Abstract:

Objective The severe water crisis has become a critical problem for human development because of the rapidly expanding population and water contamination. Considerable promising but energy-intensive devices such as membrane distillation, electrodialysis, and reverse osmosis have been expended for collecting clean water. Solar steam generation (SSG) is a low-carbon, cost-effective, and portable solution for desalination and purification. During SSG, the bulk brine is transported to the evaporation interface through the water supply material, and the heat generated by the photothermal conversion is concentrated on desalination. Conventional devices employ a thermal concentration strategy to reduce heat loss, but has theoretical efficiency limits. Therefore, an extra ambient heat harvest solution for promoting stereoscopic multi-stage evaporation through open thermal management is proposed, aiming to enhance energy utilization efficiency.

Method In this work, carbon black (CB) particles with broadband solar absorption properties were selected as the photothermal materials, and cotton fabrics/rods with outstanding hydrophilicity and flexibility were used as the substrate for the water supply. After functionalizing treatment and structural engineering, the 3-D carbon black cotton evaporator (CBC) was constructed to absorb ambient heat via heat transfer for cold evaporation from the water supply layer. The particle distribution on the fiber surface became gradually denser as the CB loading increased. Apparently, the CBC with increased loading gradually displayed a darker black color. The solar absorption performances of CBCs were also progressively improved with the increase in loading concentration. The solar absorption rate of CBC-1.0 was up to about 95%, the solar absorption would not obviously enhance under further loading with higher concentration. As the loading concentration increased, the CB particles on the CBC-2.0 surface were bonded into clumps and almost covered the fiber web channels, and the redundant loading led to weakening of the moisture permeability and air permeability. CB particles were bonded with fabrics through hydroxyl group interaction. As for fabrics without thermoplastic polyurethane (TPU) loading, the CB particles were easy to fall off under ultrasonic treatment and rubbing.

Results After the loading of TPU, the CB particles were firmly encapsulated inside. The physical friction with the environment was alleviated and the adhesion of the fabric to the CB particles became stronger. The TPU-loaded reinforced CBC remained strong even after overnight ultrasonication or 100 rubbing cycles. The hydrophilicity of the CBC guaranteed rapid water transport and circulation. The hydrophilic groups on the surface of CB particles would greatly enhance the hydrophilicity of the fabric and the water droplets were quickly absorbed after contacting the fabric surface as analyzed by the water contact angle test. The hydrophilicity of the fabric was not significantly weakened even after the TPU was coated. The bulk water at the bottom wetted the top surface of cotton rods within 10 s due to the strong water-wicking effect, ensuring sufficient water supply during the evaporation process. Under 1 kW/m², the evaporation rate reached a maximum value of 1.41 kg/(m²·h) for CBC-7-0 due to the dynamic equilibrium between water supply and heat input. The 3-D CBC system was constructed and designed to further promote the utilization of ambient energy, thereby enhancing the thermal management capability of the system. The evaporation surface area of the 3-D system was significantly increased, the heat convection and heat radiation losses were significantly reduced, and the bottom cotton rods could utilize the extra ambient heat for cold evaporation. Compared with the planar system, the energy utilization efficiency of 3-D CBC was optimized and enhanced. The CBC-7-4 system exhibited remarkable evaporation performance (1.80 kg/(m²·h), 97.1%) through load optimization and energy balancing. After 15 cycles of operation, the desalination rate remained over 1.73 kg/(m²·h).

Conclusion The stable desalination performance proves that the excellent hydrophilic cotton rods and CBC ensure rapid water circulation. The salt nucleation rate is less than the water replenishment rate thus avoiding salt clogging during desalination. The purified water meets the standard of healthy drinking water, which exhibits a broad prospect. Overall, the CBC system with open thermal management provides a viable solution for green, efficient, and durable desalination.

Key words: photothermal conversion, seawater desalination, cold evaporation, thermal management, fabric