r12.7 report on solar blind and active pyrometry system...
TRANSCRIPT
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Grant Agreement No. 228296
SFERA
Solar Facilities for the European Research Area
SEVENTH FRAMEWORK PROGRAMME
Capacities Specific Programme
Research Infrastructures
Integrating Activity - Combination of Collaborative
Project and Coordination and Support Action
R12.7 Report on solar blind and active pyrometry system
Due date of deliverable: Month 26
Actual submission date: Month 52
Organisation name of lead contractor for this deliverable: CNRS
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Contents
1. Introduction ..................................................................................................................................... 3
2. Optical temperature measurements ............................................................................................... 3
Description of the method .................................................................................................................. 3
Instruments for optical temperature measurements ......................................................................... 3
Techniques to calculate the body temperature .................................................................................. 5
Specifications for equipment .............................................................................................................. 6
Calibration issues ................................................................................................................................ 6
Main error sources .............................................................................................................................. 7
Literature ............................................................................................................................................ 8
3. A CSP specific problem: Solar blind pyrometry ............................................................................... 9
Presentation of the problem ............................................................................................................... 9
First solution: solar blind pyrometry — atmospheric or glass absorption .......................................... 9
Literature .......................................................................................................................................... 12
4. The emissivity problem, a possible solution: bi-‐Color pyro-‐reflectometry .................................... 13
Presentation of the bi-‐color pyroreflectometry ................................................................................ 13
Method ............................................................................................................................................. 13
Literature .......................................................................................................................................... 15
5. An alternative to solar blind measurements: active pyrometry .................................................... 16
Pulsed thermography ........................................................................................................................ 17
Phase lock-‐in active laser pyrometry ................................................................................................ 17
Evolution: temperature measurements in solar simulators and solar furnaces ............................... 17
Literature .......................................................................................................................................... 19
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1. Introduction This deliverable presents possible methods in order to measure the temperature of materials under concentrated solar irradiation such at the focus of solar facilities. An initial
This first chapter is derived from the chapter included in SFERA deliverable R12.4 Guidelines for Testing of CSP components, but with additional precisions. Reading other chapters from R12.4 such as temperature measurements with probes should be of interest to the reader.
2. Optical temperature measurements
Description of the method The temperature of materials can be determined by measuring the amount of radiative heat emitted, typically in the infrared spectrum.
The Planck Law allows to calculate the power hemispherically radiated by a local point of a black body at a given temperature for any wavelength.
𝐿! =!"!!
!
!!∙ !
!"#!!!!"# !!
(1)
Lλ Angular spectral luminance (W·m-‐2·sr-‐1·m-‐1) λ Considered light wavelength T Surface temperature of the body cλ Speed of light in the considered medium (m/s) h = 6,626 17×10-‐34 J.s Planck constant k = 1,380 66×10-‐23 J/K Boltzman constant
Conversely, by measuring the light power over the complete hemisphere for a given wavelength, one can therefore determine the temperature of the black body.
However, actual materials are not black bodies: part of this theoretically calculated power is not emitted. The emissivity coefficient compensates for this reduced emission. The emissivity coefficient depends on the considered wavelength, the temperature of the equipment, its composition, its surface aspect (polished, grinded…). Therefore, in order to determine the temperature with optical instruments, one must:
o Measure the luminance emitted from the sample
o Determine the emissivity of the sample for the considered conditions: measuring wavelength, body temperature, surface aspect, view factor or etendue.
Instruments for optical temperature measurements Two families of instruments can be referenced:
o Spot instruments typically called pyrometers.
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o Mapping instruments, usually called infrared cameras.
Typical sensitivity of different detectors technology depending on their temperature and the incoming
wavelength, from [Legrand 2002]
The technologies used for both instruments are similar, except for their organization in the device (single detector for a pyrometer, matrix or scanner for cameras):
o Thermal detectors: incoming radiation is used to heat the detector which then provide a signal. The wavelength sensibility depends on the coating used on the detector. Different technologies have been investigated for optical temperature measurements:
o Bolometers are RTD sensors: electric resistance variation due to the temperature of the sensor. Such sensors are used in pyrometers or in cameras as described in [WOO 1993].
o Pyroelectric sensors: the sensor delivers an electrical charge depending on the received electromagnetic radiation. The sensor composition is a ferroelectric material. These sensors, despite having lower sensitivity, are interesting as they are relatively fast and have wide wavelength sensitivity, even without cooling of the sensor.
