Research Papers

Double Modulation Pyrometry Applied to Radiatively Heated Surfaces With Dynamic Optical Properties

[+] Author and Article Information
Dimitrios Potamias

Solar Technology Laboratory,
Paul Scherrer Institute,
Villigen PSI CH-5232, Switzerland
e-mail: dimitrios.potamias@psi.ch

Ivo Alxneit

Solar Technology Laboratory,
Paul Scherrer Institute,
Villigen PSI CH-5232, Switzerland
e-mail: ivo.alxneit@psi.ch

Erik Koepf

Department of Mechanical and
Process Engineering,
ETH Zurich,
Zurich CH-8092, Switzerland
e-mail: koepfe@ethz.ch

Alexander Wokaun

Energy and Environment,
Paul Scherrer Institute,
Villigen PSI CH-5232, Switzerland
e-mail: alexander.wokaun@psi.ch

1Present address: Bioenergy and Catalysis Laboratory, Paul Scherrer Institute, Villigen PSI CH-5232, Switzerland.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received February 13, 2018; final manuscript received June 4, 2018; published online August 13, 2018. Assoc. Editor: M. Keith Sharp.

J. Sol. Energy Eng 141(1), 011003 (Aug 14, 2018) (8 pages) Paper No: SOL-18-1071; doi: 10.1115/1.4040842 History: Received February 13, 2018; Revised June 04, 2018

The accuracy of radiometric temperature measurement in radiatively heated environments is severely limited by the combined effects of intense reflected radiation and unknown, dynamically changing emissivity, which induces two correlated and variable error terms. While the recently demonstrated double modulation pyrometry (DMP) eliminates the contribution of reflected radiation, it still suffers from the shortcomings of single-waveband pyrometry: it requires knowledge of the emissivity to retrieve the true temperature from the thermal signal. Here, we demonstrate an improvement of DMP incorporating the in situ measurement of reflectance. The method is implemented at Paul Scherrer Institute (PSI) in its 50 kW high-flux solar simulator and used to measure the temperature of ceramic foams (SiSiC, ZrO2, and Al2O3) during fast heat-up. The enhancement allows DMP to determine the true temperature despite a dynamically changing emissivity and to identify well-documented signature changes in ZrO2 and Al2O3. The method also allows us to study the two dominant error sources by separately tracking the evolution of two error components during heat-up. Furthermore, we obtain measurements from a solar receiver, where the cavity reflection error limits measurement accuracy. DMP can be used as an accurate radiometric thermometer in the adverse conditions of concentrated radiation, and as a diagnostic tool to characterize materials with dynamic optical properties. Its simple design and ability to correct for both errors makes it a useful tool not only in solar simulators but also in concentrated solar facilities.

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Grahic Jump Location
Fig. 1

Schematic of DMP installed at the solar simulator. Flux (I0) from ten arc lamps is attenuated by a shutter, passes through chopper 1 before it converges on the sample. Surface radiosity within a fixed solid angle is collected by a lens, chopped at ω2 and then projected on the detector who's output is analyzed by the two lockin amplifiers to give the raw signals R and S. For details, see text.

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Fig. 8

Temperatures and reflectivity measured on Al2O3 during heating of the sample

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Fig. 10

Temperatures and reflectivity measured on SiSiC during heating of the sample

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Fig. 7

Relative spectral sensitivity of the DMP calculated from the spectral sensitivity of the detector (λ), the transmissivity of the filters, and the spectral irradiance of the Xenon arc-lamp

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Fig. 6

Calibration constant g1 for different configurations of active light sources and operation wavelengths

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Fig. 5

Modulation function m1(t) determined by Monte Carlo ray-tracing for all ten lamps used

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Fig. 4

Final design of chopper wheel. Region bounding the radiative flux from the solar simulator is indicated as filled ellipse (compare to Fig. 5, top).

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Fig. 3

Pathways (solid lines) where the external radiation I0 affects the response of the radiometric sensor. α(t) (absorbance) determines the temperature T(t). ε(t) (emittance) determines the magnitude of the thermal radiation. ρ(t) (reflectance) determines the magnitude of reflectance external radiation. For opaque samples, Kirchhoff's law of thermal radiation holds and α(t) = ϵ(t) = 1 – ρ(t) (dotted lines).

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Fig. 2

Equivalent circuit diagram of DMP. Signal generation (left) and detection (right) involve different processes distinguished by color. Green: Plank's law describing emission and, in its inverted form, detection of the thermal radiation. Blue: Modulation and demodulation of I0. Gray: Emissivity and its correction. Orange: Modulation and demodulation of the radiosity. White components are solitary and have no equivalent component on the other side. Symbols are explained in the text.

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Fig. 9

Temperatures and reflectivity measured on ZrO2 during heating of the sample

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Fig. 11

Evolution of reflectance (ΔTρ) and emissivity (ΔTε) induced temperature errors extracted from data of Fig 8

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Fig. 12

Ratio of thermal to reflected signals at all operating wavelengths as function of temperature based on the data presented in Fig. 8

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Fig. 13

A simplified cross section schematic of the reactor's cavity and window. The measurement spots of double modulation (light) and solar blind (dark) pyrometers are indicated on the sidewall of the cavity as are the thermocouple locations.

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Fig. 14

Temperature distribution of the glass window. The thermal emission can be neglected since Tw,max = 533 K ≪ 1500 K. Furthermore, one can deduce by observing the surface of the chopper wheel that its surface is much colder.

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Fig. 15

Temperature measurements during two heating cycles. The location of the sensors is indicated in Fig. 13.



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