Fibre-optic thermometry to support the clean energy transition
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Stephan Krenek
,
René Eisermann
Abstract
The measurement and control of temperature plays a key role in achieving the European Green Deal targets for a low carbon energy system. Fibre-optic thermometry is an emerging technology that can improve temperature measurement in extreme environments for energy providers and industry due to its distributed sensing and immunity to electromagnetic fields. Various applications for optimisation and monitoring in the energy sector are described, covering the whole range from energy generation to transmission and consumption. However, fibre-optic thermometers have cross sensitivities to other quantities (e.g., strain and humidity) and ageing effects that need to be investigated, quantified and minimised to obtain traceable and reliable measurements. This is particularly important so that applications in critical infrastructure can benefit from future measurements that are not possible with conventional sensors. The European INFOTherm project aims to overcome the limitations that currently prevent the widespread use of fibre-optic thermometry by creating a dedicated European metrology infrastructure for research, development and calibration. First results on measurement uncertainty, improvement of measurement techniques and practical field tests are presented.
Zusammenfassung
Bei der Erreichung der Ziele des europäischen Green Deal für ein kohlenstoffarmes Energiesystem spielen Temperaturmessung und -überwachung eine Schlüsselrolle. Die faseroptische Thermometrie ist eine aufstrebende Technologie, die aufgrund ihrer verteilten Messung und ihrer Unempfindlichkeit gegenüber elektromagnetischen Feldern die Temperaturmessung in extremen Umgebungen für Energieversorger und die Industrie verbessern kann. Es werden verschiedene Anwendungen zur Optimierung und Überwachung im Energiesektor beschrieben, die den gesamten Bereich von der Energieerzeugung über die Übertragung bis hin zum Verbrauch abdecken. Faseroptische Thermometer weisen jedoch Querempfindlichkeiten gegenüber anderen Größen (z. B. Dehnung und Feuchtigkeit) und Alterungseffekte auf, die untersucht, quantifiziert und minimiert werden müssen, um rückführbare und zuverlässige Messungen zu erhalten. Dies ist besonders wichtig, damit Anwendungen in der kritischen Infrastruktur zukünftig von Messungen profitieren können, die mit konventionellen Sensoren nicht möglich sind. Das europäische Projekt INFOTherm zielt darauf ab, die Hemmnisse zu überwinden, die einen breiten Einsatz der faseroptischen Thermometrie derzeit verhindern, indem eine dedizierte europäische metrologische Infrastruktur für Forschung, Enwicklung und Kalibrierung geschaffen wird. Erste Ergebnisse zur Messunsicherheit, zur Verbesserung der Messverfahren und zu praktischen Feldtests werden vorgestellt.
1 Motivation
Mitigating the effects of climate change is one of the most important challenges facing humanity. The energy system must be transformed in order to achieve the necessary drastic reduction in CO2 emissions. On the one hand, the production of electrical energy must be made largely CO2-free, and on the other hand, other sectors (e.g., transport, heating and industrial processes) must be electrified as far as possible. These are also the goals of the European Green Deal for a low-carbon energy system by switching to a higher share of renewable energy sources [1].
In many European countries, more than half of the final energy consumption is used for heating (56 % for Germany in 2019 [2]). Space heating and industrial process heat account for about a quarter of total final energy consumption. These figures illustrate the need for accurate temperature measurement to optimise energy consumption. In addition to reducing consumption, the energy transition rests on two other pillars: first, the decarbonisation of electricity generation through the expansion of renewable energies, including energy storage. Second, the expansion of energy grids to increase their capacity and resilience to the increasing volatility of energy generation [3], [4]. In all three areas, temperature measurement plays an important role in optimisation and monitoring. Fibre-optic temperature sensors have a number of unique advantages over conventional electrical sensors that make them particularly suitable for these tasks.
2 Basics of fibre-optic thermometry
Conventional temperature measurement methods rely on the temperature-dependent change in electrical properties, which are measured and evaluated directly. Thermocouples measure the voltage caused by the thermoelectric effect and resistance thermometers measure the temperature-dependent resistance. While these established methods provide accurate measurements, they are highly susceptible to electrical noise and require the sensors to be wired individually. They are therefore unsuitable, for example, for monitoring high-voltage components or a temperature distribution over a long distance with many measurement points.
Fibre-optic sensors, on the other hand, rely on the temperature-dependent change in refractive index and thermal expansion of optical fibres. The resulting change in optical path length is analysed using a variety of methods. Point sensors are typically based on Fibre Bragg Gratings (FBG) or Fabry-Pérot Interferometers (FPI) [5]. Distributed Temperature Sensing (DTS) is roughly based on a fingerprint created by the backscattering of infrared radiation in the fibre, whose temperature-dependent change is used to measure the temperature distribution along the fibre [6].
FBGs are the most widely used fibre-optic sensors in which a periodic change in refractive index is generated in the fibre by a UV laser or by femtosecond laser pulses (fs-FBG). The measurement of the temperature dependant reflection spectra is shown in Figure 1. Like a dielectric mirror, the FBG reflects the optical radiation in a narrow band around a resonance wavelength λ B. This depends on the effective refractive index n eff of the fibre and the grating period Λ, both of which depend on the temperature T:
The shift of this wavelength is usually around 11 pm/K, so that with modern interrogators, which have a scanning range of up to 100 nm, several FBGs in a fibre can be clearly distinguished using wavelength multiplexing. Depending on the measurement task, 20 or more point sensors can therefore be easily accommodated in a fibre that is only about 250 µm thick and several kilometres long. Figure 2 shows a multi-point sensor with three FBGs in one fibre.