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However, the sensor is sensitive only to variation of the incoming electromagnetic flux, so absolute measurements are not common with this technology.
o Thermopile sensor: the heated element is a thermocouple or a serie of thermocouples. Therefore, the incoming radiation leads to the measure of a voltage which depends on this incoming heat and a reference temperature (cold junction).
o Quantic detectors: the detector delivers a signal proportional to the number of received photons. Different signal can be delivered:
o Photovoltaic sensors: the incoming photons beam leads to a flow of electrical current. This is the technology used for everyday cameras: the technology is typically cheap, extremely fast, but usually for short wavelengths (visible or near infrared) thus high temperature.
o Photoconductivity: as the detector sensitive material absorbs incoming photons, its conductivity changes. This is the principle used in PbS, PbSe or InSb detectors.
Every technology has different characteristic of cost, speed, wavelength sensibility, signal to noise ratio, and R&D efforts are ongoing to improve all these aspects.
Techniques to calculate the body temperature The signal from the instruments, whatever their technology, are used in different ways in order to calculate the temperature of the considered body. Beyond the problem to get a trustable signal related to the luminance of the body in the chosen wavelength band, the problem of the lack of real knowledge of the emissivity property of the body has lead to the development of different techniques:
o Pyrometers are used to determine the average temperature of the sensed surface and infrared cameras to determine local temperature maps. The user has to provide the suitable emissivity from reference books or dedicated measurements.
o Bi-‐ or tri-‐color pyrometers can be used without knowing the emissivity. These devices are pyrometers operating at 2 or 3 wavelengths, and under the assumption that the emissivity of the material is the same at all these wavelengths, the redundant measurements allow to determine the temperature. However, this assumption is not always true despite the usual precautions (nearby wavelengths). The tri-‐color pyrometers are used to at least check this assumption, where bi-‐color pyrometers would always provide a result without hinting if it is realistic or not.
o Bi-‐color pyro-‐reflectometer are bi-‐color pyrometers that additionally measure the directional reflectivity of the material at both wavelengths. If the material is opaque and the bidirectional reflectance distributions are proportional between both wavelengths (whatever the incoming and outcoming radiation direction, the ratio is constant between the reflectivity taken at each wavelengths), the system of equation can be solved to determine the temperature without knowing the hemispherical emissivity. These assumptions, despite
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being weaker than for bi-‐ or tri-‐color pyrometry, are still not always validated, for example in case of coated materials.
Development is currently under way engineering cameras based on bi-‐color pyrometry and on pyro-‐reflectometer principles to determine temperature maps with fewer assumptions on the emissivity of the materials for more robust instruments than the current common single wavelength or bandwidth infrared cameras.
Specifications for equipment To choose a pyrometer:
-‐ Define your temperature range. Define both the minimal and maximal expected temperature. Few instruments are able to cover both very high temperature (> 2000K) and near ambient temperatures (< 500K).
-‐ Define your geometry. Define both the desired probed spot size and the distance from which you will install the instrument. Be aware that low cost pyrometers (less than a few thousand euros/dollars) typically have a low quality alignment of the provided aiming lasers and the actual position of the probed spot. You can check this by moving a diaphragm in the beam while probing a large hot surface such as a radiative plate. The source to instrument beam characteristic is the etendue.
-‐ Define your wavelength(s). Depending on both your material properties, the expected temperature and your environment condition, near or medium infrared are best suited. Generally, the lower the temperature, the longer the wavelength due to the Planck law: the body will radiate more energy hence a higher signal. The Wien’s displacement law can be used as a hint, as it gives the peak wavelength of the thermal emission of a perfect black body depending on its temperature:
𝜆!"# ∙ 𝑇 = 2898 µμ𝑚.𝐾
𝜆!"# peak wavelength of the black body thermal emission in µm
T temperature of the black body in K
If you plan front side measurements, that is from the side irradiated by the concentrated solar energy, choose solar blind wavelength without reflected light. Typical solar blind wavelengths are 2.7 µm (about 100 nm wide, due to water in atmosphere) or 4.3 µm (similarly narrow band, due to CO2 in atmosphere) and 5 to 7 µm (due to water and at the start to mirrors’ glass absorption).
Calibration issues While calibrating your pyrometer with a black body or checking in it with a radiative plate:
-‐ Always include the windows and filters that will be in your experimental setup.
