Double-Pass Solar Collectors with Porous Media Suitable for Higher-Temperature Solar-Assisted Drying Systems

A high-performance solar collector suitable for a solar-assisted drying system was designed, fabricated, and tested. Solar-assisted drying systems consist of the solar collector array, auxiliary heater, drying chamber, and the air distribution systems. The solar collector was a double-pass solar collector and has upper and lower channels. The lower channel of the solar collector was filled up with porous media, which also acts as a storage medium. The outlet temperature and the performance of the system increased with the presence of the porous media. The collector width and length are 120 and $240\mathrm{cm}$ , respectively. The upper channel depth is $3.5\mathrm{cm}$ and the lower channel depth is $10.5\mathrm{cm}$ . The solar collector array consists of six solar collectors, arranged as two banks of three collectors in series. The collectors were joined in series using internal manifolding. A temperature of $90°\mathrm{C}$ can be reached at a solar radiation level of $800\mathrm{W}∕{\mathrm{m}}^2$ , ambient temperature of $30°\mathrm{C}$ , and mass flow rate of $0.06\mathrm{kg}∕\mathrm{s}$ . The solar-assisted drying system was used for drying of oil palm fronds from moisture content of about 63% to moisture content of about 15%, for a drying time of about $7\mathrm{h}$ . The overall system efficiency is about 25%.