Maximize solar absorption and convection AND minimize radiation

[Sureshot]

I know that absorption without radiation and convection is impossible. 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 also aware that the absorptivity of copper is only .65 and cast iron is .95 However, cast iron in this instance won't do because of rust formation, and an external asphalt coat on copper (at .90 absorption) will more than likely melt in the Arizona desert heat. A typical summer day in Phoenix is 112°F at 6' off theground and in the shade.

Based on the 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 approximately 7-8 kWh/m2/day.

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. The estimated effectiveness of the parabolic dish used is 10% 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 me regarding a suitable coating?

 

[kenvlach]

see thread332-55676

 

[pennpoint]

Sureshot;
We use a Epoxie material on our space simulator surfaces. I'll post the info 5/13/03. It only takes $$, theres an answer out there for everyone.

pennpoint

 

[hacksaw]

blanketing the surface with suitable greenhouse gasses works.

 

[phex]

hacksaw: is greenhouse-gas-blanketing done in practice? it sounds like a good idea, though. how much gas-space is needed and whats the net gain?

 

[IRstuff]

No need to get quite that drastic. Speaking hypothetically, the standard greenhouse works very well because the glass is transparent to the bulk of the solar radiation, but is not at all transmissive in the IR.

This suggests that one possible option would be to black material surrounded by a layer of glass. Not sure if you need an air gap or not, but it would cut down on the convective heat loss on the outside.

TTFN