Fiber Optic Distributed Temperature Sensor Research Articles

A Case Study of Field Thermal Response Test and Laboratory Test Based on Distributed Optical Fiber Temperature Sensor

To design an efficient ground source heat pump (GSHP) system, it is important to accurately measure the thermophysical parameters of the geotechnical layer. In the current study, a borehole is tested in detail using a combined thermal response test system (CTRTS) based on a distributed optical fiber temperature sensor (DOFTS) and a laboratory test. Real-time monitoring of the stratum temperature according to depth and operation time and the geothermal profile and thermal conductivity of each stratum are obtained. The results show that the undisturbed ground temperature is 10.0 °C, and the formation temperature field within 130 m can be divided into variable temperature formation, constant temperature formation (9.13 °C), and warming formation (geothermal gradient is 3.0 °C/100 m). The comprehensive thermal conductivity of the region is 1.862 W/m·K. From top to bottom, the average thermal conductivity of silty clay, mudstone, argillaceous siltstone, and mudstone is 1.631 W/m·K, 1.888 W/m·K, 1.862 W/m·K, and 2.144 W/m·K, respectively. By comparing the measurement results, the accuracy and effectiveness of the CTRTS are verified. Therefore, it is recommended to use the thermal conductivity obtained by the CTRTS to optimize the design of the borehole heat exchanger (BHE). This study provides a case for establishing a standard distributed thermal response test (DTRT).

Research Progress of Applying Distributed Fiber Optic Measurement Technology in Hydraulic Fracturing and Production Monitoring

With the exploration and development of unconventional oil and gas resources, downhole environmental monitoring and data-analysis technologies are becoming more and more important. Distributed fiber optic measurement technology, as a new monitoring technology to obtain accurate data, has a wide range of applications in hydraulic fracturing and production monitoring. It mainly includes: distributed fiber optic temperature sensors (DTSs) to monitor gas lift, identify in-flow fluid types, interpret flow profiles and monitor production enhancement operations; distributed fiber optic acoustic sensors (DASs) to monitor low frequency strain and microseismic and hydraulic fracturing operations; and distributed fiber optic stress sensors (DSSs) to characterize fractures in the near-well area, which have been well applied in the field. This paper describes the current application status of DASs and DSSs in hydraulic fracturing and production monitoring, respectively, from the principle of distributed fiber optic measurement technology. It also points out the limitations of these measurement technologies and the direction of future development. Distributed fiber optic measurement technology has been making technical breakthroughs in recent years, providing strong technical support for the development of unconventional oil and gas resources.

Studies on thermal profile measurement and fire detection in a power supply cable of a synchrotron radiation source by Raman optical fiber distributed temperature sensor system

The paper presents the work on development and application of Raman optical fiber distributed temperature sensor (ROFDTS). ROFDTS has been used for monitoring the temperature profile and detecting the progress of unforeseen fire events anywhere in a 160 m long power supply cable (current 760 A, DC) of dipole magnets in the Indus-2 synchrotron radiation source (SRS). Indus-2 is a national facility and is being operated in round the clock mode. The sensing fiber could survive in the harsh environment of Indus-2 SRS that consists of bremsstrahlung radiation of 6.7 Rad/h, high magnetic field of 1.5 T and radio frequency (RF) field of 1.27 MV at 505.812 MHz. The fire detecting capability of ROFDTS was first tested on the mock test setup in the laboratory by artificially creating the four hot zones. Each hot zone has the width of 1.0 m in the sensing fiber at the spatial locations of 26 m, 48 m, 70 m and 92 m respectively. An audio-visual alarm was generated when the temperature of the hot zones exceeded the set temperature limit of 50 °C within 30 s of the start of the fire. Finally, ROFDTS system was deployed to measure the temperature profile of power supply cable during ‘no operation’, ‘injection’ of the electron beam current (109 mA with the energy of electrons @ 550 MeV) and at ‘normal operation’ of the electron beam current (145 mA with the energy of electrons @ 2.5 GeV) in Indus-2 SRS. The peak temperature of power cable reached from 26 °C to 44 °C from ‘no operation’ to during the ‘normal operation’ condition of the Indus-2 SRS.

