# Questions - Pop Can Solar Heater



## MoonRiver (Sep 2, 2007)

I'm having trouble understanding the science behind these heaters. 

Are the cans that helpful for heat exchange? Why not just an empty box?

Some models use a fan and others don't. Is a fan only needed when you don't have a short input and output duct? Or when you are trying to pump cold air uphill?

How important is depth? Is 12" depth better than 4"?

I'm thinking about building 2 heaters of identical size, one with cans and one without, and see if there is any difference in heat output.

I saw where someone did a quick study and found that the type of hole cut into the can made little or no difference. I also saw one model that just had slats to divert the air rather than cans and it seemed to heat up fine.


----------



## TnAndy (Sep 15, 2005)

Purpose of the cans is to increase the surface area exposed to the sun, thus increase the amount of heat. All other things equal ( air flow, sunlight, etc ) the can model would perform better.

Same principle as a car radiator. You could make a simple metal box of the same dimensions, but by using tubing and the fin network, you increase the surface area tremendously compared to the amount of water IN the radiator, and thus exchange the heat better.


----------



## MoonRiver (Sep 2, 2007)

TnAndy said:


> Purpose of the cans is to increase the surface area exposed to the sun, thus increase the amount of heat. All other things equal ( air flow, sunlight, etc ) the can model would perform better.
> 
> Same principle as a car radiator. You could make a simple metal box of the same dimensions, but by using tubing and the fin network, you increase the surface area tremendously compared to the amount of water IN the radiator, and thus exchange the heat better.


But if my glass is 3' x 4', the same amount of light (heat) gets in no matter whether I have cans inside or not. That's what I don't understand. It seems to me the only thing the cans do is retain some of the heat inside the box.


----------



## Ky-Jeeper (Sep 5, 2010)

With alum. The heat transfer is very quick.


----------



## SolarGary (Sep 8, 2005)

MoonRiver said:


> But if my glass is 3' x 4', the same amount of light (heat) gets in no matter whether I have cans inside or not. That's what I don't understand. It seems to me the only thing the cans do is retain some of the heat inside the box.


Its true the same amount of heat enters the collector. Its also true that if you paint the cans, or whatever is there to absorb the heat black, than nearly all the solar heat will get absorbed. 

So, any of the approaches you mention will accept the solar input, and absorb it well. From here that heat goes to two places 1) it gets absorbed by the air passing through the collector, and ends up in the room you are heating (this is the path you want), 2) it gets lost back out the glazing (this is the path you don't want). Its a zero sum game, so you want to maximize the path that ends in the room and minimize the path that is heat loss out the glazing.

The hotter the collector absorber runs, the more heat you put out the wrong path (out the glazing). So, the idea of have a lot of absorber area and a good air path that with and even distribution of a healthy flow of air over that absorber is to get the heat out of the absorber, and keep it running cooler. Pop cans are pretty good because they have quite a bit of surface area to transfer the solar heat to the air, and they usually have a good manifold system to deliver the air evenly to the full collector area.

If you are not careful about the air collector design, you end up with uneven flow over the absorber -- dead spots that get little airflow get very hot and lose a lot of heat out the glazing. You also want a lot of absorber area with air flow over it because air has both low density and a low heat capacity -- you have to flow a lot of air in good contact with the absorber to get the solar heat out -- if you don't, the absorber just heats up until it gets hot enough to lose the heat out the glazing.

Pop cans are certainly not the only way to do this.
I use two black screen:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
this works well.

Similar to the pop cans, you can use gutter downspouts. Much less work, and probably more efficient in that they are heavier gage and will transfer the solar heat around the whole circle better. 

The back flow design is very popular. You have a solid metal sheet absorber with airflow behind it. The flow channel is pretty shallow so that the airflow is turbulent and scrubs the absorber well.

If you look here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#ActiveAir
There is a lot of good material on air collectors.

The first book that's listed is very good.

The TEA collector that is next is very well and carefully designed -- the plans (you can download for free) are very detailed.

There are a bunch more air collector designs shown with quite a bit of detail.

One thing to watch out for is performance claims. There is very little out there on good performance measurements or accurate comparisons of these designs. Most of what is shown is BS.

My 2 cents would be that the can collector can probably give you good results. There are a lot of other designs that probably give as good or better performance and are less work to build. The empty box with a black back surface you mention will give you poor efficiency for the resons mentioned above: airflow dead spots, and not enough absorber area to transfer the heat to the air efficiently.

The other thing that people tend to forget is that size really does matter in air collectors -- the bigger the better.

Yet another thing to remember is that air collectors that are putting out really hot air are inefficient collectors. You want the collector to have a temperature rise of about 50F from inlet to outlet --this is about the minimum that is useful for space heating. You want to adjust the airflow to get about this rise. If you set the airflow to low, then you will get really really hot air out the outlet, but not much of it, and the high temps cause most of the heat to be lost out the glazing.

So, there is air collectors 101 in 5 paragraphs 

Gary


----------

