Laboratory scale electrical resistivity measurements to monitor the heat propagation within porous media for low enthalpy geothermal applications

Transcription

1 32 CONVEGNO NAZIONALE Novembre 2013 TRIESTE Laboratory scale electrical resistivity measurements to monitor the heat propagation within porous media for low enthalpy geothermal applications N. Giordano 1, L. Firmbach 2, C. Comina 1, P. Dietrich 2, G. Mandrone 1, T. Vienken 2 1 Dipartimento di Scienze della Terra Università degli Studi di Torino 2 Department of Monitoring and Exploration Technology UFZ Leipzig

2 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. GROUND THERMAL ENERGY STORAGE 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) (Drake Landing Solar Community, Okotoks, Canada)

3 Design Dealing with low enthalpy geothermal applications, the heat propagation within the porous media is therefore a fundamental concept to be aware of Monitoring activity Laboratory tests measurements of the soil s thermal properties analogical simulation electrical resistivity surveys MULTDISCIPLINARY APPROACH 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

4 Laboratory device 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 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

5 (1) parameters describing the bulk soil: porosity (n), water content (θ) and structure Electrical resistivity (ρ) depends upon (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) TWO-PHASE SYSTEM ρ w resistivity of the fluid ρ a resistivity of the mixture F formation factor n porosity m cementation index THREE-PHASE SYSTEM ρ s resistivity of the solid phase θ water content x saturation index 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 C R is a multiplier dependent upon the gravel and sand size fraction of the soil (Singh et al., 2001)

6 1 Linear configuration with 24 electrodes in Tomography mode. Electrical surveys on the lab device 3 Network configuration with 24 electrodes. Linear configuration with 16 electrodes in Vertical Electric Sounding (VES) mode. 2

7 EXPERIMENTAL DATA STOP HEAT UP NUMERICAL DATA Porous medium Sand = 91% vol. Silt = 9% vol. Porosity = 0.46 Water content = 0, 25, 100 % (Kolditz et al., 2012) PROPERTIES Solid Air Water Therm. conduct. (W/m*K) Heat capacity (kj/kg*k) Density (t/m 3 )

8 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.

9 1 1. Introduction 2. Materials and methods 3. Results 4. Field case study 5. Conclusions Electrical tomography The increase in resistivity is progressive and it is on the average the 30% at 4h from the beginning HEAT SOURCE 15 cm EVAPORATION 1h from Archie s law (1942) 900 Ω*m Sr = 25% +30% 1,200 Ω*m Sr = 20% HEAT SOURCE 2h 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 HEAT SOURCE HEAT SOURCE EVAPORATION 3h 4h

10 2 VES mode survey 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. Water flux induced in the medium (about 10-3 l/s) HEAT UP COOL DOWN HEAT UP COOL DOWN

11 3 Network configuration

12 3 Network configuration 10% positive variation in temperature generates a 2.5% negative change in resistivity This kind of surveying is for sure appealing for a field application, where more electrodes could be used and more data could be acquired HEAT UP HEAT UP COOL DOWN COOL DOWN

13 By recalling the previous cited linear equation between electrical and thermal resistivity (Singh et al., 2001) we calculated the C R 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 ρe electrical resistivity ρt thermal resistivity with Porous medium Sand = 91% vol. Silt = 9% vol. Porosity = 0.46 Water content = 100 % C R = 2.01

14 GRUGLIASCO TEST SITE

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16 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 Thank you! Dip. di Scienze della Terra Via Valperga Caluso, TORINO nicolo.giordano@unito.it