Note: Descriptions are shown in the official language in which they were submitted.
2~
This invention relates to solar collector~ and
more particularly to a new and impro~ed solar collector
having a selective ilm of improved stabilit~ to liquid
water~
It is well recognized that the fossil fuels of
the w~rld are being depleted. Indeed in certain oil-
producing countries, the known reserv~s oF oil are o~
~uite limited magnitude~ It would thereore app~ar of a
high level of importance to utilize the energy o the sun,
i.e, solar energ~ to the extent we feasibly can to me~t
our energy requirements~ Solar energy could, in theory,
supply all o~ ~he energy requirements of this country,
Considerable research and experimentation has
been carried out heretofore in the field of solar thermal
~nerg~ conversion, and the technology o~ solar thermal
energy conversion is fairly well advanced. One of the
most important o~ a number o fairly recent discoveries
in this field that have signi~icantly improved the e~ficiency
o solar thermal energy collectors is ~he use of "selectlve"
sur~aces, which are also referred to as spectrally selecti~e
surfaces, A surace that is both a good absorber o~ solar
radiation and a poor emitter of infrared radiation is said
to be "selectivel~ or ~pectrally selective~ A selectiv~
surface can significantly improve the thermal eficiency
o~ solar thermal energy collectors or solax heat collectors~
It ts essential that the selective surface be stable or
durable under the conditions encountered in the operation
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of solar heat collector.
Prior art selective sur~aces sufer ~rom a stabi
lity or durability problem, in that liquid water contacting
the selective suraces of the solar collectors when the
S collector box is not sealed, such as the water ~ormed by
the condensation of moisture from the air within the
collector box in the cooler t~mperatures o ~vening,
d~trimentally chang~s the thickness o tha selective copper
oxide coating on the copper panel or flat plate surface
10 re~ulting in an undesirable, considerable reduction in the
selectivity of ~he black coating . Consequently, the solar
absorptIvity of ~he selective coating of th~ collector i5
considerably lowered and usually to an undesirable value
below O .90 and the thermal emissivity of the selective
coating is considerably increased and usually to an unde-
sirable value abo~e 0.20~ Such lack of desired stability
of the selective coating to contact with water is es-
pecially a problem, when the selective coating or ~ilm is a
black cGpper oxide ~CuO) coating or film on a copper or
copper alloy substrate and is produced by contacting the
substrate with a prior art aqueous blackening solution
comprising an oxidizing agent or ~he copper such as an
alkali metal chlorite, and caustic sodaO
U.S, Patents 2,364,993 and 2,460,896 disclose
the deposition of black coatings or films on copper and
copper alloy surfaces by immersing the surace in a bath
comprising an aqueous solution of alkali metal hydroxide,
e.g. caustic soda; and sodium chlorite or potassium
chlorite. U.S~ Patent 2,481j854 also discloses the0 blackening of surfaces of copper and copper alloy by con
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3tacting the surface with an aqueous solution comprising
sodium chlorite or potassium chlorite, an alkali metal
hvdroxide, e.g. sodium hydroxide. The chlorite utilized
in preparing the blackening solution is uniformly fixed
in the water of hydration of the alkali metal hydroxide.
The publication entitled "Spectral and Directio~al Thermal
Radiation Characteristics of Selective Suraces for Solar
Collectors" by D. XO Edwards, J. T. Gier, K. E~ Nelson
and Ro D~ ~oddich, presented to the United Nations
Conference on New Sources of Energy, April 20, 1961,
discloses that promising low temperature collector suraces
appear to be copper or steel treated by the commercial
"EBO~OL" (a Tradb Mark) processes. Ihis last-mentioned publica~n also
discloses that commercial chemical dip treatments of
copper and skeel are shown to give ~el~ctive characteristics
desirable fo~ low temperature collectors such as solar
water heaters, and that a copper surface dipped 5 minutes
in "EBONoL" (a Trade Mark), C blacke~er solution at 175F. ~x~Nced a
coating on the surface having a solar absorptivity of 0.91
and a thermal emissivity of 0~16 for collectors in spaoeD
Prlor art solar energy collectors and heaters are
disclosed ~y U~S. Patents 1,425,174; 1,888,620t 1,889,238;
1,971,242~ 2,202,756; 2,208,789; 2,931,578; 1,~34,465;
2,553,307; 3,176,678 and 629,122.
