CA1090223A - Solar collector having selective film of improved stability to liquid water condensate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Solar collector including a copper or copper alloy base and a selective solar heat energy-absorptive coating or film of improved stability to liquid water on the base.
The selective film is characterized by having a solar absorptivity of 0.90 or higher, infrared emissivity no more than 0.20, and a stability such that its selectivity is not significantly reduced after contact with liquid water condensate over a prolonged, cumulative, non-continuous time in excess of 15,000 hours.

Description

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 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 the 0 blackening of surfaces of copper and copper alloy by con

-3-3tacting 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 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 ~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.

~ 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 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%

~lackener Concentrate B
hy ~eight Sodium hydroxide... ~.................. ~...... 50~
Sodium chlori~e.... ~................. ..,.... 50%

Claims (30)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A solar collector comprising a base of copper or copper alloy, and a stable, selective, solar heat energy-absorptive coating on the base, said selective solar heat energy-absorptive coating comprising a multiplicity of discrete projections of copper oxide, said selective, solar heat energy-absorptive coating having been rendered stable by contacting the selective, solar heat energy-absorptive coating on the base with a chromic acid solution comprising chromic anhydride (CrO3) and water for a time sufficient to impart to the selective coating a stability such that its selectivity is not significantly reduced after contact with liquid water over a prolonged, cumulative, non-con-tinuous time in excess of 1600 hours.

2. The collector of claim 1, wherein the selective, solar heat energy-absorptive coating on the base has a solar absorp-tivity of at least 0.90 and an infrared emissivity no more than 0.20 prior to the stabilizing treatment with the chromic acid solution.

3. The collector of claim 2, wherein the stability imparted to the selective film is such that its selectivity is not significantly reduced after contact with liquid water con-densate over a prolonged, cumulative, non-continuous time in excess of 15,000 hours.

4. The collector of claim 1, wherein the chromic acid solution comprises sodium phosphate, chromic anhydride, and water.

5. The collector of claim 4, wherein the chromic acid solution comprises, per each 100 gallons of water about 1 to about 20 lbs. of a concentrate composition comprising, by weight, about 30% to about 60% of chromic anhydride and about 70% to about 40% of the sodium phosphate.

6. The collector of claim 1, wherein the chromic acid solution comprises about 0.1 to about 5.0 lbs. of chromic anhydride per each 100 gallons of aqueous liquid.

7. The collector of claim 4, wherein the sodium phosphate is sodium tripolyphosphate.

8. The collector of claim 5, wherein the sodium phosphate is sodium tripolyphosphate.

9. The collector of claim 1, wherein the temperature of the chromic acid solution during the stabilizing contacting treatment is in the range of from room temperature to boiling temperature.

10. The collector of claim 1, wherein the selective coating is contacted with the chromic acid solution for a time in the range of about 10 seconds to about 60 seconds.

11. The collector of claim 3, wherein the selective coating is contacted with the chromic acid solution for a time in the range of about 10 seconds to about 60 seconds.

12. The collector of claim 1, wherein the selective, solar heat energy-absorptive coating on the base is prepared by a process comprising contacting the copper or copper alloy base with a hot aqueous solution comprising an alkali metal chlorite, an alkali metal hydroxide and water at a solution temperature in the range of about 140°F. to about 220°F. for a time sufficient to obtain a selective film on the base having a solar absorptiv-ity of at least 0.90 and an infrared emissivity no more than 0.20.

13. The collector of claim 12, wherein the alkali metal chlorite is sodium chlorite and the alkali metal hydroxide is sodium hydroxide.

14 The collector of claim 12, wherein the chromic acid solution comprises sodium phosphate, chromic anhydride and water.

15. The collector of claim 14, wherein the chromic acid solution comprises, per each 100 gallons of water about 1 to about 20 lbs. of a concentrate composition comprising, by weight, about 30% to about 60% of chromic anhydride and about 70% to about 40% of the sodium phosphate.

16. The collector of claim 13, wherein the chromic acid solution comprises sodium phosphate, chromic anhydride and water as solvent therefor.

17. The collector of claim 16, wherein the chromic acid solution comprises, per each 100 gallons of water about 1 to about 20 lbs. of a concentrate composition comprising, by weight, about 30% to about 60% of chromic anhydride and about 70% to about 40% of the sodium phosphate.

18. The collector of claim 12, wherein the chromic acid solution comprises about 0.1 to about 5.0 lbs. of chromic anhydr-ide per each 100 gallons of water.

19. The collector of claim 15, wherein the sodium phosphate is sodium tripolyphosphate.

20. The collector of claim 17, wherein the sodium phosphate is sodium tripolyphosphate.

21. The collector of claim 20, wherein the temperature of the chromic acid solution is in the range of from room temperature to boiling temperature.

22. The collector of claim 21, wherein the selective coating is contacted with the chromic acid solution for a time in the range of about 10 seconds to about 60 seconds.

23. The collector of claim 20, wherein the temperature of the chromic acid solution during the contacting is in the range of room temperature to boiling temperature.

24. The collector of claim 23, wherein the selective coating is contacted with the chromic acid solution for a time in the range of about 10 seconds to about 60 seconds.

25. The collector of claim 20, having a plurality of metallic tubes adapted to contain heat-transfer fluid in contact with the selective surface.

26. The collector of claim 25, wherein the fluid is water.

27. The collector of claim 25, having a layer of thermal insulation material on the side of the base opposite the selec-tive coating.

28. The collector of claim 25, wherein a sheet of trans-parent material is retained above the selective surface, and in spaced relationship thereto.

29. The collector of claim 25, also having the stable, selective, solar heat energy-absorptive coating on at least the exposed external surfaces of the tubes.

30. The collector of claim 25, wherein the tubes are of copper or copper alloy.