The Development of a Solar Heated Kiln for Drying New Zealand Hardwoods

The design of a simple greenhouse-type solar drying kiln for drying New Zealand hardwoods is described. Special features include split construction to ease loading and automatic venting with cheap humidity sensors to give control over drying conditions. The results of two kiln trials are presented, in which 25 mm thick boards of red beech and silver beech were dried. Drying times were approximately halved compared with the air drying times for these species reported by other workers. The wet-bulb depression, a measure of the driving force for drying, was doubled in the solar kiln compared with open-air drying. An analysis of the economic viability of the solar kiln compared to air drying showed that the Net Present Values of air drying and drying in the solar kiln were within 15% of each other, so the benefit in strictly financial terms was marginal. This variation was smaller than the effect of changing the annual discount rate from 10% to 15% which was 20% of the Net Present Value.