Diagram of a fibre Bragg grating, which reflects optical radiation in a narrow band whose wavelength shifts to longer wavelengths with increasing temperature.
Photograph of a temperature sensor with three fibre Bragg gratings in a silica fibre protected in a glass tube to investigate multi-point sensing. Red light is coupled into the fibre, scattered at the FBGs and decoupled at the end of the fibre so that the respective positions are clearly visible. The furnace underneath the sensor glows due to its high temperature set point and is used to study ageing, for example.
Distributed fibre optic sensors can quantify temperature or strain changes along the entire length of an optical fibre by measuring changes in the back-scattered optical radiation (see Figure 3). They typically operate in the infrared, around the common telecom wavelength of 1300 nm or 1550 nm, which is referred to here as ‘light’ for simplicity.
Schematic of typical scattering intensity as a function of frequency ν within an optical fibre resulting from temperature changes ΔT and induced strain Δɛ.
Rayleigh-based methods utilise the elastic scattering of light caused by microscopic inhomogeneities within the fibre. This scattering occurs at the same frequency as the incident light and is sensitive to strain and temperature. Brillouin scattering involves inelastic interactions between light and acoustic phonons. This results in frequency shifts (Stokes and anti-Stokes components), which depend on strain and temperature. Raman-based techniques are inelastic and involve interactions with molecular rotational-vibrational states. Such interactions result in Stokes and anti-Stokes bands, where the intensity ratios are highly sensitive to temperature. Further information can be found in [7], [8].
These three scattering effects can be measured using a variety of techniques operating in the time or frequency domain. This results in a wide range of measurement systems available to suit specific applications. Some are capable of high resolution in the millimetre and millikelvin range, others can cover distances of up to 100 km [6].
3 Current and future applications in the energy sector
As a consequence of the optical working principle previously described, fibre-optic sensors offer unique advantages in being immune to electromagnetic fields and providing distributed sensing. This facilitates numerous optimisation and monitoring applications within the domain of energy infrastructure, which are inaccessible with conventional electric temperature sensors. The following sections provide an overview of these different applications, which can be categorised as follows (see Figure 4): Energy generation and storage (e.g., heat storage and geothermal energy), Energy transmission (electricity grid infrastructure) and Energy consumption (energy intensive industrial processes). This comprehensive range covers the entire spectrum of energy generation to consumption.
Sketch illustrating the applications of fibre-optic thermometry (shown in red) in the energy sector, from energy generation and storage to energy transmission and consumption.
Despite the unique advantages of fibre-optic thermometry, there are still a number of challenges that currently prevent its widespread use. On the one hand, there is no calibration infrastructure, so certified SI traceable sensors are not available. This is particularly important if the method is to be used to monitor critical energy infrastructure. As with all measurement methods, there are cross sensitivities to other quantities, such as strain or moisture. These need to be metrologically characterised in detail and minimised as far as possible so that measurements can be made with a known uncertainty budget. In addition, there is ongoing research into the further development of fibre-optic thermometry, for example for use at higher temperatures. These challenges are being addressed by the European research project INFOTherm, described in Section 4.
3.1 Energy storage and generation
3.1.1 Nuclear energy
Nuclear energy plays a significant role in the transition to net-zero emissions, currently contributing approximately 10 % to global electricity production [9]. In order to ensure the safe operation of existing and future nuclear fusion or fission power plants, as well as the operation of nuclear fuel recycling or transmutation facilities and the long-term monitoring of nuclear waste, the use of radiation-resistant thermometers is essential [9], [10], [11]. Despite the progress achieved in reducing the impact of ionising radiation, high temperatures and process gases on conventional contact thermometers, there remains a significant demand for more robust thermometers in hazardous environments. In comparison with alternative solutions, fibre-optic thermometry has exhibited substantial advancements under these conditions; however, further validation and stability monitoring is necessary [10], [11]. A responsible nuclear energy usage necessitates the establishment of underground repositories for radioactive waste. Given the significant dissipation of thermal energy from stored waste, the necessity of temperature monitoring of such subterranean structures is paramount to ensure their structural integrity and radiological containment [11]. As fibres are to be buried within such structures, it is imperative to investigate fibre ageing and in situ temperature calibration systems
3.1.2 Geothermal energy
Geothermal energy has the potential to function as a heat source for heat pumps or for direct geothermal energy utilisation. Furthermore, thermal energy can be stored seasonally in subterranean areas via multiple boreholes [2]. The monitoring of such boreholes has been demonstrated to greatly improve system operation and integrity; however, reliable temperature measurements with high spatial resolution are required along a path, and not merely at a few points. Distributed Temperature Sensing has also been used for geophysical studies and exploration for geothermal energy [12]. Sensing fibres are typically not removable, and interrogators cannot be taken offline for recalibration. This underscores the necessity to develop on-site calibration artefacts and solutions to maintain accuracy over extended periods. Initial steps towards this objective have been taken with the development of fibre artefacts for SI traceable calibration of the distance measurements of DTS systems [13].
3.1.3 Thermal energy storage
The development and deployment of thermal energy storage technologies constitutes a pivotal component of the European Green Deal [1], [14], [15]. This can provide much-needed flexibility across timescales from hours to several days, which is essential for the transition to a system dominated by variable energy supply from renewables. The necessity for advanced thermal storage solutions is evident in various applications, including building heating, industrial heating, concentrated solar power, power-to-heat Carnot batteries, and zero-carbon industrial heat. The temperature range for latent and sensible heat storage extends from sub-zero levels (ice slurries) to temperatures in excess of 500 °C (molten salt) [14].
