- Email: [email protected]
Measurement of thermal conditions in rock massif
9th IFAC Workshop on Programmable Devices and Embedded Systems Roznov pod Radhostem, Czech Republic, February 10-12, 2009
Measurement of thermal conditions in rock massif Jiri Koziorek*. Bohumil Horak*. Radovan Hajovsky*. Petr Bujok**.
*VSB-Technical University of Ostrava, Faculty of Electrical Engineering and Computer Science, Czech Republic (e-mail: jiri.koziorek@ vsb.cz, bohumil.horak@ vsb.cz, radovan.hajovsky@ vsb.cz ). **VSB-Technical University of Ostrava, Faculty of Mining and Geology, Czech Republic (e-mail:petr.bujok@ vsb.cz). Abstract: This paper brings information about research activities of temperature measurement in boreholes. The described research is focused on boreholes used for heat pumps applications. The first part of the contribution deals with long time measurement of the temperatures in boreholes used in heat pump installation. A system for monitoring and evaluation of temperatures in measurement polygon is presented. The second part is focused on measurement of thermal profile of the borehole. The paper GRHVQ¶W HYDOXDWH JHRWKHUPDO SURSHUWLHV RI WKH ERUHKROHV EXW LW GHVFULEHV WKH PHDVXUHPHQW LVVXHV DQG methods. Keywords: Thermal profile, Heat pump, Borehole, Temperature measurement, Temperature monitoring, Temperature sensor.
1. INTRODUCTION In 2006, the construction of a new conference building at VSB - Technical University of Ostrava was finished. The building has installed a unique system of heating and cooling. The system is based on application of heat pump. The installed heat pump system is the biggest in Czech Republic. The system contains 6 heat pumps with overall power 700kW. The heat pumps use 110 boreholes in surrounding of the building. The depth of boreholes is 140m. The system is used for heating during winter period and for cooling during summer period.
The heat pumps are able to use low-temperature energy sources with temperatures between 0-&6XFKJHRWKHUPDO energy is ecologically clean and practically inexhaustible. The heat pumps in described case use boreholes in which is installed a closed system of PE pipes for heat transfer. The heat pump raises a thermal imbalance in the surrounding of the boreholes, so that there is continuous heat transfer between the rock massif and the boreholes. During winter period, the energy is obtained from the rock massif so the massif is permanently cooled. It depends on properties of the massif as well as on the heating-transfer surface in borehole, if surrounding temperature of the borehole decreases under zero (the borehole is frozen). A temperature of the rock massif is influenced by solar energy, geothermal energy and decay of the minerals. The solar radiation is usually mentioned as main influence in depths where are boreholes for heat pumps used.
Fig. 1. New conference building at VSB-Technical University of Ostrava, Czech Republic.
978-3-902661-41-8/09/$20.00 © 2009 IFAC
During the design and installation of heat pumps at VSBTUO, the research activities focused on thermal behavior of rock massif were also considered. The system of heat pumps has as mentioned 110 boreholes, 140m deep. A concrete part RI WKHVH ERUHKROHV ZDV GHFODUHG DV ³0HDVXUHPHQW SRO\JRQ´ and it was equipped by of the temperature measurement system. The temperature measurement allows monitoring of influences of the heat pump operation on rock massif, influence of solar radiation and seasons. The measurement polygon allows as well a testing of different types of temperature sensors in such applications, a testing of sensors protection and evaluation.
226
10.3182/20090210-3-CZ-4002.0042
2. MEASUREMENT POLYGON The measurement polygon contains two types of the boreholes: -
-
Technological boreholes ± these boreholes used by heat pump for its operation. There are 10 technological boreholes. Each is equipped by pair of PE-collectors with 32mm diameter. There is also a set of 4 temperature sensors at input pipes (colder side) in depths 20, 50, 100 and 140m and a set of 2 temperature sensors at output pipes (warmer side) in depths 20 and 100m.
The schematic drawing of measurement polygon is displayed at figure 3.
