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

Freshwater scarcity is a global challenge, with nearly 2 billion people projected to face absolute water scarcity by 2025. Atmospheric water harvesting (AWH) offers a ubiquitous freshwater source, and sorption-based AWH (SAWH) is highly adaptable, operating across a wide range of relative humidity (10–100% RH). While advanced sorbents such as MOFs, hydrogels, and salt-based composites have improved material-level performance, practical devices often suffer from rudimentary heat/mass transfer designs, low total daily yield, and sensitivity to solar intermittency (for passive systems), or excessive weight and size (for active systems). The study aims to realize a portable, stable, and high-yield water harvester that can achieve hundred-gram-level daily production in semi-arid climates by combining scalable LiCl-based composite sorbents with an operation strategy that addresses the mismatch between sorption and desorption dynamics.

Recent progress in SAWH materials includes MOFs, hydrogels, and salt-based composite sorbents. Salt-based composites (e.g., LiCl, CaCl2 in porous matrices) offer wide RH range uptake, low cost, simple synthesis (impregnation at mild conditions), and scalability, contrasting with MOFs and hydrogels that often require costly materials and complex synthesis. Device-level strategies have improved yields, including thermal insulation (silica aerogels), selective solar absorbers, latent heat recovery, and multicycle operation. Reported outdoor productivities per kilogram of sorbent improved from ~0.07 to >1 L kg−1 day−1 (e.g., 0.7 L kgMOF−1 day−1 in desert climate; 1.05 L kg−1 day−1 with eight solar-driven cycles). However, passive photothermal systems typically deliver only tens of grams per day and are solar-dependent, while active systems are heavier and bulkier. The literature seldom accounts for the dynamic mismatch between slow sorption and fast desorption rates in scaled sorbents, nor the degradation of performance from milligram-scale tests to bulk device conditions, highlighting a gap this work addresses.

Materials and sorbent synthesis: Li-SHC sorbents were fabricated by impregnating active carbon fiber felt (high surface area, hydrophilic, porous channels) with LiCl solutions (0.05–0.45 g mL−1), followed by vacuum treatment (to remove trapped air), mild heating/drying, and encapsulation within a breathable waterproof porous PTFE membrane (3 μm) to prevent solution leakage and corrosion. The target LiCl loading was ~90 wt% to balance sorption capacity and dynamics, based on dynamic vapor sorption tests across 40–95 wt% loadings and linear driving force model fits. Characterization: PXRD confirmed LiCl/LiCl·H2O phases at 30–110 °C. SEM/EDS verified uniform LiCl distribution among fibers and channels. Sorption isotherms at 30 °C showed multi-step uptake: (1) chemisorption below LiCl deliquescence (~11% RH), (2) deliquescence of LiCl·H2O to saturated solution, and (3) absorption into concentrated/saturated LiCl solution (dominant at RH >60%). Dynamic sorption tests were performed at 30%, 60%, and 90% RH for 12 h. Cycling stability was assessed over >186 h without leakage. Bulk-scale performance assessment: To capture realistic device-relevant behavior, a 2-mm-thick, 250×250 mm sorbent (~gram-scale, nearly 10,000× larger than mg-scale samples) was tested in a semi-arid-like chamber (15 °C, 65% RH) with slight airflow for 12 h sorption, followed by desorption on a 110 °C hot plate. Derivative mass-change analysis revealed a strong mismatch between sorption (peak ~0.5 g g−1 h−1) and desorption (~5 g g−1 h−1) rates. Device design and optimization: A portable, modular harvester was built with a 25×25 cm2 electrical heating plate (12 V DC), sorbent container (29×29×2.4 cm, aluminum construction), and a removable roof-shaped condensation cover (37° angle). A feedback control maintained plate temperature within ±2.5 °C. COMSOL simulations guided thermal/fluid design; large natural convection in unblocked cavity regions raised condensation temperatures and reduced temperature gradients. A reflective polystyrene-foam/aluminum-foil heat insulation panel above sorbents mitigated convection and radiative coupling, enhancing condensation temperature drop and water collection. Operation strategy: To reconcile slow sorption vs. fast desorption, a batch-processed alternating mode was implemented. Multiple sorbent pieces were exposed simultaneously overnight to ambient high RH, then desorbed one-by-one during daytime, maintaining high desorption rates and quasi-continuous water production. Eight cycles per day were targeted, each with ~60 min heating at ~110 °C and ~20 min standby. Field testing: Conducted in a semi-arid environment at Xiagouya Mountain, Lanzhou, China (September 2021). Nighttime sorption (20:00–08:00; RH up to ~65%) was followed by daytime desorption cycles (from ~08:00; daytime RH as low as ~15%; ambient ~30 °C). Device volume: 5.6 L; weight: 3.2 kg. Data acquisition used K-type thermocouples, Agilent 34970A, and current transmitters. Water quality was analyzed via ICP-OES and ion chromatography. Additional methods detail: gas sorption (Micromeritics 3Flex), TGA with humidity control, climate chamber tests (KMF-115, Binder).

