Engineering high environmental robustness in solar evaporation to bridge the lab-to-field performance gap

Domain: biology
Published: 2026-03-26
Editorial outcome: accept

Abstract
Downward solar evaporation with multistage configurations is a promising off-grid solution for high-efficiency potable water production. However, a major, yet often overlooked, barrier to practical application is the significant, unquantified performance gap between laboratory benchmarks and field operation, arising from the complex environmental factors. To diagnose this gap, this work first introduces the Environmental Robustness Index (ERI), the ratio of field-to-lab normalized water productivity (P'), as an essential metric. A comprehensive framework is then developed to precisely quantify the effect of key environmental factors, including wind, sky cooling, and ambient temperature, on the ERI. Guided by the framework, we present the spectrally selective air lock strategy as a universal principle to suppress environmental heat losses and improve ERIs. Implementing this strategy significantly enhances downward solar evaporator’s ERIs from 0.55 to 0.98, effectively closing the gap. This study establishes a framework for solar evaporation to move beyond reporting P' alone and utilize (P', ERI) as the dual metrics for advancing real-world applicability.

Introduction
Increasingly frequent extreme weather events1, compounded with population growth2, are driving a global intensification of water scarcity.3, 4 This crisis is particularly pressing in regions such as the Middle East, Northern Africa, and Sub-Saharan Africa, where water resources are already constrained and population are expanding rapidly.4, 5 Solar evaporation technologies present a promising and sustainable approach to securing clean water supply. By harnessing solar energy to facilitate freshwater production from non-potable water sources, such as seawater and wastewater, this technology is especially advantageous for remote and off-grid communities lacking infrastructure for conventional water treatment systems.6

Despite the potential, its practical application remains hindered by several challenges, one of the most significant being the relatively low water productivity.7 Under 1 kW·m−2 solar flux, the thermodynamic limit of water evaporation rate is 1.47 kg·m−2·h−1.8 Since high throughput is a critical requirement for the viability of most clean water supply applications, developing strategies that enhance water productivity to approach this fundamental threshold is of significance.

Over the past decade, considerable research efforts have been concentrated on enhancing the water productivity of solar evaporation, among which modular designs incorporating downward evaporation, wherein vapor flows downward to the condenser, have demonstrated superior energy efficiency by minimizing optical losses due to condensate deposition.9-12 Furthermore, downward evaporation architecture enables multistage distillation processes, which have revolutionized system performance through efficient latent heat recovery during vapor condensation and enabling successive evaporation cycles that dramatically enhance water production rates.13 Such multistage solar distillation (MSD) systems have demonstrated exceptional productivity that frequently surpasses 3.00 kg·m−2·h−1, far exceeding the thermodynamic limit inherent to single-stage systems.14-22 This rapid progress in the laboratory has spurred a trend towards scale-up and real-world demonstration23-25 However, in this critical transition from laboratory to field, a challenge emerges, whereby significant performance degradation are often observed. The water productivity can decrease by 23% and up to 57% compared to laboratory benchmarks even under equivalent solar irradiation and operation conditions.14, 16, 18, 22, 26-29 This discrepancy arises from the complexities introduced by outdoor environments, where factors such as wind, atmospheric transmittance, solar flux, and surrounding temperature can all significantly affect the evaporation module’s thermal efficiency. If these impressive laboratory performance metrics cannot be preserved in field applications, the practical and economic value of the technology is significantly diminished. This suggests that the pursuit of robustness under real-world conditions is perhaps more critical for practical deployment than the attainment of record-high performance metrics in controlled environments. A system with high robustness would exhibit nearly identical performance between laboratory and field conditions.

To date, this critical lab-to-field disparity, and the robustness it implies, has not been formally quantified. We therefore propose the Environmental Robustness Index, defined as the ratio of normalized water productivity in the field, P' , to that in standard lab conditions (1 kW·m−2, 25°C, 0 m·s−1 wind), P' , as a critical metric to evaluate and benchmark a system's real-world viability. To understand and predict this ERI, a comprehensive heat and mass transport framework is developed to model the coupled impacts of key environmental factors. This analysis identifies detrimental factors that cause low ERIs, such as wind, sky cooling, and varying solar fluxes. Conversely, high ambient temperature is identified as a key positive factor that can be leveraged to achieve an ERI > 1.

To mitigate these environmental impacts and engineer a high-ERI system, a spectrally selective air lock (SSAL) strategy is proposed and investigated. This approach counters radiative losses through spectrally selective materials that are transparent to sunlight but opaque to thermal radiation, while simultaneously suppressing convective and conductive losses via engineered pores or thin air layer. The versatility of this strategy is demonstrated with three distinct configurations: a monolithic mesoporous silicone aerogel, and two cost-effective composite alternatives comprising a PMMA or soda-lime glass cover with a stagnant air layer. Strategy effectiveness was validated by a direct comparison of the ERI. In continuous outdoor tests, the SSAL-equipped MSD module demonstrated an ERI consistently above 0.9, confirming its environmental robustness. In contrast, the ERI of the conventional, uncovered module seldom surpassed 0.8 and at times dropped below 0.5, indicating environmental fragility. Moreover, confirming our model's prediction, when field-tested at elevated ambient temperature of 38°C, the module implementing SSAL outperformed its laboratory benchmark, reaching a productivity of 6.53 kg·kWh−1 and demonstrating an ERI of 1.67. The comprehensive analysis framework and the validated SSAL strategy presented herein are crucial for enhancing solar evaporation module’s robustness and closing the laboratory-field performance gap. Finally, we encourage the solar evaporation field to move beyond reporting P’ alone and adopt (P’ , ERI) as the dual metrics for performance, which ensures future research is grounded in real-world applicability.

Results
Identifying the Source of Environmental Fragility

In a downward solar evaporation system, downward heat transfer drives the evaporation process, while upward heat transfer (predominantly via radiation and convection) represents parasitic loss to the environment. To quantify the heat loss from the solar absorber in the evaporation module under representative environmental conditions, we evaluated the respective contributions of convection and radiation, assuming an absorber operating temperature of 60°C in a 25°C environment. The extent of radiative heat to the ambient environment or through sky cooling is illustrated by the spectral radiance curves in Figure 1a. In indoor, laboratory settings, radiative heat loss occurs from the absorber to the surrounding walls and ceiling, whose emissivity is assumed to be 1. The radiative heat loss was calculated as the difference in the area under the respective curves for the absorber and the surrounding structures.

However, in outdoor, field conditions, the sky exhibits an atmospheric transparency window particularly in the 8 – 13 μm range (represented by the blue-shaded area in Figure 1a), which largely overlaps with the peak blackbody emission spectrum of the solar absorber. This spectral matching enables direct radiative heat transfer to the cold sink of outer space (−270°C), known as sky cooling.30, 31 The effect is most pronounced under dry, cloudless skies, creating a crucial, counter-intuitive trade-off: the very conditions conventionally thought ideal for high solar flux also create the strongest radiative heat loss. This overlooked loss mechanism, which is completely absent indoors, is a key culprit for the lab-to-field gap and results in a substantial environmental penalty on performance.