A preliminary study on wellbore flow interpretation of fiber optic vibration signals based on K-means clustering algorithm

The wellbore flow analysis of optical fiber vibration signal depends on distributed optical fiber logging. Distributed optical fiber logging technology identifies the fluid in the well through distributed optical fiber acoustic sensor (DAS) and distributed optical fiber temperature sensor (DTS). Distributed optical fiber sensor has the advantages of small underground interference, high efficiency and low cost. In this paper, the wellhead data extracted by the distributed optical fiber acoustic sensor is used to calculate the upper bound of the fluid sound frequency band in the pipe by nonlinear least squares fitting. The K-means clustering algorithm is used to cluster the optical fiber vibration signals in the low frequency band. According to the clustering results, the ratio of the optical fiber signal eigenvalues of each production layers is obtained, and the trend of the ratio of the optical fiber signal eigenvalues of each production layers is judged to be close to the trend of the water absorption intensity. Compared with traditional acoustic logging, the wellbore flow analysis using distributed optical fiber acoustic sensor can quickly determine the production contribution of each layer and the change of fluid phase state in the production cycle. Combined with traditional production logging technology, distributed optical fiber logging shows its reliability and accuracy in data collection, logging interpretation and production application. Starting from the principle of distributed optical fiber acoustic sensing technology, this paper briefly expounds the properties of distributed optical fiber acoustic sensor and the principle of injection profile logging, systematically introduces the processing of distributed optical fiber acoustic data, and emphatically introduces the accuracy of K-means clustering algorithm for analyzing distributed optical fiber acoustic signal and qualitative judgment of production layer, which provides a new idea for judging the accuracy of production layers.

Characterizing soil water content variability across spatial scales from optimized high-resolution distributed temperature sensing technique

Fiber-optic Distributed Temperature Sensing, when combined with the Single-probe Heat-pulse technique can measure soil moisture (θ) across spatial scales. The key limitation of this system is in obtaining the relationship between soil thermal conductivity (λ) and θ for a specific field. Using the Department of Energy Atmospheric Radiation Measurement (ARM) site, this study tested a new methodology to account for the spatial variability in the λ-θ relationship using a Gaussian processes model. The resulting accurate θ measurements (RMSE = 0.03 m3m−3) were used to characterize the spatial variability of θ across scales and to develop an empirical equation that can correct for the changes in the θ spatial variability observed at different spatial resolutions. In addition, the number of required samples to accurately characterize θ and its variability over scales ranging from 5 m and 350 m were estimated. These findings provide key information to scale soil moisture from centimeters to hundreds of meters for process understanding.

Design and implementation of an early-stage monitoring system for iron sulfides oxidation

Sulfur corrosion is one of the significant concerns that could cause potential hazards in the petrochemical industry, and the traditional periodical inspection techniques are insufficient to support timely and reliable monitoring of iron sulfides oxidation. The emerging data-driven fault detection models and innovative sensing technologies provide new opportunities to process safety monitoring. This article proposed an integrated approach that employed fiber-optic distributed temperature sensing (FO-DTS) system, electrochemical gas sensors, embedded systems, and neural networks to detect the exotherm of early-stage iron sulfides oxidation in complex scenarios. Specifically, the sulfides oxidation exotherm is simulated using programmed electrical heating devices, and a software simulation is conducted to optimize the heating rods power selection; the exothermic chemical reaction is carried out with the oxidation of dimethyl sulfoxide (DSMO) by hydrogen peroxide (H2O2) in a simulated stainless-steel reactor. The continuous temperature data and its spatial distribution of the targeted reactor surface are generated by the DTS, and the SO2 concentration is collected as an additional criterion. The edge computing gateway can handle the field data collection from different types of protocols and other auxiliary tasks. Furthermore, the performance of the field sensing system and the anomaly detection neural networks are tested. The result shows that the proposed method is able to distinguish the simulated iron sulfides oxidation exotherm from the chemical reaction exotherm with an acceptable accuracy rate. The details of the system components are also demonstrated as a reference for deploying similar sulfides oxidation monitoring tasks in practice.

