Reliable access to safe drinking water is a critical global challenge, recognized as a priority in the UN Sustainable Development Goals. Traditional approaches to provide safely managed drinking water (SMDW) – defined as improved, on-premises sources, available when needed, and free from contamination – are costly, estimated at US$114 billion annually. Bottled water presents environmental concerns, while household-level interventions face adherence challenges. Atmospheric water harvesting (AWH) presents a promising alternative, especially if a cost-effective, off-grid device can be developed and scaled. Existing AWH designs include passive devices (relying on dew or fog) and active devices (using external energy). Active, sorbent-based AWH devices, specifically continuous-mode devices, are particularly promising because they reduce sorbent mass and device size, but their effectiveness at lower RH levels remains a question. This study assesses the global potential of solar-driven, continuous-mode AWH (SC-AWH) using global climate data to determine the feasibility and impact of this technology in addressing the global water crisis.

The literature reveals a range of AWH device types, categorized by energy source and operational mode. Passive devices are limited geographically, while active devices, including sorbent-based and cooler-condenser designs, offer broader applicability. Sorbent-based devices, operating in diurnal or continuous modes, differ in their energy consumption and water output. Cooler-condenser devices actively cool air below its dew point but suffer energy conversion losses. Previous assessments of AWH have been limited in scope, focusing on specific locations or regions, or employing limited datasets. This study addresses this gap by using global climate data and a newly developed geospatial tool to analyze the potential of SC-AWH worldwide.

This research employs a geospatial tool, AWH-Geo, built in Google Earth Engine, to assess the global potential of SC-AWH. AWH-Geo uses the ERA5-Land climate reanalysis (2010-2019) at a 9km resolution to account for interannual variability in global horizontal irradiance (GHI), relative humidity (RH), and air temperature. The tool takes as input the instantaneous rate of water output as a function of GHI, RH, and temperature. Water output can be entered as areal harvesting rates or expected yield of a real device. The workflow involves mapping the distribution of people without SMDW using WorldPop data and JMP data on drinking water service levels. AWH-Geo then maps theoretical upper bounds of solar-driven AWH using SY (liters per kilowatt-hour) data from literature, including thermodynamic limits from Kim et al. and performance data for cooler-condensers and sorbent devices (MOFs and TRPs). The analysis assesses the mean daily water output, operational hours per day (OPH), and the impact on the population lacking SMDW, considering various parametric thresholds for GHI and RH. The study further examines seasonal variability and explores trade-offs between device performance and reach using target curves representing different numbers of users and device sizes.

The study finds that SC-AWH has significant global potential. Mapping reveals considerable water production potential, especially in tropical regions. Analyzing the coincidence of sunlight and humidity above various parametric thresholds highlights key spatio-demographic patterns, particularly in the tropical savannas of sub-Saharan Africa and the Ganges River Valley in India. A device operating above thresholds of 30% RH and 600 W/m² GHI for 3-5 hours daily could theoretically serve over half the world's population lacking SMDW. Comparison of different device types (cooler-condensers and sorbent-based) and materials (MOFs and TRPs) shows that a 1m² device with a SY profile of 0.2-2.5 l/kWh (0.1-1.25 l/kWh for a 2m² device) could meet the SMDW needs of approximately 1 billion people, assuming 2-3 hours of daily operation under sufficient sunlight and RH. The shape of the SY curve is critical; a linear profile prioritizes low-RH performance, while a logistic profile maximizes yield at high RH. Advanced sorbent materials like TRPs show the most promise for achieving large-scale impact. Seasonal variability analyses are provided but not detailed here, showing that shortfalls might be addressed through water storage or alternative sources.

This study demonstrates that daytime climate conditions are sufficient for continuous-mode AWH operation in regions with the highest need. Focusing device design on maximizing impact and reducing costs is crucial. The findings address the research question by quantifying the potential of SC-AWH to provide SMDW on a global scale. The significance lies in showcasing the potential of a decentralized, sustainable, and off-grid solution to a major global challenge. The results are relevant to the fields of water resource management, renewable energy, and sustainable development, offering a pathway towards achieving SDG 6.1. The use of a novel geospatial tool allows for a more comprehensive assessment compared to previous studies.

This study provides a comprehensive assessment of the global potential for solar-driven, continuous-mode atmospheric water harvesting to provide safely managed drinking water. A 1m² device with appropriate SY profiles could serve the needs of a billion people. Future research should focus on optimizing device design for various climatic conditions, developing cost-effective manufacturing processes for advanced materials, and conducting user-centric research to ensure wide adoption and effective integration into existing water access practices.

The study relies on modeled climate data and hypothetical device performance. Real-world device performance may vary due to factors not considered in the model, such as maintenance, dust accumulation, and material degradation. The analysis assumes a constant daily water requirement per person, which may not reflect actual consumption patterns. Furthermore, the study focuses solely on drinking water and doesn't consider other household water uses. The hydro-ecological impact is assessed as negligible, but more detailed studies may be needed to address local environmental effects.