Figure 1b illustrates the heat loss pathways for an absorber operating indoors. (For detailed calculations please refer to Method S1) Under both windless and windy (2 m·s−1) conditions, convection is the dominant pathway of heat loss. Notably, even in the absence of wind, natural convection driven by density gradients above the heated absorber causes substantial heat loss (361 W·m−2). Introducing a moderate wind (2 m·s−1) drastically increases the convective heat loss to 1247 W·m−2. When combined with the radiative loss (242 W·m−2), this total heat loss of 1607 W·m⁻2 is ~ 60% higher than the standard 1-sun input (1 kW·m−2), making it impossible to sustain the target temperature even before accounting for evaporation.

In contrast to indoor operation, exposing the absorber under a clear sky in the field significantly increases radiative heat loss due to sky cooling. As illustrated in Figure 1c for the windless scenario, radiative heat loss rose to 463 W·m−2, with sky cooling contributing by 219 W·m−2. When wind is introduced (2 m·s−1), the combined convective and radiative losses lead to an even greater overall heat loss, reaching 1710 W·m−2. These results quantify the physics of environmental fragility. The combined heat losses, driven by wind-induced convection and sky cooling, dramatically exceed the available solar input. This imbalance is the direct cause of the catastrophic performance drop and a critically low ERI in unprotected modules.

Notably, the atmospheric transmittance used in this study was obtained under a clear sky and low humidity32 and represents a worst-case scenario for radiative heat loss. Under cloudy or humid conditions, atmospheric transmittance is reduced, which would lessen this environmental penalty.31 Therefore, our calculation is an upper-bound estimate, while the indoor condition represents the lower bound.

To achieve a high ERI, the absorber requires effective insulation against the impact of convective flow and sky cooling. Ideally, such insulation would exhibit high spectral selectivity, characterized by perfect transmittance in the visible spectrum to maximize solar energy harvesting, and zero transmittance in the infrared band to obstruct radiative heat exchange with the environment. Meanwhile, convective heat transfer, which arises from the macroscopic movement of a fluid, in this case, air, can be suppressed by confining air within a porous structure or narrow space. When the characteristic size of the voids is sufficiently small, the bulk air motion is impeded, thereby eliminating convection as a significant heat loss mechanism.33 We term a hypothetical material possessing these combined properties as Spectrally Selective Air Lock. The potential efficacy of the SSAL in engineering a high ERI system is demonstrated by the calculation presented in Figure 1d. Employing this idealized insulation renders radiative and convective heat loss negligible, and conduction becomes the dominant heat loss mechanism. Assuming a thermal conductivity characteristic of insulating foams (0.03 W·m−1·K−1), the calculated conductive heat loss is merely 113 W·m−2 under windless condition and 135 W·m−2 with a 2 m·s−1 wind. This suppressed heat loss, representing over 85% of incoming energy being available for evaporation, is how a high ERI is achieved. The analysis thus highlights that engineering a high ERI is synonymous with suppressing these environmental loss pathways. The SSAL strategy is shown to be an effective approach to engineering environmental robustness and close the lab-field performance gap.

SSAL Design Principles for Environmental Robustness

Based on the preceding analysis, engineering a high-ERI system requires an insulating solution that is transparent to sunlight, opaque to thermal radiation, and convection-free. This is the core function of the SSAL strategy. Three practical implementations of this strategy were designed and evaluated. The first, mesoporous silicone aerogel, represents a near-ideal case study whose nanostructure embodies the SSAL properties. Although prior works have explored aerogel16, 21, systematic field evaluation has been limited. The silicone aerogel34 evaluated here perfectly demonstrates the three required SSAL functions (detailed synthesis procedures of the aerogel are provided in the Methods section). A high solar-weighted transmittance (SWT) of 95% (at 6 mm) ensures maximum solar energy input (Figure 2a, b). Near-zero transmittance in the long-wave infrared band (Figure 2a, c) due to absorption by –OH groups and Si―O―Si vibrational modes35, which blocks thermal radiation from the absorber and directly counters the radiative loss pathway.36, 37 Its mesoporous structure (Figure 2d) yields a thermal conductivity (0.022 W·m−1·K−1) lower than stagnant air, which suppresses conductive and convective heat losses (Figure S1). A hydrophobic surface34 (Figure S2, Note S1) also imparts durability for outdoor applications.

Note that while increasing aerogel thickness enhances robustness against sky cooling, it simultaneously reduces solar transmittance. This highlights the core engineering trade-off of any SSAL design. Aerogel with 6 mm thickness was selected, as theoretical analysis (Figure S3) showed it to be the point of diminishing returns, balancing heat loss suppression with solar gain.

The SSAL strategy is a material-agnostic engineering principle and not reliant on one single advanced material. To demonstrate this, two composite configurations were designed using common PMMA and soda-lime glass which posses the requisite spectral selectivity (Figure 2e). Their limitation, however, is high thermal conductivity (0.18 W·m−1·K−1 for PMMA and 1.09 W·m−1·K−1 for glass), which can be overcome not by material discovery, but through thermal design: implementing a composite air lock structure. This approach involves a precisely dimensioned air layer between the cover and absorber (Figure 2f, inset) that is thick enough to be a strong thermal insulator but thin enough to suppress the onset of natural convection. This critical thickness (H) is a calculable design parameter governed by the critical Rayleigh number, as shown in Equation (1).

Ra = (gβΔT H^3) / (ν α) (1)

where Ra represents the critical Rayleigh number, beyond which the fluid motion transitions from conduction-dominated heat transfer to convection as the fluid currents form. For a horizontal layer of fluid heated from below with no-slip, rigid boundaries, Ra is 1708.38 ν, α, and β are the kinematic viscosity, thermal diffusivity, and thermal expansion of air, respectively. g is gravitational acceleration, and ΔT represents the temperature difference across the air layer. Assuming initial absorber and ambient air temperature to be 60°C and 25°C, respectively, the calculated H is approximately 8.4 mm. A smaller thickness was used in this study as excessive heat losses through the sidewalls of the air layer were observed when the thickness approached H. Calculations indicate that optimal configuration, balancing thermal insulation with solar gain, consists of a 6 mm air layer coupled with a 3 mm thick PMMA or glass sheet (Figure S4). As shown in Figure 2f, the engineered composites achieve total thermal resistance comparable to the monolithic 6 mm aerogel. While aerogel is the highest-performing configuration, the composite design is highly competitive, proving that a high ERI can be successfully engineered using accessible, conventional materials (Note S2).

Predictive Modeling and Quantification of ERI

To predict the ERI of a given module design under any environmental conditions, a theoretical heat and mass transfer model was developed (Method S1 and S2). The model, illustrated schematically in Figure 3a, accounts for all primary heat transfer pathways, including convection (Qv), radiation (Qr), conduction (Qd), and spectral reflection losses (Qs) from the surface of the absorber to the ambient environment. Heat input from the sun, Qin, when reduced by Qv, Qr, Qd, and Qs is the available heat for water evaporation, Qevap (see Method S1 and S2 for detailed equations). Crucially, the modeling framework is designed to accept environmental variables (wind speed, atmospheric transmittance, Tamb, solar flux) as inputs to predict the steady-state absorber temperature, water productivity, and, thereby, the module's ERI. For this analysis, the total number of stages was fixed at six, as preliminary studies indicated that additional stages yield diminishing returns on water productivity relative to the increased material cost (Figure S5). All other modeling parameters are listed in Table S1. The model was then used to systematically quantify the water productivity and ERI of the four configurations (no cover, 6 mm aerogel, PMMA-air, and glass-air) under the impact of each critical environmental variable.

