solar kiln extra heat

[logman 219]

need more heat in solar kiln !!! any ideas

[Broncman]

Pics? How is it built? Is the collector area painted black? How is it insulated?

[logman 219]

walls 3 1/5 thick with fiberglass and 1inch foam'''south wall and roof insulated patio door panels''painted black inside -- two attic fans seat to come at 100 degrees --roof slants at 45 degrees

[doc henderson]

Are the patio doors cloudy, or do the have a solar reflective coating.  Is this a new problem, or has it never got hot enough?  I assume you mean the room is panted black inside not the back of the glass doors.  If you put a hand on the glass is it very hot.  The energy has to go through the glass and get absorbed inside.  You have to convert solar to heat (black surface absorption) and keep it (Insulation).  If the glass is very hot, it is capturing the heat.  I know pics are a learning curve issue, but that might help us with the diagnosis.  how hot does it get inside the kiln?

[logman 219]

right now it is 98 outside !! inside the kiln it is  125    !!!

[trimguy]

Can you raise the temperature of the fans coming on so more heat stays in ?

[Sod saw]

.


Why is it that you believe that you need more heat ?

A solar kiln will dry just fine with the inside temperature about 20 degrees F (or more) warmer than the outside air temperature.

If you are attempting to sterilize the wood, , ,  that's another issue.

As for the fans, I assume that they are used to circulate the air within the kiln chamber and not exhaust fans.

What state (country) are you in?  It helps to know these things.


.

[doc henderson]

any pics?

[logman 219]

to sterilize    load ,, indiana,, kiln based on vermont model,,

[scsmith42]

For sterilization purposes, I added a propane torpedo heater to one of my solar kilns.  See response 9 in this post for more details.

https://forestryforum.com/board/index.php?topic=122494.msg1989144#msg1989144

Scott

[tacks Y]

Did you just load it? I know mine with a fresh load does not get real hot. I can tell how dry the wood is by how hot it gets.

[Oth]

Been thinking about this myself. Built a solar kiln a few years ago, been going strong but certainly slows down in the winter here in CT. My kiln has no power, fans are 12v direct wires to solar panels. In that same line of thought I was considering direct wiring some 12v silicone block heater mats to solar panels. 100 Watt pad goes for around 15 bucks. Have to scrounge some panels somehow. Thoughts?

[Ianab]

I think it would need a lot of solar panels to power enough heat mats to make a difference? 100 watts of heat isn't going to do much to a kiln load of wood, maybe 10 of them might, but then you need 1,000W of panels to power them. The heaters are only going to work fully during good sunlight, and there is less of that in the Winter, which is of course the main reason the kiln slows down.


Maybe an auxiliary solar collector beside the kiln could collect more heat, and only need an extra solar fan / panel to blow what heat it does collect through a vent into the main chamber? If there is enough sunshine for the PV panels to be useful, an extra collector panel is going to do some good as well. The Aux collector could also be angled to make the best use of what Winter sun you get, rather then the "average" angle that's normally used. It could also be angled towards the sunrise a little, which would help get the kiln up to temp sooner in the day? Basically the solar collector in the kiln designs are sized to not over-cook things in summer, in winter more collector area should be safe.


As an aside, a local guy has worked out a relatively simple solar hot water heating system that uses PV panels and requires no changes to the plumbing. Basically he replaces the existing mains heater and thermostat with a dual element heater, one for 240v mains and the other for 12v from standard  solar panels. The thermostat then has a micro-controller that monitors the water temp, solar power output and time of day, and will switch in the mains heat if the tank isn't up to temp at the end of the day, or the hot water is all used before the sun comes up. Because there is no plumbing going to the solar panels it's much simpler and cheaper to install, basically just the electrics, and because it's "smart" managed it only turns on the mains heater when it's actually needed.  He claims it will cut your hot water power use by ~75%, and still keep the water hot enough.


So the idea of PV cells hooked to a simple heating element isn't crazy, just it needs a whack of PV cells to power a serious heater. When what you really want is just warm dry air to keep the kiln working.

[Oth]

A friend of mine works in solar and tips me he can get me 425W panels for a very reasonable price. We'll see what they turns out to be but 2 of those would run 8 mats for 800w, nothing to sneeze at I think.

