Integrated triboelectric nanogenerator and radiative cooler for all-weather transparent glass surfaces

Domain: earth_science
Published: 2024-08-02
Editorial outcome: accept

Abstract
Sustainable energies from weather are the most ubiquitous and non-depleted resources. However, existing devices exploiting weather-dependent energies are sensitive to weather conditions and geographical locations, making their universal applicability challenging. Herein, we propose an all-weather sustainable glass surface integrating a triboelectric nanogenerator and radiative cooler, which serves as a sustainable device, harvesting energy from raindrops and saving energy on sunny days. By systematically designing transparent, high-performance triboelectric layers, functioning as thermal emitters simultaneously, particularly compatible with radiative cooling components optimized with an evolutionary algorithm, our proposed device achieves optimal performance for all-weather-dependent energies. We generate 248.28 Wm−2 from a single droplet with an energy conversion ratio of 2.5%. Moreover, the inner temperature is cooled down by a maximum of 24.1 °C compared to pristine glass. Notably, as the proposed device is realized to provide high transparency up to 80% in the visible range, we are confident that our proposed device can be applied to versatile applications.

Introduction
Sustainable energy scavenged from abundant natural resources is considered to be the foremost and fundamental step towards achieving carbon neutrality and transitioning to an ecologically benign source of power. Among all the clean energy sources associated with climate, e.g., sunlight, raindrops, and wind, are the most ubiquitous and non-depleted in our daily lives. Therefore, previous studies, which have focused on converting natural energy into usable energy, especially electrical energy, e.g., solar cell1–3, piezoelectric4–6, and triboelectric nanogenerators (TENG)7–9, are receiving increasing attention.

Among them, solar cells have garnered spotlight with extensive study owing to their high efficiency10,11. However, solar cells have limitations associated with various weather conditions and pose numerous challenges requiring large areas and high costs. Meanwhile, radiative cooler (RC), an energy saving device that can cool down terrestrial objects without energy input, has gained tremendous attention with an urgent and ever-increasing need to withstand global boiling. Due to the spectral overlap between black body radiation and the atmospheric window (AW, 8–13 μm), the heat from objects can be directly transferred to the cold universe in the form of thermal radiation. By maximizing the emissivity in the AW and the reflectance in the solar spectrum (0.3–2.5 μm), daytime radiative cooling can be achieved even under direct sunlight. While early studies focused on maximizing the cooling effect using photonic structures12–15, more recently, devices considering practical applications are being actively proposed including temperature adaptive16,17, textile18,19, and transparent or colored RC16,20,21. Specifically, transparent RC has significant potential for application as self-cooling systems, by integrating transparent RC into vehicles or buildings for an energy saving strategy that can replace the conventional glass. However, the inherent limitation of RC is the weather-dependency. Because water is highly absorptive, RC becomes inefficient in rainy weather affecting the transparency of the atmosphere. The inconsistency in cooling performances and regional dependency decreases its versatility, hindering integration in real-life scenarios.

Meanwhile, studies on energy conversion beyond solar resources have been actively pursued to address weather-dependent challenges. Globally, approximately two-thirds of the year have clear days, whereas the remaining one-third experiences rainfall22. Traditionally, to harness energy from rain including abundant renewable energy, hydroelectric power plants are adopted23,24. However, these approaches tend to be bulky and involve astronomical infrastructure costs. Recently, a methodology has been proposed to directly convert mechanical energy from liquid–solid contact electrifications and electrostatic induction during the raindrop process25,26. TENG, distinguished by notably high instantaneous power density compared to other energy converting elements such as piezoelectricity and pyroelectricity27,28, emerges as an effective inducible energy source. Moreover, TENG has advantages in terms of being lightweight, cost-effective, highly reliable, and providing a wide range of material choices. With limitless possibilities for advancement, study is being conducted to elucidate the mechanism of raindrop TENG and explore structural developments29,30. In particular, efforts are underway to integrate raindrop TENGs with solar cells to enable functionality even in sunlight conditions31,32. However, no existing study explores the integration of RC into raindrop TENG.

Herein, we propose a hybrid energy harvesting and saving glass surface that integrates TENG and RC (TeRC) for all-weather conditions (Fig. 1a). The proposed TeRC consists of multiple layers designed for various purposes (Fig. 1b). The upper part on the glass substrate consists of layers for triboelectricity generation and electrodes, while the lower part includes layers for reflecting solar spectrum and emitting thermal energy. Moreover, by systematically designing transparent (Fig. 1c), triboelectric layers particularly compatible with radiative cooling components, TeRC can expand horizons of weather-dependent energy while providing versatile self-powered applications, e.g., wireless sensors, Internet of Things, and barren environment monitoring.

Results
Design of TENG part in TeRC
To understand the principle by which TeRC generates electricity through raindrops, it is essential to investigate charge carrier, the predominant mechanism of liquid–solid contact electrification (i.e., triboelectrification). According to the hybrid electric double layer (EDL) model26,33,34, during which the first droplet collides with an electrically pristine solid surface, the interatomic potential barrier decreases due to the overlap of the electron cloud when the interatomic distance is within the repulsive region, resulting in electron transfer (Supplementary Fig. 1). The transferred electrons and the chemically absorbed ions on the solid surface form an electrostatic field, resulting in the formation of an electrostatically induced opposite charge of equal magnitude on the lower electrode. When the next droplet collides with an accumulated negatively charged solid surface, the positively charged ions contained within the droplet are attracted to the solid surface due to the electrostatic induction effect, forming an EDL at the liquid–solid interface (Fig. 2a).

A triboelectric layer, an electret material with high charge storage capability, determining TeRC performances must possess a high hydrophobicity which promotes easy droplets sliding, leaving minimal residual droplets. To identify the optimal layer, various transparent polymer films are compared based on contact angle and surface roughness. The contact angle measurement reveals that fluorinated ethylene propylene (FEP) exhibits the highest contact angle of 111.8° (Fig. 2b and Supplementary Fig. 2), which suggests that FEP film is the most hydrophobic, enabling droplets to roll off its surface effortlessly. Additionally, surface roughness for each film is evaluated using scanning electron microscopy (SEM) (Fig. 2c and Supplementary Fig. 3) and atomic force microscopy (AFM) (Fig. 2d and Supplementary Fig. 3). The results definitively demonstrate that FEP film has the most substantial surface roughness among the measured films, which implies that increased roughness plays a crucial role in enhancing surface hydrophobicity, significantly improving triboelectrification. Therefore, throughout this study, we adopt FEP film as the triboelectric layer.

To investigate the behavior of electrons between TeRC and droplets, we represent schematics of the droplet sequence flowing on TeRC (Fig. 2e) and captured corresponding snapshots (Fig. 2f). The proposed TeRC adopts a two-electrode configuration to extract electrical energy by combining electrostatic induction and triboelectrification, transforming interfacial to bulk effect to avoid interfacial screening effect, while also reducing structural complexity26; a thread-like conductive tape is attached as a top electrode, while the lower electrode is composed of an indium tin oxide (ITO) layer sputtered on a glass substrate. The two electrodes are directly connected to the load resistance RL, forming a closed-loop system with the droplet. After the droplet collides with the FEP surface, the triboelectricity generation is categorized into five stages based on the droplet movement: (i) When the droplet contacts with an electron-accumulated FEP surface, the cations in the droplet bind with the electrons on the FEP surface, forming a pseudo capacitor CF at the contact interface. (ii) While the droplet flows along the surface, charge transfer to the external circuit does not occur until it contacts the top electrode, maintaining a switch-off state. (iii) When the droplet contacts the top electrode, new pseudo capacitors are formed between the triboelectric layer and droplet CE1, as well as between the top electrode and droplet CE2. Consequently, a closed-loop is established with the droplet resistance Rdrop, transitioning to a switch-on state. Then, electrons previously stored in CF rapidly flow to the lower electrode (Supplementary Fig. 4). (iv) During droplet shrinkage, electrons return to the top electrode, and CF is recharged by CE1 and CE2. The spreading and shrinking of droplets represent the charging and discharging phenomena of the pseudo capacitors. (v) As the droplet continues to flow away from the top electrode, the electricity generation cycle ends. To measure triboelectric performance of TeRC, we configure the experimental setup to release droplets at the drop height hdrop and the drop angle θdrop (Fig. 2g).

Furthermore, TeRC has a self-cleaning and antifouling ability from outdoor contamination owing to strong hydrophobic surface, which effectively prevents the reduction in triboelectric performance (Fig. 2h). To assess this, chalk powders are employed as dust, and the ink-ed water to facilitate easy identification of residue on the surface. When water is dropped on the conventional glass, it tends to stay, hindering effective cleaning in the presence of dust and leaving residue on the surface. In contrast, when water is dropped on TeRC, droplets easily roll off, carrying away the powder, in this case, TeRC can be effortlessly cleaned with droplets.

TENG performance of TeRC
Throughout this work, we keep RL, film thickness d, hdrop, θdrop, frequency, droplet volume, and water type fixed at 10 MΩ, 250 μm, 45 cm, 45°, 2.5 Hz, 40 μL, and tap water, respectively, to explore the influence of each parameter. In this configuration, we measure the electrical signal of a single droplet in open-circuit conditions (Fig. 3a). When the droplet touches the top electrode, an instantaneous pulse-like electrical signal is measured, reaching up to 415.43 V, and electrons are released from CF. After discharging, a notable observation reveals the detection of a negative signal with the shrinking of droplet, indicating the backflow of electrons attributed to the recharged CF by CE1 and CE2. To calculate the output power, we sweep RL (0.1–10 MΩ), to extract voltage signals (Fig. 3b). By dividing the output power by the maximum spreading area of the droplet Sdrop, we calculate the power density, reaching a maximum value of 248.28 Wm−2 at 0.5 MΩ. Moreover, the transfer charge q also reaches the maximum value of 30.53 nC at the identical RL (Fig. 3a and Supplementary Fig. 5). Furthermore, TeRC produces 2.95 μJ of energy from a single droplet at identical RL, serving as an indicator for evaluating harvested energy (Supplementary Fig. 6).

The selection of the triboelectric layer is a crucial factor in determining the triboelectric performance. The voltages are extracted using various polymer films previously employed for the comparison of hydrophobicity (Fig. 3c). The peak voltage of the droplet over 10 s is highest for FEP of 317.57 V, while the output voltage from other films aligned with the hydrophobic tendency. Notably, the electrical signal using polyethylene terephthalate (PET) is not detected after the first droplet, because the prolonged shrinkage time of the droplet leads to the merging of subsequent droplets, resulting in the electrical cancellation effect (Supplementary Fig. 7). Consequently, we verify that higher hydrophobicity generates a more stable and higher triboelectric performance (Supplementary Fig. 8).

If we assumed that FEP film is much thicker than EDL, the voltage loaded CF can be simplified35,36:

V = qd / (ϵFEP Sdrop)

where ϵFEP represent the permittivity of FEP film. The output voltage is proportional to d, and we experimentally confirm that the output voltage increases with d of FEP film (Fig. 3d). Moreover, in terms of RC part of TeRC, a thicker FEP film functions as a thermal emitter with higher absorption capability in the infrared (IR) range (Supplementary Fig. 19).