Figure 3b demonstrates the impact of wind. The uncovered module shows extreme environmental fragility. Its water productivity plummets from 2.55 at 0 m·s−1 to just 0.86 kg·m−2·h−1 at 2 m·s−1 wind, and nearly halts at 6 m·s−1 (0.37 kg·m−2·h−1). This collapse in output is mirrored by its ERI, which plummets to 0.34 and 0.14, respectively. In stark contrast, all three SSAL configurations exhibit remarkable environmental robustness. The 6 mm aerogel-insulated module, for example, achieves a high peak productivity of 4.88 kg·m−2·h−1 (windless) and maintains 4.41 kg·m−2·h−1 even at 6 m·s−1 wind — a mere 9.6% decrease. This excellent retention is reflected in its high ERI, which remains above 0.9 (Figure 3b), as well as the temperature profile of the solar absorber, which remains consistently high and homogeneously distributed (Method S3, Note S3, Figure S7). The PMMA-air and glass-air composites are similarly effective, validating the air lock as a core design principle for achieving wind robustness. Their ERIs approach that of the aerogel, consistent with their comparable thermal resistance (Figure 2f). The PMMA-air composite's lower water productivity is, as predicted, a direct result of its lower SWT (Figure 2e).

Sky cooling represents a second, distinct environmental penalty. Its impact was isolated by comparing the windless indoor benchmark to a windless outdoor condition (Figure 3c). The unprotected module's ERI drops to 0.76 from sky cooling alone. Conversely, all three SSAL configurations demonstrate near-perfect immunity, with both their water productivity and ERI remaining virtually unchanged, as their infrared opaqueness effectively decouple the hot absorber from the cold sky sink. When a 2 m·s−1 wind is also applied, the uncovered module’s performance collapse entirely – a 76% reduction from the indoor benchmark. The ERIs of SSAL-insulated modules, in contrast, remain above 0.9. Figure 3d provides a clear diagnosis for this fragility. For the uncovered module, transitioning from indoor to outdoor (2 m·s−1) conditions causes total heat losses to skyrocket, consuming nearly 92% of the incoming solar energy. The model identifies the culprits: a large increase in radiative loss (Qr) from sky cooling, compounded by a massive increase in convective loss (Qv) from wind. The SSAL-insulated modules, however, successfully suppress these environmental penalties, capping total heat loss below 40%. With the dominant convective and radiative losses attenuated, the primary performance differentiator among SSAL designs becomes their respective spectral properties (i.e., SWT).

A key finding from the predictive framework is the module’s strong, positive coupling with ambient temperature (Tamb). As shown in Figure 3e, both water productivity and ERI increase with Tamb across all configurations. This phenomenon is rooted in the exponential dependence of saturated vapor pressure on temperature. As Tamb elevates the module’s internal temperature profile, the vapor pressure gradient driving mass transport between evaporator and condenser is enlarged for the same temperature difference.20 This mechanism underpins the ERI > 1 observation: in high-temperature environments, the module can leverage ambient heat, enabling its normalized field performance to surpass the 25°C laboratory benchmark.

Finally, the framework reveals the impact of solar flux (Figure 3f). In terms of water productivity, the uncovered module’s output declines sharply to < 0.3 at low flux (< 0.5 kW·m−2), becoming nearly non-operational. In contrast, the SSAL-insulated modules maintain moderate output even at a low 0.3 kW·m−2 flux. The ERI, however, shows a more nuanced trend. The ERI for all configurations declines at lower fluxes since, at reduced energy inputs, the system operates closer to its activation threshold, where even suppressed heat losses constitute a significant fraction of the total energy budget. Despite this decline, the SSAL-insulated modules maintain functionality in low-flux periods (e.g., during morning and late afternoon) whereas the uncovered module becomes ineffective. This capability is critical for maximizing total daily water yield. Overall, this theoretical analysis underscores that engineering robustness with SSAL strategy is vital for sustaining performance and reliability in dynamic, real-world environments.

Validation under Simulated Sunlight and Wind

To experimentally validate the theoretical predictions, a six-stage MSD module was fabricated and tested under controlled laboratory conditions (detailed material specifications and assembly procedures are provided in the Methods section). The module design and operation are illustrated in Figure 4a, featuring a crossflow configuration15, 18 where source water is wicked into each stage’s evaporator, heated by the solar absorber (absorbance spectra shown in Figure S8), and partially evaporates. Concentrated brine exits from the opposite side, while the generated vapor diffuses downward across a small air gap to condense on a cooler stainless-steel plate. This condensation process releases latent heat, which conducts directly to the evaporator of the subsequent stage immediately below, thus enabling efficient latent heat recycling. These stages were repeated to drive multiple water evaporation–condensation cycles, achieving solar-vapor conversion efficiency exceeding 100%.13 Fresh distillate is collected and transported from each condenser via capillary wicks to an external reservoir. To minimize sideward heat losses, the module sidewalls were insulated with thick polyurethane foam. The experimental setup (Figure 4a) simulated environmental conditions using a solar simulator with filters and a variable-speed fan (not shown). Real-time water productivity and absorber temperature (Tabs) were recorded.

Figure 4b details the experimental validation of wind robustness. The uncovered module showed extreme sensitivity to wind, with water productivity collapsing from 2.37 to mere 0.53 kg·m−2·h−1 as wind speed increased from 0 to 6 m·s −1, corresponding to ERI of only 0.22. Its absorber temperature concurrently plummets from 55°C to 34°C, indicating massive convective heat loss.

In stark contrast, all three SSAL configurations demonstrated experimentally validated robustness. The aerogel-insulated module’s water productivity remained high and stable, decreasing only slightly from 3.90 to 3.15 kg·m−2·h−1 across the same wind speed range, which corresponds to ERI of above 0.8, with its absorber temperature dropping only from 64°C to 57°C. While glass-air composite showed slightly lower productivity and PMMA-air composite yielded the lowest among the three SSAL configurations, they exhibited similarly exceptional ERI (> 0.7), confirming that the SSAL strategy effectively insulates the module from realistic wind. This observed performance hierarchy is in excellent agreement with the preceding optical-thermal analysis and modeling predictions. Importantly, the solar absorber must be fully covered by SSAL materials to ensure maximum efficiency. Experiments revealed that 20% exposed surface can lead to considerable convective heat loss and thus a noticeable reduction in water productivity (Figure S9).

Figure 4c investigates the performance of the MSD module under varying solar flux. As the model predicted, the SSAL-insulated modules maintained higher productivity and absorber temperature at lower fluxes down to 0.5 kW·m−2, proving their superior heat retention. At 0.3 kW·m−2, however, the water productivity of all modules converged to a similarly low level (0.36 for the uncovered and 0.46 – 0.48 kg·m−2·h−1 for the SSAL-insulated modules). The absorber temperature all fell below 40°C, indicating that the module has an activation threshold of ~0.5 kW·m−2, below which there is insufficient energy to sustain the evaporation-condensation process. The results define the operational limit of the MSD system itself. Nevertheless, as indicated by the modeling (Figure 3e), elevated ambient temperature can significantly enhance water productivity, potentially compensating for the performance loss under low-light conditions. This suggests that external heat sources could be leveraged to sustain operation below this solar threshold.

