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A method for spectral analysis of concentrated solar radiation
Pergamon PII:
Renewable Energy, Vol. 10, No. 2/3, pp. 197-201, 1997 Copyright IQ 1996 Published by Eisevier Science Ltd Printed in Great Britain. All rights reserved so%&1481(%)ooo63-8 09%1481/97 S15.00+0.00
A METHOD FOR SPECTRAL ANALYSIS OF CONCENTRATED
SOLAR
RADIATION
A. Neumann, J. Kaluza
Deutsche Forschungsanstalt fi,ir Luft und Raumfahrt e.V. (DLR) Hauptabteilung Energietechnik D-5 1140 Kiiln
ABSTRACT In the year 1994 the DLR in Cologne inaugurated a 20 kW solar furnace for technological and scientific experiments. One of the main applications of the solar furnace is the research on photochemical reactions using concentrated sunlight. For this purpose exact knowledge of the spectral composition and its local distribution is essential. The evaluation of experiments and the validation of computer codes rely on these informations. The solar radiation that is concentrated to high flux densities (up to 4 MW/m2) shows shorttime fluctuations and therefore a special measurement technique is required. It is shown how to use the imaging properties of a Czemey-Turner monochromator in combination with video camera detection to achieve a fast spectral analysis. This measurement is actually done along an intersection line through the focus. Moreover the calibration is described and first test pictures are analysed. Copyright 0 1996 Published by Elsevier Science Ltd.
KEYWORDS Solar furnace, spectral analysis, imaging spectrometer, astigmatism of monochromators INTRODUCTION Starting in the year 1991, a high flux solar furnace was built at the DLR research center in Cologne to allow high flux experiments in the field of solar chemistry and material research. The solar furnace uses a 52m2 flat heliostat to track the sun and a concentrator composed of 147 spherical mirror facets to focus the solar radiation in a spot of about 13cm diameter. At an
197
198
A. NEUMANN and J. KALUZA
insolation of 850 W/m2 it is possible to collect a total power of 20 kW with peak flux densities of 4 MW/m2. The measurement system that is used for temperature and flux density measurement in the solar furnace is called FATMES [I] . The flux density mapping is done by the white target technique. This technique is based on recording the image of the beam generated on a target screen. This target is coated with a diffuse reflecting surface and is positioned in the light path. The reflection pattern is monitored with a calibrated camera.
THE PROBLEM The solar chemistry research investigates with the solar furnace photochemical reactions under high concentrated radiation. Besides the total beam power the spectral content of the beam is of great importance. Actually, a computer code is used to calculate the spectrum of the concentrated solar radiation. But this simulation does not give information about the local distribution of the flux. Furthermore, up to now there are no means to validate the simulation. The main optical processes that affect the solar radiation on its way through the atmosphere, passing the heliostat and the concentrator, and reaching the focus are wavelength dependent. Therefore, a homogeneous local and spectral distribution may not be expected. Within this work a spectrometer was developped that resolves the spectrum in the wavelength band which is of importance for the photochemistry (300 nm < h < 900nm). In addition, the spectrometer gives information about the local energy distribution. Two main problems must be solved: coupling of the high intensities into the spectrometer high measurement speed because of changing solar spectral irradiance A possible input device can be an optical fiber head. For enabeling the profile scan the front end must be mounted on a scanning device. This optical fiber device has major disadvantages. As the damage threshold for a clean optical fiber surface is of the order of 100 W/cm2 (1MW/m2) the radiation must be attenuated and the inlet must be cooled. The other disadvantage is that the wavelength scan needs some time (l-10 mm) and the profile scan needs time too. Combining both would lead to excessive measurement time. The measurement speed could be increased by the use of a circular variable filter (CVF). These devices are interference filters deposited on circular substrates. The transmission wavelength varies linear with the angle of rotation. The half bandwith of a CVF is about 17 nm. A problem is that CVF’s are usually employed in the infrared spectroscopic. There are no CVF’s available for wavelengths shorter than 400 nm. Besides this CVF are very expensive and require additional research on their optical properties. Another way of increasing the measurement speed is the use of the Fast Fourier Transform (FFT) spektroscopy. This would be the best way to build a fast imaging spektrometer. But the commercial availability is very poor and such a system is rather expensive.