In addition, we analyze the output voltage according to hdrop, observing an increase in output voltage with hdrop (Fig. 3e). As hdrop increases, the velocity where the droplet impacts the FEP film also increases. Then, with the increasing Weber number, more electrons participate in triboelectrification26,37. Notably, the droplet inherently carries gravitational potential energy, and the energy conversion ratio η is a crucial factor to evaluate the renewable energy source. By comparing the electrical energy derived from the output power, η records a consistent value about 2.5% regardless of hdrop, which confirms that potential energy is stably converted into electrical energy (Supplementary Fig. 9).

Triboelectric performance also depends on θdrop. The peak voltage increases with θdrop, reaching its peak at 45°, and decreases as θdrop becomes larger, because the contact electrification is closely associated with Sdrop and the electricity discharging time38,39 (Fig. 3f). As θdrop increases, due to the droplet skimming the surface, Sdrop decreases, leading to a reduction in the involvement of electrons in electrification. On the other hand, as the discharging time decreases, the time available for the electrification reaction diminishes (Supplementary Fig. 10). Thus, these two variables synthetically influence the triboelectric performance.

TeRC can be applied to various situations beyond raindrops, such as ocean environments. However, previous studies have reported a reduction in triboelectric performance owing to seawater, but the investigation into the underlying causes has been insufficient40,41. To assess this potential, we evaluate the triboelectric performance based on the potential of hydrogen (pH) of droplets (Fig. 3g). We observe an increasing trend in the output voltage as the pH shifted from acidic to alkaline. This may be attributed to the higher concentration of hydron with decreasing pH, inducing a screening effect that hinders electrostatic induction42,43. However, considering that the pH of global seawater is ~8.1, it can be inferred that the poor triboelectric performance of seawater may not be primarily influenced by pH. Therefore, we compare electrical signals by adjusting salinity using sodium-chloride (NaCl), and observed that as the practical salinity unit (psu) increases, the triboelectric performance deteriorates (Fig. 3h). This is because dissolved NaCl dissociates into Na+ and Cl−, and as psu increases, the ion concentration rises. The excessive free ions in the droplet, caused by the high ion concentration, induced a screening effect. Additionally, we evaluate the potential of TeRC system in real environments by verifying its triboelectric performance using rainwater, and confirmed its ability to maintain excellent electricity generation capabilities (Supplementary Fig. 11).

Design of RC part in TeRC
To cool down an enclosed space under direct sunlight while maintaining visible transparency, the following optical properties should be met; (i) High reflectivity in ultraviolet (UV) and near-infrared (NIR) regions, (ii) high transmittance in the visible region, (iii) high emissivity in the AW, and (iv) high thermal radiation absorption of the inner enclosed system (Fig. 4a). To this end, RC is composed a solar reflector with high AW emissivity material and thick polydimethylsiloxane (PDMS) layer. The reflector consists of two parts; the dielectric-metal-dielectric (DMD) layer, a thin metal film sandwiched between dielectric layers, and the bottom Bragg mirror (Fig. 4b). The permittivity of the dielectric used in the DMD layer is much greater than that of the metal, restricting the surface plasmon coupling and reducing absorption, making it transparent in desired frequencies and strongly reflecting NIR light44,45. Here, we have chosen titanium dioxide (TiO2) and silver (Ag) for high permittivity contrast. To further improve NIR reflection, we devise an additional Bragg mirror layer at the bottom with alternating silicon dioxide (SiO2) and TiO2 layers. Keeping the thickness of the Ag layer fixed to 24 nm, the rest of the thickness profile of the multilayer structure is optimized to have high transparency in the visible and high reflection in UV and NIR using the particle-swarm-optimization algorithm (Supplementary Figs. 14 and 15). Finally, the PDMS layer is placed at the bottom which is responsible for the IR radiation absorption of the system beneath it for effective cooling of an overheated enclosed space46.

The optimized TeRC shows a high visible transparency maximum of 95.7% and an NIR solar reflectance maximum of 88.3% (Fig. 4c). The fabricated TeRC exhibit slightly reduced visible transparency maximum of 80.0%, and NIR solar reflectance maximum of 70.9%, and overall higher absorptance than the optimized structure (Fig. 4d). This discrepancy is attributed to inherent losses in the actual materials including TiO2, Ag, and ITO (Supplementary Fig. 16). Notably, considering that the TENG-only part in TeRC registers a measured visible transmittance below 84.7% (Supplementary Fig. 17), it is expected that the fabricated sample aligns with or falls below this threshold. However, despite the observed discrepancy, the fabricated TeRC maintains commendable level of visible transmittance, enabling clear visibility (Fig. 1c).

Furthermore, the measured emissivity covers all of the AW indicating effective thermal radiation to outer space (Fig. 4e). The angular emissivity of TeRC shows omnidirectional radiation (Fig. 4f). The expected colors and transmitted images are visualized based on the simulated and measured spectra (Fig. 4g and Supplementary Fig. 18). The color coordinates are (0.3288, 0.3669) for the simulated and (0.3376, 0.4074) for the measured TeRC, showing a greenish color.

RC performance of TeRC
To characterize the cooling potential of TeRC, the cooling power is calculated based on the IR optical spectrum (Supplementary Note 22). To consider TeRC as an alternative to conventional glasses, we evaluate radiative cooling potential by assuming the structure is placed on an absorbing chamber, mimicking transparent glazing of vehicles or buildings where most of the non-reflected light is absorbed inside. We compare the optical and cooling properties of TeRC with TENG-only sample and a conventional bare glass. The radiated power from the sample Prad, power absorption from the incident atmospheric emission Patm, and the incident solar flux absorbed by the sample Psun are simulated under ambient temperature Tamb = 303 K (Fig. 5a). Since the non-reflected light is all absorbed inside the chamber, we set the emissivity ε = 1 − R where R represents reflectance. There is no significant difference for Prad and Patm because of the identical sample temperature and effectively high AW emissivity for all three structures (Supplementary Fig. 19). The most notable difference comes from Psun. The NIR reflecting layers of TeRC contribute to decreased Psun of 411.54 Wm−2, which is significantly lower than that of TENG-only, and the glass samples which are 710.8 Wm−2 and 827.54 Wm−2, respectively. Consequently, TeRC shows the highest cooling power than the two references at various temperatures. (Fig. 5b). Equilibrium temperatures where the cooling power is zero at various non-radiative heat transfer coefficient hcc are also calculated (Fig. 5c). Due to the difference in solar absorption, the equilibrium temperatures of TeRC are lower than those of TENG-only and the glass samples by more than 20 and 40 °C, respectively.

To demonstrate the cooling effect of TeRC, we conduct an outdoor measurement with a chamber setup resembling a perfectly absorbing inner system (Fig. 5d and Supplementary Fig. 21). To compare the cooling effect of TeRC to glass and TENG, two reference boxes and an ambient box without the testing sample are measured simultaneously (Fig. 5e). The amount of AW flux emitted by four comparisons are visualized as thermal-IR images (Fig. 5f). The temperature variations are measured over 3 days at the four points; inner temperature of the absorbing chamber in TeRC box TTeRC and in the two reference boxes Tglass and TTENG, the temperature outside the absorbing chamber in the ambient box Tchamber, and ambient temperature outside the acrylic boxes Tamb (Fig. 5g). Notably, TTeRC < TTENG < Tglass shows the TeRC can cool down inner temperature of an enclosed space under direct sunlight by reflecting undesired energy in the non-visible solar spectrum range. TTeRC < Tglass also attributed to strong solar reflection and the large covering area of TeRC sample. The temperature difference between samples is significantly high during the daytime. Specifically, ΔT1 = Tglass − TTeRC reaches maximum 24.1 °C and average of 8.2 °C, while ΔT2 = TTENG − TTeRC reaches maximum 18.0 °C and average 5.7 °C (Fig. 5h).

Prospective application of TeRC
The triboelectricity generated through TeRC can be applied to versatile low-power devices. First, we charge capacitors using a rectifier diode circuit (Fig. 6a). By charging the capacitor, a stable electrical supply is achieved, and we successfully charge a 1 μF capacitor to 2 V in ~50 s (Fig. 6b). Moreover, we demonstrate the potential of TeRC by powering a hundred light-emitting diodes (LEDs) using raindrop energy (Fig. 6c, Supplementary Fig. 22, and Supplementary Movie 1). Furthermore, to demonstrate the potential real-world applicability of TeRC, we arrange multiple TeRCs in an array and measure their electricity generation, proving that the TeRC array produces electrical energy at a much higher frequency than a single TeRC (Supplementary Fig. 23). In addition, we evaluate the geographical dependency of TeRC using global long-term-mean precipitation in July47 (Fig. 6d). The precipitation is predominantly concentrated near the equator, known for dense tropical rainforests. In these regions, TeRC can be utilized for supplying power to harsh environments.

Subsequently, the global radiative cooling power is estimated. The cooling power highly depends on the sample temperature and the incident solar irradiation, meaning local weather conditions can substantially affect TeRC performance. To evaluate the cooling effect compared to the pristine glass, the cooling flux difference ΔPcool = Pglass − PTeRC is calculated based on the Earth ambient air temperature and solar irradiance in July48. The sample temperature is set equal to the Earth ambient air temperature (Supplementary Fig. 24). The calculated map portrays strong geographical dependency due to regional climate differences (Fig. 6e). Regions with high solar power tend to have higher differences owing to strong reflecting properties of TeRC in the non-visible solar range. ΔPcool shows a similar distribution pattern with the solar irradiance map, meaning that it has higher potential in regions under strong sunlight (Supplementary Fig. 25). Consequently, we have confirmed that TeRC, with its functionality as both TENG and RC, can perform multifunctionality in the same region according to climate conditions. Additionally, we have identified its potential to be more appropriately applied, considering geographical conditions.

Methods
Fabrication
To fabricate the RC part of the proposed TeRC system, titanium dioxide (TiO2), silver (Ag), and silicon dioxide (SiO2) layers are deposited on the backside of indium tin oxide (ITO) glass substrates using electron beam evaporation (KVE-ENS4004, KVT). The 10 × 10 × 0.07 cm3 ITO glass exhibit a resistance of 8 Ω sq−1 and an ITO thickness of 150 nm (TMALAB). The thicknesses of the TiO2 layers were 29, 25, 92, and 94 nm for the first, second, third, and fourth layers, respectively. Between the first and second TiO2 layer, a 24-nm thick Ag layer was deposited. The thicknesses of the first and second SiO2 layers were 112 nm and 168 nm, respectively. A 50-μm thick IR absorber was created using a mixture of polydimethylsiloxane (PDMS) (Sylgard 184A, Dow Corning) and its curing agent (Sylgard 184B, Dow Corning) in a 10:1 weight ratio. The mixture was spin-coated onto the TiO2, Ag, and SiO2 multilayer at 1400 rpm for 60 s, and then cured at 70 °C for 2 h.

