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This work describes the thermal behavior and evaluation of kinetics drying of natural rubber sheets using two solar drying technologies: direct and indirect. The solar drying tests were carried out simultaneously in order to compare the behavior under same weather conditions. The moisture content of the rubber sheets was reduced from 45.8 to 0.59% and from 49.7 to 0.33% on a dry basis (d.b.) for direct solar drying and indirect solar drying, respectively. The direct drying mode was carried out for 12 days with average temperature and relative humidity inside the greenhouse, during sunny hours, of 37.04 °C and 14.1%, respectively. Meanwhile, the indirect drying operation was carried out in 9 days (61.62 h intermittently) with 61 °C and 4.77%, respectively. The average global energy efficiency of the field of SAH was 33.77%. Thirteen thin layer models were used to evaluate the drying behavior of which the Modified Henderson and Pabis model was the best to describe the solar drying of rubber sheets for the direct and indirect system. As a conclusion, the direct greenhouse-type solar dryer is recommended for small producers and the indirect tunnel-type solar dryer for companies with high and continuous production.

This paper presents the thermal and energy analysis of a Solar-LP gas hybrid drying plant built in Zacatecas, Mexico. It is focused on the dehydration of agricultural products. The drying system is a forced convection hot air type with a drying chamber. For the air heating required in the drying process, there are two solar thermal systems: a direct air heating system (48 solar air heaters with an area of 111.1 m2) and an indirect air heating system (40 solar water collectors with an area of 92.4 m2 and 6000 L of thermal storage). Also, there is a backup system of conventional energy (LP gas burner). Two hybrid tests were carried out to dry Nopal (Opuntia ficus-indica) initially operating with the direct system, after that with the indirect system and complementing the energy demand of drying with the conventional system. The effective drying time was 14.3 and 11.7 h, removing a total of 179 and 183 kg of water, respectively. The air temperature at the entrance of the drying chamber was around 59 °C. The solar fraction obtained for this hybrid mode of operation was around 80%. The thermal analysis of the direct and indirect air heating system, together with the conventional system, are presented and discussed. The performance of the solar systems is finally compared with a conventional LP gas drying system via a cost and return on investment analysis.