soda can passive hot air panel, worth the hassle?

[J.P.M.]

I have been in a lot of 3rd world countries over the years and have yet to see the water bottle in the roof thing applied. That is really at the bottom of the ladder.

The turbulent air flow in the tube (cans) is important. One can always make up some type of insert to create the same condition in a straight tube. Fairly common in heat recovery systems.

Because of the relatively poor film coefficients of air or most other gases at/near atmospheric pressure when compared to liquids, turbulence is important but a large surface area is usually more important. Both are usually two common features of air cooled collectors. Since the relative power difference required to pump a given mass of two different fluids is in large part, but not entirely, proportional to the ratio of the specific volume of the fluids, another big consideration for air cooled collectors is required pumping power and its cost. One goal of good heat exchanger design, including solar collector design is a balance of tradeoffs, often making screen/mesh/fins/spirals/etc. materials with a high surf to vol. ratio and large void fraction usually better than relatively smoother surfaces, provided any increase in the pressure drop can be managed.

Bottom line: Air collectors need more surface area, and pumping costs (and noise) will be higher than liquid collectors for similar mass flow rates. But, big advantages: air collectors seldom freeze and if they're not leak proof, no one knows or cares much.

[Sinestro]

So I'm planning on building one of these soda can solar heaters and have just about got all the cans I need. Before I get started however, I have a query. Has anyone tried snaking the air through the cans, ie:drilling a hole towards the top half of 2 cans(side by side) and linking them with either PVC pipe or some metal tubing & then drilling a hole on the opposite side of the can and joining it on that side the the next can & so on and so forth, snaking the air all the way up through all the cans?

If so what were the results?
What are peoples thoughts on this, pros & cons?

[J.P.M.]

So I'm planning on building one of these soda can solar heaters and have just about got all the cans I need. Before I get started however, I have a query. Has anyone tried snaking the air through the cans, ie:drilling a hole towards the top half of 2 cans(side by side) and linking them with either PVC pipe or some metal tubing & then drilling a hole on the opposite side of the can and joining it on that side the the next can & so on and so forth, snaking the air all the way up through all the cans?

If so what were the results?
What are peoples thoughts on this, pros & cons?

Series vs. parallel flow ? It may increase turbulence by increasing local velocity. Increased turbulence will improve heat transfer from the metal to the air. Increased turbulence also costs pumping power. It will also slow down the mass velocity some while increasing local velocities in some/most areas - overall, probably more heat transfer. However, the lower mass velocity (lower throughput) will likely increase the overall collector average temp., decreasing the collector efficiency. To offset this, more pumping power is required. The increased pumping power will increase parasitic losses, and decrease overall efficiency. Lots of things interact with one another. Part of the fun of heat transfer analysis.

Bottom line - There ain't no free lunch, but on balance, increased velocity is often better than slower a lot of the time but not always, depending. There are no simple answer - only tradeoffs.

[russ]

What are peoples thoughts on this, pros & cons?

Just have a header on both ends - simpler and easier

[rhawkman]

Came back to this idea today and was browsing threads here. While I have seen the comparison videos and all, I don't know where anyone is finding filter media or black aluminum screen 'cheap'. Lowes has the screen, cheapest one in 28" x 100' for about $100. We used to use tons of the filter rolls on our big air handlers and never considered cost. You (the taxpayer) paid for it all since it was a USAF system. But it is harder to find than I anticipated and a 20' roll costs $60. Hell, two 10' downspouts and elbows will cost nearly $50! I can see why soda cans are popular.

I was contemplating downspouts and loosely stuffing steel wool inside for surface area heat transfer. And to make it even crappier looking, I am thinking about putting a piece of plywood in a partially opened window with two holes cut in it for inlet and outlet, kinda like a window AC.

Oh, and I am not about to pay someone for one of these ideas. It reminds me of the how to get rich plan someone will send to you if you send them $20 from the back of Pop Mechanics or something.

[J.P.M.]

