The energy absorbing properties of solar heating panels are improved by depositing a black chrome coating of controlled thickness on a specially prepared surface of a metal substrate. The surface is prepared by depositing a dull nickel on the substrate, and the black chrome is plated on this low emittance surface to a thickness between 0.5 micron and 2.5 microns

Mcdonald, G. E.

English

NASA Technical Reports Server

United States Patent 
McDonald 
1191 4,055,707 
1451 Oct. 25, 1977 
SELECTIVE COATING FOR SOLAR PANELS 
Inventor: Glen E. McDonald, Strongsville, 
Ohio 
Assignee: The United Stated of America IS 
represented by the Administrator of 
the N i t i o d  Aeronautics urd Space 
A- tion, Washington, D.C. 
Appl. No.: 643,041 
Filed: Dec. 22,1975 
Int. 0 . 2  ......................... F24J 3/02; B32B 15/18; 
B32B 15/20; BO5D 3/12 
U.S. c1. .................................... 428/6!5& 126/270; 
Field of Search .......... 126/270, 271; 350/1; 
427/162, la0; 428/652, 667,679 
427/160; 42a1667; 428/679 
Refemas Cited 
U.S. PATENT DOCUMENTS 
2,917,817 12/1959 Tabor ................................... 126/270 
3,920,413 11/1975 Lowery ................................ 126/270 
OTHER PUBLICATIONS 
NASA Technical Memo X-3136, Dec. 1974. 
NASA Technical Memo X-71596. 
“Spacecraft Radiators” in S’ce/Aeronautics, pp. 76-82, 
Jan. 1962. 
Perveev, et al. “Antireflecting Coatings on Light-Ab- 
41, Aug. 1974, pp. 453-455. 
Primary Examiner-James R. Hoffman 
Attorney, Agent, or Finn-N. T. Musial; G. E. Shook; 
John R. Manning 
sorbing Materials. .... in Sov. J. Opt. Technology, vol. 
1571 ABSTRACT 
The energy absorbing properties of solar heating panels 
are improved by depositing a black chrome coating of 
controlled thickness on a specially prepared surface of a 
metal substrate. The surface is prepared by depositing a 
dull nickel on the substrate, and the black chrome is 
plated on this low emittance surface to a thickness be- 
tween 0.5 micron and 2.5 microns. 
8 cwms, 2 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19780011656 2020-03-20T12:55:44+00:00Z
U.S. Patent October 25, 1977 4,055,707 
FIG. I 
22 
20 
FIG. 2 
1 
4,055,707 
2 
~ 
with high absorptivity and low emissivity while exhibit- 
ing long term durability under solar radiation. SELECTIVE COATING FOR SOLAR PANELS 
ORIGIN OF THE INVENTION 
A further object of the invention is to provide im- 
moved solar heating panels having a black chrome 
The invention described herein was made by an em- 5 coating of controlledthickness. 
-
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
BACKGROUND O F  THE INVENTION 
This invention is concerned with improving solar 
heating panels. The invention is particularly directed to 
an improved coating for solar heating panels. The coat- 
ing has high absorptivity for visible solar radiation and 15 
low emissivity for infrared radiation. 
the sun for conversion to heat and electrical power. 
These devices utilize flat composite panels tb collect the 
energy. 
Selective coating of the heating panels has been sug- 
gested to increase the energy absorbing properties. By 
way of example, a solar heating panel may be coated 
great amount of solar energy. However, it has been 
Black paint also enables much of the absorbed energy to 
As shown in FIG. 2 each composite panel 14 has a be lost by emittance. 
