Welcome to the first newsletter of the INFOTherm project

One year ago, the European project "Integrated European research, calibration and testing infrastructure for fibre-optic thermometry" (INFOTherm) was launched with the objective of overcoming the limitations that currently prevent the widespread use of fibre-optic thermometry.

In this newsletter you will find the following: 

• Introduction to the project 
• Handling high temperatures with Razor Sharp Measurements
• DTS can help increase the efficiency of alternative energy sources, and thus contribute to the European Green Deal
• Fibre-optic thermometry is forging a metrological path for critical infrastructure monitoring

Introduction 

Importance of Temperature Monitoring
The measurement and control of temperature is a crucial factor in achieving the objectives outlined in the European Green Deal with regards to the transition towards a low-carbon energy system. Fibre-optic thermometry is an emerging technology offering distinctive measurement capabilities, largely due to the ability to conduct fully distributed sensing and its immunity to electromagnetic fields.

This makes these measurement methods an excellent solution to meet some of the challenges associated with the energy transition. The INFOTherm project addresses a comprehensive range of application areas, spanning the entire energy chain, from energy production and storage to the monitoring of the energy grid and power electronics, and finally to industrial consumers with energy-intensive processes.

Applications in Energy Storage and Geothermal Systems
For instance, energy from wind and solar resources can be stored when production is high and released when less electric energy is produced. This can be achieved through thermal energy storage, such as tanks of molten salt at temperatures up to 560 °C. To quantify the loading state and minimize losses through mixing, it is necessary to measure the temperature distribution in the tank.

It is also necessary to provide geothermal energy and seasonal storage of thermal energy in underground areas through multiple boreholes. The monitoring of such boreholes can significantly enhance system operation and integrity. However, to ensure reliable temperature measurements with high spatial resolution, they must be taken along a path, rather than just at a few points.

Monitoring Electrical Grid Infrastructure
Fluctuations in renewable energy production caused by local weather conditions increase the dynamics of the electrical grid and put additional stress on its components, such as power cables and transformer stations.

Thus it is becoming increasingly important to monitor temperature in order to ensure the efficiency and resilience of existing infrastructure. And it is therefore evident that measurements of this kind require instruments that are not affected by electromagnetic fields; fibre-optic thermometers represent an ideal solution in this regard.

High-Temperature Process Control
Furthermore, precise temperature monitoring and control is essential for the efficient operation and product quality of some high-temperature processes, such as silicon production for solar cells or semiconductors.

Conventional high-temperature thermometers, primarily thermocouples, have significant limitations due to ageing and drift. There is a clear need for stable contact thermometers capable of withstanding temperatures above 1600 °C.

Need for Traceability and Standardization
The lack of traceability of existing commercial fibre-optic systems to the International System of Units (SI), despite the evident need and the advantages of fibre-optic thermometers, underscores the necessity for further work to validate fibre-based thermometry and to make its use in these key industrial and public sector applications more attractive.

This will require independent verification and accredited calibration services. It is also important to note that fibre-optic thermometers, like all sensors, are susceptible to cross-sensitivities to other quantities. To fully leverage the capabilities of fibre-optic sensors, it is essential to investigate, minimise and quantify these cross-sensitivities, ensuring reliable results through the further development of standards and calibration guides.

The aim of the INFOTherm project is to meet these needs by: 

• Developing accurate methods for quantifying the sources of measurement uncertainty and cross sensitivities of existing fibre-optic thermometers.
• Developing accurate and validated distributed temperature sensing (DTS) techniques for large-scale applications, based on Rayleigh, Raman and Brillouin scattering or multiplexed fibre FBGs, with an expanded target uncertainty of 3 °C up to 500 °C
• Developing validated fibre-based thermometry for high-temperature process control with an expanded target uncertainty of 1 °C up to 700 °C (silica fibres) and 3 °C up to 1600 °C (sapphire fibres)
• Demonstrating the application of these techniques through case studies in key application areas for fibre-optic thermometry, including monitoring of the loading state of heat storage tanks and geothermal heat storages, monitoring of electrical power cables and other parts of the grid, and control of energy-intensive high-temperature processes.
• Providing validated information on suitable fibre-optic thermometry techniques for specific applications, temperature ranges, spatial temperature resolution, and achievable measurement uncertainties.

This newsletter outlines the steps we are taking to achieve our long-term goal of removing barriers to the widespread use of fibre-optic thermometry in various sectors. We are doing this by providing novel, traceable temperature measurement solutions where temperature measurement and control play a key role but are inaccessible to conventional electrical methods. The aim is to make the energy infrastructure more resilient and reliable in the face of future challenges.