To develop the TENG part of the proposed TeRC system, a 250-μm thick fluorinated ethylene propylene (FEP) film (FEP0250/004, ALPHAFLON) was attached to all edges of the ITO glass substrate using commercial tape. Subsequently, thread-like conductive fabric tape (GYT12-5020, GT-Wave), consisting of nickel and copper, was attached to the FEP film surface as the top electrode (10 × 0.1 cm2). An identical conductive tape was attached to the ITO surface to form two electrodes. For electricity generation, we opted for conventional conductive tape to enhance durability, as the electrostatic induction on ITO rather than the top electrode plays a dominant role. A syringe pump controlled by a flow regulator was installed at the height hdrop to release water droplets at regular intervals, and the TeRC sample was positioned at the angle θdrop.

Simulation
An in-house rigorous coupled-wave analysis (RCWA) code was used for the optical simulations. The monthly long-term-mean precipitation was obtained from CMAP precipitation data provided by the NOAA PSL, Boulder, Colorado, USA. The data for the long-term-mean Earth skin temperature and solar irradiance in July were obtained from the NASA Langley Research Center POWER Project funded by the NASA Earth Science/Applied Science Program.

Measurement
To assess the hydrophobicity of the polymer films, the contact angles were measured by using droplet analyzer (SmartDrop, FEMTO-BIOMED). The surface roughness of the polymer films that served as the dielectric layer in the TENG part was measured using SEM (Regulus 8100, Hitachi) and AFM (Park XE7, Park Systems). The AFM images were recorded in non-contact mode and at room temperature, with a scanning speed of 5 Hz. The droplet size and time sequences were captured using a high-speed camera (FASTCAM SA3, Photron). The output electrical signal was measured using an oscilloscope (DSOX1204A, Keysight) equipped with a 1:10 voltage attenuator (GTP-100A-4, GW Instek). To adjust the pH of the droplets, citric acid was used to lower the pH, and sodium hydrogen carbonate was used to increase it. Sodium chloride was dissolved in the water to adjust salinity.

A UV-visible-NIR spectrometer (Solidspec 3700, Shimadzu) equipped with an integrating sphere was used to measure the transmittance and reflectance of the TeRC sample and the references in the solar spectral range from 0.3 μm to 2.5 μm. Spectralon (Certified Spectralon White Diffuse Reflectance Standard, Edmund Optics) was used for comparison. In the IR regime from 2.5 μm to 15 μm, the spectra were measured using Fourier-transform IR spectroscopy (Nicolet IS-50, Thermofisher Scientific). For all the measurements, the absorptivity was calculated by subtracting the sum of the transmittance and reflectance from unity.

The size of the samples was 100 × 100 mm2. For the measurement chamber, a wooden frame chamber that was 120 mm in height and 100 mm × 100 mm in width, with a 300-mm high and 100 × 100-mm wide acrylic plate was used. The samples were carefully fixed onto a polystyrene sample cradle, to minimize heat conduction. The acrylic plate had holes on its side to allow air to flow. The wooden chamber was covered with aluminum tape, to reflect sunlight, and the top of the chamber was covered with a low-density polyethylene film to avoid convection. To prepare an absorbing chamber with dimensions of 45 mm (length), 45 mm (width), and 15 mm (height), the exterior of the chamber was painted with commercial white paint, to achieve high solar reflectance; black commercial paint was painted on the interior to allow all incoming light to be absorbed in the chamber. The samples were covered with absorbing chamber which was located in a wooden chamber. The inner temperature of the absorbing chamber was measured using Copper wire format K-type thermocouples inside the chamber for the cooler and reference boxes. In the ambient box, the inner temperature of the acrylic box was measured using the same type of thermocouple. The measurement point of the ambient temperature was located outside of the chamber, where free air flowed and sunlight was blocked. All temperatures were measured at intervals of 30 s, using a data logger (OM-CP-OCTTEMP-A-2000, OMEGA Engineering). The wind speed, relative humidity, and dew point were measured using a weather station (HD52.3D, Delta OHM). The IR camera was calibrated for an emissivity of 0.95.

Data availability
The data that support the findings of the study are available from the corresponding author upon request.

