FIRST THERMAL RESPONSE TEST IN BULGARIA

Transcription

1 FIRST THERMAL RESPONSE TEST IN BULGARIA A. Georgiev, R. Popov, S. Tabakova Technical University of Sofia, Branch Plovdiv PO Box 7, 4023 Plovdiv, Bulgaria Tel: ABSTRACT The Thermal Response Test (TRT) is an effective method for an in-situ determination of the ground thermal properties. The objective of the present paper is to present the built mobile station for TRT determination as well as the first TRT that is realized in Bulgaria. The constructed in 2007 TRT rig combines both options: an investigation installation and lodging. A borehole with a depth of 41 m was constructed in November 2008 in the courtyard of the Technical University of Sofia, branch Plovdiv. The first TRT in Bulgaria was performed in January The aim of the team is to create a cadastre of the ground thermal properties of Bulgaria that will be used in future geothermal projects, based on Ground Source Heat Pumps (GSHP). 1. BACKGROUND Ground Source Heat Pump (GSHP) systems deliver three or four times more thermal energy (heat) than it is used in the electrical energy driving the system. The heat source part of the system, the ground heat exchanger, can be developed as an Aquifer Thermal Energy Storage (ATES) or as a Borehole Thermal Energy Storage (BTES). The hydro-geological restrictions are less for the BTES system, thus it has a bigger potential for application. The knowledge of the underground characteristics is very important to design an installation using BTES. The most effective method to determine the ground thermal characteristics in-situ is the Thermal Response Test (TRT). The TRT was first presented by Mogensen (1983) - his installation was designed as an immobile system. Later, a conductivity measurement mobile installation appeared in Sweden (Eklöf & Gehlin, 1996). A similar installation was created by Austin (1998) in USA. The theoretical bases of the Thermal Response Test are presented by Hellström (1991), Gehlin (1998) and Kavanaugh & Rafferty (1997). Some quality requirements are proposed by Zervantonakis & Reuss (2006) on the base of more than 60 thermal response tests. The TRT described in this work is the first one in Bulgaria, but the second test of the team leader, A. Georgiev. He took part in the realization of the first TRT performed in South America as a participant of research groups of Chile, Argentina and Bulgaria (Roth et al., 2004). The cooperative work of the team led to a charge/ discharge experiment with solar collectors on a shallow single borehole (Georgiev et al., 2006a). Some activities in Bulgaria preceded the first official TRT (Georgiev et al., 2006b). There is a progress in the installation and the borehole construction, as well as in the implementation compared with the TRT by Roth et al. (2004): - a mobile installation was constructed in Bulgaria (against a stationary one in Chile); - the borehole at the Technical University of Sofia, branch Plovdiv is 41m deep (against the 17m depth of the one in Chile) - it is no more a shallow borehole heat exchanger; - the data logging process was fully automated (against the semi automated one in 2003); - there is already an information of the borehole body - 5 sensors were mounted inside the borehole (all the mounted sensors in the borehole in Chile did not work because of bad quality).

2 Figure 1: Upper part of the borehole with the mobile installation 2. MOBILE INSTALLATION The mobile installation for the TRT was constructed in 2007 at the Technical University of Sofia, branch Plovdiv (Georgiev et al., 2009). The platform sizes are 4,25m х 1,96m x 2,2m and its loading capacity is about 890 kg. The laboratory has two parts combining two functions: an investigation installation and lodging (Figure 1). The first part (the bigger one) consists of a gas stove for heating, a gas cooker, refrigerator, sink, table and 2 beds (it is foreseen that the investigators will live and work during the TRTs). The second one contains the working installation - the mechanical, measuring and control parts of the system (Figure 2). Figure 3 shows the installation set-up. The following parts are presented: electrical boiler 1, calorimeter 2, pressure watch 3, expansion tank 4, thermo manometer 5, filter 6, circulation pump 7, deaeration pipe 8, quick couplings 9, valves 10 and electrical unit 11. Figure 2: Photo of the measuring device