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-‐ Reproduce as best as possible the geometry from your experiment such as distances, angles between the probed body and your instrument.
-‐ Atmospheric conditions: especially for solar blind pyrometry, check accordingly the ground H2O or CO2 levels during calibration to have them similar to those prevailing during your experiment.
Main error sources -‐ Emissivity
The emissivity is the ratio of light emitted by a body compared to the ideal black body. Ideally, the emissivity of your material should be measured. Tables from bibliography only give a possible range of the parameter, surface properties can make it change up to 50%, hence the determined temperature by similar ratio. Emissivity depends on:
o Chemical composition of the material.
o Physical state, including the crystal structure.
o Surface roughness: both low and high frequency shape compared to the pyrometer wavelength have an effect such as diffraction in the grooves.
o Temperature of the body.
o Coatings. Low thickness coating (a few microns) can be transparent or semi-‐transparent at your wavelength. You may measure emitted light from the coating, the body, or both; and there can be a thermal gradient between the body’s surface and the coating’s surface, leading to a complex mix of signals measured by the pyrometer.
-‐ Solar-‐blindness If your pyrometer is not completely solar blind (e.g. using an industrial pyrometer for the glass industry at 4.7 – 5 µm), residual reflected concentrated light will be added to the light emitted by your material. If the flux concentration and the material surface properties do not change, this can be dealt with as a systematic error and corrected, otherwise the temperature will be overestimated.
-‐ Dirt As any optical measurements, any dirt on the optics will change the results. As it is an infrared measure, there can be dirt not visible by the human eyes, such as residues from unsuitable cleaning liquids (soap…). Alcohol based liquid are advised, preferably iso-‐propanol, but check the suitability with your windows materials (some infrared windows could be dissolved) and your working wavelengths (no residues).
-‐ Noise Parasite reflection can impede the optical measurement, notably for low temperature bodies. Electric noise should be reduced as always with proper grounding and shielding.
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-‐ Multi color pyrometry Depending on the distance between the several wavelengths used by the pyrometer, all the above cited errors may be similar if the wavelength are near (no more than a few hundred nanometers) or the errors may be different if the wavelength are far from each other (several microns). In the latter case, the errors should be corrected or at least check individually for each wavelength used.
Literature D. Hernandez, G. Olalde, A. Beck and E. Milcent (1995): Bicolor pyroreflectometer using an optical fiber probe ; Rev. Sci. Instrum., 66, 5548.
P.B. Coates (1977): Wavelength Specification In Optical and Photoelectric Py-‐ rometry; Metrologia, 13, 1.
P.B. Coates (1981): Multi-‐Wavelength Pyrometry; Metrologia, 17, 103.
M. Schubnell, H.R. Tschudi and Ch. Muller; Temperature measurement under concentrated radiation; Sol. Energy, 58, 69, (1996)
SFERA Deliverable R12.4: Guidelines for Testing of CSP components
Etendue: http://en.wikipedia.org/wiki/Etendue
Woo R.A., Foss R.A. (1993): Micromachined bolometer array achieves low cost imaging, Laser Focus world, vol 6, pp101-‐106.
Legrand A.C. (2002): Thermographie multispectrale haute et basse temperature, Application au contrôle non destructif, PhD thesis, Univ. Bourgogne.
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3. A CSP specific problem: Solar blind pyrometry
Presentation of the problem If the equipment for which we want to know the temperature is irradiated by concentrated solar energy, the aforementioned instrument will measure both the self-‐emitted light from the equipment and reflected concentrated solar light. This is true even for apparently non-‐reflective materials or cavities (except if completely designed as a black body). If the intensity of the self-‐emitted light is relatively low even the poorly reflected intense incoming concentrating light can dominate it.
The bright spot at the focus of the concentrating solar facility comes both from reflection of the incoming strong light and from the thermal emission of the heated material: different methods must be used in order to sort the signals, such as choosing wavelengths with only on of the signal (solar blind pyrometry) or adding a controlled heating modulation (active pyrometry).
First solution: solar blind pyrometry — atmospheric or glass absorption
For such conditions, the wavelength for the measurements must be chosen for which there is no or little concentrating solar light. This is the case either in Earth’s or Sun’s atmosphere absorption bands, or on the concentrating optical system absorption bands as illustrated in the figure below.
