Simultaneous atmospheric water production and 24-hour power generation enabled by moisture-induced energy harvesting

The study addresses the dual global challenges of freshwater and electricity scarcity, particularly acute in arid and remote regions where traditional technologies (rain collection, artificial precipitation, desalination) are impractical. Atmospheric water harvesting (AWH) leverages the vast atmospheric moisture reservoir, with sorption-based AWH (SAWH) effective at low relative humidity due to strong sorbent–water vapor affinity. Although solar-driven SAWH is feasible in arid climates, its water productivity lags behind solar interfacial evaporation/desalination due to low sorbent capacity at low RH and low device thermal efficiency. Metal–organic frameworks (MOFs) such as MOF-801, MOF-303, Co2Cl2(BTDD), and MOF-333 offer higher capacity and lower regeneration temperatures, and low-cost sorbents (hydrogels, ionic solutions, salt-based composites) have also emerged. In parallel, off-grid sustainable power generation is needed; hybrid solar systems for steam and electricity are not viable in water-scarce regions. SAWH coupled with thermoelectric generators (TEGs) can co-generate freshwater and power from solar thermal energy, but prior systems cannot sustain nighttime generation without energy storage. Radiative cooling enables nighttime power but with small temperature differences and low power density. The research proposes a moisture-induced energy harvesting approach that synergistically exploits sorption/desorption thermal effects with daytime solar heating and nighttime radiative cooling to achieve efficient water production and continuous (24-hour) thermoelectric generation.

The paper reviews AWH technologies: chiller-based dew collection, radiative cooling condensation, fog harvesting with hydrophilic surfaces, and SAWH. SAWH is highlighted for arid climates due to sorbent–vapor affinity. Traditional sorbents (silica gel, zeolites) are outperformed by MOFs (e.g., MOF-801, MOF-303, Co2Cl2(BTDD), MOF-333) for higher water uptake and lower regeneration temperatures. Cost-effective materials include hydrogels, ion solutions, and salt-based composites. A key unresolved issue is managing endothermic/exothermic sorption heats, which reduce energy efficiency. For power, prior hybrid solar systems combine evaporation/distillation with photovoltaics or salinity gradient but rely on water availability. SAWH-TEG coupling has been proposed but lacks nighttime generation due to solar intermittency. Radiative cooling TEGs demonstrate nighttime generation yet with limited power density due to small temperature differences. These gaps motivate a hybrid system that uses sorption heating/cooling to augment TEG temperature differences day and night.

Design and principle: A hybrid SAWH–TEPG device is developed comprising a dual-functional coating layer (black paint on a thin copper plate; ~95% solar absorptance and ~90% emissivity in 8–13 μm), a thermoelectric generator (TEPG) module, an SAWH module (sorbent packed on an aluminum thermal framework), an air-cooling condenser, thermal insulation, and a polyethylene cover. The SAWH module is thermally coupled to the TEG cold side via an aluminum block. Daytime operation uses solar heating on the absorber to heat the TEG hot side while sorbent desorption provides strong endothermic cooling at the TEG cold side, enlarging ΔT across the TEG and enabling water release and condensation at an air-cooled condenser. Nighttime operation exposes the absorber to the sky for radiative cooling at the TEG cold side, while sorption heating from water uptake in the sorbent warms the TEG hot side. The TEG heat flux is modeled as Q_TE = AΔT_TE/R_TE, and the theoretical maximum power density P_max = (n(SpΔT_TE)^2)/(4R_L), emphasizing ΔT_TE as the key determinant. Sorbent synthesis and characterization: MIL-101(Cr) is chosen for its high uptake, fast kinetics, and low desorption temperature. MIL-101(Cr) is synthesized hydrothermally by dissolving 1 mmol CrCl3·6H2O and 1 mmol terephthalic acid in 7.2 mL deionized water and reacting at 190 °C for 24 h, followed by removal of recrystallized ligand and washing with DMF and ethanol. To improve heat/mass transfer, MIL-101(Cr) powders are coated onto copper foam (CF) via in-situ solution impregnation to form MIL-101(Cr)@CF. Characterizations include SEM (uniform coating), water sorption–desorption isotherms (S-shaped; uptake ~1.2 gwater gMOF−1), temperature-insensitive sorption behavior, N2 adsorption (BET ~3224.6 m2 g−1), kinetics, TGA-DSC (sorption–desorption thermal energy density ~2500 kJ kg−1), thermal conductivity (MIL-101(Cr)@CF 3.54 W m−1 K−1 vs powder 0.12 W m−1 K−1), cyclic stability, mechanical and chemical stability. The composite exhibits up to 10 °C self-heating over ambient during sorption due to exothermic heat release. Device prototypes and testing: A proof-of-concept hybrid SAWH–TEPG device is assembled in a layer-by-layer structure (dual-functional coating/TEPG/MIL-101(Cr)@CF), with a referenced device lacking the sorbent for comparison. Indoor nighttime tests are conducted in a chamber at 25 °C and 65% RH; radiative cooling is simulated by a water-cooled heat sink 5 °C below ambient. Temperature evolutions, ΔT_TE, open-circuit voltage (VOC), short-circuit current (ISC), and P–V characteristics are measured. Indoor daytime tests use a solar simulator at 500, 750, and 1000 W m−2; TEG temperatures, VOC, Pmax, and water desorption rates are recorded. Outdoor demonstrations are performed on a rooftop in Shanghai, with real-time monitoring of ambient conditions, MOF temperature, TEG temperatures, VOC, and water collection via an air-cooled aluminum condenser. Energy flux analyses for radiative cooling/heating, sorption heating, desorption cooling, and convective losses are provided (Supplementary Notes), including estimates of radiative cooling power, sorption heating power, and desorption cooling power. A week-long continuous outdoor test tracks 24-hour power generation and daily water uptake. Annual water production is modeled for six cities using climate data.

