Harnessing Water from Thin Air: The Future of Desiccant Atmospheric Water Harvesters
Imagine being able to generate tens of liters of clean drinking water using nothing but the moisture present in the air. In this article, we’ll explore the science behind desiccant atmospheric water harvesters (AWH), uncovering how they can improve the lives of over 2 billion people worldwide. We’ll examine the remarkable materials capable of absorbing water several times their own weight and compare commercial products with cutting-edge research developments.
In recent years, interest in atmospheric water harvesting has surged. While cooling condensation-based AWH systems are already available, there is a pressing need for more cost-effective and efficient alternatives as freshwater scarcity worsens globally. One promising solution lies in desiccant or sorbent-based AWHs. Unlike traditional condensation-based systems, these units can operate using passive energy sources like solar power. Moreover, they perform effectively even in regions with lower humidity, making them particularly suitable for arid and semi-arid climates where sunshine is abundant, humidity is limited, and water scarcity is severe.
Advanced Materials for Water Absorption
A variety of hygroscopic and desiccant materials are being investigated for their ability to extract water vapor from the air. These include carbon-based sorbents, deliquescent salts, metal-organic frameworks (MOFs), zeolites, hydrogels, and liquid desiccants. While many of these materials are readily available, each requires a unique framework to optimize its properties. Their absorption capability is measured by the maximum water retention relative to their own weight, typically expressed in grams of water per gram of material.
- Silica Gel: Absorbs up to 0.3 times its weight in water. For example, 1 kg of silica gel can retain around 300 grams of water.
- Zeolites: Depending on temperature and humidity conditions, they can retain between 0.3 and 4 times their weight in water.
- Metal-Organic Frameworks (MOFs): Synthetic compounds capable of absorbing up to 0.4 times their weight under optimal conditions.
- Hydrogels: Have the potential to absorb 10 to 100 times their weight in water. However, their ability to capture water vapor directly from the air is limited, requiring them to be used alongside deliquescent salts such as lithium chloride.
The Two-Step Process of Water Extraction
Capturing water vapor in sorbent materials is just the first phase of a two-step process. Once the material reaches saturation, the trapped water must be extracted—a step that typically requires heat. This is where solar energy becomes a game-changer. By heating the sorbent material, the absorbed water is released as vapor, which is then condensed into liquid form. However, the more hydrophilic a material is, the more energy it requires to desorb the captured water.
The choice of sorbent material depends on environmental conditions such as humidity and solar energy availability.
- In high-humidity, low-solar environments, materials with lower water retention perform better.
- In arid regions with high solar energy, sorbents with greater water retention capacity are preferred.
Desorption Temperatures and Efficiency
Different sorbent materials require different temperatures to release absorbed water:
- Silica Gel: Releases water at temperatures between 100–120°C.
- Hydrogels: Loosely bound water is removed at 30°C, but strongly bound water requires heating up to 150°C.
- MOFs (e.g., MOF-801 – Zirconium Fumarate): Highly efficient for water harvesting, capable of operating in environments with just 20% relative humidity. They release water at a much lower temperature range (50–80°C), which can be achieved using solar thermal devices.
Comparing Atmospheric Water Harvesting Technologies

The effectiveness of solar-powered AWH systems is often measured using water mass flux, which refers to the amount of water collected per day per square meter of surface area.

- Passive solar AWH systems: Currently yield between 0.77 and 2.89 liters per square meter per day—significantly lower than cooling condensation-based systems, which can produce around 20 liters per day within the footprint of a typical water dispenser.
- Most sorbent-based AWH devices capture moisture only at night, when relative humidity is higher, and release it during the day when solar energy is available—resulting in a single daily cycle of water extraction.
- Some advanced systems are designed to perform multiple absorption-desorption cycles per day, significantly boosting water yield but requiring continuous energy input.
Comparison of different MOF materials
MOF | Water Uptake | Optimal RH Range | Desorption Temperature | Key Advantage |
MOF-801 | 25% | 20–40% | ~50–80°C | Efficient in low RH conditions |
MOF-303 | 30–40% | 20–50% | ~60°C | Stable, eco-friendly |
MIL-101(Cr) | 40% | 30–80% | ~80–100°C | High adsorption capacity |
The Future: Hybrid AWH Systems
Hybrid AWH devices that combine both active and passive energy sources are expected to offer the highest efficiency. These systems could integrate radiative cooling at night with solar heating during the day, supplementing their operation with active heating and cooling as needed.
For widespread adoption, the efficiency of AWH systems must improve. A study suggests that at 50% relative humidity, an AWH unit should yield at least 4.4 liters of water per day per square meter. At 80% RH, this number should exceed 9 liters per day. While some commercial devices are approaching these benchmarks, they consume excessive energy—using approximately 1 kWh of electricity per 3 liters of water produced, which limits their scalability.

However, there is room for optimism. Research by the Korea Institute of Science and Technology has identified the thermodynamic limits of adsorption-based AWH systems, revealing that in many regions, it’s theoretically possible to harvest over 100 liters of water per square meter per day. Achieving even one-fifth of this potential could significantly alleviate the global water crisis.

Conclusion: A Path Toward a Water-Secure Future
The field of atmospheric water harvesting is rapidly advancing, and its potential to address global water shortages is immense. With continued research and innovation, we can create efficient, scalable solutions that provide clean drinking water to communities worldwide. By working together, we can build a future where access to freshwater is no longer a challenge.