Solar Air Insulation Question

[Glen]

Hi Everybody,

First of all, thanks for a great site and for sharing a tremendous amount of useful information!

I am currently building a solar air space heater. The design is a little different than what I've seen searching the web. The south side of my house faces the front in a neighborhood and the window I'm ducting it through is low. I also didn't want to cut any holes in my home (this time!). Anyway, I'm building a unit that is 12' long by 30" wide that I can store hanging on my garage wall during the warm monthes. I'm building it out of 1X6s with twinwall polycarbonate glazing and polyiso insulation. The absorber is 3" aluminum flexible duct and will be painted black (as well as the entire interior of the box). The fan part is still up in the air, I've got a 102 cfm DC fan (future PV powered?), but may go for a more powerful AC unit depending on what kind of airflow I can achieve. Will be using a 110-90 degree snap switch. Am going to snake the duct through the box and airflow will be through the duct.

Finally the question:

I'll be using multiple layers of the polyiso insulation. I planned on having the foil side of the interior layer facing the inside of the box. Should I face the layer closest to the wood toward the interior or exterior of the box?

Thanks,

Glen

[GaryGary]

Hi,

If you can face the foil layer on the outside layer toward the house it will insulate a little better because anything that is highly reflective also has a low emissivity, so it will lose less heat by radiation.  This is assuming that that the foil layer faces an airspace -- if it is in direct contact with something, then it does not make any difference.

But, most of your losses will be out the front, so I would not sweat it too much. Most commercial collectors have something like one inch of polyiso on the back side of the collector.  

Try to keep the airflow through the collector high enough so that the temperature rise from inlet to outlet is not more than 50 to 60F -- this will keep your losses down and efficiency up.

http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

Gary

[Glen]

Hey Gary,

Thanks for the response and the link. I've been reading alot of the buildit website and I have to say its probably one of the reasons I've started this project. Been reading like crazy. Liked the idea of a passive heater, but it evolved into my latest design, which requires forced air due to lots of ducting turns. I'll go with the outside foil facing out, there will be a very small adhesive airspace. Also I'll try to keep the temps down, seems counter-intuitive but you can't argue with the numbers. Anyway, the main box got built last weekend and I'll see how far I can get this weekend, taking some pictures and will try to post the results if they're not too ugly.

Thanks Again,

Glen

[GaryGary]

Hi Glen,

Sounds like a good project -- looking forward to seeing the pictures.

Gary

[Glen]

Hey Everyone,

Finally got around to finishing this project. Are they ever really

finished?? There is always room for improvement. Here are some pictures

and performance details:

http://h1.ripway.com/glensolar/GlensSolarHeater.htm

Glen

[ghurd]

Very nice!

I wondered about the fan too.

Fans are different.  Some are for lower pressure, some for higher pressure.

Might get more air for less power with another fan.

Google duct booster fan or inline duct fan?

G-

[Glen]

Thanks!

I'll be shopping for a squirrel cage blower with higher output for next winter.

Glen