Energy, environmental and economic analyses of an indirect cocoa bean solar dryer: A comparison between natural and forced convections

Abstract

The objective of the research is to present, the numerical simulation modelling of cocoa bean dryer in natural, forced and combined forced and natural convection to predict their effectiveness in a tropical environment. Cost analysis and CO2 mitigation potential of the solar dryer were also presented. The finite difference and fourth-order Runge Kutta method were used to resolve the iterative equations. The drying kinetics, drying rates, CO2 emissions mitigation, thermal efficiency and payback period are estimated in Yaoundé climate in Cameroon. It was difficult to have the moisture content of the product in equilibrium with ambient air using only natural convection. The best mode of functioning is to use forced convection during sunny period and natural convection during not sunny period. With an initial moisture content of 1.2 kg/kg (db), it took 32 h of drying time to obtained final moisture content of 0.15 kg/kg (db) using forced convection during the day and natural convection during the night for each month. The gradients of humidity and temperature are sufficient to have a homogeneous drying in the established product layers. The specific energy consumption range between 5 and 15 kWh per kg of humidity extracted for the combined natural and forced convection. The thermal efficiency of the solar collector is above 30%, while the global thermal efficiency ranges from 5% to 18%. The CO2 emissions mitigation potential per mass of evaporated water ranged between 15 and 25 g of CO2 per kg of water evaporated in a day. The calculated payback period was 2.19 years.