Performance of a solar tunnel dryer with microcomputer control

A solar tunnel dryer has been built and tested in the Sunraysia district of Northern Victoria, Australia. A novel feature of the dryer is the microprocessor system used to control the exhaust fan. A tray containing a small sample of the crop is suspended from a load cell, and the output from this is used to continuously calculate the current moisture content of the crop. Using this estimation and inputs from sensors measuring the relative humidity of the air inside and outside the dryer, an appropriate decision is made on whether to activate the exhaust fan or not. The dryer was used to dry two loads of untreated grapes in the summer of 1992. Drying time was reduced by approximately 40% compared to sun drying in both trials. Using a two stage control algorithm, fan operating time was reduced by 67% compared to continuous fan operation, and by 34% if a light sensitive switch had been used to control the fan. Reduced fan operation also maximised the air temperatures achieved in the dryer. Peak daily values were usually between 50 and 60°C, and the average operating temperature of the dryer was approximately 10 degrees above the average ambient dry bulb temperature.