Drying of red pepper slices in a solar greenhouse dryer and under open sun: Experimental and mathematical investigations

The present work is mainly focused on the study of the solar drying of red pepper grown in the North of Tunisia. A new mixed mode solar greenhouse dryer (SGD) with forced convection was used in the drying experiments. To further understand the characteristics of the SGD, an open sun drying experiment was conducted under the same climatic condition. The results show that the constant rate period is absent from the experimental drying curves. The experimental drying curves show only the falling rate period. Thus, the drying time which was three days in the open sun decreased to two days in the SGD. The drying data were fitted to eight different mathematical models. The fit quality of the proposed models was evaluated by using the coefficient determination (R2), reduced chi-square (chi(2)) and root means square error (RMSE). Among the models, Modified Henderson and Pabis model for the forced solar drying and the open sun drying was found to best explain the thin-layer drying behaviour of pepper slices. The instantaneous thermal efficiency of the solar collector varied between 30% and 80%, at a mass flow rate of 0.047 kg/s. The overall energy efficiency of the solar dryer was 34%. Finally an economic evaluation was calculated using the criterion of payback period which was found very small 1.02 years compared to the life of the dryer (SGD) 20 years.