A NEW METHOD TO DETERMINE in situ GROUND THERMAL CONDUCTIVITY IN BOREHOLE HEAT EXCHANGERS

1. A NEW METHOD TO DETERMINE in situ GROUND THERMAL CONDUCTIVITY IN BOREHOLE HEAT EXCHANGERS Ernst Rohner1), Ladislaus Rybach1,2) , Ulrich Schärli3), Thomas Kohl1) 1) GEOWATT AG Zürich, 2) Inst. für Geophysik ETH Zürich, 3) Dr. U. Schärli – Geologie + Geophysik, Zürich • Purpose • Probe construction • Field and laboratory measurements • Data processing, applicational example • Conclusions, outlook

2. Goal and purpose • Ground thermal conductivity l is a decisive property for dimensioning of borehole heat exchangers (BHE): the specific heat extraction rate (W per m BHE length) is directly proportional to l. • Important for the design of BHE groups: optimization I.e. determining BHE number and depth must be done immediately after the availability of l information. • Although l can be determined on rock samples from the borehole in the laboratory or in situ by a customary “Response Test” (a BHE circulation experiment), both methods need special equipment and are time-consuming. Therefore we developed a quick and simple method.

3. Rock thermal conductivity and its effect on BHE performance

4. Wireless Probe • We developed and built a small, light, and wireless probe with pressure and temperature sensors. The probe contains also a programmable microprocessor in a closed metal case, water-tight up to 100 bar. • The wireless probe is lowered into one tube of the U-shaped BHE where it sinks under its adjustable weight (=velocity) and records pressure (=depth) and temperature while going down, at pre-selected time (=depth) intervals. • After the probe has reached the U-tube bottom it stops there and is flushed back to the surface by a small pump for recovery and data retrieval. A measurement run in a 300m deep BHE takes less than 60 minutes.

5. Electronics: P and T -Sensors Probe case 235 x 23 mm weight 99.8 g Wireless Probe (“Fish”)

6. Wireless Probe (“Fish”) • Block diagram • of probe

7. The built-in electronics (ADC with 16bit resolution) consists of Analog/Digital Converter, microprocessor EEPROMs (=electric erasable/ programmable read-only memory) for data storage. The software consists of three parts to perform the following tasks: controlling the probe operation communication probe  laptop (for data retrieval) calibration. Wireless Probe (“Fish”)

8. The controlling part runs in a programmable microprocessor (Basic Stamp BS2pe), performs the measurements and stores the data (time, pressure, temperature). The communications part consists of a macro in an Excel Workbook, reads the data into an Excel table and enables the configuration of the wireless probe, to set the measurement mode as well as the setting of time, pressure and temperature intervals. Data deletion, synchronization of probe, and laptop timing are also accomplished. Also the calibration part consists of a macro in an Excel Workbook; it enables to calibrate individual probes. The calibration parameters are stored in the probe. Thus the measurements can immediately be started, after configuration, with any of the probes built. Wireless Probe (“Fish”)

9. Laboratory tests und calibration • Function checks, after testing for water-tightness, have been performed under laboratory conditions. • The temperature calibration in a thermostat vessel, the pressure calibration by hydraulic means. • The probe has high temperature resolution (± 0.003 C) and can store three times 16’000 measurements (time, pressure, temperature). • The time constant of temperature measurement is 3.5 sec. • Depth resolution is, due to the time constant, about ± 0.5 m. • The calibration functions for pressure and temperature are described by second grade polynoms of the form y= a + bx + cx2. The calibrations constants a, b and c of each individual probe are stored in the probes.

10. Temperature calibration (range: 0 – 50 °C)

11. Field operation with probe catcher and pump

12. Field measurement reults: • Drillhole profile • Temperature log • Gradient logs • (Raw data with Dz=1.1m, • smoothed with Dz=13m) • BHE drillhole in Bülach • near Zurich, Switzerland

13. Field measurements

14. Calculation of the thermal conductivity profile • Assumption: pure conduction (disturbing effects like palaeclimate, groundwater flow to be corrected for beforehand) • From the temperature log T(z) the gradient log T(z) is calculated depth sectionwise (i); • Finally the thermal conductivity profile, with the local heat flow qloc from

15. Calculated thermal conductivity profile (full line), Laboratory measurements (vertical bars) BHE drillhole in Bülach/CH m

16. Petrophysical data, samples from BHE drillhole in Bülach/CH lm: Thermal conductivity of rock matrix, rm: Matrix density, Feff: effectiveporosity, lf: Thermal conductivityof water-saturated sample, ±D: measurement error

17. CONCLUSIONS • Since probe manufacturing in 2003, several BHE boreholes have been logged in Switzerland. The results are fully satisfactory; the method is now routinely used to design large BHE arrays; • The data processing yields the detailed l distribution over the borehole profile; • The wireless measurement offers several advantages: higher resolution, better signal/noise ratio than with logging cables; • The measurement time for a 300m deep BHE is less than 60 minutes, whereas a Response Test takes at least 50 hours and requires large-scale equipment; • The time saving is highly relevant under construction site conditions.

18. OUTLOOK • The probe can be used, besides for thermal conductivity determination: • Lithologic subdivision of borehole profiles, • Data acquisition for palaeoclimatic studies, • Identificationofgroundwater flow.