off-grid lumber drying ventilation/solar kiln

[nybhh]

Hi all. I've been running in circles around this forum for a while now and need to go ahead and get some informed feedback before I confuse myself any further!

I think I have one of the few uses where it makes sense to keep it simple and not use batteries. I basically need to add some fans to a barn where I season/dry lumber and this setup will eventually be moved to a solar kiln when I get around to building it PLANS HERE

The goal for the fans are to run "as much as possible". The more it/they run, the faster the lumber will dry and the less the chance for mold and mildew related staining but there is no minimum required for comfort. cloudy periods, etc. While having batteries and some LED lights in the barn would be nice, I think getting the timing down for the fans where I run them as much as possible without over-draining the batteries would be a serious challenge unless the system was sized much larger than I think makes sense for this.

Ideally I'd direct wire a couple of 12v fans to 1-2 100W 12volt panel, let them run when they run and be done with it. The more reading I do here however, the more I start second guessing that. Do I need 24v panels? Should I buy batteries and a MPPT charge controller? Will I need to limit the voltage coming off the panels to 12V if I direct wire? Things seem to spiral out of control and get complicated and expensive real quick.

There is a problem with the "simple" direct-wire option also in that there really doesn't seem to be many options out there for moving a descent amount of air on 12V, especially oscillating where a "typical" $40 oscillating room fan would do the job perfectly.

Can someone plant me in reality here? This thread was super helpful but I feel this person ultimately needed a bit more out of their system than I do.

Thanks for reading.

[J.P.M.]

How much moisture to you intend to evaporate from how much wood ?

Do you intend to have backup for the PV powered air circ.?

Ever consider a passive solar dryer w/nat. convection circ. ? Works for kilns too.

[nybhh]

JPM,
Thanks for responding, real good questions. This is a small scale hobby operation so not much. The exact amount of moisture is species dependent and varies with how recent the wood was milled so it varies but volumes are generally 1500 board feet or less and that is what the kiln will be designed around. Think 1 inch thick lumber in a stack 4 ft wide by 10-12 ft long x 6 ft high with 3/4" stickers/spacers between each vertical row.

Backup ventilation is not needed since the worst case scenario is dying too fast which I can control. If power shuts down or cloudy weather, moisture will just build up and drying will stop but temps are generally lower so less moisture to deal with anyway.

Too much moisture over an extended period can lead to mildew and staining issues but that can happen outdoors in humid/still air as well.

I have not looked into passive convection but will research it, sounds interesting enough.

Thanks again.

[Mike90250]

I thought wood had to dry slowly to prevent cracking and warping, or is it clamped in place in the kiln ?

[J.P.M.]

JPM,
Thanks for responding, real good questions. This is a small scale hobby operation so not much. The exact amount of moisture is species dependent and varies with how recent the wood was milled so it varies but volumes are generally 1500 board feet or less and that is what the kiln will be designed around. Think 1 inch thick lumber in a stack 4 ft wide by 10-12 ft long x 6 ft high with 3/4" stickers/spacers between each vertical row.

Backup ventilation is not needed since the worst case scenario is dying too fast which I can control. If power shuts down or cloudy weather, moisture will just build up and drying will stop but temps are generally lower so less moisture to deal with anyway.

Too much moisture over an extended period can lead to mildew and staining issues but that can happen outdoors in humid/still air as well.

I have not looked into passive convection but will research it, sounds interesting enough.

Thanks again.

You're most welcome.

Passive solar drying of wood is not only interesting but it's been around a very long time - much longer than PV and workable. Designs abound and are region/climate type dependent. Essentially, the wood is dried in a passively heated solar structure with the ventilation provided mainly by natural convection that's sometimes or often aided by forced convection. Some think such means are more appropriate technology. It's probably less expensive and more reliable than throwing whiz bang PV at an application.

[nybhh]

I thought wood had to dry slowly to prevent cracking and warping, or is it clamped in place in the kiln ?

That is correct but each species varies in how fast is too fast. Oak, for example, needs to dry very slow but also is not prone to staining. Eastern white pine is at the opposite end of the spectrum however and can handle rapid drying but is also very prone to stain if too wet or humid. I had about 700 board feet develop mold and stain in 3 days this summer during a wet, humid, still period. Until I get the kiln built, I am basically looking for away to move moist air off the lumber when the weather is not cooperating.

The solar kiln is designed to heat up well but is insulated and enclosed so a fan is needed to remove moist air and the speed which the moisture is removed is controlled through operable vents. The schematic I linked to illustrates the cconcept pretty well.

Solar kilns are far from newfangled technology but fans are critical to their operation and grid electrical just is not feasable at this location. The kiln is very similar to an attic venting situation actually while my barn fans will hopefully speed things up a bit and help prevent mildew during still humid periods.

