Continuous Solar Vapour Generation and Salt Harvesting from Seawater

Fresh water on our planet is locked in the ocean as seawater, which has to be desalinated for domestic, industrial, and agricultural use. Solar vapour generation using photothermal materials to efficiently convert solar energy into heat for water evaporation is an emerging technology for seawater desalination. It is promising to mitigate the worldwide water scarcity problem in a green and sustainable way. However, developing high-efficient continuous solar evaporators for practical desalination applications still remains challenging due to the salt accumulation issue on the photothermal materials, the difficulties in scaling up the evaporators due to the limitation of water supply driven by capillary force, and the insufficient understanding of multiphase transport and salt harvesting mechanisms inside porous structures. To address these limitations, this work aims to develop high-efficient solar evaporators that can achieve continuous solar vapour generation and salt harvesting concurrently from seawater, with improved salt harvesting mechanisms understanding. For this, the thesis primarily focuses on the following four parts: i) light-trapping photothermal material development, ii) evaporator structure improvement for continuous concurrent solar vapour generation and salt harvesting, iii) salt harvesting mechanism exploration, and iv) scale-up of the evaporator for large area applications. In this work, a light-trapping nanofiber photothermal coating was proposed by copolymerization pyrrole with dopamine, which can be rapidly synthesized at room temperature by ultrasonic spray coating. The highest solar absorptance reached 97.73%. This nanoscale coating significantly improved solar absorption at different incident angles across the full solar spectrum, achieving the highest solar evaporation rate of 1.385 kg•m-2•h-1 under 1 sun. Then an umbrella evaporator was developed, which achieved high efficiencies of continuous fresh water production and salt harvesting concurrently via double-sided evaporation. The evaporation process was simulated to reveal that better mass transfer condition of the umbrella evaporator by double-sided evaporation greatly improves the evaporation rate, and the influence of environmental conditions, surface height and opening angles on the evaporation performance were also analysed by this simulation. The salt nucleation, growth and falling mechanisms were also revealed. The salt mobility on the evaporation surface determines if the salt accumulates at the edge to fall, affected by water supply, salinity, and Mg2+ and Ca2+ contents. The salt on the edge grows from the front by wicking the water through its porous structures inside and falls due to the dissolution of its connecting parts. Salt creeping behaviours on a glass slide can indicate if the pre-treated seawater fits for salt harvesting. To address the water supply and distribution problems of the umbrella evaporator for large area application, a double-sided suspending evaporator with top water supply and surface water distribution systems was developed. Top central water supply gets away from the limitation of capillary force and allows larger area application. Its evaporation rate achieved 1.40 kg•m-2•h-1 with deionized water under 1 sun (95.7% energy efficiency), with a remarkable low surface average temperature (28.2 ºC). Then the salt distribution process on the surface was simulated to develop a novel floriform evaporation surface with a radial arterial water distributor to forcedly expose salts at the edge for harvesting and efficiently distribute the water on a larger evaporation surface. This work shows great potential of the polypyrrole-dopamine nanofiber light-trapping coating as photothermal materials for continuous vapour generation, provides new ideas for the structure design of the evaporators that achieve solar vapour generation and salt harvesting concurrently, and allows us to better understand and control the salt harvesting process

Carbonized mangrove wood as photothermal material for solar water desalination

The investigation into the physical properties of carbonized mangrove wood (CMW) is essential for its development as an efficient solar heat absorber. This study explores the physical characteristics of CMW and its potential application in solar desalination. Initially, the mangrove wood was cleaned with running water, followed by ultrasonication at a frequency of 42 kHz in 96% ethanol for 5 minutes, and then heated at 125 °C for 2 hours. The carbonization process was conducted in a furnace for 1 hour at temperatures of 400, 500, and 600 °C. The physical properties of CMW were analyzed using an X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive spectroscopy, and scanning electron microscopy (SEM). The findings revealed the formation of a carbon structure at 2 theta angles of approximately 24.08, 23.26, and 23.16°, with carbon contents of 45.05, 36.86, and 39.37%, respectively. CMW was identified as a porous material, making it highly effective for sunlight absorption in seawater evaporation. The hydroxyl content within the CMW structure enhanced its water evaporation capabilities. In experimental investigations aimed at desalinating seawater, a 300-watt halogen lamp was positioned 15 centimeters above the CMW's surface, resulting in an evaporation rate of 5.33 kg.m-2.h-1. CMW shows significant promise as a solar evaporator

Double-Sided Suspending Evaporator with Top Water Supply for Concurrent Solar Evaporation and Salt Harvesting

