Curved Carbon Fabric Evaporator Turns Sunlight and a 3-Volt Jolt into Fresh Water

Fresh water scarcity is one of the defining challenges of the coming decades, and one of the most elegant proposed solutions is also one of the simplest: float a dark, water-loving material on the surface of seawater, let the sun heat it, and collect the vapor that rises. This approach, known as solar interfacial evaporation, promises desalination without the massive energy budgets of reverse osmosis or thermal distillation plants. Yet the technology has a stubborn Achilles heel. When the sun sets or clouds roll in, the evaporation stops, and the salt that accumulates on the evaporator surface can choke the whole process. A new study from researchers at Pohang University of Science and Technology in South Korea offers a way to keep the water flowing around the clock, and in doing so reveals a surprisingly overlooked design variable that can make or break performance.

Writing in Environmental Science and Pollution Research, Younghoon Suh and Sang Joon Lee describe a curved, fabric-based evaporator that combines two heating mechanisms: sunlight and electricity. The device is built from carbon fabric that has been surface-treated to become strongly hydrophilic, meaning it draws water into its fibers through capillary action. The treated material is remarkable in its own right. It absorbs an incoming water droplet in less than 100 milliseconds, so fast that the fabric essentially never appears dry, and it captures more than 85 percent of incident light across an extraordinarily broad spectrum spanning 200 to 2500 nanometers, from ultraviolet through the visible and deep into the infrared. That broadband absorption is exactly what a photothermal material needs, because it converts as much of the solar spectrum as possible into heat right at the water surface rather than letting it reflect away.

The key structural innovation is the geometry. Instead of laying the fabric flat on the water, the team draped it over two blocks of melamine sponge, creating an arched, tunnel-like shape. This curved configuration does two clever things at once. First, it exposes both faces of the fabric to the air, effectively doubling the evaporation area relative to the footprint of the device. Second, it allows water to be supplied from both ends of the fabric simultaneously, so the capillary wicking that feeds the hot central zone has two sources rather than one. In flat evaporators, the distance that water must travel from the reservoir to the center of the hot zone is often fixed by the device size, and engineers have rarely treated it as a tunable parameter. Suh and Lee made it the centerpiece of their study.

That tunable parameter is the span distance, the gap between the two sponge supports. The researchers fabricated evaporators with span distances ranging from 3 to 6 centimeters and systematically compared their performance. The physics involved is a delicate three-way trade-off. A wider span means more of the fabric is suspended in the arch, exposed to light and air, which increases the effective heating and evaporation area. But a wider span also means water must travel farther through the fabric’s capillary network to reach the center, and if the supply cannot keep pace with evaporation, the hottest and most productive zone dries out, its temperature climbs without producing vapor, and efficiency collapses. A narrower span keeps water close at hand but wastes potential evaporation area on the submerged or supported portions of the fabric.

The experiments showed that the sweet spot sits at a span distance of 4 centimeters. At this width, the balance among solar absorption, Joule heating, and capillary water transport is optimized: the fabric’s central zone stays wet and cool enough to evaporate water continuously while maximizing the illuminated area. The team then pushed the optimized device, an 8 by 2 centimeter strip of treated carbon fabric, under combined illumination equivalent to one sun and an applied voltage of just 3 volts. The electrical contribution matters because it provides Joule heating, the same resistive warming that makes a toaster glow, which can operate independently of weather. Under these dual-drive conditions, the evaporator achieved an evaporation rate of 7.81 kilograms of water per square meter per hour using deionized water, and 7.23 kilograms per square meter per hour using artificial seawater at 3.5 weight percent salinity, which closely matches the salt content of the real ocean.

