Solar Wood Kiln -Anyone have Experience building/using one?

[owsnap]

I usually get my timber around 18-20% ''dry'' as it's up to 2x less expensive than a kiln dried one and more readily available,usually I just stock up and always have dry timber for making stuff , leaving the fresh one to dry out in a room temperature overtime.

I did came up an article about a solar wood kiln http://www.popularwoodworking.com/projects/solar-kiln and it intrigued me to dig some more info about solar kilns as they seem quite popular in other parts of the world.

Was wondering maybe anyone have any experience building one or have thought about building one?
and If so can you really dry timber just with a power of sunshine ( in the article they say you can dry green timber 25% down to 8% in 4-5weeks which sounds just great) And it doesn't seems hard at all to build a wooden shack with a clear top with a fan inside to rotate the air.

Sounds like a great way to control the quality of the timber you need for dirt cheap without relaying on anyone.

 

[Sgian Dubh]

The effectiveness of solar kilns depends significantly on latitude, and work most efficiently or effectively nearer the equator. Coupled with the latitude factor is the amount of sunlight that hits the reflector. Here in the UK I can think of less than a handful of people operating solar kilns commercially. One operator I know reckons it can take from October to March to dry a batch, whereas he can dry four batches of wood between April and the end of September.

I suspect you would find a similar seasonal pattern, although even more pronounced than here in the UK with Finland being further north, with close to 24 hour darkness in the depths of winter. Of course, there'd be near 24 hour daylight in June/ July. You would want to capitalise on summer sun as much as possible, so setting the collector somewhere between 50 and 60º from horizontal would be typical, all depending on your latitude (between 60 and 70ºN). A typical solar collector to wood volume allowance is 1m² of solar collector per 0.25m³ of 25 mm thick wood in the drier.

I'm rather sceptical about the likely effectiveness of pure solar kilning at such high latitudes if the intention is a commercial proposition, but for a hobby woodworker or small scale operator without commercial pressure I can see it might be worth further investigation. I'd start by trying to find out if anyone else is operating solar kilns in similar high latitudes, and if there are, endeavouring to learn from their experience. There might exist a few, and it wouldn't surprise me to find they have to supplement solar power with heat from wood waste, electricity, oil burners, etc. Slainte.

 

[owsnap]

Thanks man you cleared up something more , I don't run any large commercial operation so don't need a lot of it, just need a couple m3 in stock to keep me going, doing this as a hobby and to make some spare $ when I don't have anything better to do.
was really thinking to use it only in spring/summer months when there is lots of sun to help speed up the drying process as it takes ages just to air-dry something + it sounds like a great way to get the exact quality timber you need without setting up a large scale kiln which would be a total waste of time and energy due to my small volume.

 

[Droogs]

AS Sgian Dubh has said the most important fact is your latitude. In order to make the solar kiln effective and efficient , your collector (ie the glass area) has to be built on a sloping plane that matches your latitude. This is due to the fact that the glass will have impurities in it that will refract the light and energy passing through sufficiently to reduce the kilns capacity by up to 60% for even a few degrees of misalignment. In order for the kiln to work properly the light must pass through the glass at as close to 90 degrees to the surface plane as possible.
Sighting the kiln is also very important, you need to calculate where on the horizon sunup and sundown are for you on the shortest daylight day you have in order to have the collector facing it's best possible aspect, this ties in with the azimuth(angle of the sun above the horizon) at it's highest point over the period you intend to use the kiln as you then need to factor this angle in to the slope angle of your collector.
So, once you know your latitude and the azimuth of the sun you need to get a blalance that will have the slope of the collector as close to perpendicular to the sun as it travels across the sky during the period you intend to use the kiln. Also if possible use glass with the lowest iron oxide content that you can find in order to keep up efficiency.
If you do build a kiln insulate it to the same standard as you would a house in you location and have the internal walls a smooth as possible. You also need a good dehumidifier if you live in an area that his high humidity in summer as relying on just your venting fan can be a bit hit and miss. You should ideally have at least 3 fans in your design 2 for venting to the atmosphere and one to provide airflow over the back/bottom are of the kiln.

