A review on recent developments in natural convective solar dryer for agricultural products: Methods, collector design, influencing factors, performance and challenges

Solar drying is the best sustainable practice in preserving agricultural produce as open sun drying is prone to contaminations of food grains. Although a number of conventional technologies are in practice, they pollute the environment, increase the carbon foot print and require huge investment. To meet the sustainable development goal of UN 3, 7 and 12, the world focuses more on developing green technologies particularly natural convective solar drying. This review explores natural convective solar dryer (NCSD) that utilizes flat plate collector solar air heaters (FPCSAH) with heat harnessing element, resulting similar to 70 % in thermal efficiency and similar to 50 % reduction in drying time. Numerous traits and practices for improving effectiveness of solar drying are studied. Subsequently, this study identifies the convincing factors that influence the construction, working and performance evaluation correlations of NCSD. Besides, this review provides informative insights about the methods, detailed procedure to design FPCSAH. Also, the applications of NCSD by distinguishing the types of operations, food products and process parameters are discussed in detail. Eventually, the summary and outlook gathers all the factors and challenges including low mass flow, conversion rate, nano-PCM composite feasibility, robustness, customer perception and recommends solution with advancement for future directions, which will be a data mine to the scientific community in the developing nations for more economical drying process and procedures in NCSD.