soda can air heating

[Stewart Corman]

Met a fellow who has built an inexpensive 4x8 solar collector constructed of soda cans. He is busy measuring and upgrading the air handler. Will try to get him to introduce himself and contribute to this thread with his results.

The idea is not new, but implimentations vary.
Basic design is a frame with insulation at the back, cans glued together and painted black, headers for input/output, fan/blower for circulation, polycarbonate sheet for cover.
Bottom line is that you need to force air through the cans to get heat out.

A small solar panel to power the blower is a thought ...cans only gets hot while the sun is out, but a blower which draws 1 amp, only costs about 1.5 cents/hr to run ...hard to justify the expense of a panel

Just as a reference, I looked up a few accepted values for power availability.
Avg power from sunlight is 432 BTU/hr/sqft
so , for a 32sqft panel, this equates at 100% to 13,800 BTU/hr

or
1365 watts/m^2 = 126watts/sqft or again for 32sqft equates to 4KW/hr

As far as how much a single unit can heat a house, I can relate to an experiment I did with a resistance heating element in my forced air system. A 4KW element running full time, was only barely able to maintain constant temp for 1400sqft of house with ouside temp just below freezing. It would require 10KW to warm the house up and probably need to run >50% of the time. A HHO furnace runs at 100,000 BTU outpu at a 30% duty cycle to heat the house up to 68F.

There is a commercial unit which is quit expensive @ $2745:
http://www.cansolair.com/index.php

here is a project using thin walled aluminum drier flex tubing:
https://sites.google.com/site/glenssolarheater/
he charts some data which is shows 70F temp rise
note: temp rise is a function of air throughput,
whereby output is flow (cfm) x temp rise (F)

here is a fairly sophisticated implementation of the can idea:
http://www.freeweb.hu/napenergia/gyak/szp/sztgyi_en.htm
in above he has 23sqft and produces 700 watts or about 25% efficiency

Above uses an insert to create turbulence, but this will affect volume flowrate.
The idea is sound, because to maximise heat transfer, you want to draw as much heat from the can sidewall as possible. IMHO, would use a squirel cage blower over a muffin fan, since they produce a much high static pressure. Also, if you used 1/2 as many cans and covered the backing with shinny aluminum foil, you'd get 360degrees of the can exposed to sunlight, rather than just 180degrees, and reducing can count increases flow rate ie less pressure drop.

IMHO, instead of using single pane polycarbonate, I would procure thermopane glass
from surplused sliding glass patio doors (Craigslist?)...around here a set goes for <$75 for the pair

I would presume that the 30% loss from a fixed vs tracking PV is applicable here as well.

If anyone on the board has built, researched, or seen one of these units, any comments appreciated

I have a neighbor who has a fixed three panel air solar collector constructed of black tubing up for 30+ years ...still working!

Stew Corman from sunny Endicott

[Peter Klaassen]

I have seen a cansolair at Reachview farm in Prince Edward County. They are using it on a south facing wall, drawing air out of the basement and into the main floor. The owner held a demonstration day a few years ago - he is off grid also with a Whisper xxx and solar panels, and said he had to open windows in the winter sometimes due to overheating with this operating. I gathered he was pleased with it. The surface of it was slightly curved and it looked ok on the wall - sort of like a tinted bay window.
P Klaassen

[Brian McGowan]

I am using black polyester felt for an absorber and 1.5" thick packing foam for the sides and double wall polycarbonate for the face on my newest one it is based on a 4x8' sheet of plywood and has 1 3/8 fiberboard insulation. This year is the first year I have vented it into the actual living space. I made a board that has the duct nipples in it to clamp in the window.

I only have one picture of it on my website here. It is very behind where I am and needs some serious updating but here it is.
http://home.comcast.net/~bigvid/
4th picture down.
So far it's working pretty good but is shaded by the trees behind the house. Since we are past the winter solstice the sun is on the rise and the shadows will be oof the box. Last year I was getting a 35 degree rise at about 100cfm I think and was venting it into the basement.

Brian

[Terry Murphy]

I think the first of this type of heater was designed by two guys from Newfoundland and they used beer cans because they had more fun collecting the cans.

I built a small prototype out of a storm door and put it on my garage as an experiment and it works fine and now I plan to build a larger 4'x8' one using a thermopane glass I acquired from an old picture window. I followed an improved design that uses flexible aluminum dryer vent instead of the cans. The improvement is supposed to be because there is more surface area to absorb the sun on the flexible tube as opposed to the smooth skin surface of the aluminum cans. Also you eliminate soldering all the cans together. I have a thermostat inside the box that turns the fan on and off at 100 degrees F. During full direct sunlight the inside of the box maintains a temp. of 120F to 130F even on a cold day like today here in Nova Scotia.

[Stewart Corman]

Terry,

I am in the camp that recycled thermopane glass is better than polycarbonate single pane sheeting

I followed an improved design that uses flexible aluminum dryer vent instead of the cans. I have given it some thought since my first post.
In the link that showed the aluminum dryer tubing has some intrinsic flaws.
The reason for soda/beer cans is three fold:

1) smaller diameter = more heat transfer
same reason for using 1/2" baseboard radiators vs 3/4" ...for the same volumetric flow rate inside, the sidewalls have more surface area per cross-section

2) thinner walls = better/faster heat transfer ...
less thermal resistance
you wouldn't want 1/4" wall aluminum piping , would you?

3) better turbulence ...directs air flow to the can walls to suck out heat, rather than shooting down the middle

I think the other link that showed the inserts inside cans for turbulence may be overkill.
I can see punching several small holes in can ends with a flat screwdriver and then twisting it to make small vanes ...OR make a slit with a matte knife and shove a thick cold chisel into it to spread the vanes ......not enough holes and you bog down the air flow

Also you eliminate soldering all the cans togethersoldering??? these are aluminum cans ....a cheap tube of construction caulk/adhesive is used to atach the bottom of one can to the next ...using a "V" ( length of angle iron?) as the form for straight alignment while glue dries.

Stew

[Terry Murphy]

Thermopane glass adds a lot of weight but that doesn't bother me for the insulation value of the double glass as well as the cost savings of recycling something that is free.

You may be right about the air flow in the smaller diameter cans but the aluminum flexible dryer vent material is a lot thinner than pop cans and very delicate to work with without tearing holes in it so I think the heat transfer is more efficient than cans. I would also be concerned that the glue used to join the cans together might eventually break down from the high temperatures it will be exposed to, but if it's been done and proven that may not be an issue. Anyway, all I really know is that the small panel I built works way beyond my expectations so I'm going to build a larger one using the same design and materials with just some small modifications to handle the extra weight of the thermopane glass and the wooden frame and box. Oh yeah, and the big one is going on my house this time, not the garage.

Terry

[Stewart Corman]

Terry,
You make several good points , so IMHO, I will add another thought to your implementation.
I presume that like the link I provided, you are planning one continuous loop, rather than several short pieces and a header at top and bottom??

The large 4" diameter of the dryer tube allows bulk air to travel down the middle and do little work for the long stretches. Did you consider instead to make short horizontal serpentines along the shorter dimension to minimize straight runs and increase the turbulence? That dryer tube is very flexible and many "U" turns could be an improvement.
The only problem I see in not using multiple paths w/headers, is that the end of the single loop is hottest and would therefore not be as efficient as the cold input end ....maximum heat transfer is when the temp diff is maximized

I don't know w/o personal experimental experience whether a combo of above would be an significant benefit ....header at top and bottom, with three very short horizontal serpentines winding from bottom to top ...each using 1/3 of the available width.

Stew