Code availability
The simulation and optimization code are available from the corresponding author upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors reported a new window design by combining triboelectric nanogenerator (TENG) and radiative cooler (RC). Different from conventional energy harvesting window which mainly utilizes photovoltaic material, the application of TENG and energy harvesting from raindrops opens a new venue for energy efficiency window design. Meanwhile, the data provided is rich and comprehensive. Therefore, I recommend publishing this manuscript in Nature Communication after the authors address the following questions.\n1. It is suggested to explain the rationale for adding an IR absorption layer at the back side (Interior faced side) of the window. Based on the previous research (Nano Energy 89, 106440 (2021).; Science 374, 1501-1504 (2021). ), the front side of window should have a high AW emissivity to prompt RC, while the back side of window should have a low AW emissivity. Therefore, I suggest that the authors should reconsider the necessity of adding IR absorption layer at the back side of window.\n2. In Fig. 5f, the author showed IR images for glass, TENG, TeRC, and ambient and claimed that the IR images represented the surface temperature of the samples. I suggest that the authors should take precautions when measuring surface temperature via IR camera. As the AW emissivity of glass, TENG, and TeRC are different, the amount of IR emitted by these materials may not represent their actual surface temperature. Therefore, the author may wish to change the statement from \"surface temperature of four comparisons\" to \"amount of AW emitted by four comparisons\".\n3. Based on Fig. 6d and e, the regions with high application potentials for TeRC include North America, Europe, and part of Asia. As these regions may have both cooling demand in summer and heating demand in winter, the authors should comment on fulfilling both cooling and heating demand in these regions.\n4. I am curious about the energy harvesting efficiency of TENG in real-life situations. As a supplementary, the author may consider measuring energy harvesting efficiency of TENG on a rainy day. This suggestion is not compulsory. However, I believe that an on-site demonstration may be helpful to better illustrate the advantage of TeRC window.\n5. It is understandable that the manuscript was transferred from Nature Sustainability as Fig. 1C, the photo of the sample has a background with the word \"Nature Sustainability\". However, as this paper is currently in the reviewing process (and may be published) in Nature Communication, the authors should change the background to avoid ambiguity.", "reviewer_2": "The authors proposed windows that achieve sustainable functionalities in all-weather conditions by combining energy harvesting using triboelectric nanogenerators leveraging liquid-solid electrification in rainy conditions, and energy saving using radiative cooling technology for selectively reflecting, transmitting, and absorbing the solar spectrum energy to regulate indoor temperature. The integration of seemingly unrelated engineering disciplines to address energy challenges has the potential to pioneer new research fields, sparking interest not only within the scientific community but also garnering significant attention from social and economic communities. I think this work is quite interesting and timely and may contribute to the related community. So, I raise some issues and would like to provide some advice on the manuscript before considering the process of publication in Nature Communications as follows.\n1. In Figures 4c and 4d, the authors present the simulated and measured results of reflectance, transmittance, and absorptance spectra of the designed TeRC. However, while both results show a similar trend, they cannot be considered entirely reasonable. It’s important to sufficiently explain the cause of these differences or demonstrate through further experiment that the simulation results adequately support the experimental findings.\n2. In the process of validating the functionality of the TeRC as a triboelectric nanogenerator, the authors utilized a thread-like conductive tape composed of nickel and copper as the top electrode. Was there a specific criterion for selecting the top electrode? Does the performance of the triboelectric nanogenerator vary when using different types of electrodes?\n3. The authors designed the TeRC with a dielectric-metal-dielectric (DMD) reflector-Bragg mirror underneath the glass substrate for radiative cooling functionality. Was there a specific reason for arranging the two systems in this order? Would the optical performance remain the same if the other of the two systems were reversed?\n4. To effectively utilize the electrical energy harvested from the TeRC in real applications, current is considered a crucial factor. However, the manuscript primarily described the performance of the TeRC in terms of voltage values. It is requested to provide performance metrics regarding the current of the triboelectric nanogenerator.\n5. The TeRC sample used for measuring radiative cooling performance appears quite purple in Figure 5e. In my opinion, the color due to reflection spectra might be more visually prominent than transmission. Could you include calculations for the color based on reflectance?\n6. In Figure 5h, it is stated that the TeRC can lower the inner temperature up to a maximum of 24.1 degrees compared to conventional glass. However, in Figure 5f, the surface temperature shown does not appear significantly lower for the TeRC compared to other samples. Is there a reason for this discrepancy? Please explain how the cooling effect on the interior is achieved.\n7. Please ensure that the explanations are aligned with the time reference set for illustrating the droplet sequence in Figure 2f and Figure 3a. Based on the Figure 2f, the electrical signal is detected at the moment the droplet contacts the electrode at 6.5 ms. However, it seems that Figure 3a employs a different time scale.\n8. Please provide the optical refractive index (n) and extinction coefficient (k) values for the materials used in the design of the TeRC.\n9. Please check the logo of the journal used to indicate the transparency of the TeRC sample in Figure 1c with the logo corresponding to Nature Communications.\n10. The title used in Supplementary Information is different from the main manuscript. Please check it carefully.\n11. The following recommendations should be considered for the next submission: There is a lack of mention and references related the broad field of smart windows. It is recommended that references can be added to enrich the content for the readers, especially those with thermal-, strain-, and/or combined stimulus-responsive materials for windows: [1] ACS Energy Lett. 2023, 8, 5, 2300–2307; [2] Mater. Horiz., 2018, 5, 1000-1007; [3] Joule 2019, 3 ,858–871; [4] ACS Energy Lett. 2020, 5, 4, 1159–1166; [5] Adv. Mater. 2015, 27, 2489–2495; [6] Adv Funct Mater 2023, 33, 2305998.\nThe concept put forward this manuscript is relevant to the sustainable energy, radiative cooling, and triboelectric nanogenerator literature and the results presented are good to aid the future research in this regard. In my opinion, this manuscript can be accepted for publication in Nature Communications after rounds of revision by considering the aforementioned issues, where several points need clarifying and certain statements require further justification for the benefit of the readers. These are as far as I could get in this manuscript. I am pleased to have taken a review of this admirable work.", "reviewer_3": "Review of all-weather sustainable windows for energy harvesting and saving by Geon Lee et. al.\nThe work by Lee et. al presents a window structure comprising a exterior-facing tribolelectric nanogenerator and an interior facing radiative cooler. The design is novel and on the materials front, the theory and the characterizations are reasonable. However on the applications front, there are some major issues, detailed below, that I would suggest the authors to meaningfully address before I can recommend their work for publication. In particular, it appears that their design is better suited for vehicles than buildings. Specifically:\n1. It seems that the authors missed the fact that the TENG itself is a sufficient radiative cooler, and that there is no need for the PDMS reflective cooler in most situations. The authors claim that an emissive surface like PDMS is needed to remove the heat from indoors. This assumes that the indoors are hotter than the window. However, their window has significant solar absorption, which would make it hot. Also buildings that can afford to employ sophisticated systems like theirs are actively cooled to temperatures that can be quite a bit cooler than the outdoor air, and the window. The role of radiative coolers in that case becomes minimizing heat flow from outdoors rather than enabling heat flow from indoors. In that case, having a low-E surface on the interior may be more beneficial than having a radiative cooler.\nWhere I see their system being useful are cars, whose dark interiors and significant stationary periods under the sun make the interiors suitable for their design. But buildings seem more suited to a case where they have their TENG radiative cooler coupled with an indoor facing low-E surface. The authors are asked to comment on this, and adjust the scope of their paper accordingly.\n2. The authors should provide the R, T and E values of their real design (line 244), and not just the simulations, as that is important to validate the performance of their designs.\n3. The authors should explain the mechanism of the triboelectricity generation in more detail, especially since this paper will be read by people from various disciplines. For instance, why do the polymer films in Line 103 have a negative charge? Is it true for all polymers? If so, why? How is the charge regenerated so quickly after the water drops leave?\n4. The authors are requested to add details about the optical properties of the TENG in the main text, as that is probably more critical for cooling. Would the authors please clarify if the properties account for all components, like the conductive tape, which covers a fraction of the area of their design? Is it solar absorptive?\n5. While I do not doubt the functionality of their design, it is unclear if the authors have done the best experiments to show their design’s benefits. For instance, why is there a PE film above their design in Fig. 5? Windows don’t have PE film above them? If the experiments represent car front windows, then they are fine, but if they represent building windows, should they not be vertical? Is it representative to have a setup that absorbs all transmitted sunlight to become very hot (not true for buildings)? How are the indoor temperatures measured – are the thermocouples shaded from the sun?\n6. Also, their cooling power modelling seems to be incorrect. For instance, antarctica has a 0 cooling power in Fig. 6, while Greenland has high cooling power. That does not seem right. Also, using the earth skin temperature for calculations is not correct, they should use ambient air temperatures as a reference. There are ample examples of this. I also would like to suggest that to make calculations simpler, the authors could just do the calculations for a few major cities – a global map is not necessary, and in their case, inaccurate.\n7. It was not clear to us whether the authors have included a comparison of their TENG, and an outdoor radiative cooler (which would be just the optical part of the TENG), that would help readers understand the benefit of the triboelectric part.", "reviewer_4": "In this manuscript, by rationally integrating transparent triboelectric layer and radiative cooler on a glass substrate, the authors demonstrate the combination of triboelectric nanogenerator and radiative cooling in a single window, enabling all-weather energy harvesting and saving. The design is novel and inspirational, but I don’t think the manuscript meet with the high requirement of nature communications.\n1. The discrepancy of real RTA from the simulated results, as explained by the authors, arises from inherent losses in FEP and ITO. I wonder how the FEP film was attached onto the ITO side of the glass. If there is an air gap left between them, the optical performance would be deteriorated.\n2. The authors did not give demonstration of the durability and real-environment performance of TENG, evaluation of possibility of scaling up the sustainable window, and comparison of energy saving and harvesting amount of RC and TENG part. Prospective applications should be enriched to better support the authors’ outlook.\n3. For TENG, peak power density is measured, but total energy output of a single raindrop is not given. As the resistance of the load increases, it is expected that the release of charges takes up a long period, and so the total energy output may increase. It is suggested that the total energy output should be measured for different load resistance. The discussion part gives an unrealistically high value of 0.446 kWh in an hour. This value should be recalculated based upon total energy output instead of upon peak power density to reveal real output capability.\n4. At the beginning of Result section. The description on the electrification mechanism of triboelectric layer is confusing. Triboelectrification (or contact electrification) refers to the microscopic separation or generation of positive and negative charges caused by contact or friction, which originates from the overlap of electron wave function of different materials (refer to: 10.1016/j.mattod.2019.05.016), instead of due to electrical discharge as implied in your description. Triboelectrification produces the negative charges on the polymer film, not the opposite. According to literature (10.1038/s41586-020-1985-6), the continuous impinging of droplets generates the negative charges. The electric discharge is occurred at the stage that the droplet contacts the top electrode, which forms the electric signal in external circuit. This is the working mechanism of droplet-based TENG, not the triboelectrification process.