- Email: [email protected]
4.3 Vacuum evaporated thin films for solar collectors
4.3 Vacuum evaporated thin films for solar collectors L E Flordal and R Kivai$i," Institute of Optical Research, S- 100 44 Stockholm, Sweden
Selectively absorbing surfaces are important for the efficient use of solar energy. The main characteristics aimed at for this type of coatings are the highest possible absorptivity for solar radiation and the lowest possible emissivity for thermal radiation. (Temperatures up to 100-200°C). There are several ways to create surfaces with these properties. In the present work evaporated semiconductor films with suitable anti-reflection coatings were used on different metals. For the laboratory tests, the metals were vacuum evaporated on glass substrates. The evaporations were performed with an electron beam gun in a diffusion pumped system. The evaporated thicknesses and the rate of evaporation were controlled by a quartz crystal monitoring system. The selective coatings were studied on flat and rough metal surfaces. Sinusoidal, metal covered gratings with different periods were used as the rough absorber surfaces. This simplified the measurements of absorptivity and emissivity and gave a more thorough understanding of the basic thin film phenomena. It is possible to use gratings because the general rough surface can be looked upon as a superposition of different sinusoidal gratings. A comparison is made between the properties of the layers on flat and rough surfaces. The coatings were subjected to high humidity and high temperature tests. They were also cycled between the intended working temperature and ambient temperature to check possible degradation. Results concerning film adherence and stability during these environmental tests are given.
Introduction The use of solar energy in Sweden might seem very optimistic during cloudy and dark autumn and winter days. However, the total insolation per year on to a horizontal surface is 900 kWh/m =, which means a mean value over the year of 100 W]m =. This is more than 35% of the insolation at the most intensively illuminated desert areas of the earth. 1 The insolation on to an oblique area in Sweden is thus during spring and summer quite comparable to that in e.g. Sahara. The radiation we receive is, however, rather different from that which is falling on most of the sunny areas. The main differences are that our insolation is diffuse to a high degree and that it is unevenly distributed over the year. The use of solar energy therefore puts somewhat special demands in our country. Thus, concentrating systems can hardly be used because of the many cloudy days. What we need are absorbers which work well even with diffuse light. The construction should be made in such a way that they work efficiently without tracking mechanisms. Another very important demand for the practical use of solar energy in Sweden is cheap and efficient systems for energy storage, preferably from one season to another. This is not dealt with in the present work. However, the primary element in all systems for direct use of the Sun as a continuous source of energy is an efficient absorber of some type. The present study has the goal to find some possibilities to absorb solar energy thermally in thin film coatings.
temperatures are shown in Figure 1. This shows clearly the separation in wavelength between the two spectra. The boundary wavelength lies roughly at 2.5/~m for temperatures below 500 K. Because of this separation, it is possible to make a surface which absorbs sunlight efficiently, but which does not re-emit the absorbed energy at low temperatures. Such a surface, a selective absorber, should have the highest possible absorptivity for wavelengths below 2.5 #m but a low emissivity for longer wavelengths. 2 A selective absorber placed in sunlight will thus reach a much higher temperature than a common black surface. This is important, since thermal energy is more valuable at high temperatures than at low. A system consisting of a thin semiconductor layer with a simple anti-reflection coating on a metal with high reflectivity in the infrared was found to fulfil the d e m a n d s ) The high metal refiectivity
~E.:~1"5
c E
~
i
-= 1.0
£
ir mass 2
.5
/
Physical background The spectral distributions of the incident sunlight at the surface of the earth and of the emitted black-body radiation at different * Permanent address: University of Dares Salaam, P.O. Box 35063, Dares Salaam, Tanzania. Vacuum/volume 27/number 4.
0
.3
.5
2 Wavelength =n m i c r o n s
5
10
Figure 1. Solar irradiance at the Earth's surface and blackbody radiation at different temperatures.
