No.60, 2008 A Solar Powered Lumber Kiln by Robert H. Rehder (ON)
A hundred years ago fresh cut lumber was air dried outdoors. The elapsed time for the drying process was at least six months and as long as two years depending on the type of wood. The wood must be dry to be able to machine and dress the wood.
Hope Mill is a circa 1835 water driven mill located on the Indian River near Lang, 15 km east of Peterborough, Ontario. In 1892 the mill was converted from a wool carding mill to a saw mill and new water turbines were installed at that time. It provided power to cut the logs and to plane and process the wood after drying. The mill operated commercially to 1966. The Otonabee Conservation Authority purchased the property and with the help of government funding and volunteer help, converted it to a demonstration sawmill and a museum for antique wood working tools. In 1990 the mill funding was eliminated and the mill went into disrepair and became derelict.
In 2000 a group of 16 retirees accepted the challenge to refurbish the mill and to again cut logs using the original water power turbines and old machinery. By 2006 the volunteers had the log saw operational and using donated logs log cutting demonstrations were presented. We piled the cut lumber in the basement of the mill and in various neighborhood barns for it to air dry. We continued to cut logs in 2007 but we were running out of space for storing and drying the lumber because of the long drying time.
One of the volunteers, using the internet, found a reference to solar powered kilns to dry lumber. It was decided that we would use some of our lumber and build our own kiln. This would reduce the drying time to two or three weeks for the soft wood and some hard wood species. It would make our lumber more saleable as it would be kiln dried. We could thickness plane and joint it using our antique machinery.
We finalized a design of our own with a floor plan of 6 ft. by 12 ft. The building would be on skids as we wanted to be able to move it easily if there was any public objection to having such a modern concept structure near an historic site. The kiln will hold a 4 ft. by 4 ft. pile of 10 ft. long planks, the planks being spaced with wood stickers to allow air to pass through. The roof is transparent corrugated plastic and sloped at 38 degrees. The roof hinges up and the low front wall hinges out and down to provide loading and unloading access. There are no doors or windows. There are two shuttered vents in the back wall, one high and one low, to get rid of the moisture and to provide fresh incoming air. Across the top of the wood pile is a horizontal sheet of plywood painted black. This will absorb the heat from the sun. Two 16 inch diameter fans are located high and near the back of the kiln. These fans will draw in air from outside and blow it toward the front of the kiln and pick up heat off the black plywood. The now warm air goes down the front wall, then back through the spaces in the lumber pile, picking up moisture from the wood as it goes. At the rear, some of the now moist warm air will pass out through the lower vent but some of it will go up to the top of the structure and mix with the incoming fresh air and hence to the fans for recirculation. The upper and lower shuttered vents at the back will be adjusted to optimize the moisture transfer depending on the time of year.
Power for the two fans is provided by two solar power sources located above and at the back of the structure. Control is simple. The power panels are connected directly to their own fans. When the sun is out the fans operate. When there are clouds the fans stop. We don’t require any complex controls such as humidity or temperature switches. If the sun is not out, we don’t have a heat source so we don’t need to move the air.
We will be opening the kiln once a week and checking the moisture content of the wood. When the wood is down to 6% moisture, we will remove the wood and reload with new. We will try to load with similar species so that wood can be loaded as a batch. Variation in drying times can be as extreme as a few weeks to many months (oak).