geotechnical design of heat exchanger piles

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


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


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)


• 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


(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


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� Power withdrawn from the soil: 500 kW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


� 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� Power withdrawn from the soil: 2700 MWh/yr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


� Lainzer tunnel, Vienna

� Piled retaining wall with piles

� Power withdrawn from the soil: 144 MWh/yr

Main realizations


• 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.





4. Conclusion


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


• 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


-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)


• 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


• 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:





4. Conclusion


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


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


The ultimate bearing capacity is reached when the plateau value qs of all the

modelled springs is reached.

ThermoPile – ultimate bearing capacity


- 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


Test 1:

Mechanical load P = 0 kN

(no Building)

DT = 21.8

Test 7:


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


EPFL CASE STUDY

Test 1:

Test 7:Test 1:


(no Building)

∆T = 21.8

Test 7:


Stiffness Pile – Structure = 2 GPa/m

∆T = 14.3°C



Cooling:

∆T = -19°C


Heating:

∆T = +9°C

P


LAMBETH COLLEGE CASE STUDY


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:


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:


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.





4. Conclusion


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/


Thank you for your attention.

geotechnical design of heat exchanger piles

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