While the experimental results confirm the theoretical trends and the benefits of the SSAL strategy, the measured water productivities tended to be lower than model predictions. This quantitative discrepancy is primarily attributed to unmodeled heat losses on module sidewalls, particularly at the water inlet and outlet openings, which were intentionally left unsealed to avoid obstructing water transport. Nevertheless, as the primary objective of this study was to investigate and mitigate heat loss through the top surface (which is where the most heat losses occur) and to close the laboratory-field performance gap, further optimization of water productivity was not pursued.

Field Validation of ERI Using Real Seawater

To validate the SSAL strategy and quantify the ERI in a real-world setting, outdoor field tests were conducted. Two parallel six-stage MSD modules (one uncovered, one with 6 mm aerogel, Figure 5a) were operated using real seawater, modified for continuous operation (See Methods and Figure S10 – S11 for full details).

Real-time data from representative day (November 27th, 2024, Tamb ≈ 24°C) immediately demonstrated SSAL strategy’s impact. The uncovered module struggled against sky cooling and mild wind (≤ 2 m·s−1), its absorber temperature peaking at only 42.8°C (Figure 5b). In contrast, the aerogel effectively shielded against convective and radiative losses, enabling the absorber to reach a peak of 60.0°C. This thermal advantage directly translated into higher, more consistent water production throughout the day (Figure 5c). Over a seven-day test encompassing both clear and overcast conditions, the SSAL-insulated module consistently doubled the daily water productivity of its coverless counterpart (Figure 5d). On the sunniest day in November, it achieved a peak daily yield of 10.88 kg·m−2, corresponding to a higher normalized productivity of 3.83 kg·kWh−1 and an ERI of 0.98 (Figure 5d). Ionic concentrations of produced water decreased from ~104 ppm in feed seawater to below 10 ppm, a three-order-of-magnitude reduction that comfortably satisfies WHO drinking water guidelines on salinity (200 mg·L−1 for Na+, 250 mg·L−1 for Cl− and SO4 2−, Figure S11).39

Heat loss analysis elucidates the performance differences between laboratory and field conditions. Under 0.3 m·s−1 wind speed and 669 W·m−2 solar flux (12:30–12:45 pm, Nov 27th, 2024), the uncovered module’s normalized water productivity (P') dropped from 1.66 to 0.91 kg·kWh−1, representing a poor ERI of 0.55 and proving its environmental fragility. This degradation primarily resulted from sky cooling. As illustrated by Figure 5e (left panel), the solid yellow bar and the dashed outline represent the range of radiative heat losses. The yellow bar represents the lowest possible radiative heat loss (105 W·m−2), which occurs indoors or under high humidity and cloud coverage. The peak value, 327 W·m−2 indicated by the dashed outline, occurs under the highest atmospheric transparency and the most pronounced sky cooling. Hence, total upward heat loss increased from 269 W·m−2 under laboratory settings to a maximum 512 W·m−2 during clear day, low humidity outdoor operation, which can decrease thermal efficiency and water productivity considerably. In contrast, the aerogel-insulated module demonstrated exceptional environmental robustness, with normalized productivity decreasing slightly from 3.62 kg·kWh−1 in the lab to 3.26 kg·kWh−1 in the field, demonstrating an ERI of 0.90. Heat loss remained minimal, increasing from 86 W·m−2 to 98 W·m−2, with sky cooling effect suppressed to negligible 12 W·m−2, validating the SSAL strategy’s effectiveness in closing the laboratory-field performance gap.

Furthermore, a second outdoor test at a high ambient temperature of 38°C (August 23rd, 2025, Figure S12) experimentally confirmed the ERI > 1 discovery predicted by our model. As shown in Figure 5d, both the SSAL-insulated and the uncovered modules surpassed their 25°C laboratory benchmarks, achieving a remarkable water productivity of 6.53 and 2.78 kg·kWh−1, corresponding to an ERI of 1.68 and 1.17, respectively. A detailed comparison of the heat losses (Figure 5e, right panel) reveals that, despite the higher operating temperature, total heat losses from the module's top surface actually increased compared to the 24°C test. This is attributed to the significantly higher absorber temperature, which increases temperature gradient that drives heat transport to the environment. However, this penalty was outweighed by the exponential increase in vapor flux with the entire system operating at a higher temperature profile.20 These results corroborate the theoretical predictions from Figure 3f and demonstrate that under specific extreme field conditions, water productivity can substantially exceed values obtained in controlled laboratory settings. Moreover, the SSAL strategy consistently maintains superior productivity due to its exceptional heat insulation capability.

Figure 5f benchmarks this work against prior studies14, 16, 18, 26-29, 40, 41 by plotting ERI against laboratory-reported normalized productivity (P'lab). To facilitate comparison, the average ambient temperatures at 12:00 – 14:00 recorded during the respective field tests are indicated in the figure legend. Many existing systems exhibit high P'lab yet a low ERI of 0.4 – 0.7. Our SSAL-insulated module breaks this trade-off, demonstrating both a high P'lab and a near-unity ERI (0.98) at 24°C. Moreover, our test at 38°C occupies a previously unachieved space on the plot, demonstrating ERI > 1. Collectively, this analysis underscores the imperative of employing SSAL strategies and underscores the necessity of accounting for all major environmental variables to accurately predict and optimize real-world performance.

Finally, to translate the demonstrated lab-to-field performance gap into practical economic implications, a preliminary techno-economic analysis was conducted to estimate the price of the produced water (Note S4, Table S3). Based on the productivity obtained from controlled laboratory conditions, the economic distinctions among uncovered and SSAL-insulated modules appear modest (Figure 6a). After one year, the projected water prices for the uncovered and SSAL-insulated modules range from $0.011 to $0.016 per liter, all substantially lower than average bottled water price ($0.43 L−1).

However, a distinct economic picture emerges when the analysis is based on realistic outdoor performance data (Figure 6b). At Tamb = 24°C, the water price for the uncovered module doubles to $0.038 L−1 due to its low ERI. In contrast, the price for the SSAL-insulated module remains stable, proving its environmental and economic resilience. Furthermore, at Tamb = 38°C, the water prices for both the uncovered and SSAL-insulated modules decrease substantially owing to enhanced productivity. This analysis critically highlights that relying on laboratory benchmarks can lead to severely misleading economic projections, and the crucial role of the SSAL strategy in ensuring a stable and competitive water price for practical deployment.

Methods
Materials
Methyltrimethoxysilane (MTMS, ≥ 98.0%) was purchased from Sigma-Aldrich (USA). Pluronic P84 (EO19 PO43 EO19) was purchased from Guangdong Fangxin Biotechnology (China). Aqueous tetramethylammonium hydroxide (TMAOH, 25%) was purchased form Macklin (China). Sodium chloride (≥ 99.5%) was purchased from Guangzhou Chemical Reagent Factory (China). Acetic acid (1.00 Normal) was purchased from Aladdin (China). Methanol (≥ 99.5%), ethanol (≥ 99.7%), and isopropanol (≥ 99.7%) were purchased from General-Reagent (China). Carbon dioxide (≥ 99.9%) was supplied by G-gas (China). Nano carbon copper foil tape was purchased from Shenzhen Huayan Adhesive Products Co., Ltd. (China). Nonwoven fiber cloth TX®609 Technicloth® was purchased from Texwipe (USA). The polysaccharide-based anti-scaling agent was acquired from Shandong Deshunyuan Petroleum Sci & Tech Co. Ltd. Ultrapure deionized water (resistivity ≥ 18 mΩ·cm) was used throughout the synthesis and experiments.

