[Greenbuilding] Question about a window solar heater

nick pine nick at early.com
Fri Dec 2 09:52:35 PST 2011

"Bob Waldrop" <bwaldrop at cox.net>

>I recently came across this interesting take on the traditional solar window heater.

>In discussing this elsewhere, however, someone mentioned that since it hangs on the inside of a window, it blocks sunlight from the window.

Views too.

>That person thought that the heat resulting from the flow of sunlight into a room without any obstruction in the window would be greater than the heat resulting from the operation of this device in a window, which would have the incidental effect of blocking some sunlight from the room.
Is that correct? 

Yes, but it also increases the heat loss from the window when the sun is shining. 

http://rredc.nrel.gov/solar//old_data/nsrdb/1961-1990/bluebook/data/14821.SBF says 780 Btu/ft^2 falls on a south wall on an average January day in Columbus, OH with a 26 F outdoor temp. With no solar heating panel, a 1 ft^2 R2 window with 80% solar transmission in a 70 F house would gain 0.8x780 = 624 Btu and lose 24h(70-26)1ft^2/R2 =  528, for a net gain of 96 Btu/day. With a 143 F panel next to the glass for 6 hours, it would lose 6h(143-26)1ft^2/R2 + 18h(70-26)1ft^2/R2 = 747 Btu, for a net  loss  of 123 Btu/day, or more, if the panel reflects some sunlight back out of the window. 

And how can Mike circulate solar warm air through the rest of the house with the furnace blower if he leaves the return vent open but closes the supply vents to the solar room? Airflow requires return and supply paths. Where's the supply path to the room?

If Mike knew more about airflow or heatflow, he might suggest hanging huge ugly boxes and cutting holes in house walls or turning a south window into an air heater with a foamboard insert that collects less solar heat than the window but greatly reduces the heat loss at night...

If a 2' wide x 3' tall window gains 6ft^2x624/6h = 624 Btu/h by day and a 1" R10 foil-foamboard panel with 1.5" slots at the top and bottom with dark high-temp BBQ paint facing the window and foil facing the room loses about 18h(70-26)6ft^2/R12 = 396 Btu at night and 16.6(24in^2/144in^2/ft^2)sqrt(3’(70+dT-70)) = 7.2dT^0.5 cfm of 70+2dT (F) air flows out of the top slot and provides (70+2dT)-70)7.2dT^0.5 = 14.4dT^1.5 Btu/h to the room and 624 = (70+dT-26)6ft^2/R2+14.4dT^1.5 = 132+3dT+14.4dT^1.5, ie dT = ((492-3dT)/14.4)^(2/3). Plugging in dT = 10 F on the right makes dT = 10.07 on the right, then 10.07, with 23 cfm of 80 F airflow and 230 Btu/h of heatflow, for a net gain of 6hx230-396 = 984 Btu/day, vs 96x6ft^2 = 576 for the window alone, or a net loss of 738 with Mike’s panel. 

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