Analysis of thermal dilution experiments with distributed temperature sensing for fractured rock characterization

Thermal dilution experiments with Fiber-Optic Distributed Temperature Sensing (FO-DTS) were conducted at the In-situ Stimulation and Circulation (ISC) rock laboratory, at the Grimsel Test Site (GTS) in Switzerland. The experiment consists in replacing the total volume of a borehole with a warmer water and monitoring the rate of temperature increase and decrease during the heating and cooling periods, respectively. The changes in temperature monitored in depth and time under various hydraulic conditions and in different boreholes, are used to investigate the information provided by thermal dilution experiments in terms of groundwater flow and thermal properties in low-permeable fractured crystalline rock. The data analysis, and the use of analytical and numerical solutions for reproducing this data in the context of pure diffusion and advection–diffusion scenarios, lead to the following improvements and conclusions. (i) The formation thermal conductivity is estimated along the borehole by inverting the data collected under ambient conditions with a simple analytical solution. The estimated values are consistent with laboratory estimates. The method presents the advantage of requiring much shorter experiments than existing methods based on standard active-line-source (ALS) experiments, i.e., several hours versus the traditional 1–2 days. (ii) Hydraulically active fractures connecting boreholes are detected from experiments conducted under cross-borehole forced hydraulic conditions. (iii) The formation thermal conductivity and fracture flow velocity have a distinguished impact on the temperature anomalies for some ranges of these property values, implying that both properties can be estimated from well-parametrized experiments.

Combining passive and active distributed temperature sensing measurements to locate and quantify groundwater discharge variability into a headwater stream

Abstract. Exchanges between groundwater and surface water play a key role for ecosystem preservation, especially in headwater catchments where groundwater discharge into streams highly contributes to streamflow generation and maintenance. Despite several decades of research, investigating the spatial variability in groundwater discharge into streams still remains challenging mainly because groundwater/surface water interactions are controlled by multi-scale processes. In this context, we evaluated the potential of using FO-DTS (fibre optic distributed temperature sensing) technology to locate and quantify groundwater discharge at a high resolution. To do so, we propose to combine, for the first time, long-term passive DTS measurements and active DTS measurements by deploying FO cables in the streambed sediments of a first- and second-order stream in gaining conditions. The passive DTS experiment provided 8 months of monitoring of streambed temperature fluctuations along more than 530 m of cable, while the active DTS experiment, performed during a few days, allowed a detailed and accurate investigation of groundwater discharge variability over a 60 m length heated section. Long-term passive DTS measurements turn out to be an efficient method to detect and locate groundwater discharge along several hundreds of metres. The continuous 8 months of monitoring allowed the highlighting of changes in the groundwater discharge dynamic in response to the hydrological dynamic of the headwater catchment. However, the quantification of fluxes with this approach remains limited given the high uncertainties on estimates, due to uncertainties on thermal properties and boundary conditions. On the contrary, active DTS measurements, which have seldom been performed in streambed sediments and never applied to quantify water fluxes, allow for the estimation of the spatial distribution of both thermal conductivities and the groundwater fluxes at high resolution all along the 60 m heated section of the FO cable. The method allows for the description of the variability in streambed properties at an unprecedented scale and reveals the variability in groundwater inflows at small scales. In the end, this study shows the potential and the interest of the complementary use of passive and active DTS experiments to quantify groundwater discharge at different spatial and temporal scales. Thus, results show that groundwater discharges are mainly concentrated in the upstream part of the watershed, where steepest slopes are observed, confirming the importance of the topography in the stream generation in headwater catchments. However, through the high spatial resolution of measurements, it was also possible to highlight the presence of local and highly contributive groundwater inflows, probably driven by local heterogeneities. The possibility to quantify groundwater discharge at a high spatial resolution through active DTS offers promising perspectives for the characterization of distributed responses times but also for studying biogeochemical hotspots and hot moments.

