[Ok-sus] Heat your home with aluminum cans ???

Bob Waldrop bob at bobwaldrop.net
Sun Dec 15 21:59:02 UTC 2013

Passive solar is a great and very under-utilized resource.  

The generally accepted value of the "solar constant", which is the energy value of sunlight when it hits the atmosphere, is 429 btu/square foot.  By the time it gets to the ground, it loses about 30%, so we're looking at 320 BTUs per square foot at ground level.  So when my 120 square feet of south-facing glass are in full sunlight in the winter, they're being blasted with 38,400 BTUs per hour (120 X 320).  I'd have to do some archeological work, but IIRC the transmissivity of my south facing windows is about 70%, which means that 70% of the 38,400 gets inside the house so I get 26,880 btus per hour of full sunlight.  That's not an insignificant amount of heat.  Of course, the sun doesn't shine 24 hours/day, and generally, my south facing windows will all be the sun no more than 2 hours a day, but each square foot of window gets additional light starting earlier in the day on the southeast side of the face and gradually moving to the southwest late in the day.

Because we have done, and continue to do, a good job of weatherization, and we have 9 inches of insulation in the walls, 14inches in the attic, and R-20 coverings for our windows, we hang onto more of our heat. 

Devices like this soda pop can heater aren't magic.  They aren't going to somehow get more than 320 BTUs/sq ft/hr times whatever the transmissivity of the covering is.  But adding more windows to the south face of my house would be expensive.  Making one of these soda can heaters effectively increases the size of the collection area by turning the flat surface in front of a window into a solar collection system.  Last winter I made "Mothers Heat Grabber", from plans downloaded from the Mother Earth News. It worked as advertised, that is, it produced a steady by low flow of hot air into the house when the sun shone on it, but it wasn't big enough to make a significant difference.  It was only about 2 ft wide.  If I had one that was ,e.g. 8 feet wide, and 6 feet long, that's 48 square feet times 320 btus which is somewhere around 15,000 btus/hour at peak operating time.

Aside from the initial capital investment, passive solar pays dividends for a long time in the form of free heat.  During our recent snow storm, when it was cloudy several days in a row, we needed supplemental heat.  But as soon as the sun came out, we let the wood stove fire die.

In the summer, we don't want this kind of heat coming into the house. Fortunately, I have an old house and the south overhang is properly sized for this latitude, so the direct sunlight never hits my south facing windows in the summer. And in the summer, we put up our insulated window shades during the day, whereas in the winter, they go up whenever the sun isn't shining through that window (during the winter, even if it is light out, if the sun is not shining directly into a window, it is losing heat.

Bob Waldrop, http://www.ipermie.net where you can learn how to calculate the heat loss (or gain) from your dwelling

-----Original Message-----
From: Ok-sus [mailto:ok-sus-bounces at lists.oksustainability.org] On Behalf Of Crews, Andrew J.
Sent: Saturday, December 14, 2013 12:35 PM
To: ok-sus at lists.oksustainability.org
Subject: Re: [Ok-sus] Heat your home with aluminum cans ???

passive solar is awesome, and something anyone can do with some time and determination.  While there won't be the resources to add new centralized energy systems built from the ground up, there will be plenty of salvage lying around for people to do this stuff.
From: Ok-sus [ok-sus-bounces at lists.oksustainability.org] on behalf of Paula Brennecke [paulabren at cableone.net]
Sent: Friday, December 13, 2013 10:52 AM
To: ok-sus at lists.oksustainability.org; okc at lists.sustainableokc.org; RunningOnEmpty2 at yahoogroups.com
Subject: [Ok-sus] Heat your home with aluminum cans ???

What do you think of this idea?


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