Pergamon Press/Printed in Great Britain
399
L E Flordttl and R Kivai$i: Vacuum evaporated thin films for solar collectors
corresponds directly to a low infrared emissivity. Aluminium, nickel, chromium and copper were investigated as base metals. Copper was later abandoned due to oxidation even at moderate temperatures. The metal was covered with a thin semiconductor coating in order to increase the absorptivity of solar radiation. If the semiconductor has a band-gap of about 2.5 ~,m, this can be done without increasing the thermal emissivity, since the semiconductor is transparent for infrared radiation. Germanium and lead sulphide were chosen as suitable semiconductors. The semiconductor coating absorbs incident solar radiation, but has a rather high reflection loss because of its high refractive index. The absorptivity can be increased by covering the semiconductor with a simple antireflection coating. Silicon monoxide was chosen for this purpose since it is rather resistant and easy to evaporate. This gives the basic design: metal-semiconductoranti-reflection coating.
~ [
.ss
GO n m
,,i
I
60 n m $ i 0
.80
.75
AI
50 n m S i O ~
Tneocetical e u i ~ l a t l o ~ Different combinations of metal substrates (aluminium and nickel) and coatings were investigated theoretically. The absorptance versus wavelength at different angles of incidence was calculated for many different layer thicknesses. One example is shown in Figure 2. The solar absorptance was then calculated
2o
'
3'0
'
Semiconductor
20
t ~ c k n e $ $ in n m
Figure 3. Theoretically calculated solar absorptance versus semiconductor thickness at normal incidence. }.
0o Q
Ni-PbS-S;O Ai-Ge-SiO
60 n m
", \
6
o
i,oo
I ,,s
.~.
0 .2
0
"-..
.3
. . . . . . . -5
1.
2
II '
5
~0
.2
.~ .6 Solar absorptance
.8
1.
4. Theoretically calculated solar absorptanc¢ versus angle of incidence for two optimized coatings. Semiconductor thickness 30 nm, silicon monoxide thickness 60 nm.
Wavelength rn m i c r o n s
Figm~ Z. Theoreti~lly calculated absorptance at different angles of incidence for one example of the two layer coatings. using the irradiance curve (Figure 1) as a weighting function. The method with selected wavelengths was used for this purpose (20 wavelengths were used). Figure 3 shows the variation of solar absorptance when the semiconductor thickness is varied. It is obvious from these curves that the solar absorptance is not very sensitive to changes in coating thicknesses around the optimum values. This is very important for practical use. Figure 4 shows together with Figure 2 that this type of selective absorber works efficiently in diffuse light. The solar absorptance is almost constant from normal to 60 ° angle of incidence. The theoretical calculations also showed that the absorption in the systems is due to two different mechanisms. An example of this is shown in Figure 5 where the absorption is divided between the metal substrate and the coating. In the wavelength range where the semiconductor is non-transparent, it is antireflected by the single silicon monoxide layer. Thus, short wavelength radiation is absorbed mainly in the semiconductor. 400
"
/
.2
o.3 _.4'_.5
\
L
I "~s 13°~
f
"" 1. 2 Wave|ength in m i c r o n s
5
:::
10
Figure 5. Theoretically calculated absorptance in the coating and the metal substrate at normal incidence. Fifty-six per cent of the incident energy is absorbed in the semiconductor layer, 35 % in the metal.
L E Flordal and R Kivaisi." Vacuum evaporated thin films for solar collectors
The wavelength of maximum absorptivity can be chosen at will by adjusting the single anti-reflection layer thickness." in the region where the semiconductor is transparent, it gives an antireflection coating of the base metal together with the silicon monoxide layer. Here, the radiation is absorbed in the base metal. The wavelength of maximum absorptivity can be chosen to a certain degree, and is given by the compound optical thicknesses of the layers. The optimum anti-reflection layer for all the combinations was 60 nm silicon monoxide, which gives anti-reflection at roughly 550 rim. The optimum thicknesses for the semiconductor were between 20 and 40 nm, depending on base metal and semiconductor. These thicknesses are well suited for vacuum evaporation, which was used to make the selective absorber samples. Experimental For measurement purposes, both the two layers and the base metals were produced by vacuum evaporation. The metal was evaporated on a smooth glass to get an even surface. From the optical point of view this is equivalent to a semi-infinite metal substrate, as soon as the metal film is thick enough to be nontransparent for all radiation under consideration. The flat, mirror-like surfaces simplified the subsequent practical measurements. The evaporations were performed in a conventional, diffusion-pumped system with a working pressure o f roughly 10-s torr. The substances were evaporated from an electron beam gun with maximum 5 kW power. The evaporation rates differed from 1 to ].5 nm s - t for germanium and silicon monoxide. Lead sulphide films were obtained at very low rate, about 5 to 6 nm m i n - 1 , to get good film properties. A quartz crystal detector, which was earlier calibrated with the actual sul~stances, was used to control rate and thickness. During the evaporations, where four samples were coated in each run, the samples were rotated to get even thicknesses o f the coatings. Several series with different thicknesses were produced to be able to study the influence on performance caused by thickness variations.
