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Author Topic: supplemental heat in a solar kiln????  (Read 657 times)

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Offline WLC

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supplemental heat in a solar kiln????
« on: November 04, 2017, 06:55:48 pm »
Anyone ever added supplemental heat to a solar kiln?  Living in Alaska makes me wonder if a solar kiln will be able to heat up enough to dry wood in a timely manner. I know my window for using a solar kiln would be limited to probably late April/early May to sometime in late September because of our climate.  I would not to dry wood with it outside of this timeframe.  Could supplemental heating be added?  If so, any ideas on the "with what" and "the how" aspects of it?   
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Offline GeneWengert-WoodDoc

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Re: supplemental heat in a solar kiln????
« Reply #1 on: November 04, 2017, 10:32:57 pm »
A solar kiln collector using the VT design where the roof is a two-layered collector is very energy inefficient when the sun is not shinning.  So added heat would be wasted with this roof design.  Rather build an insulated box for the kiln and then choose to supply heat with solar hot air panels, dehumidification, or even a wood furnace like is used to heat a house.

Note that the collector for solar will face southward, so early morning sun and late afternoon sun will do you no good, as the sunlight will not be perpendicular or close to perpendicular on the collector.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Savannahdan

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Re: supplemental heat in a solar kiln????
« Reply #2 on: November 05, 2017, 06:34:41 am »
Dr. Gene, if he has the solar kiln constructed couldn't he add insulated roof panels over his regular roof panels during the winter months and then use supplemental heat?  I'm working on building a small solar kiln using plans featured in American Woodworker magazine and have thought about the same thing.  These plans are based on your VT solar kiln design just downscaled to handle around 300 bd. ft.
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Offline GeneWengert-WoodDoc

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Re: supplemental heat in a solar kiln????
« Reply #3 on: November 05, 2017, 08:04:45 am »
Because of the low sun angle and short duration of sunlight, the roof will lose so much heat in spring and fall, the kiln will work only about 3 months a year, and because the sun is so far to the NE and NW in the morning and evening, it is likely that the kiln would dry one load per year.  Add to this that the roof angle is equal to the latitude, so the roof would be steep.

Financially, a $2000 cost and one load a year for 5 years means drying costs of $275 per MBF, plus electricity, labor, stacking, etc.  Totally unreasonable.

A blanket can be used indeed, but by the time such extras are added, plus the overall poor performance, it is my opinion that the VT design is not a good choice.  So, build a kiln that is totally insulated and then add solar, DH, hot water, or what ever system you want.  This will give an option of running 12 months a year, if desired.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Savannahdan

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Re: supplemental heat in a solar kiln????
« Reply #4 on: November 05, 2017, 10:19:10 am »
Yep, looked at the sunlight hours for his location and it isn't much during January.  Thank you for the information.
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Offline WLC

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Re: supplemental heat in a solar kiln????
« Reply #5 on: November 05, 2017, 03:06:08 pm »
Savannahdan, we don't have much sunlight now.  With turning the clocks back an hour last night we will be getting dark at 4-5pm now.  My biggest concern was as Dr Gene stated.  Having the cost of a solar kiln and only being able to dry maybe one load a year.  Our highest temps, unless it is an odd weather summer, are generally high 60's to mid 70's.  My greenhouse will get close to 100, but not for long. I wondered if some type supplemental heat would be advisable.  I've got to rethink my plans.  I need a kiln and I don't think the local sawmill will dry for the public.
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Offline Savannahdan

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Re: supplemental heat in a solar kiln????
« Reply #6 on: November 05, 2017, 03:28:43 pm »
WLC, when you look at the way Dr. Gene presented as to amount of use versus outcome (board feet processed) and expense it makes sense for you to go with a full blown, insulated kiln.  Fortunately you have options for getting heat into the kiln including solar.  The small kiln that I'm putting together will be using a solar powered fan.  Our weather today made me realize that not all days will have sun and that I may need to add something to keep the drying process going.  For now I'll just concentrate on the main kiln and see what happens.  I'm like you that the folks locally with kilns don't want to interrupt their schedules for my lumber.  I'm fine with that because it helps me get more into the full cycle of turning logs into lumber and turning lumber into projects.  Good luck.
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Re: supplemental heat in a solar kiln????
« Reply #7 on: November 05, 2017, 03:36:59 pm »
Just putting this out there . I am by far not an expert . If winter is like ours the RH is very low . Is there a way to move the cold dry air through the kiln and dry by freezing ? just asking .
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Offline GeneWengert-WoodDoc

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Re: supplemental heat in a solar kiln????
« Reply #8 on: November 06, 2017, 05:08:18 pm »
Actually, the average outside relative humidity in MN is 65% RH in the winter and summer too.  It is dry in a heated home because when you heat air, the RH drops quickly.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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Re: supplemental heat in a solar kiln????
« Reply #9 on: November 06, 2017, 05:46:38 pm »
I don't know what chart you looked at but in northern MN in the summer you can hardly get hay to dry and it is not unlikely to see RH at 95 % on a warm day . In the winter the days that are near 0 RH will be at 20% with the frost freezing our of the air .
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Offline Don P

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Re: supplemental heat in a solar kiln????
« Reply #10 on: November 07, 2017, 07:07:50 am »
Not sure where you are metering but one example of relative humidity,
If it is 0F @ 65% rh the air contains 3.58 grains of moisture/lb
If the same air is warmed to 65F the RH is 4% and it contains... 3.58 grains of moisture/ lb

Offline pineywoods

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Re: supplemental heat in a solar kiln????
« Reply #11 on: November 07, 2017, 09:16:16 am »
If you need more heat in a solar kiln, try this...Obtain some old sheet metal roofing a foot or so shorter than the kiln glazing. Paint it flat black. Fasten it to the under side of the rafters under the glazing, leaving a gap at the top and bottom. Cheap, simple, and it made a huge difference on my kiln...
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Offline Savannahdan

