Yet another solar kiln build started

[SawyerBrown]

Bought materials last fall, and they've been waiting patiently (??) for someone (me) to get started.  Cold and snow finally broke, so it's time.  Starting with the deck, frame built and vapor barrier added


 

Now some treated 3/4 ply for the floor:


 

Trimmed and finished:


 

Flipped on the underside and adding 2" of insulation:


 

Tomorrow I'll finish plywood on the underside, then add the pairs of 2x10 "skids" (want to be able to move this if I need to, so the skids will allow me to pull it up onto my trailer and fit between and over the fenders)

Now if this little guy will just stop distracting me, maybe I can get some work done!


 


 

[Andy White]

SB,
That is going to be a fine looking kiln. What are the dimensions inside? Skids are a good idea for sure. Your helper looks like he's taking his job seriously! You will have a great companion for a long time. Keep us updated on your progress.   8) 8)    Andy

[SawyerBrown]

Thanks Andy.  It will be just under 16' long by 6' wide inside.  I needed it to fit on my 16' trailer, both from a length and weight standpoint, and I wanted to use as much standard 16' materials (2x's, plywood, etc) as possible.  So I won't be able to dry 16' boards, but not too often you're doing that with hardwoods anyway was my assumption (they get DanG heavy!).  Besides, I still want to focus primarily on custom sawing and offer drying as a side.

By the way (fortunately or unfortunately, as the case may be), the puppy is not mine.  I rent a crib and tool shed out on a farm place, and there's a family that rents the house and garage -- and he's theirs.  I guess it's kind of like grandkids -- you can play with them and enjoy them ... and then send them home for somebody else to take care of!   :D

[Peter Drouin]

Looking good 8) 8)

[YellowHammer]

What are you going to dry?  I'm looking at the background of the photos and only see one little bump, I think it's a tree, in the far distance :D

Just kidding, looks like you've a good handle on things.  Solar kilns are fun builds because when you get done with construction, they just work. 8). I guess it's like a windmill, as soon as it gets up in air, it starts turning.  Same thing with a solar kiln. 
YH

[Tree Dan]

Looking good Mr. Brown 8)....i better get cracking or your gona pass me :D

[SawyerBrown]

Now that I have the base almost finished, I'm wondering if I should anchor it somehow.  Would the wind ever blow this thing over??  East/west (on the ends) shouldn't be a problem.  The north is somewhat protected by the tool shed, but from the NE might sneak around tool shed and impact the biggest face.  The south is wide open to winds comin' sweepin' down the plain.  I guess the good thing about the south with its 45deg roof is that there is also down-pressure from any wind, and a relatively short vertical face.  I'm making it light enough to move with a tandem-axle trailer ... but is it too light?  Thanks for letting me think about this out loud, let me know if you have any advice.  I guess if all else fails, I start doing some figuring ... but where is the center of gravity? ...

[pineywoods]

Sawyer, if you keep it full of lumber, wind won't be a problem  ;D

[Tree Dan]

I would only worry about it if it's empty like pineywoods says.
And if it moves when its loaded up, then more buildings are going with it.

What you could do if you have a auger, is drill 6 posts in there...add some dry cement...pack it in there, then lag bolt flat bars or angle irons from the posts to your outside base wood.

Looking good....too windy to work on mine.

[SawyerBrown]

What are you going to dry?  I'm looking at the background of the photos and only see one little bump, I think it's a tree, in the far distance :D

YH, wha choo talkin' 'bout?  Trees right around here are a little sparse, but they're big and really straight.  Consistent too.  Same three branches on all of 'em.  Can't wait to cut one down and see what it looks like inside!   ;D 


 

Got the top of the bottom  say_what finished today, and added the 2x10 rails.  They'll allow me to slide it onto the trailer (over the fenders) if I ever need to move it.  Hopefully not ...


 

OK, now how to get it flipped over.  No braun left, so brains is all I got.  Inch it over a little bit at a time.  Gosh I love that jack!


  

  

  

  

 
With Sonny's help, we got it flipped over and leveled.  Whew!  Tree Dan, he's holding it down until we can get the posts in.   :D


 

Sonny was tired, so we rested and called it a day.  Maybe tomorrow we start building walls.


 

[Tree Dan]

Look at the Paws on that little carpenter helper.

Floor looks good, wheres the walls 8)

[Planman1954]

Nice progress photos! Don't let up! This is fun to watch.

[SawyerBrown]

Look at the Paws on that little carpenter helper.

Oh I know!  They're bigger than their other dog, a full grown female black lab.  He's gonna be a big boy!

