DIY solar air heater 2011 fall
Channel: zawy
Date: 22/10/2011

See comments below for all the calculations for these type boxes. This project did not turn out nearly as well as the math promised, so I'm scrapping this project and trying a heliostat to use the reflection 4x8 to direct sunlight directly into the house windows.  I'm not allowing comments to be added so the calculations at the top will remain.  Youtube would not allow the calculations to be added here.

UPDATE: someone pointed out that it's a lot less efficient than I calculated because I did not account for the black body radiation coming from the board.  It's just plain high temp paint, not Solkote, so the board is like a black body and Stefan-Boltzman equation says energy loss could be W/m^2 = s*(e1*T1^4 - e2*T2^4) where e1 and T1 are the emissivity of the paint (~0.9) and temp in Kelvins (about 340 K, 155 F) and e2 and T2 are the emissivity and temp of ambient.  I do not know what e2 is, but the equation gives at least 1,584 W for my 6 m^2, which explains the missing 1,610 W.  s=5.67E-8.  The paint in these hot air boxes really matters, costing me nearly half the energy.  This is another reason keeping airflow high to keep temps low is important, but even with good air flow, my black surface stays too warm.

Update: i tested 1 4x8 box with 1 layer plastic, without the reflectors.  Efficiency was about 39% (1,173 Watts at noon).  The black part of the insulation board was 145 F at the bottom and 165 F at the top. Outlet temp was 100 F,  inlet temp was 68 F, 110 CFM.  Calculation: (100-68)*110/3 = 1173 W.  Error in air flow measurement filling up large bags is about 15%.  1 layer plastic loses about 13% from reflection (not 8% said in the video).