A review on the use of sorption materials in solar dryers

Solar drying has received great interest as a viable alternative solution to fossil fuel dryers. However, due to the intermittency of the solar resource, solar dryers suffer from discontinued drying and thus, long drying time. In this paper, the use of sorption materials to enhance the performance of solar dryers is reviewed. The main aspects regarding their use as thermal energy storage or dehumidification materials are highlighted. The paper shows that solid adsorbents, especially silica gel, are the most used materials. The use of composite materials, for instance the mixture of bentonite, CaCl2, vermiculite, and cement proved to be promising. The incorporation of a sorption dehumidifier in a solar dryer, usually at the solar collector inlet, generally leads to a reduction of the drying time of 15–30%, although values up to 50% and even 64% have been reported. On the other hand, introducing sorption materials as thermal storage in a solar dryer, mostly integrated at the top in the drying chamber, usually leads to a reduction of the drying time in a range of 30–45%. There are still various aspects to be investigated in depth before a large-scale utilization of these techniques in solar drying technology.