Kind regards, 

Dr. Stephan Krenek
Coordinator of the INFOTherm project

Further details can be found at www.infotherm.ptb.de.

The project INFOTherm (22IEM07), founded by the European Union, involves a multidisciplinary team forming a consortium (https://www.infotherm.ptb.de/consortium) with several European National Metrology Institutes, University and Research Centers and industrial partners with their specific requirements to measure temperature in High-Voltage cable joints, transformers, power electronics and measurement during chemical processes.

Handling high temperatures with
Razor Sharp Measurements

INFOTherm designs hair-thin temperature sensors for demanding industrial environments. Fabry-Perot sensors promise 50 mK precision up to 800 °C.

Temperature measurements play a key role in ensuring product quality, optimizing processes and protecting human health. Optical measurement methods and systems provide new approaches to the measurements of non-electrical quantities that are often associated with challenging technological, biomedical, production, scientific, transport, environmental, security and other application fields and systems. 

Lack of appropriate standards for optical sensors
Despite very intense research in the past, only a limited number of optical-fibre sensing technologies have made a successful transition into practical applications. This is a consequence of complex and mass production incompatible concepts that often result in poor cost-to-performance ratios of existing fibre-sensing technologies, which has also led the absence of appropriate standards for optical sensors, such as for example those available for RTD temperature sensors and thermocouples.

However, the continual evolution of temperature measurement technologies contributes to improved accuracy, efficiency, and adaptability, making them indispensable tools in a variety of sensor systems.

As measuring of high temperatures with high resolution in different harsh environments continues to present a major and broad challenge, research group from University of Maribor decided to look for a possible solution in the form of a Fabry-Perot (FP) temperature sensor. The sensor shown in Fig. (a) utilizes Fabry-Perot Interferometer (FPI) constructed between two semi-reflective mirrors, placed at the tip of a large 200 m lead-in single mode optical fibre (SMF).

Sensor design and flexibility
Large lead-in fibre was extended with short section of standard 125 m SMF, which ensures an airgap between the capillary wall and the sensor wall (Fig. (c), thereby ensuring no strain transfer to the sensor cavity in different packaging applications. FP temperature sensors offer flexibility in miniaturization and signal interrogation which is a major limitation in reducing the overall cost of optical sensor systems today.

Temperature sensor: (a) sensor design, (b) fabricated sensor packaged in sealed quartz capillary.

Expected results and timeframe
The expected timeframe for finishing the project is end of August 2026, by when we commit to deliver results on Fabry Perot temperature sensors that are fully dielectric, miniature in size (sensor tip diameter slightly larger than human hair and shorter than 1 mm), can reliably operate in the temperature range up to 800 °C with a resolution at least of 50 mK.

DTS can help increase the efficiency of alternative energy sources, and thus contribute to the European Green Deal 

Becoming the world’s first climate-neutral continent by 2050
is a once in a lifetime opportunity to modernize the EU economy and society and re-orient them towards a just and sustainable future. 

Molten salt as energy storage system 
Advances in renewable energy production require an efficient and sustainable energy storage system. The Plataforma Solar de Almeria (PSA)in Almería, Spain is a research centre owned by the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) whose main goal is the development and testing of new technologies based on solar energy. 

The focus for this test will be the thermal storage tanks found there. The purpose of these infrastructures is to store large quantities of molten salt, saving the energy produced in concentrated solar power plants.

MOSA facilities in Plataforma Solar de Almeria (CIEMAT), Spain. In the right, we can see the molten salt storage tank to be monitored with DTS.

Measuring of temperature 
One of the key issues is properly measuring the temperature of the contents inside the tanks. Traditionally, thermocouples have been used for this purpose. The INFOTherm project proposes to replace this outdated temperature monitoring configuration with a new distributed temperature sensing (DTS) system based on fibre optics. The Centro Español de Metrología (CEM) and CIEMAT are responsible for this activity.  

Some of the advantages of installing a DTS system include achieving a temperature resolution of tenths of a degree with a spatial resolution of less than a centimetre. This means that in five metres of fibre, we would obtain more information than with fifty thermocouples distributed over the same length.

Another advantage is that thermocouples require cumbersome and difficult-to-maintain wiring and connections, whereas with a DTS system, we only need a fibre and a stand-off cable to connect the sensible fibre to the interrogator, which is the device used to stimulate the fibre to obtain the temperature measurements.

As a result of this substitution, we expect a reduction in cost and occupied space, along with an increase in spatial resolution, all without compromising the quality of the measurements.  