30 substrate 16 forming a high thermal conductivity base. 
heating panels. Black nickel is a good heat eonductor, The substrate 16 is a metal having good heat exchange 
but coatings of this material are vulnerable t b  moisture. properties, such as steel, aluminum, or copper. The 
Various attempts have been made to improve the substrate 16 has at least one surface 18 exposed to solar 
black nickel coatings. For example, a substrate is first energy. 
coated with bright nickel and then overlaid with a layer 35 An important feature of the invention is that the sur- 
tive coating has been difficult to control in the deposi- having the solar selective properties required for the 
tion of black nickel coating in order to achieve the solar collector 10. This black chrome coating has high 
desired optimum Propertie and coating thickness. As absorptance across the solar spectrum and low emissiv- 
the black nickel is being deposited, the substrate cannot 40 ity in the 250" F blackbody thermal spectrum. ne 
be removed from the plating bath without the require- preparation of the surface 18 prior to coating is an im- 
ments for stripping the black nickel and restarting the portant feature of the invention. 
deposition process again. Also, the black nickel, de- Black chrome was plated in accordance with the 
posited, is still subject to corrosion from the atmosphere invention on aluminum tube sheet panels, after an ap- 
and other sources. 45 propriate zincating procedure. The black chrome was 
Other thin exPoEd light-absorptive and heat trans- also plated on cold rolled steel panels that were buffed. 
parent Surface coatings are disclosed in U.9. Pat. NO. The panels were 4 by 6 inches having a thickness of 
2,917,817 to Tabor. These coatings are in heat conduc- 0.035 inch. The application of black chrome was deter- 
tive relation to a bright metal base. Sol& heating panels mined to be equally feasible on aluminum or steel. 
utilizing these coatings have been expensive and dim- 50 Each panel was first electrolytically cleaned. An alka- 
cult to produce. line electrolytic cleaner interspersed with acid was used 
Still another object of the invention is to provide 
improved solar heating panels which have coatings 
with improved stability and corrosion resistance. 
These and other objects of the invention will be ap- 
10 parent from the specification which follows and from 
the drawings wherein like numerals are used through- 
out to identify like parts. 
DESCRIPTION O F  THE DRAWING 
FIG. 1 is a perspective view of a solar collector em- 
bodying the features Of the invention; and 
line 2-2 in 
Solar collectors are devices that collect energy from FIG* 2 is an enlarged vertical section view the 
1. 
DESCRIPTION O F  THE PREFERRED 
EMBODIMENT 
Referring now to the drawing, there is shown in FIG. 
1 a solar collectnr 10 that is utilized to collect radiant 
energy from the sun for conversion to heat. A fluid, 
by this energy. The solar collector 10 includes at least 
accordance with the invention. 
20 
with paint which is jnexpensive and a 25 such as water or air, flowing through tubes 12 is heated 
found that paint flakes, chips, and is not very durable* one flat composite panel 14 constructed in 
has been wed as a coating for 
Of black nickel. However, this bright-black nickel selec- face 18 of the substrate 16 is coated with black chrome 
SUMMARY O F  THE INVENTION at 190" F at 70/80 amps/ftz. Two cycles were used with an acid diD between them. An electrolytic alkaline ma- 
These problems have been solved by the present in- terial known commercially as a Kelating cleaner has 
vention wherein black chrome is utilized as the selective 55 been satisfactory. 
solar coating. The black chrome deposit has a high A layer 20 of bright nickel was deposited onto the 
absorptivity in the visible range and a low emissivity in surface 18 of the substrate 16 to produce low emittance 
the infrared range when the coating is between about and to prevent the steel from rusting. The layer 20 was 
0.5 micron and about 2.5 microns. approximately 0.0005 inch thick and was plated in 15 
60 minutes at 40 A/ftz. A bright nickel plating solution 
known commercially as Zodiac prepared by the Har- OBJECTS O F  THE INVENTION 
It is, therefore, an object of the present invention to shaw Chemical Company has been satisfactory for the 
provide improved solar collectors having heating pan- plating of the bright nickel. 