Solar evaporation of seawater is promising to mitigate the fresh water scarcity problem in a green and sustainable way. However, salt accumulation on the photothermal material prevents the system continuous operation, and the water supply driven by capillary force severely limits the scale-up of the evaporators. Here, we demonstrate a double-sided suspending evaporator with top water supply and a surface water distributor for high-efficient concurrent solar evaporation and salt harvesting for large area applications. Both sides of the evaporator can evaporate water with automatic salt harvesting from the edge concurrently. Top water supply gets away from the limitation of capillary force for a larger area application and completely cuts off the heat leak to the bulk water below for higher efficiency. The energy conversion efficiency reaches 95.7% at 1.40 kg·m–2·h–1 with deionized water under 1 sun with a remarkable low surface average temperature (28.2 °C). Based on the simulation and experiment, a novel radial arterial water distribution system is developed to efficiently distribute water on a larger evaporation surface. The water distribution system alters the water transport path in the evaporation surface, leading to salt accumulation on the surface body, where salt is unable to be harvested by gravity automatically. This problem is further resolved by cutting out the salt accumulation area (16.4%) on the surface to create a floriform evaporator, which forcedly exposes the salt at the edge for harvesting. Up to70 h continuous solar evaporation from salt water at a rate of 1.04 kg·m–2·h–1 with concurrent salt collection on this floriform evaporator is achieved. This work resolves water supply and salt accumulation problems in scaling up the solar evaporators and advances the structural design of evaporators for high-efficient large area applications

Optimizing the Marangoni effect towards enhanced salt rejection in thermal passive desalination

Amid escalating water scarcity and rising energy prices, the scientific community strives to propose innovative and efficient water treatment solutions. In this context, solar passive technologies have attracted much attention. Furthermore, recent studies have experimentally revealed that the Marangoni effect, when leveraged in well-designed passive devices, may be a promising pathway towards long-term stable performance. This study presents a comprehensive numerical exploration of applying the Marangoni effect to mitigate salt accumulation, a challenge in long-term system operation. Through an extensive sensitivity analysis, we evaluate the solute molar outflow induced by the Marangoni effect, as different parameters vary. Specifically, the Marangoni effect induces enhanced mass transport, outperforming pure diffusive flow by over three orders of magnitude, under nighttime isothermal conditions. Furthermore, we provide a semi-empirical equation describing accurately the mass transfer versus the Marangoni number. Hence, nighttime brine discharge simulations show rapid salt reduction from the evaporator, reaching seawater-like salinity levels within two hours, setting stage for optimal daytime performance. To the best of our knowledge, this discharge time is the lowest reported in the literature under equivalent conditions. In conclusion we believe that the still poorly explored Marangoni effect may offer a durable mean of providing freshwater, particularly in emergencies

Farm-waste-derived recyclable photothermal evaporator

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tian, Y., Liu, X., Li, J., Deng, Y., DeGiorgis, J. A., Zhou, S., Caratenuto, A., Minus, M. L., Wan, Y., Xiao, G., & Zheng, Y. Farm-waste-derived recyclable photothermal evaporator. Cell Reports Physical Science, 2(9), (2021): 100549, https://doi.org/10.1016./j.xcrp.2021.100549Interfacial solar steam generation is emerging as a promising technique for efficient desalination. Although increasing efforts have been made, challenges exist for achieving a balance among a plethora of performance indicators—for example, rapid evaporation, durability, low-cost deployment, and salt rejection. Here, we demonstrate that carbonized manure can convert 98% of sunlight into heat, and the strong capillarity of porous carbon fibers networks pumps sufficient water to evaporation interfaces. Salt diffusion within microchannels enables quick salt drainage to the bulk seawater to prevent salt accumulation. With these advantages, this biomass-derived evaporator is demonstrated to feature a high evaporation rate of 2.81 kg m−2 h−1 under 1 sun with broad robustness to acidity and alkalinity. These advantages, together with facial deployment, offer an approach for converting farm waste to energy with high efficiency and easy implementation, which is particularly well suited for developing regions.This project is supported by the National Science Foundation through grant no. CBET-1941743. This project is based upon work supported in part by the National Science Foundation under EPSCoR Cooperative Agreement no. OIA-1655221

Recent strategies for constructing efficient interfacial solar evaporation systems