Synthesis of mesoporous silicone aerogel
5.0 mL of 5 mM acetic acid and 5.0 mL of MTMS were mixed and stirred at room temperature for 15 min to facilitate the hydrolysis of MTCMS. Subsequently, 4.0 g of Pluronic P84 and 4.0 mL of H2O were added to the hydrolyzed solution, and the mixture was continuously stirred until a homogeneous sol was obtained. The sol was then cooled in an ice bath for 20 min with moderate stirring, after which 3.0 mL of 0.5 M TMAOH solution was introduced drop by drop. The mixture was stirred for an additional 1 min and transferred to an airtight container to allow for gelation at room temperature for 1 h. Following gelation, the gel was maintained at 60°C in oven for 3 days to undergo further aging. Subsequently, the aged gel was removed from the airtight container to undergo washing and solvent exchange. The gel was soaked in a series of solvents at 60 °C in oven for at least 8 h. The solvent exchange was conducted in a sequential manner to avoid cracking due to osmotic pressure: first by gradually replacing water with methanol in volume ratios of H2O:methanol = 1:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:1, with the pure methanol exchange process repeated five times; followed by the replacement of methanol with isopropanol, with the isopropanol exchange process repeated three times. The final alcogel obtained from this process was then transferred to a supercritical dryer (SCD-380A, Shianjia Technology, China), in which isopropanol was first exchanged with liquid carbon dioxide at 5°C under 4.5 MPa, and dried supercritically at 35°C under 8 MPa for 2 h.

Module assembly
Nonwoven fiber cloth (0.2 mm thick) as the capillary wick for the evaporator was cut into 60 × 230 mm pieces with three parts: a wick to transport the feed water in (20 × 110 mm), the evaporation center (50 × 60 mm), and a tail to transport the concentrated brine out (20 × 70 mm). For the condenser, the cloth was cut into 60 × 180 mm pieces with two parts: a condensation center (50 × 60 mm) and a tail (20 × 130 mm) to transport the distilled water out. The evaporation center and condensation center of the cut cloth were attached to the cleaned stainless-steel sheets (60 × 60 × 1 mm) using heat conductive tape, forming the evaporators and condensers, which were separated by 3D-printed TPU spacers of 3.5 mm. The condenser tail was protected with plastic wrap to prevent water loss to the ambient. These components were assembled as shown in Figure 3a. The sidewalls were sealed by epoxy glue to prevent vapor leakage. At the module top surface, nano carbon copper foil tape was cut to 50 × 50 mm and affixed as the solar absorber. The monolithic silica aerogel was directly placed on top of the solar absorber. For PMMA-air and glass-air composites, the PMMA or soda-lime glass sheet was placed on a designated thickness of polystyrene foam, creating a thin air layer between the PMMA or glass sheet and the solar absorber. An aluminum heat sink was attached to the bottom of the module to facilitate condensation heat dissipation to the surrounding environment. To minimize heat loss from the sidewalls, the module was surrounded with 10 – 15 mm thick PU foam which has a thermal conductivity of 0.03 – 0.04 W·m−1·K−1.

Water production evaluation
For the laboratory experiment, the setup is illustrated in Figure 3a. The module was placed on a 3D-printed ABS support enabling efficient heat dissipation by the bottom heat sink. The source water reservoir was placed on one side of the module which allowed the capillary wicks to access the source water. An automatic water filler was attached to the reservoir to ensure consistent water level throughout the experiments. The concentrated brine container was placed on the opposite side of the module and slightly lower than the source water reservoir to prevent backflow. The condensed water was transported out of the condenser by capillary wick and collected by an acrylic container which was covered by plastic wrap to minimize water evaporation to the ambient. The mass of the produced water was recorded by a balance with 0.01 g resolution (ME2002/M, Mettler-Toledo, USA). Homogeneous solar irradiation at 1 kW·m-2 was provided by a solar simulator (CEL-AAAS100, CEAuLight, China) with a standard AM 1.5G spectrum optical filter. The solar flux was measured using an optical power meter (CEL-NP2000-2(10)A, CEAuLight, China). Neutral density filters (Phtode, China) were used to provide lower solar fluxes without modifying the solar spectrum. Temperature data from K-type thermocouples was recorded at 0.5 s intervals by Arduino Nano with MAX6675 thermocouple modules. Mass change data from the balances was recorded at 5 min intervals by LabView. Water productivity was calculated from the mass change curve using linear fitting. Wind was supplied by a chassis fan (120× 120 × 38 mm), and the wind speed was measured by a handheld anemometer (ST866A, Smart Sensor, China). Each experiment was repeated three times separately.

The module setup for field experiments is shown in Figure S9. The concentrated brine was transported by the capillary wick to the bottom of the module to absorb the condensation heat released from the last stage’s condenser. A nonwoven fiber cloth was affixed to the bottom of the module by heat conductive tape to collect crystallized salt left behind by the evaporated brine. This setup omitted the need for the aluminum heat sink while achieving zero liquid discharge. In field experiments, seawater (~35 g·L−1) collected from Baisha Bay, Shenzhen, China (114°30’E, 22°39’N) was used as the source water. Non-toxic, biodegradable antiscalant was added to the source water to prevent salt clogging on the evaporation surface and ensure uninterrupted operation.17, 52 Total organic carbon (TOC) analysis confirmed that no detectable carryover of the anti-scaling agent in the produced water (Figure S10). The experimental setup is shown in Figure 5a. Two modules were used – one with SSAL insulation and the other without. The modules were placed on 3D-printed ABS support, which was tiled 22.9° to face the sun. The experiments were conducted on a rooftop (Eastern Campus of Sun Yat-Sen University, Guangzhou, China) to ensure no blockage of sunlight by the nearby buildings throughout the day (9:00 − 17:00, November 24th – 30th, 2024 and August 23rd, 2025). In addition, the balances were properly insulated to minimize fluctuation. The solar flux, surrounding temperature and wind speed were recorded at 15 min intervals by a weather station 2 m away from the modules.

Sample characterization
Optical transmittance measurements were performed using an Agilent Cary 5000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere. Transmittance spectra were recorded in the range of 250 to 2500 nm, with a spectral resolution of 1 nm. The solar-weighted transmittance was calculated according to JIS R3106:2019 in the range of 0.3 – 2.5 μm. Transmittance spectra in the range of 2.5 – 15 μm were collected using Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific, USA), at a resolution of 1 cm⁻¹ and an accumulation of 32 scans to ensure high signal-to-noise ratio. Background spectra were recorded under the same conditions and subtracted from each sample measurement. The morphologies of aerogels were observed by field-emission scanning electron microscopy (FESEM, Gemini500, Zeiss, Germany) and transmission electron microscopy (TEM, Tecnai G2 Spirit 120kV, Thermo Fisher Scientific, USA). For TEM imaging, the aerogel was gently rubbed with ultrafine sandpaper (10,000 grit), and the resultant powder was collected and dispersed in ethanol. The dispersion was dropped onto a copper mesh and imaged at an accelerating voltage of 120 kV. The porous structure of the aerogel was characterized by nitrogen adsorption–desorption isotherms at 77 K using Micromeritics ASAP 2460 (USA). Prior to measurement, samples were degassed at 80°C for 12 h. The pore size distribution was determined via the original density function theory method from the adsorption branch.