- Synergistic thermal effects: Coupling sorption/desorption with radiative cooling/heating enlarges ΔT across the TEG, enabling continuous 24-hour power generation without energy storage and enhancing SAWH performance by heat consumption that lowers sorbent temperature during sorption. - Indoor nighttime (simulated radiative cooling): Combined sorption heating + radiative cooling yields maximum VOC = 115.8 mV and Pmax = 42.6 mW m−2, outperforming radiative cooling alone (VOC = 62.1 mV, Pmax = 12.3 mW m−2) and sorption heating alone (VOC = 20.7 mV, Pmax = 1.6 mW m−2), corresponding to a 346% improvement over radiative cooling-only TEGs. - Indoor daytime (solar simulator): Under 500/750/1000 W m−2, maximum VOC = 383.3/553.8/729.4 mV with corresponding Pmax = 467.3/968.3/1726 mW m−2. With MIL-101(Cr)@CF, the TEG cold side runs cooler (e.g., bottom temperature 63.4 °C under 1 sun, 9.3 °C lower than reference), raising ΔT and VOC (average VOC improved from 492.3 to 571.9 mV at 1 sun). Water desorption productivity reaches ~150 g m−2 h−1 and increases with irradiance. - Outdoor nighttime: Hybrid device achieves VOC up to ~80 mV and higher ΔT_TE than reference; Pmax enhanced by ~200% over reference. Estimated radiative cooling power up to ~90 W m−2 and sorption heating power up to ~130 W m−2. - Outdoor daytime: Peak VOC = 433.7 mV; maximum Pmax = 685 mW m−2. Desorption cooling power up to ~300 W m−2. Solar-driven water harvesting efficiency η ≈ 21.7%. Water condensed on an air-cooled condenser and collected as droplets. Hybrid device shows stable water collection across repeated outdoor cycles. - All-day performance: Reported all-day thermoelectric power density up to 685 mW m−2 (day) and 21 mW m−2 (night). Water production reaches 750 g m−2 per cycle. Week-long continuous outdoor tests deliver continuous voltage (max VOC 505 mV) and average water uptake ~800 g m−2 per day. - Scalability and projections: Predicted annual water production 295.7–612.6 L m−2 across climates in six cities. The system can be scaled by assembling multiple SAWH and TEG units. - Sorbent/material properties: MIL-101(Cr) uptake ~1.2 g g−1 (composite ~0.93 g g−1), BET ~3224.6 m2 g−1, thermal energy density ~2500 kJ kg−1, composite thermal conductivity 3.54 W m−1 K−1 (≈28× higher than powder), sorption-induced temperature rise up to 10 °C; dual-functional coating with ~95% solar absorptance and ~90% mid-IR emissivity.

The hybrid SAWH–TEPG leverages moisture-sorption exothermy and desorption endothermy to actively manage the TEG hot and cold sides, significantly increasing ΔT compared with conventional designs. At night, sorption heating elevates the TEG hot-side temperature while radiative cooling lowers the cold side, producing useful power without energy storage and simultaneously accelerating sorption by removing sorption heat through the TEG. By day, desorption cooling reduces the TEG cold-side temperature while solar heating warms the hot side, enhancing power output and enabling water release and collection. These synergistic interactions directly address the limitations of prior SAWH (low thermal efficiency) and radiative-cooling TEGs (limited ΔT and low power). The demonstrated power densities (up to 685 mW m−2 day and 21 mW m−2 night) and water production (up to 750 g m−2 per cycle) indicate practical feasibility for off-grid applications. The approach is compatible with simple, low-cost materials (black paint absorber, air-cooled condenser) and can be scaled modularly to power small electronics (e.g., LEDs with a boost converter, sensors) while producing potable water, offering relevance for arid, remote regions.

The study introduces and validates a moisture-induced energy harvesting strategy that couples SAWH and TEGs with dual-functional radiative surfaces to achieve simultaneous atmospheric water production and continuous 24-hour power generation. By exploiting sorption/desorption thermal effects with radiative cooling/heating, the hybrid device substantially enhances TEG ΔT and power density and accelerates AWH. Outdoor and indoor experiments confirm daytime Pmax up to 685 mW m−2, nighttime up to 21 mW m−2, and water production up to 750 g m−2, with annual yields projected at 296–613 L m−2 depending on climate. The system is modular and scalable for off-grid water and power co-generation. Future work should optimize sorbents (capacity, kinetics), thermal management, and device architecture to reduce equilibration times between sorption/desorption and further increase both water productivity and power output.

The study notes a long equilibration duration between sorption and desorption processes, indicating room for improvement in sorbent kinetics and device thermal/mass transfer design. Some indoor nighttime evaluations used a water-cooled sink to simulate radiative cooling, which, while referenced to literature, is an approximation of real sky cooling. Nighttime power density, though enhanced, remains modest relative to daytime, and performance inherently depends on ambient RH, temperature, and sky conditions.