3 Mechanical part of the system We present here some of the unit elements. Two tubular electrical heaters are situated in the well insulated steel body of the boiler. Two thermostats are available: a boiler thermostat which regulates the water temperature in the boiler body and a damage thermostat avoiding the boiler overheating. A pump with a wet rotor (three phase circulation pump with 3 speeds) of the German company WILO is chosen (corresponding to a borehole depth till 120m). This pump gives an opportunity for flow rate regulation based on frequency (thus a very precise flow rate and pressure regulation is reached). Figure 3: Installation set-up The calorimeter МEGATRON2 (SIEMENS production) is installed in the unit. It has a memory and a display, which reports the consummated and measured energy values as functions in time. The pressure and temperature of the heating medium, after leaving the electrical boiler and before entering the borehole, are measured with a thermo-manometer. The pump and the installation boiler are protected from high pressure or from overheating of the fluid by means of a pressure watch. It is a WATTS production, Italy - PM/5 model and acts with two positions. An expansion tank of membrane type with capacity of 8 liters is used to balance the small variations of the heating medium in the system. It is produced by the company ELBI, Italy. A filter of net type is used to eliminate all rigid parts entering the unit. It allows a passage only in the working fluid flow direction. The pipeline used in the installation (production of the German company AQUATHERM ) is characterized with absolute corrosion stability, chemical stability, high impact stability, low tube roughness, very good welding characteristics and high thermal stability. AEROFLEX insulation is used in the mobile system - the tube insulation is 1.5 mm and the insulations of the input and output of the borehole are 3 mm. Measuring and control system A fully automated system for data archiving is used, that is produced by the company SIGMATECH EOOD, Plovdiv, Bulgaria. The multiprocessor system SН700 automates the

4 measurements and processes the obtained data (twenty temperature points are measured). The controller has a memory, where the measuring process is recorded (the process is shown in real time and represented on a file, ready to be used in computer applications after process end). An electrical generator of the company SUBARU, model EH36B is chosen (sometimes the tests are done on places without electricity; it is used to deliver electricity in urgent cases, too). The generator is programmed to switch on automatically when needed. Some sensors and measuring elements are mounted in the mobile station: - the flow rate through the borehole is measured by a calorimeter sensor and the electronic unit controls the pump performance; - inductive sensor with a code disc for motor revolution measurement of the electrical generator; - sensor for temperature measurement of the generator motor oil; - controller for parameter measurement and control of the system motor generator; - measuring and archiving system of the installation parameter values SIGMATECH production; - computer analyzing the collected information (supplied with processing software); - temperature elements for PT100 (8 pieces), which have a sensibility of about +/-0,05 C in the temperature range 0 С-120 С; they are used to measure the inlet and outlet fluid borehole temperature (the sensors situated on the input and output of the borehole are orientated under an angle of 45 in direction of the passing fluid - as shown by Zervantonakis & Reuss, 2006), the ambient air temperature and the temperature in depth of the borehole. 3. BOREHOLE EXCHANGER Figure 4: Ground sorts 3m to 42m under the surface. The building III garden of the Technical University of Sofia, branch Plovdiv was chosen as a test place. One perforation with a diameter of 180 mm was done to a depth of 42 m in November The soil at the site consists of some different layers, which are shown on Figure 4 (the soil content for every 1 meter layer thickness is presented from the 3rd to the 42nd m under the

