Modelling Pile Capacity and Load-Settlement Behaviour of ...
The importance of the concrete in thermal pile behaviour · importance of the concrete in thermal...
Transcript of The importance of the concrete in thermal pile behaviour · importance of the concrete in thermal...
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The importance of the concrete in thermal pile behaviourFleur LoveridgeGSHPA, 27 September 2012
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Outline• Introduction & traditional approach
• A transient approach to pile concrete
• Numerical study using real heat pump temperatures
• Initial site data
• Concrete thermal properties
• Conclusions
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Pile Thermal Resistance• Temperature change across concrete usually captured using
a (steady state) resistance term
• Empirical database of experience is absent
• Rpconv & Rpcond relatively “easy” to calculate
• Rc is often largest part of resistance due to volume of concrete
• Depends on pipe arrangements and thermal conductivity of concrete
cpcondpconvb RRRR
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Time to Approach Steady State
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Time to Approach Steady State
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Time for a transient approach?
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Piles with Centrally Placed Pipes
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% o
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c upper bound case:1200 mm pile; central pipes;c=2W/mK, g=1W/mK
lower bound cases:300mm pile; central pipes;c=1W/mK, g=2W/mK
brt / 2
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Piles with Pipes near the Edge
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upper bound case:1200 mm pile; pipes near edge;c=2W/mK, g=1W/mK
lower bound cases:300mm pile; pipes near edge;c=1W/mK, g=2W/mK
brt / 2
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Example: Steady State vs Transient
• Assumptions:
– Transient G function for ground temperature changes
– Transient G function for pile concrete (as % of steady Rc)
– Steady state heat transfer within and across pipes
– 600mm dia pile, 20m long (AR=33.3); 4 pipes near the edge
• c=1W/mK; g=2W/mK; g=1E-6m2/s
• Rc=0.075mK/W; Rp=0.025mK/W
gg
ccpf GqGqRqRT2
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Thermal Pile G-function
0.0
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0.01 0.1 1 10 100 1000 10000Fo
g
lower bound
upper bound
AR=50
AR=33
AR=25
AR=15
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Thermal Pile G-function
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AR=25
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Pile Concrete G-function
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upper bound case:1200 mm pile; pipes near edge;c=2W/mK, g=1W/mK
lower bound cases:300mm pile; pipes near edge;c=1W/mK, g=2W/mK
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Thermal Loads
‐120
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0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Hours (in Year)
heatin
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mand W/m
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Thermal Loads: Daily Variation
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heatin
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mand W/m
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Results: Components
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Results: Totals
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Results: Totals
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Results: Totals
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Real Thermal Loads
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Numerical Model(2D ABAQUS)
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Results: Temperatures
c=g=3W/mK
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Results: Heat Flux
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Siemens USC Site Data
Central thermistors
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Siemens USC Site Data
Thermistors on pile cage
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Consequences• Importance of concrete for storage not just transfer of heat
• Thermal buffering, preventing extreme temperatures reaching the ground
– Effect greatest when c lower than g
– Impact on geotechnical design
• More important to determine concrete thermal properties (not just Rb)
• Concrete properties has greatest impact in largest diameter piles as furthest from steady state
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Concrete Thermal Properties• Thermal conductivity: 1.2
to 4W/mK
• Volumetric heat capacity:2 to 3 MJ/m3K
• Depends on:
– Moisture content– Aggregate type and
ratio– Additives, cement
replacement products• Is rapid heat transfer
desirable?
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Conclusions• Under constant q piles may take days to approach steady
state. – Caution with thermal response tests
• Pile concrete is rarely at a thermal steady state during thermal pile operation
• Pile is being used as an energy store
• Pile is protected the ground against extreme temperatures
• Need more emphasis on determining pile properties
• Treating the pile as transient during design will improve thermal efficiency
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Acknowledgements• Professor William Powrie
• Engineering and Physical Sciences Research Council
• Steering Group: Mott MacDonald, Golder Associates, Cementation Skanska, WJ Groundwater Ltd
• Siemens USC: Arup, Geothermal International Ltd, Siemens, ISG, Balfour Beatty Ground Engineering, Foundation Developments Limited