Those rates are impressive by the standards of interfacial evaporation, but laboratory performance can be misleading. Real desalination devices must survive repeated exposure to saline water without fouling, and salt accumulation is the classic failure mode: as water evaporates, dissolved ions concentrate at the evaporation front and crystallize, clogging pores and degrading performance. To test durability, the researchers ran five consecutive evaporation cycles using the same specimen with artificial seawater, rinsing the fabric with water between cycles. After all five rounds, the device retained approximately 99 percent of its initial evaporation rate. That near-perfect recovery suggests that the curved, double-fed water supply helps flush salts away from the critical zones and that simple rinsing is enough to restore full function, a practical advantage for real-world deployment where maintenance must be cheap and easy.

The team also took the device outdoors, which is where many promising evaporators falter. Under natural sunlight, the optimized evaporator produced water collection rates above 3 kilograms per square meter per hour. More importantly for the goal of producing drinkable water, the researchers measured the concentrations of major ions in the collected condensate. The results were striking: the concentrations of the major dissolved ions were reduced by three to four orders of magnitude relative to the feed seawater, meaning the collected water was dramatically purer than the source. While the study reports ion reduction rather than a full potability certification, a four-order-of-magnitude drop in major ion content puts the condensate in a range that conventional post-treatment could readily finish, or that could serve for agriculture and other non-potable uses in water-stressed regions.

What makes the study resonate beyond its specific numbers is the framing of span distance as a first-class design parameter. In the growing literature on photothermal fabrics, hanging evaporators, and 3D structures, researchers have explored materials ranging from graphene architectures to MoS2 aerogels to hierarchical nanowire coatings, often chasing marginal gains in absorption or thermal localization. Suh and Lee’s results suggest that for curved geometries, simply adjusting the mechanical span of the same material can shift the balance between water supply and heat input enough to dominate performance. The finding is a reminder that in interfacial evaporation, transport physics, how fast water wicks toward the hot zone, how far it must travel, and how the wetted area interacts with the illuminated area, can matter as much as the choice of nanomaterial.

The photoelectrothermal strategy also addresses the fundamental limitation of solar-only systems: intermittency. Because the carbon fabric is electrically conductive, applying a modest 3 volts keeps the evaporation surface warm during darkness or overcast conditions, allowing continuous operation. Previous work by other groups has explored Joule heating in graphene sponges, braided fabrics, and nickel foam-based evaporators, and the POSTECH team’s earlier studies included heliotropic solar steam generators assisted by Joule heating. The new work integrates that dual-mode heating into the curved, double-fed architecture and, crucially, identifies the geometric optimum that lets both heating modes and the capillary supply work in concert rather than against one another.

The research, funded by a National Research Foundation of Korea grant, points toward evaporator designs that are cheap, robust, and operable day and night. Carbon fabric is an inexpensive, flexible, and scalable substrate compared with many engineered nanomaterial platforms, and the melamine sponge supports are commodity materials. The demonstration that a single specimen can endure repeated saline cycles with nearly complete performance recovery, combined with outdoor water collection above 3 kilograms per square meter per hour and ion removal of three to four orders of magnitude, makes a credible case that curved photoelectrothermal evaporators deserve attention as practical desalination units. For coastal communities facing brackish or saline groundwater and unreliable grid power, a device that harvests sunlight when available and switches to a few watts of electricity when it is not could be a meaningful step toward water security. The lesson from Pohang is that sometimes the breakthrough is not a new material at all, but knowing exactly how far to stretch it.

Subject of Research: Optimization of curved photoelectrothermal carbon fabric evaporators for solar- and electrically driven seawater desalination

Article Title: Optimization of span distance in photoelectrothermal-based curved-type hydrophilic carbon fabric evaporators for seawater desalination

Article References: Suh, Y., & Lee, S. J. (2026). Optimization of span distance in photoelectrothermal-based curved-type hydrophilic carbon fabric evaporators for seawater desalination. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38284-9

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38284-9

Keywords: seawater desalination, solar interfacial evaporation, photoelectrothermal, carbon fabric, Joule heating, capillary water transport, span distance, curved evaporator, hydrophilic surface, salt resistance, water purification, photothermal conversion