off-grid lumber drying ventilation/solar kiln

[nybhh]

Mike90250 - Thanks for controller recommendation! I looked at it a bit this morning and it looks like it has some real nice features that could work great for in this application. I'll need to dig in with a bit more detail but a great lead for sure. I believe you misread the timing however as it is (approximately) 2 hours AFTER sunrise until 2 hours AFTER sunset so the length of a day basically which is what I used for the calculations in post #10 which indicates 12V at 300W is plenty. Do you see an error or misunderstanding in my calculations?

littleharbor - I've looked at some of these radiator fans and they are look interesting and are much cheaper than the other options I've been looking at. I've had a hard time getting CFM ratings for them though and if the kiln is well insulated and the vents are closed, interior temps can get to 160 which can melt some plastics so I've been looking for all-metal, brushless options but those are expensive and the plastics on these are probably designed to handle a bit more heat given radiators can get pretty hot. I may have to pick up one or two of these to experiment with and meter before investing in something a lot more expensive. It is interesting though that you are running 2 on a 65 watts panel though. I was using the full 80w each in my calculations which was requiring a much bigger system.

J.P.M. - Wow, you clearly have a much better handle on the engineering of these than I do! I appreciate your interest but am reluctant to stray too far off topic here? Not really sure what is appropriate for this forum.

Anyway, the exact design I have decided on is here... http://pubs.ext.vt.edu/content/dam/p...20-030_pdf.pdf
and I don't really plan on making any significant changes as this design is well tested and one of the designers, Dr. Gene Wenget, is active on another forum I read and he is the one that suggest about 3,000 CFM for this size kiln. It is easy to miss but the diagram on the bottom right corner of page 3 is important in understanding how these work. They are designed to be pretty simple and idiot proof (thankfully) to operate as long as solar collector size does not exceed 10 SF per 100 BF. There are lots of examples of people adding additional controls and automation but there is a point where it begins to loose its simplicity and becomes more of its own hobby rather than a tool to perform a job.

For other interested parties, the basics of these are:
Vents open - air doesn't recirculate as much with more fresh air intake so internal temps are cooler and moisture coming off the wood is exhausted to the exterior quicker.

Vents closed - air recirculates more and the internal temps get much hotter but moisture evaporating off the wood builds up and slows drying. The more humid interior however coincides with a higher MC of the wood itself so it is sort of self-regulating since the early stages with higher MC has the most risk of drying defects. As the wood dries and moisture slowly escapes to outside, the humidity in the kiln drops accordingly and the temps rise even higher as less energy is used for evaporation.

The vent gradient in between full-open and full-closed is where the user dials things in based on experience, kiln specifics, a speed vs risk decision, insulation, glazing type, etc. It is the "art" of it.

In the early stages of drying, air flow is critical and does the most work while the later stages when wood gets down near the limit of "outdoor" equilibrium, more heat is required to go lower. In my area, 15% is about as low as we can go air drying.

So pine for example, vents wide open - moisture is evaporated as quickly as possible to prevent staining. Once the wood gets down to about 15-20%, the vents can be closed to heat up the kiln and push the wood down to 6-8% with the added benefit of killing insect and larvae and setting pitch.

White Oak on the other hand is the most difficult native species to dry and that is where the 10SF per 100BF rule comes in which is what these designs are based around. A partial load of oak requires covering up some of the the "windows" to stay below 10:100. For oak, you basically close the vents all the way if you have a "leaky" kiln or slight cracked if you are tight well insulated kiln and just let the sun work. The wood should dry as fast as weather conditions allow while never exceeding the safe rate of about 2-2.5% per day.

In both cases, turning the fans off at night is recommended as they aren't accomplishing enough then to justify the electricity and allowing the wood to "rest" and retake up a bit of moisture at night eases internal drying stresses and prevents case-hardening. Better commercial hardwood kilns use steam to accomplish the same thing. Any PV advise?

[J.P.M.]

[USER="1076"]
J.P.M. - Wow, you clearly have a much better handle on the engineering of these than I do! I appreciate your interest but am reluctant to stray too far off topic here? Not really sure what is appropriate for this forum.

Not really. Probably/mostly that I've made more mistakes than you, Including starting an early collector design on fire before I went back to school. Otherwise, just related equipment design experience that may be transferable to other things.

One/several final thoughts, then I'll shut up :

1.) If the constuc. material is wood as appears in the design, I'd make it kiln dried and maybe avoid some of the dimensional changes when that wood get REALLY kiln dried during use.