Calibrating the fibre optics system
One of the biggest challenges is the calibration of DTS fibre optics system. CEM is firmly committed to developing DTS calibration facilities and exploring new protocols to perform the calibration.

Their strategy consists of two stages: In the first stage, influencing variables in controlled environments will be studied and characterized, evaluating the impact they could have on the measurements. This type of thermometers is very susceptible to strain in the fibre, harsh environmental conditions and vibrations. 
 

Calibration facilities at CEM, Spain. In the pictures we can see a liquid bath and a thermostatic chamber used for DTS characterization and calibration. (left: thermostatic chamber. Right: Liquid bath).

In the second stage, CEM will perform a calibration by comparison in the range of 0°C to 600°C using type R thermocouples and PT-100 thermoresistors, calibrated at fixed points up to aluminium (660,3°C), providing traceability to the measurements. 
 
Installation in the tank
Another significant challenge is the installation of the fibre in the tank. Molten salt is highly corrosive and is in constant motion due to convection currents and pump suction at the bottom of the tank. The sensing fibres must be protected from the salt and placed in a location where the movement of the liquid is mitigated.

Additionally, the protection must be chosen so that the differences between the protected fibre and the bare fibre are negligible.  

In this project, it is planned to install the fibre through a stainless steel tube in the centre of the tank. However, in the future, other options may also be considered, such as attaching the fibre to the walls or the bottom of the tanks where temperature gradient induce thermal stress that may lead to tank rupture.  

Evaluation and analysis of data
The expectation is to install the DTS in 2026. From that date, CEM and CIEMAT will focus on evaluating and analysing the data obtained, comparing it with the data from the previous monitoring system. If this project is successful, it is expected to result in a new calibration method and facilities for DTS systems calibration at CEM.

Additionally, it will provide a way to simplify temperature monitoring in molten salt tanks, potentially opening a research line for CIEMAT on the use of fibre for energy loss quantification during storage. In the future, this could help in building more efficient storage tanks and components. 

Fibre-optic thermometry is forging a metrological path for critical infrastructure monitoring

The INFOTherm consortium seeks to establish a metrological infrastructure that supports the use of fibre-optic thermometry for monitoring various types of critical infrastructure.

Ultimately, this new approach will enable a common and time-efficient testing standard for distributed temperature interrogators that could be used by calibration laboratories across Europe.

Lacking metrology infrastructure
The applications of fibre-optic thermometry are strongly focused on monitoring the well-being and operation of critical infrastructure. Nevertheless, national metrology- and designated institutes in Europe are completely in lack of calibration services in the field, meaning that end-users must rely solely on the Q&A of the fibre-sensor manufacturers. Before the technology becomes even more widespread, this lack of third-party accredited testing is a challenge we must start addressing. 

Consisting of national metrology institutes, technological institutes, and research institutes across Europe, the INFOTherm consortium seeks to establish a metrological infrastructure that supports the use of fibre-optic thermometry for monitoring various types of critical infrastructure.

Fiber-artefact standards: a new metrological tool for distributed sensing
Led by DFM, the national metrology institute in Denmark, we have come up with a new metrological method, based on a fibre artefact standard, that allows us to simultaneously calibrate all the most relevant aspects of distributed temperature sensors in a single interrogation run.

Beyond calibration of sensor temperature response, the fibre artefact approach importantly also allows traceable calibration of the distance output of the measurement device and a quantification of the achievable spatial resolution. 

The future of temperature measurement
Fiber-optic thermometry is a disruptive technology, which, although still in a maturing phase, finds a growing number of application areas every year. With simultaneous large spatial coverage and high density, long lifetime, and electro-magnetic passivity and immunity, fibre-optic thermometers hold some remarkable advantages over conventional temperature point sensors.

These features have allowed the technology to be implemented for diverse applications such as distributed fire- and heat detection in tunnels, high-voltage power cable monitoring, and underground geothermal investigations. 

Time-efficient testing standard coming up
As of right now, the new fibre-artefact methodology has only been implemented in DFM’s lab in Hørsholm, Denmark, and tested using a home-built version of a Brillouin optical time-domain reflectometer.

In future work to be carried out within the timeframe of INFOTherm, we will focus on tests with commercial interrogators, construct additional fibre artefacts for other INFOTherm partners, carry out bi- and multilateral inter-comparisons, and tweak the fibre artefacts to other forms of distributed temperature sensing based on Raman- and Rayleigh scattering.

In the end, this new approach will enable a common and time-efficient testing standard for distributed temperature interrogators that could be used by calibration laboratories across Europe.