Interfacial solar evaporation (ISE) is a promising technology to relieve worldwide freshwater shortages owing to its high energy conversion efficiency and environmentally sustainable potential. So far, many innovative materials and evaporators have been proposed and applied in ISE to enable highly controllable and efficient solar-to-thermal energy conversion. With rational design, solar evaporators can achieve excellent energy management for lowering energy loss, harvesting extra energy, and efficiently utilizing energy in the system to improve freshwater production. Beyond that, a strategy of reducing water vaporization enthalpy by introducing molecular engineering for water-state regulation has also been demonstrated as an effective approach to boost ISE. Based on these, this article discusses the energy nexus in two-dimensional (2D) and three-dimensional (3D) evaporators separately and reviews the strategies for design and fabrication of highly efficient ISE systems. The summarized work offers significant perspectives for guiding the future design of ISE systems with efficient energy management, which pave pathways for practical applications

Progress in interfacial solar steam generation using low-dimensional and biomass-derived materials

The pressing concern of escalating water scarcity has spurred the creation of advanced technologies, such as interfacial solar steam generation (ISSG), to tackle the challenge. ISSG employs solar energy for efficient water desalination and purification. This comprehensive review delves into various aspects of ISSG, primarily focusing on elucidating its mechanisms, optimizing substrate materials, implementing thermal management strategies, and exploring applications. The study dissects the intricate mechanism of ISSG, highlighting photothermal behaviors across different materials, including the significant role of nanoparticles in vapor generation. The impact of substrate composition and shape on solar evaporation efficiency is investigated, with multi-surface evaporators considered for environmental energy harnessing. To enhance performance, thermal management strategies, including innovative water transport paths for improved heat distribution, are assessed. Addressing key challenges like salt accumulation, biofouling, corrosion, and oil fouling, the review offers insights for issue mitigation. Practically, ISSG is spotlighted for its role in seawater desalination, wastewater treatment (e.g., dye and heavy metal removal), oil-water separation, and sterilization, extending its relevance across industries and healthcare. By comprehensively examining ISSG's mechanisms, substrate considerations, thermal strategies, and applications, this review advances its implementation as a transformative solution for global water challenges

Development of Sustainable Superhydrophobic Surfaces for Water Manipulation

The thesis focuses on developing sustainable superwettable surface systems for efficient water harvesting and manipulation. Inspired by nature, we developed superhydrophobic surfaces with adjustable hydrophilicity via self-assembly of CNC stabilized Pickering emulsions. These surfaces exhibited outstanding water harvesting performance, and we investigated their applications in continuous water harvesting systems. Additionally, we studied the unique switchable wettability and adhesion properties of smart surfaces that were constructed by pollen particles, triggered by external stimuli, such as temperature. By leveraging these properties, we demonstrated their potential for precise control over water droplet behavior and efficient water transport in solar-driven evaporation systems

Sustainable utilization of desalination concentrate

Lack of water availability is a global crisis. Many arid countries are turning to desalination technologies in order to fulfill their water needs. Hypersaline water, brine, is the byproduct of desalination and can be dangerous to the environment if disposed of in an unsustainable manner. Research surrounding brine management focuses on improved methods of direct disposal, strategies of volume minimization, and reuse strategies. However, the mentioned brine management methods revolve around chemical and mechanical techniques requiring high technological skills, know-how and energy. This thesis aims to find biological solutions that use brine, with minimum resources, in low-cost, low-energy conditions to generate economic value and to minimize the negative effects of brine by attempting to reducing brine salinity. After conducting a thorough literature review, two possible organisms, with the potential of living in brine, algae and Artemia were selected. Different algae species are able to withstand high saline environments and uptake minerals from concentrated solutions; thus decreasing its overall salinity. Artemia sp. thrives in high saline conditions producing cysts of large economic value. This thesis aims to demonstrate that Artemia can live in brine and is a viable method for revenue generation and that the algae species Nannochloropsis sp. can also be adapted to live in brine and uptake nutrients, somewhat decreasing the salinity of brine. The Artemia and algae biomass can be sold, generating additional economic benefits and minimizing the cost of such a system, thus allowing for an economically, socially and environmentally safe way to utilize desalination waste. Marine microalga Nannochloropsis sp. was tested for its salt stress tolerance and salt accumulation capability in mediums of sea salt and brine with different concentrations and nutrients. In sea salt experiments, the alga grew best in salinity 80,000 mg.l-1 with F/2 nutrients where it reached an increase of 4-fold. In brine test BH with F/2 nutrients, excluding the vitamin stock solution and substituting NaNO3 with urea, the alga was able to reach a higher growth of 5-fold. Salt accumulation was minimal and thus will not decrease the TDS of brine. Nevertheless, the optimum conditions for growth of Nannochloropsis sp. in brine were identified, the biomass may be utilized for biofuel production, to generate economic value. Artemia experiments demonstrated the organism’s ability to survive in brine. The Artemia was able to survive for two months in a medium of 100% brine, indicating that a larger brine project may be conducted using this organism to generate economic value