Solar drying of green oak (Quercus spp.) lumber

A comparison was made of the rates and quality of solar and air drying from the green condition of a mixture of red and white oak 5/4 inch lumber during winter conditions in Blacksburg, Virginia. The solar kiln was of a semi-greenhouse type, having a capacity of 150 board feet and provided with an air circulating fan. Both the south facing vertical wall and the south facing sloping roof (45° to the horizontal) were double glazed with a special ultra-violet resistant transparent polyester film. All other walls and the floor were of insulated frame construction. Temperature both inside and outside the kiln was monitored. Data were also available for calculating the mean horizontal solar radiation during the drying period.

Both the rates of drying and the final quality of the solar-dried lumber were superior to those for the air-dried lumber. It required 75 percent as much time to solar dry the lumber to 20 percent moisture content compared with air drying. Furthermore, the solar kiln permitted drying as low as 6 percent compared with a minimum of 14 percent for the air dried material.

The overall efficiency for utilizing the total solar available energy was approximately 29 percent in. terms of the fraction of this energy which was used to actually evaporate the water from the wood. This decreased from 36 percent at the early stages of drying (early winter) to 8 percent at the end of drying (early spring).