3. TEMPERATURE SENSORS An analysis of temperature sensors preceded their installation in the measurement polygon.
Special boreholes - these boreholes used for measurement only. There are 5 special boreholes. Each is equipped by PE-collector but the collector is not used for any technological purpose. There is installed a set of 4 temperature sensors in depths 20, 50, 100 and 140m.
0m -10m -20m -30m -40m
Fig. 3. Copy from the project of boreholes situation with highlighted positions of the technological (circles) and special (triangles) boreholes. The signals from all thermometers are linked to an underground concentrator where remote I/O system is installed. The concentrator is also visible in the figure.
-50m -60m -70m -80m
Temperature sensors can be divided into two groups - selfgenerating and modulating sensors. The self-generating VHQVRUV GRQ¶W UHTXLUH DQ\ VRXUFH RI SRZHU Temperature sensors based on thermocouples belong to this category. The temperature sensors based on the thermal modulation of electric currents or voltages supplied by auxiliary energy sources are considered to be modulating sensors. Resistors, diodes and transistors belong to this group.
-90m -100m -110m -120m
The following temperature sensors were taken into account: -130m
-
Metal resistance sensors ± all metallic conductors change their resistivity with the temperature. The change of resistance is quite linear over a large range of temperature; deviation from a straight line is more significant at higher temperatures. Materials usually used for such sensors are Platinium and Nickel. They are use usually in range -WR&
-
Semiconductor resistance sensors ± thermally sensitive resistor that is made from semiconducting ferromagnetic ceramic materials. They have good temperature sensitivity, small dimensions, easy connection but they have non-linear characteristics. They are use usually in range -WR&
-140m
Fig. 2. Technological and special borehole. The aim of such system of boreholes is obtaining relevant data for modeling and evaluation of thermal processes during a heat pump operation. Another aim is testing and design of temperature sensors for longtime measurement in boreholes. The sensors taken into the consideration are based on electrical and optical basis.
227
-
-
Monolytic p-n temperature sensors ± they are based on temperature dependence of p-n junction. They have temperature range from -WR&7KH\DUH XVXDOO\ integrated with circuit for signal processing.
4.2 Monitoring application The monitoring application is created in SCADA system Promotic. The application has following features:
Thermoelectric sensors, thermocouples ± they are based on Seebeck effect. Thermocouples are always composed of two conductors or semiconductors and they are able to generate a voltage. They are used for measurement of temperature differences. The materials for thermocouples are standardized. They can be used for temperatures between ± 180 to &
The parameters, availability, accuracy and ways of signal processing of these temperature sensors were compared and metal resistance sensor PT1000/3850, class A in 4-wire connection were selected. They had the best accuracy & QHDU & VWDQGDUGL]HG SURFHVVLQJ RI PHDVXUHG signal, and a compensation of length of connection, good availability and price.
-
Displaying actual temperatures.
values
-
Displaying historical trends or tables of selected temperatures.
-
Storing of data into database.
-
Displaying values of temperatures in selected level of depth.
-
Scheduling and setting of measurement.
-
Remote access to the application.
The other methods of the temperature measurement, as crystal thermometers or optical fibers temperature measurement were not considered at this stage because they require more sophisticated signal processing.
4. MONITORING SYSTEM The monitoring system measures temperatures of 10 technological boreholes and 5 special boreholes, displays actual values and trends, stores all data to archives and provides remote access to data. 4.1 Mesurement The temperatures are measured by I/O system ICP DAS. The system contains three devices, remote inputs and outputs that are connected by Ethernet communication network to PC computer. There is the monitoring application on the computer that reads measured values from remote I/Os. The connection between PC computer and remote devices is realized by OPC communication.
Fig. 4. Screen displaying actual values.
The temperatures are measured in concrete time intervals. The intervals can be set between 10s and 1day. The temperature changes in rock massif are very slow. So the measurement is also relatively slow, the measurement system GRHVQ¶W QHHG to meet hard requirement for real-time performance. The combination of PC computer and remote I/Os is satisfactory in this case. The temperatures are measured by 4-channel RTD Input Modules with 16bit A/D converter. The temperature sensors Pt 1000 are connected by 4-wire link. Accuracy of the module is satisfactory in comparison to used sensor and length of the cable.