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Such measurement are called solar blind as they don’t see solar radiation per design. Refer to [Hernandez 2004] to the investigation to multiple wavelengths suitable of high concentration solar furnaces where the problem is exacerbated.
Solar-‐blind temperature measurement developments for tower applications [Ballestrin 2011]: Solar spectrum based on a MODTRAN simulation, solar reflected spectrum, two band-‐pass filters, black-‐ body radiance at several temperatures, mirror reflectance, and quartz transmittance.
Different wavelengths are usually considered for solar blind measurements thanks to atmospheric absorption bands:
o From about 2.5 to 2.8 µm (typically called the 2.7 µm band): atmospheric absorption from CO2 and H2O.
o From about 4.2 to 4.4 µm (typically called the 4.3 µm band): atmospheric absorption due to CO2.
o From 5.5 to 7.3 µm: atmospheric absorption due to H2O.
o Beyond 14.2 µm: atmospheric absorption from CO2 and H2O.
open with direct solar irradiance of 1000 W m-2, peak irradiance at the focus [12] is 3034 kW m-2, total power is 69 kW and the 90 % power focal diameter is 26.2 cm.
3. Solar-Blind IR camera prototype
A new IR camera based on an InSb detector has been designed. This detector works in the 1.5-5µm spectral range and a software-controlled filter wheel enables the centered band-pass filters (Table 1) to be used on these two wavelengths bands (Fig. 2) creating a solar-blind IR camera.
Filter properties Filter 1 Filter 2
CW, Central wavelength (nm) 3320 4720
HW, Full width at half maximum (nm) 60 90
Table 1. IR camera band-pass filters.
Figure 2 shows the solar spectrum based on a MODTRAN code simulation [13], the solar reflected spectrum, two band-pass filters, black-body radiance at several temperatures, mirror reflectance, and quartz transmittance. Once the solar radiation is reflected by the heliostat mirror and concentrator, some solar radiation from the sample is reflected onto the IR camera (Fig. 1). This IR camera has two special band-pass filters, which avoid or minimize the reflected solar radiation from the sample. The low reflectivity of the mirrors over 3000 nm [14] allows wavelength bands to be defined where the solar radiation is almost negligible (Fig. 2).
Fig. 2. Solar spectrum based on a MODTRAN simulation, solar reflected spectrum, two band-pass filters, black-body radiance at several temperatures, mirror reflectance, and quartz transmittance.
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Atmospheric absorption bands in the infrared spectrum
These absorptions wavelengths depends obviously on the content level of the considered gas in atmosphere and the length of the light path through this atmosphere: for example, to measure the temperature of receivers installed at the top of solar towers, the instrument is usually installed hundred of meters away. For such applications, the H2O bands are not advised as the amount of water in the atmosphere changes a lot. Corrections are possible as it is relatively easy to measure the ground humidity, but it leads to greater uncertainties of the results. The CO2 bands are better for such applications, as the CO2 level is pretty stable in open space conditions.
When considering the light path from the Sun to the solar receiver, an other component can absorb part of the solar energy radiated by the sun: the mirrors. Indeed, even if the typical total thickness of the glass on the light path is just a few millimeters, this adds at least two possible windows for solar blind measurements:
o UV measurements
o 4.7 to 8 µm measurements, with confirmation depending on the actual glass used.
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Typical glass optical properties, from the users manual of the Heitronics KT15 pyrometer
When using such absorption bands, the change of the optical components should be monitored carefully. For example, when replacing broken mirrors with new ones, their thickness and their chemical composition may be different, leading to a different absorption of the solar radiation and errors on the temperature measurements.
Literature D. Hernandez, G. Olalde, J.M. Gineste, C. Gueymard (2004): Analysis and experimental results of solar blind measurements in solar furnaces; Journal of Solar Energy Engineering, Vol. 126, pp 645-‐653.
J. Ballestrína, A. Marzoa, J. Rodrígueza, I. Cañadasa, F. J. Barberob (2011): Two wavelength bands for IR thermometry, SolarPaces 2011.
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4. The emissivity problem, a possible solution: bi-‐Color pyro-‐reflectometry
Presentation of the bi-‐color pyroreflectometry The pyroreflectometry is a punctual technic which allow to measure simultaneously the true temperature and the emissivity of an opaque surface. The method has been developed at the CNRS-‐PROMES at two wavelengths. The principle of the pyroreflectometry is to get the true surface temperature of metallic target (with ε ≠ 1) by measuring both the bidirectional reflectivity, ρ’’, and the temperature radiance, TR, at two wavelengths. If these two wavelengths are chosen in solar blind windows, this method is destined to exhibit the best measurement performance for the surface temperature of CSP components. The development of such solar blind bi-‐color pyro-‐reflectometer is currently conducted at CNRS-‐PROMES.
