Exceptional water production yield enabled by batch-processed portable water harvester in semi-arid climate

Freshwater scarcity is a critical global issue, with nearly 2 billion people projected to face absolute water scarcity by 2025. Sustainable solutions are urgently needed, particularly in arid regions and emergency situations. Atmospheric water harvesting (AWH), utilizing ubiquitous atmospheric water vapor, offers a potential solution. Among AWH technologies, sorption-based AWH (SAWH) stands out due to its adaptability across various humidity levels. SAWH systems depend heavily on sorbent water sorption capacity. Recent advancements in sorbents, including metal-organic frameworks (MOFs), hydrogels, and salt-based composites, have improved SAWH performance. Salt-based composites, particularly those using lithium chloride (LiCl), offer advantages in terms of cost-effectiveness, ease of synthesis, and broad humidity range applicability compared to MOFs and hydrogels. Despite advancements in sorbent materials, practical SAWH devices often suffer from low efficiency and limited water yield due to simplistic designs. Strategies like thermal insulation, selective solar absorbers, and multi-cyclic operation aim to improve efficiency and yield, but challenges in achieving high and stable water production persist. Active SAWH systems, while offering higher yields, are often too bulky and heavy for portable applications. This research focuses on developing a portable, high-yield, and stable SAWH system suitable for various scenarios.

The literature extensively documents the development of high-performance sorbents for atmospheric water harvesting. MOFs, known for their high surface area and tunable properties, have shown promising results. However, their high cost and complex synthesis limit scalability. Hydrogels, offering high water uptake capacity, are also being explored, but challenges remain in terms of long-term stability and mechanical strength. Salt-based composite sorbents, especially those using LiCl and CaCl2, have emerged as a cost-effective and easily synthesized alternative. These materials leverage the hygroscopic nature of salts and the porous structure of the supporting matrix to achieve high water uptake. Many studies have focused on enhancing the efficiency of SAWH systems by optimizing thermal design, incorporating solar energy, and employing multi-cycle operation strategies. Despite these advances, the total daily water yield in passive systems remains limited, often in the tens of grams, and they exhibit sensitivity to solar energy fluctuations. Active systems, while producing higher yields, lack portability.

This study developed a portable and modular water harvester using LiCl-based hygroscopic composite (Li-SHC) sorbents. Li-SHC was synthesized by impregnating LiCl salt onto active carbon felts. The carbon felt provides a high surface area and capillary force for efficient water uptake. The LiCl content was optimized to balance water uptake capacity and sorption dynamics. The sorbents were encapsulated within a water-vapor breathable but waterproof PTFE membrane to prevent leakage. The water uptake isotherms and dynamic sorption curves of Li-SHC were characterized under various humidity and temperature conditions. A rationally designed batch processing mode was introduced to address the mismatch between water capture (relatively slow) and release (relatively fast) rates. This involves multiple Li-SHC units simultaneously capturing water vapor at night and then releasing it alternately during the day. The portable water harvester design includes an electrical heating plate, a condensation cover, and a heat insulation panel. The heating plate temperature was controlled by a feedback system to optimize desorption. The system's performance was modeled using COMSOL software to optimize the thermal design and minimize convection effects. Finally, the performance of the harvester was evaluated under real semi-arid climatic conditions in Lanzhou, China, by using eight desorption cycles in a single day and analysing collected water quality.

Li-SHC sorbents demonstrated exceptional water uptake capacities of 1.18 g g⁻¹, 1.79 g g⁻¹, and 2.93 g g⁻¹ at 15%, 30%, and 60% RH, respectively. The batch processing mode, combined with the optimized thermal design, enabled the portable water harvester to achieve a remarkable water production yield of 311.69 g day⁻¹ and 1.09 g g_sorbent⁻¹ day⁻¹ in a semi-arid environment with extremely low relative humidity (-15%). The harvester, with a volume of 5.6 L and weight of 3.2 kg, is easily deployable by a single person. The water quality met WHO drinking water standards. The system’s performance was further tested in various simulated climates over six days, demonstrating both stability and adaptability. An analysis of the trade-offs between water productivity, cost, weight, and volume highlighted the device's superiority concerning weight and space.

This study successfully addresses the challenge of achieving high-yield atmospheric water harvesting in semi-arid climates. The combination of high-performance Li-SHC sorbents and the innovative batch processing strategy significantly improved water production compared to existing SAWH technologies. The portable and modular design makes the system practical for diverse applications, including emergency water supply and rural communities. The results demonstrate the feasibility of large-scale, reliable water production from the atmosphere even under challenging environmental conditions. The findings contribute significantly to the development of sustainable water solutions, particularly in regions facing severe water scarcity.

This research demonstrated a high-performance portable water harvester capable of producing a record-breaking amount of water in semi-arid conditions. The combination of optimized Li-SHC sorbents and a novel batch-processing strategy achieved exceptional water production yield. The system's portability, low cost, and ease of use make it a promising solution for addressing water scarcity. Future research could focus on further improving sorbent materials, integrating solar energy for power generation, and exploring advanced thermal management techniques.

The current system relies on electrical heating for desorption, which necessitates a power source. While solar PV systems can be integrated, this adds to the system's complexity and cost. The study focused on a specific semi-arid climate; further testing in diverse climates is needed to fully assess the system’s adaptability. The long-term durability and stability of the Li-SHC sorbents under continuous operation need further investigation.