Current temperature mapping relies on base metal thermocouples, which suffer from drift and complex installation. Distributed sensing by fibre-optic thermometry has the potential to replace conventional sensors; however, suitable calibration capabilities are currently in development. DTS will provide crucial insights into the loading status of a heat storage tank and instances of inefficient operation due to undesired mixing during thermal loading or unloading processes [14], [16].
Two examples of DTS applications that are currently undergoing further development to ensure reliable operation at elevated temperatures in molten salt storage are [16]: Concentrated solar power plants have the capacity to provide a continuous supply of electricity in southern European countries by heating salt and storing it for use at night to generate steam for conventional steam turbines [17]. It is imperative to monitor storage tanks and pipes transporting the molten salt to ensure its liquid state at 300 °C to 550 °C [16]. The presence of solidified salt along the pipes or evaporated salt would necessitate costly maintenance.
An similar example is that of so-called Carnot batteries, which are a set of storage technologies that store electricity in the form of thermal energy [18]. This makes them well-suited to balancing the electricity grid and enabling direct sector coupling with heating applications, unlike battery storage. During charging, an electric input is used to generate a temperature difference between two thermal reservoirs. During discharge, this temperature difference powers a cycle that produces electricity. The characteristics of Carnot batteries depend on the processes used for the conversion between electricity and heat, and on the thermal energy storage concept implemented [18]. One concept is to retrofit conventional coal or gas power stations by replacing their heat infrastructure with molten salt storage tanks while retaining the electrical components. This makes thermal energy storage one of the most cost-effective solutions [19]. Recent projects will directly benefit from sector coupling and utilise the stored heat for industrial processes [20]. In all the aforementioned applications, knowledge of the temperature distribution in the storage tanks is essential for optimising energy efficiency, calculating the loading state and monitoring the safety of the facilities. Future research will focus on increased measurement density, establish a spatial measurement grid, and align temperature sensing with complex geometric structures (e.g., heat exchangers) for further efficiency gains.
3.2 Energy transmission
The transformation of the European energy system, characterised by an increased use of renewable energy sources, necessitates enhanced transmission capacities that approach maximum transmission power levels. With an expected increase in electricity consumption of around 60 % between 2023 and 2030 [3]. This transformation also demands a substantial increase in flexibility with respect to large load fluctuations [4], [15], [21]. In order to ensure the durable and fail-safe operation of the energy infrastructure, close-meshed, reliable and disturbance-free temperature monitoring is required [22], [23]. Studies indicate that operating electrical infrastructure beyond its rated power can relieve grid congestion and improve power utilisation, but this requires precise, time- and location-resolved temperature monitoring [4], [24].
State-of-the-art temperature sensors, such as resistance thermometers and thermocouples, exhibit high sensitivities to electromagnetic interferences and are not well suited for establishing measurement chains for monitoring temperature distributions. Conversely, DTS has exhibited superior performance due to its capacity for spatial resolution and immunity to interference, rendering it ideal for temperature monitoring of electric sea or underground cables with a single continuous strand of an optical fibre [22], [25]. However, the lack of reliable validation and metrological characterisation of existing techniques hinders the widespread use of such techniques in critical infrastructure [23], which is to be resolved through joint European research [26].
An additional challenge for the energy network is the accelerated ageing of the network infrastructure due to an increasing number of prolonged summer heatwaves [27]. This has been shown to lead to increased outages in Italy’s medium-voltage grid [28]. Stressed components, such as transformers and cable joints, need to be monitored to enable predictive replacement and keep failure rates low [4], [23], [27]. Conventional electric temperature sensors cannot be used to monitor high-voltage applications due to the risk of sparkover. Even monitoring the outside of cable joints is impractical in field applications involving long wiring connections to the sensors. In contrast, fibre optic sensors allow direct contact with high-voltage systems and can have feed fibres of up to 100 km in length. For example, FBGs can be glued to transformer windings or embedded in the silicone of cable joints. However, in order to enable the use of sensors within critical infrastructure, they need to be validated. Therefore, the necessary calibration infrastructure for fibre-optic sensors must be established to ensure long-term reliability. The example of outages in Italy’s medium-voltage grid shows that, in the future, there will also be a need for cost-effective interrogation schemes for lower-value but more ubiquitous installations, in addition to monitoring high-value applications such as submarine cables.
3.3 Energy consumption
Reducing energy consumption is a cornerstone of the transition to clean energy. A substantial contribution can be made by increasing the efficiency of energy-intensive, high-temperature processes (e.g., glass production, additive manufacturing) as well as widely used processes around room temperature [29]. Achieving efficient operation of energy-intensive high-temperature processes while meeting product quality requirements in the metal or ceramics industry, cement production or the heat treatment of metals necessitates the use of accurate and drift-free thermometers. In the absence of superior alternatives, thermocouples are employed in the vast majority of applications. However, these instruments have been observed to demonstrate significant instability or drift in certain high-temperature processes, such as silicon production, due to the harsh conditions prevalent in these contexts. Fibre-optic thermometry can be employed due to its reduced sensitivity to various environmental effects. Consequently, novel approaches based on specialty fibres for very high temperatures are currently under research, with technical developments for enhanced reliability in industry [30], [31].