\n5. You mentioned raindrops carry positive charges before reaching the polymer. That’s true. However, even if the raindrops are strictly electrostatically neutral, they become positively charged after impinging the polymer, during which triboelectrification leads to charge transfer from the droplet to the polymer. This process takes place not due to electrical discharge between raindrop and polymer, but due to microscopic solid-liquid interaction and charge transfer. As raindrops continuously collide, triboelectric negative charges on the polymer accumulate till saturation. As a result, the original positive charges on the raindrops barely contribute to this process. It is safe to conclude that the effect of the positive charges on the raindrops due to colliding with air particles do not contribute to triboelectrification. The triboelectrification process should be rewritten.\n6. “the geographical dependency of the two climate conditions exhibits a complementary effect,” the complementary effect of the two climate conditions also seems that the “multifunctional” of TeRC is unnecessary, because there is only one part (TENG or RC) workable in some places."}, "author_response": "Response Letter for\nAll-weather sustainable windows for energy harvesting and saving\nGeon Lee1,10, Hyunjung Kang1,10, Jooyeong Yun1,10, Dongwoo Chae2, Minsu Jeong1, Minseo Jeong3, Dasol Lee3, Miso Kim4,5, Heon Lee2, and Junsuk Rho1,6,7,8,9,*\n(POSTECH), Pohang 37673, Republic of Korea\nRepublic of Korea\nKorea\n(SKKU), Suwon 16419, Republic of Korea\n(SKKU), Suwon 16519, Republic of Korea\n(POSTECH), Pohang 37673, Republic of Korea\n(POSTECH), Pohang 37673, Republic of Korea\nPohang 37673, Republic of Korea\n* Corresponding author: Junsuk Rho (jsrho@postech.ac.kr)\nOverall response to Editor and reviewers\nWe appreciate the Editor and reviewer’s invaluable time and effort in offering positive\nsuggestions for strengthening our manuscript. Thus, it is with great pleasure that we submit\nour revised manuscript for further consideration. We have made substantial changes based on\nthe suggestions you so kindly provided. Additionally, we believe that our edits and responses\nadequately address the Editor’s and reviewer’s suggestions. The revision’s \"responses\" and\n\"modifications\" are highlighted in \"blue\" and \"green\" respectively.\nResponse to Reviewer #1’s comments\nGeneral Comment: In this manuscript, the authors reported a new window design by\ncombining triboelectric nanogenerator (TENG) and radiative cooler (RC). Different from\nconventional energy harvesting window which mainly utilizes photovoltaic material, the\napplication of TENG and energy harvesting from raindrops opens a new venue for energy\nefficiency window design. Meanwhile, the data provided is rich and comprehensive.\nTherefore, I recommend publishing this manuscript in Nature Communications after the\nauthors address the following questions.\nResponse: We extend our heartfelt gratitude to the reviewer for dedicating the reviewer’s\nvaluable time to providing constructive comments, which have greatly enhanced the quality\nof our manuscript and deepened our understanding of study findings. We have carefully\nconsidered and actively incorporated the reviewer’s comments, addressing each point raised\nand resolving the concerns expressed. We have provided a point-by-point response to each\ncomment and have reflected these changes in the revised manuscript. We would like to\nexpress our gratitude to the reviewer for once again acknowledging the novelty and\noriginality of our manuscript and actively recommending it for publication in Nature\nCommunications.\nComment 1: It is suggested to explain the rationale for adding an IR absorption layer at the\nback side (Interior faced side) of the window. Based on the previous research (Nano Energy\n89, 106440 (2021).; Science 374, 1501-1504 (2021).), the front side of window should have a\nhigh AW emissivity to prompt RC, while the back side of window should have a low AW\nemissivity. Therefore, I suggest that the authors should reconsider the necessity of adding IR\nabsorption layer at the back side of window.\nResponse 1: We would like to express our gratitude to the reviewer for providing insightful\ncomments that can enhance the value of our study. Based on the raised questions by the\nreviewer, we would like to clarify our intensions and application. The motivation behind\nadding the PDMS layer at the bottom is to increase the inner heat absorption. The PDMS\nlayer of 50 µm-thick acts as a broadband absorber capable of absorbing thermal radiation at\nhigh temperatures, especially in black-colored inner space. Such an idea has been\ndemonstrated in other studies as well particularly for cooling heated enclosed space [R1.1.1].\n[R1.1.1] Heo, S. Y. et al. A Janus emitter for passive heat release from enclosures. Sci. Adv.\n6(36), eabb1906 (2020).\nNonetheless, to address the reviewer’s concern and emphasize the excellence of our proposed\nTeRC system, we have conducted additional experiments to compare the effect of the PDMS\nlayer, which functions as an IR absorber, on the radiative cooling performance.\nFigure R1.1.1. a Temperature variation during the rooftop measurement. T , T ,\nglass TENG\nT , T represents inner temperature of the absorbing chamber on\nTeRC,with PDMS TeRC,without PDMS\nwhich the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box and T is the\namb\ntemperature outside the acrylic boxes. b Temperature difference between T and\nTeRC,without PDMS\nT .\nTeRC,with PDMS\nAs shown in Figure R1.1.1a, the inner chamber heated by solar irradiation reaches 84.6°C\nwith conventional glass sample, whereas the TENG and TeRC samples demonstrate greater\ncooling effect of the inner temperature. A notable aspect of this additional experiment is the\ndifference in radiative cooling performance with and without the PDMS layer. Therefore, we\ncalculate the temperature difference in the TeRC samples with and without the PDMS layer\n(Figure R1.1.1b). Consequently, we demonstrate that the TeRC sample with the PDMS layer\ncan reduce the inner temperature by up to 3.9 °C compared to the TeRC sample without the\nPDMS layer. This confirms that the PDMS layer functions positively as an effective IR\nabsorber. We have reflected the reviewer’s constructive comment and emphasized the\nvalidity of our proposed TeRC system’s design strategy by adding the following statements\nto the revised manuscript and supplementary information.\n(Revised manuscript, page 17)\n“Additionally, we verify the strong validity and potential of TeRC for real-life applications\nby measuring its radiative cooling performance under various conditions: absence of IR\nabsorbing PDMS layer (Supplementary Fig. 26), low solar energy absorptive white indoor\nenvironment (Supplementary Fig. 27), and when tilted (Supplementary Fig. 28).”\n(Revised supplementary information, page 33)\n“In our proposed design of TeRC, the PDMS layer, functioning as an IR absorber, is applied\nto enhance radiative cooling performance by absorbing heat within the chamber, especially in\nhigh-temperature scenarios such as a black-colored enclosed chamber. To verify the effect of\nthe PDMS layer, we measure radiative cooling performance with and without PDMS. The\nexperimental results show that TeRC samples significantly surpass conventional glass and\nTENG sample in terms of radiative cooling, effectively lowering the chamber’s inner\ntemperature (Supplementary Fig. 26a). More specifically, comparing the TeRC samples with\nand without PDMS layer, the TeRC sample with PDMS reduces the inner temperature by up\nto 3.9°C more than the TeRC without PDMS layer (Supplementary Fig. 26b). The\nexperimental result indicates that the PDMS layer contributes to effective indoor radiative\ncooling acting as an IR absorber.”\nSupplementary Figure 26. a Temperature variation during the rooftop measurement. T ,\nglass\nT , T , T represents inner temperature of the absorbing chamber\nTENG TeRC,with PDMS TeRC,without PDMS\non which the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box and T is the\namb\ntemperature outside the acrylic boxes. b Temperature difference between T and\nTeRC,without PDMS\nT .\nTeRC,with PDMS\nMoreover, in cases where inner temperatures are assumed to be lower or similar to the\nambient such as windows in buildings, PDMS layers may not be necessary. In these\nenvironments, low-E coatings in the inner side can be preferable as they help block infrared\nlight from penetrating the glass, improving the cooling/heating power efficiency. For these\nreasons, to evaluate the effect of the PDMS layer in environments where inner temperatures\ndo not rise excessively, we additionally measure the radiative cooling performance with and\nwithout the PDMS layer of TeRC in a white chamber.\nFigure R1.1.2. a Schematic and b photograph of the rooftop measurement setup using white\nchamber. c Temperature variation during the rooftop measurement. T , T ,\nglass TeRC,with PDMS\nT represents inner temperature of the absorbing chamber on which the\nTeRC, without PDMS\nreference and the cooler sample are placed, respectively. T denotes the temperature\nchamber\noutside the absorbing chamber but inside the acrylic box. d Temperature difference between\nT and T .\nTeRC,without PDMS TeRC,with PDMS\nAs shown in Fig. R1.1.2a, we applied a white inner chamber to prevent the rapid increases in\ninner temperature. To simulate more realistic conditions and consider both radiative and non-\nradiative effects, we removed the PE film from the original setup and measured the cooling\nperformance. The results indicate that the inner temperature of the white chamber with the\nglass sample did not rise as sharply as in the original black chamber. Nevertheless, even with\nthis experimental setup, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Fig. R1.1.2c). Compared to the glass sample, the TeRC cooled down the inner\ntemperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS layer\neffectively radiates internal heat in this configuration, we compared the radiative cooling\nperformance with and without the PDMS layer (Fig. R1.1.2d). The TeRC sample with the\nPDMS layer lowered the inner temperature by up to 2.1°C more than the sample without the\nPDMS layer, confirming that the PDMS layer serves as an effective IR absorber and aids in\nthe dissipation of internal heat.\nIn summary, according to target applications and working environment, these optimal design\nspecifications may be subjected to change. However, we would like to stress our motivation\nbehind adding PDMS layer was to promote its usage in a broad range of applications not only\nas building windows but as windows in cars or other vehicles. Consequently, to emphasize\nthe broad applicability of TeRC and its suitability for various applications, we have included\nthe details of the additional measurements and analyses in the revised supplementary\ninformation.\n(Revised supplementary information, pages 34-35)\n“To demonstrate the efficacy of TeRC in practical applications and to explain the\nfunctionality of PDMS layer, we conduct additional cooling experiments on a clear day under\nvarious conditions resembling an environment of actual real-life use. First, we applied a\nwhite inner chamber to prevent the rapid increases in inner temperature. Second, to simulate\nmore realistic conditions and consider both radiative and non-radiative effects, we removed\nthe PE film from the original setup and measured the cooling performance. Compared to the\noriginal black chamber, the inner temperatures do not rise as high in the white chamber. Even\nin such conditions, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Supplementary Fig. 27c). Compared to the glass sample, the TeRC cooled down\nthe inner temperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS\nlayer effectively radiates internal heat in this configuration, we compared the radiative\ncooling performance with and without the PDMS layer (Supplementary Fig. 27d). The TeRC\nsample with the PDMS layer lowered the inner temperature by up to 2.1°C more than the\nsample without the PDMS layer, confirming that the PDMS layer serves as an effective IR\nabsorber and aids in the dissipation of internal heat.”\nSupplementary Figure 27. a Schematic and b photograph of the rooftop measurement setup\nusing white chamber. c Temperature variation during the rooftop measurement. T ,\nglass\nT , T represents inner temperature of the absorbing chamber\nTeRC,with PDMS TeRC, without PDMS\nwhich the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box. d Temperature\ndifference between T and T .\nTeRC,without PDMS TeRC,with PDMS\nComment 2: In Fig. 5f, the author showed IR images for glass, TENG, TeRC, and ambient\nand claimed that the IR images represented the surface temperature of the samples. I suggest\nthat the authors should take precautions when measuring surface temperature via IR camera.\nAs the AW emissivity of glass, TENG, and TeRC are different, the amount of IR emitted by\nthese materials may not represent their actual surface temperature. Therefore, the author may\nwish to change the statement from \"surface temperature of four comparisons\" to \"amount of\nAW emitted by four comparisons\".\nResponse 2: We appreciate the constructive comment provided by the reviewer, which can\nenhance the completeness of our work. We fully agree with the reviewer’s comment that due\nto the different AW emissivity of glass, TENG, and TeRC, the emitted IR quantity may not\nnecessarily represent the actual surface temperature. Therefore, we have diligently revised the\nstatement from “surface temperature of four comparisons” to “amount of AW emitted by four\ncomparisons” to better convey accurate information, as suggested by the reviewer.\nMoreover, to convey more accurate information to the readers, we have revised the\ndescription to clarify that Figure 5f represents the AW emissivity of each sample as measured\nby the IR camera, instead of the surface temperature calculated based on the measured AW\nemissivity.