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Re: supplemental heat in a solar kiln????
« Reply #12 on: November 07, 2017, 12:33:17 pm »
Thanks, Piney, I'll look at incorporating some metal panels in my small, solar kiln in the works.  I'm making it small enough that I can disassemble for moving to a different location easily.  I have a dinky yard and some live oak trees that claim a lot of it and also shade it.
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Offline GeneWengert-WoodDoc

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Re: supplemental heat in a solar kiln????
« Reply #13 on: November 07, 2017, 04:16:25 pm »
If you check the following publication, you will see the average monthly humidity for five MN cities including International Falls.  Actually the values are EMC and you can change them to RH using Table 1 and you should get 65% RH approximately.

https://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf

Note that 50% average RH will be 9% MC.  65% RH is 12% MC. 
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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Re: supplemental heat in a solar kiln????
« Reply #14 on: November 07, 2017, 05:47:52 pm »
Ok this guy is in Alaska  He may get 4 hrs. of sun that will not even top the trees in Dec. and Jan. Like I said He isn't going to get enough sun in the winter . HEAT ? Cold? WLC how much sun on Jan . 1 .
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Offline WLC

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Re: supplemental heat in a solar kiln????
« Reply #15 on: November 07, 2017, 11:33:59 pm »
Ok this guy is in Alaska  He may get 4 hrs. of sun that will not even top the trees in Dec. and Jan. Like I said He isn't going to get enough sun in the winter . HEAT ? Cold? WLC how much sun on Jan . 1 .

5.5 hours of sun above the horizon on Jan 1.  We are gaining light by then though and it increases rapidly.  Average Jan temps are 23 for the high and 7 for the low.  Our warmest months are June, July, and August with average highs at +-70 and lows in the mid 40's( I don't agree with the lows though.  seem too low.  I would estimate at least in the low-mid 50's from my observations.) 

Our cooler average temps when we have sun and "warmer" temps (+-April-Sept) is the reason I asked about supplemental heating in a solar kiln.  Trying to take advantage of any extra "free heat" from that big ole nuclear reactor in the sky I can get. 
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Offline Crusarius

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Re: supplemental heat in a solar kiln????
« Reply #16 on: November 08, 2017, 12:26:28 pm »
This is probably a dumb question, but here goes. I know you need heat to sterilize wood. But do you really need heat to dry it? It is so much dryer during the winter months. Usually very dry cold air, and almost always a dry breeze to accompany that. I thought drying wood it was more important to have proper air flow than temperature?
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Re: supplemental heat in a solar kiln????
« Reply #17 on: November 08, 2017, 01:17:15 pm »
Given the location, it may be better to rephrase it as a standard kiln with supplemental solar...

WLC, what else would you use for fuel?  Or would you run DH?

This design may encompass both things you are looking for.  Insulated, but with solar.  Has been used in Wisconsin, and the PNW.  Will it work in AK? ???

http://timbergreenforestry.com/Solar%20Cycle%20Kilns.html
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Offline Don P

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Re: supplemental heat in a solar kiln????
« Reply #18 on: November 09, 2017, 04:31:20 pm »
This is probably a dumb question, but here goes. I know you need heat to sterilize wood. But do you really need heat to dry it? It is so much dryer during the winter months. Usually very dry cold air, and almost always a dry breeze to accompany that. I thought drying wood it was more important to have proper air flow than temperature?

Hopefully Gene will correct this;

Try an experiment, make maple syrup using only a fan, then try using heat to get that water excited. Which one is going to land on your pancakes first? Another, leave a pan of water outside but out of the rain during winter and during summer, which will take longer to evaporate?

Wood equalizes in relation to the relative humidity of its surroundings. Looking at Doc's linked emc table tells the story on how "dry" winter air is... the relative humidity doesn't vary as much as you think. My average emc varies about 3% through the year with winter not being the driest. In MN a good bit of winter is the highest rh.

Warm air can hold much, much more moisture than cold, that is why when you take that 0 air at 65% RH and warm it up to 65 degrees the RELATIVE humidity crashes to 4%. Humidity is measured relative to the amount of moisture it can hold at that temperature.

Another way to think about it,
at 0F and 100%rh (fully saturated, all the moisture it can hold at that temp) the air will hold .48 grains of water per cubic foot.
at 120 and 100%rh ... 35 grains of moisture per cubic foot. about 70 times more moisture can be carried away in the warm air compared to the cold
at 160 and 100%,  90 grains cuft, the air can hold 180 times more moisture compared to 0

Offline YellowHammer

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Re: supplemental heat in a solar kiln????
« Reply #19 on: November 09, 2017, 05:32:27 pm »
Building on what Don and others have said, heat is generally required for two things. 

1.  It is used to provide energy to release the bound moisture in the wood. Unbound comes out easily, such as during air drying at ambient temperatures.  Bound moisture is that last few percent that is much more difficult to remove, and it takes a little extra kick.

2.  Once the moisture is released, it's important to have a carrier medium to transport the moisture away from the wood, which is the warm air flowing over the pack of lumber.  Warmer air can carry much more moisture than cooler air as shown by Absolute Humidity vs Relative Humidity charts. 

The reason it's important to have proper airflow is because since the air is serving as a vapor carrier, the airflow pattern and velocities should be optimized to provide even and consistent patterns.



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Re: supplemental heat in a solar kiln????
« Reply #20 on: November 09, 2017, 05:35:44 pm »
I knew there was a good reason :)

Thanks :) That actually doe help understand the process better as well.
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