X Phys. IV France 9 (1 999) Presentation of a solar reflectometer coupled with a solar concentrator D. Hernandez, G. Olalde, J.M. Gineste, D. Antoine, P. Seimpere and M. Clement* lnstitut des Materiaux et Procedes (IMP), Centre National de la Recherche Scientifique (CNRS), BP. 5, 66125 Font-Romeu, France * Societe Europeenne de Propulsion (SEP), FoBt de Vernon, BP. 802, 27208 Vernon, France Abstract : Devices and methods are presented in which an optical fiber reflectometer and a solar concentrator are used to determine solar reflectivity and absorptivity for opaque and diffuse materials. The measurements can be taken at high temperature, up to 25OO0C. First the specific reflectometer is presented. We will then describe the whole experimental hardware (solar installation, associated devices) and the method used to determine solar reflectivity and absorptivity. Finally, we will present examples of results obtained on a metallic sample. 1. INTRODUCTION In the fields of space, aeronautics and high technology, the optimal use of material and the control of physical phenomena are important challenges and need a very complete characterisation. In particular, the estimation of thermal radiative exchanges requires the knowledge of the thermoradiative properties in condition of use. The problem of determining these properties at high temperature remains largely unresolved [l] but it is notable that interesting capabilities exist for solar installations [2], [3]. Procedures and methods for joining a solar reflectometer and a solar installation are applied to determine solar absorptivity from ambient temperature to 2500 C for opaque and dffise materials. The solar installation is used as a source of heating and illumination, the optical fiber probes and detection devices, for directional reflectivity and temperature measurements. 2. THE OPTICAL FIBER SOLAR REFLECTOMETER The solar reflectometer enables the measurement of the solar directional reflectivity of the sample illuminated with concentrated solar radiation. The diagram of the reflectometer (Fig. 1) includes five main parts. The sam~le suvvort (A) holds the sample inside the focal volume of a solar concentrator during the measurement. It is protected by a water cooling system, so that it can operate under high flux (1400W/cmZ). A sample (3 mm thick and 25 mm diameter) (4) is located inside a specific hole which allows heating through concentrated solar radiation (67, on one side, and measurements without parasitic solar radiation on the other side. On the measurement side there is also, near the sample and with respective faces in the same plane, a reflectivity reference (5) for the measurement of the reflectivity by comparison between both. The principles of the measurement method are given in [4] and [5]. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1999397
Pr3-612 JOURNAL DE PHYSIQUE IV The ovtical fibers removable support (B) is a water cooled device and it has two displacement functions, one to move the probes from the heated sample to the reflectivity reference (a), the other to control the distance to the sample (b). The optical fibers (1 and 2), inside mechanical protection (3), are arranged with their observation optical faces tangent to the same circle. One of the fibers (I), with normal position (0 ), is linked to the emitting module (C) and illuminates the sample, the others (2), with positions in step intervals of 10" (from 0" to SO0), collect the reflected fluxes and are linked to the detection module @). lgg.1 Solar Reflectometer Diagram A: Sample suppo* B: Optical fibers removable support, C: Emitting module, D: Detection module, E: Computer 1:Illumination O.F., 2:Obse~ng O.F., 3:Mechanid protection of O.F., 4:Sample, %Reference, 6:Parallel solar beams, 6':Concentrated solar beams, 7:Convergent lens, 8:Beams shutter, 9:Thermopile, l0:signal amplification electronics, I 1 :Computer connection. The emitting module (C) is a convergent lens (7) that concentrates parallel solar radiation (6) (reflected by the heliostat (Fig.2)) into the illumination fiber (1) which is in a normal position relatively to the sample. This allows to obtain a rather intense source (0.3W) which can be switched on or off by using a removable shutter (8). The detection module (Dl consists of observing optical fibers is connected to a water cooled photopile (9) selected for its flat spectral sensitivity (thermal detector). The photopile is the transducer, the electronics (10) amplifies and then transmits the proportionnal signals to the computer (1 1). The comvuter E) records the measured signals, analyses them to determine reflectivity and absorptivity, and controls the displacement of the optical fibers removable support (B) and the beam shutter (8). 3. DETERMINATION OF THE SOLAR ABSORPTMTY AT HIGH TEMPERATURE 3.1 Description of the solar installation and associated devices The set up used to measure solar reflectivity and absorptivity is presented in Fig.2. It is essentially composed of a solar installation coupled to the reflectometer.