Observations of coherent transverse wakes in shoaling nonlinear internal waves

Abstract Space- and time-continuous seafloor temperature observations captured the three-dimensional structure of shoaling nonlinear internal waves (NLIWs) off of La Jolla, California. NLIWs were tracked for hundreds of meters in the cross- and along-shelf directions using a fiber optic Distributed Temperature Sensing (DTS) seafloor array, complemented by an ocean-wave-powered vertical profiling mooring. Trains of propagating cold-water pulses were observed on the DTS array inshore of the location of polarity transition predicted by weakly nonlinear internal wave theory. The subsequent evolution of the temperature signatures during shoaling was consistent with that of strongly nonlinear internal waves with a large Froude number, highlighting their potential to impact property exchange. Unexpectedly, individual NLIWs were trailed by a coherent, small-scale pattern of seabed temperature variability as they moved across the mid- and inner shelf. A kinematic model was used to demonstrate that the observed patterns were consistent with a transverse instability with an along-crest wavelength of ∼10 m – a distance comparable to the cross-crest width of the wave-core – and with an inferred amplitude of several meters. The signature of this instability is consistent with the span-wise vortical circulations generated in three-dimensional direct numerical simulations of shoaling and breaking nonlinear internal waves. The coupling between the small-scale transverse wave-wake and turbulent wave-core may have an important impact on mass, momentum, and tracer redistribution in the coastal ocean.

The Namib Turbulence Experiment: Investigating Surface–Atmosphere Heat Transfer in Three Dimensions

Abstract The Namib Turbulence Experiment (NamTEX) was a multinational micrometeorological campaign conducted in the central Namib Desert to investigate three-dimensional surface layer turbulence and the spatiotemporal patterns of heat transfer between the subsurface, surface, and atmosphere. The Namib provides an ideal location for fundamental research that revisits some key assumptions in micrometeorology that are implicitly included in the parameterizations describing energy exchange in weather forecasting and climate models: homogenous flat surfaces, no vegetation, little moisture, and cloud-free skies create a strong and consistent diurnal forcing, resulting in a wide range of atmospheric stabilities. A novel combination of instruments was used to simultaneously measure variables and processes relevant to heat transfer: a 3-km fiber-optic distributed temperature sensor (DTS) was suspended in a pseudo-three-dimensional array within a 300 m × 300 m domain to provide vertical cross sections of air temperature fluctuations. Aerial and ground-based thermal imagers recorded high-resolution surface temperature fluctuations within the domain and revealed the spatial thermal imprint of atmospheric structures responsible for heat exchange. High-resolution soil temperature and moisture profiles together with heat flux plates provided information on near-surface soil dynamics. Turbulent heat exchange was measured with a vertical array of five eddy-covariance point measurements on a 21-m mast, as well as by collocated small- and large-aperture scintillometers. This contribution first details the scientific goals and experimental setup of the NamTEX campaign. Then, using a typical day, we demonstrate (i) the coupling of surface layer, surface, and soil temperatures using high-frequency temperature measurements, (ii) differences in spatial and temporal standard deviations of the horizontal temperature field using spatially distributed measurements, and (iii) horizontal anisotropy of the turbulent temperature field.

Study on temperature distribution prediction of horizontal wells during fracturing treatment

In recent years, fiber-optic distributed temperature sensors (DTS) have been widely used for the dynamic monitoring of horizontal well during fracturing treatments. It is proposed to address the frequent issues of unknown artificial fracture initiation location, unclear crack extension pattern, unknown fracturing fluid injection success, and difficult fracturing effect evaluation in the hydraulic fracturing of horizontal wells. The temperature prediction model is the basis for fracturing diagnosis based on DTS monitoring, however, the quantitative prediction of temperature distribution throughout the horizontal well fracturing process remains a challenge. In this paper, a set of temperature-distribution prediction model for multi-stage fracturing in horizontal wells is established, with various micro-heat effects taken into account, and the models are solved through coupling. The temperature distribution during multi-cluster fracturing, and the temperature distribution characteristics during staged fracturing of horizontal wells have been analyzed. Moreover, the influence of fracturing fluid displacement, formation filtration coefficient, fracture width, and height on temperature distribution has also been clarified. The findings of this study provide theoretical support for fracturing performance diagnosis, fracture initiation identification, and analysis of the whereabouts of fracturing fluids using DTS monitoring. It has a great significance on fracturing effect evaluation and fracturing design optimization for horizontal wells.

