Global potential for harvesting drinking water from air using solar energy

Ensuring reliable access to safe drinking water is a global priority under SDG 6.1, yet 2.2 billion people lack safely managed drinking water (SMDW). Traditional approaches to provide on-premises safe water are costly and logistically challenging, and reliance on bottled water has environmental and equity drawbacks. Atmospheric water harvesting (AWH), particularly solar-driven systems, offers a decentralized, potentially off-grid solution if devices can achieve sufficient yields under daytime conditions when solar energy is available. This study investigates whether solar-driven, continuous-mode AWH (SC-AWH) can meet household drinking water needs globally by mapping climate suitability, quantifying potential outputs, and relating device performance targets to the spatial distribution of people without SMDW.

The paper reviews AWH categories: passive (dew/fog) systems that require favorable microclimates and are geographically limited, and active systems that use external energy. Among active systems, sorbent-based devices can operate in diurnal single-cycle or continuous modes, while cooler-condenser devices use electrical work, often from PV, to cool air below the dew point. Prior analyses examined limited locations or specific regions and device types, and highlighted low specific yields at daytime RH, raising doubts about performance. Thermodynamic limits (Kim et al.) set upper bounds on achievable SY (approximately 5–50 l kWh−1), and device-level studies (e.g., Peeters for cooler-condensers; Zhao for thermo-responsive polymers; MOF-based harvesters) provide benchmarks across RH ranges. However, a comprehensive, spatially continuous global assessment of SC-AWH potential had been lacking, motivating this study.

The authors developed a geospatial assessment tool (AWH-Geo) in Google Earth Engine to estimate SC-AWH potential globally. Climate inputs are from ERA5-Land reanalysis at ~9 km spatial resolution and hourly temporal resolution over 2010–2019. AWH-Geo ingests output tables defining water yield as a function of binned environmental variables: global horizontal irradiance (GHI, W m−2), relative humidity (RH, %), and air temperature (°C). Output can be expressed as areal harvesting rates (l h−1 m−2) or expected device yields (l h−1) and is aggregated across the multiyear time series to produce maps of mean daily output and variability statistics (including optional P90 availability metrics). Population in need was mapped by applying WHO/UNICEF JMP regional proportions of population without SMDW to WorldPop (2017) 1 km population counts, yielding a weighted global distribution of people lacking SMDW at 1 km resolution. Analyses performed: - Theoretical upper bounds: Mapped thermodynamic SY limits (5–50 l kWh−1) and maximum outputs for design classes using literature-based SY profiles (e.g., cooler-condensers per Peeters; sorbents such as thermo-responsive polymers and MOFs), overlaying population need. - Coincidence analysis: Computed mean daily operational hours per day (ophd) when GHI and RH exceed paired thresholds (e.g., GHI >400 or >600 W m−2 with RH >10/30/50/70%), producing global maps and cumulative population curves by ophd. - Impact-weighted performance: Ran collections of specific-yield (SY) curves for existing devices (e.g., cooler-condensers evaluated by Bagheri; SOURCE panel specs) and hypothetical target SY curves through AWH-Geo. Outputs were normalized by horizontal collection area (l d−1 m−2) to relate performance to the number of people without SMDW potentially served at specified daily drinking water output (5 l d−1 per person). Device area scaling (1 m2 vs 2 m2) was analyzed to show linear scaling of output and halving of SY requirements when area doubles. Assumptions: SC-AWH is considered for drinking water only (not replacing other domestic uses). Device performance targets focus on continuous-mode operation during coincident sunlight and humidity windows. Results were summarized as annual means, with seasonal/diurnal variability discussed qualitatively and in extended data.