High-temperature sensors based on FBGs in single-crystal sapphire fibres have already demonstrated practical applicability up to 1600 °C [31]. A prototype sapphire-based thermometer was tested in an industrial production process involving roughly 24-h cycles between 25 °C and 1600 °C, over a period of more than 3 weeks. No ageing was detected within the measurement uncertainty [31]. In addition to the challenge of inscribing the FBG with femtosecond laser pulses, the high multimode light propagation in the fibre leads to complex reflection spectra (see Section 4.3.3). Current research is aimed at reducing the resulting uncertainty, with the long-term goal of outperforming thermocouples at temperatures above 1600 °C [26].
Accurate temperature monitoring in the −70 °C to 150 °C range is applicable to a wide range of industrial applications, including aviation (e.g., icing detection [32]), cold storage (e.g., food and medicine [33]), and process and machine monitoring. Distributed or quasi-distributed fibre-optic temperature sensors facilitate comprehensive monitoring using a single fibre [6]. However, for applications such as accurate process and machine monitoring, measurement uncertainty must be reduced to below 100 mK for meaningful data (e.g., by higher order FBGs). For other applications, such as aviation and the storage of delicate goods, a complete quality management chain is indispensable.
Some processes require rapid temperature measurements for optimisation. For example, in cryogenic wet-ice blasting [34] or avionic applications, air temperature needs to be measured at a high rate. Fibre sensors are ideal for this, as their small heat capacity provides a fast response time. Increased data sampling of calibrated interrogators is required to enhance temporal resolution of the complete measurement set-up. A significant proportion of monitoring applications rely on contact surface temperature measurements, where fibres are ideal due to their minimal heat capacity and conductivity. To this end, there is a clear need for further optimisation, characterisation, calibration and demonstration of these sensor technologies.
4 The INFOTherm project
4.1 Overview and need
The European project “Integrated European research, calibration and testing infrastructure for fibre-optic thermometry (INFOTherm)” [26], coordinated by PTB, brings together ten national metrology institutes, six research institutes, three universities, and seven industrial stakeholders to combine their expertise in temperature measurement and optical sensing to address these challenges (Figure 5). In addition to the further development of sensors, their characterisation and application in field tests, the aim is also to establish the associated calibration infrastructure by supporting standardisation processes. The project has been allocated funding from the European Partnership on Metrology programme for a period of three years.
The map illustrates the geographical distribution of the participating countries and institutions in the INFOTherm project. These include national metrology institutes (NMI) and designated institutes (DI), in addition to universities and research institutes, as well as manufacturers. Furthermore, the locations of the test cases at scientific test facilities and in industrial application are indicated, in addition to stakeholders supporting the project from its inception (Legend see figure).
As outlined in Section 3, fibre-optic thermometry has the capacity to address several crucial measurement challenges in the context of the clean energy transition. However, it should be noted that existing commercial fibre-optic systems are not traceable to the International System of Units (SI). Consequently, further research is necessary to validate fibre-based thermometry and enhance its appeal for utilisation in pivotal industrial applications. This is particularly true for applications related to critical infrastructure monitoring and control, advanced manufacturing processes, and quality control, where the maturity of the measurement techniques used, including independent verification, is paramount. This necessity is best met by calibration services provided by accredited calibration laboratories, National Metrology Institutes (NMIs), or bodies approved under the Measuring Instruments Directive (MID).
Fibre-optic thermometers have been demonstrated to be effective tools for measuring temperatures in the cases mentioned above. However, as already mentioned these sensors are susceptible to cross-sensitivities to other quantities, such as strain and humidity. The word clouds shown in Figure 6 provide a visual representation of the relevant quantities to be investigated for a full uncertainty budget. In order to exploit the full potential of fibre-optic sensors (e.g., Fibre Bragg Gratings (FBGs) in silica or sapphire fibres) or distributed sensing techniques (using Rayleigh, Brillouin or Raman scattering), it is necessary to investigate, minimise and quantify these cross-sensitivities so that reliable results can be obtained. Standards, calibration guides and services are required to integrate fibre-based thermometry into the existing energy system and infrastructure.
Illustration of the uncertainty budgets in fibre-optic thermometry for distributed methods and point sensors. The word clouds illustrate the respective contributions that need to be investigated, minimised and considered when providing measurement results (black text).
4.2 Objectives
The overarching objective of the INFOTherm project is to develop a research, calibration and testing infrastructure for fibre-optic thermometry measurements, with the specific aims listed below [26].
The development of accurate methods to quantify the sources of measurement uncertainty and cross sensitivity of existing fibre-optic thermometers. These will include: (i) disturbance effects, including thermal expansion, strain, vibration, ambient humidity or pressure; (ii) ageing effects; and (iii) the influence of sensor mounting.
The development of accurate and validated distributed temperature sensing (DTS) techniques for large-scale applications based on Rayleigh, Raman and Brillouin scattering or multiplexed fibre FBGs (see Section 2). Determination of the measurement uncertainty and spatial resolution of DTS methods, with the research objective of achieving an expanded uncertainty (k = 2) of 3 °C for temperatures up to 500 °C.
The development of validated fibre-based thermometry (e.g., FBG sensors) for high temperature process control with an expanded target uncertainty of 3 °C up to 1600 °C and accurate temperature monitoring at elevated temperatures (expanded target uncertainty of 1 °C up to 700 °C).