The reflectivities of the finished samples were measured in spectrophotometers at close to normal incidence between 0.35 and 40 t=m wavelength. Since the samples were non-transparent, these measurements gave the absorptivity or emissivity directly as unity minus the reflectivity. The solar absorptivity of each sample was calculated by weighting the measured absorptivity at different wavelengths with incident solar irradiation at two airmasses. The method with 20 selected wavelengths was again used for this purpose. The thermal emissivity was calculated in a similar manner using black-body radiation curves at 400 and 500 K. The measurements were made at room temperature, under the assumption that the optical properties were not changed considerably at elevated temperatures. However, this assumption has to be investigated by measurements. The measurements showed reasonable agreement with the theoretical predictions. One example of this is given in Figure 6. The deviations were probably caused by the uncertainty of the optical constants of the materials used in the calculations. Thus, ]. ""..'"
'~
/ ~
~ Theory ...... Experim,n,
l'osl 'oom
t .3
\\J., .5
1
1. 2 Wavelength in microns
5
10
Fi~,wre 6. Comparison between calculated and measured absorptance at normal incidence.
Table I. Solar absorptances =, and therma] cmittances ~ of some selective absorbers. Silicon monoxide thickness 60 nm.
Design AI-Ge-SiO
AI-PbS-SiO Ni-Ge-SiO Ni-PbS-SiO Cr-Ge-SiO Cr-PbS-SiO
Semiconductor •thickness nm 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40
% theory
=, exp
0.77 0.84 0.79 0.75 0.83 0.84 0.91 0.89 0.85 0.91 0.92 0.89
0.75 0.79 0.74 0.77 0.89 0.87 0.88 0.85 0.82 0.92 0.93 0.93 0.93 0 90 0.86 0.93 0.94
e,ooK
*SOOK
"/~,=0OK
=/eSOOK
0.012 0.012
0.014 0.014
63 66
54 56
0.018 0.018
0.018 0.02
43 52
43 44
0.035 0.04
0.044 0.052
25 21
20 16
0.041 0.043
0.054 0.055
22 22
17 17
0.11
0. i 2
8
8
0.11 0.12
0.13 0.14
8 8
7 7
0.94
401
L E Flordal and R Kivaisi: Vacuum evaporated thin films for solar collectors
Systems with germanium showed 3 to 4 % lower absorptivity in practice, while the designs with lead sulphide gave somewhat higher absorptivities than the calculated. These results are summarized in Table i. The differences were mainly due to changes in the absorption edges of the semiconductors. The highest measured solar absorptivities were for A I - G e - S i O 79%, AI-PbS--SiO 89%, N i - G e - S i O 88%, Ni-PbS--SiO 93%, C r - G e - S i O 93% and Cr-PbS--SiO 94%. The samples with chromium as the base metal showed the highest infrared emissivities 10-15 %, while these were only 1-2% with aluminium. Nickel showed intermediate emissivities of 4 to 5 % at 400 K. The difference in emissivity means that chromium is suited for the low working temperatures, while aluminium is more efficient for high temperature applications. The ratio between solar absorptivity and emissivity at 400 K was 50--60 for aluminium, about 20 for nickel but only 8 for chromium as the base metal. Stability test
A simple environmental test of the coatings was also performed. The samples were subjected to high relative humidities and were heated to 250°C in air several times to check possible deterioration. The performances of the absorbers were measured before and after the treatment and were compared. The investigations are summarized in Table 2 and showed that the absorbers with Table 2. Stability of selective coatings. Semiconductor thickness 30 nm, silicon monoxide thickness 60 nm. After heating in air, 240°C
After humidity test, 90--95 0t s
Design
=, untreated
0t s
AI-G¢-SiO AI-PbS-SiO Ni-Ge-SiO Ni-PbS--SiO Cr--Ge--SiO Cr-PbS--SiO
0.79 0.79 0.85 0.92 0.90 0.94
0.76 0.69 0.85 0.88 0.93 0.94
An' s
about the stabilities. A far more extensive investigation with homogeneous metal substrates is needed to give the long term stability of the coatings.