I should've been a little more clear in my initial post that I'm looking for the supplemental heat in the winter to get to temperatures that will sterilize the wood so I only need it that high for a few hours, doesn't matter to me if it's off at night. Speeding up the process is just a side effect in my eyes.

I thought a while about an auxiliary collector like a bunch of black PVC coiled up behind glass then piped in but it just ends up being too complicated for what it's trying to achieve. If they had 12v electric baseboard I would do that, no hesitation. These mats just seem like the closest thing to simple and cheap, assuming i can get panels for a reasonable price. That hot water heater setup sounds  pretty slick not planning on installing a hot water heater in my kiln though or running power to it for that matter hahah.

[doc henderson]

100 watts of heat is not much.  think of the 1500-watt heaters are the little ones my scribes use under a counter at work cause they are always cold.  also, energy is neither created or destroyed.  with each change in energy, there may be inefficiency like light produced when you want heat, or sound.  you could consider increasing your area of collector, but it will be hotter in the summer as well.  also, amps x volts = watts.  so, 12 volts takes more amps to produce the same watts.  so going through an inverter can make 110 volts, but there is usually a fan, and heat loss so you are losing some energy.  the amps drop proportionately to the volt increase minus losses so have less heat produced.  If just for sterilization, I would conder wood heat to forced air furnace.  could make a separate sterilization chamber better insulated (i.e. no solar collector).  I would consider a generator for a day to run big electric heat.  the dryer elements are cheap online. 

[doc henderson]

to dry wood, you just need the kiln temp 20° higher than the outside temps to drop the relative humidity.  It will be slow but will dry.  to sterilize you need the whole mass of the kiln and wood to get to over 133° to the core, so the standard is to tun at 160° for a day.  heating a solar kiln means (especially at night), the loss through the glaze may exceed your ability to produce heat.  It may take you 3 days to get the kiln up to 160 and then need to stay there for a day.

[Oth]

Doc, generator is a great idea especially since I already have i good sized portable unit. I can throw some cheap baseboard heaters or dryer elements in and run them for a day or two. My kiln is just out in a cleared area with no power. I don't have a mill like most folks on here so unless I spend a lot more time clearing trees a wood furnace just isn't in the cards for me, good idea for guys with a mill though.

Thanks- I'll be wheeling my generator out there in a few weeks, much simpler.

[doc henderson]

think safety.  wooden building full of wood and heat source. 
many folks use the hotter than heck halogen work lights.  threads abound.  add up the watts to just under what you gen. will pull
can throw moving blankets over the collector to insulate from massive heat loss.  If you have a generator, good chances you may already have some of the above as well.  good luck.

[Sod saw]

.


Lets look at some math and science for a moment.

Many of us have heard of a British Thermal Unit (BTU).  Most of us have no idea what one is.  For this discussion, it doesn't matter what one is or how it relates to physics.  Just know that most heating/air conditioning
guys as well as engineers use them as a commonly accepted means to measure the flow of heat from one place to another.

If you contact the company who made your clear solar collector "glass" roof you can find out how many BTU's per hour will pass (be lost to the nite air) thru your roof.  Write that number down.

Next find out the thickness of your walls and look up how many BTU's are lost per hour based on your wall thickness insulation type.  Next - do the same for your floor.  What kind of insulation is in your floor and how thick is your floor insulation?

Next, do the same for each of your doors.  How many BTU's are lost thru each door?  Write each of these numbers down.

Next add a fudge factor because you have leaks around each door because it is not a solid wall.  Nothing is perfect .

Now you have a vague idea as to how many BTU's you need to provide each hour to replace all of those that are lost thru your walls, floor, roof, etc.  I believe that the number will be quite high due to the roof being a poor insulator.

After you know each hours worth of lost BTU's you need to decide how many hours you will be without sun to warm the kiln chamber up to 150 degrees (F) as that is the accepted goal so that the center of each board within your kiln will reach a temperature of 133 degrees (F).  YellowHammer likes to leave his lumber at 150 degrees for 24 hours.  Remember It should not fall below that goal at nite, otherwise you may need to start the clock over again the next morning.  How many hours of BTU's are lost each nite?

Now, we need to decide how we are going to actually make the replacement heat to replace all that is lost each hour.

If you have a generator, like mine, you can make electric heat from 115 or 230 volts.  I chose 230. volts.

