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External coating to maximize solar absorption and convection

External coating to maximize solar absorption and convection

External coating to maximize solar absorption and convection

(OP)
I want to maximize absorption and convection but minimize radiation.  The pipes are copper, 3" in diameter, and filled with glass marbles to increase heat capacity.

I'm aware that the absorptivity of copper is only .65, while cast iron is .95   However, cast iron in this instance won't do because of rust formation, and an external asphalt coat on copper (a .90 absorption) will probably melt in the Arizona desert heat.

Based on data provided by  NREL (National Renewable Energy Laboratory) the annual average for a North-South Axis Tracking Concentrator tilted at Latitude here in AZ., is 7-8 kWh/m2/day.

In the preliminary testing phase, a prototype pipe coated with flat black high temperature paint and irradiated by a handmade aluminum foil parabolic dish, attained a temperature of 230°F + in less than 45 minutes at approximately 11AM.
The April day was sunny, 78° and with a slight breeze. Average Phoenix temp. in summer 6' off the ground and in the shade is 112°F.   The estimated effectiveness of the parabolic dish used is 10-12% at best.

In the finished product each pipe will have a parabolic reflector manufactured of DuraBrightTM aluminum with ~.99 reflectivity.   Does anyone have input they can give regarding a suitable coating that will accomplish the task?

RE: External coating to maximize solar absorption and convection

Black chromium plating comes close to being the ideal solar collector coating:
    High absorbance to incident solar radiation with wavelength below 3000 nm,
    Low emission of longwavelength thermal radiation,
    Good thermal stability (no effect below 480oC, usable long-term up to 590oC), and
    Good corrosion resistance (even better with a satin nickel undercoat).

Information primarily from: The Canning Handbook.  Surface Finishing Technology, 23rd Edn., p. 464 (1982).

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