BACKGROUND OF THE INVENTION

Solar collectors are routinely formed with a black, solar receptive surface, to increase the absorptance of the collector. A common material for this coating is black paint. While black paint has a high absorptance, in the range of about 0.95, this material suffers from a high emittance, which is also in the range of about 0.95. Thus, while a black painted solar collector collects a great amount of solar energy, black painted solar collectors also permit much of this energy to escape. Black paint has often been used as a solar collector coating, however, due to its low cost.

It is, therefore, the primary object of the present invention to produce a solar collector coating material which provides a high absorptance level and maintains the low cost achieved by black paint, while reducing emittence levels, to thereby increase the amount of energy retained by the collector and transferred for solar heating.

THE PRESENT INVENTION

According to the present invention, these objectives have been obtained. The selective solar coating of the present invention comprises an organic compound or mixture of compounds characterized by a high carbon content and a high molecular weight. Either animal or vegetable oils, fats and waxes, as well as petroleum products (hydrocarbons) and synthetics have been found to produce a carbon pigmented varnish on a metallic surface following a controlled pyrolysis. The selective solar coating of the present invention has a high absorptance for solar energy, preferably in excess of about 0.9, and a low emittance of thermal energy of about 0.3 to 0.4. The low emittance results from the thinness of the coating after pyrolyzing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, the surfaces of a solar collector which are to be exposed to solar energy are first coated with an organic compound or substance having a high carbon content and a high molecular weight. Vegetable oils, such as castor, corn, tung, soy and palm, have all been found to be suitable coating materials. Likewise, animal materials such as lard, butterfat and beeswax and certainly petroleum hydrocarbons also produce satisfactory solar coatings. Synthetic materials, such as vinyl polymers, can also be pyrolyzed to produce solar selective coatings. The coating material could be 100% of the organic compounds or mixtures thereof or could be diluted in a suitable solvent, such as toluene, kerosene or mineral spirits. The percentage of the solvent in the coating composition is selected to produce a desired viscosity for coating of the composition onto the solar collector. The composition may be coated onto the collector by means such as spraying, roller coating or brush coating, each of which requires a different viscosity for proper application. Thus, the solvent could be present as an amount of from 0-90% by weight of the total composition.

After coating, the collector is subjected to heating at a temperature which may range from about 600° to 750° F. (315.6° to 398.9° C.) for a period of from about 5 to 30 minutes. This heating operation pyrolyzes the coating, producing a carbon black pigmented varnish.

Clearly, the solar collector must be formed from a material which can withstand the temperatures of pyrolysis. Suitable solar collector materials include metals, such as aluminum and steel.

The resulting coating has an absorptance preferably in excess of about 0.9 and an emittance preferably less than about 0.4. The low emittance results from the thinness of the coating after pyrolysis. After pyrolysis, the coating has a thickness of less than 2 micrometers, and preferably about 0.5 micrometers.

Additionally, a silicone or fluorocarbon may be added to the coating composition, in an amount up to about 10% by weight of the coating composition. These materials produce a hydrophobic surface, producing increased corrosion resistence for the collector.

EXAMPLES

______________________________________Ex-                                   Corrosionample Coating  Pyrolysis    α                            ε                                 Test*______________________________________1     1        15 min. @ 750° F.                       .922 .430 sl. spalling2     2        10 min. @ 600° F.                       .810 .315 no change3     2        20 min. @ 600° F.                       .860 .335 no change4     2        30 min. @ 600° F.                       .853 .340 no change5     2        10 min. @ 650° F.                       .905 .337 no change6     2        20 min. @ 650° F.                       .900 .271 no change7     2        30 min. @ 650° F.                       .910 .307 no change8     2        10 min. @ 700° F.                       .901 .250 no change9     2        20 min. @ 700° F.                       .912 .234 no change10    2        30 min. @ 700° F.                       .908 .198 no change11    2        10 min. @ 750° F.                       .912 .201 no change12    2        20 min. @ 750° F.                       .908 .263 sl. fade13    2        30 min. @ 750° F.                       .908 .161 no change14    3        10 min. @ 750° F.                       .916 .369 no change15    3        10 min. @ 750° F.                       .916 .280 mild fade16    4        10 min. @ 750° F.                       .921 .395 30% spalling17    4        10 min. @ 750° F.                       .920 .393 not tested18    5        15 min. @ 750° F.                       .933 .370 not tested19    6        15 min. @ 750° F.                       .926 .441 not tested20    7        15 min. @ 750° F.                       .915 .280 not tested21    8        15 min. @ 750° F.                       .922 .413 not tested22    9        15 min. @ 750° F.                       .929 .410 not tested23    10       15 min. @ 750° F.                       .923 .380 not tested24    11       15 min. @ 750° F.                       .924 .465 not tested25    12       15 min. @ 750° F.                       .928 .365 not tested______________________________________ Coating Material 1. Tung oil 2. 50/50 (by volume) palm oil and mineral oil in toluene 3. 50/50 (by volume) palm oil and mineral oil 4. 60/40 (by volume) palm oil and mineral oil in toluene 5. Soybean oil 6. Corn oil 7. Castor oil 8. Butterfat 9. Lard 10. Beeswax 11. Heavy petroleum oil 12. Vinyl polymer *18-day condensinghumidity corrosion test.

From the foregoing, it is clear that the present invention provides a simple, yet effective and low cost selective solar coating for solar collectors.

While the present invention has been described with reference to certain specific embodiments thereof, it is not intended to be so limited thereby, except as set forth in the accompanying claims.