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.


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[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.