Soil Heat Survey Field Determination of Subsoil Thermal Conductivity Soulton Hall, Wem, Shropshire
for Mr John Ashton Soulton Hall
by Soil Heat Carbon Zero Consulting Ltd Seaton Grange Offices Grange Lane Seaton Rutland LE15 9HT Tel. 01572 338307 info@carbonzeroco.com
January/February 2013
Soilheat
Carbon Zero Consulting
Contents 1.0 2.0 3.0 4.0 5.0
INTRODUCTION.................................................................................................. 1 SOILHEAT SURVEY ............................................................................................ 1 SURVEY DETAILS .............................................................................................. 2 SURVEY RESULTS ............................................................................................. 2 REFERENCES .................................................................................................... 4
Tables Table 1 Table 1 Table 2
Summary description of soils found on site Summary of measurements Summary of results Appendices
Appendix A
Theory of operation of the thermal needle system
Completed by CA
Date January 2013
Checked by JF
Date 5/2/13
Soilheat
Carbon Zero Consulting
February 2013
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Soilheat Survey Field Determination of Soil Thermal Conductivity Soulton Hall, Near Wem, Shropshire
1.0
INTRODUCTION
Carbon Zero Consulting has been contracted to perform a ‘Soilheat’ survey. The introduction of MIS3005 guidelines and look-up tables requires knowledge of soil thermal conductivity and mean ground temperature for a proposed closed loop array installation. This Soilheat survey has been undertaken to obtain a site specific value for each parameter at the intended depth of loop burial. 2.0
SOILHEAT SURVEY
2.1
Method of measurement
The survey has been carried out using a Hukseflux FTN01 probe. A short technical explanation of how this is achieved is provided in the Appendix. A hand auger is utilised to excavate to a depth in the range of 0.8m – 1.4m. The instrument is then pushed in to place to ensure the thermal needle is in good contact with the soil at the excavated depth. The instrument is allowed to equilibrate at the soil temperature prior to power being applied. The undisturbed soil temperature is recorded. A voltage of 3V or 4V is applied to the thermal needle depending on whether the soil is moist or fully saturated. A pre-survey calibration is performed in a medium of known thermal properties (Glycerine). The measurement result should be 0.25 and 0.35W/mK for the system to be working correctly. The measured result is shown in Sections 3 and 4. 2.2
The survey area
Soils in the area are described by the BGS as being Glaciofluvial deposits consisting of deep sandy to clayey loam. The survey area comprises the northern part of field 4300, a 2.69ha arable field to the south of Soulton Hall. To the north, the field is bounded by the historic paddock adjacent to the Hall. From the field entrance the land rises along its western edge towards the southern boundary, delineated by a former hedge line marked by mature trees. The field slopes to the east, falling between 3m and 4m over its (average) 150m width, with the centre part of eastern boundary marked by a circular pond. Beyond this boundary, the land falls away a further 2m towards Soulton Brook. The sub-soils are highly varied within the survey area and are summarised in Table 1:
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Depth
Soil Summary
Description
0 – 0.3m 0.3 – 0.5m 0.6-1.2m+
Light/medium topsoil Coarse sand Varied mix of sand/gravel/clay
Arable, recently wheat and potatoes. Mini-till cultivation. Often with gravel and bands of clay Highly variable sub-soil ranging from sand and gravel dominant along western boundary with increasing silt/clay towards east. Distinctive bands of clay, often dense run roughly NW to SE.
Table 1. Summary description of soils found on site It should be noted that the weather in the months preceding the survey were characterised by torrential rain followed by frost and snow. The year of 2012 was the wettest ever recorded in England. As such, soils throughout the country are at field capacity with water standing on the surface in many places. The movement of groundwater within the soil can have an impact on the recording of thermal conductivity – requiring a larger number of readings to be taken to obtain high quality data. During the initial site visit on the 26 January 2013, water was present beneath a depth of about 0.5m - 0.7mbgl. By the following week, the depth of water had fallen to below 0.9mbgl along most of the upper western half of the survey area. The soils in the lower part of the field were ‘running’ and unstable due to presence of groundwater below 0.3mbgl. The owner, Mr John Ashton, advised that the field remains moist for most of the year can rarely be considered ‘dry’. 3.0
SURVEY DETAILS
Date of measurements Total number of measurements Measurements within auger holes Number of measurements where standard deviation <0.10 (accepted measurements) Pre-survey calibration pass/fail
26/1/13 and 2/2/13 35 35 17 Pass
Table 2. Summary of measurements.
4.0
SURVEY RESULTS
In order to obtain a representative determination of the bulk thermal conductivity of a site, research and previous trials have suggested that a minimum of 10 - 12 thermal conductivity determinations are required. A summary of results is given in Table 3. Results with a standard deviation of greater than 0.10 (5% of the measured value) have not been utilised.
