Development and performance analysis of a mixed mode solar thermal dryer for drying of natural rubber sheets in the north-eastern part of India

In the north-eastern geographical setting of India, lies Tripura, the second largest state of India in terms of natural rubber cultivation. Often, the unpredictable weather and lack of appropriate drying methodology deteriorates the quality of the dried natural rubber fetching less income to the unorganized small scale farmers. Installation of modular solar thermal dryers in this region can resolve this problem. In this study, a 6 kg/batch mixed-mode solar thermal dryer was developed and installed at Agartala for performance investigation. Moisture content of the natural rubber sheets dried in the solar thermal dryer was reduced from 40% to 4% vis-a-vis 11% in open sun drying condition in 3 days' time on wet basis. Six drying models were studied and validated against the experimental data. The average energy and exergy efficiencies of the system for natural rubber sheet drying were 9.09% and 2.48% respectively. Quality of the solar dryer dried sheets were found to be superior to the open sun dried ones. Techno-economic analysis showed a payback period of 2.2 y. Innovative features of the developed solar thermal dryer include (a) recirculation of spent dehumidified air to minimize thermal energy loss (b) 90% UV cutoff in the drying chamber leading to better colour retention of the product (c) solar photovoltaic operated dehumidifier for use during night to prevent reabsorption of moisture. The higher throughput, better quality and ease of operation can make this mode of natural rubber sheet drying an attractive option among akin communities.