Fig. 5. Screen displaying trends.
228
of
80
measured
4.3 Solving problems with instability of measured values
Why there is a leak resistance between two independent sensors? There are two reasons:
After commissioning of the monitoring system, problems with instability of some measured values appeared. The values changed in time intervals of seconds or minutes within UDQJH & 7KH FKDQJHV ZHUH LUUHJXODU LQ WLPH 7KH temperature progress in rock massif is very slow so such fast changes had to be caused by another reason. All thermometers were tested in long period before installation the monitoring system. The testing was made following way ± there were measured resistance of supply cables and resistance at output contact of each thermometer. The results GLGQ¶W LQGLFDWH DQ\ SUREOHP DQG WKH Yalues were stable in time. The first hypothesis explaining the instability was an influence of disturbance. The cables that connect sensors to I/O devices are quite long ± between 50 and 200m so the disturbance could be expected. To verify this hypothesis, additional measurements using oscilloscope were done. Certain level of higher frequency disturbances was detected. But the sensors that have stable measured value and the sensors with unstable values have similar level of the disturbance. Moreover applying filters to suppress GLVWXUEDQFHV KDG PLQLPDO HIIHFW 7KH GLVWXUEDQFH GRHVQ¶W seem to be a cause of instability of some measured values. The second hypothesis was a malfunction of input modules on remote I/Os devices. This reason was also rejected because when modules that measures stable values were used for the sensors with unstable values, situation was the same. During the measurement of signals using oscilloscope, an interesting property of the system was found out. The system measures the temperatures such way that a current supplying a thermometer is not constant but pulsed. The 4-channel RTD Input Modules measures only one input in the moment ± the others are without supply current. The measurement sequence is a cycle ± from first input to fourth and again. This is the fact that is not mentioned in documentation of the module. During further measurement, another fact was detected ± there is a finite resistance DSSUR[ Nȍ between two independent sensors, for example between sensors in different boreholes. This fact was determined as the main reason of the problem with stability and accuracy of the measurement. While one sensor is measured, start of measurement of another, independent sensor evokes a step change of measured value. U1
1.
There is a box at the top of each borehole that concentrates cables from each thermometer to one multi-wire cable that leads to the monitoring system. These boxes are place approximately one meter under ground. It seems that humidity gets into some of these boxes. This hypothesis was confirmed when several such boxes were uncovered.
2.
Second reason is a penetration of humidity into sensors or their cables. Certain level of such penetration is also supposed in this installation but the influence is much smaller than influence of leak resistance in the boxes. Verification of the sensor and cables is not possible because their extraction from the borehole is not possible.
The influence humidity is great problem that must be solved when temperature sensors should be installed in boreholes and should be used for long-time period. A risk of penetration of humidity into the sensors increases with the time. The ways of protection of the sensors and ways of their installation in boreholes are subject of our further research activities.
5. BOREHOLE THERMAL PROFILE The thermal profile is a temperature distribution within the borehole. It is an important parameter that can refer to properties of rock massif. The thermal profile is also important for evaluation of thermal response test measurement. There is two general ways how to measure the thermal profile: -
Continuous measurement ± the thermal conditions of the borehole can be monitored continuously using similar system as described above. A set of sensors must be installed in the borehole and the temperatures are then measured in specified intervals. The sensors have to be installed in several levels within the borehole ± the thermal profile can be approximated when number of temperatures in different depth is known. The main advantage of this solution is that the measurement can be done continuously and so temperature changes can be monitored during long period. The measurement can be done during usage of the borehole. Disadvantages ± higher costs and complicated installation.
-
Irregular measurement ± the thermal profile can be checked by irregular measurements. Usually, a sensor is dropped down into the borehole and temperatures are measured in required levels. Advantages are following ± the measurement system is very easy (a sensor with appropriate protection, a cable and a measurement device), the temperature can be easily measured in high number of levels, the sensor can be replaced when it is broken. Disadvantages ± the measurement is irregular, the
U2
I1
I2
IE
Fig. 6. Leak resistance between independent sensors.