\n(Revised manuscript, page 13)\n“The amount of AW emitted by four comparisons are visualized as thermal-IR images (Fig.\n5f)”\n(Revised manuscript, page 14)\nFig. 5 Cooling power simulation and rooftop measurement of the samples on top of an\nabsorbing chamber. … f Photograph captured using an infrared (IR) camera for measuring\natmospheric window (AW) flux.\nComment 3: Based on Fig. 6d and e, the regions with high application potentials for TeRC\ninclude North America, Europe, and part of Asia. As these regions may have both cooling\ndemand in summer and heating demand in winter, the authors should comment on fulfilling\nboth cooling and heating demand in these regions.\nResponse 3: We thank the reviewer for providing insightful comments for enhancing the\ncompleteness of our manuscript. Indeed, the presented world map of cooling power covers all\nregions of the world, even where radiative cooling technology is not necessary. Our intention\nis to show the regional dependency of radiative cooling potential of our TeRC system and\nassess its efficacy. Nonetheless, in regions where both summer and winter are present\nswitchable or temperature-adaptive radiative cooling would be desirable for actual practical\nuse. However, we are confident in the novelty and originality of our proposed TeRC system,\nwhich combined radiative cooling technology with a triboelectric nanogenerator to create a\nsustainable energy device with multifunctionality on a single platform. Our work has the\npotential to introduce new principles and strategies by leveraging advancements in the fields\nof radiative cooling and triboelectric nanogenerator, thereby enabling real-world applications.\nNonetheless, to address the reviewer’s valuable comment, we have added the following\ncomments as future perspectives regarding state-of-the-art radiative cooling technologies\nincluding temperature-adaptive phase change materials and switching mode designs in the\nDiscussion section.\n(Revised manuscript, page 17)\n“Moreover, TeRC can be coupled with other energy devices such as solar cells or\npiezoelectric elements to utilize additional energy sources. In terms of radiative cooling, in\nregions such as North America, Europe, and some parts of Asia, where TeRC is suitable for\napplication but sometimes requires heating, the practicality and generality of the TeRC\nsystem can be enhanced through strategies such as incorporating temperature-adaptive phase\nchange materials [R1.3.1-3] and switching mode designs [R1.3.4-6]. Furthermore, by\nintegrating with systems such as electrochromic materials [R1.3.7-9] or elastomeric\nnanocomposites based on the nanoparticle [R1.3.10-12], TeRC could potentially achieve\neven more versatility within a single platform.”\n[R1.3.1] Tang, K. et al. Temperature-adaptive radiative coating for all-season household\nthermal regulation. Science 374(6574), 1504-1509 (2021).\n[R1.3.2] Wang, S. et al. Scalable thermochromic smart windows with passive radiative\ncooling regulation. Science 374(6574), 1501-1504 (2021).\n[R1.3.3] Yang, M. et al. Phase change material enhanced radiative cooler for temperature-\nadaptive thermal regulation. ACS nano 17(2), 1693-1700 (2023).\n[R1.3.4] Zhao, H. et al. Switchable cavitation in silicon coatings for energy-saving cooling\nand heating. Adv. Mater. 32(29), 2000870 (2020).\n[R1.3.5] Fei, J. et al. Switchable surface coating for bifunctional passive radiative cooling and\nsolar heating. Adv. Funct. Mater. 32(27), 2203582 (2022).\n[R1.3.6] Li, X. et al. Integration of daytime radiative cooling and solar heating for year-round\nenergy saving in buildings. Nat. Commun. 11(1), 6101 (2020).\n[R1.3.7] Fang, H. et al. Multifunctional hydrogel enables extremely simplified\nelectrochromic devices for smart windows and ionic writing boards. Mater. Horiz. 5(5),\n1000-1007 (2018).\n[R1.3.8] Cai, G. et al. Molecular level assembly for high-performance flexible electrochromic\nenergy storage devices. ACS Energy Lett. 5(4), 1159-1166 (2020).\n[R1.3.9] Ren, R. et al. Tunable interaction between Zn2+ and superstructured Nb18W16O93\nbimetallic oxide for multistep tinted electrochromic device. ACS Energy Lett. 8(5), 2300-\n2307 (2023).\n[R1.3.10] Ge, D. et al. A robust smart window: reversibly switching from high transparency\nto angle-independent structural color display. Adv. Mater. 27(15), 2489-2495 (2015).\n[R1.3.11] Ke, Y. et al. Adaptive thermochromic windows from active plasmonic elastomers.\nJoule 3(3), 858-871 (2019).\n[R1.3.12] Ke, Y. et al. Bio-inspired, scalable, and tri-mode stimuli-chromic composite for\nsmart window multifunctionality. Adv. Funct. Mater. 33(46), 2305998 (2023).\nComment 4: I am curious about the energy harvesting efficiency of TENG in real-life\nsituations. As a supplementary, the author may consider measuring energy harvesting\nefficiency of TENG on a rainy day. This suggestion is not compulsory. However, I believe\nthat an on-site demonstration may be helpful to better illustrate the advantage of TeRC\nwindow.\nResponse 4: We would like to express our gratitude to the reviewer for providing critical\ncomments that can enhance the value of our study. We wholeheartedly agree that conducting\nan on-site demonstration along with the validation of TeRC window’s TENG functionality\nwould effectively convey the significance and impact of our work to the readers. However,\nwe acknowledge that on-site demonstrations involve numerous external variables, which may\ndeviate somewhat from the conceptual proposal of TeRC presented in this manuscript.\nCurrently, research on TENG utilizing liquid-solid contact electrification is at a very early\nstage, with fundamental studies underway and various promising conceptual designs being\nproposed, and most of these previous papers have also utilized laboratory-level setups to\nvalidate the performance of TENG [R1.4.1-3].\n[R1.4.1] Xu, W. et al. A droplet-based electricity generator with high instantaneous power\ndensity. Nature 578(7795), 392-396 (2020).\n[R1.4.2] Xu, X. et al. A leaf-mimic rain energy harvester by liquid-solid contact\nelectrification and piezoelectricity. Nano Energy 90, 106573 (2021).\n[R1.4.3] Ye, C. et al. An integrated solar panel with a triboelectric nanogenerator array for\nsynergistic harvesting of raindrop and solar energy. Adv. Mater. 35(11), 2209713 (2023).\nNevertheless, we agree with the reviewer’s point that on-site demonstration is an essential\nstep for the practical application of TeRC. Hence, to evaluate its potential for practical\napplications, we have included information on the stability of TeRC’s electricity generation\ncapability. Consequently, we have confirmed that stable electricity of over 300 V was\nmeasured under the same conditions for 10 seconds using the FEP film.\n(Revised manuscript, page 9)\n“Consequently, we verify that higher hydrophobicity generates a more stable and higher\ntriboelectric performance (Supplementary Fig. 8).”\n(Revised supplementary information, page 12)\n“To evaluable the stability of our TeRC system’s electricity generation capability, we\nmeasure the output voltage signal over an extended period for a single droplet. The total\nperiod was 10 seconds, which is sufficiently long to assess the TeRC’s stability, considering\nthat the electricity generation cycle for a single droplet is 40 ms. To assess this, we set load\nresistance, film thickness, drop height, drop angle, drop frequency, droplet volume, and water\ntype fixed at 10 MΩ, 250 μm, 45 cm, 45°, 2.5 Hz, 40 μL, and tap water. Over a period of 10\nseconds, we detect a total of 25 peaks, with maximum, minimum, and average values\nmeasured at 334.37 V, 310.25 V, and 317.57 V, respectively. Consequently, we verify that\nour TeRC system possesses stable electricity generation capability.”\nSupplementary Figure 8. Output voltage signal for a long period.\nMoreover, to investigate the influence of water type dropped in real environments, we\ncollected actual rain water and compared its performance with tap water commonly used in\nthe previous experiments. Compared to tap water, rain water contains numerous impurities,\nwhich induces electric screening effect and results in performance degradation. However,\neven when compared to tap water, we confirmed that rain water still exhibits excellent\nperformance, reading 75% efficiency. This validates the superiority and potential of our\nproposed TeRC system. Consequently, we have added the following statements regarding\nthese discussions in both the revised manuscript and supplementary information.\n(Revised manuscript, page 10)\n“Additionally, we evaluate the potential of TeRC system in real environments by verifying its\ntriboelectric performance using rain water, and confirmed its ability to maintain excellent\nelectricity generation capabilities (Supplementary Fig. 11).”\n(Revised supplementary information, page 15)\n“To assess the potential applicability of the TeRC system, which operates based on raindrop\ncollisions, in real-world environments, we collect rain water and conduct a comparison of its\nelectricity generation capability with tap water. To assess this, we set load resistance, film\nthickness, drop height, drop angle, drop frequency, and droplet volume fixed at 10 MΩ, 250\nμm, 45 cm, 45°, 2.5 Hz, and 40 μL, respectively. Consequently, during a 10-second\nmeasurement from rain water, the maximum, minimum, and average values of the peak\nvoltage and corresponding current are measured at 255.78 V, 231.66 V, 239.98 V,\nrespectively. The decrease in electricity generation performance from rain water can be\nattributed to the potential influence of impurities and high ion concentration between the\ndroplet and the triboelectric layer, resulting in deterioration compared to tap water.\nNevertheless, considering that the average value for tap water is 317.57 V, we confirm that\nrain water exhibits a respectable performance of 75.6% compared to tap water.”\nSupplementary Figure 11. Comparison of peak voltage and current according to the water\ntype.\nFurthermore, we believe that verifying the electricity generation capability of the TeRC\nsystem in a scaled-up scenario is crucial for its practical application. Therefore, we arrange\nthe TeRCs in a 3 × 3 array and integrate an electric circuit to form a TeRC array\n(Supplementary Fig. 23a). Through this experimental setup, we confirm that a stable voltage\noutput above a certain level could be consistently extracted from each TeRC over an\nextended period (Supplementary Fig. 23b). Consequently, we demonstrate that the electric\nsignal measured from the TeRC array can produce electric energy at a much higher frequency\ncompared to a single TeRC system.\n(Revised manuscript, page 15)\n“Furthermore, to demonstrate the potential real-world applicability of TeRC, we arrange\nmultiple TeRCs in an array and measure their electricity generation, proving that the TeRC\narray produces electrical energy at a much higher frequency than a single TeRC\n(Supplementary Fig. 23).”\n(Revised supplementary information, pages 29-30)\nSupplementary Figure 23. a Picture of the experimental setup for TeRC array measurement,\nand b the measured electric signal from TeRC array.\n“TeRC has the drawback of limited electric energy generation within a given time due to its\nlow operating frequency. To overcome this limitation and further emphasize the practicality\nof the TeRC system, we create a scaled-up structure composed of a 3 × 3 TeRC array\n(Supplementary Fig. 23a). The entire TeRC array is assembled by cutting 10 × 10 cm2 holes,\nthe size of the TeRCs, into a 40 × 40 cm2 acrylic plate. Then, the TeRC array, tilted at 45°,\nallows droplets to collide from drop heights of 45 cm, 36 cm, and 27 cm. The drop frequency,\ncontrolled by a flow regulator, is set to 2.5 Hz, and droplets with a volume of 40 μL are\nreleased. Consequently, the electric circuit, connected with conductive tape, is routed out to\nmeasure the electric output signal through a load resistance set at 10 MΩ (Supplementary Fig.\n23b). Examining the extracted output voltage signal reveals that electric energy is generated\nat a much higher frequency compared to the voltage signal from a single TeRC at the same\ndrop frequency, with various voltage levels depending on the drop heights. Furthermore, by\nextracting a consistent output signal over 100 seconds, we demonstrate that the TeRC array\ncan produce stable electric energy.”\nComment 5: It is understandable that the manuscript was transferred from Nature\nSustainability as Fig. 1C, the photo of the sample has a background with the word “Nature\nSustainability”. However, as this paper is currently in the reviewing process (and may be\npublished) in Nature Communication, the authors should change the background to avoid\nambiguity.\nResponse 5: We would like to extend our sincere appreciation to the reviewer for the kind\ncomments. Additionally, we apologize for any ambiguity caused by including an incorrect\nbackground image to express the transparency of our proposed TeRC, which may have\nimpeded the reviewer’s understanding of the manuscript. We are currently in the reviewing\nprocess at Nature Communications, having transferred from Nature Sustainability, and there\nis a possibility of future publication. As such, we have replaced the background image with\nthe journal logo of Nature Communications.\n(Revised manuscript, page 4)\nFig. 1 Concept of the raindrop triboelectric nanogenerator and radiative cooler (TeRC)\nsystem as a sustainable energy device. … c Picture of the fabricated TeRC at outdoor and\nindoor.\nResponse to Reviewer #2’s comments\nGeneral Comment: The authors proposed windows that achieve sustainable functionalities\nin all-weather conditions by combining energy harvesting using triboelectric nanogenerators\nleveraging liquid-solid electrification in rainy conditions, and energy saving using radiative\ncooling technology for selectively reflecting, transmitting, and absorbing the solar spectrum\nenergy to regulate indoor temperature. The integration of seemingly unrelated engineering\ndisciplines to address energy challenges has the potential to pioneer new research fields,\nsparking interest not only within the scientific community but also garnering significant\nattention from social and economic communities. I think this work is quite interesting and\ntimely and may contribute to the related community. So, I raise some issues and would like to\nprovide some advice on the manuscript before considering the process of publication in\nNature Communications as follows.