Research @ the Solar Energy Group

Sputtered Solar Absorbing Coatings

1. Solar selective absorbing coatings

All bodies emit thermal radiation. An ideal solar selective surface absorbs most of incident solar radiation while simultaneously suppressing emittance itself. Commonly quoted parameters of performance are solar absorptance and thermal emittance. Most solar selective coatings use metal-dielectric composites, known as cermets, as the absorber of solar energy. Electroplated black chromium is most popular solar selective absorber. Nicked-pigmented anodic Al2O3 solar absorber is another popular solar selective absorber. The costs of the production of these two kinds of solar coatings are low, so they are widely used in flat plate solar collectors for solar hot water application. However the emittance of both coatings is high, around 0.1 - 0.2 at 100°C.

2. Graded SS-C cermet solar coatings

Magnetron sputtering technology has also been used for the deposition of solar selective absorbing coatings with improved solar performance, in particular, lower emittance and less environmental pollution than electrochemical methods. The School of Physics within the University of Sydney have achieved several innovations of solar selective coatings in the last two decades. Selective surfaces, incorporating DC reactively sputtered stainless steel-carbon (SS-C), were intensively studied in this school during 1970s to 1980s. The all-glass evacuated solar collector tubes, incorporating DC reactively sputtered graded SS-C cermet selective coatings, have been mass-produced in Japan. A solar absorptance of 0.88-0.92 and emittance of 0.05 - 0.07 at 80°C have been archived for mass-produced SS-C cermet solar collector tubes.

3. Graded Al-N cermet solar coatings

Reactive sputtered Al-N cermet solar coating was initiated in this school in 1980s. The all-glass evacuated solar collector tubes, incorporating DC reactively sputtered graded Al-N cermet selective coatings, have been mass-produced in China in large capacities, more than 10 million tubes a year in 1999. A solar absorptance of 0.88-0.92 and emittance of 0.06 - 0.10 at 80°C have been archived for mass-produced SS-C cermet solar collector tubes. Cylindrical DC magnetron sputter coater has been used to manufacture economically the SS-C and Al-N cermet solar collector tubes.

4. Graded Mo-Al2O3 cermet solar coatings

Graded Mo-Al2O3 cermet solar collector tubes that were manufactured by Luz Co. Israel, were used for solar thermal power plants constructed in Southern California during 1984 -1990. Nine plants with a total net electricity capacity of 354 MW are under operation to feed electricity in the grid of the regional utility. Mo-Al2O3 cermet solar coatings were deposited by using conventional magnetron sputtering technology. The Mo metal component in the cermet is deposited using DC sputtering and the ceramic component is deposited by RF sputtering. A solar absorptance of 0.96 and emittance of 0.16 at 350°C were archived.

5. Double cermet layer solar coatings - Achieving the highest solar performance

Recently we have made two very significant innovations related to solar selective coatings, double cermet layer structure for achieving the highest solar performance and metal-aluminium nitride (M-AlN) solar selective coatings deposited by a novel two-target DC magnetron sputtering technology for reducing the costs of high-temperature solar coatings.

Through fundamental analysis and computer modelling, we have developed a double cermet film structure for solar selective surfaces and achieved theoretically and experimentally better solar performance than surfaces using a homogeneous cermet layer or the conventional graded film structure. In the double cermet layer solar coatings, solar radiation is effectively absorbed internally and by phase interference. This innovation has been patented in Australia, USA and other several countries. A typical double cermet film structure, as schematically shown in Fig. 1, from substrate to surface consists of:

1. a metal infrared reflecting layer;
2. an absorbing layer composed of two homogeneous cermet sub-layers, the layer near the metal infrared layer has a high metal volume fraction (HMVF) and the layer near the anti-reflection layer has low metal volume fraction (LMVF); and
3. an anti-reflection layer composed of a transparent dielectric material.

We have developed a computer modelling for design of solar selective coatings. As an example, a predicted normal reflectance spectrum of a Mo-Al2O3 cermet selective surface is shows in Fig. 2. The reflectance spectrum corresponds to a solar absorptance of 0.965 and normal emittance of 0.031 at 20°C. A near normal reflectance spectrum of a deposited film (dot points) is also shown in Fig. 2. This reflectance spectrum corresponds to a solar absorptance of 0.96, and near normal emittance of 0.03 at room temperature. Both calculated and deposited films have the same film structure, Al2O3/Mo-Al2O3(LMVF)/Mo-Al2O3(HMVF)/Cu.

6. M-AlN cermet solar coatings by a novel two-target DC sputtering - Reducing the costs of high-temperature solar coatings

We have invented a series of new cermet materials for solar selective coatings deposited by a novel two-target DC magnetron sputtering technology. The ceramic and metallic components in the cermet are simultaneously deposited by DC sputtering, running two metallic targets. Ceramic component is deposited by DC reactive sputtering and metallic component by DC non-reactive sputtering. A patent application describing these new solar selective coatings has been lodged.

A series of metal-aluminium nitride (M-AlN) cermet solar coatings have been deposited using a planar magnetron sputter apparatus at the laboratory. An Al metal target is used to deposit AlN ceramic component in the cermet by DC reactive sputtering in a gas mixture of argon and nitrogen. Other metallic targets, including SS, W, Ni80Cr20 and Mo based alloy TZM, which have good nitriding resistance, are used to deposit the metallic component in the cermet by DC non-reactive sputtering in the same gas mixture. The solar absorptance of 0.94 - 0.96, and normal emittance of 0.03 - 0.05 at room temperature has been achieved for the deposited M-AlN cermet selective coatings with the double cermet layer structure.

