I finally figured out the black body calculations when there is a glass front and it is a bit complicated (I could not find a reference, but it's a differential equation). The end result is that the losses will not be as much as I've indicated because the glass is blocking all the radiation heat from immediately escaping, but it matters on cold days, especially you let the black part get hot. The primary goal for efficiency in the design of these things is high air flow to not let it get hot.
@Yaheuben66 the unpainted side feels hotter to touch because without the paint as a thermal insulator, the metal is transferring heat to your skin better. I don't think the unpainted side will heat flowing air better than the painted side, especially because with the higher air flow it is not supposed to get too hot. I like the black plastic trash bag method due to being cheap and light. A problem is that you can't let the air flow go to zero while exposed to sun or it melts.
WoW!! Thank you, thank you, thank you for putting this information out.. I have been bouncing this problem around in for a year now... Great information!
I have thought about the "collector"-- I thought that aluminum foil (painted black with a "naked" back) placed mid-channel would be best-- aluminum is a good heat sink/shedder
I have a piece of aluminum plate painted black-- the back side gets WAYYYYY hotter than the black-painted front (tested by conduction/touch, anyway) in full sun...
I finally figured out the black body calculations when there is a glass front and it is a bit complicated (I could not find a reference, but it's a differential equation). The end result is that the losses will not be as much as I've indicated because the glass is blocking all the radiation heat from immediately escaping, but it matters on cold days, especially you let the black part get hot. The primary goal for efficiency in the design of these things is high air flow to not let it get hot.
HudsonKnowsEconomics 1 month ago
@Yaheuben66 the unpainted side feels hotter to touch because without the paint as a thermal insulator, the metal is transferring heat to your skin better. I don't think the unpainted side will heat flowing air better than the painted side, especially because with the higher air flow it is not supposed to get too hot. I like the black plastic trash bag method due to being cheap and light. A problem is that you can't let the air flow go to zero while exposed to sun or it melts.
HudsonKnowsEconomics 1 month ago
Thank You!!! The equation helps me judge my design so much easier.
rlidrazzah 1 month ago
WoW!! Thank you, thank you, thank you for putting this information out.. I have been bouncing this problem around in for a year now... Great information!
I have thought about the "collector"-- I thought that aluminum foil (painted black with a "naked" back) placed mid-channel would be best-- aluminum is a good heat sink/shedder
I have a piece of aluminum plate painted black-- the back side gets WAYYYYY hotter than the black-painted front (tested by conduction/touch, anyway) in full sun...
Yaheuben66 3 months ago