With the CNRS-‐PROMES current non solar blind apparatus, the bidirectional reflectivity, ρ’’ is measured from pulsed laser diodes working at λ1 = 1.3 μm and λ2 = 1.55 μm. The radiance temperature, TR, is deduced directly from the total flux emitted by the target and collected by the photodiode by using the Planck’s law and a calibration of the apparatus with a blackbody.
The bicolor pyroreflectometry is not based on standard hypothesis such as Lambertian sources or grey body. Indeed the method introduces the diffusion factor η: the ratio of the hemispherical reflectivity ρ’∩ to the bidirectional reflectivity ρ’’. π corresponding to illumination direction of the target with a laser and the observation direction of the pyroreflectometer.
The main hypothesis is to consider that the diffusion factor is independent with λ1 and λ2. This key parameter is a thermo-‐optical property directly linked to the surface state:
o When η converges to 1, the surface is Lambertian: ρ’’ is constant for any incident and viewing direction.
o When η converges to 0, the surface is specular: ρ’’ is strongly dependent on the incident and viewing direction.
The invariance of η with wavelengths of measurement has been experimentally validated at the solar furnace with thermocouple for temperature validation and multi-‐directional reflectometer on several metallic surfaces [Hernandez 2009] (notably with the DISCO instrument).
Method Thanks to this approach, the advantage is to express the emissivity as a function of the diffusion factor:
ε = 1-‐ ρ’∩ =1-‐π.η.ρ’’ (2)
The measured radiance can be written has following for each wavelength:
R(T,λ) = (1−π.η(T).ρʹ′ʹ′(T,λ )).R(T,λ) (3)
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The Wien approximation of the Planck law is used in our case since this approximation is true as soon as λ•T << 14400 μm.K. The T as well as η can be then deduced by solving a system of two equations, respectively for λ1 and λ2 with two unknown parameters (T and η):
1/T =1/TRλ1 + λ1 /C2 ln(ελ1) =1/TRλ1 + λ1 /C2 ln(1−π.η(T).ρʹ′ʹ′(T,λ1)) (4)
1/T =1/TRλ2 + λ2 /C2 ln(ελ2)=1/TRλ2 + λ2 /C2 ln(1−π.η(T).ρʹ′(T,λ2)) (5)
��with C2 = (h.c)/kB = 1.4388.10-‐2 mK where h is the second Planck constant, c is the speed of light and kB is the Boltzman constant.
The solution converges to T*= T and η.
In order to study the sensitivity of the method on bidirectional reflectivity measurement, the error calculation has been evaluated from (4) and gives the relation:
∆!∗
!∗=
!∗∙∆!!,!
!!,!! + !∙!∙!
∗
!! ∙!∙ 𝜂 ∙ ∆𝜌" + 𝜌" ∙ ∆𝜂 (6)
In the analysis presented in [Delchambre 2011], it has been concluded that the technic is sensitive to the reflectivity measurement and the accuracy on T increases when the measurement of the bidirectional reflectivity, ρ’’, is optimum. In [Delchambre 2012], the parasitic effects of the radiation emitted from a plasma are studied: as this situation has similitudes to the use of non solar blind windows, the reader should refer to this work for further information, including pyro-‐reflectometry camera and tri-‐color pyro-‐reflectometry.
The choice of the working wavelength range is also of a main interest. Indeed, on the contrary to the standard bicolor thermography and/or active pyrometry where ∆λ = (λ2 – λ1) has to be maximised to reduce the uncertainty on the surface temperature [Loarer 2011], the error on temperature with the pyroreflectometry method depends on the choice of λ1 and λ2 (see equ. 6).
It is worth noting that the choice of wavelength was initially “due to their proximity (for diffusion factor invariance), to the silica optical fibers and the availability of industrial components of low cost in the spectral range” [Hernandez 2009]. However the choice of wavelength range can be modified and the choice of wavelengths (in order to keep the method applicable) has to take into account, in priority, the bi-‐directional reflectivity of the material.
