geotechnical design of heat exchanger piles
TRANSCRIPT
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Geotechnical Design of Heat Exchanger Piles
Hervé PERON (speaker)Lyesse LALOUISwiss Federal Institute of Technology Lausanne
With the contribution of:Christoph KNELLWOLFMathieu NUTHClaire SILVANI
GSHP Association Research Seminar Current and Future Research Into Ground Source EnergyJanuary 21st 2010, National Energy Centre, Milton Keynes, United Kingdom
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
1. The energy pile technology today
2. Some features of the thermo-mechanical behaviour of the system energy pile / surrounding ground
3. Geotechnical design tool for heat exchanger piles, validation and optimal design
4. Conclusion
Outline
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Principle of Heat Exchanger Piles
- The heat exchange is made via the foundations of the building
-The energy transfer is made via a fluid circulating in pipes cast in the piles
- Optimal efficiency: heat extracted in winter re-injected in summer
Return of the heat
transfer fluid
Inflow of the heat
transfer fluid
Bored pile
Reinforcing
cageHeat exchanger
tube
Heat pump
Heat exchanger pile
Heat distribution
(Source: Lippuner & Partner AG, Grabs)
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
• Cost efficiency and reliability:
Local source of energy: almost no energy transportation, secure and rational supplying.
Minor modifications in the foundation, no additional structure is required (compared to classical geothermal structures):
Minor additional energy supply (electricity for the heat pump).
• Environmentally friendly technology:
Example: Zürich airport terminal E, 75 % of the used energy for heating and cooling comes directly from the energy piles.
Most of the energy is simply the heat naturally present in the ground
Advantages of the system
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
(Source: Lippuner & Partner AG, Grabs)
Constructions using thermal piles (about 40 in Switzerland):
• Finkernweg (75 thermal piles)
• Lidwil (120)
• Pago (570)
• Photocolor at Kreuzlinger (93)
• Etc.
Main realizations
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Ma
in T
ow
er
- F
ran
kfu
rt
� Main Tower – Frankfurt am Main
� 112 energy piles (1.5 m in diameter and 30
m in length) + diaphragm wall composed by
110 heat piles
� Soil affected volume: 150’000 m3
� Power withdrawn from the soil: 500 kW
Main realizations
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
� Zurich Airport – Dock Midfield terminal
� 300 energy piles (90 and 150 cm in diameter and 30 m in length)
� Soil affected volume: 200’000 m3
� Power withdrawn from the soil: 2700 MWh/yr
Main realizations
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
� Lainzer tunnel, Vienna
� Piled retaining wall with piles
� Power withdrawn from the soil: 144 MWh/yr
Main realizations
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
• The design today: adapted from geothermal probe design
From the building energy demand and soil/concrete thermal (and hydraulics) properties, design of the heat pump + piles.
Lack of understanding…
• No adapted geotechnical design of the energy piles!
• Safety factors are twice usual (without heat exchange) systems.
•Typical behaviour: increase the number of piles, width and length, which increases the cost.
NEED FOR UNDERSTANDING of heat exchanger pile / soil systemmechanical behaviour.
Challenge in geotechnical engineering
Source: www.geotherm
ie.ch
NEED FOR DESIGN TOOL based on a sound knowledge of the processes.
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
1. The energy pile technology today
2. Some features of the thermo-mechanical behaviour of the system energy pile / surrounding ground
3. Geotechnical design tool for heat exchanger piles, validation and optimal design
4. Conclusion
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
p’c(T)
Main aspects:
Change in friction angle (soil strength)
Thermo-elasticity
New processes: irreversible contraction upon heating � THERMOPLASTICITY
Constitutive model ACMEG-T developed at EPFL-LMS
Modelling: influence of temperature on mechanical behaviour
TEMPERATURE induces COMPLEX NON-LINEAR PROCESSES relating to the SOIL MECHANICAL BEHAVIOUR.
1 0
5
0
1 5
3 0
2 5
2 0
T e m p er a tu re
Fri
ctio
n a
ngle
at
crit
ical
sta
te (
°)
Strength
Stiffness
MC ClayAxial strain 0.1%
0
20
40
10
30
50
Temperature
Seca
nt
mod
ulu
s (M
Pa)
OCR=1
OCR=2.2
OCR=4
OCR=8
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
• Numerical tool: Finite Element code
Coupled formulation: displacements, pore water pressure and temperature are the field variables.Requires boundary and initial conditions
Modelling: application to soil-pile system
Single pile
Group of piles
11m
ACMEG-TS
3-phase THM field equations
T HWater and Gas flow
Heat conduction
Effective stressSuction change
Desiccation cracks
Porosity
Heat convectionThermal conductivity
Specific heat
Fluid densityviscosity
PorosityThermal strainDesiccation
cracksM
stress strain framework
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
-60
-50
-40
-30
-20
-10
0
-5 -4 -3 -2 -1 0
∆T=0°C (Initial state)
∆T=21°C (12 days)
∆T=3°C (28 days)
Dep
th (
m)
Vertical effective stress (MPa)
Heating
Cooling
Modelling: soil - single pile system
Et. 0
Et. 1
Et. 2
Et. 3
Et. 4
TEST 0 TEST 1
TEST 2
TEST 3
TEST 4
TEST 5
TEST 6 TEST 7 (TEST 8)
Mec
han
ical
lo
adin
g
Time
TEST 1 TEST 2TEST 3TEST 4 TEST 5TEST 6TEST 7(TEST 8)
temps
∆∆ ∆∆T
[°
C]
LAYER A1
LAYER A2
LAYER B
LAYER C
LAYER D
PILE
Heat induced deformations (x1425.15)
0
1
2
3
4
5
0 4 8 12 16 20 24 28 32
SimulationLevellingExtensometersOptical fibers
Ve
rtic
al h
ead
dis
pla
cem
en
t (m
m)
Time (days)
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
• Influence of the range of temperatures variations
Plastic zones
∆T = 60°C
Under slab center
z = -1m
• Influence of cycles on behaviour of energetic geostructure ∆T=60 (°C)
∆T(°C)
0
3 months
6
months1.5 months
1 year 2 years
Modelling: soil – group of piles system
11m
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
• Increase of the overall load within the pile upon heating
• Decrease of the overall load within the pile upon cooling
• Additional pile displacements
• Modification of the shaft friction mobilization along due to pile uplift or settlement
• Complex non-linear processes relating to the SOIL mechanical behaviour (thermo-hydro-mechanical couplings): cyclic thermo-plasticity. Assessment of the temperature range to avoid irreversible settlements
Conclusions
• “Exceptional situations”, e.g. heat pile failure, “non-conventional”loadings. Assessment of differential settlements and the excess of stresses that may lead to injuries to the structure.