els which absorb and retain a greater amount of solar A black chrome layer 22 was then deposited onto the 
energy by being coated with a material which is readily 65 bright nickel layer 20. The black chrome layer 22 was 
available and easily applied. plated at 24 volts and 200 amps/ftZ for three minutes. A 
Another object of the invention is to provide im- black chrome electroplating solution made from a mix- 
proved solar heating panels having low cost coatings ture known commercially as Chrome-Onyx manufac- 
4,05 5,707 
3 4 
tured by the Harshaw Chemical Company has been The importance of the preparation of the surface 18 
satisfactory. for coating is illustrated by panels A, C, and D as com- 
Black chrome electroplating solutions can also be pared to the previously described coating B. Black 
prepared in accordance with the teachings of a publica- chrome coating A was plated over dull nickel while 
tion entitled METAL FINISHING, p. 268, by Metals 5 coatings C and D were plated over surfaces roughened 
& Plastic Publications, Inc, dated 1974. This publication by grit blasting. In contrast, black chrome coating B 
lists a plating bath as follows: was plated over bright nickel on steel buffed to a RMS 
roughness of less than 0.125 pm (0.5 pin.) as previously 
described. 
The panels were the same size as panel B and were 
electrolytically cleaned in the same manner. Panel A 
had a layer 20 of dull nickel applied instead of a layer of 
bright nickel as panel B. The layer 20 of dull nickel was 
approximately 0.0005 inch (0.0125 m) thick and was 
15 plated in 15 minutes at 40 A/ft*. A plating solution 
known commercially as NUSAT prepared by the Har- 
shaw Chemical Company was satisfactory. 
The general appearance of the black chrome on dull 
nickel coating on panel A was that of a matte diffuse 
20 surface as contrasted with the mirrorlike surface of the 
black chrome on bright nickel combination on panel B. 
Panels C and D appeared shiney but rough as con- 
trasted with the dull flat appearance of panel A. 
Spectral measurements were measured in the manner 
The black chrome coated panel was then water-rinsed, 25 previously described. The roughening of the surface 18 
rinsed again in alcohol, and air dried. either by underplating with dull nickel or by physical 
Visible and infrared spectral reflectance measure- abrasion decreases the reflectance in the visible spec- 
ments were measured for the black chrome coated trum by approximately a factor of 2. The reflectance of 
panel B, several black nickel solar selective coating on the coating on panel A was indistinguishable from those 
panels E and F, and a black paint coating G. The spec- 30 of the coatings on panels C and D. However, the infra- 
tral reflectance was measured with spectrophotometers red reflectances of the coatings on panels C and D are 
with spherical diffuser reflector attachments. A MgO lower than that of either panels A or B. 
surface was used as standard for measurements in the The increased absorptance of panels A, C and D is 
range of 0.35 to 2.1 microns while an evaporated gold believed to be the result of a physical effect of light 
film was used as the standard from 3.0 to 18.0 microns. 35 trapping. However, the roughness produced in the dull 
The general appearance of the black chrome coating nickel is apparently of such a small scale that it has no 
was indistinguisable from the black nickel coatings. The effect on the infrared reflectance. The larger scale 
reflectances of the black chrome, the two black nickels, roughness produced by grit blasting produces a Hohl- 
and the black paint integrated over the solar spectrum raum effect for the infrared reflectance. 
for air mass 2 were 0.132,0.123,0.133 and 0.033, respec- 40 Table I1 shows the variation of the solar selective 
tively. The reflectances of these same coatings ink- properties of the black chrome on dull nickel coating on 
grated over the blackbody spectrum for 250" F from 3 panel A with coating time. 
to 15 microns were 0.912, 0.934, 0.891, and 0.033, re- TABLE I1 
spectively. 
BODY EMI-ITANCE OF SOLAR SELECTIVE BLACK 
10 
Chromic acid 33-40 oz/gal 
Acetic acid 28.2 oz/gal 
Temperature 
Current density 40-90 amp/ft2 
Barium acetate 1 oz/gal 
90-115' F 
Another plating both listed in this publication is: 
Chromic acid 33 oz/gal 
Fluorsilic acid 0.033 oz/gal 
Current density 150-450 amp/ft2 
Temperature 80-95' F 
SOLAR SPECTRUM ABSORPTANCE & 250" F BLACK 
Chrome ON DULL Nickel 
Plating Time Absorptance Emissivity 
15 Sec 0.64 0.04 
30 Sec 0.87 0.06 
These reflective measurements indicate absorptivity- 45 
to-emissivity (a/€) values set forth in Table I. The val- 
ues of absorptance and emissivity are also given in 
Table I. The black nickel coatings on panels E and F 
showed considerable differences in reflectance charac- 1 Mm 0.96 0.10 
teristics which were evidently the result of differences 50 2 Min 0.96 0.12 
4 Min 0.95 0.17 
in application procedures 10 Min 0.94 .34 
TABLE I. 