229
measurement have to be done, when the borehole is not connected to the technology, the measurement takes some time (30 min - 1 hour), pulling out of the sensor could be difficult. Each way of measurement is convenient for specific cases. Both of them are realized during research activities at VSBTUO. The continuous measurement system was described in chapters 2-4. There is also another shallow borehole for measurement of influences of solar radiation on rock massif. The depth of this borehole is only 20m but there are installed 29 sensors in certain levels. The example of the measurement is at fig 7.
6. CONCLUSIONS The measurement of the temperatures in boreholes is an issue that brings a lot of technical problems to solve. The main question is how to protect the sensors against humidity, water, mechanical influences during installation and using. The experiences show that protection of sensors (especially when they are based on analog electrical principle) could be difficult in the long term. This is reason why there are research activities now at VSB-TUO that are focused on digital temperature sensors and the sensors with optical fiber. These measurement methods should be less sensitive to humidity and other influences. The other direction of research activities is measuring of thermal profile of the boreholes and automation of this measurement process.
Fig. 7. The thermal profile of a 20m borehole.
The thermal profile of special borehole MV03 that was taken by irregular measurement is presented in figure 8 (the position of MV03 is depicted in figure 3). The measurement was realized using temperature sensor PT1000 with cable 140m long. The sensor was manually dropped down into the borehole and pulled back. The temperature was measured in each meter.
Fig. 8. The thermal profile of a borehole.
ACKNOWLEDGEMENT This work was solved on the VSB-TU Ostrava, Czech Republic. This work was supported by the Grant "Research
230
of rock massif thermal conditions during heat pumps operation LQWKHDUHDRI$8/$DQG&,79â%-TU Ostrava"
REFERENCES %XMRN39UWHN0+RUiN%+iMRYVNê5+HOOVWURP* (2005). Study of thermal response of rock massif for installations of heat pumps. Study for Czech Energetic Agency, Ostrava 2005. Kreidl, M. (2005). Temperature measurement. Ben, Prague, Czech Republic, 2005. Burkhard, S., Mands, E., Sauer, M., Grundmann,E. (2007). Technology, development status, and routine application of Thermal Response Test. In Proceedings European Geothermal Congress 2007, Germany.
231
Measurement of thermal conditions in rock massif Jiri Koziorek*. Bohumil Horak*. Radovan Hajovsky*. Petr Bujok**.
*VSB-Technical University of Ostrava, Faculty of Electrical Engineering and Computer Science, Czech Republic (e-mail: jiri.koziorek@ vsb.cz, bohumil.horak@ vsb.cz, radovan.hajovsky@ vsb.cz ). **VSB-Technical University of Ostrava, Faculty of Mining and Geology, Czech Republic (e-mail:petr.bujok@ vsb.cz). Abstract: This paper brings information about research activities of temperature measurement in boreholes. The described research is focused on boreholes used for heat pumps applications. The first part of the contribution deals with long time measurement of the temperatures in boreholes used in heat pump installation. A system for monitoring and evaluation of temperatures in measurement polygon is presented. The second part is focused on measurement of thermal profile of the borehole. The paper GRHVQ¶W HYDOXDWH JHRWKHUPDO SURSHUWLHV RI WKH ERUHKROHV EXW LW GHVFULEHV WKH PHDVXUHPHQW LVVXHV DQG methods. Keywords: Thermal profile, Heat pump, Borehole, Temperature measurement, Temperature monitoring, Temperature sensor.
1. INTRODUCTION In 2006, the construction of a new conference building at VSB - Technical University of Ostrava was finished. The building has installed a unique system of heating and cooling. The system is based on application of heat pump. The installed heat pump system is the biggest in Czech Republic. The system contains 6 heat pumps with overall power 700kW. The heat pumps use 110 boreholes in surrounding of the building. The depth of boreholes is 140m. The system is used for heating during winter period and for cooling during summer period.
