Borehole Diameter ...…………………………………………… 6 inches
Drill Log .................... Sand and gravel 0'-6'
Gravel with some silt matrix 6'-57'
Sand with some silt and occassional wood 57'-170'
Interbedded sandy silt/silty sand 170'-320'
U-bend Size …………………………………………………….. 1 inch HDPE
U-Bend Length ..……………………………………………….. 315 ft
Grout Type ...……………………………………………….…… 3/8” gravel
Grout Solids …………………………………………………..… NA
Grouted Portion ………..……………………………………..… NA
Note: Bore backfilled with bentonite chips from 10-20 ft.
Test Data
Test Duration ...……………….………………………………… 18.4 hrs.
Average Voltage …………………………………………..…… 240.3 V
Average Power …………………………………………….…… 6,044 W
Total Heat Input Rate ……………………………………..…… 20,628 Btu/hr
Calculated Circulator Flow Rate …...…………………….…… 5.9 gpm
Standard Deviation of Power …………………………………. 0.08%
Maximum Peak Variation in Power …………………..………. 0.32%
GRTI
February 2, 2009 FTC Test and Data Analysis
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0 6 12 18 24
Time (hours)
45
50
55
60
65
70
75
80
85
Temperature (F)20.00
20.50
21.00
21.50
22.00
(Thousands)Heating Rate (Btu/hr)Juneau International Airport, Juneau, Alaska
January 20-21, 2009
Loop In
Temp
12-18.4hr
Average
Loop Out
Temp
Heat
Rate
Figure 1: Temperature versus Time Data
GRTI
February 2, 2009 FTC Test and Data Analysis
Page 7 of 9
Line Source Data Analysis
-1 0 1 2 3
ln(Time)
45
50
55
60
65
70
75
80
85
Temperature (F)20.00
20.50
21.00
21.50
22.00
(Thousands)Heating Rate (Btu/hr)Juneau International Airport, Juneau, Alaska
January 20-21, 2009
Loop In
Temp
12-18.4hr
Average
Loop Out
Temp
Heat
Rate
Figure 2: Temperature versus Natural Log of Time
Thermal
Time Period Slope Average Heat Input Conductivity
(Btu/hr-ft) (W/ft) (Btu/hr-ft-°F)
12 – 18.4 hrs 5.81 65.5 19.2 0.90
The temperature versus time data was analyzed using line source analysis. Results from a short
test should not be relied upon to determine an exact thermal conductivity value as the results are
more sensitive to fluctuations in ambient temperature and supplied power. However, data of high
quality can be used to give an indication of the true value. The supplied power for this test was
quite stable with a standard deviation of 0.08% and a maximum variation in power of 0.32%. An
average linear curve fit was applied to the data from 12 to 18.4 hours. The slope of the curve
was found to be 5.81. The resulting thermal conductivity was found to be 0.90 Btu/hr-ft-°F.
GRTI
February 2, 2009 FTC Test and Data Analysis
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Estimated Thermal Diffusivity
The reported drilling log for this test borehole indicated that the formation consisted primarily of
sand, gravel and silt. A weighted average of heat capacity values based on the indicated
formation was used to develop an average heat capacity for the formation. An estimated
diffusivity value was then found using the calculated formation thermal conductivity and the
estimated heat capacity. The thermal diffusivity for this formation was estimated to be
approximately 0.65 ft2/day.
Est. Average
Heat Capacity
(Btu/ft3-°F)
Thermal
Conductivity
(Btu/hr-ft-F)
Est. Thermal
Diffusivity
(ft2/day)
33.0 0.90 0.65
GRTI
February 2, 2009 FTC Test and Data Analysis
Page 9 of 9
Frequently Asked Questions (FAQ’s) Regarding FTC Testing
Q:Thermally-enhanced grout is specified for the final loop field design. The test bore was
grouted with a low conductivity, 20% solids, bentonite grout. How do I adjust the thermal
conductivity value to account for this?
A:While the conductivity of the grout is important for the loop field design, it is not important
for determining formation thermal conductivity. We use the “line source” method to
analyze data, which assumes an infinitely thin line rejecting heat at a constant rate into
an infinite medium. The initial ten hours, which is influenced by the bore dimensions and
grout conductivity, is ignored in the analysis. However, once the heat has penetrated into
the formation, the temperature rise of the formation approaches steady-state. It is the
slope of the temperature rise that is used in the analysis. Hence, no adjustment to the
reported formation thermal conductivity is required.
Q:The software I use to design the loop field requires that I input a value for “soil
conductivity”. Is this the same as formation thermal conductivity?
A:Absolutely. Formation, soil, and ground are all used interchangeably to describe the
conditions in which the u-bends will be installed. The use of the word “formation” simply
implies that the installation conditions may be soil, rock, or some combination of the two.
Q:I’ve just received your report. I have a formation conductivity of 1.54 Btu/hr ft °F. How do
I translate that into a loop length requirement, in terms of bore depth (in feet) per ton?
A:The formation thermal conductivity test provides values for three key parameters required
for the ground loop design. These are the “Undisturbed Formation Temperature,
Formation Thermal Conductivity, and Formation Thermal Diffusivity.“ These parameters,
along with many others, are inputs to commercially available loop design software (e.g.
GchpCalc, available at GeoKiss.com/software). The software uses all of the inputs to
determine the required loop length in bore depth per ton.
Q:Is the “Undisturbed Formation Temperature” listed in the report the temperature that I
enter into my loop design software where it calls for the “Deep-Earth Temperature”?
A:Generally, yes. The “Undisturbed Formation Temperature” is the constant temperature
of the formation. We attempt to determine this value by measuring the temperature of
the water entering the test unit at the beginning of the test. However, the value we
measure and report may be inaccurate if the test is initiated too quickly after the
installation of the test bore, or if the testing operator failed to activate the data acquisition
unit prior to energizing the heating elements. If you suspect the temperature we are
reporting to be too high or too low, we recommend that you investigate further through
other sources.