Solar Lumber Kiln Using Radiator Fans

SystemSystem Posts: 2,511 admin
Hi all - I'm a newbie here, so please redirect if I'm out of line. I searched as best I could determine on this topic and didn't see anything.

I'm an urban forester who saves hardwood trees for lumber. My sawyer has had to dismantle his kiln, so I need to make a new one at my remote site (i.e. off the grid) so that I can continue to dry lumber. Of course, I'd like to do it in the most ecological and economical way possible. So I thought I'd take a few old 12V radiator fans to provide the airflow, hook them to a solar panel (and perhaps a car battery and charger system for more consistent running) and voila! However, I've searched far and wide on the internet for a real nuts and bolts description of how to do this and have come up empty.

Anyone here have any clues for me? I'm looking for advice on which PV panel to use, any components that would be required, and possible pitfalls.

If there's a useful hardcopy reference manual out there, I'd be up for recommendations on something like that, too.

Thanks much in advance!


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Solar Lumber Kiln Using Radiator Fans

    i don't know as i had no trouble with a quick search and found this:
    about the only things that weren't addressed that i foresaw was the size of the fan in relation to the size of the kiln in cubic ft/min and similar parameters for the fan such as its diameter and wattage. it must have enough power to push that hot air downward to the lumber.
    i might have thought to design the kiln in such a way as to maximize the solar natural flow from hot to cold and heat rising by placing the lumber in an area to the rear and slightly above or equal to the solar heated area. fans should need less power to do the same job and being heat naturally rises the lumber being treated shall stay in heated air longer in the sun's absence.
    as to the pvs used in this it all depends on the draw of the fan motors. some voltage regulation may be employed as the voltages on pvs can vary widely enough to possibly harm coil windings depending on the fan motor's specs. no need to keep batteries imho as the kiln does not operate well at night anyway.
  • System2System2 Posts: 6,290 admin
    Re: Solar Lumber Kiln Using Radiator Fans

    Thanks for your time and reply. I know all about this site, it's the standard solar kiln design and much like the one I intend to build. But it says nothing about the use of solar powered fans.

    What I'm looking for is more like a cookbook for powering something using PVs. I would prefer the kiln fans to run constantly during the daylight hours (up to a certain relative humidity), then shut off at night to let the wood relax. For this reason, it seems to me that the PVs need to be charging a battery or batteries instead of being hooked up to the fans directly. But, again, I can't find the right info to help me make the right buying decisions.

    The radiator fans will be 12v, obviously. So what kind of PV will I need if I want to charge the 12V battery(s) to a level necessary to run the fans for, say 12 hours a day? Does there need to be some sort of interface between the panel and the battery to help it charge correctly?

    I thought that this kinds of scenario (e.g PV-->battery-->device) would be fairly common. So any advice, particularly first-hand, would be greatly appreciated!

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Solar Lumber Kiln Using Radiator Fans

    the kiln works by the sun so the fans could too. if there's no sun then there's not that much heat that needs pushed out of the kiln. it is simply the current drawn by the loads(fans) plus some extra for losses. for example a fan that draws say 1 amp need that initial 1 amp and upwards of another .25 amp (just guessing as losses vary by your choice of wires, connectors, switches, and anything else that would affect it) with a simple system like i proposed, this would automatically shutdown in the sun's absence taking the complexity, losses, and power draw of other items out of the mix such as timers for example. you now are talking of the potential for it to run all night long in a failure of a timer thus ruining the batteries too that aren't needed during none sunny times anyway in addition to not being needed during the cooler nights.
    if you elect to add the batteries to this mix you add a more expensive regulator to be employed (charge controller), batteries; extra wiring, connectors, and fuses; and the timer that add more costs needlessly and more to go wrong with the extra worry that goes with it all. up to you though. let us know if you do this and what you did with the results.

    edited to add, did you understand what i meant about modifying their kiln for better natural flow?
    as to cook books i guess there are books out there to read and even magazines. or you could read here for free too if you get the time as there's that much reading.
    for the record- your idea of using solar powered fans i deem to be a good idea, but just try to keep it simple. i guess i should add that the pvs should not be on top of the kiln due to the heat and you don't want to block the sun from the kiln either. maybe off to the side or back somewhere, but still in a good position to get good sun exposure as well. we don't want them competing for the same thing or being a detriment to the other.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Solar Lumber Kiln Using Radiator Fans

    As I see it, the first issue is the design of the kiln itself.

    The current design has the fan blowing the hot air down--if possible, it would be better if you can use the fact that heat rises to help move the airflow in the direction you need--would reduce the size of the fans needed.

    If you can build a glass "green house" area in "front" of the kiln at ground level to maybe 1/2 way up--with the cold air inlet at the "toe" of the design. That way, the hot air wants to rise and move into the kiln area on its own.

    Next, to find a fan motor that would work well on DC. If you can avoid the whole battery thing--you will save a bunch of money... No batteries and no losses due to battery charging--will allow you to use about 1/2 the wattage of solar panels.

    The issue with DC motors that seem to be readily available for running fans--they are usually brushed DC motors--and long life is probably 5,000 hours maximum... So, at ~5 hours per day at full power, you probably will need to replace the motor every 1-3 years.

    Here is one place that lists DC fans (don't know anything about the product or the vendor)--at least it will give you an idea of the size, power requirements, and the costs.

