UV Transmission through glass of commercial mirrors

Solar Eneroy Vol. 25, pp. 575-576

0038-092X/80/1201-0575/$02.00/0

© Pergamon Press Ltd., 1980. Printed in Great Britain

TECHNICAL NOTE UV Transmission through glass of commercial mirrors1" F. L. BOUQUET Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91103 U.S.A.

(Received 1 November 1979; revision accepted 8 July 1980)

With the advent of increasing interest in using the sun's energy to produce power, high performance concentrators appear promising to achieve high solar intensities [1]. One of the most readily available types of reflectors being considered is a second-surface silvered glass mirror. High quality silvered mirrors, when exposed outdoors, have initial normal solar reflectances in the 88-96 per cent range [2]. However, the reflectance vs wavelength curve shows decreased reflectance near 300 nm [3, 4] which has been called by one author the "silver hole" [5]. It is the purpose of this paper to present new measurements of UV transmission through glass and silver metallization. The nature of corrosive forces on the reflectance of silvered mirrors, exposed outdoors, is currently inexactly known. However, a number of research paths are being tThe research described in this paper was carried out at the Applied Mechanics Technology Section, Jet Propulsion Laboratory, California Institute of Technology and was sponsored by the Department of Energy through an agreement with NASA. lO

pursued by this and other laboratories to develop longlived solar mirrors. One approach is to measure the intensity of the ultraviolet radiation penetrating the glass, silver, and copper metallization backing of commercial mirrors. The rationale is to determine if the UV could be the driving force for potential corrosion mechanisms. The transmissivity was measured for two types of unpainted commercial mirrors using a Cary Type 219 Spectrophotometer and the results are shown in Figs 1 and 2. The UV component is found to penetrate both the glass and metallization and therefore can impinge upon the paint coating. The variation of the peak intensities, 2.1 per cent 4.7 per cent, is typically observed batch-to-batch due to non-uniformity in the metallization. The decreased transmission of sample 1 (0.3 per cent peak) is assumed to be the result of slightly thicker glass (2.89 mm vs 2.197 mm) and improved metallization. Other experiments have shown that UV is capable of causing degradation of similar organic systems. Therefore, the possibility that the small but finite UV intensity could be responsible for basic mirror corrosion paths should be researched.

t

I

I

I

I

A • Instrument Cary 219

8

eSodo lime gloss thickness 2 . 8 9 mm (0.114 in.)

C

eCoating Silver -861 m g / m z (0.176m I b / f t z ; • Copper - 215 m g / m z ( 0 . 0 4 4 m I b / f t z) eSample t (Buchmin)

Glass-Ag-Cu

I. 4 Per cent (350A) c::

loss

B Gloss + A g

Q. tn

(340A) C Gloss -t-Ag ÷ C u

0.I

f 300

320

340

360

380

400

420

Wavelength, nm

Fig. 1. Spectral transmission vs wavelength for a typical mirror. 575

440

460

576

Technical Note 10

I

I 4 . 7 Per cent

I

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e Soda lime glass • Coating Silver- 861 mg/m z {0.176m I b/ft z)

E

2.1 Per cent

~

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Sample 3 8 (Binswanger)

~

Glass: 2.197mm (0.0865in.)

E

o

Cu: 2 6 9 mg/m 2 (0.055m I b / f t z) o

Sample37

(Rinswanger)

O3 0 . 3 Per cent

~

Sample I (Buchmin) Glass: 2 . 8 9 mm (O, II4in.)

C

Glass: 2.197 mm (0.0865 in. ) Cu: 269 mg/mZ (0,

u: 215 mg/mZ(O.O44m I b / f t z)

0.1

3O0

320

340

360

380

400

420

440

460

Wavelength, nm

Fig. 2. Spectral transmission vs wavelength for mirrors with constant silver metallization.

REFERENCES

l. E. A. Igel and R. L. Hughes, Optical analysis of solar facility heliostats. Solar Energy 22, 283 (1979). 2. F. L. Bouquet, Glass for solar concentrator applications. JPL Rep. 5102-105 (April 1979).

3. W. G. Driscoll and V. William, Handbook of Optics, pp. 8-92. McGraw-Hill, New York (1978). 4. R. Kingslake, Applied Optics and Optical Engineerin9, Vol. III, p. 313. Academic Press, New York (1965). 5. C. Fabry, Optical Workshop Principles 2nd Edn, p. 347. Hilger & Watts (1954).