Water is an essential resource for human survival and a primary focus of the United Nations Sustainable Development Goals. Despite this, approximately two-thirds of the global population lacks regular access to freshwater, experiencing water scarcity for at least one month each year.
This scarcity is further exacerbated by pollution of freshwater sources, contributing to the death of a child every two minutes from diseases related to water, sanitation, and hygiene. To mitigate these challenges, new methods for producing freshwater are being explored, with solar steam-based desalination emerging as a promising green technology.
Among various water treatment technologies, solar steam generation has gained significant attention. This method utilizes interfacial solar absorbers to convert solar energy into heat, effectively removing salt from seawater to produce freshwater. By concentrating absorbed energy at the surface, these absorbers minimize heat loss to the bulk water, enhancing efficiency.
Crucially, solar absorbers can operate off-grid, making them suitable for remote areas where potable water access is unreliable. However, a common obstacle to scaling up these technologies is salt crystallization on the solar absorber, which diminishes light absorption and reduces the surface area, thereby hindering continuous water supply.
Researchers in China have addressed the salt crystallization challenge by developing a waffle-shaped solar evaporator (WSE). Constructed from a graphene-like porous monolith via a zinc-assisted pyrolysis process using biomass and recyclable zinc, the WSE incorporates unique design features to enhance performance.
Led by Yanjun Wang and Tianqi Wei from Nanjing University, the team designed the WSE with a basin, ribs, and additional sidewalls—features absent in conventional plane-shaped solar evaporators. These sidewalls induce the Marangoni effect, which involves fluid movement from areas of low to high surface tension, driven by gradients in solute concentration and temperature.
In the WSE, faster evaporation and more efficient heat consumption on the plateaus create solute concentration and temperature gradients. The sidewalls generate a surface-tension gradient, inducing Marangoni flows in the same direction. This dual Marangoni effect increases fluid convection within the device, accelerating salt ion transport and diluting the maximum salinity below the critical saturation point, thereby preventing salt crystallization. As a result, the WSE maintains continuous salt rejection with minimal fouling.
The WSE achieves a solar absorption rate of 98.5% and high evaporation rates of 1.43 kg/m²/h in pure water and 1.40 kg/m²/h in seawater. Outdoor experiments with a prototype WSE treating brine solution demonstrated freshwater production up to 2.81 l/m² per day and continuous operation for 60 days without cleaning.
The WSE effectively addresses three primary obstacles in solar desalination device design: efficient water evaporation, condensation, and salt fouling prevention. Its ability to mitigate salt crystallization, coupled with cost-efficiency, suggests potential for commercial scalability. However, the current evaporation rate is limited by the single-stage evaporator design. Researchers are now focusing on developing a multistage evaporator to enhance solar-to-water efficiency and freshwater yield, aiming to further improve the device's performance.
The innovative waffle-shaped solar evaporator represents a significant advancement in solar steam-based desalination, offering a practical solution to global water scarcity.