- Li-SHC sorbent performance: Equilibrium water uptake at 30 °C of 1.18 g g−1 (15% RH), 1.79 g g−1 (30% RH), and 2.93 g g−1 (60% RH); >846% water content at 90% RH in 12 h. Dynamic sorption reaches equilibrium within ~6 h at 30% and 60% RH (mg-scale tests). Sorption capacity largely determined by LiCl; porous matrix mainly facilitates mass transfer/storage. Cycling stability maintained over >186 h without leakage. - Bulk-scale dynamics and mismatch: In realistic semi-arid conditions (15 °C, 65% RH, 2-mm thickness), sorption is slower than mg-scale; desorption on a 110 °C hot plate is much faster. Peak derivative rate mismatch: sorption ~0.5 g g−1 h−1 vs. desorption ~5 g g−1 h−1 (10× difference). Despite slower sorption, bulk Li-SHC releases ~2.37 g g−1 per capture-release cycle. - Device-level performance: Portable harvester (5.6 L volume, 3.2 kg) with thermal optimization and batch-processed alternating mode achieved eight daytime desorption cycles following overnight sorption. Field tests in Lanzhou (semi-arid; daytime RH ~15%, nighttime RH ~65%) produced 311.69 g day−1 total water, corresponding to 1.09 gwater gsorbent−1 day−1 (accounting for all eight sorbent pieces). Each cycle: ~60 min heating, ~20 min standby; condensation surface temperatures ~71→50 °C during cycles. - Energy and cost: Average energy consumption per cycle reduced due to device heat capacity; estimated production cost ~$0.19 L−1, with specific yield ~448.5 mL kWh−1. - Water quality: Collected water met WHO drinking water standards for metals and ions. - Portability and deployment: Lightweight sorbents and modular device allow single-person deployment; device avoids complex auxiliaries via advanced thermal design and operation strategy. - Global potential: Estimated daily production >1000 mL in more humid locations (e.g., Birmingham, UK), ~150 mL in the most arid month in Kharga, Egypt; conservative global mapping suggests >350 mL day−1 in most regions, excluding extreme areas like the Tibetan Plateau and parts of North Africa.

The study addresses the key practical barrier in SAWH devices: the mismatch between slow sorption (ambient, bulk-limited) and fast desorption (thermally driven). By introducing a batch-processed alternating operation, multiple sorbents fully exploit high nighttime RH for uptake and stagger daytime desorption to keep high vapor generation rates, thereby maximizing diurnal yield. Thermal/fluid optimization—particularly suppressing natural convection and radiative coupling with a reflective insulation panel—improves condensation temperature gradients and water collection efficiency. The resulting system achieves hundred-gram-scale daily water production in a real semi-arid climate while remaining portable (5.6 L, 3.2 kg) and low-cost. The findings demonstrate that careful alignment of material properties (high LiCl loading on conductive carbon felt with PTFE encapsulation), realistic bulk-scale characterization, and device-level operation strategies can bridge gaps between material-level metrics and field-scale productivity. The demonstrated WHO-compliant water quality and favorable energy economics support relevance for emergency and off-grid scenarios. The approach scales with available electrical energy, and PV integration is feasible in solar-rich arid regions, supporting stable, weather-agnostic operation compared with passive solar thermal devices.

This work presents a portable, modular water harvester employing scalable, low-cost LiCl-based composite sorbents (Li-SHC) and a batch-processed alternating operation mode to overcome sorption-desorption rate mismatch. Li-SHC delivers high uptake across low to moderate RH, bulk release of ~2.37 g g−1 per cycle, and stable cycling. The optimized device achieves 311.69 g day−1 and 1.09 gwater gsorbent−1 day−1 in a semi-arid field test (daytime RH ~15%), with low energy cost and WHO-compliant water quality. The system’s compactness and modularity indicate strong potential for emergency and rural water supply. Future directions include integrating PV/PVT and thermal storage for all-day harvesting, improving system energy efficiency, and developing next-generation composite sorbents with built-in joule-heating, enhanced thermal conductivity, and tailored porosity to further improve kinetics and reduce losses.

- Performance depends on ambient RH and temperature; yields are lower in extremely arid conditions (e.g., ~150 mL day−1 estimated in the driest month in Kharga, Egypt). - Bulk sorption kinetics are slower than mg-scale measurements due to vapor diffusion and heat transfer limitations; device performance may vary with sorbent thickness, packing, and airflow. - Operation relies on electrical (joule) heating; while PV compatibility is discussed, field performance depends on energy availability and storage for continuous operation. - Field validation was conducted in one semi-arid location and season; broader multi-season, multi-location field trials would strengthen generalizability. - Maximum recommended daily cycles (eight) impose operational constraints (e.g., manual handling, timing), which may affect practicality without automation.