Indeed the equation system can be solved only when the bidirectional reflectivity is different at both wavelengths in particular when ρ’’ λ1 < ρ’’ λ2 (λ1 < λ2) where mathematically there is only one solution (which is observed on most of the metallic surfaces) [Hernandez 2005].
When ρ’’ʎ1 > ρ’’ʎ2, two calculated solutions can be obtained. Thus, it is necessary to use a third wavelength to determine the convergence to a single point.
When ρ’’λ1 = ρ’’λ2, the surface is a grey body and the surface temperature can be calculated with the color temperature, TC, deduced from the standard bi-‐color pyrometry method:
TC = (1/λ1 – 1/λ2) / (1/λ1T1 -‐ 1/λ2T2)
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Literature The content of this chapter is greatly derived from the following paper with minor modifications:
E. Delchambre, MH. Aumeunier, T. Loarer, D. Hernandez, Y. Corre, E. Gauthier (2012): Performance of the Pyroreflectometry in Magnetic Fusion Devices for Plasma Wall Interaction monitoring, 11th International Conference on Quantitative InfraRed Thermography, 11-‐14 June 2012, Naples Italy.
The following report is a great and complete introduction and description of the bi-‐color pyro-‐reflectometry and the practical instrument developed:
D. Hernandez (2010): Bases de la pyroréflectométrie bicolore et presentation du pyref.
D. Hernandez et al., Rev. Sci. Instr. 80, 094903 (2009)
E. Delchambre et al., Phys. Scr. T145 (2011) 014078 (4pp)
T. Loarer, Contrib. Plasma Phys. 51, No. 2-‐3, 201–206 (2011)
D. Hernandez et al., Measurements 42 (2009) 836-‐843
D. Hernandez et al., Rev. Of Sci. Instr. 76, 024904 (2005)
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5. An alternative to solar blind measurements: active pyrometry Active pyrometry regroups 3 different methods, all based on studying the impact of a known heat solicitation in addition to the steady state of the body:
o Pulsed thermography: amplitude based (PT)
o Pulsed thermography: phase based (PPT)
o Lock-‐in thermography: phase based (PLT)
Application for temperature measurement for CSP applications is still under research: it should be noted that a main application of these methods is to detect the presence of defects in materials, by assuming good knowledge of their thermal properties: non destructive testing (NDE). This is valid for fields where the material abuse history is known with good confidence.
Pulsed thermography: log plot of the relative temperature increase vs time decay as typically used for non destructive testing, from [Balageas 2010].
The envisioned application here for CSP applications is mostly its counterpart: the materials are assumed perfect or nearly perfect (anisotrop, no defects), but with unknown or high uncertainty on their thermal properties, as it is the case for materials abused in operating facilities with little consistent knowledge of the extents of stress (thermal gradients due to cloud, environmental pollution…). The usual approach can also be used in order to determine the real optical and thermal properties of the material and then use these properties with pyrometers.
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Pulsed thermography A pulsed laser is used to induce surface heating in the materials: 1 – 50 ms long single shot or with low rate repetition. Fast pyrometers or imaging cameras are used to observe the induce thermal radiation emission from the component. 3D modeling is used to fit the observed behavior of the component to a theoretical but accurate thermo-‐optical model. [Shepard 2007][Rashed 2006][Li 2010].
Phase lock-‐in active laser pyrometry A pulsed laser (1 Hz – 1 kHz sinusoidal or square modulation) is used to induce surface heating in the materials. Fast pyrometers are used to observe the induce thermal radiation emission from the component. Lock-‐in amplifier are used to measure the amplitude and the phase of the observed thermal waves which brings knowledge about the material properties and state:
o The depth penetration of the thermal waves depends on their frequency. The faster the modulation, the more in surface the wave stays.
o The amplitude of the thermal waves depends on the surface emissivity, intensity of the heating laser modulation, and thermal properties of the heated material.
Principle of the lock-‐in method and thermal wave description as presented in [Melyukov 2010]
Evolution: temperature measurements in solar simulators and solar furnaces
Choosing solar blind windows may prove to be difficult or impractical in the field. One alternative is to modulate the incoming concentrated energy in order to identify the reflected component.
This method shares some similitudes to the phase lock-‐in active pyrometry, but the modulated energy does not heat the surface sample because it is chosen much faster than the surface thermal response of the body.