Spectral analysis of concentrated solar radiation
TIE
199
SPECTROMETER
The DLR has a positive experience with design and construction of video camera based systems. Therefore, the solution was to implement this experience in the development of the new system. The setup that we propose here combines the imaging properties of a simple monochromator with the advantages of the video camera technique.
Video
Camera
Focussing
Mirror
Monochromator Fig. 1: Measurement setup. The beam image on the lambertian target is transfered into the monochromator which selects the wavelength and the location of the beam profile cross-section The concentrated beam is directed onto a cooled target that is coated with a diffuse reflecting target. With a mirror-based imaging optics the target is reproduced on the entrance slit of the monochromator. A focused image of the beam shape is present at that plane and the entrance slit cuts out a line. Behind the exit slit of the monochromator the video camera records an image of the entrance slit at the monochromator wavelength. The spectral analysis along a cross section through the beam can be performed with a resolution of about +l mm. The front end optics must be transparent in the required wavelength band. Therefore, normal photo lenses made of glass or plastic material cannot be used. A quartz lens or a metallic mirror must be used. A comparison of the investigated entrance optics is shown in Fig. 2. A single lens is affected by chromatic aberration. By using a metallic mirror this disadvantage can be overcome. Despite the fact that the mirror system shows astigmatism it is the best compromise. The necessary f#-number matching is possible by the selection of off-the-shelf mirrors.
A. NEUMANN and J. KALUZA
200
00
-xix-x-x
320
300
NX’ /
/
_IX---Ex==x~&i
340
360
380
/
400
420
4
Wawlengthbxnl Fig. 2: Measured transmission of lenses of different material. For our purpose only the Quartz Lens is usable. The two-dimensional imaging quality of an ordinary monochromator is quite poor because of astigmatic effects. The tangential image has the form of a pencil parallel to the entrance slit and it lies in the plane of the exit slit[2] . This allows the selection of the wavelength. The sagittal image is perpendicular to the first one. Its plane is not identical with the former image plane. The CCD array of the video camera is arranged to record the sag&al image. By choosing the width of the exit slit a determination of the wavelength resolution is possible. The entrance slit determines the width of the examined line which is the cut through the target picture. The resolution dependence is shown in Fig. 3. Choosing smaller slits increases accuracy but decreases light flux through the monochromator which is problematic for calibration purposes.
<”
--
.
.
,
,
Exit Slit (Spectral Resolution Power -+-Entrance Slit (Width of the Examined
25
[nm.]) Line [cm])
’
20 15 10 5 0
0
1
2 Slitwidth
3
[mm]
Fig. 3: Influence of slitwidth on measurement properties. The entrance slit determines the width of the analysed line and the exit slit chooses the spectral resolution.
Spectral analysis of concentrated
201
solar radiation
CALIBRATION AND TEST a light source with a flat spectrum (tungsten halogen lamp) is used which is positioned in front of the target. The large diference in intensity between the concentrated sunlight and the calibration source requires a detecting range of 60.000: 1. To cover this dynamic range a special camera was selected that allows the exposure times from 1/10.000 set to 300 set which gives a maximum ratio of 3.000.000: 1.
For the calibration
-
I,-------______
\
40
20
0
0
50
100
150
200
250
300
Cut Along Line [mm]
Fig. 4: Spectral Intersections through the Green-Red Target ,Fig. 4 shows the result of a first experiment with an artificial light source. A red and a green spot light giving half a circle in red and half one in green. This arrangement was used to simulate the concentrated solar beam. On the left hand the cross section shows the green area at 5.50 nm. On the right hand the red area at 650 nm is visible. As the sources are not monochromatic intensity is also present at 600 nm. The next step in the project will be the experiment in the concentrated sunlight.