The heat pumps are able to use low-temperature energy sources with temperatures between 0-&6XFKJHRWKHUPDO energy is ecologically clean and practically inexhaustible. The heat pumps in described case use boreholes in which is installed a closed system of PE pipes for heat transfer. The heat pump raises a thermal imbalance in the surrounding of the boreholes, so that there is continuous heat transfer between the rock massif and the boreholes. During winter period, the energy is obtained from the rock massif so the massif is permanently cooled. It depends on properties of the massif as well as on the heating-transfer surface in borehole, if surrounding temperature of the borehole decreases under zero (the borehole is frozen). A temperature of the rock massif is influenced by solar energy, geothermal energy and decay of the minerals. The solar radiation is usually mentioned as main influence in depths where are boreholes for heat pumps used.
Fig. 1. New conference building at VSB-Technical University of Ostrava, Czech Republic.
978-3-902661-41-8/09/$20.00 © 2009 IFAC
During the design and installation of heat pumps at VSBTUO, the research activities focused on thermal behavior of rock massif were also considered. The system of heat pumps has as mentioned 110 boreholes, 140m deep. A concrete part RI WKHVH ERUHKROHV ZDV GHFODUHG DV ³0HDVXUHPHQW SRO\JRQ´ and it was equipped by of the temperature measurement system. The temperature measurement allows monitoring of influences of the heat pump operation on rock massif, influence of solar radiation and seasons. The measurement polygon allows as well a testing of different types of temperature sensors in such applications, a testing of sensors protection and evaluation.
226
10.3182/20090210-3-CZ-4002.0042
2. MEASUREMENT POLYGON The measurement polygon contains two types of the boreholes: -
-
Technological boreholes ± these boreholes used by heat pump for its operation. There are 10 technological boreholes. Each is equipped by pair of PE-collectors with 32mm diameter. There is also a set of 4 temperature sensors at input pipes (colder side) in depths 20, 50, 100 and 140m and a set of 2 temperature sensors at output pipes (warmer side) in depths 20 and 100m.
The schematic drawing of measurement polygon is displayed at figure 3.
3. TEMPERATURE SENSORS An analysis of temperature sensors preceded their installation in the measurement polygon.
Special boreholes - these boreholes used for measurement only. There are 5 special boreholes. Each is equipped by PE-collector but the collector is not used for any technological purpose. There is installed a set of 4 temperature sensors in depths 20, 50, 100 and 140m.
0m -10m -20m -30m -40m
Fig. 3. Copy from the project of boreholes situation with highlighted positions of the technological (circles) and special (triangles) boreholes. The signals from all thermometers are linked to an underground concentrator where remote I/O system is installed. The concentrator is also visible in the figure.
-50m -60m -70m -80m
Temperature sensors can be divided into two groups - selfgenerating and modulating sensors. The self-generating VHQVRUV GRQ¶W UHTXLUH DQ\ VRXUFH RI SRZHU Temperature sensors based on thermocouples belong to this category. The temperature sensors based on the thermal modulation of electric currents or voltages supplied by auxiliary energy sources are considered to be modulating sensors. Resistors, diodes and transistors belong to this group.
-90m -100m -110m -120m
The following temperature sensors were taken into account: -130m
-
Metal resistance sensors ± all metallic conductors change their resistivity with the temperature. The change of resistance is quite linear over a large range of temperature; deviation from a straight line is more significant at higher temperatures. Materials usually used for such sensors are Platinium and Nickel. They are use usually in range -WR&
-
Semiconductor resistance sensors ± thermally sensitive resistor that is made from semiconducting ferromagnetic ceramic materials. They have good temperature sensitivity, small dimensions, easy connection but they have non-linear characteristics. They are use usually in range -WR&
-140m
Fig. 2. Technological and special borehole. The aim of such system of boreholes is obtaining relevant data for modeling and evaluation of thermal processes during a heat pump operation. Another aim is testing and design of temperature sensors for longtime measurement in boreholes. The sensors taken into the consideration are based on electrical and optical basis.
227
-
-
Monolytic p-n temperature sensors ± they are based on temperature dependence of p-n junction. They have temperature range from -WR&7KH\DUH XVXDOO\ integrated with circuit for signal processing.