7. Mass-produced double Al-N cermet layer solar collector tubes

We have made very successful commercial development of above two new innovations in collaboration with Peking University and Himin Solar Energy Company, China.

Figure 3 (a photograph) shows two cylindrical DC magnetron sputter coaters, installed at a production line in China, for depositing Al-N cermet solar coatings onto batches of tubes. A cross-section schematic of the vacuum chamber is shown in Fig. 4.

Borosilicate glass tubes are used in mass-production. During sputter coating the glass tubes are given planetary rotation to ensure coating uniformity. Al-N cermet solar selective coatings with double cermet layer film structure, AlN/Al-N(LMVF)/Al-N(HMVF)/Al/Al-N(adhesion) are deposited onto batches of tubes using the sputter coater. A solar absorptance of 0.95 - 0.96 and a thermal emittance of 0.06 - 0.09 at room temperature have been achieved.

8. Mass-produced double SS-AlN cermet layer solar collector tubes

Commercial-scale cylindrical DC magnetron sputter coater has been developed for deposition of SS-AlN solar selective coatings onto batches of tubes, one installed at Peking University, another installed at the University of Sydney. A cross-section schematic of the vacuum chamber is shown in Fig. 5. A solar absorptance of 0.95 - 0.96, and emittance of 0.05 - 0.08 at 80°C has been achieved for the SS-AlN cermet selective surface coatings with the Al or Cu metal infrared reflector layer. The absorptance variation along the length of tubes is less than 1%. Figure 6 shows the reflectance spectrum of a deposited solar selective surface with double SS-AlN cermet layers. This film has the solar absorptance of 0.95 and emittance of 0.05 at 20°C. These SS-AlN cermet solar collector tubes are stable at 300 - 350°C.

Himin Solar Energy Company, China has mass-produced advanced Al-N and SS-AlN cermet solar collector tubes under a licence agreement with the University of Sydney. Currently Himin Solar Energy Company has manufactured three different series of advanced solar collector tubes, TS-AA, TS-SA and TS-SC. Enquiries for more information about solar collector tubes should be directed to

Himin Solar Energy Company, China Website: http://www.himin.com/english/index.htm (functions best using Internet Explorer)

9. Double W-AlN and Mo-AlN cermet layer solar collector tubes

Using above commercial-scale coater, it would be possible to produce high-temperature W-AlN and Mo-AlN cermet solar collector tubes. The procedure for deposition of the W-AlN and Mo-AlN cermet solar coatings is similar to that for SS-AlN cermet coatings. In order to improve further the thermal stability of the solar coatings at high temperature, It is also possible to deposit a metallic W or Mo layer as the infrared reflector. The W-AlN and Mo-AlN cermet solar coatings are being commercially developed.

The costs of the M-AlN cermet solar coatings are expected to be slightly higher than the Al-N cermet solar coatings. These M-AlN cermet solar collector tubes have the potential for widespread applications, in particular, for solar thermal electricity. The availability of these low-cost high-temperature M-AlN cermet solar collector tubes will open up the possibility for the design of a high-temperature solar collector system for solar thermal electricity generation at low cost than previously contemplated. Recently a Compact Linear Fresnel Reflector (CLFR) configuration has been proposed for solar thermal power plant. Flat mirror strips direct solar radiation to both sides of a rack of vertically oriented evacuated tubes.

[1] Very Low Emittance Solar Selective Surfaces Using New Film Structure. Q.-C Zhang and D. R. Mills, J. Appl. Phys. 72, 3013-3021(1992).

[2] High Efficiency Mo-Al2O3 Cermet Selective Surfaces for High-temperature Application. Q.-C Zhang, Y. Yin and D. R. Mills, Sol. Energy Mater. Sol. Cells 40, 43- 53(1996).

[3] Direct Current Magnetron Sputtered W-AlN Cermet Selective Surfaces. Q.-C. Zhang, J. Vac. Sci. Technol. A vol.15, pp.2842-2846(1997).

[4] Metal-AlN Cermet Selective Surfaces Deposited by Direct Current Magnetron Sputtering Technology. Q.-C. Zhang, J. Physics D: Appl. Phys. 31, 355-362(1988).

[5] A Cylindrical Magnetron Sputtering System for Depositing Metal-AlN Cermet Solar Coatings onto Batches of Tubes. Q.-C. Zhang, K. Zhao, B.-C. Zhang, L.-F. Wang, Z.-L. Shen, Z.-J. Zhou, D.-Q. Lu, D.-L Xie and B.-F. Li, J. Vac. Sci. Technol. A16, 628-632 (1998).

[6] High Performance Al-N Cermet Solar Coatings Deposited by a Cylindrical DC Magnetron

Sputter Coater. Q.-C. Zhang, K. Zhao, B.-C. Zhang, L.-F. Wang, Z.-L. Shen, Z.-J. Zhou, D.-Q. Lu, D.-L Xie and B.-F. Li, Vac. Sci. Technol. A17, 2885-2890 (1999).