SELECTIVELY ABSORBING COATINGS

SELECTIVELY ABSORBING COATINGS

J. Vuletin, P. Kuli¡ik, M. Bosanac

To cite this version:

J. Vuletin, P. Kuli¡ik, M. Bosanac. SELECTIVELY ABSORBING COATINGS. Journal de

Physique Colloques, 1981, 42 (C1), pp.C1-191-C1-194.

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SELECTIVELY ABSORBING COATINGS

J. Vuletin, P. ~uliSi6 and M. Bosanac

F a c u l t y of E l e c t r i c a l E n g i n e e r i n g , S p l i t , Y u g o s l a v i a

RBsm6.- Quelgues revgtements ?our absorbeur s6lecti.f ont 6t6 pro- duits par galvanisation et conversion chimique d'une couche ap- pliquee en surface sur une plaque mgtallique. Du chrome noir a 6t6 obtenu par galvanisation d'oxyde de chrome sur des sous-cou- ches de cuivre (fer ou bronze) nickels, et du cuivre noir (oxyde de cuivre) a Bt& pr&par& au moyen d'hydroxyde de sodium et persul- phate de potassium. La reflectance du spectre de ces recouvrements a 6tg analys6 et leur application B des collecteurs solaires a

6tg discut6. On a de meme fait des recherches sur la stabilite S

llhumidit&, les changements de temperature et la dgformation mB- canique de ceux-ci.

Abstract.- Some selective absorber coatings were produced by elec troplating and by chemical conversion of a surface layer on the metal plate. Black chromium was obtained by electroplating chro- mium oxide onto nickel coated copper (iron or brass) substrates and black copper (copper oxide) was prepared by means of sodium hydroxide and potassium persulphate. The reflectance spectra of these coatings have been investigated and their applications on solar collectors were discussed, Stability on humidity, tempera- ture changes and mechanical deformation have been investigated.

1. Introduction.- Most optically efficient selective solar absorber

coatings combine high solar absorptivities with low thermal emissivities. The selective absorption of such coatings is due to the presence of a

surface layer which absorbs appr~ximately the whole solar spectrum whi-

le it almost does not permit infrared frequencies reemission.

2. Selective absorber coatings.- Several selective absorber coatings

are produced by electroplating and by chemical conversion of a surface layer on the metal plate. Black chromium is obtained by electroplating chtomium oxide onto nickel coated copper (iron or brass) substrates. Black copper (copper oxide) is prepared by means of sodium hydroxide add potassium persulphate.

Visible and infrared spectral reflectance is measured using spectropho- tometers (Perkin-Elmer) with adequate reflectance attachment. Figure 1 shows some results obtained from measurements of reflectance of black chromium samples. One can see that even small variations in technologi- cal processes cause a change in the selectivity of the sample.

JOURNAL DE PHYSIQUE

Fig.1.- Reflectance as a function of wavelengths for black chromium samples prepared under various electrolytic condi- tions.

Calculations are made to obtain the values of absorptance (averaged over about the total spectrum from 0.3 to 2.5 pm) and emittance (avera- ged over a blackbody wavelength distribution for the corresponding tem- peratures)

.

The maximum solar absorptance of 0.95 is obtained for black chromium with the minimum emittance of 0.20. This high solar absorptance and low thermal emittance are achieved by controlling the coating thickness through variations in plating time and in current density.

An excellent absorptivity of 0.98 with a very satisfactory emissivity

of 0.10 is obtained for black copper samples by varying the duration of

chemical reactions (Fig.2). These selective properties are improved as compared to those reported in literature

/a/.

The coatings are subjected to a heating test to determine their thermal stability, resistance to humidity and mechanical deformation. The test results indicate that there are no significant changes in these proper- ties under the conditions that may be expected in applications. Black chromium samples show better thermal stability than black copper sam- ples ; however, the latter have the advantage of being produced more readily and in a simpler way.

The efficiency curves of flat plate collectors are determined using the Hottel-Whillier-Bliss equation. The usual values of collector's parame- ters are taken /7/. Figure 3 shows collector efficiency as a function

on t h e c o l l e c t o r s u r f a c e p e r u n i t a r e a ) f o r black chromium, black cop- p e r and b l a c k p a i n t a b s o r b e r . I t can be seen t h a t t h e e f f i c i e n c y o f a c o l l e c t o r w i t h s e l e c t i v e c o a t i n g i s much b e t t e r t h a n t h a t of a c o l l e c - t o r w i t h black p a i n t . The e f f i c i e n c y of a black copper c o l l e c t o r i s about 3% h i g h e r than of a black chromium c o l l e c t o r .

Fig.2.- Reflectance a s a f u n c t i o n o f wavelength f o r black copper samples.

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401 3 M '@7 W a'/

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JOURNAL DE PHYSIQUE

3. Conclusion.- The produced black chromium and black copper samples have a high solar absorptance and low thermal emittance. The test re- sults indicate that black chromium shows better thermal stability, higher resistance to humidity and mechanical deformations than black copper samples, but the production of black copper absorber is more simple.

References

/1/ McVeigh, J.C., Sun Power (Pergamon Press, Oxford) 1977

/2/ Tabor, H., Selective Surface, First International

Symposium on Non-conventional Energy, Trieste (1979) /3/ Harding, G.L., Thin Solid Films,

38

/4/ Vuletin, J., ~arselja, E., ~uliZi6, P. and Bosanac, M., Energija, 11-12 (1977) 425

/5/ Duffie, J.A. and Beckman, W.A., Solar Thermal Processes

(John Wiley, New York) 1974

/6/ ~uligid, P., Bosanac, M. and Vuletin, J., Zbornik radova "Primjena sun8eve energije u turisti8kim objektima", Trogir, 1979

,

/7/ Kulizic', P., Bosanac, M. and Vuletin, J., Zbornik radova "Solarna energi ja u agroindustri jskom kompleksu"

,

,

/8/ Kreith, F. and Kreider, J.F., Principles of Solar Engi-