2.) On that note, think safety. Some day (one that's warm, clear, dry, and not windy) Murphy's law says air flow will cease for some reason. The kiln construction material will have dried out by then (several years from now ?). The kiln will heat up and reach stagnation temp. under no flow conditions (where the incoming solar radiation equals the overall heat loss coeff. of the structure. If the construction wood has REALLY dried out, there is a small b ut non zero chance some of it may be heated by a combination of direct irradiance and elevated air temps. to above its (now much lower) kindling temp.

3.) I'd suggest considering using light paint colors for the interior of the kiln. That will "bounce" the incoming irradiance around so that more of it will settle on the product. (It'll also keep the construction wood cooler).

4.) On the paint, two things: a.) All paint will off gas as it dries and for some time thereafter somewhat asymptotically for a month or more depending on the content. The vapors will coat the inside of the kiln, including and maybe particularly the glazing. That will reduce the transmittance of the glazing, lower the amount of energy transmitted and reduce the drying potential of the kiln. b.) I'd consider either doing the required painting about a month or more before construction as much as possible so the paint has time to outgas, or buying VERY low VOC paint, or better yet, both.

5.) I'll harp on this one: blowers in service will not produce their unrestricted (rated/published) flowrate. If I was doing it, I'd build the kiln and then use a turbine meter to find/est. the flowrate, and have the design flexible enough to be able to increase the blower size/number. FWIW, my experience with blowers/air movers is that in service, and a s a SWAG/starting guess only, I planned for about half the rated flow as an actual output, installed 2X capacity and then throttled the flow as necessary.
If you want 3,000 CFM actual, start with/buy higher capacity air movers unless/until you have a better handle on actual flows.

Good luck,

J.P.M.

[nybhh]

J.P.M. With the possible exception of #3, which conflicts with Dr. Wenget, that is all truly great advise and I really appreciate your insight into this. #4 in particular had not occurred to me nor have I seen any discussion about that on the kiln related forums.

Re: #3, no direct sunlight actually reaches the lumber/product as the baffle and "solar collector" (diagram on page 3) catches all the direct light, shading the lumber and the rest of the kiln. Air flow over these solar heated surfaces is how that energy is spread around the kiln. There is no "direct" light to bounce around the kiln behind those surfaces so I don't know how much difference it really makes but the general guidelines is to paint the interior black to absorb as much heat as possible. If we were relying on bounced light, I would agree with you but direct sunlight on a board's surface causes very uneven drying and badly warped lumber.

Thanks again!

[bcroe]

I am going to add, if you must have uniform circulation over hours and days, batteries will
be needed. If not, I (mr over paneled) suggest you consider increasing the number of
12V paralleled panels by perhaps 4 times, so full fan power will be available even under
modest levels of clouds, and some circulation virtually any day the panels are not covered
with snow. Tilt sets of paralleled panels E and W to extend hours of operation on sunny days.

If this may sound extravagant, do your battery system calculations. You are going to need
to collect enough energy to run the system, in the end it may take all those panels anyway
to get that energy. UNLESS, you are willing to constantly monitor the system and bring out
a generator VERY PROMPTLY to make up the energy shortcoming and SAVE your expensive
batteries. That is how batteries can work, they do not add energy, just time shift it. Bruce Roe

[J.P.M.]

J.P.M. With the possible exception of #3, which conflicts with Dr. Wenget, that is all truly great advise and I really appreciate your insight into this. #4 in particular had not occurred to me nor have I seen any discussion about that on the kiln related forums.

Re: #3, no direct sunlight actually reaches the lumber/product as the baffle and "solar collector" (diagram on page 3) catches all the direct light, shading the lumber and the rest of the kiln. Air flow over these solar heated surfaces is how that energy is spread around the kiln. There is no "direct" light to bounce around the kiln behind those surfaces so I don't know how much difference it really makes but the general guidelines is to paint the interior black to absorb as much heat as possible. If we were relying on bounced light, I would agree with you but direct sunlight on a board's surface causes very uneven drying and badly warped lumber.

Thanks again!

You're most welcome, but on # 3, I'll explain later why other surfaces inside a passively heated space ought to be light colored. Up to my best intentions just now.