Kalt, A, M. Becker, H.G. Dibowski, U. Groer and Neumann, A (1993): The New Solar Furnace of the DLR, Ktiln, Germany, Specifications and First Test Results. In: Proceedings of the 7th International Symposium on Solar Thermal Concentrating Technologies, Moscow, September 26-30,pp.llB1130. Hecht (1989): Optics, Addison-Wesley,
New York
Renewable Energy, Vol. 10, No. 2/3, pp. 197-201, 1997 Copyright IQ 1996 Published by Eisevier Science Ltd Printed in Great Britain. All rights reserved so%&1481(%)ooo63-8 09%1481/97 S15.00+0.00
A METHOD FOR SPECTRAL ANALYSIS OF CONCENTRATED
SOLAR
RADIATION
A. Neumann, J. Kaluza
Deutsche Forschungsanstalt fi,ir Luft und Raumfahrt e.V. (DLR) Hauptabteilung Energietechnik D-5 1140 Kiiln
ABSTRACT In the year 1994 the DLR in Cologne inaugurated a 20 kW solar furnace for technological and scientific experiments. One of the main applications of the solar furnace is the research on photochemical reactions using concentrated sunlight. For this purpose exact knowledge of the spectral composition and its local distribution is essential. The evaluation of experiments and the validation of computer codes rely on these informations. The solar radiation that is concentrated to high flux densities (up to 4 MW/m2) shows shorttime fluctuations and therefore a special measurement technique is required. It is shown how to use the imaging properties of a Czemey-Turner monochromator in combination with video camera detection to achieve a fast spectral analysis. This measurement is actually done along an intersection line through the focus. Moreover the calibration is described and first test pictures are analysed. Copyright 0 1996 Published by Elsevier Science Ltd.
KEYWORDS Solar furnace, spectral analysis, imaging spectrometer, astigmatism of monochromators INTRODUCTION Starting in the year 1991, a high flux solar furnace was built at the DLR research center in Cologne to allow high flux experiments in the field of solar chemistry and material research. The solar furnace uses a 52m2 flat heliostat to track the sun and a concentrator composed of 147 spherical mirror facets to focus the solar radiation in a spot of about 13cm diameter. At an
197
198
A. NEUMANN and J. KALUZA
insolation of 850 W/m2 it is possible to collect a total power of 20 kW with peak flux densities of 4 MW/m2. The measurement system that is used for temperature and flux density measurement in the solar furnace is called FATMES [I] . The flux density mapping is done by the white target technique. This technique is based on recording the image of the beam generated on a target screen. This target is coated with a diffuse reflecting surface and is positioned in the light path. The reflection pattern is monitored with a calibrated camera.
THE PROBLEM The solar chemistry research investigates with the solar furnace photochemical reactions under high concentrated radiation. Besides the total beam power the spectral content of the beam is of great importance. Actually, a computer code is used to calculate the spectrum of the concentrated solar radiation. But this simulation does not give information about the local distribution of the flux. Furthermore, up to now there are no means to validate the simulation. The main optical processes that affect the solar radiation on its way through the atmosphere, passing the heliostat and the concentrator, and reaching the focus are wavelength dependent. Therefore, a homogeneous local and spectral distribution may not be expected. Within this work a spectrometer was developped that resolves the spectrum in the wavelength band which is of importance for the photochemistry (300 nm < h < 900nm). In addition, the spectrometer gives information about the local energy distribution. Two main problems must be solved: coupling of the high intensities into the spectrometer high measurement speed because of changing solar spectral irradiance A possible input device can be an optical fiber head. For enabeling the profile scan the front end must be mounted on a scanning device. This optical fiber device has major disadvantages. As the damage threshold for a clean optical fiber surface is of the order of 100 W/cm2 (1MW/m2) the radiation must be attenuated and the inlet must be cooled. The other disadvantage is that the wavelength scan needs some time (l-10 mm) and the profile scan needs time too. Combining both would lead to excessive measurement time. The measurement speed could be increased by the use of a circular variable filter (CVF). These devices are interference filters deposited on circular substrates. The transmission wavelength varies linear with the angle of rotation. The half bandwith of a CVF is about 17 nm. A problem is that CVF’s are usually employed in the infrared spectroscopic. There are no CVF’s available for wavelengths shorter than 400 nm. Besides this CVF are very expensive and require additional research on their optical properties. Another way of increasing the measurement speed is the use of the Fast Fourier Transform (FFT) spektroscopy. This would be the best way to build a fast imaging spektrometer. But the commercial availability is very poor and such a system is rather expensive.