\nThe concept put forward this manuscript is relevant to the sustainable energy, radiative\ncooling, and triboelectric nanogenerator literature and the results presented are good to aid\nthe future research in this regard. In my opinion, this manuscript can be accepted for\npublication in Nature Communications after rounds of revision by considering the\naforementioned issues, where several points need clarifying and certain statements require\nfurther justification for the benefit of the readers. These are as far as I could get in this\nmanuscript. I am pleased to have taken a review of this admirable work.\nResponse: We extend our heartfelt gratitude to the reviewer for dedicating the reviewer’s\nvaluable time to providing constructive comments, which have greatly enhanced the quality\nof our manuscript and deepened our understanding of study findings. We have carefully\nconsidered and actively incorporated the reviewer’s comments, addressing each point raised\nand resolving the concerns expressed. We have provided a point-by-point response to each\ncomment and have reflected these changes in the revised manuscript. We would like to\nexpress our gratitude to the reviewer for once again acknowledging the novelty and\noriginality of our manuscript and actively recommending it for publication in Nature\nCommunications.\nComment 1: In Figures 4c and 4d, the authors present the simulated and measured results of\nreflectance, transmittance, and absorptance spectra of the designed TeRC. However, while\nboth results show a similar trend, they cannot be considered entirely reasonable. It’s\nimportant to sufficiently explain the cause of these differences or demonstrate through further\nexperiment that the simulation results adequately support the experimental findings.\nResponse 1: We appreciate the constructive comment provided by the reviewer, which can\nenhance the completeness of our work. We acknowledge the discrepancy between the\nsimulated reflectance, transmittance, and absorptance (RTA) spectra of the designed TeRC\nand the experimentally measured results featured in Figure 4c and 4d, respectively. In\nresponse to the reviewer’s concern, we have additionally conducted the following analysis.\nFor the simulated results of RTA spectra in the original manuscript, we have utilized optical\nproperties obtained from various references and literatures. Among these, the extinction\ncoefficient of materials used in our designed TeRC, especially TiO , approached zero.\nreflection of actual extinction coefficient values, and hence, we additionally measured the\noptical properties of the actual deposited layers.\nFigure R2.1.1. The measured optical refractive index and extinction coefficient of the\nmaterials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick SiO\n2 2\nlayer (100 nm), c thin TiO layer (35 nm), d thick TiO layer (100 nm), e Ag layer (38 nm),\n2 2\nand f ITO layer (150 nm).\nAs a result, upon examining the refractive index and extinction coefficient of the materials\nused in the designed TeRC, especially TiO , Ag, and Indium Tin Oxide (ITO), we confirmed\n2\nthe discrepancies were mainly caused by the actual material losses which we have overlooked\ninitially. Based on this finding, we have conducted additional simulations of TeRC to obtain\nRTA spectra considering the material losses. Consequently, we confirmed that the RTA\nspectra calculated with updated material losses matched better with the measured RTA\nspectra of the TeRC compared to those calculated without considering the material losses.\nTherefore, we have included the contents mentioned above in the revised supplementary\ninformation to enhance the completeness of our manuscript.\nFigure R2.1.2. a Simulated reflectance, transmittance, and absorptance (RTA) spectra of the\ndesigned TeRC, b simulated RTA spectra of the designed TeRC considering material losses,\nand c measured RTA spectra of the designed TeRC.\n(Revised supplementary information, page 19)\nSupplementary Figure 15. The measured optical refractive index and extinction coefficient\nof the materials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick\n2 2 2\nnm), and f ITO layer (150 nm).\n(Revised supplementary information, page 20)\n“The optical refractive index and extinction coefficient of the various materials used to\ndesign the TeRC are highly dispersive and sensitive to different fabrication conditions\n(Supplementary Fig. 15). Especially, the discrepancy of extinction coefficients can cause\nadditional material losses, negatively affecting the TeRC designed for high transparency in\nthe visible range and high reflectance in the near-infrared (NIR) range. Therefore, we\ncalculate the reflectance, transmittance, and absorptance (RTA) spectra of TeRC based on the\nupdated, measured optical refractive index and extinction coefficient (Supplementary Fig. 16)\nto improve our experiment-simulation consistency. As a result, we achieve RTA spectra with\na maximum transmittance of 81.0% in the visible range and a maximum reflectance of 80.3%\nin the NIR range, confirming that the results align with our desired optical responses.”\nSupplementary Figure 16. Simulated reflectance, transmittance, and absorptance (RTA)\nspectra of the designed TeRC considering material losses.\nComment 2: In the process of validating the functionality of the TeRC as a triboelectric\nnanogenerator, the authors utilized a thread-like conductive tape composed of nickel and\ncopper as the top electrode. Was there a specific criterion for selecting the top electrode?\nDoes the performance of the triboelectric nanogenerator vary when using different types of\nelectrodes?\nResponse 2: We want to extend our heartfelt thanks to the reviewer for offering critical\ncomments that can greatly improve the expertise and quality of our manuscript. As a\ncomponent to impart functionality to TeRC as a TENG, we used a thread-like conductive\nfabric tape consisting of nickel and copper as the top electrode, which was attached to the\nFEP film surface. As the reviewer mentioned, alternative materials or configurations could be\nadopted for the top electrode, but some cases have reported insensitive effect on electricity\ngeneration. As a representative works, various comparisons involving materials such as\naluminum, gold, and copper, as mentioned in the reference below, have demonstrated that the\nmaterial, position, and size of the top electrode have little to no effect on the output voltage.\n‘Output voltages remain constant when the aluminum electrode is replaced by a gold or\nsilver electrode, suggesting that electricity generation is not sensitive to the specific electrode\nmaterial.’\n‘Moreover, the output is insensitive to the size and spatial location of the aluminum electrode\nand to the electrode material.’\n‘This makes sense because the source of electricity generation is the electrostatically induced\ncharge on the ITO, rather than on the aluminum.’\nFigure R2.2.1. Examples of previous studies using various electrodes for measuring droplet-\nbased electricity generating performance. a [R2.2.1], and b [R2.2.2].\n[R2.2.1] Xu, W. et al. A droplet-based electricity generator with high instantaneous power\ndensity. Nature 578(7795), 392-396 (2020).\n[R2.2.2] Zhao, L. et al. Boosting power conversion efficiency by hybrid triboelectric\nnanogenerator/silicon tandem solar cell toward rain energy harvesting. Nano Energy 82,\n105773 (2021).\nFor these reasons, we chose to adopt conventional conductive fabric tape as the top electrode.\nFurthermore, the conductive tape we used possesses adhesiveness compared to conventional\nelectrodes, which can prevent the detachment of the electrode from the FEP film due to the\nimpact of droplets, thereby increasing the durability of TeRC. By adding these details to the\nrevised manuscript, we strive to provide readers with a more comprehensive understanding of\nthe experimental setup process and enhance the quality of the manuscript.\n(Revised manuscript, page 18)\n“For electricity generation, we opted for conventional conductive tape to enhance durability,\nas the electrostatic induction on ITO rather than the top electrode plays a dominant role.”\nComment 3: The authors designed the TeRC with a dielectric-metal-dielectric (DMD)\nreflector-Bragg mirror underneath the glass substrate for radiative cooling functionality. Was\nthere a specific reason for arranging the two systems in this order? Would the optical\nperformance remain the same if the other of the two systems were reversed?\nResponse 3: We are grateful to the reviewer for providing insightful comments that have\nhelped to enhance the justification of the design used in our work. To address the reviewer’s\ninquiry, we have conducted additional simulations on the optical performance by reversing\nthe order of the dielectric-metal-dielectric (DMD) structure and Bragg mirror layers applied\nin TeRC. Consequently, based on the calculated optical properties, the reflectance,\ntransmittance, and absorptance (RTA) spectra indicate that the originally designed TeRC\nexhibits high transmittance in the visible range and high reflectance in the NIR range,\npredicting excellent radiative cooling performance and visible transparency (Fig. R2.3.1a).\nFor comparison, we recalculate the optimized thickness of the deposition layers using the\nparticle-swarm-optimization algorithm after reversing the order of the DMD structure and the\nBragg mirror. The optimized thicknesses are as follows: from the glass, [SiO ; TiO ; SiO ;\n2 2 2\nTiO ; TiO ; Ag; TiO ] = [93; 104; 174; 138; 31; 24; 30] nm. As shown in Fig. R2.3.1b,\n2 2 2\nalthough the general trend is maintained, the overall performance deteriorates compared to\nthe original TeRC. This demonstrates that the order of the deposition layers in the original\nTeRC is advantageous in terms of the targeted optical properties and radiative cooling\nperformance. As a result, to provide readers with a comprehensive understanding of the\nTeRC design process and ensure accurate information delivery, we have included this\ndetailed explanation in the revised supplementary information.\nFigure R2.3.1. RTA spectra of TeRC. a Order to DMD-Bragg mirror, and b Bragg mirror-\nDMD.\n(Revised supplementary information, page 17)\n“The deposited layers applied in designing the RC part of TeRC affect its optical properties\ndepending on their order. Therefore, we have obtained the simulated results of the optical\nproperties for TeRC with different orders of the dielectric-metal-dielectric (DMD) structure\nand Bragg mirror applied for radiative cooling. For comparison, we recalculate the optimized\nthickness of the deposition layers using the particle-swarm-optimization algorithm. The\noptimized thicknesses are as follows: from the glass, [SiO ; TiO ; SiO ; TiO ; TiO ; Ag; TiO ]\n2 2 2 2 2 2\n= [93; 104; 174; 138; 31; 24; 30] nm. As shown in Supplementary Fig. 13, the reflectance,\ntransmittance, and absorptance (RTA) spectra show similar trends to the original TeRC, but\nexhibit overall reduced performance when considering the initial objectives. Consequently,\nwe demonstrate that the order of the deposition layers in the originally designed TeRC is\nadvantageous in terms of targeted optical properties and radiative cooling performance.”\nSupplementary Figure 13. The reflectance, transmittance, and absorptance (RTA) spectra of\nTeRC calculated by changing the order of dielectric-metal-dielectric (DMD) structure and\nBragg mirror.\nComment 4: To effectively utilize the electrical energy harvested from the TeRC in real\napplications, current is considered a crucial factor. However, the manuscript primarily\ndescribed the performance of the TeRC in terms of voltage values. It is requested to provide\nperformance metrics regarding the current of the triboelectric nanogenerator.\nResponse 4: We would like to express our sincere gratitude to the reviewer for providing\nvaluable and constructive comments that significantly contributed to improving the quality of\nour manuscript. We fully agree with the reviewer’s suggestion that comparing the\nperformance of electrical energy harvested by TeRC and providing information on output\ncurrent for real-life applications are crucial factors. In our original manuscript, we have\nillustrated the output voltage of TeRC based on various parameters, including film type,\nfilm thickness, drop height, drop angle, potential of hydrogen, and practical salinity unit,\nin Figure 2. Following Ohm’s law, we have represented this data in terms of current and have\nincluded the details in Supplementary Fig. 12 of the supplementary information.\n(Revised supplementary information, page 16)\nSupplementary Figure 12. Comparison of peak current according to a film type, b film\nthickness, c drop height, d drop angle, e droplet potential of hydrogen (pH), and f droplet\npractical salinity unit (psu).\nComment 5: The TeRC sample used for measuring radiative cooling performance appears\nquite purple in Figure 5e. In my opinion, the color due to reflection spectra might be more\nvisually prominent than transmission. Could you include calculations for the color based on\nreflectance?\nResponse 5: We would like to express our sincere gratitude to the reviewer for providing\ninsightful comments. We agree with the reviewer’s critical opinion that the color represented\nin Figure 5e of TeRC appears to be close to purple compared to the greenish colors depicted\nin Figure 1c and 4g. As the reviewer mentioned, this is attributed to the pronounced effect of\nthe reflectance spectra from the TeRC sample. Depending on the location and the intensity of\nthe illumination source and which side of the sample you look at, the color appearance of the\nsample varies as the spectrum reaching our human eye changes. In response to the reviewer’s\nfeedback, we additionally calculate the color based on reflectance, and have included these\nresults in the revised supplementary information.