Design and construction of a solar – biomass hybrid dryer for cocoa beans.

Abstract

In Uganda, cocoa has been grown for all the years and farmers have always relied on open sun drying as the main drying method however, this method has proven to ineffective during rainy season since high yields of Cocoa pods are normally received during rainy seasons, and also associated with a serious risk of contamination with wind-borne dirt and dust, dead insects and animal droppings thus lowering the quality of the cocoa beans produced. Innovations of solar dryers have been made and put on market to reduce losses related to drying of grains and also improve the drying conditions of agricultural products. But these systems also face similar challenge as the open sun drying due to dependency on weather conditions and lacking a means of regulating temperatures within the drying chamber with low drying effectiveness factor. This project aims at design and construction of a solar – biomass hybrid dryer using locally available materials at relatively low cost. The dryer is composed of solar collector for optimizing the amount of solar energy for drying, back-up heater which acts as the backup energy source to enable drying to continue till the night hours and reduce on the drying period, airflow system for carrying heat energy to the drying chamber and a temperature regulating system, this controls the heat energy inside the drying chamber automatically. The design was based on the study area of Imanyiro sub county located Mayuge district, where cocoa is being grown by many farmers. The average ambient conditions were; 25°C air temperature and 75% relative humidity with daily global solar radiation incident on horizontal surface of about 5 MJ/m2/day. A minimum of 0.2m2 solar collector area was required to dry a batch of 6kg of cocoa beans in 26h with the dryer from the initial moisture content of 50.8% to final moisture content of 7.75%. The system has an effectiveness factor of 1.52 which makes is 52% more effective than the open sun drying and a drying efficiency of 11.6% which is low because of the non-uniform drying temperature on the trays due to un even distribution of heat, heat losses though the joints, heat loss due to poor insulating material and also the heat obtained by burning the biomass does not directly come in contact with the cocoa beans, but instead heats the air duct which in turn heats the drying air. However, preventing leakage in the air supply system, use of better lagging (insulating) material, and increasing the size of the flat plate collector will increase the efficiency of the system.