To use the techniques developed in Objectives 1 to 3 to carry out 12 case studies in key areas of application for fibre-optic thermometry: (i) monitoring of power cables and other parts of the network, (ii) control of energy intensive high temperature processes, (iii) monitoring of thermal storage loading conditions, (iv) monitoring of geothermal heat storage, and (v) NMI intercomparisons. To provide validated information on suitable fibre-optic thermometry techniques for specific applications, specific temperature ranges, spatial temperature resolution and achievable measurement uncertainties.
The long-term establishment of an integrated European metrology infrastructure is imperative, incorporating certified European training centres and accredited calibration laboratories for fibre-optic thermometry. This will include the development of testing and calibration guides. In addition, the project will facilitate the uptake of the technology and measurement infrastructure developed by the measurement supply chain (accredited laboratories, certification and accreditation bodies), standards developing organisations (IEC) and end users (sensor manufacturers, industry and energy sectors).
4.3 First results
4.3.1 Measurement uncertainty and cross sensitivity
Fibre-optic temperature measurement techniques, whether distributed or point sensors, are likely to be sensitive to a variety of parameters related to the conditions of their implementation. Regardless of the technique considered, it is appropriate to separate the two elements of the chain consisting of the sensing optical fibre or sensor (e.g., FBG or FPI) on the one hand, and the optoelectronic system (interrogator) used to interrogate and detect the response of the optical fibre sensor on the other.
Various factors have been identified that can influence the temperature measurement by fibre-optic sensors (see Figure 6), such as the relative humidity of the medium to be measured, the pressure applied to the sensor, mechanical constraints and in particular the strain or torsion applied to the measuring fibre or the sensor (whether localised or spatially extended), and discontinuities in the reflection of the measuring fibre due to the presence of connectors or splices necessary to connect the sensor to the interrogator.
To briefly discuss the underlying physical mechanisms of cross-sensitivities, FBG sensors will be used as a basis for comparison to keep the explanation clear:
The effect of strain along the fibre axis is twofold as it first affects the length of the fibre leading to change of the grating periode in FBGs (see Eq. 1). Second the reflective index changes with strain due to the elasto-optic effect. This typically results in a sensitivity of 1 pm/µɛ for a bare silica FBG, where a microstrain µɛ is defined as a relative length change of 10−6.
Humidity affects the fibre primarily through the coating of the fibre due to water absorption. This results in volume variation, which creates strain on the silica fibre and affects the measured signal, as explained above.
Pressure causes geometric changes to the fibre, resulting in strain. Due to the elasto-optic effect, this changes the refractive index. Furthermore, the polymer coating can act as a transducer, converting the applied transverse load (pressure) into axial strain. This influences the measurement, as explained above. The pressure sensitivity of the sensor depends on the Young’s modulus of the fibre and polymer, as well as its shape and diameter.
Similar effects occur with Rayleigh- and Brillouin-based sensing, as changes in geometry and refractive index affect the temperature measurement (see Section 2). In contrast, Raman-based measurements exhibit hardly any cross-sensitivity to strain and are mostly affected by changes in transmission along the length of the optical fibre. Therefore, attenuation caused by connectors, splices or bending results in perceived temperature steps in Raman-based DTS systems. In order to estimate the sensitivity of the different fibre temperature measurement techniques to these identified influencing parameters, the partner laboratories considered different approaches. Specific experimental means and associated methods were developed, as well as methods based on numerical modelling. A selection of these approaches and first results are described below.
In France, a measurement set-up has been developed to apply controlled strain to a section of optical fibre and simultaneously control the temperature up to 100 °C in a 6 m long tubular calibration furnace. The length of the homogeneous region allows the spatial resolution of almost any distributed temperature measurement system to be covered. This makes it possible to separate the thermal (temperature) and mechanical (strain) effects to which the measurement fibre is sensitive, for example due to variations in the refractive index and density of the core. In the case of interrogators that detect and analyse Brillouin scattering, where changes in temperature (ΔT) and strain (Δɛ) are two correlated variables, the independent variation of both allows accurate measurements of the temperature (C T ) and strain (C ɛ) sensitivity coefficients intrinsic to the sensing fibre. For example, in the case of a single mode fibre of the common type SMF-28, the change of the Brillouin frequency is
with the sensitivity coefficients measured to be
In order to characterise the influence and cross sensitivity of pressure and relative humidity of the surrounding atmosphere on point temperature measurement techniques, different set-ups have been developed in Slovenia, the Czech Republic and Germany, covering different parameter spaces as shown in Table 1. The sensitivity of fibre-optic sensors to pressure or volumetric strain (dn/dp) can be predicted using Young’s modulus, allowing the pressure sensitivity of temperature measurements using such sensors to be quantified before they are built. The predicted linear pressure dependence is currently being investigated in the parameter space shown in Table 1 as a function of temperature.
The parameter range of the set-up within the consortium to characterise the influence of different atmospheres (including pressure and humidity) surrounding the fibre on the temperature measurement.
| Temperature/°C | Pressure/bar | Atmosphere |
|---|---|---|
| 23 | 1…1000 | air |
| 23…700 | 1…7 | air, (10…90) %rH |
| 23…1000 | 0…80 | N2, Ar, N2+5 % H2 |
| (Purity: 99.999 %) |
These set-ups are also used to determine the effect of temperature and humidity on the ageing or drift of the fibre sensors. For temperature calibration and drift analysis, the fibre coatings (acrylate or polyimide) are chemically removed prior to packaging. Experiments with three different capillaries were carried out at the ice-point (see Figure 7(a)). Results show a stability over 24 h significantly smaller than the noise level of about ±20 mK for all three packaging solutions (see Figure 7(b)).
