A comprehensive review of hybrid solar dryers integrated with auxiliary energy and units for agricultural products

Abstract

Solar energy as a clean and abundant energy source has been widely utilized in the drying domain. Much research has focused on the drying system driven by solar energy to improve the performance and sustainability of the drying system. In this review, the construction, working principles and related studies of various solar drying technologies including direct, indirect, mixed and hybrid solar dryers were summarized and compared. In particular, we reviewed hybrid solar dryers integrated with electrical heating, biomass energy, thermal energy storage and wind energy, and then concluded their advantages and disadvantages. It was found that hybrid solar dryers can achieve a stable and continuous drying operation, which can effectively improve the performance of the dryers and the quality of the products. Among the four hybrid solar dryers, the solar dryer integrated with thermal energy storage has strong scalability and applicability, because thermal energy storage materials can integrate with solar collector and drying chamber. Furthermore, the investment cost of this hybrid solar dryer is low, with a payback period within 3 years. For solar-biomass and solar-wind dryers, they are driven by biomass and wind energy, which dramatically improve the utilization ratio of renewable energy sources.