Extending Atmospheric Water Harvesting Beyond Daylight Through Thermal Energy Storage
- Amin Mojiri
- 2 hours ago
- 6 min read
Original Authors: Weicheng Chen, Yangxi Liu, Mingyun Luo, Yuxuan Tan, Jinze Yao, Bingzhi Chen, Zhixuan Chen, Muthusankar Ganesan, Xiaolong Zhao, Ci Lin, Tingting Qin, Yutang Fang, Shuangfeng Wang, Wanwan Fu, Bingqiong Tan, Ting Zou, Yanshu Luo, Sai Kishore Ravi, Dennis Y. C. Leung
Original paper is accessible at: https://doi.org/10.1002/adfm.202516624
Why nighttime operation matters in atmospheric water harvesting
Solar-driven atmospheric water harvesting has made important progress, particularly through the use of sorbents such as metal–organic frameworks (MOFs) that can release captured water at relatively low temperatures. However, most solar-powered systems still depend directly on daylight for desorption.
This creates a major operational limitation: during nighttime, when solar energy is unavailable, desorption stops. As a result, a large portion of the day remains underutilized, limiting total daily water production.
Electrical or thermal heating can overcome this limitation, but such approaches introduce ongoing energy costs and may be impractical in remote or economically disadvantaged arid regions.
This study addresses the problem by asking a different question:
Can solar energy collected during the day be stored and later used to produce water at night?
A thermal battery for nighttime AWH
The researchers developed a water-generation unit that combines two separate components:
a MOF-303 sorption layer for atmospheric moisture capture
a composite phase-change material thermal battery for solar-energy storage
During the day, the thermal battery absorbs sunlight and stores heat through a solid–liquid phase transition. At night, the MOF layer first captures water vapor from the atmosphere. The charged thermal battery is then brought into direct contact with the MOF layer, transferring stored heat and driving water desorption.
This operating sequence separates solar-energy collection from water release and allows the system to produce water even after sunset.
Design of the composite phase-change material
The thermal battery is based on sodium acetate trihydrate, CH₃COONa·3H₂O, as the phase-change material.
Because the pure material has limited light absorption and low thermal conductivity, it was combined with expanded graphite to form a composite phase-change material, or CPCM.
Expanded graphite performs three functions:
enhances photothermal conversion
increases thermal conductivity
provides a porous structure that retains the molten phase-change material and prevents leakage
The optimized composite contained 16% expanded graphite, which maintained structural stability at temperatures up to 70–85°C.
Thermal-storage performance
The pure phase-change material exhibited:
a phase-change enthalpy of 256.90 J/g
a phase-change temperature of 57.40°C
After incorporating 16% expanded graphite, the resulting CPCM retained:
a phase-change enthalpy of 215.40 J/g
a phase-change temperature of 57.16°C
The enthalpy decreased by approximately 16.15%, closely corresponding to the expanded graphite content, while the phase-change temperature changed by only 0.24°C.
This temperature range is well aligned with the regeneration requirements of many MOF-based sorbents, including MOF-303, which typically desorbs water at approximately 40–60°C.
The thermal conductivity increased from approximately:
0.512 W·m⁻¹·K⁻¹ for the pure PCM at 200 kg/m³
to:
2.612 W·m⁻¹·K⁻¹ at 200 kg/m³
4.674 W·m⁻¹·K⁻¹ at 500 kg/m³
5.077 W·m⁻¹·K⁻¹ at 600 kg/m³
This substantial improvement was essential for transferring heat rapidly from the thermal battery to the MOF layer.
Photothermal charging during the day
Pure PCM absorbed sunlight poorly and reached only about 43°C after one hour under 1 sun, which was insufficient to complete phase transition.
In contrast, the expanded-graphite composite showed strong absorption across the UV, visible, and near-infrared regions. Under 1 kW/m² solar irradiation, the CPCM reached approximately:
84°C after 1.5 hours
Its photothermal conversion efficiency as a powder was calculated at:
66.48%
When the CPCM was compressed into a dense thermal-battery block, performance improved further. The complete thermal battery achieved a photothermal conversion efficiency of:
87.99%
This was 21.51% higher than the powder-form CPCM because the compact structure enabled faster heat transfer into the deeper layers of the battery.
Reduced supercooling and controlled heat release
Phase-change materials can suffer from supercooling, in which stored heat is released at a temperature much lower than the desired phase-change point.
The authors reduced the supercooling degree to only:
3.0°C
by adding a small amount of sodium phosphate and using the heterogeneous nucleation effect of expanded graphite.
This is important because MOF desorption requires heat to be released within a specific temperature range. Excessive supercooling could reduce desorption efficiency or prevent crystallization and heat release altogether.
MOF-303 adsorption under nighttime humidity
MOF-303 was selected because of its strong hydrophilicity, hydrothermal stability, and ability to adsorb water at low relative humidity.