Thermal conductivity of the aerogel was measured on Hot Disk TPS2500S (Sweden) using the transient plane source technique at 25°C, while the water contact angle was measured on Powereach JC2000A-Plus (China). Ionic content of the feed and produced water was quantified using the Shine CIC-D100 ion chromatograph (China). Total organic carbon content (TOC) of the produced water was measured using Shimadzu TOC-L CPH (Japan). TOC results were calculated by the difference between the concentrations of total carbon and inorganic carbon. Prior to ion chromatography and TOC analyses, samples were filtered through 0.22 µm nylon membrane filters (Labshark, China) to remove particulate matter, and diluted with deionized water to fall within the instrument’s linear concentration range.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work reported an aerogel as an insulator for solar absorbers from lab to field and showed good thermal management performance based on this designed aerogel. The manuscript needs to be revised before publication.\n1. The preparation process of the aerogel could to be more detailed. And the cost and maintainability of the scale-up production of SSAL aerogel could be provided.\n2. The thermal conductivity and long-term stability (e.g. salt spray, UV aging test) of the aerogel should be provided.\n3. It is necessary to detailly explain the corresponding relationship “five-fold increase” and “twice that” in the abstract.\n4. The reverse multistage distillation system lacks detailed construction process of the integrated system and material type.\n5. Some terminology is confusing (e.g., “sky cooling”/“radiative heat loss to outer space”, “water productivity”/“distillate flattening”…).\n6. Incomplete references in the main text (e.g., Fig. S6 refers to “20% exposed surface leading to heat loss” but is not cited in the result; Fig. S12 aerogel-air layer design is not expanded in the discussion).\n7. As the most central link from lab to field, the economic feasibility is lacked. The discussion part can introduce the quantitative relationship between the cost of aerogel production (e.g., cost of material per square meter) and the benefit of freshwater output to enhance the application reference value.\n8. As a study of the system thermal management, the recovery of latent heat of steam, the water supply temperature of seawater, and the thermal management of brine should be mentioned.", "reviewer_2": "The utilization of silica-based aerogel films in multistage solar distillation is not new as acknowledged by the authors. A recent study by Li et al., (Energy Environ. Sci. 2025) found out that silica aerogel-based cover resulted in evaporation rate of 6.25 kg m−2 h−1 under 1 sun with similar thickness of 6 mm adopted by authors of this study. The study by Li et al. also confirms that covering the top is crucial, indicating that wind speed and heat concentration play a vital role. The evaporation rates achieved in this study (3-4 kg m-2 h-1) are also lower compared to other studies as well. The module design (6-stage, crossflow configuration) replicates established layouts without structural or functional improvements, thus no innovation there as well.\nThis study offers a meticulous analysis of environmental heat losses in MSD and demonstrates that SSAL aerogels can nearly eliminate lab-field disparities. The authors’ experimental and computational analysis of heat-loss pathways is rigorous and provides actionable insights for field deployment. However, the moderate performance gains and reliance on established materials limit its suitability for Nature Communications. While the study is technically sound, it does not meet Nature Communications’ threshold for novelty or performance superiority. I therefore, reject this paper for publication in Nature Communication.", "reviewer_3": "This work focuses to reduce the heat loss in multistage solar desalination devices, mostly through adding solar transparent and IR absorbing aerogel. The authors conduct comprehensive work in material developments, modeling, device optimization, indoor characterization and field tests. It is very impressive with the amount of efforts included in the work. However, there are a few concerns to address, especially when the work is aiming for a high impact journal publication.\n1. The authors mentioned spectrally selective air lock (SSAL) aerogel, which is transparent in solar spectrum, and absorbing in IR spectrum, to reduce convection and radiative heat loss. However, in a previous work (citation 16), a silica aerogel has already been added on the solar absorbing surface. The aerogel is supposed to be with similar performance. Could authors elaborate how this work provides a significant difference compared to the previous work?\n2. The title mentioned “advanced thermal management”. In the actual manuscript, the main effort is associated with adding an aerogel on the solar absorbing surface, as compared to some other multistage devices without aerogels. I feel like specifying the main approach “aerogel” will be more suitable as the title. Otherwise, current title feels much broader and vague, sounding like a review paper to me.\n3. Compared to high performance aerogel, could authors note whether a sealed double-wall window can achieve similar effect? Such window panel is transparent in sunlight and black in IR, while being extremely low cost and easy to fabricate.\n4. Authors may want to reduce the caption length. For example, caption for Figure 3 is almost a full page long.\n5. Page 18, line 480, I believe that there is a typo “2.37 to 1.22 kg·m−2·s−1”. I assume the unit should be kg·m−2·h−1.", "reviewer_4": "In this manuscript, the authors investigated heat input and output during solar evaporation and proposed a strategy to minimize heat loss in field applications by employing SSAL aerogel as a selective solar-transmitting window. The study is quite interesting and has the potential to attract a broad readership of Nature Communications. I would recommend publication of this article after addressing the following minor issues:\n1. The authors refer to SSAL aerogel as a selective solar-transmitting material. Is this material newly developed by the authors? If so, please provide a more detailed analysis and explanation of the physical mechanism responsible for its selective transmittance. If not, please cite appropriate references.\n2. Were the internal temperatures of the evaporating unit measured and incorporated into the thermal analysis? The internal temperature can vary depending on the solar flux intensity and may differ significantly with and without the SSAL aerogel. Since the degree of radiative loss is strongly dependent on the temperature difference between the surface and the surrounding air, this information seems important. Also, there can be a vertical temperature gradient in the device due to the evaporative cooling.\n3. A recent report discussing the observation of evaporation behavior under different surrounding temperatures may be relevant and worth citing:\nhttps://doi.org/10.1016/j.desal.2024.118077\n4. It is generally accepted that convection over an evaporation surface enhances the evaporation rate due to more vigorous heat acquisition from the surroundings, which appears to contradict the results presented in this study. Could the authors clarify this discrepancy? This phenomenon is also strongly correlated with the temperature difference between the evaporation surface—reduced by evaporative cooling—and the surrounding air.\n5. In a multi-stage evaporation system, both the number of evaporating layers and the distance between them can significantly influence the overall evaporation performance. Were these parameters optimized in this study?"}, "author_response": "Response to Reviewers\nWe sincerely thank the reviewer for the time and effort invested in evaluating our\nmanuscript. The feedback provided was extremely constructive and has been\ninvaluable in helping us to improve the quality, clarity, and physical depth of our work.