Spectral analysis of concentrated solar radiation
TIE
199
SPECTROMETER
The DLR has a positive experience with design and construction of video camera based systems. Therefore, the solution was to implement this experience in the development of the new system. The setup that we propose here combines the imaging properties of a simple monochromator with the advantages of the video camera technique.
Video
Camera
Focussing
Mirror
Monochromator Fig. 1: Measurement setup. The beam image on the lambertian target is transfered into the monochromator which selects the wavelength and the location of the beam profile cross-section The concentrated beam is directed onto a cooled target that is coated with a diffuse reflecting target. With a mirror-based imaging optics the target is reproduced on the entrance slit of the monochromator. A focused image of the beam shape is present at that plane and the entrance slit cuts out a line. Behind the exit slit of the monochromator the video camera records an image of the entrance slit at the monochromator wavelength. The spectral analysis along a cross section through the beam can be performed with a resolution of about +l mm. The front end optics must be transparent in the required wavelength band. Therefore, normal photo lenses made of glass or plastic material cannot be used. A quartz lens or a metallic mirror must be used. A comparison of the investigated entrance optics is shown in Fig. 2. A single lens is affected by chromatic aberration. By using a metallic mirror this disadvantage can be overcome. Despite the fact that the mirror system shows astigmatism it is the best compromise. The necessary f#-number matching is possible by the selection of off-the-shelf mirrors.
A. NEUMANN and J. KALUZA
200
00
-xix-x-x
320
300
NX’ /
/
_IX---Ex==x~&i
340
360
380
/
400
420
4
Wawlengthbxnl Fig. 2: Measured transmission of lenses of different material. For our purpose only the Quartz Lens is usable. The two-dimensional imaging quality of an ordinary monochromator is quite poor because of astigmatic effects. The tangential image has the form of a pencil parallel to the entrance slit and it lies in the plane of the exit slit[2] . This allows the selection of the wavelength. The sagittal image is perpendicular to the first one. Its plane is not identical with the former image plane. The CCD array of the video camera is arranged to record the sag&al image. By choosing the width of the exit slit a determination of the wavelength resolution is possible. The entrance slit determines the width of the examined line which is the cut through the target picture. The resolution dependence is shown in Fig. 3. Choosing smaller slits increases accuracy but decreases light flux through the monochromator which is problematic for calibration purposes.
<”
--
.
.
,
,
Exit Slit (Spectral Resolution Power -+-Entrance Slit (Width of the Examined
25
[nm.]) Line [cm])
’
20 15 10 5 0
0
1
2 Slitwidth
3
[mm]
Fig. 3: Influence of slitwidth on measurement properties. The entrance slit determines the width of the analysed line and the exit slit chooses the spectral resolution.
Spectral analysis of concentrated
201
solar radiation
CALIBRATION AND TEST a light source with a flat spectrum (tungsten halogen lamp) is used which is positioned in front of the target. The large diference in intensity between the concentrated sunlight and the calibration source requires a detecting range of 60.000: 1. To cover this dynamic range a special camera was selected that allows the exposure times from 1/10.000 set to 300 set which gives a maximum ratio of 3.000.000: 1.
For the calibration
-
I,-------______
\
40
20
0
0
50
100
150
200
250
300
Cut Along Line [mm]
Fig. 4: Spectral Intersections through the Green-Red Target ,Fig. 4 shows the result of a first experiment with an artificial light source. A red and a green spot light giving half a circle in red and half one in green. This arrangement was used to simulate the concentrated solar beam. On the left hand the cross section shows the green area at 5.50 nm. On the right hand the red area at 650 nm is visible. As the sources are not monochromatic intensity is also present at 600 nm. The next step in the project will be the experiment in the concentrated sunlight.
Kalt, A, M. Becker, H.G. Dibowski, U. Groer and Neumann, A (1993): The New Solar Furnace of the DLR, Ktiln, Germany, Specifications and First Test Results. In: Proceedings of the 7th International Symposium on Solar Thermal Concentrating Technologies, Moscow, September 26-30,pp.llB1130. Hecht (1989): Optics, Addison-Wesley,
New York