If you chose electric baseboard heat, you will find that the baseboard manufacturers have included a thermal cut off that is below 150 degrees.  This will limit the baseboard heat from getting uo to 150 degrees (F).  You can disconnect that theremal cut off switch but then you risk overheating (melting) the wires within each baseboard heater themselves (not a great idea).

I have disconnected my baseboard heaters from the kiln and installed normal outlets throughout the kiln chamber and use Quartz Halogen Lights (work lites) that just plug into those outlets.  You will need to actually do your own math to determine how many watts of heat you will need.  Those outlets are connected to a thermostat to prevent the kiln chamber from heating past 150 degrees (f), but turn on the heat source if the temperature falls below 148 degrees (F).  Yes, I do leave the generator(s) running all day while the sterilizing cycle is in progress to power the thermostats and heaters.

My internal kiln chamber fans are always powered by the solar panels as yours are.  However, they do not work after dark so I have included a 12 volt DC power supply connect to the generator so that those fans will circulate the kiln chamber air all nite even though no sun shines. Of course assuming that the generators are running. 

After knowing how many BTU's are lost each hour, you can look up how many watts of electric heat it will take to replace those BTU's.  That conversion is available on line.  If you chose to use propane, you can look up how many gallons or pounds of gas it will take to replace those lost BTU's of heat.  If you use oil as a heating fuel, you can look that up or wood fuel, and on and on.

I use quite a few thousand watts of electric heat each hour after the sun sets to keep my kiln chambers warm enough to prevent the wood from cooling down at nite.  I do not attempt to sterilize wood in fall or winter or early spring.   But please remember that we do not sell lumber, only use it ourselves and do not require an efficient year round kiln operation.

I believe that your math will discover that solar electric heat will not be affordable for your operation, but I would love for you to show me the math to prove that I am wrong.  I am used to being wrong, , , just ask my daughter.

Good luck with you kiln project.

.

[doc henderson]

the dryer elements generally go higher and mount on the housing.  you may want a fan.  they are often 220 V and have a thermostat and thermal fuse that prevent overheating.  These come with but you can buy them separate and choose the amount of temp you want.  these usually mount to the housing so you may want it higher than the 160 so it keeps heating. what is the Watts of your generator?  you should be able to choose one slightly less than max. 

[doc henderson]

to be a little crazy, you could run the generator exhaust through a conduent pipe that runs through your kiln to get that heat in there as well.  current price on amazon is 36 bucks for a 5500 watt 220 v element with a series of safety temp limiters that fit the housing.   the one I looked at was 260° max in the housing.  the one I bought I think was at the 160° range, and I planned to separate it from the housing and element.  these are cheap and come in all sorts of temp. ranges.

[doc henderson]

https://m.media-amazon.com/images/W/MEDIAX_792452-T1/images/I/71dhgR6tXPL._AC_SX679_.jpg

[Oth]

I won't set out to prove you wrong but I will do the math. I may take my time since I've got more pressing things to attend to than recreational math for a little while.

[Oth]

Doc, my generator is 13k watts dual fuel. I've definitely aimed the exhaust at my propane tanks that run it in the winter to keep them from freezing... probably not in the operators manual. ;D Thanks for the link. I would definitely need a fan, it would be nice if I could just unplug the fans from the panels and plug them into the generator but that whole voltage thing... could rig up a little transformer for each fan i suppose.

[doc henderson]

yes, and running exhaust through a building is a fun theoretical, but if you got a leak in the metal Conduent, there is a risk of CO poisoning.  pics of your kiln would be fun for us.  the most common inverter is 12 v to 110, but you can check with a heating and air provider, and they commonly have low voltage control stuff.  add up the amps and it may turn out to only need one for all the fans.  or you could get a couple of Walmart box fans (over 20 bucks each now).  use these when you sterilize.  check that other components of your kiln can withstand the heat.  good luck.

[Oth]

 

 

 

 

Here's some pics, Doc.

I've done some homework and according to my calculations at 30 F I'll need 3860Whr to keep it at 150 F. If i throw some 2" foam on top of the glazing temporarily I would need 2024Whr. This is assuming 2 ACH, which I felt was conservative. Did calculations 50f also which is 100f off 150 so I can easily multiply that result by say .9 to get 60F results or 1.3 to get 20F.