\nIn response to the reviewer’s remarks, we have made several revisions to the\nmanuscript. We clarified how elevated ambient temperatures enhance the vapor\npressure gradient and thus the mass transport, allowing field performance to exceed\nlaboratory benchmarks (ERI > 1). We have included new ERI heat maps as a function\nof module dimension, stage number, and environmental factors to guide module scale-\nup and optimization in real-world deployment. Finally, to avoid overcrowding the main\ntext and to better focus the manuscript on the practical value of the ERI in real\napplications, two of the material-centric subsections have been moved to the\nSupplementary Notes.\nIn the following sections, we provide a detailed response to the reviewer’s remarks,\nclarifying how the manuscript has been revised accordingly. All changes made are\nindicated in orange in the revised manuscript. Within this response document, our\nreplies are presented in blue text, while the corresponding revised text in the\nmanuscript is presented in orange. We hope that the changes made have met the\nreviewer’s expectations and further strengthened our work’s contribution to the field.\nReviewer #3\nI would like to appreciate the significant efforts that went into this revision from the\nauthors, along with the insights into the solar desalination, beyond what has been\npresented in previous manuscript. I think the ERI can be a valuable metric to guide\nthe industrial adoption when translating lab-scale work into a much large scale.\nThe authors pointed out that ERI can rise above 1, due to higher ambient\ntemperature over 25C. I believe that that is because saturated vapor pressure does\nnot rise linearly with temperature. The vapor pressure difference of 30C/25C is\nsmaller than 35C/30C. With identical temperature rise, higher ambient temperature\ncan produce higher mass transport for water evaporation. Please feel free to correct\nme if I am wrong. I assume that this explanation may provide more in-depth physical\nperspectives.\nWe are grateful for the reviewer’s positive assessment and the appreciation of our\nrevision efforts. We also agree that, the non-linear, exponential relationship between\n1\nsaturated vapor pressure and temperature (according to the Clausius-Clapeyron\nequation) is indeed the fundamental driver behind the observed enhanced water\nproductivity and ERI under elevated ambient temperatures. As the reviewer correctly\nnotes, because the vapor pressure curve steepens at higher temperatures, an identical\ntemperature difference between evaporator and condenser yields a significantly larger\nvapor pressure gradient at 40°C than at 25°C. This leads to higher vapor mass\ntransport rates and, thus, higher water productivity. We have modified the manuscript\nto explicitly include this physical explanation.\nLine 339, page 13: This phenomenon is rooted in the exponential dependence of\nsaturated vapor pressure on temperature. As T elevates the module’s internal\namb\ntemperature profile, the vapor pressure gradient driving mass transport between\nevaporator and condenser is enlarged for the same temperature difference.\n1. Should ERI be defined against a specific 25C lab test condition, or the lab test\ncondition should share the same ambient conditions with field tests? ERI quantifies\nhow sensitive it is to non-ideal conditions including sky cooling and convection in\nfield tests. Adding transparent aerogel insulation, for example, increases ERI for a\nsolar desalination design, making it more resilient to ambient field conditions. If the\nERI fluctuates a lot with ambient temperatures, the metric may not be very\nuniversally adopted. If ERI is set to compare lab vs field at same ambient\ntemperatures, it sounds more reasonable to me, with a numerical range of 0 to 1.\nWe appreciate the reviewer’s thoughtful critique regarding the definition and\nuniversality of ERI. After careful consideration, we have decided to maintain the current\ndefinition, referencing a fixed laboratory benchmark (25°C, windless, no sky cooling),\nfor several reasons that we believe are central to the metric’s utility:\n1. Standardization: In both research and industry, solar thermal modules are\nrated based on standardized laboratory benchmarks. We believe the primary\nvalue of the ERI lies in its role as a lab-to-field comparator. If the denominator\nwere to shift with every change in field temperature, the ERI would essentially\nrevert to a conventional efficiency metric, losing its specific utility as a tool to\npredict how a rated module will behave in a complex, real-world environment.\n2. Robustness Signaling via Fluctuation: For an ill-insulated module, the ERI\nwill fluctuate significantly across different environments. This fluctuation is itself\na valuable signal of environmental fragility. As shown in our study, a module\nimplementing SSAL insulation exhibits a much more stable ERI across varying\n2\nconditions. Therefore, the stability (or lack thereof) of the ERI informs the\neffectiveness of the module’s thermal management.\n3. Site Assessment: Maintaining a fixed benchmark allows the ERI to signal\nperformance boosts. An ERI > 1 scenario informs a project developer that a\nspecific deployment site, such as a high-temperature arid region, offers\nenvironmental conditions that will actually boost productivity beyond the\nmodule’s nominal rating, a critical insight for economic modeling.\nHowever, for fairness and transparency of module-to-module comparisons, they are\nbest performed under identical environmental settings to determine which module\ndesign and configuration is intrinsically more resilient. We acknowledge that the ERI\ncomparisons across prior works in Figure 5f involve different ambient temperature\nconditions. To address this, we have modified Figure 5f and the associated text to\ninclude the average ambient temperature recorded during the field tests for each study.\nThe Supplementary Table S2 is also updated accordingly.\nLine 429, page 16:\nLine 512, page 19: To facilitate comparison, the average ambient temperatures at\n12:00 – 14:00 recorded during the respective field tests are indicated in the figure\nlegend.\nSI Line 390, page 17:\nTable S1. Previous studies on MSD comparing the water production rate obtained in\nlaboratory settings with that obtained outdoors. The water production rates have been\nconverted to kg·kWh−1 to account for the variation of solar irradiation. The ambient\ntemperature in outdoor tests is taken from the averaged recorded temperature at 12:00\n– 14:00 of each study. T : ambient temperature. WP: water productivity.\namb\namb\nwater number WP, WP, outdoor\nkg·kWh−1 kg·kWh−1 test, °C\nSeawater 21\nChiavazzo, 0.6\n(Ligurian 10 3.28 2.07\n202020 3\nSea) 33 0.4\nXu, 202021 3.5% NaCl 10 5.78 2.6\nSeawater 3 2.00 1.25\n202022 3\nHuang, 37 0.6\nSeawater 6 2.20 1.45\n202023 6\nSeawater 29 0.4\nWang, 202124 8 3.85 1.66\n(Red Sea) 3\n23 0.9\nWang, 202225 Seawater 4 2.38 2.19\n2\n18 0.4\nWang, 202226 Seawater 5 3.00 1.44\n8\n1.09-1.25 25 0.43-\nWang, 202227 Seawater 6 2.54\n0.49\nSeawater 34 0.8\nTian, 202428 8 2.25 2.0\n(Huanghai) 9\nNot 0.7\nLuo, 202429 Seawater 6 4.65 3.74\nprovided 7\n2. Could authors provide a heat map of ERI or a similar approach, with possible\ndevice design choices as influencing factors? This way, readers/engineers can\ndirectly see how much impact aerogel/thermal insulation/sky cooling affect the ERI\n4\nof the specific design. This will provide a direct visual illustration how much each\nfactor dominates the overall performance.\nWe find the reviewer’s suggestion to visualize the interplay between device design and\nenvironmental factors to be very valuable. In the revised manuscript, we have added\na sensitivity analysis presented as ERI heat maps (Figure S15). We selected two\ndevice design parameters for this analysis: module dimension and stage number. The\nformer is a necessary design choice to consider when attempting scale-up, while the\nlatter shifts its optimum with ambient temperature. We demonstrate that as module\ndimensions increase, the relative impact of sidewall heat losses with respect to solar\nenergy input decreases. This leads to improved ERI for large-scale modules, even\nunder high wind speeds. The heatmaps also reveal that for a given wind speed at a\npotential deployment site, there exists a threshold module dimension required to\nmaintain an ERI ≈ 1.