Enhancing natural convection solar drying of high moisture vegetables with heat storage

Abstract

African Leafy Vegetables hold a high potential as an income source for resource poor rural dwellers in Cameroon but this potential has not been realized because of lack of appropriate post harvest packages resulting in high losses. A solar tunnel dryer was designed and constructed using local materials and evaluated for drying leafy vegetables and other agricultural products.  Four drying trays made of wood and plastic mesh with a total surface area of 3.25 m² were used for drying.  The dryer was south facing with an inclination of 6º, and the solar radiation falling on the dryer surface was estimated at 12.13 kJ/m² per day.  At sunset during the wet harvesting period, the temperature inside the dryer was 5 ºC above the ambient because of additional heating due to heat storage.  The complete dryer could dry 17 kg of sliced cabbage from 95% moisture content wet basis down to 9% in five days in a period characterized by intermittent downpours and permanent cloud cover.  The overall dryer efficiency was 17.68%, with a moisture extraction efficiency of 79.15% and airflow of 9.68 m³/hr.  The relative humidity of the air inside the dryer varied from 75% in the morning down to about 35% at noon.  Tests on other high moisture products showed that the dryer could reduce the drying time by 30 to 50% depending on the product and the final product was acceptable in taste and colour.

Keywords: leafy vegetables, post harvest losses, solar dryer, natural convection, sun drying, Cameroon