Using heated chimneys and reduced collector air gap height to improve the performance of indirect passive solar dryers

Abstract

The performance of indirect passive solar dryers tends to be rather poor because of the low air flow rates which occur through such dryers. These low air flow rates can cause the efficiency of the solar collector to be low and also limit the rate of drying that can be achieved with such systems. Two ways of trying to improve the performance of such dryers are; to use waste fuel to increase the buoyancy forces by heating the air in a chimney attached to the dryer, and to reduce the air gap between the upper transparent cover and the absorber plate in the collector in order to increase heat transfer to the air. This paper, therefore, presents the results of an experimental study in which the effects of heating the air in a chimney fitted to the dryer and the effects of varying the collector gap on the dryer performance have been measured. The tests were done under outdoor conditions in Senegal on two dryers, using four chimney configurations heated with waste sawdust, and using two collectors with air gap heights of 4 and 5 cm. Results indicate that the performance of the dryer can be improved if temperatures in the chimneys are maintained over 50°C above the ambient for more than 4 hrs and if the chimney is at least 2 metres high. Mean drying rates in the dryer can also be improved by reducing the collector air gap height which should not be greater than 4 cm. An order of magnitude analysis which confirmed the overall effect of reducing the collector air gap height is presented.