Solar Drying of Agriculture Products: A Critical Review of Hybrid Technologies, Thermal Energy Storage, Performance Evaluation and Future Research Directions

Solar drying is a promising renewable-energy technology for preserving agricultural products, reducing post-harvest losses and lowering dependence on conventional fuel-based drying methods. However, the intermittent availability of solar radiation, variations in ambient conditions and non-uniform airflow can affect drying time, product quality and overall system performance. Recent research has therefore focused on improved solar collectors, hybrid heating systems, thermal energy storage, computational modelling and intelligent monitoring. This review critically examines the major types of solar dryers, their construction and operating principles, the integration of sensible and latent heat storage, and the application of supplementary energy sources. It also discusses the drying of fruits, vegetables, grains, spices and other agricultural materials, together with the technical, economic and environmental indicators used to evaluate system performance. The literature indicates that no single dryer configuration is optimal for every product or climatic condition. Appropriate selection depends on product characteristics, required drying temperature, local weather, production capacity, investment cost and quality requirements. Important research needs include improved airflow distribution, cost-effective thermal storage, standardized performance testing, product-specific operating conditions and long-term field validation. Solar dryers integrated with suitable storage and auxiliary heating can provide a practical pathway towards more reliable and sustainable agricultural processing.