R o u ~ surfaces The two layer coating systems were also studied on rough metal surfaces. This study was simplified by using gratings with different periods as prototypes for rough surfaces. The gratings were made by exposing holographic plates in interfering laser beams. After development, the gelatin on the plates had a sinusoidally varying thickness. With the gelatin side as a substrate metals were evaporated to give rough metal surfaces with known roughness. The two coatings were evaporated on those rough metals and measured almost as the flat surfaces. The measurements showed that these rough selective absorbers had higher solar absorptivities than the flat ones. The thermal emissivity, however, was the same. The increase in absorptivity was a few per cent, depending on the grating period and the coating system. A possible use of rough surfaces could be to simplify the selective absorber to one single coating. Work is going on in this direction. Discussion This investigation shows that it is possible to make good selective absorbers by vacuum evaporation of coatings. A practical use of these coatings to capture thermal energy from the Sun requires very large areas. These coatings may be possible to produce at a competitive cost in large-scale vacuum plants in the future.
~.0C s
Acknowledgements 0.03 0.10 0.00 0.04 -0.03 0.00
0.77 0.70 0.84 0.88 0.90 0.94
0.02 0.09 0.01 0.04 0.00 0.00
chromium as the metal base were not affected at all. The combination nickel-lead sulphide lost 4% in absorptivity. The Joss was of the same magnitude for the combination germaniumaluminium, but as high as 10% for lead sulphide and the same metal. The test was by no means complete, but gives some hints
402
One of us, R Kivaisi, was a participant to the International Seminar in Physics sponsored by S I D A (Swedish International Development Authority) during this work. The work was also supported by N F R (Swedish Natural Science Research Council).
Refe~ences
i H E Landsberg, Solar Energy, 5, 1961, 95. 2 D M Mattox and R R Sowell, J Vac Sci Technol, 11, 1974, 793. J .lurisson, R E Peterson and H Y Mar. J Vac Sci Technol, 12, 1975, 1010.
Selectively absorbing surfaces are important for the efficient use of solar energy. The main characteristics aimed at for this type of coatings are the highest possible absorptivity for solar radiation and the lowest possible emissivity for thermal radiation. (Temperatures up to 100-200°C). There are several ways to create surfaces with these properties. In the present work evaporated semiconductor films with suitable anti-reflection coatings were used on different metals. For the laboratory tests, the metals were vacuum evaporated on glass substrates. The evaporations were performed with an electron beam gun in a diffusion pumped system. The evaporated thicknesses and the rate of evaporation were controlled by a quartz crystal monitoring system. The selective coatings were studied on flat and rough metal surfaces. Sinusoidal, metal covered gratings with different periods were used as the rough absorber surfaces. This simplified the measurements of absorptivity and emissivity and gave a more thorough understanding of the basic thin film phenomena. It is possible to use gratings because the general rough surface can be looked upon as a superposition of different sinusoidal gratings. A comparison is made between the properties of the layers on flat and rough surfaces. The coatings were subjected to high humidity and high temperature tests. They were also cycled between the intended working temperature and ambient temperature to check possible degradation. Results concerning film adherence and stability during these environmental tests are given.
Introduction The use of solar energy in Sweden might seem very optimistic during cloudy and dark autumn and winter days. However, the total insolation per year on to a horizontal surface is 900 kWh/m =, which means a mean value over the year of 100 W]m =. This is more than 35% of the insolation at the most intensively illuminated desert areas of the earth. 1 The insolation on to an oblique area in Sweden is thus during spring and summer quite comparable to that in e.g. Sahara. The radiation we receive is, however, rather different from that which is falling on most of the sunny areas. The main differences are that our insolation is diffuse to a high degree and that it is unevenly distributed over the year. The use of solar energy therefore puts somewhat special demands in our country. Thus, concentrating systems can hardly be used because of the many cloudy days. What we need are absorbers which work well even with diffuse light. The construction should be made in such a way that they work efficiently without tracking mechanisms. Another very important demand for the practical use of solar energy in Sweden is cheap and efficient systems for energy storage, preferably from one season to another. This is not dealt with in the present work. However, the primary element in all systems for direct use of the Sun as a continuous source of energy is an efficient absorber of some type. The present study has the goal to find some possibilities to absorb solar energy thermally in thin film coatings.