5 surface). The lithological variety, the density and the hydro-geological characteristic of every layer are presented in Table 1. A U-pipe of high density polyethylene (HDPE) with an external diameter of 25 mm was installed in the perforation. It consists of two pipes, which are connected with two knees in the lower part. A mixture consisting of 11% bentonite and 2% cement was used to grout the borehole. The cement is used to harden the mixture because of the considerable quantity of underground water in Plovdiv. Five temperature elements of the type PT100 were mounted at depths of 41, 31, 21, 11 and 0.30 m. The sensors must be used to measure the borehole temperature at the mentioned depth under the surface. In order to reduce the further ambient influence on the system, a surface area of about 4 m² on top of the store was insulated with a layer of 0.1 m of high density polystyrene covered with aluminum folio. The upper part of the U-tube is about 50 cm over the insulation (Figure 1). Table 1: Geological section Thickness, m Depth, m Lithological variety Density, t/m³ Hydro-geological characteristic Medial grainy sand 1,2 aquifer Clay 2 moist Medial grainy sand 1,2 aquifer Clay 2 moist Medial grainy sand 1,4 aquifer Clay 2,1 moist Coarse grainy sand with gravel 1,2 aquifer Clay 2,1 moist Coarse grainy sand with gravel 1,1 aquifer Clay 2,1 moist Medial grainy sand 1,4 aquifer Clay 2,2 moist Coarse grainy sand with some gravel 1,1 aquifer Clay 2,2 moist Coarse grainy sand with gravel 1,1 aquifer Clay 2,1 moist 4. IMPLEMENTATION OF THE THEMAL RESPONSE TEST The test was implemented in January, 2009 with a duration of 10 days, namely from 11 to 21 January Both tubes (containing the sensors foreseen to measure the inlet and outlet borehole temperatures) were connected to the upper part of the borehole. They were additionally insulated to avoid the influence of rain, solar radiation and wind. The measuring equipment was calibrated prior to the test. The test started with the determination of the undisturbed ground temperature s T. This temperature was calculated as the average of the measured outlet fluid borehole temperature values. The recording interval was fixed to 10 s, after the experience of Zervantonakis & Reuss (2006). The following parameters were measured: the inlet and outlet fluid temperatures of the

6 borehole, the ambient air temperature and five temperatures in the borehole body. The measurement interval was chosen to be 60 s. The electrical power (about 1500 W) was measured and regulated to be constant during the whole test. The flow rate of water was 4,06 l/min; it was controlled and measured during the full process period. The electrical pump worked at the mentioned conditions with a power of about 100 W. 5. TREATMENT OF THE EXPERIMENTAL DATA There are some methods to evaluate the experimental data and to calculate the unknown thermal conductivity λ and borehole thermal resistance R b. The Line Source Model (LSM) is the widely used and the most simple method. The delivered heat is considered as coming from an infinite line source (the borehole) - Eklöf & Gehlin (1996). The following equation represents the heating process: Q Q 1 4a T + + f,m = ln( t ) + ln γ R b T s 4πλH H 4πλ 2 rb where T = (T T ) / 2 - medium fluid temperature, K; f,m f,i + f, o Q delivered heat power, W; λ - thermal conductivity, W/mK; H borehole depth, m; t time from start, s; a - thermal diffusivity, m 2 /s; rb - radius of the borehole, m; γ = 0,5772 Euler s constant; R b borehole thermal resistance, mk/w; for 5r t (1) a b 2 T s initial soil temperature, K. Let us simplify the equation (1) by T, m where c and d are constants. = c.ln( t ) d (2) f + Figure 5: Temperature response during the TRT The value of c is determined as the inclination of the line in the plot of the medium fluid

7 temperature versus ln of time. Then, the conductivity λ is computed from the graph slope: Q λ = (3) 4πcH We can calculate the borehole thermal resistance R b after rearranging equation (1). Both temperatures - the medium fluid temperature and the ambient temperature are plotted in Figure 5 during all 240 hours of the test. After that the thermal conductivity is determined by means of the Line Source Model (1) (3). From (3) it is seen that the thermal conductivity is related to the slope of the resulting line in a logarithmic time plot of the mean fluid temperature in the borehole. Figure 6 presents the measured data curve and a sloped line (the regression line). Usually, some of the first test hours have to be cut (the approximate solution used in deriving the method approaches the exact solution for large times) - we ruled out the first 10 hours. The results of the TRT are as follows: regression coefficient is R²=0,9694, thermal conductivity is λ = 0,86 W/mK and borehole thermal resistance is R b = 0,54 mk/w. Figure 6: Regression line of the test based on the experimental data (excluding the first 10 h) 6. CONCLUSIONS The presented quality requirements of a Thermal Response Test by Zervantonakis & Reuss (2006) were taken into account during the first TRT in Bulgaria. The following conclusions and recommendations can be done on the base of the described above TRT: - the information about the temperatures in the borehole body is a good precondition for future simulations; - the relative low value of the thermal conductivity (0,86 W/mK) is due to the ground structure - mainly sand and clay; - the relative high value of the borehole thermal resistance (0,54 mk/w) is caused by the material grouted in the perforation and forming the borehole, the bentonite solution; - the material for borehole grouting has to be changed with a material possessing higher thermal conductivity;