The modulation is not an additional laser but the modulation of the incoming concentrated solar beam: either modulation of the lamps electrical supply for a simulator or the use of a fast shadowing
PHASE LOCK-IN LASER ACTIVE PYROMETRY FOR SURFACE LAYER CHARACTERISATION OF TOKAMAKS WALLS
by D. Melyukov*, C. Sortais*, A. Semerok*, P.Y. Thro*, X. Courtois**, D. Farcage*
*CEA Saclay. DEN/DANS/DPC/SCP/LILM **CEA Cadarache, DSM/IRFM/SIPP Abstract
A lock-in method of surface layer characterization of tokamak walls is presented for determining the thermo-physical properties of a layer (like adhesion, thickness and others) deposited on a substrate, by comparing the experimental results of the phase shift between an excitation signal from a modulated laser and the thermal response of the layer, with the predicted phase shift obtained by a theoretical 3D model. A fast method of phase shift calculations for 3D analytical model was developed. The model validation is done on the stainless steel sheets of different thicknesses and the main experiments were performed on an ITER-like sample. 1. Introduction
In modern fusion reactors, the erosion of the plasma facing surfaces results in dust deposition on the tokamak “cold” surfaces, in form of layers which could trap tritium. Metallic surface coatings are also used on some components. To provide efficient operation of tokamaks, it is essential to characterize such layers. In-situ fast surface characterization without the reactor component disassembly is required. The lock-in pyrometry appears as a very suitable method: together with a laser (1 Hz - 1 kHz repetition rate frequency), applied for surface heating it can be used to characterize the micrometric layer properties, for example its adhesion to the substrate, its thickness and others.
The lock-in method is based on the propagation of thermal waves in the material and its interaction with discontinuities or non homogeneities. The depth penetration of thermal waves depends on their frequency. This makes it possible to study thermo-physical properties of materials. Any thermal wave has amplitude and phase. In case of contactless temperature measurements by pyrometry method, the amplitude strictly depends on the surface emissivity, heat flux intensity and other optical parameters that are unknown. But the phase of thermal wave is independent of these parameters. This advantage is often used in the thermal characterization of the materials.
Local laser heating of a layer causes a 3D thermal conduction regime and thus a 1D model becomes not valid. On the other hand the 3D theoretical models calculation takes very long time especially in the case of numerical solution. A fast method of phase shift calculations for the analytic theoretical 3D model of the laser heating of the layer was developed [1,2]. Comparison of both theoretical and experimental results makes it possible to determine the main layer parameters. The lock-in pyrometry system was developed and tested on a WLOH�VDPSOHV������ȝP�:-layer on CFC substrate and graphite tile with deposited layer of 10-���ȝP�WKLFNQHVV�RI�different adhesion).
Figure 1. Principe of the lock-in method and thermal wave description.
Laser t
Substrate
Layer
Reflected thermal wave
Passed thermal wave
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modulator for solar facilities. A fast pyrometer is used to observe the luminance from the sample, which is an addition of:
o The thermal emission of the sample;
o The reflectance of the incoming concentrated solar or lamps beam;
o Part of the time reflectance from the modulated additional energy from the concentrated beam.
From [Alxneit 2011]: Setup of the experiment with a solar simulator. (1) sample, (2) vacuum furnace, (3) lens, (4) optical fiber, (5) mechanical chopper, (6) diaphragm, (7) narrow bandwidth transmission filter, (8) PbS detector, (9) Hg/Xe short arc lamp, (10) function generator.
A lock-‐in amplifier at the frequency of the modulation allows to discriminate the thermal emission from the sample from the reflected light, allowing the calculation of the temperature of the material by assuming knowing its emissivity.
As this method doesn’t rely on absorption bands, it can be applied to determine the temperature of irradiated surfaces even with solar simulators which lamps have thermal continuous spectrum.
This method will be developed in SFERA II, WP12, with planned tests with a solar simulator and a solar furnace.
The complete principle is presented in:
Alxneit I. (2011): Measuring temperatures in a high concentration solar simulator – Demonstration of the principle, Solar Energy 85 (2011) 516–522
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Literature D. Melyukov, C. Sortais, A. Semerok, P.Y. Thro, X. Courtois, D. Farcage (2010): Phase lock-‐in laser active pyrometry for surface layer characterisation of tokamaks walls, ������������������������������������������������������������������ �����������������������������������������������������������������10th International Conference on Quantitative InfraRed Thermography, Quebec.
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