Drift of the measured FBG wavelength at the ice point for three capillaries used to protect the fibres. (a) Overview of 3.5 days. After three days, a significant proportion of the ice point has melted, leading to a substantial temperature increase. This makes the difference in heat dissipation between the capillaries apparent. (b) The 24-hour section shows that the stability over this period is significantly better than the noise level of about ±20 mK.
The data collated during this research will enhance our comprehension of the various sensitivities and cross-sensitivities of fibre-optic temperature measurement systems. The quantification of these sensitivities will facilitate the consideration of uncertainty budgets and recommendations to ensure the reliability and control of temperature measurements carried out using fibre-optic measurement systems, whether located or distributed. This work thus serves as a foundational basis for the subsequent research outlined in Sections 4.3.2–4.3.4.
4.3.2 Validated distributed temperature sensing
Various validation and calibration methods have been developed for DTS techniques, focusing on key sensing parameters such as spatial resolution and measurement uncertainty. For instance, a fibre-optic artefact has been developed to enable traceable distance calibration and to characterise the sensing performance of DTS systems working in time domain [13]. This artefact essentially functions as a fibre loop, but with the addition of an isolator, which permits light transmission in only one direction, as shown in Figure 8(a).
Fibre artefact used for distance calibration: (a) Scheme of the fibre artefact, (b) Illustration of the signal obtained from the artefact.
During calibration, an optical pulse is launched into the loop, traveling through multiple round trips, with the backscattered light recorded over time. The isolator allows the optical pulse to pass through, but blocks backscattered light from the fibre section after the isolator. Consequently, only the light that is backscattered before the isolator can be detected, resulting in temporal traces with a distinct interval between successive round trips, as shown in Figure 8(b).
Ideally, the temporal trace length should exactly match the loop section between the loop entrance and the isolator. However, discrepancies arise due to the sampling rate and pulse width used in the sensing systems. By comparing the actual loop length with the measured trace length, the spatial accuracy of different DTS systems can be determined and calibrated [13]. Furthermore, the signal decreases with each round trip due to loop losses, causing a corresponding reduction in the SNR. Analysing the SNR changes aids in evaluating the measurement uncertainty of distributed temperature sensing systems. This artefact has been successfully employed to assess both self-developed and commercially available DTS systems based on Rayleigh, Brillouin, and Raman scattering [13].
The measurement uncertainty of DTS systems is extensively investigated and characterised under various temperature conditions as part of the INFOTherm project [26]. Many DTS systems encounter cross-sensitivities, so the sensing performance, like uncertainty and precision, is influenced not only by the SNR of the system but also by other quantities. The impact of strain and humidity on distributed temperature measurements is studied both theoretically and experimentally across different types of sensing fibres. These findings on the dependence of measurement uncertainty and spatial resolution on the system SNR will offer valuable guidelines for achieving precise and accurate temperature measurements.
Additionally, the sensing performance of a DTS system with millimetre spatial resolution was compared with an FBG sensor over a broad temperature range, from 0 °C to 600 °C. A more comprehensive comparison is planned to assess the performance of various DTS systems and a quasi-distributed system that multiplexes a series of FBGs over a sensing fibre. Different systems will be evaluated in terms of measurement uncertainty, spatial resolution, measurement time, etc. Ultimately, the performance will be summarised in a matrix, providing a useful reference for end-users to select the most suitable sensing system based on their specific requirements and working conditions.
4.3.3 Fibre-optic thermometry for high temperatures
For the practical use of fibre sensors in a wide range of applications, different fibre optic sensing schemes for silica and sapphire fibres are being tested. An expanded uncertainty of 1 °C up to 700 °C is targeted for silica-based FBGs and 3 °C up to 1600 °C for sapphire-based FBGs.
A calibration set-up has been developed for the simultaneous calibration of several FBGs at high temperatures using a radiation thermometer. A graphite block is used to provide a uniform temperature inside a tube furnace. One side of this block is suitable for radiation thermometer based temperature measurement. The other side provides multiple slots for FBG sensors or capillaries. Alumina and silica based capillaries have been selected, each providing at least four slots for FBG fibres. In total, this setup provides the capability to calibrate 15 fibres in parallel.
Sapphire fibres are suitable for temperatures up to 1900 °C and are commercially available in lengths of several metres. There are a number of challenges to overcome for the practical use of sapphire FBG-based sensors. The air clad fibre and the resulting step index profile of the fibre, together with the large core size of 80 µm to 100 µm result in over 30000 modes theoretically transmitted by the fibre. In reality this number is slightly decreased due to surface imperfections of the sapphire fibre leading to a loss of near-surface modes. In practice, the number of modes also depends on the length of the sapphire fibre. This is because the multimode fibre used to connect to the sapphire fibre cannot excite all of its possible modes. The number of higher-order modes increases with the length of the sapphire fibre.
Figure 9 shows the spectrum of a single FBG in a sapphire fibre (S-FBG) and two FBGs in a silica fibre. Both sensor fibres have a comparable lenth of 75 cm. The full-width at half-maximum of the asymmetric S-FBG peak is around 4 nm compared to the 0.5 nm FWHM of the silica fibre.
Fibre-Bragg-grating spectra of a fibre optical sensor cable of around 75 cm length using a sapphire fibre (black) or a silica fibre (red).
Initial results show that calibration of these sensors is possible up to 1500 °C with an expanded uncertainty (k = 2) of 10 °C. In these experiments the reflection spectrum of the S-FBG was interrogated with a setup based on a superluminescent LED (SLED), a spectrometer, and a mechanical mode mixing unit (see Ref. [31] for further details).