New design tool: ThermoPile
Outcome of the THM simulations:
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
1. The energy pile technology today
2. Some features of the thermo-mechanical behaviour of the system energy pile / surrounding ground
3. Geotechnical design tool for heat exchanger piles, validation and optimal design
4. Conclusion
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Finite difference scheme
Pile rigidity
Base rigidity
Head rigidity
Building
(Bedrock)
Shaft - Soil
rigidity
Pile shaft – soil rigidity(after Frank and Zhao 1982)
Pile base – soil rigidity(after Frank & Zhao 1982)
ThermoPile: main assumptions
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Step 1: Displacement / stress calculation under mechanical loading
(corresponds to a conventional settlement calculation after Coyle and Reese)
Step 2: Displacement / stress under thermal loading (heating / cooling)
2a
2b1
ThermoPile – mechanical and thermal loading
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
The ultimate bearing capacity is reached when the plateau value qs of all the
modelled springs is reached.
ThermoPile – ultimate bearing capacity
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
- Standard calculation of
bearing capacity and settlement
- Determination of the additional stresses and strains
in pile due to thermal loading
- Modelling of multi layer soils
Includes:
- 2 theories to asses the
bearing capacity (DTU, Lang and Huder)
- 2 Load transfer curves
- Possibility of manual entry
ThermoPile – other features
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Test 1:
Mechanical load P = 0 kN
(no Building)
DT = 21.8
Test 7:
Mechanical load P = 1000 kN
Stiffness Pile – Structure = 2 GPa/mDT = 14.3°C
Et. 0
Et. 1
Et. 2
Et. 3
Et. 4
TEST 0 TEST 1
TEST 2
TEST 3
TEST 4
TEST 5
TEST 6 TEST 7 (TEST 8)
Mec
han
ical
load
ing
Time
TEST 1 TEST 2TEST 3TEST 4 TEST 5TEST 6TEST 7(TEST 8)
temps
∆∆ ∆∆T
[°
C]
ThermoPile – validation
EPFL CASE STUDY
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
EPFL CASE STUDY
Test 1:
Test 7:Test 1:
Mechanical load P = 0 kN
(no Building)
∆T = 21.8
Test 7:
Mechanical load P = 1000 kN
Stiffness Pile – Structure = 2 GPa/m
∆T = 14.3°C
ThermoPile – validation
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Cooling:
∆T = -19°C
Mechanical load P = 1200 kN
Heating:
∆T = +9°C
P
ThermoPile – validation
LAMBETH COLLEGE CASE STUDY
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Pile Length: L = 10 m
Pile Diameter: D = 0.5 m
Thermal loading: ∆∆∆∆T = +15 ºCUltimate shear stress capacity: qs = 50 kPa
Ultimate bearing capacity at pile base: qb = 0 kPa
Menard Modulus: EM = 20 MPa
Bearing capacity can be reached
Heating
ThermoPile – Design of a floating pile
Main aspect to consider during the design:
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Pile Length: L = 10 m
Pile Diameter: D = 0.5 m
Thermal loading: ∆∆∆∆T = -15°CUltimate shear stress capacity: qs = 50 kPa
Ultimate bearing capacity at pile base: qb = 0 kPa
Menard Modulus: EM = 20 MPa
Tensile forces
Cooling
ThermoPile – Design of a floating pile
Main aspect to consider during the design:
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Optimization of hat exchanger pile systems
With the developed tool, we know the evolution of the efforts and the mobilized friction in the pile for different temperatures.
Proposed methodology:
- The pile system can be first designed with respect to the energy demand only.
- This leads to a given number of piles with a certain length and some temperature profiles along the piles.
- The safety of the pile can then be verified: in case of failure, the geotechnical design of the system can be re-assessed using the new tool.
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
1. The energy pile technology today
2. Some features of the thermo-mechanical behaviour of the system energy pile / surrounding ground
3. Geotechnical design tool for heat exchanger piles, validation and optimal design
4. Conclusion
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
ACHIEVEMENTS OF THE PROJECT
Higher and larger buildings are now equipped with heat exchanger piles. But there is currently no method for the geotechnical design taking into account the effects of temperature cycles applied to the pile/soil system.
The numerical tool ThermoPile is an alternative to cumbersome FE modelling and expensive in situ tests. It can be considered as a handy and practice-oriented tool, offering a standard calculation method, which provides with useful results in a short time.
ThermoPile paves the way for an optimized geotechnical design of heat exchanger piles, therefore to reduce the safety factors applied (larger than the values usually employed for classical piles).
The software will be available from May 2010Information on http://lms.epfl.ch/
GSHP meeting – Jan. 21 2010 – Milton Keynes, UKHeat exchanger piles – Péron H, Laloui L
Thank you for your attention.