I read a few of the pieces of published literature I could find out there which all said the 30 minutes at 133 inside the wood. Read more on this forum and the only thing I'm confused by is how 24hrs at 150 became the rule of thumb. I can see that for very thick slabs but it seems like massive overkill for most batches. Is it just belt and suspenders or am I missing something?

[scsmith42]

Wood is an insulator.  When you crank the heat up inside the kiln the exterior of the lumber might reach 140F pretty quickly, but the core may still be below 125.  When I'm finishing a run in my Nyle and the temp has been at 120F for a week or more, then I only need a 13 degree rise in the core oof the boards.  This takes less time versus a "finish off and sterilization cycle" load that I put in for sterilization purposes only.  Typically this is for lumber that has air dried down to EMC.  If the air dried load goes in when it's 70F outside, I need to raise the core by 63 degrees - not just 13.  So it's going to take more time. 

Moisture content plays a role here as well, due to the cooling effect of the evaporation process.

What I've observed is that 6% lumber will reach targeted sterilization temperatures much more quickly than 15% lumber (such as hardwood for woodworking versus pine for structural use).

24 hours at 150 allows plenty of time for the core to reach 133F (presuming it's at the end of a drying cycle and not a fresh air dried load just placed in the kiln), which is why most of us follow that rule.

Per Gene Wengert, the 30 minutes at 133F inside the wood is incorrect.  All you need is for all portions of the wood to reach 133.  Keeping it there for 30 minutes is an extra safety margin, especially considering that kilns may not have consistent airflow totally throughout them and the heat is conveyed through the hot air.

[Oth]

Yes, for sure only trying to sterilize at the end of the drying cycle. So the 24 hrs is if you're putting it in the kiln just to sterilize it. If it's already been in the kiln drying you still do the 24hrs out of habit or are you shortening that period since, as in your example, you only need to make up 13 degrees?

[doc henderson]

Well, if you pull your car in the garage and turn on the forced air heat, you can get the air to 70° in no time.  It will take many hours to get the concrete and cars up to that temp.  Temperature is the result of heat applied to and object.  It depends on density so air heats fast.  think of temperature as heat density in an item.  It will take a while to heat a 5-gallon bucket of water.  in fact, the definition of a BTU is the energy to raise temp of 1 ml of water 1 degree centigrade.  a cord of hedge wood is 32 million BTUs.  all forms of fuel will tell you about the BTUs per unit volume or weight.  the other factor is that the temp increase is linear with the exception of a phase change.  the phase change from solid to liquid, or liquid to gas, requires energy itself, so the change in temp is halted, but the energy is consumed.  So not only do you have to heat the mass of wood but also any remaining water.  this is why it is probably easier to sterilize wood that is relatively dry.  So, a temp of say 160° for 24 hours is presumed to get 4/4 lumber that is stickered and with fans going to at least 133 degrees internal.  You can measure temp and MC of wood that is in a kiln.  but temp is a big probe in theory.  A IR temp gun will tell you the surface temp.

[doc henderson]

how can you measure the core temp?  well, if you cannot the industry standard is 24 hours.  You could correct for the fact that the wood is dry and relatively warm.  but the liability is on the producer.  I will try to find a little home experiment I did with a meatore probe.

[doc henderson]

https://en.wikipedia.org/wiki/British_thermal_unit#:~:text=The%20British%20thermal%20unit%20%28BTU%20or%20Btu%29%20is,part%20of%20the%20United%20States%20customary%20units.%20

https://forestryforum.com/board/index.php?topic=112021.msg1755589#msg1755589

[Oth]

Yea, I understand the physics behind the whole endeavor. I saw your meat probe trial, I think I'll replicate it when it comes time. Thanks for everyone's responses. Enough theory, I'll just play around when the time comes and see the results.

[doc henderson]

sounds good!  nice job on the kiln.  My comments were in response to your Question about time if you are already warm, and I think the answer is we do not know.  but if the wood is for you, you can make that determination.

[Sod saw]

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Oth,    Two thoughts:

There are a number of manufacturers who make thermometers and/or thermostats that have the thermocouple dangling at the end of a small wire (3 or 4 feet long).