Came back to this idea today and was browsing threads here. While I have seen the comparison videos and all, I don't know where anyone is finding filter media or black aluminum screen 'cheap'. Lowes has the screen, cheapest one in 28" x 100' for about $100. We used to use tons of the filter rolls on our big air handlers and never considered cost. You (the taxpayer) paid for it all since it was a USAF system. But it is harder to find than I anticipated and a 20' roll costs $60. Hell, two 10' downspouts and elbows will cost nearly $50! I can see why soda cans are popular.

I was contemplating downspouts and loosely stuffing steel wool inside for surface area heat transfer. And to make it even crappier looking, I am thinking about putting a piece of plywood in a partially opened window with two holes cut in it for inlet and outlet, kinda like a window AC.

Oh, and I am not about to pay someone for one of these ideas. It reminds me of the how to get rich plan someone will send to you if you send them $20 from the back of Pop Mechanics or something.

Around my area consulting fees go for about $250/hr. I won't be sending you a bill.

One way to increase both the surface area and turbulence is screening or mesh. However, unless the mesh, or screen has a heat source (the sun), it will not heat up and will, therefore be unable to transfer any heat to the fluid (in this case air), and thus be unable to increase the air's temp. Putting or cramming steel wool or anything else where it will not be exposed to solar irradiance is almost entirely counterproductive. As a practical matter, buried in a dark conduit, it will only increase pressure drop and, unless the blower size is increased, will slow down the bulk (mass) velocity and will not provide any additional heat transfer surface because, and almost by definition, a solar collector's heat (energy) collecting surface is limited to those surfaces which "sees" the sun. Mesh in a closed conduit does not fit that definition. Not a good idea. I wouldn't do that.

If you browse a bit more you may find a thread where I briefly described an air cooled collector I made back in the day that used steel wool spray painted flat black. That absorber cost something like $.25 to $.50/ft.^2 - I forget exactly how much, but I was a lot poorer in the '70's, so it couldn't have been much. The collector worked fine as a prototype until it met a fiery demise. The resulting phoenix was my further education.

[rhawkman]

Around my area consulting fees go for about $250/hr. I won't be sending you a bill.

One way to increase both the surface area and turbulence is screening or mesh. However, unless the mesh, or screen has a heat source (the sun), it will not heat up and will, therefore be unable to transfer any heat to the fluid (in this case air), and thus be unable to increase the air's temp. Putting or cramming steel wool or anything else where it will not be exposed to solar irradiance is almost entirely counterproductive. As a practical matter, buried in a dark conduit, it will only increase pressure drop and, unless the blower size is increased, will slow down the bulk (mass) velocity and will not provide any additional heat transfer surface because, and almost by definition, a solar collector's heat (energy) collecting surface is limited to those surfaces which "sees" the sun. Mesh in a closed conduit does not fit that definition. Not a good idea. I wouldn't do that.

If you browse a bit more you may find a thread where I briefly described an air cooled collector I made back in the day that used steel wool spray painted flat black. That absorber cost something like $.25 to $.50/ft.^2 - I forget exactly how much, but I was a lot poorer in the '70's, so it couldn't have been much. The collector worked fine as a prototype until it met a fiery demise. The resulting phoenix was my further education.

Thanks JPM, check's in the mail

Yeah, well, just so's ya don't think I'm completely dense, I had planned the steel wool to be very un-densely packed. I saw you mentioned it but didn't look for that post cuz I am in making some pulled pork and adjusting my sauce. Fiery demise does not sound good. I tend to learn things the hard way myself. I really do imagine whatever I finally make won't look too purdy, what with plywood in the window and probably some dryer vent going down to the floor for the cold air return. But I want some function, not so much aesthetics. My furnace is resistance heat so my Daka wood furnace will be getting a workout. Just want to do something fun and useful. Thanks again.


Ron

[russ]

For your parts and pieces - do some internet search - it really pays off for hot water system parts I know. For this I haven't tried.

[rhawkman]

For your parts and pieces - do some internet search - it really pays off for hot water system parts I know. For this I haven't tried.