- Potential scale: AWH could provide safely managed drinking water for about 1 billion people under plausible device performance. A hypothetical 1 m2 SC-AWH with a specific-yield (SY) profile of 0.2–2.5 l kWh−1 across 30–90% RH (or 0.1–1.25 l kWh−1 for a 2 m2 device) can meet an average drinking water target of 5 l person−1 day−1. - Geographic potential: Thermodynamic-limit and device-class projections indicate substantial output potential across much of the world, especially the tropics, aligning with regions where two-thirds of people without SMDW live. - Coincidence of resources: Coastal regions commonly provide 2–4 operational hours per day (ophd) with RH >50%. Operating below 30% RH yields only marginal gains (1–2 h) in arid zones compared with 30% RH, showing diminishing returns at very low RH. Key threshold ranges (RH 30–50%, GHI 400–600 W m−2, ophd 3–5 h) correspond to climatic transitions where the majority of unserved populations live (e.g., sub-Saharan savanna, Ganges valley). Devices operating above these thresholds could theoretically serve more than half of the remaining population lacking SMDW. - Device and material performance: Existing commercial cooler-condensers and some sorbent devices show approximately linear SY vs RH, underperforming target curves for reaching ≥0.5–1.0 billion users. Thermo-responsive polymer gels (TRP) exhibit high yields at high RH (logistic profile), offering the best promise to reach up to 2.0 billion users when scaled. Doubling device area from 1 m2 to 2 m2 halves the SY required to hit target impacts at a given RH. - Environmental impact: Supplying all 2.2 billion people at 10 l d−1 would require ~8 km3 yr−1, only ~0.20% of net global cropland water extraction (4,000 km3 yr−1) and ~0.01% of terrestrial evapotranspiration (65,500 km3 yr−1), indicating negligible hydro-ecological impact. - Cost and manufacturability: SC-AWH architectures can be simple and potentially low-cost; achieving cost targets likely requires mass manufacturing of advanced sorbents (e.g., MOFs, TRP), for which high-volume methods are emerging. - Variability: Seasonal and diurnal variability can be substantial; shortfalls may necessitate storage or supplemental sources (e.g., rainwater), especially in monsoon climates.

The study demonstrates that daytime climatic conditions—specifically the overlap of sunlight and moderate humidity—are sufficient in many high-need regions to enable continuous-mode solar-driven AWH. By linking device-specific yield profiles to spatially resolved climate and population need, the analysis translates engineering targets (SY vs RH, collection area) into human impact metrics (people served at 5 l d−1). The results highlight the importance of the SY curve shape: achieving higher yields at moderate-to-high RH (logistic profiles, as seen with TRP gels) maximizes impact in humid tropical regions, while improving performance at lower RH benefits populations in climate transition zones (e.g., Sahel, western India). Device area linearly scales output, providing a straightforward pathway to relax SY requirements. Findings reinforce that SC-AWH could complement, not replace, other water sources, with storage or seasonal switching mitigating variability. Socioeconomic and behavioral factors (affordability, financing, user acceptance, and safe storage) are critical for real-world adoption. The negligible atmospheric water budget impact supports sustainability at scale. Overall, the analysis closes a key knowledge gap by providing a global, spatially continuous assessment that can guide design trade-offs and R&D priorities for maximum impact.

This work introduces AWH-Geo and delivers the first global, spatially continuous assessment of solar-driven, continuous-mode atmospheric water harvesting potential tied to population need. It shows that a 1 m2 device with an SY profile of 0.2–2.5 l kWh−1 across 30–90% RH, operating 2–3 h d−1 at GHI >600 W m−2 and RH >30%, could provide 5 l d−1 of drinking water for around 1 billion people. The study quantifies climatic thresholds and highlights material/device profiles (e.g., TRP gels) most likely to achieve large-scale impact within thermodynamic limits, while emphasizing linear scaling with device area. Future research should: (1) validate device performance in diverse outdoor field conditions with published output tables; (2) refine device architectures to improve SY at key RH ranges and reduce losses; (3) develop cost-effective mass manufacturing of high-performance sorbents; (4) incorporate additional climate variables and longer-term trends; and (5) integrate socio-economic adoption studies, financing mechanisms, and safe storage practices into deployment strategies.

- Metrics emphasize long-term mean outputs; seasonal, weekly, and diurnal variability can affect reliability and adoption. Periods of shortfall, especially in monsoon climates, may require storage or alternative sources. - Results rely on reanalysis climate data (ERA5-Land) and a deterministic mapping of JMP proportions onto WorldPop at 1 km; uncertainties in these datasets and assumptions could affect spatial accuracy. - Device performance inputs are based on literature SY profiles and hypothetical targets; real devices will incur additional losses and may underperform laboratory/material benchmarks. - The analysis focuses on drinking water and does not address other domestic water needs or comprehensive techno-economic assessments (costs, maintenance, supply chains). - Cooler-condenser results involve PV conversion assumptions; performance may vary with technology choices and local conditions.