University of Wisconsin Center for Cooperatives

Putting nature's powerhouse to work

by Rick Barrett of the Wisconsin State Journal
January 29, 2000
Used with permission from the Wisconsin State Journal

A Spring Green area timber cooperative is firing up a large solar kiln for the first time next week to dry oak lumber for furniture-making and flooring.

Through heat generated from solar collectors, the kiln will be able to remove 94 percent of the moisture from stacks of fresh-cut lumber.

It won't be fast, taking a month of heating to dry wood in a natural way. But the kiln along Highway 14 in Lone Rock is expected to process 100,000 board feet a year—enough to build about 15 houses, said Jim Birkemeier, who designed the kiln for Sustainable Woods Cooperative.

Most important' Birkemeier said, the slow-dried wood won't split or warp as it sometimes does in fast, gas heated kilns.

It cost about $50,000 to build the solar kiln. It is, essentially, a building 108 feet long and 32 feet wide, with a roof that has 2,000 square feet of solar collectors.

The collectors are made from double layers of black greenhouse plastic and corrugated aluminum. They will change sunlight to usable heat, which will be circulated through three insulated rooms by six powerful electric fans.

Solar heat will raise the inside temperature of the rooms—housing stacks of lumber—50 degrees above the temperature outdoors.

On cloudy days, and at night, the wood will cool down naturally— keeping it from overheating and splitting.

"Cloudy days actually help because they reduce the wood stress and keep it in better condition," Birkemeier said.

Freshly cut 8-foot oak boards (6 inches wide and 1-inch thick) contain a gallon of water and must be dried to a 6 percent moisture content before they are suitable for flooring and furniture.

Boards will be stacked in an openair portion of the kiln building for three months to remove most of the moisture.

Then they will be placed in the insulated heating rooms for a month to finish drying them to the 6 percent level. Without the heat, the boards would not be usable.

It will cost about a penny per board foot to dry boards in the solar kiln, and the payback will be a dollar per board foot in improved, high-quality lumber.

"Furniture makers are going to love the wood from this kiln," Birkemeier said. "Because it isn't baked and steamed, the natural color will be preserved."

Sustainable Woods Cooperative serves about 100 timber owners in Central Wisconsin. Most of the members are not timber professionals, but they live in rural areas and own trees that are suitable for lumber.

The cooperative recently had an old-fashioned "kiln raising" staffed with volunteers to put up the building.

"We used sawed lumber from the cooperative's members," Birkemeier said. "And we used an aluminum roof from an old shed for part of the solar collection system. It was free and it was the perfect material."

Solar kilns come in many sizes.

A timber group in Vermont built a solar kiln and used solarpowered fans to circulate the heat, but Birkemeier said it cost several thousand dollars more than the Spring Green kiln and was more complex to operate.

Birkemeier's company, Timbergreen Forestry of Spring Green, has sold kiln plans to timber users in Maine, Vermont Alaska, Australia and Costa Rica.

There are plenty of saw mills in Wisconsin, he said, but there's a shortage of kilns.

The Sustainable Woods Cooperative kiln is on 4.5 acres, that also includes a saw mill, owned by the cooperative.

The first finished wood is expected this summer, and the kiln could be open to noncooperative members if there's excess capacity, Birkemeier said.

"It's a simple design, but it has taken more than 12 years to refine it," he said.


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