8 - the Line Source Model can be used as an effective and fast method for treatment of experimental results; - the implementation accuracy of the Thermal Response Test will bring to better evaluation of the ground thermal parameters; - some evaluations with the hours-off effect were done - 7, 10, 15, 20 and 24 h (the thermal conductivity varies between 0,84 and 0,86 W/mK) - that means that the elimination of only 10 hours brings to good results; - the curve of the mean fluid borehole temperature is very fluent - it is not influenced by the ambient temperature (because of the relative big depth of 41 m and the precise implemented TRT); - the research team of the Technical University of Sofia, branch Plovdiv believes to have the leading position when a cadastre of the ground thermal properties is created in Bulgaria. ACKNOWLEDGMENTS This work has been financially supported by a research project No BY-TH-212/ 2006 of the National Science Fund, Ministry of Education and Science, Bulgaria, which is gratefully acknowledged by the authors. REFERENCES Mogensen P. (1983). Fluid to Duct Wall Heat Transfer in Duct System Heat Storages. Proceedings Int. Conference on Subsurface Heat Storage in Theory and Practice, Stockholm, Sweden, June 6-8, 1983, p Eklöf, C. & S. Gehlin, S. (1996). TED A Mobile Equipment for Thermal Response Test. Master s Thesis 1996:198E. Luleå University of Technology, Sweden. Austin, W. A. (1998). Development of an In-Situ System for Measuring Ground Thermal Properties. Master s thesis. Oklahoma State University. Stillwater, Oklahoma. Hellström, G. (1991). Ground Heat Storage. Thermal Analysis of Duct Storage Systems. Part I Theory. University of Lund, Department of Mathematical Physics. Lund, Sweden. p Gehlin, S. (1998). Thermal Response Test - In-Situ Measurements of Thermal Properties in Hard Rock. Licentiate Thesis, Luleå University of Technology, Department of Environmental Engineering, Division of Water Resources Engineering. 1998:37. p Kavanaugh, S. P. & Rafferty, K. (1997). Ground-Source Heat Pumps: Design of Geothermal Systems for Commercial and Institutional Buildings. ASHRAE, Inc., Atlanta, p Zervantonakis, I., & Reuss, M. (2006). Quality requirements of a Thermal Response Test. Proc. ECOSTOCK 06, 10th Int. Conference on Thermal Energy Storage, Stockton, USA, May 31 - June 2, Roth, P., Georgiev, A., Busso, A., Barraza, E. (2004). First In-situ Determination of Ground and Borehole Thermal Properties in Latin America. "Renewable Energy", V. 29 (12) p Georgiev, A., Busso, A., Roth, P. (2006). Shallow Borehole Heat Exchanger: Response test and Charging - Discharging test with solar collectors. "Renewable Energy", V. 31 (7) p Georgiev, A., Pekov, O., Angelov, A., Popov, R., Urchueguía, J., Witte, H. (2006). First steps of ground accumulation in Bulgaria. Proc. of the World Renewable Energy Congress-IX, Italy, Florence, August Georgiev, A., Tabakova, S., Popov, R., Todorov, Y. (2009). Bulgarian Variant of a Mobile Installation for Ground Thermal Properties Determination. Acc. for publication in the journal "ISESCO Science and Technology Vision".