The ~olar collector of the present invention com-
prises a ba~e or 8ub9trate of copp~r or copper base alloy,
and a stable, selective, solar heat energy-absorptiv~
coating on the base~ This selective, solar heat energy-
Absorptive coating has been rendered ~table by contacting
the select~ve, ~olar heat energy-absorptive coating on
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the base with a chromic acid solution for a tima sufficient
to impart to the selective coating a stability such that its
selectivity is not significantly reduced after contact with
liquid water, ordinarily liquid water condensate, over a
prolonged, cumulative, noncontinuous time in excess of 1600
hours.
~ he stable, selective, solar heat energy-absorptive
coating on the base, is characterized by having a good or
high solar absorptivity, usually of 0.90 or higher, and a
poor or low infrared emissivity, usually no more than 0.20.
Further, the stability of this coating is such that it
ordinarily retains its high solar absorptivity of 0.90 or
higher and its low thermal emissivity of no more than 0.20
after the contact with the liquid water over such prolonged
period in excess of 160n hours.
The temperature of the chromic acid solution during
the contacting of the selective, solar heat energy-absorptive
coating on the base to render the coatinq stable, can be
within the range of room temperature to the boiling temp-
erature of the chromic acid solution.
In a specific embodiment, ~he selective, solarheat energy-absorptive film or coating of the solar collectors
herein is prepared by a process comprising contacting the
copper or copper alloy base, with a hot aqueous solution
comprising an oxidizing agent capab~e of oxidizing copper to
cupric oxide, an alkali t and an aqueous liquid, usually
water, for a time sufficient to obtain the selective coating
or film on the base having the solar absorptivity o 0.90
or higher and the infrared emissivity no more than 0.20.
It is to be understood, however, that the selective,
.- --5--
solar heat energy-absorp~ive aoating or ilm of the ~olar col-
lectors of this invention can be provided on the copper or
copper base alloy by any suitable process, method or procedure.
More specifically, the aqueous solution of the
aorementioned specific embodiment for producing the selective
film on the copper alloy surface, for example the copper or
copper alloy flat plate or shee~ surface and, if desired, on
the surfaces of the metallic tube~, when utilizad, adapted
to contain the heat transfar liquid, such as the copper or
copper alloy water tubes, is an aqueous blackening solu~ion
comprising an alkali metal chlorite as the oxidizing agen~,
an alkali me~al hydroxide as the alkali, and water. The
coppar or copper allo~ surface is contacted with such aqueous
blackening solution comprising the alkali metal chlorite,
alkali metal hydroxide, and water while the solution is hot,
and or a time sufficient to obtain the selective film or
coating on the surface having the solar energy absorptivity
of 0.90 or higher and the thermal emissivity of no more than
0.20. The stabilizing treatment of the selective coating
~y contacting same with the chromic acid solution is then
carried out as previously disclosed herein.
~ he oxidizing agent constituent o the aqu~ous
solution for producing the selective film on the copper or
copper alloy can be any suitable oxidizing agent capable
of oxidizing copper to cupric oxide. We have obtained good
results in blackening the surfaces of copper and copper
alloys with use of an alkali metal chlorite, e.g. sodium
chlorite or potassium chlorite, as the oxidizing agent.
The alkali constituent of the selective-film producing
aqueous solution is ordinarily caustic alkali, i.e. an
_~_
alkali metal hydroxide, e.g. sodium hydroxide or pota~sium
hydroxide. The selective film or coating produced by this`
aqueous solution is a black coating which may be lighk, medium
or deep black. This coating may also be of another dark color,
such as, for example, dark brown.
By "copper alloy" as used herein is meant a copper
base alloy containing more than 50% by weight copper and i~
exempli~ied by copper base alloys containing, by weight, from
65% ko more than 99% by weigh~ eopper. Specific example~ o
such copper ba~e alioys are: rad bra~s, 85% Cu 15% Zn; yellow
brass, 65% Cu 35% Zn; admiralt~ brass, 71% Cu 28~ Zn 1% Sn;
leadad bronze, 8~% Cu 9.25% Zn 1.75% Pb; and beryllium copper,
Cu 2% Be 0.2S% Co or 0.35% Ni. The term~ "selective" and
"selectively" are use~ herein ko mean a surface or surface~ of
a coating or film that is a good ab~orber of solar radiation
and a poor emitter of infrared radiation, a~ exemplified by a
solar absorptivity of .95 and an infrared emissivity of .05.
