A Review on Recent Advancements in Indirect Solar Drying of Agricultural Products

Preserving food from harvest to consumer level is a challenge in the agriculture sector. Drying is a crucial post-harvest technique that lowers moisture to levels suitable for storage. Solar drying is a traditional renewable energy drying process. Different solar drying methods have been developed to speed up the drying process and maintain the product's nutritious content. Indirect solar drying is one of the efficient drying methods that has better control over the drying temperature. Indirect solar drying has developed into a desirable, effective, and environmentally responsible drying technique when combined with solar collectors and thermal storage. Flat plates, evacuated tubes, and concentrated solar collectors are used in indirect solar dryers along with direct air heating or thermal storage systems. This study aims to review the improvement in the drying rate with different air heating mechanisms. Flat plate collectors with liquid working fluid are employed to heat the air, whereas in evacuated tube collectors, the air is directly heated passing through the tubes. Working fluids, air temperature, air velocity, and solar radiation are important dryer parameters affecting the drying rate. The paper also discusses the usage of heat storage devices for continuous drying operations. The drying time is greatly reduced through integration with latent and sensible storage technologies. Products that have been dried using indirect solar dryer and appropriate drying models are tabulated. Aspects of indirect solar drying and challenges in drying time reduction are also reported.