There are various methods of drying food materials, with each having its specific advantages based on applications. The method adopted for a particular drying process is dependent on the availability of the drying medium, nature of the food material and the purpose which the dried product is meant to serve. About 20–30% of energy demand in agricultural sector consumed for drying process, refers to remove the moisture content of wet products to increase the shelf life of agricultural products such as fruits and vegetables (Alimohammadi et al., 2020). Among many such drying methods, sun drying is the most economical method but causes problems associated with food contamination by dust, wind, rodents, insects, deterioration, rain, and humidification at night can cause poor quality. The poor quality of dried product could not be acceptable for the market purposes and the price may decrease significantly. To control this microbial contamination and damage of foods like grains, fruits, and vegetables, etc., solar drying, an interesting alternative drying method can be used. Solar drying can reduce dependence on fossil fuels, protects food from environmental conditions and solar radiation, increases drying efficiency and the quality of food products is better in terms of nutrients and colour. Solar energy has the potential to meet energy demands for drying, especially in subtropical countries like India, that receive sufficient solar radiation throughout the year. In rural areas, farmers generally see drying, as a means of food preservation. Since solar drying generates high air temperatures and produces low air relative humidity, and reasonably economical and can be easily accessed by local farmers.
Solar drying systems must be adequately designed to meet the requirements of specific crop drying and to provide satisfactory performance against the requirements of energy. The solar dryers can be divided according to the way in which solar energy is used to heat. According to the operation principle, natural or forced convection mode; classified again into active and passive (direct or indirect) solar drying systems, and mixed type dryers.
- Direct Solar drying by radiation, where the products to be dried are subjected directly to the effect of solar radiation. Direct solar dryers are simple and consist of a glass box containing racks where the products to be dried are placed. The airflow required to dry the product is due to natural CONVECTIVE force or buoyancy force. These dryers are based on the greenhouse effect that allows more efficient heating of the walls of the solar dryer and thus the interior atmosphere of the dryer (Krabch et al., 2022).
- Indirect drying by convection, where the foods to be dried are not exposed directly to solar radiation, so they are protected from the harmful effects of UV rays. Here drying is carried out indirectly by convection between the heated air and the products to be dried. This mode of drying has the advantages of shorter drying period, best quality products, lower loss of raw materials and larger scale of production (when compared with traditional direct solar drying or open-air drying)
- Indirect passive solar dryers don’t depend on any energy source other and any storage system. Most solar food dryers use two compartments: a drying chamber and an air solar collector (to heat the air and inject it into the drying chamber). The air collectors may increase the overall cost of the dryer, thus limiting its use by farmers (Krabch et al., 2022).
- In mixed type solar dryers (MSD), a flat solar collector for air heating, a drying chamber with transparent polycarbonate cover and a chimney for the humid air outlet are used. There are no fans or any forced convection system, thereby called as the passive dryer. This type of dryer acquires heat energy from the beams of sun that enters through the solar collector and through the drying chamber. In an indirect mode, the sun’s heat is first collected by the solar collectors and then is passed onto the dryer chamber, where the drying occurs. The air is heated by the solar collector which flows in natural convection into the drying chamber, then the air flows between the fruit/ vegetable pieces. Due to heat and mass transfer phenomena, the air gives heat to the food, and the food gives moisture to the air. Subsequently hot and humid air leaves the drying chamber through the upper part of the chimney Compared with indirect mode dryers, MSD provides more efficient drying performance, since transparent polycarbonate cover promotes higher heating of the food materials by the direct incidence of solar irradiance in the food and a lower relative humidity inside the drying chamber (Cesar et al., 2020).
Developments in solar drying systems
Indirect solar dryer based on solar collectors that produced thermal energy can be used for drying agricultural wet products. Solar collectors are the most used thermal systems to convert solar radiation to thermal energy and the produced energy consumed in solar dryers. The major problems encountered in indirect solar dryers (ISD) is that the dehydration process is interrupted at night or under a low radiation, resulting in a poor quality of the dried product. To overcome this issue, one approach is the use thermal storage units. The thermal energy unit uses storage materials that captures and stores excess thermal energy during the sunshine hours and use it during the off-sun (night) hours. The dryer systems with the thermal energy storage can be classified into two forms, solar drying systems using sensible heat storage (SHS) methods and the solar dryers using latent heat storage (LHS) methods (Khadraoui et al., 2017).
In SHS method, sand, rock bed, gravel, iron, and aluminum filings, are used as sensible heat storage (SHS) materials (Natarajan et al., 2017). In LHS method, the storage material changes its phase during charging and discharging cycle. During these cycles, LHS materials have been reported to possess sufficiently high energy storage density with small thermal gradient. Phase change materials (PCM) have been noticed to work nearly isothermally due to their phase change mechanism. This proves PCM as the most promising materials for thermal storage applications. Paraffins have been reported as better PCM owing to its properties such as non-corrosiveness, less expensive, chemical stability below 65 °C and minor volume changes on melting (Bhardwaj et al.,2020). Other PCM materials like Lauric acid, glycerine are being used.
Indirect solar dryer (ISD) with thermal storage Unit operated in two drying modes: a daytime and a night-time. In the daytime, a fraction of the total solar radiation transmitted by the transparent cover is absorbed by the black absorber plate of the solar air collector (solar energy accumulator and solar air panel). The solar energy accumulator solar air collector with phase change materials (PCM)) consists of an insulator, a PCM cavity filled with paraffin wax, and a glass cover. The solar energy accumulator is used as an alternative source to heat the interior environment of the drying chamber during the off-sunshine period. At daytime, the outlet opening (valve V₁) of the solar air panel is opened whereas the outlet opening (valve V₂) of the solar energy accumulator is closed. The air passes through the free space between the glass cover and the absorber plate of the solar air panel and gets heated. After that, the heated air enters the drying chamber and distributes over perforated trays with food materials. At the same time (charging process) a fraction of the total solar radiation transmitted by the transparent cover absorbed by the black absorber surface of the PCM cavity: the absorbed thermal energy is stored as the sensible and the latent heat forms into the solar energy accumulator. At night-time, the outlet opening (valve V₁) of the solar air panel is closed whereas the outlet opening (valve V₂) of the solar energy accumulator is opened. The fan blows air across the PCM cavity and extracted the stored heat to heat air being passed to drying chamber. Thereby, the stored heat is utilized during the off-sunshine hours to continue the drying process. The ISD with paraffin wax as energy storage material is an effective design to yield more favourable conditions for the drying process compared to the ISD without energy storage (Khadraoui et al., 2017). In the recent years, nano-scale materials are becoming noteworthy and attain scientific and industrial significance. Indirect and aesthetic based nanoparticle coated solar reflectors are used for the reduction in consumption of air conditioning energy. The nano particle based black paint coating can enhance the solar absorptivity, which can be used for drying of various agro products (Sivakumar et al., 2020)
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About Author: Sai Prasanna N received her BTech degree in Agricultural Engineering (2017) from Acharya N G Ranga Agricultural University (Andhra Pradesh), India and MTech degree in Food Process Engineering (2019) from IIT Kharagpur, India. Currently, she is pursuing her PhD under the joint supervision of Prof. KSMS Raghavarao and Dr. Nilesh Choudhary, at Chemical Engineering Department, Indian Institute of Technology Tirupati, India. She received the most prestigious Prime Minister’s Fellowship (sponsored by SERB-CII, India) in 2023. Her research is focused on Nanotechnology and membrane processing for food applications, Post-Harvest Engineering, and Extraction of bioactive compounds from bio-waste.