The Design and Fabrication of the Multistage-Membrane Distillation Device Integrated with Solar Cell for Simultaneous Water and Electricity Production via Sunlight

Abstract

Freshwater scarcity and clean energy shortage are two grand challenges to global sustainable development. The inextricably interconnected water-energy nexus is being increasingly felt globally owing to the massive water used for electricity generation and huge amount of energy consumed in water desalination. This dissertation investigated the utilization of the waste heat of the solar cell to produce fresh water. This is achieved by constructing a multistage membrane distillation device (MSMD) at the backside of the solar cell to efficiently utilize its heat and it is capable of recycling the latent heat of the vapor condensation in each distillation stage. The first generation photovoltaic-membrane distillation (PV-MD) device exhibits a clean water production rate of 1.64 kg/m2 h with the solar cell temperature of 58 oC in a 3-stage device under one-sun radiation. However, some concentrated seawater can be produced from the PV-MD owing to its cross-flow design. To this end, an evaporative crystallizer is designed beneath the PV-MD, which can reuse the low-grade latent heat of vapor condensation in the last stage of the MSMD to evaporate the produced concentrated seawater, realizing zero liquid discharge. In addition, a theoretical model was also established to enhance the clean water production rate and reduce the solar cell temperature, which guides us to select a hydrophobic membrane with a thickness of 0.1 mm and porosity of 0.86 to fabricate the second generation photovoltaic-membrane distillation-evaporative crystallizer (PV-MD-EC) device. We experimentally demonstrate that a 5-stage PV-MD-EC device can desalinate seawater at a rate of ~2.45 kg m-2 h-1 with a lower solar cell temperature of ~48oC. The electricity generation efficiency of the solar cell is also enhanced by ~8% owing to its reduced temperature. A trade-off exists between the clean water production performance and material cost of the MSMD because a higher energy efficiency is at the expense of more stages applied. A low-cost and highly flexible 8-stage paper-based MSMD (P-MSMD) is further designed and fabricated and it showed a clean water production rate of 3.61 kg/m2 h for seawater desalination. This work sheds light on the design and fabrication of a composite system capable of achieving the simultaneous production of electricity and clean water with solar energy as an only energy source. Owing to their low barrier of entry, the devices reported in this dissertation are well suited to provide off-grid electricity and freshwater in a decentralized manner for point of consumption locations especially off-grid communities and communities with small- to medium-sized population even with challenging economic conditions.