    DC radiator fans may be worth trying--Although, I do not know what kind of motor they are (brushed, electronic?). And depending on the motor design, you may need to watch the maximum voltage (12 volt solar panels usually are ~17 volts - 20 volts--which could damage an electronically commutated 12 volt motor).

    You can use the PV Watts program to estimate how many hours per day of "full sun" you will get. And the program has the hour by hour solar panel wattage output too for the average 365 day year--so you can pick the minimum sized solar panel wattage vs how many hours per day you want the unit to run. And you can compare seasonal variations too.

    So, to use PV Watts program... Assuming Roanoak Virginia is somewhat near you... Put in 1 kW of solar panel (minimum number program accepts, you can scale up or down later). Use everything else as a default for now.

    You will get an average of ~3 to 5.5 hours of sun per day across the year.

    Now, I would pick a fan... Say, for example a 16" fan running at 24 volts and 2.7 amps (FO16 rated 1,600 CFM at 24v)... Take the PV Watts program output and say you want this to run a minimum of 6 hours per day 9 months of the year. That would be February through October (94 kWhrs per month of 1kW solar panel power--not using these numbers, just identifying the "darkest" month we expect the system to operate).

    Now click on "Output Hourly Performance Data"--and look at February's numbers. If we look towards the end of February, we see that the hours of sun starts getting useful for our 6 hour minimum at full power:
    1973, 2, 28, 07:00, 0
    1973, 2, 28, 08:00, 1
    1973, 2, 28, 09:00, 100
    1973, 2, 28, 10:00, 184
    1973, 2, 28, 11:00, 229
    1973, 2, 28, 12:00, 263
    1973, 2, 28, 13:00, 264
    1973, 2, 28, 14:00, 416
    1973, 2, 28, 15:00, 595
    1973, 2, 28, 16:00, 539
    1973, 2, 28, 17:00, 345

    1973, 2, 28, 18:00, 108
    1973, 2, 28, 19:00, 0

    So, for a 1 kW array, 263 watts from Noon to 5pm is the minimum power for our six hour window.

    The fan we looked at was a 24 volt at 2.7 amps or:

    P= 24v*2.7a = 64.8 watts average power

    And to scale the 1,000 watts (1 kW) of solar panels to your need:

    64.8W / 263W * 1,000 watts of solar panels = 264 watts of solar panels (24 volt Vmp solar panel).

    Lastly, you should look at a "24 volt Linear Current Booster" to increase the total number of hours the fan runs in a day. Basically, a "LCB" takes the High Voltage / Low Current of a Solar Panel that is not in full noon time sun, and turns it into Low(er) Voltage and High(er) Current that the DC motor prefers (better power matching between the electrical characteristics of a solar panel to the electrical characteristics of a DC motor).

    The above is just the way I would start to look at your needs for drying wood. If you need the kiln to run 12 months out of the year, or need more than ~6 hours of operation at the beginning and end of the season--then the design may need to get more complicated.

    It is also possible, that you will need much less (or even much more) power than I have documented here... The system, as I attempted to design is optimized to provide as much air circulation as possible (6 hour minimum) at the beginning and ending of the 8-9 month season)--you can adjust the above calculations to your needs/location.

    Also, you may try building one kiln first--and just purchase the components for one fan--and then purchase the rest of the components based on what you find out you really need (example, more fans, but fewer solar panels--so you can optimize to your needs).

    Just for a sanity check--you could always compare the costs of a solar powered system (and fan replacement motor costs) to purchasing (for example) a Honda eu2000i genset (reasonable fuel efficiency between 400-1,600 watts), AC powered fans, and fuel...

    $900 Honda eu2000i
    $310 16" 400Watt 120 VAC Exhaust fan (probably CFM equal to 2-5 of the DC fans)
    $342 = 0.5gpd * 274 days per year (9 months) * $2.50 per gallon of gas
    Assume generator and fan lasts two seasons:

    ($900 + $310 + 2*$342)/2= $1,552 per year (genset)

    The solar system... Assume $5 per watt for solar panels (and 20 year life for panels and LCB), Fans last 2 years:

    $1,320 = 264 Watts * $5 per watt (20 year life)
    $400 = 4x$100 for Linear Current Booster (20 year life)
    $1,500 = 4x$375 fans (replace every 2 years)

    1,320*/20yr-life + $400/20yr-life + 1,500/2yr-life = $836 per year costs (solar)

    Note that AC fans typically are rated CFM vs back pressure (typically 0-0.4" inches of water for a simple exhaust fan). The DC fans are probably rated in open air--so their performance probably is seriously degraded when trying to pull air through the restrictions of the lumber and if it is against the natural tendency of hot air to rise (original design).

    Anyway--my 2 cents and a first guess. The above design/costs may make sense, or not, for your your needs--comparing DC vs AC based design more or less done as a sanity check.

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Solar Lumber Kiln Using Radiator Fans

    The major purpose of a kiln is not to dry lumber, but to dry lumber evenly so it does not warp or crack. This is especially true for hardwoods. That is one main reason that some premium hardwoods, like Walnut, are often air dried over a 1-3 year period.

    If you are not in a hurry you might be better off just using a greenhouse type setup with good air circulation to keep the drying even, which will probably mean a fair amount of fans and/or vents.

    Also see
  • halfcrazyhalfcrazy Solar Expert Posts: 720 ✭✭✭
    Re: Solar Lumber Kiln Using Radiator Fans

    Check out these guys can and will answer all of your questions. guaranteed some one there has done this already. Plus if you are into wood in any way it is a great place to hang out.
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