Solar thermal dehydrating plant for agricultural products installed in Zacatecas, México

Abstract

This paper presents a hybrid thermo-solar plant for the dehydration of foods, built in Zacatecas, Mexico. The plant is integrated by a semi-continuous drying chamber with a capacity of up to 2 700 kg of fresh product. The thermal energy required for the drying process is provided by two solar powered thermal systems: an air heating system with 48 collectors (111.1 m2) and a water heating system with 40 solar water heaters (92.4 m2), a thermal insulated storage tank, and a fin and tube heat exchanger. The plant also has a fossil energy backup system (LPG) to heat air. The monitoring system measures and records 55 process variables (temperature, relative humidity, water pressure, solar irradiance, air velocity, volumetric and mass flow rates). Experimental results obtained with the solar air heaters and water collectors are reported. The average efficiency of the solar air heaters field was ˜45% with a maximum increase of the air temperature of 40.8 °C and for the water solar collectors field the average efficiency was ˜50% and the water in the storage tank reaches 89.6 °C in two days of operation. A batch test was carried out on September 26 with the thermos-solar systems for the dehydration of 282 kg of Nopal (Opuntia ficus-indica) with an initial moisture content of 85.28 % (wet basis), reaching a final moisture content of 9.13 % (wet basis) in 9.7 hours. Both thermo-solar systems are able to deliver the temperature required in the food drying process (temperatures above 50 °C). According to the results obtained, different kind of food products can be dried on the thermo-solar plant designed, resulting in substantial fuel savings and environmental benefits. The development and implementation of solar drying systems technology, initially at demonstration scale, in the Mexican industrial sector could allow a technical and economic maturation of the technology, and the benefits could be presented in the short term.