Solar Cycle Kilns Increase Sales

Our family has sold our annual timber harvest at Timbergreen Farm as manufactured products, direct to our customers, for over 20 years. Our unique Solar Cycle Kilns have been the single most important factor in our success. Everyone talks sustainable forestry and energy conservation. In one glance, our customers can see for themselves that we actually do what we say. This is the most important factor in our sales.

Lumber from the Solar Cycle Kiln is also the best quality possible, due to the natural daily heating and equalizing - ‘conditioning’ - cycle. Our customers have reported back to us from the start that our lumber was much better than anything they could buy in other lumber stores. Easier to work, flatter, less stressed, less damaged, and brighter colored. Having the best lumber on the market is another key to successful sales for a business. Manufacturing wood products on the farm, we appreciate the quality everyday.


I first experienced the power of the sun in a solar kiln back in 1980 at Mazomanie, WI. It is like sitting in a car on a sunny day, times one hundred. Scott Stokes had built three large kilns that opened from the front, allowing loading/unloading with a forklift. I took his improvements and researched the other solar kilns, them designed and built our first Solar Cycle Kiln in 1988. We have increased the efficiency of the design by 1000% since the first model, focusing on better insulation and air control.

Our new kilns have 4 rooms, three for pre-drying and one for the kiln chamber. The rooms are sized for the common lengths of lumber that we use. The rooms are open on both sides for pre-drying, then enclosed with insulated sliding doors to create the kiln chamber.

Pre-drying the lumber under a roof was an important factor. Why put wet wood in a kiln when you can pre-dry it for free? The wood is protected from the sun and rain, and the ends of the boards are not exposed to the problems of end-checking. Our design has a very tall roof that naturally accelerates the air flow through the building. Like an airplane wing, the air flows faster as it passes around the shape.

One inch thick oak lumber will pre-dry to 12 % Moisture Content in about 3 months in a pre-drying room of our kiln building. In our climate in Wisconsin, that is as dry as lumber gets when exposed to the outside humidity levels. Full pre-drying removes 90% of the water for free. Since we are only removing a small amount of water in the closed heated kiln room, there is never high humidity or corrosion. The daily natural moisture equalization – ‘conditioning’ – means there is never a need to steam “condition” at the end of the kiln cycle. This means that we can use inexpensive building materials including plywood, OSB, sheetrock, nails, etc. There is no need for Aluminum or Stainless Steel.

A concrete slab forms a flat foundation for each pile. Once a pile of lumber is complete in a pre-drying chamber, we put 2” nylon ratchet straps around the top 15-20 layers of boards. A strap is placed above every other sticker column, with a 2X4 across the top of the pile. Fifteen foot long straps are used for a 4 foot wide pile. This lets us put about 4 tons of pressure on the lumber to hold it flat while it is drying. There is enough weight in that bundle of wood to hold the boards below it flat also.

For products that will be used indoors - flooring, furniture, cabinets, etc. we need to kiln dry our lumber to 6% MC. This requires adding heat, and we chose to use solar energy.

When a pile has completely pre-dried to 12%MC, the chamber is enclosed by the sliding doors, and air ducts are opened to the solar collectors. In perfect weather the Solar Cycle Kiln will dry 3,000 BF of oak lumber from 12%MC to 6%MC in one week. On the average though, one month is needed. When the batch is dry, the doors slide to the next chamber, changing that room from pre-drying to kiln-drying. We carefully stack the lumber one time, pre-dry it then kiln-dry it without moving the wood again.

Four Parts of the Solar Cycle Kiln
A clear window facing the sun. We use two layers of greenhouse poly tarp, inflated by a tiny blower. Inflating the space between the two tarps produces an insulated window whose rounded shape also helps shed snow in winter. The inside surface of the window stays about 20 degree F warmer than a single layer, reducing heat loss from the solar collector. It is 6 mil thick and UV treated, and available from any greenhouse supply store. The tarps last about 6 years if they have good UV protection. It takes four people about an hour to install the tarps on a calm day, and the cost is about $200. Glass and polycarbonate sheeting are more expensive, usually coated to reflect heat away, and have lots of joints - compared to our simple poly tarp roof. Our kilns have about 600 square feet of collector surface and that will collect about 500,000 BTUs each sunny day.

Black Metal Collector Surface. We hang a layer of used metal roofing – painted black – on the bottom of the wood rafters that support the window. Corrugated metal has more surface area and stirs the air, heating the air more than a flat sheet. Metal will heat up and transfer more heat to the air than wood. (All surfaces of the solar collector room that receive sunlight should be painted black.) Air flows up both sides of the metal surface to capture the energy so the heat can be blown down into the kiln chamber.

Fans to Circulate the Air. We use two 1/3hp propeller type fans to circulate the air in each Solar Cycle Kiln. You need enough air-flow to move the heat from the metal surface into the piles of wood in the chamber below. A simple thermostat and relay turns on the fans when the air in the collector room reaches 80 degrees in winter (90 degrees in summer), then turns them off when the available heat has been used. No other controls are needed. Electric costs using 120/240 volt AC are less than a half cent per board foot. Photovoltaic cells and DC motors can be used also to circulate the air and have a higher initial investment.

