Advancements in packing materials for humidification-dehumidification desalination systems: a systematic overview

Humidification-dehumidification (HDH) desalination technology, leveraging low-grade waste heat or renewable and sustainable energy sources (e.g., solar, wind, and geothermal energy), represents a promising method for producing freshwater. A significant trend in current research involves the development of hybrid systems that integrate HDH with electricity or energy generation technologies, such as photovoltaics, mechanical vapor compression, and organic Rankine cycles. These hybrid systems aim to simultaneously address water and energy demands. However, challenges remain in scaling these systems for broader applications, particularly concerning cost-effectiveness and maximizing freshwater yield across diverse environmental conditions. Optimizing the packing materials, which serve as the core component where approximately 90% of the heat and mass transfer occurs, is critical for advancing the commercialization of HDH technology. The packing materials within humidifiers play a fundamental role by maximizing water holdup and sustaining mass-transfer capacity through enhanced contact area and prolonged interaction time between water and air streams. This study systematically reviews and classifies the packing materials and structures utilized in HDH humidifiers. Packing materials exhibit substantial diversity, encompassing cellulose papers, plastic packings, fiber-based materials, metal wire mesh, ceramics, wood-based materials, and biomimetic structures. Analysis indicates that structured packings, such as corrugated plates, have dominated long-term HDH applications despite their higher costs and maintenance complexities due to their compact design and superior thermal performance. Material utilization statistics reveal that cellulose-based packings are the most prevalent, followed by plastics and fiber-based materials, collectively accounting for approximately 60% of deployments. Cellulose and wire mesh demonstrate superior humidification performance, while structured corrugated plates and biomimetic configurations are favored for their structural advantages. To further enhance humidification efficiency and overall system performance, future packing designs should not only emphasize high humidification efficiency but also prioritize key characteristics, such as wettability, durability, and environmental sustainability. This review provides a comprehensive classification and overview of packing materials to assist researchers in improving HDH system efficiency and advancing its commercialization potential.