Fanless Solar Kiln?

Natural convection is not a practical way to power air flow through a drying kiln. August 1, 2011

I have been running a 1,600 bd ft solar kiln for about 8 years now. Originally I built it to cure the white cedar for my boat project and have been renting time to other woodworkers ever since. One of my customers is a tree service and has a Wood-Mizer. He wants me to build him a kiln. I stopped by today and this guy has a pile of logs maybe 90' long and 20' high. 28" chestnuts, 24" black walnuts, 20" white oaks, rock maple and who knows what. We are talking a 4 or 5,000 bd ft kiln just to make a dent. The problem is he has no way to get power to his yard and solar powered fans for a kiln that big are way out of his budget, so I am looking for a design that uses natural convection.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
If you build a kiln on top of the solar collector, then as the air is heated it will rise and enter the kiln. Then, as it dries the wood, it will cool and flow back, possibly cooling the incoming air before the hot air can do any work. I have had people try to figure out a design, but there is none, especially for such a large kiln. The air flow provides warm, dry air around the lumber, carries the moisture away and also assures a good degree of uniformity of conditions.

The only possible design in your case is one that uses an exterior windmill that is mechanically linked to the interior fans. This design was used by Edward Peck around 1960 in Sauk City, WI. I have the article, but it is so simple, you do not need the article. The main factor to watch is the 1 sq ft of flat roof for 10 bf of lumber. Of course, a windmill connected to a generator would also work, but would be more expensive.

From the original questioner:
Thanks. That is what I figured. I tried working up an elevated kiln with a collector below and ducting the return air back to the bottom, but to get enough area the kiln would be almost impossible to load and the velocity would never be adequate. Not enough wind for a windmill either.

I am going to try to talk him into either bringing in power or springing for solar. If he doesn't do something he is going to lose a lot of valuable lumber.

From contributor S:

If you've got sun, you can have electricity. Solar powered attic fans hooked to a thermostat (from an old hot water heater?) or a battery powered timer might work. With two or more fans you could stage the turn on point depending on temperature.

Harbor Freight sells a solar panel kit (about $170) that delivers 45 watts that could power multiple muffin fans like in computers that could be turned on in stages with the same hot water heater thermostats.

Vents with a flap used on propane heaters from trailers could be used for exhaust. Very little pressure to open them because of balance. Greenhouse vents that open based on temperature could also be used. All designed to withstand kiln temperatures.

From the original questioner:
That was my first idea, but a 5,000 bd ft kiln with a mix of 4/4 and 8/4 boards needs about 4500 cfm air at about .10" WC to maintain 150 fpm. That is about 290 watts using four 16" Snapfans. That is about $1,400 in fans and $1,000 in solar panels. That's what made him choke.

From contributor J:
Solar panels are dropping in price quite rapidly. Currently here in Ontario, Canada the average going rate is $2-2.50 per watt, and is predicted to drop to $1.50 a watt soon with our new government incentive program.

If you are in Ontario I would suggest applying for a micro fit setup and sell the solar energy back to the government at $0.802 per kilowatt. Then take the profits and buy another system to use on your kiln. If you qualify for the micro fit program, you will be guaranteed a loan from any financial institution as the contract is for 20 years and the payback is less than 8 years.

From Professor Gene Wengert, forum technical advisor:
Something strange here, contributor J, as electricity costs 8 to 15 cents per kWh, not 80 cents. Electric companies may buy clean electric at a few cents per kWh. Many small operations do not have the correct frequency consistently, so cannot sell.

The small solar kiln might require 10 kW per day, so a solar panel should be 2kWh to assure enough electricity at medium to low sun angles, or about $4000. We also need to be concerned about cloudy days and frequent dirt on the collector and other efficiency losses over time. So maybe add another 50%. This is a lot of money for a kiln costing $700 or so.