Visible Absorptance & Infrared Emissivity of Solar 
Selective Coatings 
AbSOrp- 
Absorptance Emissivity tance to 
Coating Material a E Emissivity 
Black Chrome on Nickel 
A. Dull Nickel 0.923 0.085 10.9 
Bright Nickel Surface Roughness 
B. Bright Nickel 0.0125 p m (0.5 pin) 0.868 0.088 9.8 
Preparation Buffing 
C. Bright Nickel 0.75 m(30 p in.) 0.930 0.12 7.8 
Preparation Grit $lasting 
D. Bright Nickel 25- pm/100 -pin.) 
Preparation Grit Blasting 
Black Nickel 
E. 
F. 
Black Paint 
G. Nextel 
0.877 0.066 13.3 
0.867 0.109 8.0 
0.967 0.967 1 .o 
4,055,707 
5 6 
For plating time of 15 seconds to 10 minutes, the 
general appearance of the panel varied from almost 
purely dull-nickel appearance at 15 seconds to a velvet 
black at 10 minutes. Only a very faint deposit of black 
chrome could be observed on the 15 second panel by 5 
visual observation. 
Spectral measurements were made. Both the visible 
and the infrared reflectances decrease with increasing 
plating time. There is an increase in black-chrome thick- 
ness with increasing plating time. There is first a rapid 10 strate is aluminum. 
increase in visible absorptance without a significant 
increase in infrared emittance until a nearly maximum 
value of absorptance is reached, after which the emit- 
tance rapidly increases with no appreciable increase in 
absorptance. In fact, as plating is continued, there is 15 
even a slight decrease in absorptance across the solar 
spectrum. 
The optimum Plating time is determined by the radia- 
tive energy balance of the collector plate. It has been 
found that the optimum plating time iS approximately 20 
one minute at 180 A/ft2. This produces a coating having 
a thickness between about 0.5 micron and about 2.5 
microns . 
While the preferred embodiment of the invention has 
been described, it will be appreciated that various modi- 25 
fications may be made without departing from the spirit 
of the invention or the scope of the subject claims. 
a layer of dull nickel on said one surface to lower the 
emissivity of said surface, and 
a coating of black chrome on said layer of dull nickel, 
said coating having a thickness between about 0.5 
micron and about 2.5 microns and being highly 
absorbtive in the visible range. 
2. Apparatus as claimed in claim 1 wherein the sub- 
3. Apparatus as claimed in claim 1 wherein the sub- 
4. Apparatus as claimed in claim 1 wherein the layer 
5. A method of making a composite heating panel for 
depositing a layer of dull nickel to a thickness of about 
0.0005 inch on a surface of a high thermal conduc- 
tivity substrate to lower the emissivity thereof, and 
coating said layer of dull nickel with black chrome to 
a thickness between about 0.5 micron and about 2.5 
microns to raise the absorbtivity of said surface in 
the visible range. 
6. A method of making a composite heating panel as 
claimed in claim 5 wherein the surface of the substrate 
is roughened. 
7. A method of making a composite heating panel as 
claimed in claim 6 wherein the substrate surface is 
roughened by physical abrasion. 
What is claimed is: 8. A method of making a composite heating panel as 
1. In apparatus for Collecting solar energy ofthe type claimed in claim 7 wherein the substrate surface is grit 
having a composite heating panel for absorbing said 30 blasted to a roughness of between about 0.75 pm and 
solar energy and converting the same to heat, the im- ’ about 25pm. 
provement comprising 
a metallic substrate forming a high conductivity base 
for said panel, said substrate having at least one 
strate is steel. 
of dull nickel has a thickness of about 0.0005 inch. 
a solar collector comprising the steps of 
* * * * *  surface exposed to said solar energy, 35 
40 
45 
50 
55 
60 
65 


Selective coating for solar panels

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