Experimental drying characteristics of a portable solar dryer for apple slices in consecutive and continuous tests

Abstract

A portable solar dryer constructed with cost-effective materials, operated in sunny and cloudy conditions for off-grid farmerswas successfully used for extending the shelf life of apple slices in three different experiments. The performance of the portable dryer was evaluated in three 8-hour consecutive and three 32-hour continuous drying tests. Parameters that influenced the dryer’s performance were the solar radiation, relative humidity, wind speed, and drying temperatures. The top tray exhibited the lowest final moisture ratio with a drying time of around 10 h, and a final RH of around 16 %. The average drying rates were 0.050, 0.058, and 0.066 kg/h, whereas the maximum values of specific energy consumption (SEC) were 8.31, 12.02, and 15.10 kWh/kg for three 8-hour tests. The effective moisture diffusivity (D_eff) values of dried apple slices were 1.89×10⁻¹⁰, 1.68×10⁻¹⁰, and 1.78×10⁻¹⁰ m²/s for the three 8-hour tests. The rehydration ratios (RR) values were 3.10, 3.25, and 3.31, while the shrinkage factors were found to be 43.2, 42.64, and 41.36 %. Modelling showed that the Page model with an average R² value of 0.9985 was the best fit to describe and predict the drying behaviour of apple slices. In the 32-hour drying test, the weather conditions affected the thermal profiling of trays and walls during daytime, while temperatures of the dryer’s components were lower than the ambient temperature at night. The final moisture ratios were 0.1029, 0.0971, and 0.0857 on the top trays, while the SEC was 10.30, 10.86, and 11.38 kWh/kg for the drying tests.