Experimental investigation of an integrated solar driven wastewater treatment system for trigeneration applications

Abstract

Hydrogen is a promising alternative as an energy carrier (and carbon-free fuel) to meet the global power demand. Hydrogen production systems can efficiently harvest energy from renewable sources. Even though photo-electrochemical systems offer an attractive potential for both hydrogen production and wastewater treatment systems, their application in industrial scales is not satisfactory. This thesis study proposes a trigeneration system for electricity, hydrogen and clean water production. TiO2 photocatalyst is used in a photoelectrochemical (PEC) reactor, which cleans the water via Fenton-like process and produces hydrogen. A novel solar thermoelectric generator (TEG) unit drives the reactor. For the hot side, the phase change material (PCM) which is heated by the concentrator feeds TEG, and the wastewater stream provides the cold surface. The PEC system is investigated experimentally while the design and calculations of the TEG-PCM sub-system are analyzed theoretically in this study. Moreover, in this study, the synergistic effects of advanced oxidation reactions (AOPs) in a combination of TiO2 photocatalysis are comparatively investigated for hydrogen production and wastewater treatment applications. The synergistic effects of Fenton, Fenton-like, photocatalysis (TiO2/UV) and UV photolysis (H2O2/UV) are investigated individually and in a combination of each other. The effects of various parameters, including pH, type of the electrode and electrolyte and the UV light, on the performance of the combined system are also investigated experimentally. A numerical modeling study is performed to predict the temperature, heat transfer rate and generated power distributions on the thermoelectric generator which is included in the integrated system. The thermodynamic properties of the matter flows at each stage and state point are obtained for the integrated system. The overall energy and exergy efficiencies of the integrated system are 5.2% and 5.5% respectively. The total cost rate of the system is determined to be 0.28 $/h from exergoeconomic analysis results. After 40 minutes of the operation time of the PEC reactor under 2.3V of practical cell potential and 60°C of cell temperature, the reactor produces 22.6 mg of hydrogen and removes 87% of COD. The proposed TEG unit configuration generates 551.2 W of electricity with 7.46% heat to electricity efficiency for the case when the temperature of wastewater is 25°C, and the molten salt is at phase changing temperature of 323°C.