[nybhh]

If this may sound extravagant, do your battery system calculations. You are going to need
to collect enough energy to run the system, in the end it may take all those panels anyway
to get that energy. UNLESS, you are willing to constantly monitor the system and bring out
a generator VERY PROMPTLY to make up the energy shortcoming and SAVE your expensive
batteries. That is how batteries can work, they do not add energy, just time shift it. Bruce Roe

Thanks Bruce. My battery calcs in post #10 suggest with "5" days of backup (2.5 with FLA @ 50%), I need 208 ah of battery juice in Dec and 338 ah in July. I haven't gotten any push back on those calculations so I assume they're good but would love to hear otherwise if not. This saw mill is at a weekend property so I'm generally not around during the week so a controller that could shut things down if batteries drop below a certain threshold would be a good fail-safe to preserve batteries if I hit a long cloudy period, or enough backup power to get me to a weekend without over-discharging. The batteries can be pulled for grid-charging on weekends if needed. I have grid power is about 1/4 mile away. Adding more panels so that I am only rarely tapping into batteries though is an interesting idea as well. Several ways to skin this cat I suppose.

[J.P.M.]

J.P.M. With the possible exception of #3, which conflicts with Dr. Wenget, that is all truly great advise and I really appreciate your insight into this. #4 in particular had not occurred to me nor have I seen any discussion about that on the kiln related forums.

Re: #3, no direct sunlight actually reaches the lumber/product as the baffle and "solar collector" (diagram on page 3) catches all the direct light, shading the lumber and the rest of the kiln. Air flow over these solar heated surfaces is how that energy is spread around the kiln. There is no "direct" light to bounce around the kiln behind those surfaces so I don't know how much difference it really makes but the general guidelines is to paint the interior black to absorb as much heat as possible. If we were relying on bounced light, I would agree with you but direct sunlight on a board's surface causes very uneven drying and badly warped lumber.

Thanks again!

OK, back to it.

I got a chance to look at the design with some spare time.

1.) Aside from safety issues or unless I'm part of a design review, I'm not critical of others' designs. I'll just say, that this design appears adequate, but I'd have done it differently in many respects.

2.) On the color - dark vs. light for the interior: After reviewing the design, if it was my design, and I kept the cavity absorber principle as used here (more on that in a minute), I think I'd make the back wall and the horizontal portion of the "collector" some non wood product such as black poly carb or flat black painted aluminum. I'd then paint the E & W walls white. Note: SAFETY ISSUE: Covering the E-W walls w/alum. foil or leaving the alum. side of batt insulation exposed can be a safety issue where/when non planar and non rigid semi specular reflecting surfaces like alum foil can act as solar concentrators and like a magnifying glass, start fires or leave roast marks.
The White E-W interior walls will reflect sunlight to the black absorbers at times away from solar noon (assuming the glazing is oriented pretty much due south). Eventually, most all the light reflected from light colored surfaces will find its way to a black surface. Long story, but the design will be a bit more efficient and probably have more uniform temp. distribution over the day with light colored E-W interior walls.

3.) Time for some simplified thermal magic. This kiln design can be thought of as a form of "cavity absorber". Classic textbook cavity absorber: Think of a hollow sphere that's well insulated whose interior is painted flat black. Now, imagine a relatively small hole in the sphere, say, 10 % or less as wide as the diameter of the sphere. Now, shine some sunlight onto the sphere normal the hole. Here's what will happen: Most (>> 99%) light that enters the sphere through the hole will hit the black interior and be absorbed. Most of the non absorbed light (and not much from a flat black surface) will be reflected or reradiated with most all that light - except a very small portion that's lucky enough to pass through the hole and escape - hitting other portions of the (flat black) interior and will be reabsorbed. Some of that reabsorbed light will get reflected/reradiated again and the process continues until essentially all of the light (--->>>100%) that entered the sphere gets absorbed. The interior of the sphere (and eventually the air in it) will increase in temp. from the absorbed energy and tend to stay that way longer due to the insulation.
Now, paint half the sphere interior white. Doesn't matter which half or even if the pattern is regular or discontinuous. An equal quantity of energy will enter same as before. Only this time, about half of it will really get bounced around by the white surface - but eventually the bounced around (reflected) light will (most all of it anyway) wind up hitting a flat black part of the sphere and get absorbed. Well - and here's the magic part - eventually, all the energy that entered the sphere will be absorbed by the black portion of the sphere - the same amount as before. However, because the black portion is now smaller in area and has still absorbed all the energy, the black portions of the sphere will now be at a higher temp. than when the sphere interior was all black.