4.2 Monitoring application The monitoring application is created in SCADA system Promotic. The application has following features:
Thermoelectric sensors, thermocouples ± they are based on Seebeck effect. Thermocouples are always composed of two conductors or semiconductors and they are able to generate a voltage. They are used for measurement of temperature differences. The materials for thermocouples are standardized. They can be used for temperatures between ± 180 to &
The parameters, availability, accuracy and ways of signal processing of these temperature sensors were compared and metal resistance sensor PT1000/3850, class A in 4-wire connection were selected. They had the best accuracy & QHDU & VWDQGDUGL]HG SURFHVVLQJ RI PHDVXUHG signal, and a compensation of length of connection, good availability and price.
-
Displaying actual temperatures.
values
-
Displaying historical trends or tables of selected temperatures.
-
Storing of data into database.
-
Displaying values of temperatures in selected level of depth.
-
Scheduling and setting of measurement.
-
Remote access to the application.
The other methods of the temperature measurement, as crystal thermometers or optical fibers temperature measurement were not considered at this stage because they require more sophisticated signal processing.
4. MONITORING SYSTEM The monitoring system measures temperatures of 10 technological boreholes and 5 special boreholes, displays actual values and trends, stores all data to archives and provides remote access to data. 4.1 Mesurement The temperatures are measured by I/O system ICP DAS. The system contains three devices, remote inputs and outputs that are connected by Ethernet communication network to PC computer. There is the monitoring application on the computer that reads measured values from remote I/Os. The connection between PC computer and remote devices is realized by OPC communication.
Fig. 4. Screen displaying actual values.
The temperatures are measured in concrete time intervals. The intervals can be set between 10s and 1day. The temperature changes in rock massif are very slow. So the measurement is also relatively slow, the measurement system GRHVQ¶W QHHG to meet hard requirement for real-time performance. The combination of PC computer and remote I/Os is satisfactory in this case. The temperatures are measured by 4-channel RTD Input Modules with 16bit A/D converter. The temperature sensors Pt 1000 are connected by 4-wire link. Accuracy of the module is satisfactory in comparison to used sensor and length of the cable.
Fig. 5. Screen displaying trends.
228
of
80
measured
4.3 Solving problems with instability of measured values
Why there is a leak resistance between two independent sensors? There are two reasons:
After commissioning of the monitoring system, problems with instability of some measured values appeared. The values changed in time intervals of seconds or minutes within UDQJH & 7KH FKDQJHV ZHUH LUUHJXODU LQ WLPH 7KH temperature progress in rock massif is very slow so such fast changes had to be caused by another reason. All thermometers were tested in long period before installation the monitoring system. The testing was made following way ± there were measured resistance of supply cables and resistance at output contact of each thermometer. The results GLGQ¶W LQGLFDWH DQ\ SUREOHP DQG WKH Yalues were stable in time. The first hypothesis explaining the instability was an influence of disturbance. The cables that connect sensors to I/O devices are quite long ± between 50 and 200m so the disturbance could be expected. To verify this hypothesis, additional measurements using oscilloscope were done. Certain level of higher frequency disturbances was detected. But the sensors that have stable measured value and the sensors with unstable values have similar level of the disturbance. Moreover applying filters to suppress GLVWXUEDQFHV KDG PLQLPDO HIIHFW 7KH GLVWXUEDQFH GRHVQ¶W seem to be a cause of instability of some measured values. The second hypothesis was a malfunction of input modules on remote I/Os devices. This reason was also rejected because when modules that measures stable values were used for the sensors with unstable values, situation was the same. During the measurement of signals using oscilloscope, an interesting property of the system was found out. The system measures the temperatures such way that a current supplying a thermometer is not constant but pulsed. The 4-channel RTD Input Modules measures only one input in the moment ± the others are without supply current. The measurement sequence is a cycle ± from first input to fourth and again. This is the fact that is not mentioned in documentation of the module. During further measurement, another fact was detected ± there is a finite resistance DSSUR[ Nȍ between two independent sensors, for example between sensors in different boreholes. This fact was determined as the main reason of the problem with stability and accuracy of the measurement. While one sensor is measured, start of measurement of another, independent sensor evokes a step change of measured value. U1
1.
