The traditional open air drying of food & crop products under the sun or with smoke is generally not hygienic and time consuming as well. Moreover, in this method the food materials are not dried in an uniform manner leading to inferior quality products. In this context, solar dryer is the most suitable device that takes care of the quality of dried food products.
Different types of low cost solar dryers ranging from cabinet dryers, greenhouse dryers, step type dryers, multi-rack dryers etc. in different sizes with various configurations were fabricated for drying various agricultural products such as fruits, vegetables, medicinal plants etc. The first models of solar dryer were made of cuddapah slab (the drying chamber) covered with transparent plastic sheet and fitted with reflective metal sheets to harness sunlight and heat to dry the food materials. An improved version of this model was a step type dryer made of cuddapah slabs and covered with a transparent polycarbonate sheet. An advanced cuddapah slab model was fitted with photovoltaic (PV) panels.
This was followed by the development of a cabinet type single drying unit made up of aluminium and provided with perforated stainless racks to lay the fishes. The unit was completely painted in black that it could easily trap most of the solar radiation or heat. The device helped fisher community to dry fishes in a hygienic manner and also retained the quality of dried fishes.
With advanced technology, easy availability of PV panels and reduced cost, new model of solar dryer was fabricated with MS slotted angle frames covered by LDPE sheet on all four sides and fitted with small DC fans inside the dryer. These fans help to circulate the air inside the dryer facilitating uniform distribution of heat and drying of the material. This modification helped to reduce the drying time significantly. This solar dryer was deployed in NGOs premises where community people used this facility to dry herbal leaves like Hibiscus, Tulsi, Adathoda & Moringa leaves, and Amla fruits & cluster beans. The solar dried products had 40% more active ingredients than the conventionally dried ones indicating the feasibility of using solar dryers for herbal based medicinal preparations.
More recently, a novel dryer, harnessing solar energy using Evacuated Tubes Collector (ETC), which converts the solar energy into heat energy was developed with the help of IITM. The advantage of this unit is that it maintains the heat within a constant temperature range of 45 – 55°C even during off sunshine hours and thus, food materials can be dried continuously. This reduces the drying time by over 70% especially for high moisture food items. The food dried in this system can be stored for several months without compromising its nutrient content.