In addition to the temperature measurement using the wavelength shift of the S-FBG, thermal radiation was studied as an alternative approach. This was achieved by measuring the background spectra without illumination from the SLED and by prolonging the spectrometer’s integration time to between 1 s and 30 s, depending on the temperature. This approach enabled precise monitoring of temperature changes, however, calibration remained impossible due to the unknown origin of the captured thermal radiation [31].
The thermal radiation is also part of the background of the spectra used to determine the wavelength shift of the S-FBG. With this setup, we demonstrated a sufficient signal-to-noise ratio up to 1600 °C. To prevent the thermal radiation from outshining the S-FBG reflection at higher temperatures, the setup is currently updated to operate at a wavelength of around 900 nm.
To further reduce the uncertainty for industrial application, a fully automated mechanical mode mixing unit was developed and tested for different coupling fibre configurations (see Figure 10(a)). The automation allowed full compensation for changes in the mode profile of the coupling fibre. An ice point test showed a reduction in this uncertainty component from 10 °C to 2 °C (k = 2) using the mechanical mode mixing approach. Repeatability was tested in a molten salt calibration bath (see Figure 10(b)) and showed that the measured temperature of the S-FBG fibre agreed with the temperature of the molten salt bath within its measurement uncertainty.
Test of an automated mechanical mode-mixing unit for sapphire-based fibre Bragg grating (S-FBG) sensors: (a) Different connection fibre configurations investigated for sapphire FBG interrogation. (b) Repeatability and effective resolution tested in a molten salt bath with repeated small temperature steps.
4.3.4 Case studies
Case studies are currently being planned and implemented, using knowledge, methods and sensor hardware being developed as part of activities described in the previous Sections 4.3.1–4.3.3.
In one of the case studies involving energy storage systems, a two-tank saline tank energy storage system is being studied in Denmark. Here FBG arrays will be mounted on interconnecting pipes to detect cold spots, giving an early warning on risk for pipeline blockages, overall thermal performance. A second case study in Spain involves a vertically installed fibre in a 5 m tall saline energy storage tank. In combination with Rayleigh scattering based distributed temperature sensing, this system will provide measurements that will be used for a variety of tasks, such as monitoring mixing and the level of thermal stratification, in turn helping to optimise charge and discharge cycles. Meanwhile in Germany, fibres cables have been installed in boreholes as part of an Aquifer Thermal Energy Storage (ATES) system. Distributed temperature measurements in such systems would be potentially useful for monitoring thermal losses to the surrounding rock, and for optimising energy injection and recovery strategies. In this project the installed borehole fibres will be also monitored for potential ageing effects on the fibre over time.
Regarding electricity power transmission systems, a study in Sweden is ongoing where fibre Bragg gratings are being used for thermal monitoring of high voltage electronics, and a second case study in Germany is progressing where thermal measurement are being carried out with Rayleigh and Raman based systems to monitor the thermal profile of transition joints. Such measurements would allow for accurate load regulation, and the avoidance of excessive heat, which can potentially lead to arcing, blowouts and fires. In Norway, Raman based distributed temperature sensing datasets are being analysed from fibres installed on national and international power transmission cables, buried in the ocean floor. Such measurements are potentially useful for ensuring loads do not cause excessive heating, and verifying whether a sufficient state-of-burial for protecting the cable is being maintained. Uncertainties and long-term stability are important factors in determining the usefulness of the data, and will receive particular attention as part of this case study.
In high temperature industrial process case studies, state of the art Sapphire probes are being applied. These sensors are capable of measuring temperatures up to 1700 °C, and target an uncertainty of ±3 °C. In Germany and Norway, the sapphire sensors are being use for monitoring glass and silicon production processes respectively. Reliable temperature measurements under such conditions were until now prohibitively difficult, and the measurements provided by the Sapphire fibre sensors can potentially save enormous amounts for energy through facilitating better control of these energy intensive processes. Fibre characterisation setups for FBG and Raman DTS, systems have been established at INFOTherm partners, and intercomparison investigations concerning these are currently being planned.
5 Conclusions
We have described the basic principles of fibre-optic thermometry and its advantages over conventional sensors in specific applications. Their current disadvantages were also discussed, such as lack of SI traceability and the need for further development concerning high temperature applications. Fibre-optic sensors are beginning to be used in several applications, covering optimisation and monitoring along the entire energy value chain from generation to transmission, storage and consumption. The European INFOTherm project aims to overcome many of the current obstacles so that fibre-optic thermometry can be used even more widely in the future. A selection of the preliminary results from the project was presented, with further investigation and publication of these results in dedicated publications to follow.
The main goal of the collaborative efforts of European National Metrology Institutes, research institutions and industrial partners in the field of fibre-optic thermometry, in particular the INFOTherm project, is that fibre-optic temperature sensors become as established in the future as electrical temperature sensors are today. This will allow their unique advantages to be fully exploited in a wide range of applications, which is particularly important in the context of many applications in the field of clean energy transition. More specifically, the situation envisaged for the future is as follows:
Firstly, several European National Metrology Institutes will be offering calibration services for fibre Bragg grating based temperature sensors and Raman based distributed temperature sensing systems according to common calibration guidelines.
Secondly, secondary calibration laboratories and sensor manufacturers will be offering calibration services traceable to the SI and supported by agreed best practice guidelines and standards. This will be leading to increased demand and business opportunities for European manufacturers.
