859
Ind. Eng. Chem. Prod. Res. D8V. 1984, 23, 659-661
Solar-Selective Absorber Coatings for High-Temperature Application E. Erben' M.A.N. Neue Technolcgle, Munich, West Germany
B. A. Tihanyl Ruhrchemle AG, Oberhausen, West Germany
Experiments on the possibilities of developing solar-selective absorber systems for temperatures above 300 O C have been carried out and are reported here. This report particularly concerns work carried out by two coating techniques, galvanizing and chemlcal vapor deposition (CVD), in conjunction with the use of new coating chemicals. Two new solar-selective absorber systems on nickel and tungsten bases are presented.
Introduction The solar energy option has a key role in the solution of the long-term global energy problems. The solarthermal transformation is particularly important here because it cannot only produce hot water for direct use and heating purposes but also process heat. Several new absorber systems have been developed in a research and development project sponsored by the BMFT (German Federal Ministry of Research and Technology). The following paragraphs report on two of these systems. Coating Techniques The coating techniques used were the CVD method (Gurev, 1976; Seraphin, 1979) and electrochemical processes. The advantages of the coating techniques chosen can be described as follows. CVD. Chemical vapor deposition is a method of plating in which the deposits are produced by heterogeneous gas-solid or gas-liquid chemical reaction a t the surface of a substrate. A volatile compound of the element or substance to be deposited is vaporized and the vapor is thermally decomposed or reacted with other gases or vapors a t the substrate to yield nonvolatile reaction products which deposit on the substrate as a coating. The relatively high coating temperature guarantees that the coating bonds well to the substrate. This temperature is close to or above the later operating temperature of the absorber coating, which is why good thermal stability of these coatings up to high temperatures can be expected. It is, moreover, particularly the wide variation possible in the coating parameters and thus in the coating characteristics that makes the CVD technique so interesting in this connection. Electrochemical Process. Electrochemical processes are based on the discharge of ions a t the cathode and anode. Generally from aqueous solution the metal ion is attracted to the substrate serving as the cathode and is plated out on it. With this process both structured absorber coatings and smooth metallic reflector coatings can be obtained. Characterization of the Coatings The coatings and coating systems produced are characterized by the following: (a) thermooptical characteristics; (b) measurement of directed degree of absorption for insolation; (c) measurement of hemispherical emission E (100 "C); (d) determination of spectrally directed degree of absorption; a(X) in the 0.36-2.5 pm wavelength range; 0196-4321/84/1223-0659$01.50/0
Table I. Absorption (a)and Emission (c) of Nickel Coatings Immediately after Deposition and after Exposure Tests after deposition after stabilization 8 h, 400 O C , air exposure 100 h, 300 O C , air after exposure 100 h, 350 O C , air after exposure lo00 h, 350 O C , air after exposure 100 h, 400 O C , air after exposure 1000 h, 400 O C , air
ff
c
0.89 0.93 0.92 0.93 0.93 0.93 0.93 0.93 0.93 0.92 0.93 0.84 0.85
0.14 0.15 0.14 0.12 0.13 0.12 0.10 0.10 0.10 0.14 0.13 0.20 0.16
(e) measurement of spectral degree of directed reflection; in the 2.5-40 pm wavelength range; (f) determination of coating composition by means of energy disperse X-ray analysis (EDX); (g) surface analysis by means of scanning electron microscope (SEM) and electron spectroscopy for chemical analysis (ESCA). Experimental Results Absorber Coating on Nickel Base. Uniform black coatings were produced from strongly diluted solutions of ammoniacal nickel citrate (Erben, 1982). In detail: the nickel was deposited from a 1% ammoniacal citrate solution with the addition of NH&l as conducting salt. The backings for these coating procedures were steel substrates measuring 50 X 60 X 1 mm. These samples had been pre-nickeled and given a nickel undercoating. The nickel undercoating assumes the function of the thermal emitter, while the structured black coating deposited from the citrate solution onto this nickel layer effects the absorption of the sun's rays. The conditions for deposition of the black coatings were as follows: current density, 3-10 A/dm2; deposition period, 2-5 min; deposition voltage, 8.5-20 V; pH, 10; electrolyte temperature, 15-20 "C; 5 g/L NH&l added as conducting salt. In thermal treatment of the nickel coatings produced in this way it was found that their thermooptical values improved after brief exposure (8 h) to air a t 400 OC. This procedure is hereinafter called stabilization. ESCAanalysis (Figures 1 and 2) show that the nickel layer consists of metal after deposition; after stabilization at 400 OC in air, 8 h, nickel oxide is formed on the surface. Table I contains the results of the exposure tests. These show pl(X)
0 1984 American Chemical Society
660
Ind. Eng. Chem. prod. Res. Dev.. Vol. 23. No. 4. 1984
.E::" ..
:uti:%
it. nil
:r. . I X , 2.are
a m !e.??