Russ, I have been looking for over two years for some stuff. I read all the time how people get used sliding glass doors, water heaters, and bathtubs (for my planned greenhouse, solar batch water heater, and aquaponics respectively) for free so easy, but I have yet to run across that anywhere near me. Also I see all the videos of solar air heaters built with wood and none seem to mention that they can catch fire from getting so hot. I knew I should have just been independently wealthy so money wouldn't be an object.

[J.P.M.]

Thanks JPM, check's in the mail

Yeah, well, just so's ya don't think I'm completely dense, I had planned the steel wool to be very un-densely packed. I saw you mentioned it but didn't look for that post cuz I am in making some pulled pork and adjusting my sauce. Fiery demise does not sound good. I tend to learn things the hard way myself. I really do imagine whatever I finally make won't look too purdy, what with plywood in the window and probably some dryer vent going down to the floor for the cold air return. But I want some function, not so much aesthetics. My furnace is resistance heat so my Daka wood furnace will be getting a workout. Just want to do something fun and useful. Thanks again.


Ron

You're welcome. Forget the check. Just respect me in the morning.

[psychowolf]

Thank you for the info.

I live in Armenia and we have cold winters here and the idea of insulating homes has not been employed yet. I am in the process of trying to get enough cans to build a panel now. The biggest problem is that there are many more glass bottles used here for drinks than cans. So it is taking me a while to collect them... But thanks everyone for all the tips and information.

[russ]

Hi psychowolf - Welcome to Solar Panel Talk!

Russ

[rhawkman]

Built mine

After all my research and pondering (and procrastinating) I finished my solar screen heater that I found the plans for on builditsolar.com Finally have my solar screen heater hooked up. Don't really need it today as it is about 60F outside, but this is the temporary hook up of the fan and ducts going into living room window. 70F air at lower inlet and the fun pushing ~150F air out the top duct running a 12v boxer fan with a 5w solar panel. It might be a simple project and not the prettiest thing in the world but I am feeling pretty good about it working as planned. A higher CFM fan would help but hard to find 12v fans that run on less than .5A and are silent. The fan is hooked up directly to the solar panel with no battery so the fan will only run when there is enough sun to make heat anyway. I tried to upload pics but it says they are too big. Like I said, nothing fancy, just function.

[J.P.M.]

After all my research and pondering (and procrastinating) I finished my solar screen heater that I found the plans for on builditsolar.com Finally have my solar screen heater hooked up. Don't really need it today as it is about 60F outside, but this is the temporary hook up of the fan and ducts going into living room window. 70F air at lower inlet and the fun pushing ~150F air out the top duct running a 12v boxer fan with a 5w solar panel. It might be a simple project and not the prettiest thing in the world but I am feeling pretty good about it working as planned. A higher CFM fan would help but hard to find 12v fans that run on less than .5A and are silent. The fan is hooked up directly to the solar panel with no battery so the fan will only run when there is enough sun to make heat anyway. I tried to upload pics but it says they are too big. Like I said, nothing fancy, just function.

Congrads. The feeling I got from what sounds like a very similar experience many years ago is still vivid in my mind. Nicely done.

[bcroe]

After all my research and pondering (and procrastinating) I finished my solar screen heater that I found the plans for on builditsolar.com Finally have my solar screen heater hooked up. Don't really need it today as it is about 60F outside, but this is the temporary hook up of the fan and ducts going into living room window. 70F air at lower inlet and the fun pushing ~150F air out the top duct running a 12v boxer fan with a 5w solar panel. It might be a simple project and not the prettiest thing in the world but I am feeling pretty good about it working as planned. A higher CFM fan would help but hard to find 12v fans that run on less than .5A and are silent. The fan is hooked up directly to the solar panel with no battery so the fan will only run when there is enough sun to make heat anyway.

150F, very nice. The fan automatically runs when it needs to, an elegant design. Most fans are
far quieter when run around half voltage; maybe a bigger 24V fan would be ideal. Bruce Roe