The Forestry Forum

General Forestry => Drying and Processing => Topic started by: Florida boy on January 24, 2017, 07:09:45 PM

Title: 2" thick foam for solar kiln?
Post by: Florida boy on January 24, 2017, 07:09:45 PM
I've been milling for about 4 months now and have decided to build a solar kiln. I picked up about 30 - 2" thick 48 sheets of foam insulation used for the roof of the building I'm working on. roofers had extra and didn't feel like hauling it so they gave it to me. it's the kind that has yellow foam between fiberglass paper front and back not the bue/pink "squeeky" kind. do you think this would be good for my kiln build ?
   also I live in Northwest Florida. it gets bad hot down here in summer. I've read a lot about solar kilns on here. and have seen somewhere that because the humidity is so high it's best to rum vents closed and use a dehumidifier at night? is this correct? if someone could explain this to me it would be great. I plan on using double poly carbon on the top but may be overkill being I'm only 50 miles from Panama city florida (deep south)? i also plan on using the metal collector fastened below the polycarbon.what is a good distance for between the two?  I know I need vapor barrier in the walls and floors. Haven't figured anything on doors yet.
        I know it's a lot of questions but I know I've come to the right place to ask them. if I can figure out how to get photos from my gallery to the thread I'll post some picks of materials gathered and progress along the way. all and any advice or comments are appreciated.
Title: Re: 2" thick foam for solar kiln?
Post by: GeneWengert-WoodDoc on January 24, 2017, 11:02:47 PM
The foam should work fine.

When foggy air at 100% RH is heated 25 degrees, the humidity will drop to around 30% RH.   So, The high humidity is not a big issue.  The humidity will drop with a little sun. If you close the vents the humidity inside will rise to 100%. So open vents but not too wide. The outside RH in your area will dry wood outside in a shed to 18% without solar heat, so a solar kiln will dry much lower in your area.  No need for a dehumidifier to dry wood. You can use one if desired, but it is not needed  the high humidity at night relieves Case hardening stress.  Fans  are not run at night

The  Virginia  Tech design does not use a metal collector. Instead, the interior walls and pile cover are black

Title: Re: 2" thick foam for solar kiln?
Post by: Waterford Woodworks on January 29, 2017, 08:28:36 PM
Sorry to butt in but.... in line with your question and Gene's answer has created a question for me. Gene, what would happen if I use a black metal collector mounted to the bottom side of the rafters from the Virginia tech solar kiln? I'm building one right now and these are the type of questions that keep me up late at night. I kind of like the idea of not needing to cover the stack with a solar collector. Thanks, Jason
Title: Re: 2" thick foam for solar kiln?
Post by: GeneWengert-WoodDoc on January 29, 2017, 10:11:47 PM
The black is called the solar absorber.  It is slightly more efficient to keep it away from the clear collector covers.  Close means it gets hot and losses increase by temperature to the 4th power.  Appreciate that the incoming radiation is the same for any style of absorber.  If the absorber is flat black it will get close to 100% of the incoming.  So, the significant issue is in heat loss differences.
Title: Re: 2" thick foam for solar kiln?
Post by: pineywoods on January 29, 2017, 10:57:09 PM
I have a bit of experience with the black metal collectors. There are 3 kilns locally that use the technique. Plusses....
airflow over both top and bottom surfaces effectively doubles the amount of heat transfer surface resulting higher heat within the kiln. I have documented 180 degree temps in mine. Even on cloudy overcast days, I see a temp rise of around 40 degrees over outside temps. My kiln has lousy insulation or it would be even more.  Screwing the collector metal to the bottom of the rafters ( 4 inch gap from the glazing) seems to work well. That gap works like a plenum to conduct heated air upward to the top of the collector. There will be a fair amount of convective airflow up that plenum even without the benefit of you may be able to use smaller fans..The only problems have been fans. One builder used plastic fans, the blades actually melted and fell off..The other one used metal fans, but they all quit when the internal thermal fuses blew..