laboratory scale electrical resistivity measurements to
TRANSCRIPT
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
Dealing with low enthalpy geothermal applications, the heat propagation within the porous media is therefore a fundamental concept to be aware of
MULTDISCIPLINARY
APPROACH
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
(1) parameters describing the bulk soil: porosity (n), water content (θ) and structure
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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.
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
1
3
2
STOP HEAT UP
NUMERICAL DATA
EXPERIMENTAL DATA
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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%
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
3 Network configuration
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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 %
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
GRUGLIASCO TEST SITE
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
Dip. Scienze della Terra TORINO Nicolò Giordano GNGTS 2013
1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions
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
Thank you !