Design, construction, and evaluation of a mixed mode solar kiln with black-painted pebble bed for timber seasoning in a tropical setting

Different designs and improvements on solar kiln for drying different types of products have been an ongoing research interest, ranging from simple and small capacity to automated dryers with integrated storage systems. In this article, mixed-mode solar kiln with black pebble bed as absorber and storage is designed and evaluated at Nsukka, Nigeria, latitude 6.8°N. It combines heat from an external collector and from a transparent roof, making it efficient for wood seasoning in tropical areas. The kiln consists of a drying chamber of 0.362m3 capacity, a single glazed flat plate collector and transparent roof area of 0.54m2, a pebble bed capacity of 0.1m3, and a kiln orientation of 7°N-S facing south. At peak periods, timber stack in the drying chamber receives hot air flow from the collector and the transparent roof simultaneously. At off-peak periods, heat stored by pebble bed helps to avert reverse flow of moist air. Test results reveal that temperatures inside the dryer and solar collector were higher than the ambient temperature at all hours of the day. Maximum drying chamber temperature, which was obtained at midday, was 61.7°C. Kiln drying reduced timber moisture content from 66.27% to 12.9% whereas open air drying reduced to 20.1% dry basis in 360 hours. Also, the initial drying rates for both kiln dried wood and control were 0.205% and 0.564% per day and the final drying rates were 0.15% and 0.08% per day, respectively. The rapid rate of drying in the kiln reveals its ability to dry timber to safe moisture level without defects.