A comprehensive review of indirect solar drying techniques integrated with thermal storage materials and exergy-environmental analysis

Food processing industries generally use fossil fuels for heating and drying applications leading to increased carbon footprints in the atmosphere. The carbon emissions can be significantly reduced with the use of solar energy. Various solar drying techniques are used to dry agricultural products; however, drying can only be done during the sunshine hours. Outside the sunshine hours, drying can be performed using thermal storage materials in which thermal energy is stored during sunshine hours and utilized during non-sunshine hours. This paper aims to deliver the significance of different thermal storage materials for improving solar drying efficiency. Also, a comparative study on various modes of drying practices like natural and forced convection, with and without thermal storage materials, is presented mainly for indirect solar dryers. The crucial parameters affecting the drying rate, such as initial moisture content, air velocity, air temperature, type of food products, solar collector, and dryer efficiency, are reviewed, tabulated, and significant findings are highlighted. The challenges of using both sensible and latent storage materials were also discussed. The overall drying efficiency of the indirect solar dryers can be increased up to 25% over sun drying, and the collector efficiency can be enhanced up to 70% with thermal storage materials. A significant reduction in drying time of 6 h was noticed with thermal storage materials. The maximum solar collector efficiency of 70% was found with forced convection systems, whereas only 30% was achieved with natural convection systems. Exergy efficiency for most of the recently developed indirect solar dryers was more than 50%, which implies that the developed techniques can still be improved by minimizing the exergy losses. When the exergo-environment analysis was compared with other solar dryers, embodied energy of the indirect solar dryer was much lower when compared with other solar dryers. Therefore, the energy utilization and CO2 emission by the indirect solar dryer is significantly low. This review will guide the researchers to design efficient indirect solar driers and collectors for future applications so that net zero emission can be achievable shortly.