\nFurthermore, the analysis illustrates that the optimal stage number is strongly coupled\nto ambient temperature (T ). At elevated T , increasing the stage number provides\namb amb\na dramatic boost to both water productivity and ERI. Conversely, at low T , fewer\namb\nstages are preferred to minimize parasitic heat losses associated with the increased\nsidewall surface area. These heat maps support module design and optimization\nbased on the specific environmental factors of the targeted deployment site.\nSI Line 499, page 27:\nFigure S15. Sensitivity analysis of the Environmental Robustness Index (ERI) to\ndevice design and environmental conditions. The top row (a, b) represents\n5\nconventional module without cover, while the bottom row (c, d) represents a module\nemploying the glass-air composite as top insulation. (a, c) ERI as a function of wind\nspeed and module dimension at 25°C outdoors. (b, d) ERI as a function of ambient\ntemperature (T ) and stage number under outdoor, windless conditions.\namb\nLine 586, page 22: ERI Sensitivity to Module Design Parameters\nTo support engineering decisions regarding module design choices that directly impact\nenvironmental robustness, two influential parameters were investigated: module\ndimension and stage number. The module dimension (width) is the primary factor to\nconsider during scale-up. Due to laboratory constraints, such as the solar simulator’s\naperture size, prototypes are typically restricted to small dimensions (e.g., the 5 cm ×\n5 cm modules utilized in this study). In practical applications, larger modules are\nemployed to reduce specific material costs. A subtle consequence of scaling up is the\nreduced sidewall heat losses as a percentage of the total energy balance, driven by a\nlower perimeter-to-area ratio. Thus, beyond economies of scale, scaling up is expected\nto enhance thermal efficiency and water productivity. Figure S15a and c illustrate this\nrelationship by presenting the ERI as a function of wind speed and module dimension\nfor a module without cover and employing a glass-air composite for top insulation,\nrespectively. At constant wind speed, the ERI improves consistently with increasing\nmodule dimension, suggesting that for a given wind speed at a potential deployment\nsite, there exists a threshold module dimension required to maintain an ERI ≈ 1. Note\nthat the ERIs are calculated relative to the 5 cm × 5 cm laboratory prototype as a\nconsistent baseline, since laboratory testing is typically conducted on small-scale units.\nWhile a 6-stage configuration was selected for experimental validation at 25°C to\nbalance productivity with material costs, increasing ambient temperature shifts this\noptimum to higher stage number (Figure S15b and d). This is credited to the overall\ntemperature profile of the module shifting to higher values, exponentially increasing\nthe vapor pressure gradients in all stages and elevating overall water productivity,\nwhich justifies additional material costs. Conversely, at T < 25°C, increasing stage\namb\nnumbers actually decreases ERI as the added sidewall surface area incurs heat losses\nwithout a proportional gain in water productivity. The results indicate that in\nenvironments characterized by high ambient temperatures, increasing the stage\nnumber provides a strategic pathway to maximize water output.\nFinally, we moved two of the subsections in the earlier version’s Discussions to\nSupplementary Notes, as they are material-centric or contain too much detail on\n6\nabsorber temperature distribution. This helps to focus the main text on the practical\nvalue of ERI in real deployment.\nLine 264, page 10: While aerogel is the highest-performing configuration, the\ncomposite design is highly competitive, proving that a high ERI can be successfully\nengineered using accessible, conventional materials (Note S3).\nLine 312, page 12: This excellent retention is reflected in its high ERI, which remains\nabove 0.9 (Figure 3b), as well as the temperature profile of the solar absorber, which\nremains consistently high and homogeneously distributed (Note S5 – S6, Figure S7).\nSI Line 159, page 7:\nNote S3. SSAL Design Frontiers: High-Temperature Constraints and Mitigation\nWhile PMMA and soda-lime glass composites were demonstrated to be competent\nand cost-effective alternatives to silica or silicone aerogels for mild-temperature\napplications, their utility is limited by their low thermal stability. Extending the\napplication of SSAL insulation to higher operating temperatures, such as those\nencountered in concentrating solar power (>200°C), would render PMMA and glass\ninfeasible. In contrast, silica aerogels are thermally robust, with thermogravimetric\nanalysis indicating stability up to approximately 400°C (Figure S14). For such high-\ntemperature applications, silica/silicone aerogel is therefore the viable choice among\nthe materials studied for SSAL insulation. However, careful consideration must be\ngiven to the aerogel's spectral properties. As operating temperatures increase, the\npeak of the blackbody emission spectrum shifts to shorter wavelengths, beginning to\noverlap with silica/silicone aerogel’s inherent high-transmittance window in 3 – 5 µm\n(Figure 2a). This can lead to substantial radiative heat losses directly through the\naerogel, a pathway that is negligible at lower temperatures. This constitutes a new\n“environmental fragility” that must be engineered. To mitigate this effect, material and\nsystem-level modifications can be employed. The internal pore surface of the aerogel\ncan be modified via atomic layer deposition, where a thin layer of aluminum silicate\nhas been shown to suppress thermal radiation in this transparency window without\ncompromising solar transmittance. Concurrently, employing spectrally selective\nabsorbers, which are characterized by low thermal emissivity, would be advantageous\nto further minimize radiative heat loss, representing a departure from the broadband,\ncarbonaceous absorbers typically utilized for lower-temperature applications. This\nhighlights that \"engineering environmental robustness\" is not a static solution but a\ndynamic design principle that must be adapted to the specific operational physics of\nthe system.\nNote S6. Temperature distribution of the absorber and its implications\nFinite element analysis was employed to investigate the temperature distribution\nacross the solar absorber under varying wind speeds (Note S5). The simulations\nrevealed significant spatial temperature inhomogeneity on the uncovered absorber,\neven in the absence of wind. Natural convection alone was sufficient to induce\nnoticeable temperature gradients across the absorber, reaching magnitudes as large\nas 0.075°C·mm−1 (Figure S7). Furthermore, forced convection established a clear\ntemperature gradient aligned with the wind direction due to the variation of local\nconvection coefficient along the absorber (Note S1). The magnitude of the slope was\nhigher compared to the windless scenario, 0.107°C·mm-1 at 2 m·s−1 and 0.088°C·mm-\n1 at 4 m·s−1. Scaling up the module has been reported to exacerbate the inhomogeneity\nissue.21 Such pronounced temperature inhomogeneity is undesirable as they\npotentially enhance parasitic conductive heat losses laterally within the absorber itself\nand the evaporator. Incorporating SSAL insulation was found to mitigate this\ntemperature inhomogeneity across the absorber surface under all wind conditions. The\ngradient reduced to 0.034°C·mm−1 and 0.015°C·mm−1 under 0 m·s−1 and 2 m·s−1 wind,\nrespectively. By promoting a more uniform temperature distribution, SSAL insulation\nreduces lateral conductive losses and enhances thermal efficiency for water\nevaporation.\n8"}]

Decision letter