STCT 9 Fig.2 Measurement set up diagram l:heliostat, 2:Tracking system, 3:Parallel reflected flux, 4:Attenuator, 5:Shutter, 6:Emission module, 7:Concentrated flux, 8:Parabolic mirror, 9:Orientable table, 10: Sample support, 11 :Water cooling circuit, 12:Illumination fiber, 13:Detection module, 14:Obse~ation fibers, 15:Computer, 16:Optical fiber support, 17:Pyroreflectometer, 18: Temperature probe. The solar installation is a horizontal axis type. The incident solar radiation is collected through a square heliostat (1) (3 m side) driven by an optoelectronic tracking system (2) and the reflected flux (3) is regulated with an attenuator (4). A very small part of the parallel flux (3), which can be switched on or off by a shutter (9, is kept by the emitting module of the reflectometer (6) and the largest part is concentrated (7) through a parabolic mirror (8) (2 m diameter and 0.90 m focal distance) over an area with a 15 mm diameter (approx.). The elements of the reflectometer are located on a three axis orientable table (9). In this way, the sample support (lo), linked to a water cooling circuit (1 I), can be moved on the focal volume of the concentrator. On the front face, the sample is heated by the concentrated radiation (6) and, on the rear face, is lit by the illumination fiber (12). The observation fibers (14) link the optical fibers support (16) to a water cooled detection module (13), itself connected to a computer (15). Both are located outside the solar radiation area. The computer processes the signals sent by the detection module and drives the shutter (5) and the optical fibers removable support (16). All the measurements are made on the rear face of the sample without parasitic solar radiation (area shadowed by the emitting module and the sample support) and the temperature is determinated through a bicolor optical fiber pyroreflectometer (17) [4] whose optical fiber probe (18) is located on a normal position ofthe optical fibers removable support (16). 3.2 Measurement method for determining the solar absorptivity This system is used to determine the solar reflectivity and absorptivity at high temperature for opaque and diffusing materials.
Pr3-614 JOURNAL DE PHYSIQUE IV The method is based: - for the reflectivity, on measurements by comparison between the sample, whose properties are unknown, and a reference. More particularly, the solar bidirectionnal reflectivity corresponding to the reflectometer is defined by the equation : where Ta are the atmospheric transmittance and the transmittances of the elements, x,.f. is the optical fiber transmittance, exp(-kaw4d) giving an account of Rayleigh diffision. x, is a measured value, xo a reference value and xe a solar value. - for the absorptivity on the complementarity relation, available for opaque materials: A step by step description of the measurement procedure is given below: First step: At room temperature, the illumination optical fiber lights perpendicularly the reflectivity reference and the observing fibers collect the reflected fluxes on the different angular locations (Fig. I). The quasi-lambertian reference has a known spectral reflectivity constant on the spectral measurement range (0.3 to 2.15 ym). Thus, for each observation angle, the computer associates the signal Df(A2c3,Or) quantified by the detection module to the known reference value p2". Second steo The optical fibers removable support is located in front of a sample surface at the same distance as the reference. The sample, heated by an adjusted concentrated solar flux (attenuator and location of the sample support in a focal volume) reaches an equilibrium temperature. The light fiom the illumination fiber is switched off by the shutter and the observing fibers collect the self emitted sample fluxes. The computer stores the signals corresponding to the self emission DE(AA~,Br,T). At the same moment the pyroreflectometer determines the temperature T of the sample surface which is also stored. Third step The removing sample support is kept in front of a hot sample surface and the ligth of the illumination optical fiber is switched on by the shutter. The observing fibers collect self emitted plus reflected fluxes fiom the surface of the sample at the same temperature T. The computer memorizes the signals corresponding to both phenomena D~+~(A&, Or, T). Fourth step The computer processes the acquired measurements. A comparison between the signals obtained on the reference and on the sample gives the bidirectionnal reflectivity of the sample according to the following equation: P$ (A&, T) = ( [ D~+~(AL@, Or, T) - D~(ALB, Or, T) I I D$(A~@, or) 1~2'~ (3) By integrating reflectivity: the directionnal values the computer determines the hemispherical
