Water contained in foods allows the proliferation of microorganisms and the development of chemical reactions that deteriorate. When required preserve food for a time, a dry or dehydrate alternative is removing the water contained therein. There are several ways to achieve this issue, the simplest is to exposed the food into a stream of air with certain conditions of temperature, humidity and velocity. There are several types of solar dryers to achieve proper product treatment. The two basic elements of a solar dryer are manifold where solar radiation heats the air and the drying chamber where the product is dried by the air passing through it, these elements can be designed to integrate the team in different forms. In an indirect solar dryer the two elements are separated, the air is heated in the manifold so that no solar radiation incident on the product placed inside the opaque drying chamber, in this type of dryers the process control drying is simpler. Having a separate drying chamber manifolds facilitates handling of the product and the work of loading and unloading. Since the drying chamber is opaque, this system conveniently dry products that can be damaged by direct sunlight. This project works with an indirect solar dryer having a solar air heater built into its structure with plywood wood 1" thick, both the base and sides, with dimensions of 140 cm long and 60 cm width, a cover glass thickness of 3 mm and an inclination of 17.5° to the horizontal. The main element is a coil of copper foil matt black painted placed inside the structure, isolated base and on all four sides with fiberglass wool, has thirteen channels 10 cm x 10 cm x 10 cm, which means a final length of 5 m in the warm air path, giving a conservative 0.01 m3/s flow and a 1 m/s natural convection speed. The drying chamber is constructed of plywood wood 1/2" thickness and dimensions of 60 cm long, 40 cm wide and 55 cm high with four nylon mesh trays, allowing initial drying mass about 2 kg. The connection between the solar air heater and the drying chamber is performed by means of a neoprene hose with a very low thermal conductivity in its wall. Thermal efficiency is calculated by evaluating the productivity of the dryer measuring temperature, air flow, solar radiation and loss of product mass. Drying curves are obtained graphing Dry Basis Moisture vs. time (X vs. t) and Drying Rate vs. Dry Basis Moisture (δX/δt vs. X) for Nopal (Opuntia Lasiacanta). It is considered that this type of dryer offers several advantages over the common cabinet druer, including the drying rate and product protection from direct sunlight.
Finck-Pastrana, A. G. (2014). NOPAL (Opuntia Lasiacantha) drying using an indirect solar dryer. In Energy Procedia (Vol. 57, pp. 2984–2993). Elsevier Ltd. https://doi.org/10.1016/j.egypro.2014.10.334