Investigation of thermal performance, drying characteristics and environomical analysis: direct flow evacuated tube solar drying of okra

A direct flow evacuated tube solar dryer (DF_ETSD), a novel drying system, was used for drying pre-treated okra (Abelmoschus esculentus). The performance of DF_ETSD was analysed by determining thermal profiling, dryer and collector efficiency hourly. The maximum 3-day average ambient temperature, collector outlet temperature and solar radiation were 35.6 °C, 66.4 °C and 976 W m−2 respectively. The collector efficiency increased as solar radiation increased over time due to a higher temperature difference between the collector outlet and ambient temperature. The maximum collector and dryer efficiency observed were 30.19% and 21.47%, respectively. A pre-treatment of okra was done in hot water at 70, 80 and 90 °C for 5 min. Okra samples were dried from an initial moisture content of 87.42 ± 1.49% (wb) to a final value of 10.77 ± 1.03% (wb) in 9 h. The pre-treatment temperature of 80 °C is suitable for maximum drying rate, colour retention and rehydration ratio and minimum water activity, which signifies the longer shelf-life of okra. Midilli and Kucuk model was best fitted (highest R2, lowest χ2 and RMSE) for the control and samples pre-treated at 80 °C; however, Verma model was suitably fitted for the sample pre-treated at 70 and 90 °C. The payback period of DF_ETSD was found to be 1.27 years. Environmental analysis shows the CO2 emission and net CO2 mitigation ranged between 1.24 and 18.65 t and 9.86 and 154.05 t respectively for different selected lifecycles of the dryer. Due to its environmental sustainability and low payback period, the presented drying system is recommended for okra and other fruits and vegetables. Graphical abstract: (Figure presented.) © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2024.