So you say, and rightly so, "So WTF does any of this have to do with light colored E-W walls ?" (I can tell you're thinking here)

Well, looking at the kiln design. It's not a textbook cavity absorber, but it does have, more/less, the important attributes of a cavity absorber and - take this on faith if you can - it does act a lot like one in many ways. The way it is now, the back vertical wall and horizontal "collector" part of the kiln look like they will probably see higher air velocities than the side walls - and higher velocities are good for heat transfer. Also, sort of like (and more so than you may think) the 50% white sphere interior, the amount of energy absorbed by the kiln with white E-W walls will be about the same as if it were all black - and energy input is what drives the bus. But in this case, the kiln "collector" surfaces will get more reflected light from the side walls and so get hotter just like the black portions of the black/white sphere. That's good because the greater air velocities on those surfaces will mean more heat transfer and more drying. It also means more uniform drying throughout a day. Morning sun will reflect off the W wall and afternoon sun off the E wall. The "collector" will heat up sooner and stay warmer longer. There will still be some convective heat transfer from white walls to the air, and less than if the walls were black, but the overall result will be a hotter "collector" surface that transfers heat more efficiently because of higher air velocities, a design that may operate a bit warmer or about the same max. temp. but with smaller temp. swings mid day to tails throughout a sunny day, and it will operate a bit longer - reaching temp. a bit sooner and staying that way a bit longer in the afternoon.

4.) If I was doing it, I'd have made the sloped equator facing wall as is now, but adjusted for your latitude + 10-15 deg. or so. BTW, the higher tilt will make max. temps. warmer in winter and cooler in summer, and may slow don summer drying rates some which may/may not be desirable.

I'd also have put a black absorber directly behind the glazing and skipped the bother about interior color all together. Basically, aluminum sheet, 20/24 ga. or so, staple it the 2 X 4's on the sloped side, weatherstrip it, caulk it tight and call it done. Then, the air from the blowers can/will impinge directly on what would then be the backside of the collector. Better, faster, more efficient. Put a rollup shade on the outside for easy and somewhat linear temp. control.

5.) Lot's of other little things I'd do or not do (I'm pretty sure those listed coatings are going to coat the inside of the glazing in short order, reducing the drying efficiency- particularly the roof coatings), but as I wrote, not my design. As drawn, it's probably safe and fit for purpose.

One thing I'd consider would be to rig a scale to act as a platform for the product. Seems like that would make the measuring/drying progress an easier matter, or at least another tool to get a drying process progress. I don't have a clue how maldistributed air flow - which seems likely with this design might lead to differential drying rates or different moisture removal through a charge.

Not meant as a knock, but it's easy to tell the design wasn't done by someone who knew a lot about solar design.

I know some, but not much about wood. If I was tasked with a wood kiln design, I might ask a wood drying expert for a design review. Good engineers and designers stick to what they know and collaborate when they don't know. My guess is there's lots of little missing touches in this design that will cause problems sooner rather than later. But that's just opinion.
So much for my shutting up.

Take what you may want of the above. Scrap the rest.

[nybhh]

OK, back to it....

You had me at white E&W wall! That part immediately made 100% sense and I can't believe I haven't seen one that does that now that I think about it. I can imagine how long this took to write btw, thank you.

A lot of the modifications I've seen do some of what you described. One popular modification is many people take black painted galvanized roof tin and mount it to the inside of the sloped roof rafters creating a ~4" cavity that the circulation air is blown through. This design is sort of a hybrid in the way they mount the collector... https://goo.gl/images/4KJsQN ...and is sort of the way I was leaning.

Some people who use metal collectors directly above the wood have reported it radiates too much heat through the backside onto the top couple layers of lumber creating a lot of warping and drying defects where those top few boards receive a lot more radiant energy on the top side over the bottom so giving the collector a bit more air space or mounting the metal to something that will insulate the backside a bit would probably be a good improvement as well if used flat against the stack.

What I do like about the sloped collectors is they also allow the wood to be weighted down below the collectors rather than having to weight the collector itself with CMU blocks or something which will significantly reduce its effectiveness. It is good practice to add a few hundred pounds to weight to the top of the stack to help keep the lumber flat as it dries and I also generally crank everything down tight with several ratchet straps around the stack as well.

My winters get pretty cold in the Hudson Valley of NY so I was also considering double glazing my sloped "window" with a layer of polycarbonate roofing panel on each side of my rafters and then mounting the collector at an angle behind it like the photo above.

