Hybrid solar hydrothermal carbonization by integrating photovoltaic and parabolic trough technologies: Energy and exergy analyses, innovative designs, and mathematical Modelling

Hydrothermal carbonization (HTC) is an excellent process for transforming biomass into solid fuels. Evaluating HTC's environmental benefits requires addressing substantial energy needs for reactant heating. In this regard, two innovative solar systems for powering HTC were investigated: a parabolic trough collector (PTC) that heats a heat transfer fluid (HTF) circulating through a heat exchanger surrounding a batch reactor and photovoltaic panels (PV) that powers a heating collar. Experimental results reveal that the PV system efficiently attains subcritical conditions, whereas the PTC system faces challenges. Exergetic analysis identifies heat transfer as a primary irreversibility factor, resulting in a recorded maximum entropy of nearly 100 W/K. This caused the exergy efficiency to decrease to a minimum of 0.07, dropping the accumulated exergy below 82 W. To address this issue, a validated mathematical model was employed to investigate three optimized configurations: an immersed heat exchanger, a jacketed system, and a hybrid system combining both. The results demonstrated achieving desired HTC conditions at mixing temperatures of 185 degrees C, 195 degrees C, and 230 degrees C, respectively. In contrast, the PV system rapidly attains 220 degrees C and 40 bar in 70 min. This study proposes innovative solar HTC designs for sustainable hydrochar production using PV and PTC technologies.