In a specific embodiment of the complete method
for preparing the-stable, selective coating or film herein,
the ~opper or copper alloy surface, for Pxample, copper or
copper alloy surface~ of plates or sheets de~tined to be
solar collector panels, and, if desired, copper or copper
alloy water tube surfaces if not already clean, are cleaned
by immersion for ~ypically 3 minute~ in an alkaline cleaner
liquid at eleva~ed t~mperature of typically about 180VF. and
cbtained by æ~xing "ENPLATE" (a Trade Mark) 453 cleaner ooncentrate wi~h
water, in amount sufficient to form the alkaline cleaner
liquid of about 50% by ~olume cleaner concentrate concentra-
tion. The panels are then water rinsed. The clean copper
or copper alloy pan~ls are then immerced in a deoxidi~er
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~L~'.P~0%'~
liquid for ~ypically 1-3 minutes at room tempera~ur~ of the li-
quld and obta~d by a~xing "ENP$~TE" (a Ira~rk) AD 482 deoxldizer con-
centrate in water in amount sufficient to form the deoxidizer
liquid of about 30% by volume deoxidizer concentrate concen-
tration. The panels and tubes, when utili3ed, are then waterrinsed.
The thus-treated clean copper or copper allo~ plates
and tubes, when u~ilizad, are then immersed in, or otherwi~e
contacted with, a blackener solution comprising 1 1/2 lbs. of
"EBONOL" ta Tra~Erk) C blackener ooncentrate per gallon of wa~er, for
about 1 to 10 minutes at a solution temperature in the range
o about 140F. to about 220F. The blackener concentrate
contains a mixture of an alkall metal hydroxide, e.g. sodium
hydroxide and an alkali metel chlorite, e.g. sodium chlorite.
A black or dark film or coating of thickness within the
range of about 0.01 mil to about 0.5 mil is thereby formed
on the copper or copper alloy surace. This thickness with~n
the about 0.01 mil to about 0.5 mil thickness range i~
important for the reasons that if tha black or dark film
is significan~ly thinner than 0.01 mil, undesirable heat
radiation occurs, and if the^black or dark film is signi_
ficantly thick~r than 0.5 mil, excessive thermal emi~sivity
occurs. The panels ara then water rinsed.
The thus-treated, blackened copper or copper alloy
surface or surfaces are then immersed in, or otherwise con-
tacted with, a chromic acid solu~ion which is a solution of
chromic anhydrida, i.e. CrO3, in a suitable liquid solven~,
ordlnarily an aquaous liquid solvent such as water. A suit-
able chromic acid ~olution for u~e herein contains typically
10 lbs. of CrO3 per lnO gallons of solvent such as water.
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~ 0~'~3
The blackened copper or copper alloy plates and tubes, when
utilized, are maintained in contact with the chromic acid
solution at a temperature in the range of room temperature
to boiling temperature of the solution for a time period
sufficient to render the selectlve, black coating on the
copper or copper alloy surface or sur~aces stable. ~he usual
contact tima of the blackened aopper or copper alloy suraces
with the chromlc acid solution to render the ~elective,
blackened copper or aopper alloy ~ur~aces stable i5 about
ln to about 60 saconds at a temperature of the chromia acid
solution of about 130~.
Although it $s n~t kncwn with o~tainty, it i8 believed
the chromic acid treatment o the 3elac~ive, black or dark
coating or film render~ the same ~table by inhibiting the
migration and leaching out of copper ions from the copper
oxide coating or fllm when contacted with liquid water.
When this migration and leaching of copper ions occur~ the
thi~kness of the black or dark copp~r oxide coating is un-
desirably changed and apparently reduced, resul~ing in the
selectivi~y of the black coating being undesira~ly lowered.
~he chromic acid ~olutlon contacting treatment of the
selective, black or dark aoating film, by inhibiting this
migration and leaching of ~he copper ions from the copper
oxide coating, prevant~ or at least minimizes or inhibits
this undesirable change and reduction of thickness of the
copper oxide coati~g.
In test~ involving prolonged contact to moisture of
panels which h~d been ~ubject~d to th~ post-blackening treat-
ment with chromic acid solution in accordance with this
invention and also panels which had not been sub~ected to the
_g
z~
chromic acid solution treatment after the blackening in accord-
ance with this invention, both types of panels were placed in
a humidity cabinet and held therein for 200 hours at 100~
relative humidity. The panels which had been blackened and
then subjected to treatment with chromic acid solution in
accordance with this invention showed no change in the thick-
ness of their black films after remova~ from the cabinet.