If you can protect the brains of that thermometer from rain, mount it outside the kiln for easy reading and run the wire and thermocouple into a small drilled hole in to the center of your sample piece of wood.  This will allow you to get an idea as to the time and temperature ratio as your kiln chamber attempts to further warm up.

You might be able to compare the wood interior (especially with large and thick pieces) temperature with the actual kiln chamber temperature.  I trust that you have many thermometers scattered thru-out the kiln chamber.

Next thought:   A stand alone transformer will do a fine job of changing voltage of Alternating Current (AC).  It can change voltage to increase the output voltage up from the original input voltage.  A transformer can also change the voltage so that it is less than the original voltage.  Transformers are also used in applications where the out put voltage is exactly the same as the input voltage.

Your 12 volt fans that are operating from your solar panels run on Direct Current, not Alternating Current as is in most homes here in USA.  The polarity of the connection from solar panel to fan will determine the direction of the air flow.  If you reverse the polarity of the wires to the fan you will reverse the direction that the air flows thru your stack of wood.

What does that have to do with transformers?  The AC flowing out of the output side (load side) of a transformer reverses polarity with the change of the waveform of the input (supply) voltage.  Here in the USA the 60 cycle sine wave causes the polarity to change quite fast.

Your 12 volt transformer will attempt to have the air flow thru your stack of wood in one direction while the sine wave goes positive then will change polarity (just like you did in the example above where you swapped wires to make the air flow in the other direction).  Then when the cycle goes negative the fan will attempt to change direction and blow the air in the other direction.  Then positive, then negative, then positive, then......  you get the idea.   At 60 cycles per second of current flow the fan will attempt to change direction 120 times each second.  The result is that the fan will hum and maybe burn out (overheat) due to no air flowing past itself because it can not get turning at all.

So what to do?  My previous post talked about a power supply to drive the fans in my kiln.  Look at how many watts each of your fans draw and add those fan wattages together.  Buy a 12 volt DC power supply that operates on your generator voltage that can supply 130 % of that added fan wattage.  Or buy a separate power supply do drive each fan if you have large fans.   The math is easy.

Keep the power supply outside of your kiln chamber where it is not heated.  In a perfect world you will have a small fan built into the power supply itself.

Nice looking kiln!


.

[Oth]

DC vs AC hadn't occured to me when I said that, good catch.

I have 2 Sensor push thermometers/hygrometers that live update to an app on my phone which charts out temperature and relative humidity over time. Just put them in a couple days ago to start keeping better records. Went ahead and got a Bluetooth meat thermometer with 2 probes also. Any thermocouple that saved information that I could find was $250+ and the meat thermometer with the extra probe was just a bit over $100 and it can double as an actual meat thermometer.

I've got a guy I can get 400W solar panels from for $100 bucks. I may pick up a couple of those and wire it to some 12v heaters minimally to speed up the process in the winter. If it lets me get to sterilization temps in the summer then that's a bonus. As far as sterilization, I'm leaning towards a lp or diesel heater piped in. Seems like the least fussy option. Whatever I do, I'll collect the data.

[doc henderson]

my probe meator can actually anticipate after a period of time, when the temp should reach the target.  not sure if it works in wood but it looks at external vs internal over time and plots it out.  You can see if it close and know when to expect the internal 133° to be reached.

[Oth]

Doc, I think I'm now on a list of some sort since I just googled the R value of meat to compare it to wood

[Kit B]

I've struggled with the same issues with my solar kiln.  I started making sure everything was as tight and well insulated as possible, then added halogen lights.  Temperature was still shy of 150.  Then I added in a propane heater (with the propane outside), I surrounded this with some metal roofing panels I had to protect the kiln.  This worked, I was able to get to 150 relatively quickly.

Of course, never willing to leave well enough alone, I ran across Scott's post about using a torpedo heater which just seemed an excellent idea.  I liked that it got the heater itself outside the kiln.  So I cut a hole in the kiln, built a platform, etc.  When I hooked it up and did a trial run I realized I had made the hole a little to high in the kiln and was in danger of melting my roof.  So I added 10 ft of 6 inch duct with a series of half inch holes along the top side to try to distribute the heat.  This seems to work, but I still need to add some metal to protect the roof section above the end of the pipe.

Those torpedo heaters kick out A LOT of heat.  If you decide to use that method (which works) be careful not to place the hole into the kiln to high.