temperatures are shown in Figure 1. This shows clearly the separation in wavelength between the two spectra. The boundary wavelength lies roughly at 2.5/~m for temperatures below 500 K. Because of this separation, it is possible to make a surface which absorbs sunlight efficiently, but which does not re-emit the absorbed energy at low temperatures. Such a surface, a selective absorber, should have the highest possible absorptivity for wavelengths below 2.5 #m but a low emissivity for longer wavelengths. 2 A selective absorber placed in sunlight will thus reach a much higher temperature than a common black surface. This is important, since thermal energy is more valuable at high temperatures than at low. A system consisting of a thin semiconductor layer with a simple anti-reflection coating on a metal with high reflectivity in the infrared was found to fulfil the d e m a n d s ) The high metal refiectivity
~E.:~1"5
c E
~
i
-= 1.0
£
ir mass 2
.5
/
Physical background The spectral distributions of the incident sunlight at the surface of the earth and of the emitted black-body radiation at different * Permanent address: University of Dares Salaam, P.O. Box 35063, Dares Salaam, Tanzania. Vacuum/volume 27/number 4.
0
.3
.5
2 Wavelength =n m i c r o n s
5
10
Figure 1. Solar irradiance at the Earth's surface and blackbody radiation at different temperatures.
Pergamon Press/Printed in Great Britain
399
L E Flordttl and R Kivai$i: Vacuum evaporated thin films for solar collectors
corresponds directly to a low infrared emissivity. Aluminium, nickel, chromium and copper were investigated as base metals. Copper was later abandoned due to oxidation even at moderate temperatures. The metal was covered with a thin semiconductor coating in order to increase the absorptivity of solar radiation. If the semiconductor has a band-gap of about 2.5 ~,m, this can be done without increasing the thermal emissivity, since the semiconductor is transparent for infrared radiation. Germanium and lead sulphide were chosen as suitable semiconductors. The semiconductor coating absorbs incident solar radiation, but has a rather high reflection loss because of its high refractive index. The absorptivity can be increased by covering the semiconductor with a simple antireflection coating. Silicon monoxide was chosen for this purpose since it is rather resistant and easy to evaporate. This gives the basic design: metal-semiconductoranti-reflection coating.
~ [
.ss
GO n m
,,i
I
60 n m $ i 0
.80
.75
AI
50 n m S i O ~
Tneocetical e u i ~ l a t l o ~ Different combinations of metal substrates (aluminium and nickel) and coatings were investigated theoretically. The absorptance versus wavelength at different angles of incidence was calculated for many different layer thicknesses. One example is shown in Figure 2. The solar absorptance was then calculated
2o
'
3'0
'
Semiconductor
20
t ~ c k n e $ $ in n m
Figure 3. Theoretically calculated solar absorptance versus semiconductor thickness at normal incidence. }.
0o Q
Ni-PbS-S;O Ai-Ge-SiO
60 n m
", \
6
o
i,oo
I ,,s
.~.
0 .2
0
"-..
.3
. . . . . . . -5
1.
2
II '
5
~0
.2
.~ .6 Solar absorptance
.8
1.
4. Theoretically calculated solar absorptanc¢ versus angle of incidence for two optimized coatings. Semiconductor thickness 30 nm, silicon monoxide thickness 60 nm.
