Water scarcity is a global crisis affecting two-thirds of the population. Solar evaporation, particularly thermally localized methods, offers a promising solution for clean water production, desalination, wastewater treatment, and sterilization. However, salt accumulation at the evaporating interface significantly hinders efficiency and device longevity due to the ultralow diffusivity of salt in water compared to vapor in air. This salt accumulation leads to fouling, reduced evaporation rates, and decreased reliability. Existing thermally localized evaporators often employ capillary wick structures, but these are prone to clogging from salt crystallization. Contactless evaporators, while exhibiting superior salt rejection by avoiding wick structures, suffer from lower efficiency due to increased heat loss from the unconfined water. Therefore, a crucial challenge is achieving simultaneous thermal localization and salt rejection. This research investigates a novel approach using a wick-free confined water layer to address this tradeoff, aiming to provide a simple, low-cost solution for high-performance solar evaporation.

Previous research has explored various strategies to improve solar evaporation efficiency and salt rejection. Wick-based systems, while achieving high efficiency, struggle with salt accumulation leading to fouling and reduced performance. Recent work has focused on functional separation, macroscopic pores, and enhanced diffusion within wick structures to mitigate this problem but complete solutions remain elusive. Contactless evaporation, which avoids wicks entirely, shows improved salt rejection but suffers from reduced efficiency due to heat loss. Bio-inspired designs, including those using thin water layers within 3D-printed or hydrophobic structures, have shown promise in enhancing salt rejection. However, a simple and effective method for simultaneously achieving thermal localization and salt rejection remains a challenge due to a lack of understanding of the mechanics of salt transport under these conditions. The key challenges identified are breaking the diffusion-limited salt rejection by inducing passive convective flow and understanding the relationship between thermal localization, water confinement, and convective flow in maximizing salt rejection while minimizing heat loss.

This research introduces a wick-free, self-floating confined water layer structure for solar evaporation. This structure consists of a neutrally buoyant thermal insulation layer with macrochannels connecting the confined water layer to the bulk water below. The design facilitates thermal localization and allows for gravity-driven natural convection to passively remove accumulated salt. The macrochannel design plays a critical role in balancing heat loss and salt rejection. The authors developed a mechanistic model coupling salt transport, fluid flow, and heat transfer to optimize the macrochannel dimensions. The model considers mass and momentum conservation (Navier-Stokes equations) along with convection-diffusion equations for heat and salt transport, accounting for the brine density dependence on temperature and salinity. A uniform heat flux is applied at the bottom to simulate solar heating, and an evaporative flux boundary condition is applied at the water-air interface. The model was validated experimentally. Prototype devices were fabricated using low-cost materials like polystyrene and polyurethane foams. The dynamic stability of the floating structure was tested under extreme displacements to ensure practical operability. Experiments were conducted using a solar simulator and an IR camera to visualize thermal localization. Salinity measurements were performed using a digital refractometer. The effect of natural convection on salt rejection was experimentally verified under isothermal conditions and through visualization using food dye. The simultaneous thermal localization and salt rejection were then demonstrated and compared with models for various brine concentrations (3.5 wt%, 10 wt%, and 20 wt%). The performance of the proposed device was evaluated in the laboratory under controlled conditions and outdoors under real-world conditions, using three configurations: a standard evaporator, an evaporator with a convection cover, and a contactless evaporator.

The study successfully demonstrated a highly efficient and salt-rejecting solar evaporator using a wick-free confined water layer. The key findings are: 1. **High Efficiency and Salt Rejection:** The evaporator achieved >80% solar-to-vapor conversion efficiency and successfully evaporated brine with up to 20 wt% salinity without salt crystallization. 2. **Mechanism Elucidation:** A mechanistic model accurately predicted the observed salt transport behavior, demonstrating the effectiveness of natural convection in accelerating salt rejection. The model revealed an optimal macrochannel diameter range (2.5 mm to 3.5 mm) that balances heat loss and salt rejection, effectively achieving simultaneous thermal localization and salt rejection. 3. **Convection-Dominated Regime:** The study identified a flow regime where the mass Peclet number is greater than 1 (convection-dominated salt transport) while the thermal Peclet number remains less than 1 (conduction-dominated heat transfer). This optimal design enables significant salt rejection while minimizing heat loss. 4. **Experimental Validation:** Experimental results strongly supported the model predictions. Isothermal tests demonstrated rapid salt rejection through natural convection visualized using food dye. Solar evaporation tests showed that the evaporator reached thermal steady state within 30 minutes, achieving a confined water layer temperature of 40°C under one sun illumination. Experiments demonstrated the successful evaporation of brine with various salinities (3.5 wt%, 10 wt%, 20 wt%), with no salt crystallization observed. 5. **Broad Applicability:** The confined water layer structure proved versatile, improving the efficiency of both normal-mode and contactless-mode evaporators. The contactless mode evaporator showed an improved solar-to-vapor conversion efficiency of ~50%. 6. **Long-term Stability:** A week-long reliability test confirmed the long-term stability of the evaporator, with consistent evaporation rates and salinity levels even under continuous, high-flux operation. The experimental results were also validated through outdoor tests, demonstrating that the evaporator functions effectively under real-world conditions. 7. **Low-Cost Design:** The evaporator was constructed using low-cost, readily available materials, making it a potentially highly scalable and affordable solution.

This study successfully addresses the inherent trade-off between thermal localization and salt rejection in solar evaporators. The novel wick-free confined water layer design, coupled with a mechanistic model, offers a practical and effective method for highly efficient and salt-rejecting solar evaporation. The findings demonstrate the significance of engineering passive fluidic flow to enhance salt rejection. The identified convection-dominated regime provides valuable design guidelines for future solar evaporation systems. The broad applicability to both normal and contactless evaporators, combined with the low material cost and long-term stability, makes this technology highly promising for various applications, including desalination and wastewater treatment. The high efficiency achieved (comparable to state-of-the-art wick-based systems) while maintaining excellent salt rejection capabilities represents a substantial advancement in the field.

This work presents a novel wick-free confined water layer structure for highly efficient and salt-rejecting solar evaporation. The design leverages natural convection to overcome the limitations of previous wick-based and contactless systems. The mechanistic model provides crucial design guidance, and experimental results demonstrate exceptional performance in both laboratory and outdoor settings. Future work could focus on further optimization of the design, exploring the impact of material choices, and investigating the potential for integrating this technology with other water purification and desalination methods. The development of this low-cost, highly efficient, and stable system contributes significantly to addressing global water scarcity challenges.

While the study demonstrated excellent performance, certain limitations exist. The model assumes a uniform solar flux, which may not always be the case in real-world conditions. The long-term durability under extremely harsh conditions (e.g., prolonged exposure to high salinity, extreme temperatures, and biofouling) requires further investigation. Although the design uses low-cost materials, the scalability and manufacturing processes for large-scale applications need to be further explored. Finally, the impact of wind on outdoor performance could be more comprehensively studied.