Distributed fiber optic temperature sensor based on dynamic calibration of Raman Stokes backscattering light intensity

In a distributed fiber optic temperature sensing system, the intensity of Raman Stokes backscattering light serving as reference light increases with the increase of temperature, leading to measurement errors in the system. A novel method of dynamically calibrating Raman Stokes backscattering light intensity is proposed to improve temperature accuracy for distributed fiber optic temperature sensors. According to the real-time Stokes intensity distribution in the reference fiber, Stokes intensity curve of the whole fiber at a reference temperature is simulated, and the temperature response of Stokes light is corrected. The ratio of Raman anti-Stokes light intensity to the calculated Stokes light intensity is used to demodulate temperature along the fiber. The experimental results indicate that the temperature accuracy of the distributed optical fiber temperature sensor system after making the Stokes optical dynamic calibration is increased up to 4.3 ℃ compared with that from the conventional method. And the accuracy of temperature measurement is improved by 8.9 ℃ when combined with Rayleigh noise suppression method. This study provides a new solution for a distributed fiber optic temperature sensor system to monitor high temperature environment temperature.

Studies on Thermal Profile Measurement and Fire Detection in a Power Supply Cable of a Synchrotron Radiation Source by Raman Optical Fiber Distributed Temperature Sensor System

Studies on Thermal Profile Measurement and Fire Detection in a Power Supply Cable of a Synchrotron Radiation Source by Raman Optical Fiber Distributed Temperature Sensor System

Comparison of fiber-optic distributed temperature sensing and high-sensitivity sensor spatial surveying of stream temperature

Measuring surface water temperature spatial variability is needed to estimate the interaction between surface water and groundwater, evaluate fish habitat and thermal inertia, and to estimate streamflow frequency and duration. Fiber optic distributed temperature sensing (FO-DTS) has been used in rivers and lakes, providing high-resolution and sensitive temperature monitoring over large temporal and spatial scales. However, in streams with cobbly or bedrock-lined streambeds and variable bathymetry, use of FO-DTS to measure temperature close to the surface water and groundwater interface can be challenging if even feasible. FO-DTS can also be costly, involve difficult installations, and require an advanced understanding of the technology, calibration, and data processing. In this study, we compared FO-DTS stream temperature survey results to an alternative temperature survey method employing a towed transect of high-resolution temperature loggers spaced at 1-m and transported in the stream along the study reach, to measure the spatial distribution of stream-water temperature in East Fork Poplar Creek near Oak Ridge, Tennessee, USA. We assessed the applicability and limitations of the two methods, and quantitatively compared in-situ temperature survey results measured simultaneously with each method. Regression results showed strong temporal and spatial correlation between the two methods. Differences were only elevated near the stream banks in areas that were coincident with correlation slope deviations from unity, which was attributed to shallower water and lower data density. Kriging standard errors were also low at channel center with minor increases near the stream banks. The results suggested that the array of the individual temperature sensors can provide a practical alternative to FO-DTS for thermal characterization of surface water, providing slightly lower spatial and temporal resolution, but with higher accuracy of temperature measurement, with greater simplicity, and with a broader range of conditions where it may be applied.