The panels which had been blackened bu~ not sub~ec~ed to the
chromic acid solution treatment in accordance with this
invention, showed a material reduction in thickness of their
black coatings after removal from th~ cahinet and a material
lowering of the selectivity of their black coatings.
In the drawings:
FIGURE 1 is an enlarged, schematic cross-sectional
view, partially broken away, of a solar collector of this
invention;
FIGURE 2 is a cross-sec~ional view of a solar
collector module of this invention taken on line 2-2 of
FIGURE 3;
FIGURE 3 is a plan view of a solar collector
module o this invention.
With reference to FIGURE 1, solar collector 10
includes copper or copper alloy plate or sheet 11 and
liquid wat~r-resistant, thin, selective coating 12, which
is of black copper oxide, i.e. CuO. Selective coating 12
has been conditioned by contact with chromic acid solution
in accordance with this invention and has a stability, ~ue ~o
such condi~ioning, such that its selectivity is not signi~
ficantly reduced after contact with liquid water condensat~
~0 over a prolonged, cumulative, non-con~inuous time to excess
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g)~
of 1600 hours, usually in excess of 15,000hours, e.g., about
17,000 hours or more, but which retains a solar absoprtivity
of 0.90 or higher and a thermal emissivity of no more than
0.20 after its contaGt with the water condensate over such
prolonged time. Black copper oxide coating 12 comprises a
multiplicity o discrete, generally aonical-shaped (as
shown) projections or protuberance~ of the copper oxide which
are shown as enlarged or magni~iedin ~he enlarged schen~atic showing
of FIGURE 1. These projections or protuberance~ of copper
oxide are ordinarily of random varying shapes, lengths, and
widths and are ordinarily not of uniform shape or configur-
ation, length or width. Referring to FIGURES 2 and 3, solar
collector module 14 includes copper or copper alloy plate 11,
the cond~tioned, liquid water condensate-resistant selective
coating 12, metal tubes 15, such as copper tubes, adapted to con-
tain water mounted in direct contact with selected coating
12, transparent cover shee~ 16, e.g. of transparent glass,
and heat insulating material 17, e.g~ Styrofoam mult~-
cellu~ar polystyrene, in contact with and secured in assembly
with plate 11. ~ubes 15 are usually also coated with the
conditioned, liquid water-re~istant, selective coating or
film 12. Transparent sheet 16 is mounted and secured in
structural members 18 of wood or metal. A supply manifold
feeds cool water to tubes 15 for heating, and a withdrawal
or return manifold conducts the heated water from tubes 15
for utili~tion as desired.
Alternatively, tubes lS can be mounted beneath
copper or copper alloy plate 11 and in direct contact with
plate 11. In such case tubes 15 would be enveloped ~or the
--30 most part by insulation material 17.
The conditioned, ~electlve coating o~ ~his invention,
which has been conditloned by treatment with the chromic
acid solution, also has excellent stability to contact wlth
water in gaseous or vapor form and is thermally stable a~
elevated temperatures up to the maximum durability temperature
of the selective surace.
The chromic acid solu~lon preferably utilized ~or
the stabilizing treatment herein is a ~hromic acid solution
~ontaining alkali phosphate, chromic anhydride and an
aqueous liquid solvent such a5 wa~er. Such ohromic acid
solution is prepared by mixing together abou~ 1 to 20 lbs.
of a concentrat~ composition comprising, by weight, about
30% to 60% of chromic anhydride and about 70~ to 40% of
sodium tripolyphosphate per 100 gallons of the aqueous liquid
such as water. An especially preferred chromic anhydride-
sodium tripolyphosphate concentrate for mixing together with
water in preparing the chromic acid solution lg the following:
% by Weight
CrO3.. ~..... ~...... ~.~.. ~........ ~.... ....45
Sodium tripolyphosphateO....~ ... $5
Such concentrate is m~xed together with the aqueous liquid
such as water ln the proportions of 1 to 20 lbs. o~ the
concentrate per 100 gallons af water.
Preerred blackenlng solu~ions employed herein
are those obtained by mixing 1 lf2 l~s. of either of the solid
blackener concentrates A and B which ~ollow, per gal~on of
water.
Blackener ~oncentrate A
by ~ Wel~ht
30 Sodium hydroxide......... ~.~.. 00.~..... 66~
- Sodium chlorite.,...... ~... ~........ 0...... ~. 34%
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~lackener Concentrate B
hy ~eight
Sodium hydroxide... ~.................. ~...... 50~
Sodium chlori~e.... ~................. ..,.... 50%
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