[nybhh]

Original link is not working. Neither are apostrophes! Solar kiln schematic is here...

http://owic.oregonstate.edu/solarkiln/plans.htm

and

https://www.popularwoodworking.com/projects/solar-kiln/

[nybhh]

J.P.M. I spent a little time this AM researching passive solar and it appears to be pretty much the same exact thing as a solar kiln without the addition of some ventilation fans.

Virginia Tech has done the most work with solar kilns for lumber and their design specifies a couple of fans and all the kilns I know of use them. There are well established design guidelines for these so I am reluctant to go rougue and skip the fans.

i do own a small Woodmizer saw mill I so I do have a small but steady supply of green lumber moving through my drying space. I am willing to make an investment in a more effecient and controlled process to accelerate things and help minimize loss due to drying defects.

[J.P.M.]

J.P.M. I spent a little time this AM researching passive solar and it appears to be pretty much the same exact thing as a solar kiln without the addition of some ventilation fans.

Virginia Tech has done the most work with solar kilns for lumber and their design specifies a couple of fans and all the kilns I know of use them. There are well established design guidelines for these so I am reluctant to go rougue and skip the fans.

i do own a small Woodmizer saw mill I so I do have a small but steady supply of green lumber moving through my drying space. I am willing to make an investment in a more effecient and controlled process to accelerate things and help minimize loss due to drying defects.

Understood. Fans or other means of forced ventilation can be and often/usually are a necessary adjunct to any natural convection drying system. In spite of what treehuggers and the green mafia would like us to think, appropriate technology is not and never was meant to replace "standard" methods of living life in an all or nothing fashion.

I was once moderately familiar with process drying equipment and methods, including moisture removal from wood/cellulose products. Sometimes equipment for wood drying presented interesting situations, particularly with respect to moisture diffusion rates into/out of wood as f(wood species, growing conditions, climate, etc.). Probably still one of the (many) engineering black arts.

[nybhh]

I've done some more research on the solar kiln and I think it probably does make sense to run batteries but will stick with 12v fans to eliminate the need for an inverter.
I'd love a math/logic/sanity check if anyone is feeling generous!

Depending on the lumber type and load size, I need to be able to circulate air inside the kiln at up to 3,000 CFM and the fans generally run from about 2 hours after sunrise to 2 hours after sunset. Basically when ambient temps are higher/RH is lower in the kiln compared to outside. I've found some DC attic fans that move 1,000 CFM with 18 watts @ 12 volts so I plan to use three of them along the length of the kiln.

In my area:
June days are 15 hours long so 18W x 3 x 15 = 810 Watt Hours with 5.39 kWh/m2/day
Dec days are 9.25 hours long so 18W x 3 x 9.25 = 500 Watt Hours with 2.82 kWh/m2/day

Using a 1.5 fudge factor (MPPT charger), I need 266 panel watts in December and 225 panel watts in June. 3x 100W, 12-volt panels probably makes the most sense. Probably just get Renogy panels or something similar through Amazon.

With 5 days of backup, I need 208 ah of battery juice in Dec and 338 ah in July. The math on this makes sense because the shorter days require less watt hours but it defies intuition. A 30A charge controller will get the job done.

Probably 2x ~200ah 6-volt batteries makes the most sense for these. Any specific batteries out there that would be good for this application? The batteries can be taken off-site between kiln loads for maintenance.

Does this all make sense? I' still looking at fan options but the AC fans generally require a lot more wattage plus the addition of an inverter would add some additional inefficiencies to the system and just isn't needed as long as I can source some affordable DC fans.

I appreciate any input or red flags and I have to say the stickies in this forum rock!

EDIT:
Question: Is there a way to automatically shut the system down if the batteries get too low? Say to 50% for example? I am generally only at the site once per week or so and when I hit a period of cloudy weather, I'd rather just turn the fans off and save my batteries since weather that won't charge the batteries won't heat the kiln up much anyway.

[bcroe]

Just my opinion, but if your fans can just run when the sun shines, you would have a far
simpler system with no big maintenance needs, by running as many DC fans as needed in
daylight, with as may 12V panels as required to keep them going. Bruce Roe

[nybhh]

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.

[Mike90250]

If you plan on having fans running 2 hr before and 2 hr after, AND cloudy days, you are looking at a pretty serious bank, and it's not going to be 12V anymore.

The better charge controllers have Load control that can manage an offboard relay to cut off loads.

Morningstar has a controller with nighttime lighting features and may be able to run "loads" pre dawn and post dusk

https://www.morningstarcorp.com/products/sunsaver-mppt/

[J.P.M.]

I've done some more research on the solar kiln and I think it probably does make sense to run batteries but will stick with 12v fans to eliminate the need for an inverter.
I'd love a math/logic/sanity check if anyone is feeling generous!

