Solar energy is a type of resource that is not only available and super clean, but free of charge. Vietnam is a tropical country with total of 2000-2500 hours of annual sunshine and total radiation amount from 100-175 kcal/cm2 a year. This free source of energy could be utilized to dry produce.
Heat is trapped based on the greenhouse effect principle: the short-wavelength sunlight passes through a transparent material layer, and is absorbed by the dark-color material layer. Due to the poor reflectivity of the dark color, the sunlight would not be reflected back, and dark material layer turn into a kind of "solar trap" which absorbs radiation and converts it into to heat.