[SawyerBrown]

Floor looks good, wheres the walls 8)

Here you go, my "pile o' walls".   ;D

Oh, now I suppose you want the walls erect?  Man you're a slave driver!   :D

That's tomorrow, if it doesn't rain.  How you coming?



 

[Tree Dan]

Your doing a fine job...looks great.
I have one wall on my kiln, but got busy.

Im building mine in the shop, and I may be able to work on it this week end.

We got wood to dry, now lets get those walls up. 8)

[scot wolf]

I'll offer to help with the first load.  ;D

[SawyerBrown]

I'll offer to help with the first load.  ;D

Hey, a first customer!   8)  :D. Where you at, scot?

[chet]

Sawyer,
Dat's one dense forest ya got there in the back round.  ;)

[beenthere]

Chet, I think maybe that is just a dark cloud on the horizon... ;)   :D

[SawyerBrown]

Actually, I think it's a smudge on the camera lens ...   :D

Hey, they made fun of Noah for building a really big boat in the middle of the desert too!  Someday you guys will be sorry you made fun of me!   ;D

[SawyerBrown]

OK, walls are up, held together with baling wire and masking tape (kidding).  Quit because that damp wind was just cutting through me and it was tough to work without gloves, even though I hate using gloves.  Maybe later this week it will warm up.



 

[Tree Dan]

Looking mighty fine 8)
Is that a man door roughed in? plus are yaa putting in 2 barn doors?

The weather here has been nasty as well

Nice job...

[SawyerBrown]

Ok, question for you guys ... been trying to decide how to attach the corrugated polycarbonate roofing.  Valleys or tops?  And if tops, use what they're calling a "no-drip spacer"?  Drill first?  And thinking I need to add purlins ... maybe 3 in a 10' span?

Thoughts?

[Andy White]

SB,
Attach the screws on the ridges, not in the valleys. I used the corrugated poly filler strips under mine , mounted on purlins, and used the selftapping screws. No predrilling , mine is almost a year old now, and still no leaks! I treated the edges with flashing.


  

  

  

 
Trim length, and fold down and caulk with silicone, and done!     Andy

[Glenn1]

That is one nice looking solar kiln!!

[YellowHammer]

As said, screws through the ridges, fold or otherwise trap the edges down or the wind will get under it and try to pull it off. 
YH

[SawyerBrown]

That IS one nice looking kiln!!  Thanks for the advice.

Raining today, so no work gonna happen.  Yesterday got some more siding on and a few more 2x4's that will act as purlins, day before got a coat of flat black paint on.  (Andy, yours look nice all in a row, just have to toe-nail in one side ... I may have to rethink that ...)  Won't be long and I'll be ready for the poly roof.  So Andy, what did you do at the roof line on the tall/"north" side?  I was thinking of a short overhang with the poly I'll cut off ...


 

The north side will have double doors for loading on one end, and a "people door" on the other.


 

Sonny's been "helping" me clean up the lumber scraps   :D


 

[Planman1954]

Are you using the piney woods design like I did? It looks like mine, except for the door area. I'm proud of you! Thanks for the progress photos. You'll be glad you took them when you look back on what you've accomplished on your own.

[Tree Dan]

The kiln is looking super, and the sky is blue and the sun is shinning. 8)

Your almost there.

[WDH]

Will you be loading with a machine like a tractor or fork lift, or will you be hand loading?

[SawyerBrown]

Planman, same basic design, except instead of painting the interior black, I'm planning to put flat black painted sheet metal roofing on the underside of the rafters and run air through the gap.  I may be wrong about this, but it seems to me that since wood is an insulator, it's not going to retain much heat, but sheet metal is going to get pretty hot, and you get the added effect of radiated heat even on the underside.  (I know this from painting metal roofing as a kid ...  ;D ).   Do you agree or am I missing something, like surface area?

WDH, loading by hand, unfortunately.  I think I know where you're going with the question ... are the doors wide enough?  I've got about 10' of door opening (and, by the way, not much more between kiln and tool shed), so I could eventually pre-load at least that long if I ever got more equipment.  But thanks for asking!

[pineywoods]

Sawyer, you will like the black sheet metal. Two big advantages. Wood is a poor solar collector, metal is much better and like you said, makes a bunch more heat. Enough extra that you can skimp on the insulation without much effect. Two, you will find significant heat gain on cloudy days..Sunlight is not what heats the metal, it's infra red radiation which penetrates clouds to some extent...Looks good, leaving a wide back door, your back will thank you...