Heat transfer by seepage in sand: Influence of saturated hydraulic conductivity and porosity

Heat transfer within the soil is a complex process in the presence of seepage flow. In such conditions, the soil’s thermal behavior is influenced by the thermal and hydraulic properties of the medium as well as the initial conditions and boundary conditions to which the medium is subjected. This paper presents the experimental and numerical studies of heat transfer within the sand subjected to the seepage flow. It focuses on the influence of saturated hydraulic conductivity and the porosity of medium on the heat transfer process. The temperature distribution within the sand was monitored by the optical fiber Distributed Temperature Sensor (DTS). The experiment was performed on three types of silica-dominated sands with different saturated hydraulic conductivities and different Soil Water Characteristic Curve (SWCC). In addition to the experimental study, a coupled hydrothermal numerical model was designed in FEFLOW software and validated by comparing its results with the experimental measurements. To determine the influence of porosity and saturated hydraulic conductivity on heat transfer, we analyzed the numerical models for different values of porosity and saturated hydraulic conductivity. The numerical and experimental studies showed that the thermal velocity is higher in sand with higher saturated hydraulic conductivity and temperature declination occurs more quickly due to the heat convection process. Saturated sand with larger porosity has an overall higher heat capacity, wherefore the temperature declination started later in the measuring points but dropped down lower close to the temperature of the upstream water.

Uncertainties in Measuring Soil Moisture Content with Actively Heated Fiber-Optic Distributed Temperature Sensing.

Actively heated fiber-optic distributed temperature sensing (aFO-DTS) measures soil moisture content at sub-meter intervals across kilometres of fiber-optic cable. The technology has great potential for environmental monitoring but calibration at field scales with variable soil conditions is challenging. To better understand and quantify the errors associated with aFO-DTS soil moisture measurements, we use a parametric numerical modeling approach to evaluate different error factors for uniform soil. A thermo-hydrogeologic, unsaturated numerical model is used to simulate a 0.01 m by 0.01 m two-dimensional domain, including soil and a fiber-optic cable. Results from the model are compared to soil moisture values calculated using the commonly used Tcum calibration method for aFO-DTS. The model is found to have high accuracy between measured and observed saturations for static hydrologic conditions but shows discrepancies for more realistic settings with active recharge. We evaluate the performance of aFO-DTS soil moisture calculations for various scenarios, including varying recharge duration and heterogeneous soils. The aFO-DTS accuracy decreases as the variability in soil properties and intensity of recharge events increases. Further, we show that the burial of the fiber-optic cable within soil may adversely affect calculated results. The results demonstrate the need for careful selection of calibration data for this emerging method of measuring soil moisture content.

Modelling borehole flows from Distributed Temperature Sensing data to monitor groundwater dynamics in fractured media

Fractured aquifers are known to be very heterogeneous with complex flow path geometries. Their characterization and monitoring remain challenging despite the importance to better understand their behavior at all spatial and temporal scales. Heat and correspondingly temperature data have gained much interest in recent years and are often used as a tracer for characterizing groundwater flows. In the current work, a fast computer code is developed using Ramey and Hassan and Kabir analytical solutions which converts the temperature profile to the flow rate profile along the borehole. The method developed is validated through numerical simulations. A global sensitivity study recognizes the media thermal properties as the most influential parameters. For testing the method in the field, fiber-optic distributed temperature sensing (FO-DTS) data were used to monitor the dynamic behavior of fractured aquifers at the borehole scale at the Ploemeur-Guidel field site in Brittany, France. DTS data are used to infer the flow rates in the different sections of a fractured wellbore (flow profile) and calculate the contribution of each fracture to the total flow. DTS data were acquired for about three days in three different hydraulic conditions corresponding to two different ambient flow conditions and one pumping condition. Flow profiling using distributed temperature data matches satisfactorily with results from heat-pulse flow metering performed in parallel for cross-checking. Moreover, flow profiling reveals the daily variations of ambient flow in this fractured borehole. Furthermore, it shows that during ambient flowing conditions, shallow and deep fractures contribute roughly equally to the total flow while during the pumping condition, the deepest fractures contribute more to the total flow, suggesting a possible reorganization of flow and hydraulic heads depending on the hydraulic conditions. Thus, although the proposed method (DTS data and proposed framework) may be costlier and is based on indirect characterization through temperature measurements, it provides real-time monitoring of complex fracture interactions and recharge processes in fractured media. Thus, this method allows for a full analysis of the temporal behavior of the system with a simple and fast analytical model. Furthermore, thanks to its narrow width, DTS can be used and installed in boreholes for long-term monitoring while heat-pulse flow metering may lead to head losses in the borehole and may not be always possible depending on some borehole conditions. One of the limitations of the approach proposed is the proper knowledge of the thermal properties of media required to infer the flow rate from the temperature. Nevertheless, surface rate measurement can be useful to constrain these properties and reduce the flow profiling uncertainty. Thus, the method proposed appears to be an interesting and complementary method for characterizing borehole flows and groundwater dynamics in fractured media such as for instance, monitoring the recharge dynamic.