There is a box at the top of each borehole that concentrates cables from each thermometer to one multi-wire cable that leads to the monitoring system. These boxes are place approximately one meter under ground. It seems that humidity gets into some of these boxes. This hypothesis was confirmed when several such boxes were uncovered.
2.
Second reason is a penetration of humidity into sensors or their cables. Certain level of such penetration is also supposed in this installation but the influence is much smaller than influence of leak resistance in the boxes. Verification of the sensor and cables is not possible because their extraction from the borehole is not possible.
The influence humidity is great problem that must be solved when temperature sensors should be installed in boreholes and should be used for long-time period. A risk of penetration of humidity into the sensors increases with the time. The ways of protection of the sensors and ways of their installation in boreholes are subject of our further research activities.
5. BOREHOLE THERMAL PROFILE The thermal profile is a temperature distribution within the borehole. It is an important parameter that can refer to properties of rock massif. The thermal profile is also important for evaluation of thermal response test measurement. There is two general ways how to measure the thermal profile: -
Continuous measurement ± the thermal conditions of the borehole can be monitored continuously using similar system as described above. A set of sensors must be installed in the borehole and the temperatures are then measured in specified intervals. The sensors have to be installed in several levels within the borehole ± the thermal profile can be approximated when number of temperatures in different depth is known. The main advantage of this solution is that the measurement can be done continuously and so temperature changes can be monitored during long period. The measurement can be done during usage of the borehole. Disadvantages ± higher costs and complicated installation.
-
Irregular measurement ± the thermal profile can be checked by irregular measurements. Usually, a sensor is dropped down into the borehole and temperatures are measured in required levels. Advantages are following ± the measurement system is very easy (a sensor with appropriate protection, a cable and a measurement device), the temperature can be easily measured in high number of levels, the sensor can be replaced when it is broken. Disadvantages ± the measurement is irregular, the
U2
I1
I2
IE
Fig. 6. Leak resistance between independent sensors.
229
measurement have to be done, when the borehole is not connected to the technology, the measurement takes some time (30 min - 1 hour), pulling out of the sensor could be difficult. Each way of measurement is convenient for specific cases. Both of them are realized during research activities at VSBTUO. The continuous measurement system was described in chapters 2-4. There is also another shallow borehole for measurement of influences of solar radiation on rock massif. The depth of this borehole is only 20m but there are installed 29 sensors in certain levels. The example of the measurement is at fig 7.
6. CONCLUSIONS The measurement of the temperatures in boreholes is an issue that brings a lot of technical problems to solve. The main question is how to protect the sensors against humidity, water, mechanical influences during installation and using. The experiences show that protection of sensors (especially when they are based on analog electrical principle) could be difficult in the long term. This is reason why there are research activities now at VSB-TUO that are focused on digital temperature sensors and the sensors with optical fiber. These measurement methods should be less sensitive to humidity and other influences. The other direction of research activities is measuring of thermal profile of the boreholes and automation of this measurement process.
Fig. 7. The thermal profile of a 20m borehole.
The thermal profile of special borehole MV03 that was taken by irregular measurement is presented in figure 8 (the position of MV03 is depicted in figure 3). The measurement was realized using temperature sensor PT1000 with cable 140m long. The sensor was manually dropped down into the borehole and pulled back. The temperature was measured in each meter.
Fig. 8. The thermal profile of a borehole.
ACKNOWLEDGEMENT This work was solved on the VSB-TU Ostrava, Czech Republic. This work was supported by the Grant "Research
230
of rock massif thermal conditions during heat pumps operation LQWKHDUHDRI$8/$DQG&,79â%-TU Ostrava"
REFERENCES %XMRN39UWHN0+RUiN%+iMRYVNê5+HOOVWURP* (2005). Study of thermal response of rock massif for installations of heat pumps. Study for Czech Energetic Agency, Ostrava 2005. Kreidl, M. (2005). Temperature measurement. Ben, Prague, Czech Republic, 2005. Burkhard, S., Mands, E., Sauer, M., Grundmann,E. (2007). Technology, development status, and routine application of Thermal Response Test. In Proceedings European Geothermal Congress 2007, Germany.
231