At 25°C, the MOF layer approached equilibrium within:
90 min at 20% RH
60 min at 30% RH
40 min at 40% RH
The corresponding equilibrium water uptakes were:
0.4075 g/g at 20% RH
0.4144 g/g at 30% RH
0.4254 g/g at 40% RH
The study notes that nighttime adsorption may be kinetically favorable because nighttime relative humidity is often higher than daytime humidity in arid environments.
Rapid nighttime desorption
After daytime charging and nighttime adsorption, the thermal battery was placed in direct contact with the MOF layer.
The MOF temperature rapidly increased to approximately:
55°C within 0.10 h, or about 6 minutes
The efficient heat transfer resulted from:
direct contact between the components
the high thermal conductivity of the CPCM
the stainless-steel MOF container
the thin 0.5 mm MOF layer
Within 30 minutes, the MOF water uptake decreased from:
0.4075 g/g
to:
0.0884 g/g
This means the system released:
more than 78% of the captured water in 30 minutes
The desorption performance was faster than direct one-sun heating of the MOF layer because MOF-303 itself has relatively weak photothermal conversion.
Potential nighttime water productivity
The researchers proposed a cycle consisting of:
30 minutes of atmospheric adsorption
30 minutes of thermal-battery-assisted desorption
Over the 12-hour period from 18:00 to 06:00, the system could theoretically complete:
12 adsorption–desorption cycles
The calculated theoretical nighttime water release was:
2.5–3.8 g water per g MOF per day
The upper value of:
3.8 gwater/gMOF/day
represents the maximum theoretical nighttime output under the tested assumptions.
The authors acknowledge that actual production may be lower because residual water remains in the MOF after each cycle and sufficient charged thermal-storage material must be available for repeated desorption.
A 171% improvement over daytime-only operation
The nighttime thermal-storage strategy produced a reported:
171% improvement
over continuous daytime operation using a pure MOF layer under one-sun illumination.
This result demonstrates that thermal-energy storage can substantially extend the operating window of AWH systems rather than merely shifting the timing of water production.
From two-dimensional to three-dimensional desorption
Traditional solar-driven desorption requires sorbents to be directly exposed to sunlight. This limits the amount of sorbent that can be deployed within a given illuminated area.
The thermal-battery approach removes this restriction. Because stored heat—not direct sunlight—drives desorption, multiple water-generation units can be vertically stacked and operated simultaneously.
The authors describe this transition as a shift from:
two-dimensional solar desorption
to three-dimensional spatially parallel desorption
A custom multilayer system demonstrated an estimated output of:
≈1.95 gwater/gMOF/day
This is an important system-level advantage because it improves spatial utilization and creates opportunities for compact decentralized water generators.
Heat-transfer design controls performance
Thermal imaging and numerical simulations showed that the success of the system depends strongly on efficient heat transfer between the thermal battery and the MOF layer.
The experimental and simulated MOF temperatures differed by:
less than 3% overall
less than 0.7% during phase change
The simulations identified several important design parameters:
greater phase-change enthalpy increases stored energy
higher CPCM density increases volumetric energy storage
higher CPCM thermal conductivity accelerates heat delivery
higher MOF-layer thermal conductivity improves heat absorption
direct contact between the thermal battery and sorbent is critical
low-conductivity insulation around the thermal battery minimizes heat loss
Introducing even a small air gap weakened thermal conduction and significantly reduced the MOF temperature during desorption.
Practical limitations and opportunities
The system is best suited to arid regions with:
low nighttime humidity constraints
strong daytime solar radiation
limited access to grid electricity
In high-humidity but low-sunlight regions, thermal charging may be insufficient to raise the CPCM above its phase-change temperature. The authors suggest using solar concentrators to improve charging under weak-light conditions.
The concept could also be adapted to other MOFs. The paper identifies:
MOF-801 for sub-30% RH environments
Al-Fum for conditions above 30% RH
MIL-101(Cr) for conditions above 40% RH
These material choices could allow the thermal-storage platform to be tuned for different climates.
Key findings
Solar energy can be stored during the day and used for nighttime water desorption.
The thermal battery reached a photothermal conversion efficiency of 87.99%.
MOF-303 captured about 0.41–0.43 g/g across 20–40% RH.
More than 78% of captured water was released within 30 minutes.
The theoretical nighttime output reached 3.8 gwater/gMOF/day.
Nighttime operation improved yield by 171% relative to daytime-only MOF operation.
Thermal storage enables vertically stacked, three-dimensional desorption systems.
Direct contact and high thermal conductivity are critical for rapid regeneration.
Takeaway
The central innovation of this work is not a new sorbent, but a new way of managing solar energy.
Instead of requiring sunlight and desorption to occur at the same time, the system separates them:
sunlight is collected during the day, stored as latent heat, and used to generate water at night.
This approach extends the operating window of atmospheric water harvesters, reduces dependence on electrical heating, and opens a pathway toward compact, stacked, and round-the-clock AWH systems.
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