Critical Review on Various Solar Drying Technologies: Direct and Indirect Solar Dryer Systems

Abstract

Food industry being one of the world’s largest energy intensive industries, lack of proper preservation and storage techniques have led to huge amount of food losses and wastage. Dehydration of food and vegetables has been an effective technique of preservation as this reduces post-harvest losses, make them easier to transport, store and can prevent the growth of microbes. Abundant solar energy being available for free of cost, solar drying is desirable in terms of environment friendliness, economic benefits and is compatible for remote locations. Solar dryers optimize this process with efficient utilization of solar energy and provides higher quality products. Different configurations of solar dryers with diverse configurations and applications have been designed and implemented over the years. Based on the difference in supply and utilization of solar energy, the most prominent solar dryer configurations are direct and indirect solar dryers. This work intends to review the features, design and performance of existing direct and indirect solar dryers. Major challenges such as intermittency and unsteady availability of solar energy has been addressed through thermal energy storage by many research studies, which has also been effectively reviewed. Materials used for construction, design constraints, thermal energy storage integration and experimental results have been discussed and tabulated. The review revealed highly efficient solar collectors such as tube type absorber and evacuated tube collectors, solar concentrators employed dryers capable of achieving elevated temperatures, greenhouse and solar tent dryers with huge capacity and novel dryer and collector designs of superior performance. Various integration techniques of different thermal energy storage materials and their enhancement in performance were eminently observed in the review. As a source of technological attributes and constructional features, this review paper intends to aid the development of solar dryers and food preservation employing renewable energy.