Evaluation of Electrospun Nanofiber Membranes for Thermal-Driven Water Purification for Power to X

Power to X (PtX) is becoming an established method to store the excess renewable power into green hydrogen through water electrolysis. Hydrogen can be used to produce electricity in fuel cells, as fuel, or further refined into ammonia or methanol. However, a crucial aspect in water electrolysis is the purity of water, that must have very low electrical conductivity (< 5&#956;S/cm), to avoid corrosion and shortening of the electrolyser’s lifetime. Moreover, its high pure water consumption demands finding a sustainable, reliable, and non competitive water source, for instance seawater or reclaimed municipal wastewater. Within the various technologies to obtain such water quality, membrane distillation (MD) is gaining more and more interest, as it is a thermal-driven process able to reach 100% salinity rejection. Indeed, potentially, it can provide pure water for PtX, from seawater while being run completely by the electrolysis waste heat. This thesis project aims at proving the technical and economic feasibility of this self-sustaining design. Firstly, the potential of MD to produce pure water for PtX was investigated. To overcome one of the main bottlenecks of MD, that is progressive membrane wetting with consequent worsening of produced water, some Polyvinyldene Fluoride (PVDF) hydrophobic nanofibrous membranes were synthetized through electrospinning technique and further characterized. After some preliminary successful tests with a simple 35 g/L NaCl solution, the electrospun membrane (ENM) with the best performance was tested with Baltic Sea seawater. Compared with a PVDF commercial membrane, the ENM was more efficient, achieving a higher water flux, while maintaining comparable salt flux. In both cases, the produced water presented a quality in line with the requirement of water electrolysis. To highlight potential future improvements of the ENM, a heat and mass transfer model was first validated for this study, and subsequently employed to tune membrane parameters. Subsequently, an economic analysis was conducted to retrieve the water cost of a MD full-scale plant, producing pure water for a 100 MW alkaline electrolyser from seawater. It was proved that the MD configuration and system design play a crucial role in the determination of both the water cost and process thermal efficiency. In the best scenario analyzed, the water cost achieved was comparable with the one of a traditional seawater reverse osmosis plant, while the electrolyser waste heat was sufficient to drive the MD process.