Wavelength rn m i c r o n s
Figm~ Z. Theoreti~lly calculated absorptance at different angles of incidence for one example of the two layer coatings. using the irradiance curve (Figure 1) as a weighting function. The method with selected wavelengths was used for this purpose (20 wavelengths were used). Figure 3 shows the variation of solar absorptance when the semiconductor thickness is varied. It is obvious from these curves that the solar absorptance is not very sensitive to changes in coating thicknesses around the optimum values. This is very important for practical use. Figure 4 shows together with Figure 2 that this type of selective absorber works efficiently in diffuse light. The solar absorptance is almost constant from normal to 60 ° angle of incidence. The theoretical calculations also showed that the absorption in the systems is due to two different mechanisms. An example of this is shown in Figure 5 where the absorption is divided between the metal substrate and the coating. In the wavelength range where the semiconductor is non-transparent, it is antireflected by the single silicon monoxide layer. Thus, short wavelength radiation is absorbed mainly in the semiconductor. 400
"
/
.2
o.3 _.4'_.5
\
L
I "~s 13°~
f
"" 1. 2 Wave|ength in m i c r o n s
5
:::
10
Figure 5. Theoretically calculated absorptance in the coating and the metal substrate at normal incidence. Fifty-six per cent of the incident energy is absorbed in the semiconductor layer, 35 % in the metal.
L E Flordal and R Kivaisi." Vacuum evaporated thin films for solar collectors
The wavelength of maximum absorptivity can be chosen at will by adjusting the single anti-reflection layer thickness." in the region where the semiconductor is transparent, it gives an antireflection coating of the base metal together with the silicon monoxide layer. Here, the radiation is absorbed in the base metal. The wavelength of maximum absorptivity can be chosen to a certain degree, and is given by the compound optical thicknesses of the layers. The optimum anti-reflection layer for all the combinations was 60 nm silicon monoxide, which gives anti-reflection at roughly 550 rim. The optimum thicknesses for the semiconductor were between 20 and 40 nm, depending on base metal and semiconductor. These thicknesses are well suited for vacuum evaporation, which was used to make the selective absorber samples. Experimental For measurement purposes, both the two layers and the base metals were produced by vacuum evaporation. The metal was evaporated on a smooth glass to get an even surface. From the optical point of view this is equivalent to a semi-infinite metal substrate, as soon as the metal film is thick enough to be nontransparent for all radiation under consideration. The flat, mirror-like surfaces simplified the subsequent practical measurements. The evaporations were performed in a conventional, diffusion-pumped system with a working pressure o f roughly 10-s torr. The substances were evaporated from an electron beam gun with maximum 5 kW power. The evaporation rates differed from 1 to ].5 nm s - t for germanium and silicon monoxide. Lead sulphide films were obtained at very low rate, about 5 to 6 nm m i n - 1 , to get good film properties. A quartz crystal detector, which was earlier calibrated with the actual sul~stances, was used to control rate and thickness. During the evaporations, where four samples were coated in each run, the samples were rotated to get even thicknesses o f the coatings. Several series with different thicknesses were produced to be able to study the influence on performance caused by thickness variations.
The reflectivities of the finished samples were measured in spectrophotometers at close to normal incidence between 0.35 and 40 t=m wavelength. Since the samples were non-transparent, these measurements gave the absorptivity or emissivity directly as unity minus the reflectivity. The solar absorptivity of each sample was calculated by weighting the measured absorptivity at different wavelengths with incident solar irradiation at two airmasses. The method with 20 selected wavelengths was again used for this purpose. The thermal emissivity was calculated in a similar manner using black-body radiation curves at 400 and 500 K. The measurements were made at room temperature, under the assumption that the optical properties were not changed considerably at elevated temperatures. However, this assumption has to be investigated by measurements. The measurements showed reasonable agreement with the theoretical predictions. One example of this is given in Figure 6. The deviations were probably caused by the uncertainty of the optical constants of the materials used in the calculations. Thus, ]. ""..'"
'~
/ ~
~ Theory ...... Experim,n,
l'osl 'oom
t .3
\\J., .5
1
1. 2 Wavelength in microns
5
10
Fi~,wre 6. Comparison between calculated and measured absorptance at normal incidence.
Table I. Solar absorptances =, and therma] cmittances ~ of some selective absorbers. Silicon monoxide thickness 60 nm.