Thirdly, new specialised sensor types (e.g., based on sapphire fibres or Fabry-Pérot interferometers) will be more mature and are more widely commercially available. This will be enabling new, sophisticated applications.
Fourthly, we envisage the increased use of fibre-optic sensors for condition and operational monitoring of electrical infrastructure (e.g., cables, transformers), which will be achieved through their validation. This will be contributing to a more resilient infrastructure able to cope with volatile electricity production and consumption.
Finally, we anticipate the increased use of distributed temperature sensors in enhancing the efficiency of large molten salt storage and geothermal heat production infrastructure. The aforementioned technological advancements are key to facilitating the transition to clean energy.
Funding source: European Partnership on Metrology
Award Identifier / Grant number: 22IEM07 INFOTherm
About the authors
Stephan Krenek is leader of the working group Photonic Thermometry at the Physikalisch-Technische Bundesanstalt. He graduated in physics at the TU Dresden. Since 2008, he is researcher at PTB, where he did his PhD in the field of radiation thermometry and the measurement of thermo-physical material properties. Since 2014, he is working in the Temperature department, where his current research focus is on the application of photonics for temperature measurement, e.g., with fibre optical sensing and integrated photonic sensors. He coordinates the INFOTherm project.
René Eisermann is a research scientist at the Physikalisch-Technische Bundesanstalt. He graduated in engineering at the University of Applied Sciences Brandenburg and in physics at the Brandenburg University of Technology Cottbus. Since 2010 he works in the field of photonic integrated circuits and fibre optical sensing. He joined PTB in 2019 and his research focus on the application of fibre optical sensing for localized and distributed sensing. He coordinates the non-distributed fibre sensing work in the project INFOTherm.
Guillaume Failleau is a senior scientist at LNE. He holds a degree in physics from the University of Le Mans. After completing his doctorate at LNE in the field of contact thermometry, he took over as head of research into high-temperature contact thermometry. Since 2014, in parallel with his activities in the field of radiation properties of materials, he has been focusing on the topic of distributed temperature measurement using fibre optics and contributes to European projects, standardisation bodies and industrial partnerships.
Xin Lu received the M.Sc. degree in optical engineering from the University of Electronic Science and Technology of China, Chengdu, China, in 2011, and the Ph.D. degree in electrical engineering from the Swiss Federal Institute of Technology of Lausanne, Lausanne, Switzerland, in 2016. From November 2016 to November 2017, he was at the Institute for Astronomy, Swiss Federal Institute of Technology, Zurich, Switzerland, as a Postdoctoral Researcher. He is working as a senior researcher at Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, Germany. His main research interests include distributed fiber sensing, nonlinear effects in optical fibers and adaptive optics.
Peter J. Thomas received the M.Sc. degree in Chemical Physics from the University of Bristol, Bristol, U.K., in 2004, and the Doctor of Engineering degree in Photonics from the University of St. Andrews, U.K., in 2010. He has previously held research positions at QinetiQ, U.K., and the National Physical Laboratory, U.K. Since 2010, he has worked at NORCE Norwegian Research Centre (Christian Michelsen Research before 2018), Norway, where he is currently a Chief Scientist. His main research interests are industrial applications of distributed fiber optic sensors, and optical sensors.
Henrik Kjeldsen received his M.Sc. degree in physics and chemistry from Aarhus University, Denmark, in 1995 and later, based on activities within experimental atomic physics, Ph.D. and dr.scient. degrees. He has worked in Aarhus, Paris and Oxford within the fields of atomic physics, accelerator mass spectrometry and synchrotron radiation. For the past 7 years he has held a position as senior project manager at the metrology department at Danish Technological Institute (DTI), working withing the fields of thermometer, humidity, and flow.
Klaus Anhalt studied physics at Philipps University Marburg, the University of Manchester Institute for Science and Technology (UMIST, UK), and Freie Universität Berlin. He earned his PhD in 2008 from the Technische Universität Berlin. Since 2002, he has been involved with PTB in the field of radiation thermometry, absolute radiometric temperature measurement, the development of high-temperature fixed points, and emissivity measurement above 800 °C. Klaus Anhalt represents PTB in EURAMET and CCT committees in the areas of radiation thermometry and thermophysical properties. In 2024, he became the head of PTB’s "Temperature" department.
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Research ethics: Not applicable.
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Informed consent: Not applicable.
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Use of Large Language Models, AI and Machine Learning Tools: DeepL.com to improve language.
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Conflict of interest: The author states no conflict of interest.
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Research funding: The project 22IEM07 INFOTherm has received funding from the European Partnership on Metrology, co-financed by the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. (Funder’s DOI: 10.13039/100019599).
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Data availability: The raw data can be obtained on request from the corresponding author.
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Articles in the same Issue
- Frontmatter
- Editorial
- Special Issue: Sensoren und Messtechnik für die Energiewende
- Research Articles
- Impurity measurements in hydrogen using laser spectroscopy: carbon monoxide as a case study
- Progressing TDLAS instrumentation for SI-traceable measurements of nitrous oxide in maritime applications
- Multi-range voltage divider for wideband power measurements
- Radon sensor networks for large buildings: balancing the trade-off between energy efficiency and health
- Fibre-optic thermometry to support the clean energy transition
- Dienstleistungen der Wasserstoff-Metrologie
- The global dimension of the energy transition
- The digital transformation of the quality infrastructure supports the energy transition
- Integrating energy data from a lambda architecture into a Digital Twin