the furthest north I have helped build a kiln was near Huntly in the north of Scotland 57°26′50″N 02°47′09″W. The kiln design is based on this: pubs.ext.vt.edu/420/420-030/420-030_pdf.pdf

The kiln in Huntly is usable all year but is only really efficient enough for self/hobby use for around 8 months of the year and gives wood anywhere between 7/8% to 12% over a 4 - 5 week period

Also a factor that does help is altitude, it seems that higher is better as well.
You do need to be prepared to carry out monitoring at very regular intervals and be prepared to move the lumber around in the stack to ensure even drying and to prevent warping etc. It's also a good idea to cough up the extra cost needed to make the kiln self contained power wise and fit DC solar PV powered fan units.
hope that helps

 

[owsnap]

thank you for taking time and writing all that,btw can You use plastic film used for greenhouses in 2layers with air gap between instead of clear glass, as I have no idea where I could easily get glass locally.
Somehow I was under the impression at first that the box itself is just made from 1 sheet of plywood/osb or whatever you have but not that it's an actual timber frame insulated with insulation material between the studs as well.
Not sure why but I thought that it's more like a greenhouse at first,just with wooden walls, I have a greenhouse already and in the spring/summer it gets pretty hot there even without it being insulated much at all, Is the extra super insulation needed just for the winter months when it wouldn't be really effective anyway to get the extra drying power as I reckon it would have no problems in summer even without all the massive insulation.

 

[Jacob]

Looks a good idea.
I'd have glass rather than plastic, horticultural glass is cheap. And detail it to allow condensation run off at the eaves - then it'd work as a dehumidifier every time the temp gets low enough outside - which can happen on a clear night even in mid summer

 

[Droogs]

You can use the poly-carbonate sheets that are used on some greenhouses, but the kiln won't be as efficient. The insulation is very important if you live at latitudes greater than 22 deg from the equator, as heat loss during the hours of darkness and the subsequent rapid drop in temperature starts to affect the evenness of the drying process. This can lead to uneven drying and cause excessive splits and checks etc as well as in extreme cases a condition where the internal cells become spongy and the outside brittle (there is a name for this that I can’t remember at the moment).
The whole point is to try and keep a constant level of humidity (that you want to gradually lower over the period of drying) and a gradual rise in internal temperature in the kiln to the highest that is feasible for drying out the wood type you have. You are trying to get the wood to the optimum rate of vapour exhaust and evaporation without affecting its strength and elasticity.
Once you have it built it will take, depending on the number of different woods you will dry and how often you run the kiln, about 2 years to be up and running effectively and to be confident that what you put in will be what you want when it comes out. Take copious notes and readings and experiment a lot. This is the only way you will learn the peculiarities of your kiln. The rewards are worth it though.

 

[Sgian Dubh]

This can lead to uneven drying and cause excessive splits and checks etc as well as in extreme cases a condition where the internal cells become spongy and the outside brittle (there is a name for this that I can’t remember at the moment)

I suspect you may be searching your memory for the condition known as case-hardened, i.e., where the shell is in compression, and the core experiencing tension. The term is a misnomer because the wood at the shell is not harder than the wood at the core. During early stages of the drying process the shell of the wood tries to shrink as it loses moisture. It’s prevented from doing so by the core of the timber which is still wet and full size. By the end of the drying process, including an equalising or case-hardening relief procedure in conventional or dehumidification kilns, the wood should be equally dry throughout and the core will have shrunk. Solar kilns are neither conventional nor dehumidification kilns of course, and don't generally have the facilities to relieve case-hardening, but by the nature of operation they're generally less likely to induce it in the first place. Slainte.

 

[Jacob]

I'd think of it as just for firewood - but having a gamble on it producing joinery material. Unless you built a very big one with lots of controls.