Insulated Kiln Chamber. The kiln chamber is well insulated and isolated from the solar collector room. This saves all the collected energy in the pile of wood, so the kiln stays much hotter than other types of solar kilns. One-way air-actuated valves close the chamber when the fans turn off. We use the surface of the South facing door as a solar collector also, adding the most heat in the winter when the angle of the sun is lower.

A new Solar Cycle Kiln should operate at 85 degrees F above the outside air temperature. This means the kiln will operate at 110 degrees in winter and 165 degrees in summer here in Wisconsin. The temperature is determined by the size of the collector, the amount of wood being dried, and how much fresh air is introduced. If the vents are closed, higher temperatures can be attained if desired.

Our first kiln (20 years old) operates at 50 degrees over the ambient temperature, the next one at 55 degrees, and our latest (10 years old) better insulated kiln runs at 65 degrees over the outside temperature. I could build one new kiln, incorporating everything we’ve learned that would replace all three old ones.

The Annual Drying Cycle

Solar Cycle Kilns operate faster in the summer than winter in our climate, but some seasons – like winter 2007-2008 - have less than average sunlight. We have a large humidity-controlled sales/storage room in the loft of our old dairy barn to store our dry products. When you sell retail products to customers, an inventory is important. Our inventory is sufficient to get our business through the annual drying cycle and any cloudy season. Since we are the timber grower, we have very little cost of inventory.

The Ultimate Solar Collector

The material for the window should let the heat pass through, then reflect the radiant heat coming back from the black metal - back into the collector. The window should be insulated to minimize heat loss by contact to the colder outside air. Glazing/joints should be minimized.

The Solar Collector itself should be as light-weight as possible. Any heat used to warm up the collector room in the morning to the kiln’s operating temperature - will be lost once the fans turn off. The insulating walls of the collector room should be covered with a reflective film to bounce the radiant heat from the metal back into the collector. Foam board with an aluminum foil surface would be a good wall covering

The kiln chamber should be air tight, with one-way air pressure activated valves, and adjustable exhaust vents. Fans are sized to effectively transfer the heat from the metal surface to the wood pile.

Our kilns use a 45 degree collector facing South, receiving the sun’s heat for only part of the day, so they run from about 10am to 5pm. Several people have built Solar Kilns with windows facing all directions to capture morning and evening sunlight also. The areas of window away from the sun allow extra heat loss – minimizing the net gain. There are many variables and alternatives.

I would rather keep things simple as possible, rather than high tech and expensive. Build an inexpensive – larger collector - rather than one that is complex.

Solar Kilns in the Tropics

Warmer temperatures and a higher angle of the sun change the design of Solar Kilns in the Tropics. Gathering solar energy from overhead makes a greenhouse style kiln more effective. Insulation is not as critical in warmer climates, especially in sunny locations where the energy is basically unlimited.

A simple greenhouse with a hanging layer of black metal makes a very effective kiln. Greenhouse materials and methods are common around the world. The mass of the building is very low so it will heat up quickly and get to work. One design for the tropics recommends collecting the heat in air flowing through glass covered concrete ditches dug into the earth. The sun must first heat the hundreds of tons of earth before it heats the air – producing a reported 15 degree F temperature gain.

If insulation is desired, an insulated building with a greenhouse roof could be used to conserve and utilize the available sun’s energy.

Adding Wood Waste Heating

Adding hot air from a wood burner to a kiln is difficult to control, often resulting in damage from over-drying. Adding heat through a hot water wood furnace is a better method. If you already have a wood boiler to heat other buildings, it would be simple and effective to supplement the sun’s energy with wood waste. To add a new wood heater just to a Solar Cycle Kiln would be expensive. The larger the business, the more it would make sense to add supplemental heat. I’d rather not rush my wood in drying. Sometimes a little slower is better!

The costs of Solar Cycle Kilns

Most of the building materials can be home sawn when building this kiln. The pole shed design is easiest and strongest. Our last 20X44 foot kiln built about 10 years ago had expenses for materials of about $7,000. Concrete and roofing and insulation are the major expenditures. If you had to hire labor, that would likely double the total cost. Annual production is about 30,000 bf. The larger Super Cycle Kiln was built for about $60,000. The 30X110 foot size collects 2,000,000 BTUs per day and would dry 120,000 bf/ year.

At 54 years age, I can still stack the lumber into the kiln at a rate of 1,000 bf/hour and it unloads even faster. To make this hard work more tolerable, I think of it as stacking $20 bills – and it is good exercise! Electricity costs a fraction of a penny/bf. We have always gained about one dollar per board foot for kiln drying our lumber in this step of our wood sales business.

This design in now in use on all the continents except Antarctica. More information, Blueprints, and a DVD are available at or by calling (608) 588 7342


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