A 3D Thermal Field Restructuring Method for Concrete Dams Based on Real-Time Temperature Monitoring

Temperature monitoring and control is of great significance to ensure concrete quality during the construction of dam concrete. In this study, a 3D thermal field restructuring method is developed for the complex evolution process of thermal field in dam concrete during construction, based on real-time temperature monitoring. The proposed method is related to the ordinary Kriging interpolation and the distributed optical fiber temperature sensing (DTS) technology. Based on the temperature monitoring of the Wudongde (WDD) super-high arch dam, the 3D thermal field can be acquired effectively and fast by the proposed restructuring method. The results indicate the temperature difference between the temperature measured by thermometers and the restructured temperature is within 1 °C. The evolution process of the restructured 3D thermal field for the WDD arch dam is in good agreement with the site situation. In addition, the uncertainty, cons and pros of the proposed method and the optimal layout of temperature sensors are further discussed. Although the proposed thermal field restructuring method has some limitations, it is believed to be a valuable tool for acquiring the 3D thermal field and thermal gradient of dam concrete.

Experimental investigation of sustainable and energy efficient management of a geothermal field as a heat source and heat sink for a large office building

The new construction of a large office building in Hamburg, which is the subject of this investigation, has been closely monitored over a period of about 5 years. In addition to evaluating and optimising the energy demands and user comfort, the focus was on the management of the energy from an exceptionally large geothermal field which is used for heating and cooling the building. The possibilities for sustainable, energy efficient use of the geothermal field were investigated. The basis for the evaluations is a fibre optic Distributed Temperature Sensing (DTS) system, by means of which temperature profiles in the ground as well as in the concrete of the energy piles are recorded on an ongoing basis. With regard to the impact of geothermal energy use on the environment, the surrounding soil may be considered as undisturbed to date, based on the corresponding ground temperature measurements. The ground temperature measurements within the geothermal field show that by carrying out manual operating adjustments, ultimately the original soil temperature level could be achieved once again, after the three operating years under examination. A long-term, sustainable and therefore energy efficient use of the geothermal field is thus fundamentally possible in the present case.

Field test and numerical investigation on deep coaxial borehole heat exchanger based on distributed optical fiber temperature sensor

The thermal performance of DCBHE is superior to conventional borehole heat exchanger (BHE) systems. In the current study, the field test using a DCBHE (2044 m) is conducted, based on a distributed optical fiber temperature sensor (DOFTS). The temperature of the circulating fluid in terms of depth and operation time is monitored in real-time. The thermal extraction performance of the DCBHE is evaluated over an operation period of 60 d. Moreover, the temperature distribution characteristics, thermal process and impact scope of the formation are analyzed. The results demonstrate that the temperature of the bottom borehole is measured as 107.3 °C via DOFTS, and the average geothermal gradient is determined as 0.0507 °C/m. The thermal power and the coefficient of system performance are able to reach 238.0 W/m and 7.40 following 60 d of operation, respectively. The inner pipe made of polyethylene has exhibits a high level of thermal insulation for the outlet fluid. The thermal process between the soil and fluid is enhanced close to the borehole. And the soil temperature profiles exhibit fluctuations around the borehole within 1 m due to power failure. Furthermore, the formation impact scope is mainly within radius 5 m for operation 60 d.