[GeneWengert-WoodDoc]

If you have a black painted absorber surface, it will absorb basically all the solar energy input (visible solar energy and infrared energy) and convert it all to heat.  The key is the black paint and not what is under the paint.  So the amount of heat energy produced is the same whether the surface under the paint is metal or is wood, or any other material.  If it were different, then where would the energy be going?  Once the energy passes into the kiln, it is fully absorbed by the black paint regardless of the material under the paint.  The energy that comes into the kiln cannot just disappear.  It takes the same small amount of heat to heat a piece of metal as to heat a piece of wood.  Now, hot metal feels hotter because metal conducts heat to our fingers faster than does wood..you can handle a piece of wood at 180 F, but hot water or hot metal at 125F is too hot to handle.  Bottom line is that the material under the back paint does not matter for solar drying, but it might matter for your pocketbook if one type of material is free or lower cost.  Use whatever costs less and goes up easiest.

Note that most solar covers (glass fiberglass, plastic) do not transmit much infrared energy.  This means that once the energy in the kiln is heat, not much energy will leave the kiln as infrared energy through the clear cover.

Here is more technical info:  What is different, is that a flat absorber right under the clear collector will be hotter than if the walls and other absorbers in the kiln are painted black.  The reason is that there is more area, so the energy is dispersed, giving less temperature, but still the same amount of energy is in the kiln and energy is what drugs wood.  (Now, a lighted match would be even hotter, but the heat is in a small spot.  Likewise, a parabolic collector with the area of the solar kiln's roof is even hotter in a small spot, but the total energy is the same.  This is similar to using a magnifying glass to focus the energy on one spot and getting wood to burn.)  The flat plate right under the cover...it is hotter, but in a smaller spot than if the walls were the absorber.  But again, the overall energy input to the kiln is the same, and it is energy that evaporates water.  When cooking an egg or trying to boil water, we do need a certain temperature to do the job (water needs 212 F to boil), but for evaporating water from wood, it takes the same energy.

One small difference is that the closer the absorber is to the clear collector cover, the greater the heat loss by convection, as the air is hotter in that small space.  The hotter this absorber, the greater the infrared radiation losses as well.  For that reason, especially if the clear covers do not reflect nearly 100% of the infrared radiation (non-green glass does indeed reflect most IR; it is called solar glass), the overall efficiency is higher if the absorber is further away.  On the other hand, convection losses are reduced if the absorber is cooler (more area).  Further, the hotter the walls are if they are black and heated directly by sunlight, the greater the conduction losses.  This loss is about the same as the increased convection losses in the areas hit by sunlight.  Of course, if the fan baffles are black and the cover on the top of the lumber is black, there would be no conduction losses from these absorbers.. 

So, the bottom line is that there will be no important differences with the walls, fan baffles and lumber cover as the absorbers except the amount of black paint used (VT design for example) versus a flat plate absorber right under the clear collector covers, (so long as the walls are insulated and the clear cover reflects infrared).  That is, the black paint will absorb all the solar energy entering through the clear cover no matter what the shape or composition.  The biggest key for solar kiln design is to avoid energy losses from the kiln once this energy is in the kiln, thereby using as much of the solar energy for evaporation as possible.

[SawyerBrown]

Doc, really good explanation, thanks. 

I'm still thinking this through (and just for the sake of a good argument  ;D), isn't it really the amount of heat energy in the air, which is the only "fluid" that comes in contact with the wood?  If you could magically shut off all heat transfer from internal surfaces to the air, it would never warm up, and the wood would never dry ... all the energy would remain in the internal surfaces.  (And, I guess if there was no external heat transfer to air either, it would continue to heat up until it melted ...   :D).  So isn't the real question how much of that fixed amount of captured energy can you get into the air?  Since heat transfer is a function of delta-t (sorry, don't know how to make a "delta"), it seems like the hotter surface of the metal would result in more heat transfer to the air, via both convection and radiation.  And metal is better than wood because you get that higher delta-t on both sides.  But I'm sure I'm missing something!   :D

Sorry for all the questions, I'm just trying to learn and make sense of it all in my own head.

[Andy White]

SB,
For the flashing at the top, I took a sheet of coil stock 14" wide, and broke it with a 7" leg with a hem on both edges for strength. Put it in the brake, and bend to 135°, as the roof is at 45°. It is lapped over the side flashings forming a shingle effect, and screwed as all the sheets. The screws here go thru the sheets and the flashing into the top purling. Really watertight. The vertical leg is nailed to the siding at the top with s/s nails.   