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Point 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Carbon Zero Consulting Reading Depth m 0.9 1 1.2 1.1 0.9 1 1 0.9 1 0.8 1 1 1 1 0.9 1 1
Pre-Calibration
Average λ W/mK Standard Deviation 2.340 0.110 2.460 0.040 2.310 0.070 1.830 0.030 2.150 0.030 2.620 0.060 1.690 0.070 1.290 0.030 2.460 0.080 2.630 0.040 1.600 0.020 1.980 0.030 2.710 0.070 2.370 0.050 2.560 0.060 1.390 0.010 2.650 0.080
0.25
Start Temperature Comments Voltage 0.8m clay - wet 3.000 4 coarse sand - wet 2.980 4 coarse red sand - damp 3.200 4 red sand/clay - wet 3.460 4 0.8m firm clay with some small gravel - wet 2.420 4 0.8m sand - v wet 2.570 4 0.5m clay - wet 4.620 4 0.5m clay band. 0.7m Sand/gravel - wet 4.540 4 0.7m sand/clay -saturated 4.420 4 0.8m clay/sand - wet 4.580 4 0.8m clay/sand - wet 4.560 4 0.8m mainly sand with some clay - wet 4.480 4 0.5m+ sand - wet 4.400 4 0.8m gravel/sand/clay - wet 4.540 4 0.7m small gravel/clay - wet 4.460 4 0.7m small gravel/clay - wet 4.480 4 0.8m clay/sand - wet 4.490 4
SD % 5% 2% 3% 2% 1% 2% 4% 2% 3% 2% 1% 2% 3% 2% 2% 1% 3%
Result should be >0.25 and <0.35
Average values (All data) Average λ 2.18 W/mK Average T 4.01 °C Geomean λ 2.13 W/mK Min λ 1.29 W/mK Max λ 2.71 W/mK
Table 3. Summary of Results.
4.1
Result; Undisturbed soil temperature
At each location, the undisturbed subsoil temperature was determined prior to the commencement of the heating test. Readings were taken in the depth range of 0.8 to 1.20m. The average subsoil temperature across the site at the time of the survey is given by: Best estimate of arithmetic mean soil temperature (T) = 4 °C Note that this subsoil temperature will vary considerably with the seasons. 4.2
Result; Soil thermal conductivity
At each location, the soil thermal conductivity was determined using a voltage of 4V. Readings were taken in the depth range of 0.8 to 1.20m. Best estimate of mean soil thermal conductivity (λ) = 2.13W/mK
4.3
MCS 022: GROUND HEAT EXCHANGER LOOK-UP TABLES (MIS 3005)
Using the referenced ‘lookup’ tables, the measured value of λ can be utilised to estimate a value for heat extraction from the ground.
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The tables require a value of mean ground temperature and average ground thermal conductivity (λ). The values of temperature measured in this survey are not used in the table, however, the survey illustrates how low the undisturbed temperature of the soil at the depth of ground loop burial can fall during winter months. Assuming a mean ground temperature of 10°C, 25mm OD SDR 11 pipe in a straight pipe ground array, 0.75m minimum pipe spacing, pipe depth between 0.8m and 1.2m. • • •
5.0
Full load equivalent operation (FLEQ) of 1200 hours; Power extraction available 26 W/m Full load equivalent operation (FLEQ) of 1800 hours; Power extraction available 20 W/m Full load equivalent operation (FLEQ) of 2400 hours; Power extraction available 15 W/m
REFERENCES
Hukseflux (2010). FTN01 Field Thermal Needle System for Thermal Resistivity / Conductivity Measurement. FTN01 manual v1010. Hukseflux Thermal Sensors. Field Determination of Shallow Soil Thermal Conductivity Using a Short-Duration Needle Probe Test. Authors: David Banks, Will King, John Findlay. Quarterly Journal of Engineering Geology and Hydrogeology 2012, v45; p497-504.
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Appendix A Theory of operation
The principle of operation is that a small electrical voltage is applied to an electrical resistance element in the needle probe. This results in a constant heat power (P, in W/m) being generated, which propagates radially into the surrounding soil. The temperature T of the probe increases with time t and this increase is monitored over a heating cycle of around 5 minutes. If radial heat conduction is assumed, the temperature should increase in proportion to the logarithm of time, according to:
P ∆T = (ln t + B ) 4πλ where - ∆T = Tt − T0 = temperature rise after time t (K) -
P = heat power input in W/m
-
t = time after heating commenced
-
λ = thermal conductivity of the soil (W/m/K)
-
B = a constant
-
Tt = temperature of probe at time t
-
T0 = initial stable temperature of soil prior to heating
P . 4πλ
Thus, if ∆T is plotted against lnt, a straight line should result, whose gradient will be Knowing the power input, the thermal conductivity of the soil can be deduced.
A typical heating cycle lasts H = 300 s. In the field, the control and readout unit automatically calculates the gradient and hence the thermal conductivity for the intervals 0.5H, 0.6H, 0.7H and 0.8H. It furthermore calculates the average of these four values and a value of standard deviation. For a reliable result, these four values should be similar and the standard deviation should be low. Determinations are rejected where the standard deviation is greater than 15% of the average thermal conductivity output. In order to obtain a representative determination of the bulk thermal conductivity of a site, trials have suggested that a minimum of 10 thermal conductivity determinations are required.
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C:\CZC\PROJECTS\Soilheat\Soulton\Soil Heat Report Soulton.docx