A review of natural energy storage materials used in solar dryers for food drying applications

The application of solar energy in food drying is a well-known technology. Open sun drying has some limitations but these limitations can be overcome in solar dryers. Thermal energy storage (TES) systems for solar dryers receive wide attraction as the TES system enhances the performance of dryers. Among TES materials, the natural energy storage materials are inexpensive and easy to collect in remote areas. This work extensively reviews solar dryers with various natural energy storage materials. It discusses thermo-physical properties of various natural energy storage materials. The performance analyses of direct, indirect, mixed-mode and hybrid mode solar dryers with and without natural energy storage materials are reviewed. natural energy storage materials placed in different locations such as kind 1, kind 2 and kind 3 are discussed. The performance of frequently used natural energy storage materials such as sand, sandstone, gravel, rocks, pebbles, limestone, clay, soil, bricks, quartz, reinforced concrete and water are reviewed. The drying time saving is in the range of 9.52-47.2% using natural energy storage materials compared to conventional solar dryers. The drying efficiency (eta(d)) and thermal efficiency (eta(th)) of the direct solar dryer with natural energy storage are in the range of 2.85-42% and 9.9 -58.2%, respectively, for various food materials. The drying air temperatures inside the chamber are 5 to 20 degrees C higher than the atmospheric temperature even after sunset hours with the natural energy storage system. The progress, benefits, challenges and recommendations of natural energy storage materials for use in solar dryers are also addressed in this paper.