Solar-assisted drying of timber at industrial scale: management paper

The production of high quality timber from hardwood requires a gentle and controlled drying to a low wood moisture content. While ambient air drying depends on the weather conditions, conventional hightemperature dryers cause high investments and energy costs. Up to now, solar dryers could not be established in industrial timber production, due to their insufficient drying capacity and the lack of an adequate control of the drying conditions. Therefore, a new solar dryer was developed, which allows a controlled drying of up to 250 m3 of timber per load. The frame of the dryer is covered with a transparent, highly UV-stabilised and well isolating air bubble foil. A specially developed microprocessor control regulates air flow, temperature, humidity and a back up wood chip furnace, which provides the heat during night and unfavourable weather conditions. The drying regime is adjusted automatically according to the type of wood, the board thickness and the ambient air temperature. The paper presents the experiences gained during the operation of the two worlds largest solar timber dryers, drying up to 35000 m3 of eucalypt annually. The investigations showed, that investments, drying costs and energy consumption could be reduced by 50 % compared to conventional high-temperature drying systems. At the same time the quality of the timber was improved considerably. Southern African Forestry Journal No.192 2001: 73-7