You are right that there are a lot of improvements that could be made, side baffles when drying shorter wood than the kiln, for example but like everything I guess, it is a trade-off between being easy to make and use vs squeezing a little bit more performance out of them. These are generally used by hobby wood workers and such and drying too slow is generally safer than too fast so a little lost efficiency isn't the end of the world. A commercial kiln where every extra minute the wood is sitting in the kiln cost money is an entirely different story.

Back on topic - how do you think two T-105s would work for this?

Thanks again J.P.M. I've enjoyed this unexpected detour.

[jflorey2]

bcroe I appreciate the response. My original inclination was exactly that. It wouldn't be perfect timing wise but very close with a lot less complexity. One of the main reasons why I am now considering batteries however is related to the mildew/stain issues I described earlier. A few days of backup power during a period of very humid/muggy weather with no sun would allow me to keep the fans moving to help prevent mildew/mold staining. This is really only a danger right at the beginning of a cycle when the wood MC is the highest but with certain species like pine and maple, keeping the air moving in the early stages until the MC drops below about 30% is really critical.

Suggestion - have two systems. One that runs the fans directly off PV at full speed. This is simple and relatively cheap. The second, much smaller system provides a much lower voltage to the fans (say 6V) that causes them to barely turn over. This provides minimal ventilation all the time and runs off battery. Since it provides very little power the battery/charge controller can be smaller and cheaper. Use a charge controller that provides an LVD output so you can shut down when the battery is drained. Use a chemistry like LiFePO4 that will tolerate being discharged often and left in that state. You will also need a DC/DC to convert battery voltage to 6V.

[J.P.M.]

You had me at white E&W wall! That part immediately made 100% sense and I can't believe I haven't seen one that does that now that I think about it. I can imagine how long this took to write btw, thank you.

A lot of the modifications I've seen do some of what you described. One popular modification is many people take black painted galvanized roof tin and mount it to the inside of the sloped roof rafters creating a ~4" cavity that the circulation air is blown through. This design is sort of a hybrid in the way they mount the collector... https://goo.gl/images/4KJsQN ...and is sort of the way I was leaning.

Some people who use metal collectors directly above the wood have reported it radiates too much heat through the backside onto the top couple layers of lumber creating a lot of warping and drying defects where those top few boards receive a lot more radiant energy on the top side over the bottom so giving the collector a bit more air space or mounting the metal to something that will insulate the backside a bit would probably be a good improvement as well if used flat against the stack.

What I do like about the sloped collectors is they also allow the wood to be weighted down below the collectors rather than having to weight the collector itself with CMU blocks or something which will significantly reduce its effectiveness. It is good practice to add a few hundred pounds to weight to the top of the stack to help keep the lumber flat as it dries and I also generally crank everything down tight with several ratchet straps around the stack as well.

My winters get pretty cold in the Hudson Valley of NY so I was also considering double glazing my sloped "window" with a layer of polycarbonate roofing panel on each side of my rafters and then mounting the collector at an angle behind it like the photo above.

You are right that there are a lot of improvements that could be made, side baffles when drying shorter wood than the kiln, for example but like everything I guess, it is a trade-off between being easy to make and use vs squeezing a little bit more performance out of them. These are generally used by hobby wood workers and such and drying too slow is generally safer than too fast so a little lost efficiency isn't the end of the world. A commercial kiln where every extra minute the wood is sitting in the kiln cost money is an entirely different story.

Back on topic - how do you think two T-105s would work for this?

Thanks again J.P.M. I've enjoyed this unexpected detour.

What's a T-105 ?

[nybhh]

What's a T-105 ?

https://www.trojanbattery.com/product/t-105/

Suggestion - have two systems. One that runs the fans directly off PV at full speed. This is simple and relatively cheap. The second, much smaller system provides a much lower voltage to the fans (say 6V) that causes them to barely turn over. This provides minimal ventilation all the time and runs off battery. Since it provides very little power the battery/charge controller can be smaller and cheaper. Use a charge controller that provides an LVD output so you can shut down when the battery is drained. Use a chemistry like LiFePO4 that will tolerate being discharged often and left in that state. You will also need a DC/DC to convert battery voltage to 6V.

Hmm, I'll have to think on this and run the numbers both ways. I still may try and use PVs only initially to see how it works out and then add a battery/charge system later if I feel like I'm not getting enough ventilation or offset my schedule a bit. Interesting suggestion though, I just need to think on it a bit - thanks.