Improving Solar Dryers' Performances Using Design and Thermal Heat Storage

Solar drying is one of the most important processes used by the farmers and the agriculture producers especially in developing countries, at the same time as using free solar energy permits to reduce the cost of the operation. However, in order to face or to limit the intermittent character of solar energy, storage is proposed as a solution. It is found that two ways are used for the thermal energy storage: thermal and chemical ways. Nevertheless, thermal way is the most useful for solar drying. We present, in this paper, a classification of the common methods of thermal energy storage applied to solar drying with the presentation of the optimum design parameters for the studied dryers. It was found that the most frequent materials used for energy storage during solar drying are packed-bed storage with the use of rocks, sands, or gravels. The packed bed can be added to the drying chamber, to the solar collector, or both of them. Also, desiccants, such as a mixture of several chemical products or wheat, can find applications as storage material used for solar drying process. Water is the other proposed storage material; it is used according to its availability, cost, and some of its thermo-physical characteristics. Latent heat storage has found a little application for solar drying. In general manner and depending on the dried product, the insert of the thermal energy storage increases the efficiencies of the solar dryers and allows recovering the surplus of solar radiations during sunshine periods for a reuse during the off-sunshine periods. The study is ended by an economic analysis of some developed solar dryers.