STCT 9 Pr3-615 and finally sends the solar absorptivity in the measurement conditions according to the complementarity relation for opaque materials (2). The whole procedure is carried out under four seconds and it is repeated for different temperatures T of the sample through the solar concentrated radiation level. For serial measurements on the same material step one can be carried out only once. 4. EXAMPLE OF EXPERIMENTAL RESULTS The main purpose of this paper is to present the method and the experimental set up used to determine the solar absorptivity at high temperature usiig a solar furnace and an optical fiber reflectometer. The given results are just an illustration. Measured reflected flux (V) 90 Angle " -5 Fig.3 Example of results obtained on brass sample 1:Measurement at 20 C, 1': Representation of a lambertian behaviou corresponding to the curve 1, 2: Measurements at 530 C, 3:Measurements at 614OC, 4:Measurements at 732OC, 5:Calculated results for each of the four curves (p solar normal-hemispherical reflectivity, a solar normal absorptivity). The sample is a brass disk treated so that it has a rough surface for diffuse reflection. It has been heated at three temperature levels and results are represented in Fig.3 which shows reflected flux values as a finction of the observation angle, and two columns which give solar reflectivity and absorptivity.
Pr3-616 JOURNAL DE PHYSIQUE IV The analysis of the curve allows us to estimate the diffise behavior of the surface in order to validate the measurement. Results show that the method and system allow evolution of the brass properties to be followed closely. Because of oxidation of materials the absorptivity increases and the surface becomes more diffise. 5. CONCLUSIONS The association of a solar concentrator and an optical fiber reflectometer to determine solar reflectivity and absorptivity of opaque and diffise materials appears to be promising. In this preliminary work, measurement techniques and devices have been presented and tested successfully on diffise surfaces. However, the errors have not been evaluated. They are driven by intrinsic parameters, such as the numerical aperture of the fibers (non directionnal measurements), the spectral distribution of the source (simulation of a solar source by the illumination fiber) and the control of the geometrical configuration (comparison method) [6]. The apparatus and methods have, nonetheless, very interesting capabilities. They are well adapted to determine, for 'in situ' conditions at high temperature, parameters which are normally difficult to obtain, such as solar reflectivity and absorptivity. Currently, our main task is to improve the reflectometer in order to provide measurements on specular surfaces so that these methods can be used for any type of opaque materials. References [I] J.F. Sacadura, 'Measurement techniques for thermal radiative properties' in Proceeding of the Ninth International Heat Transfer Conference (Hemisphere, New York, 1990), Vol.1, PP. 207-22. [2] Valery V Kan, Tookhtapylat T Riskiev, Temur P Salikhov, 'The probing-flash method in the study of the radiation properties of refractory materials' High temperatures-high pressures, 1992, Vol. 24, PP 659-64. [3] J.R. Markham, A. Lewandowski & all, 'FT-IR measurements of emissivity and temperature during high flux solar processing' Journal of Solar Energy Engineering, February 1996, Vol. 1 18, PP 20-29 [4] D. Hernandez, G. Olalde & all, 'Bicolor pyroreflectometer using an optical fibre probe' Rev. Sci. Instrum, December 1995, Vol66, N012, PP. 5548-51. [5] D. Hernandez, G. Olade & all, 'Characterization of an optical-fiber reflectometer for in situ measurement applications', Applied Optics, December 1995, Vol. 34, No 34, PP. 7952-58 [6] D. Hernandez, D. Antoine & all, 'Optical fiber reflectometer coupled with a solar concentrator to determine reflectivity and absorptivity at high temperature', Journal of Solar Engineering, to be published.