laboratory scale electrical resistivity measurements to

1 Dipartimento di Scienze della Terra

Università degli Studi di Torino

2 Department of Monitoring and Exploration Technology

UFZ Leipzig

Laboratory scale electrical resistivity measurements to monitor the heat

propagation within porous media for low enthalpy geothermal applications

N. Giordano1, L. Firmbach2, C. Comina1, P. Dietrich2, G. Mandrone1, T. Vienken2

32 CONVEGNO NAZIONALE

19-21 Novembre 2013

TRIESTE

Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013

1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions

(Drake Landing Solar Community, Okotoks, Canada)

GROUND SOURCE

HEAT PUMPS (GSHPs)

The ground temperature from about 5-8 m to 100 m depth is roughly constant and it is equal to the average air temperature.

- Open loop (injection and extraction wells)

- Closed loop (borehole heat exchangers)

Heating and cooling (H&C) and domestic hot water (DHW) demand of the buildings.

The sun delivers plenty of energy which can be captured by solar thermal collectors and stored in kind of long-term accumulators.

The ground can host the heat by means of geothermal heat exchangers which are directly coupled with the solar panels boreholes thermal energy storage (BTES)

GROUND THERMAL

ENERGY STORAGE


Dealing with low enthalpy geothermal applications, the heat propagation within the porous media is therefore a fundamental concept to be aware of

MULTDISCIPLINARY

APPROACH


Design Monitoring activity

Laboratory tests

measurements of the soil’s thermal properties

analogical simulation

electrical resistivity surveys

Numerical simulation

modeling all the different configurations

predicting the heat distribution

evaluating the influence of the boundary conditions

Field tests

Monitoring the heat distribution both with direct (T-sensors) and indirect (geophysics) measurements

Evaluating the effective thermal properties and the efficiency of the system

Laboratory device

to describe the heat propagation within a porous medium

to highlight differences owing to water contents and positions of the heat source

to assess the potentiality of the electrical measurements for monitoring the heat distribution

to evaluate a multidisciplinary approach for thermally characterizing the ground



A plastic box, sized 1.0 x 0.4 x 0.4 m, was used to simulate the heat transfer within the selected porous media

- electrical resistance as heat source - 4 thermo-resistances Pt100 - 4 Watermark soil moisture sensors

Several tests were carried out and they differ

- time of heat up - static or dynamic hydraulic conditions - number, position, temperature and geometric configuration of the heat sources - position of the T-sensors - grain size distribution of the medium and its moisture content.

PURPOSES OF THE LAB TESTING


(1) parameters describing the bulk soil: porosity (n), water content (θ) and structure


Electrical resistivity (ρ)

(2) the time-invariable solid particle quantifiers: particle shape and orientation, particle-size distribution, cation exchange capacity

(3) fast-changing environmental factors: ionic strength, cation composition and temperature.

Archie’s law (1942)

ρw – resistivity of the fluid ρa – resistivity of the mixture F – formation factor n – porosity m – cementation index

ρs – resistivity of the solid phase θ – water content x – saturation index

TWO-PHASE SYSTEM THREE-PHASE SYSTEM

Under laboratory conditions some of the electrical resistivity-influencing soil parameters can be a-priori known (e.g. medium porosity and composition, water content) so that the temperature is the part which can be analyzed to understand the correlation with electrical resistivity.

A general relationship between the electrical (ρE) and the thermal resistivities (ρT) can be expected

CR is a multiplier dependent upon the gravel and sand size fraction of the soil (Singh et al., 2001)

depends upon

Electrical surveys on the lab device

Linear configuration with 16 electrodes in “Vertical Electric Sounding” (VES) mode.

Linear configuration with 24 electrodes in “Tomography” mode.

Network configuration with 24 electrodes.



1

3

2

STOP HEAT UP

NUMERICAL DATA

EXPERIMENTAL DATA



Porous medium Sand = 91% vol. Silt = 9% vol. Porosity = 0.46 Water content = 0, 25, 100 %

PROPERTIES Solid Air Water

Therm. conduct. (W/m*K) 5.0 0.024 0.58 Heat capacity (kJ/kg*K) 0.8 1.0 4.2 Density (t/m3) 3.0 10-3 1.0

(Kolditz et al., 2012)

The thermal diffusivity describes the velocity of the heat propagation

conductivity

specific heat capacity

density

Hadas (1974) takes into account the evaporation of the pore-filling water around the source

with

conductivity of the probe-soil interface

thickness of the interface



Supposing an influence radius of 0.15 m around the source, we tried to fit the experimental curves with different values of diffusivity.

1h

2h

3h

4h

HEAT SOURCE

HEAT SOURCE

HEAT SOURCE

HEAT SOURCE

EVAPORATION

EVAPORATION

15 cm Electrical tomography

from Archie’s law (1942)

900 Ω*m Sr = 25% +30%

1,200 Ω*m Sr = 20%



This means that in the portions closer to the source the evaporation influences the tests. The decrease in water content has therefore to be taken into account when processing the temperature data to evaluate the soil’s thermal properties

1

The increase in resistivity is progressive and it is on the average the 30% at 4h from the beginning

Water flux induced in the medium (about 10-3 l/s)

HEAT UP HEAT UP COOL DOWN COOL DOWN



2

The limited dimensions of the box do not allow to investigate the whole depth of the medium. Owing to this, a VES mode survey was adopted in order to reach the deepest portions and to perform measurements during the testing time.

VES mode survey



3 Network configuration



3

10% positive variation in temperature generates a 2.5% negative change in resistivity

HEAT UP

COOL DOWN

HEAT UP COOL DOWN

This kind of surveying is for sure appealing for a field application, where more electrodes could be used and more data could be acquired

Network configuration



ρE electrical resistivity ρT thermal resistivity

CR = 2.01

with

By recalling the previous cited linear equation between electrical and thermal resistivity (Singh et al., 2001) we calculated the CR value with the volumetric amount of sand and gravel of the medium (F) and other coefficients stated by Sreedeep et al. (2005) for a sandy medium

Porous medium Sand = 91% vol. Silt = 9% vol. Porosity = 0.46 Water content = 100 %



GRUGLIASCO TEST SITE



CONCLUSIONS shallow geothermal applications are increasingly applied in northern Italy; a reliable support for both design and monitoring is therefore fundamental for users and local governments

a multidisciplinary approach was performed at lab scale on a porous medium in order to check its reliability for defining the thermal properties and for monitoring the propagation of the thermal plume

focusing on geophysical results, the electrical resistivity seems to be a valid parameter for checking the temperature variation through a porous medium; a relation between electrical and thermal resistivity was also tested and good results came out

coupling direct and indirect surveys with numerical modeling could be a valid way of studying to be applied at field scale for improving the design of low enthalpy applications and to monitor the thermal plume

with a dense electrode network organised around the source, a valid 2D geophysical imaging of the heat distribution was obtained at lab scale; at field scale, with more electrodes and a better data coverage, a quasi-3D imaging could be performed and compared with the numerical simulation outcomes

geophysical surveys could be useful when the in situ thermal properties of a soil have to be evaluated for designing GSHPs and BTES systems; with an accurate data inversion the thermal properties can be estimated for all the involved ground if calibrated with direct lab measurements

Nicolò Giordano

Ph.D. Student

Dip. di Scienze della Terra

Via Valperga Caluso, 35 – 10125 TORINO

[email protected]

Thank you !

laboratory scale electrical resistivity measurements to

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