Design AI-Ge-SiO
AI-PbS-SiO Ni-Ge-SiO Ni-PbS-SiO Cr-Ge-SiO Cr-PbS-SiO
Semiconductor •thickness nm 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40
% theory
=, exp
0.77 0.84 0.79 0.75 0.83 0.84 0.91 0.89 0.85 0.91 0.92 0.89
0.75 0.79 0.74 0.77 0.89 0.87 0.88 0.85 0.82 0.92 0.93 0.93 0.93 0 90 0.86 0.93 0.94
e,ooK
*SOOK
"/~,=0OK
=/eSOOK
0.012 0.012
0.014 0.014
63 66
54 56
0.018 0.018
0.018 0.02
43 52
43 44
0.035 0.04
0.044 0.052
25 21
20 16
0.041 0.043
0.054 0.055
22 22
17 17
0.11
0. i 2
8
8
0.11 0.12
0.13 0.14
8 8
7 7
0.94
401
L E Flordal and R Kivaisi: Vacuum evaporated thin films for solar collectors
Systems with germanium showed 3 to 4 % lower absorptivity in practice, while the designs with lead sulphide gave somewhat higher absorptivities than the calculated. These results are summarized in Table i. The differences were mainly due to changes in the absorption edges of the semiconductors. The highest measured solar absorptivities were for A I - G e - S i O 79%, AI-PbS--SiO 89%, N i - G e - S i O 88%, Ni-PbS--SiO 93%, C r - G e - S i O 93% and Cr-PbS--SiO 94%. The samples with chromium as the base metal showed the highest infrared emissivities 10-15 %, while these were only 1-2% with aluminium. Nickel showed intermediate emissivities of 4 to 5 % at 400 K. The difference in emissivity means that chromium is suited for the low working temperatures, while aluminium is more efficient for high temperature applications. The ratio between solar absorptivity and emissivity at 400 K was 50--60 for aluminium, about 20 for nickel but only 8 for chromium as the base metal. Stability test
A simple environmental test of the coatings was also performed. The samples were subjected to high relative humidities and were heated to 250°C in air several times to check possible deterioration. The performances of the absorbers were measured before and after the treatment and were compared. The investigations are summarized in Table 2 and showed that the absorbers with Table 2. Stability of selective coatings. Semiconductor thickness 30 nm, silicon monoxide thickness 60 nm. After heating in air, 240°C
After humidity test, 90--95 0t s
Design
=, untreated
0t s
AI-G¢-SiO AI-PbS-SiO Ni-Ge-SiO Ni-PbS--SiO Cr--Ge--SiO Cr-PbS--SiO
0.79 0.79 0.85 0.92 0.90 0.94
0.76 0.69 0.85 0.88 0.93 0.94
An' s
about the stabilities. A far more extensive investigation with homogeneous metal substrates is needed to give the long term stability of the coatings.
R o u ~ surfaces The two layer coating systems were also studied on rough metal surfaces. This study was simplified by using gratings with different periods as prototypes for rough surfaces. The gratings were made by exposing holographic plates in interfering laser beams. After development, the gelatin on the plates had a sinusoidally varying thickness. With the gelatin side as a substrate metals were evaporated to give rough metal surfaces with known roughness. The two coatings were evaporated on those rough metals and measured almost as the flat surfaces. The measurements showed that these rough selective absorbers had higher solar absorptivities than the flat ones. The thermal emissivity, however, was the same. The increase in absorptivity was a few per cent, depending on the grating period and the coating system. A possible use of rough surfaces could be to simplify the selective absorber to one single coating. Work is going on in this direction. Discussion This investigation shows that it is possible to make good selective absorbers by vacuum evaporation of coatings. A practical use of these coatings to capture thermal energy from the Sun requires very large areas. These coatings may be possible to produce at a competitive cost in large-scale vacuum plants in the future.
~.0C s
Acknowledgements 0.03 0.10 0.00 0.04 -0.03 0.00
0.77 0.70 0.84 0.88 0.90 0.94
0.02 0.09 0.01 0.04 0.00 0.00
chromium as the metal base were not affected at all. The combination nickel-lead sulphide lost 4% in absorptivity. The Joss was of the same magnitude for the combination germaniumaluminium, but as high as 10% for lead sulphide and the same metal. The test was by no means complete, but gives some hints
402
One of us, R Kivaisi, was a participant to the International Seminar in Physics sponsored by S I D A (Swedish International Development Authority) during this work. The work was also supported by N F R (Swedish Natural Science Research Council).
Refe~ences
i H E Landsberg, Solar Energy, 5, 1961, 95. 2 D M Mattox and R R Sowell, J Vac Sci Technol, 11, 1974, 793. J .lurisson, R E Peterson and H Y Mar. J Vac Sci Technol, 12, 1975, 1010.