Depending on the lumber type and load size, I need to be able to circulate air inside the kiln at up to 3,000 CFM and the fans generally run from about 2 hours after sunrise to 2 hours after sunset. Basically when ambient temps are higher/RH is lower in the kiln compared to outside. I've found some DC attic fans that move 1,000 CFM with 18 watts @ 12 volts so I plan to use three of them along the length of the kiln.

In my area:
June days are 15 hours long so 18W x 3 x 15 = 810 Watt Hours with 5.39 kWh/m2/day
Dec days are 9.25 hours long so 18W x 3 x 9.25 = 500 Watt Hours with 2.82 kWh/m2/day

Using a 1.5 fudge factor (MPPT charger), I need 266 panel watts in December and 225 panel watts in June. 3x 100W, 12-volt panels probably makes the most sense. Probably just get Renogy panels or something similar through Amazon.

With 5 days of backup, I need 208 ah of battery juice in Dec and 338 ah in July. The math on this makes sense because the shorter days require less watt hours but it defies intuition. A 30A charge controller will get the job done.

Probably 2x ~200ah 6-volt batteries makes the most sense for these. Any specific batteries out there that would be good for this application? The batteries can be taken off-site between kiln loads for maintenance.

Does this all make sense? I' still looking at fan options but the AC fans generally require a lot more wattage plus the addition of an inverter would add some additional inefficiencies to the system and just isn't needed as long as I can source some affordable DC fans.

I appreciate any input or red flags and I have to say the stickies in this forum rock!

EDIT:
Question: Is there a way to automatically shut the system down if the batteries get too low? Say to 50% for example? I am generally only at the site once per week or so and when I hit a period of cloudy weather, I'd rather just turn the fans off and save my batteries since weather that won't charge the batteries won't heat the kiln up much anyway.

Respectful suggestions before you put the cart before the horse and get serious about PV design.

Be careful on fan flowrates. Those are probably free flow/no restriction flow rates. Depending on the circulation pattern you want to establish (More torturous path flow path >>>--- more uniform drying, but >>>--- more induced pressure drop>>> less air flow >>> --- less drying and less drying uniformity.

In any case, expecting 3,000 CFM from 3 ea. 1,000 CFM blowers in actual operation may be unrealistic due to induced pressure drop. I'd get an inexpensive but reliable turbine flowmeter and see what a fan/squirrel cage/etc. air mover produced in terms of flowrate at the kiln outlet(s) under loaded conditions before I bought the air movers./

I'd also suggest that just like designing other equipment for fluid mechanical considerations (which I spent the better part of an engineering career doing), good thermohydraulic kiln design - at least the basics - includes tradeoffs between available or required pressure drop and as good of a flow distribution (with good in this case meaning mostly even and as uniform as possible witth the goal of uniform velocities across the product as possible, and with as few bypass streams as possible - for example straight inlet to outlet with no contact with the product being dried for most of the air. It's always more than just about the absolute flowrate.

Somewhat similar to flow distribution in cooling towers (which, BTW, often and sometimes still on occasion used packed beds and layers of cedar - sound familiar ?) something called bulk flow rate or mass velocity comes into play (expressed as mass/area^2 of tower (kiln ?) cross section per sec. or other time unit. I'd think uniform mass velocity in a kiln would be one of several design goals and would lead to more uniform drying similar to the way flow maldistribution is avoided in cooling towers by striving for uniform crossectional mass velocity that avoids maldistribution of the flow.

Also, with dryers of most any type, while an important consideration, there is more to consider than just relative humidity. You are correct about higher amb. temps. having lower R.H.'s, but that's because warm(er) air has a higher capacity to hold moisture. For design of dryers, a better design parameter is the dew point. Things like absolute humidity and dew point have a role to play. As a small example, depending on the moisture content of the wood, without some flow on a day with high dew points, and somewhat depending on the location of the inlet(s), if the wood has cooled overnite, or goes in cold and the weather becomes hot/humid, you might wind up adding moisture to the wood, particularly in the early A.M. or if the wood temp. is below the (higher) dew point. I'd consider a humidistat for some of the flow control.

Just some thoughts on the Thermodynamics, heat transfer and fluid mechanical considerations of drying.

Take it for what you may think it's worth.

[littleharbor]

I run 2, 80 watt,12 volt radiator fans to help cool my garage in Baja. I simply hard wired them in parallel to a 65 watt 12 volt solar panel. The reason I have two fans in parallel is when I connected a single fan to the panel it was running so fast I didn't think it would last very long. When two were connected they still ran at a good speed, just not so fast that they would self destruct in short order.

These fans are less than $20.00 on eBay/Amazon, Mine have been running every day for nearly 4 years now and are still working without a hitch.

12 volt radiator fan.jpg