Air source heat pump drying systems in the agricultural production sector were reviewed in this study in terms of optimal designs, leading to the optimization of the heat pump drying process. Several intricate designs have been used to optimize the heat pump drying process. Multiple evaporators with multiple condensers, multiple drying chambers, cascade heat pump drying systems, hybrid heat pump drying systems, different configurations of the heat pump components, and refrigerants with lower environmental impacts have been used to accomplish optimal heat pump dryer designs and thereby optimum drying conditions for agricultural products.