[GeneWengert-WoodDoc]

Sawyer Brown.  Excellent comments.  Here is a further explanation.  The black paint will get as hot in any situation.  I think we agree on that.  Metal will conduct the heat to the other side, the dark side more easily than a piece of wood, especially if the wood is thicker.  But even so, eventually all the energy will leave the absorber and get into the air.  If it didn't leave, then the wood would have more energy, get hotter, etc.  If we had slate for the absorber, the rock would store energy, but eventually the energy will come out of the rock.  At an instant, around noon, one absorber might release more heat for that moment, but over the daylight hours, it will be equal for all absorbers if they are the same color.

You are correct that heat transfer from the collector depends on delta T.  The hotter the collector, the more heat transfer for that moment.  But now, if a cloud goes over the collector, the metal  absorber, if it is hotter, will quickly cool as it releases its energy.  Wood will release its energy more slowly and so will stay warmer for a while.  So, if we want to dry something in a few hours, metal has an advantage.  But over a days time, no difference.  By nightfall, the two collectors will have had the same energy input from the sun and will have released all this energy into the kiln.  So at the end of the day, the two will be at the same cool, energy state, ready to begin a new day the next morning.  Neither one stores or consumes energy over a 24 hour period...they both release 100% of the energy they absorbed.

Because wood is a thermal insulator, the black wood surface will be essentially the same temperature as a black metal surface if they have the same area, are facing the same direction, etc.  The difference is that the metal will feel hotter to the touch, as mentioned earlier.  The amount of energy used to heat the wood is very small, and that energy will eventually be released as night comes.  With more area, which happens when the walls and fan baffels and top cover of the lumber pile are used as absorbers, the absorber temperature will be cooler and so delta T will be smaller, but the increase in area will make up for that difference.  The formula is energy release by convection = (convection coefficient) x (temperature of surface - temperature of air) x (area). 

Consider this...if we had a metal absorber painted black and also insulated the back side really well, the top side would then be twice as hot, delta T would double, and energy convection rates would double.  The area for convection is half however, so the energy per minute is the same.  But, the total energy input, which is the amount of incoming solar would still be the same, and the energy output also, even if the absorber is hotter.  So, think about energy and not heat or hotness.  Isn't an absorber that is an insulated piece of metal really close to a piece of wood?

Although this discussion might be considered theoretical, in 1961 at the U.S. Forest Products Lab I made four small solar kilns and used one or two layers of covering and compared them, and used a metal absorber right under the clear covers and compared that to black painted walls.  The two layers of cover increased performance (much hotter air in the kiln), but the type of absorber had no effect on kiln air temperature.

There was a master thesis written at Colorado State Univeristy in about 1968 about the energy balance in a solar kiln, That has even more technical info.

[Planman1954]

SawyerBrown...you ARE using the piney woods design. It will work great. The painted black metal attached under your rafters will have your kiln heated on a hot summer day upwards to 180 degrees plus. Keep the pics coming!

[SawyerBrown]

Thanks, all!  ... Andy for the advice on overhang, Doc for the excellent explanation (which now makes perfect sense), and to Planman for the encouragement!  Hope to get more done today and next week before several days of predicted rain.

[WDH]

With your hardwoods at 180 degrees, I suspect that bad things might happen  .  Pine and hardwood, especially oak, are two different animals.

Brings to mind a quote by Lee Trevino on golf.  "You can talk to a slice, but a hook won't listen". 

Pine can stand the intense heat and rapid drying.  White oak won't listen. 

[LeeB]

Boy Danny. You sure are old. I haven't heard that name in a long time.

[Tree Dan]

Looking good,
S.B I'm pre drilling all my holes, and making the holes 1/8" larger than the screw.
Cheers

ps for the clear panels

[SawyerBrown]

Finally got a day last week when the wind wasn't blustering quite so hard (I now know why they decided to put a wind farm out here ...), so I got the polycarb sheets cut and in place and screwed down.  Now I can work on the inside putting in the insulation and building the plenum with circulation fans. 

By the way, Tree Dan, I tried just driving the screws through the polycarb sheets, and it seemed to work great.  Appears I have a good tight seal ...



 

Although we had about 1/2" of rain yesterday, I anticipate we'll be planting by Wednesday, which will put a 2- to 3-week hold on the kiln build ...

[WDH]

Nose to the Grindstone (another old saying). 

[Tree Dan]

Your almost done, and looking good.
Are the panels 8' ?
I know what you meen by the wind, Its been very windy up north here.
Have fun with the planting, It won't be long now you will be putting your
first load of wood in that kiln.