Thermodynamic analysis of forced convective solar drying of cocoa with black coated sensible thermal storage material

Abstract

Thermodynamic analysis of a forced convective solar dryer integrated with black coated sensible (firebrick) thermal storage material (STSM) for cocoa was investigated. The intermittent nature of solar radiation and its consequence on thermal processes such as drying necessitates the introduction of materials for heat supplement to keep the cocoa beans at a raised temperature above the ambient. As a steady flow thermodynamic system, the analysis was carried out based on the first and second laws of thermodynamics to account for the energy involved in the solar drying of cocoa beans. The maximum drying chamber utilization energy during the first and second daytime drying process was 0.739 and 0.724 kJ/kg. The energy utilization (EU) decreased with the increase in time from 12:00 to 18:00 h respectively. Also, the energy utilization ratio (EUR) ranged from 0.190 to 0.577, and 0.222 to 0.931 during the first and second daytime experiments respectively. The collector efficiency varied between 4.2 and 61.2% while the minimum and maximum solar radiation for the two daytimes were 49.6 and 759.6 W/m2 with an average mass flow rate of 0.032 kg/s. The maximum energy efficiencies for the two-day times obtained at 10:00 h were 28.0 and 58.2%. In the drying chamber, the maximum exergy (inflow and loss) for the two-day time drying process was 7.53 and 8.33 kJ/kg, and 4.83 and 8.175 kJ/kg respectively. The exergetic efficiency during the first daytime experiment varied between 1.52 and 65.81%, while it varied between 1.82 and 53.30% during the second daytime. Thus, the results obtained in this study will inform the choice of an appropriate solar dryer design by the relevant stakeholders in the cocoa industry.

Keywords

• Thermodynamic analysis
• Solar drying
• Cocoa
• Firebrick
• Thermal storage material
• Efficiency