Avaliação de processo em secador solar direto assistido por módulo fotovoltaico: aplicação para secagem de alimentos

Abstract

Among the thermal sources that can be used for drying food, solar energy has as its main advantage being abundant, renewable and not monopolized. The simultaneous use of solar radiation and forced convection air drying can benefit the process. For this purpose, simple and low-power fans, powered by a photovoltaic module, provide the equipment with self-sufficient energy operation. Known as direct solar dryer assisted by photovoltaic module, its configuration is resistant to use in the field, practical for locomotion and capable of processing food on a small scale by family farmers. This dryer was built, instrumented and is fully operational with the possibility of being operated with natural or forced drying air convection. In both operating conditions, the dryer was tested in April and evaluated for the drying of green onions (Allium schoenoprasium L.) in May and June (autumn), September (winter) and October (spring) of 2019. Moisture and colorimetric parameters were monitored, since they are related to quality criteria of the dehydrated product and acceptance by the final consumer. The Page and Overhults equations best described the drying curves that occurred at a falling rate with 𝑅² greater than 0.900. The experiments with forced convection in May and natural convection in June performed better each month. These results may be related to climatic conditions, since there was no control of the internal temperature. Drying experiments in September did not show satisfactory performance due to the instability of solar radiation and air conditions with high humidity and low temperature. After sampling improvements, a constant rate period was observed followed by the falling rate period, during drying experiments in October. The forced convection experiment showed a higher rate in both periods. During drying at a constant rate, external mechanisms may have controlled the process, since the average values of the bed temperature and solar radiation flow were similar for both operating conditions. For the falling rate period, the effective diffusivity values of the bed were 5.02 x 10-9 m²/s and 1.17 x 10-8 m²/s for drying with natural and forced convection runs. In the same sequence, the average efficiencies of the solar dryer were 34.2 % and 38.3 % and the specific energy consumption was 18.3 kWh/kg and 16.4 kWh/kg. Little color variation was observed between fresh and dried chives in the two operating conditions, which is consistent with the need to maintain the green tint of the material.