\n(Revised manuscript, page 12)\n“The expected colors and transmitted images are visualized based on the simulated and\nmeasured spectra (Fig. 4g and Supplementary Fig. 18).”\n(Revised supplementary information, page 23)\nSupplementary Figure 18. The calculated transmitted image through TeRC and the\nexpected colors using simulated and experimentally measured reflectance spectra and their\nCIE 1931 xyY color space.\nComment 6: In Figure 5h, it is stated that the TeRC can lower the inner temperature up to a\nmaximum of 24.1 degrees compared to conventional glass. However, in Figure 5f, the surface\ntemperature shown does not appear significantly lower for the TeRC compared to other\nsamples. Is there a reason for this discrepancy? Please explain how the cooling effect on the\ninterior is achieved.\nResponse 6: We appreciate the constructive comment provided by the reviewer, which can\nenhance the completeness of our work. We fully agree with the reviewer’s suggestion that we\nneed to explain why the performance of the surface temperature does not stand out compared\nto the excellent indoor cooling capability of TeRC. We overlooked the fact that the surface\ntemperature measured by the IR camera may not perfectly match with the actual surface\ntemperature of each sample. This discrepancy arises because the amount of IR flux emitted\nby the different materials may not accurately represent their actual surface temperature,\ndue to different emissivity of glass, TENG, and TeRC samples. Therefore, to convey more\naccurate information to the potential readers, we have modified the statement in the revised\nmanuscript from ‘surface temperature of four comparisons’ to ‘amount of AW emitted by\nfour comparisons.\nMoreover, to convey more accurate information to the readers, we have revised the\ndescription to clarify that Figure 5f represents the AW flux of each sample as measured by\nthe IR camera, instead of the surface temperature calculated based on the measured AW\nemissivity.\nFurthermore, the surface temperature of TeRC may be higher compared to that of other\nsamples, even with the highest inner system cooling capacity. The main reason for increasing\nthe surface temperature is the solar flux absorbed by the sample alone, not the entire system\nwhere energy transmitted pass the sample to the inner system is also considered as being\nabsorbed. Because of the highest absorption spectra of TeRC, the sample surface temperature\nmay rise high. However, as the inner temperature comparison of our experiment shows, this\ndoes not deteriorate indoor cooling capacity of the proposed system.\n(Revised manuscript, page 13)\n“The amount of AW emitted by four comparisons are visualized as thermal-IR images (Fig.\n5f)”\n(Revised manuscript, page 14)\nFig. 5 Cooling power simulation and rooftop measurement of the samples on top of an\nabsorbing chamber. … f Photograph captured using an infrared (IR) camera for measuring\natmospheric window (AW) flux.\nComment 7: Please ensure that the explanations are aligned with the time reference set for\nillustrating the droplet sequence in Figure 2f and Figure 3a. Based on the Figure 2f, the\nelectrical signal is detected at the moment the droplet contacts the electrode at 6.5 ms.\nHowever, it seems that Figure 3a employs a different time scale.\nResponse 7: We express our gratitude to the reviewer for providing constructive comments,\nand we apologize for any confusion caused by not using a consistent time reference. In\noriginal Figure 2f, when depicting the collision and sliding sequence of droplet’s behavior on\nthe FEP film using a high-speed camera, we set the time reference based on the moment the\ndroplet makes contact. However, in Figure 3a, when representing the output voltage signal\nover time for a single droplet, we used the time when the electrical signal is detected, i.e., the\nmoment the droplet contacts the top electrode. To ensure consistent time reference for the\nsame phenomenon, as previously applied in Figure 3a, we have adjusted the time information\nin Figure 2f accordingly, using the moment the droplet contacts the top electrode as the\nreference point.\n(Revised manuscript, page 6)\nFig. 2 Design and mechanism of triboelectric nanogenerator (TENG) part in the\nproposed raindrop triboelectric nanogenerator and radiative cooler (TeRC) system. … f\nsnapshots corresponding to each stage.\n(Revised manuscript, page 8)\nFig. 3 Triboelectric performance of the raindrop triboelectric nanogenerator and\nradiative cooler (TeRC) system. a Output voltage signal extracted a single droplet in the\nopen-circuit condition. The inset displays the current and corresponding transfer charge at the\noptimal load resistance.\nComment 8: Please provide the optical refractive index (n) and extinction coefficient (k)\nvalues for the materials used in the design of the TeRC.\nResponse 8: We would like to extend our sincere appreciation to the reviewer for their kind\nand constructive comments. We agree with the reviewer’s suggestion that the values for\noptical refractive index and extinction coefficient, which were included in the process of\nusing evolutionary algorithms to design the optical thickness of TeRC for both radiative\ncooling performance and transparency, should be provided. Accordingly, we have added this\ndata to the revised supplementary information.\n(Revised manuscript, page 11)\n“Keeping the thickness of the Ag layer fixed to 24 nm, the rest of the thickness profile of the\nmultilayer structure is optimized to have high transparency in the visible and high reflection\nin UV and NIR using the particle-swarm-optimization algorithm (Supplementary Fig. 14 and\nSupplementary Fig. 15)”\n(Revised supplementary information, page 19)\nSupplementary Figure 15. The measured optical refractive index and extinction coefficient\nof the materials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick\n2 2 2\nnm), and f ITO layer (150 nm).\nComment 9: Please check the logo of the journal used to indicate the transparency of the\nTeRC sample in Figure 1c with the logo corresponding to Nature Communications.\nResponse 9: We would like to extend our sincere appreciation to the reviewer for the kind\ncomments. Additionally, we apologize for any ambiguity caused by including an incorrect\nbackground image to express the transparency of our proposed TeRC, which may have\nimpeded the reviewer’s understanding of the manuscript. We are currently in the reviewing\nprocess at Nature Communications, having transferred from Nature Sustainability, and there\nis a possibility of future publication. As such, we have replaced the background image with\nthe journal logo of Nature Communications.\n(Revised manuscript, page 4)\nFig. 1 Concept of the raindrop triboelectric nanogenerator and radiative cooler (TeRC)\nsystem as a sustainable energy device. … c Picture of the fabricated TeRC at outdoor and\nindoor.\nComment 10: The title used in Supplementary Information is different from the main\nmanuscript. Please check it carefully.\nResponse 10: We are grateful for the reviewer’s meticulous attention, which even caught\nminor errors in our previous manuscript. We have updated the title of the supplementary\ninformation from ‘All-weather functional windows for energy harvesting and saving’,\nas originally stated, to ‘All-weather sustainable windows for energy harvesting and saving’,\naligning it with the main manuscript.\n(Revised supplementary information, page 1)\n“All-weather sustainable windows for energy harvesting and saving”\nComment 11: The following recommendations should be considered for the next submission:\nThere is a lack of mention and references related the broad field of smart windows. It is\nrecommended that references can be added to enrich the content for the readers, especially\nthose with thermal-, strain-, and/or combined stimulus-responsive materials for windows: [1]\nACS Energy Lett. 2023, 8, 5, 2300–2307; [2] Mater. Horiz., 2018, 5, 1000-1007; [3] Joule\n2019, 3 ,858–871; [4] ACS Energy Lett. 2020, 5, 4, 1159–1166; [5] Adv. Mater. 2015, 27,\n2489–2495; [6] Adv Funct Mater 2023, 33, 2305998.\nResponse 11: We are grateful to the reviewer for suggesting the inclusion of additional\nreferences to previously reported work on smart windows to emphasize the originality and\nsignificance of the new functional features introduced in our TeRC window system. This\naddition will indeed enrich the content for our potential readers. Therefore, we have actively\nincorporated the studies suggested by the reviewer into the revised manuscript and added\nreferences accordingly as follows.\n(Revised manuscript, page 17)\n“Moreover, TeRC can be coupled with other energy devices such as solar cells or\npiezoelectric elements to utilize additional energy sources. In terms of radiative cooling, in\nregions such as North America, Europe, and some parts of Asia, where TeRC is suitable for\napplication but sometimes requires heating, the practicality and generality of the TeRC\nsystem can be enhanced through strategies such as incorporating temperature-adaptive phase\nchange materials [R1.3.1-3] and switching mode designs [R1.3.4-6]. Furthermore, by\nintegrating with systems such as electrochromic materials [R1.3.7-9] or elastomeric\nnanocomposites based on the nanoparticle [R1.3.10-12], TeRC could potentially achieve\neven more versatility within a single platform.”\n[R2.11.1] Tang, K. et al. Temperature-adaptive radiative coating for all-season household\nthermal regulation. Science 374(6574), 1504-1509 (2021).\n[R2.11.2] Wang, S. et al. Scalable thermochromic smart windows with passive radiative\ncooling regulation. Science 374(6574), 1501-1504 (2021).\n[R2.11.3] Yang, M. et al. Phase change material enhanced radiative cooler for temperature-\nadaptive thermal regulation. ACS nano 17(2), 1693-1700 (2023).\n[R2.11.4] Zhao, H. et al. Switchable cavitation in silicon coatings for energy-saving cooling\nand heating. Adv. Mater. 32(29), 2000870 (2020).\n[R2.11.5] Fei, J. et al. Switchable surface coating for bifunctional passive radiative cooling and\nsolar heating. Adv. Funct. Mater. 32(27), 2203582 (2022).\n[R2.11.6] Li, X. et al. Integration of daytime radiative cooling and solar heating for year-\nround energy saving in buildings. Nat. Commun. 11(1), 6101 (2020).\n[R2.11.7] Fang, H. et al. Multifunctional hydrogel enables extremely simplified\nelectrochromic devices for smart windows and ionic writing boards. Mater. Horiz. 5(5),\n1000-1007 (2018).\n[R2.11.8] Cai, G. et al. Molecular level assembly for high-performance flexible\nelectrochromic energy storage devices. ACS Energy Lett. 5(4), 1159-1166 (2020).\n[R2.11.9] Ren, R. et al. Tunable interaction between Zn2+ and superstructured\nNb18W16O93 bimetallic oxide for multistep tinted electrochromic device. ACS Energy Lett.\n8(5), 2300-2307 (2023).\n[R2.11.10] Ge, D. et al. A robust smart window: reversibly switching from high transparency\nto angle-independent structural color display. Adv. Mater. 27(15), 2489-2495 (2015).\n[R2.11.11] Ke, Y. et al. Adaptive thermochromic windows from active plasmonic elastomers.\nJoule 3(3), 858-871 (2019).\n[R2.11.12] Ke, Y. et al. Bio-inspired, scalable, and tri-mode stimuli-chromic composite for\nsmart window multifunctionality. Adv. Funct. Mater. 33(46), 2305998 (2023).\nResponse to Reviewer #3’s comments\nGeneral Comment: The work by Lee et. al presents a window structure comprising an\nexterior-facing triboelectric nanogenerator and an interior facing radiative cooler. The design\nis novel and, on the materials front, the theory and the characterizations are reasonable.\nHowever, on the applications front, there are some major issues, detailed below, that I would\nsuggest the authors to meaningfully address before I can recommend their work for\npublication. In particular, it appears that their design is better suited for vehicles than\nbuildings.\nResponse: We extend our heartfelt gratitude to the reviewer for dedicating the reviewer’s\nvaluable time to providing constructive comments, which have greatly enhanced the quality\nof our manuscript and deepened our understanding of study findings. We have carefully\nconsidered and actively incorporated the reviewer’s comments, addressing each point raised\nand resolving the concerns expressed. We have provided a point-by-point response to each\ncomment and have reflected these changes in the revised manuscript. Specifically, to address\nthe reviewer’s concerns regarding the appropriate application of our TeRC system, we have\nconducted additional experiments and analyses to demonstrate that our proposed TeRC\nsystem is suitable not only for vehicles, as mentioned by the reviewer, but also for buildings.\nWe would like to express our gratitude to the reviewer for once again actively acknowledging\nthe novelty and originality of our manuscript and potentially recommending it for publication\nin Nature Communications.\nComment 1: It seems that the authors missed the fact that the TENG itself is a sufficient\nradiative cooler, and that there is no need for the PDMS reflective cooler in most situations.\nThe authors claim that an emissive surface like PDMS is needed to remove the heat from\nindoors. This assumes that the indoors are hotter than the window. However, their window\nhas significant solar absorption, which would make it hot. Also, buildings that can afford to\nemploy sophisticated systems like theirs are actively cooled to temperatures that can be quite\na bit cooler than the outdoor air, and the window. The role of radiative coolers in that case\nbecomes minimizing heat flow from outdoors rather than enabling heat flow from indoors. In\nthat case, having a low-E surface on the interior may be more beneficial than having a\nradiative cooler.\nWhere I see their system being useful are cars, whose dark interiors and significant stationary\nperiods under the sun make the interiors suitable for their design. But buildings seem more\nsuited to a case where they have their TENG radiative cooler coupled with an indoor facing\nlow-E surface. The authors are asked to comment on this, and adjust the scope of their paper\naccordingly.\nResponse 1: We would like to express our gratitude to the reviewer for providing insightful\ncomments that can enhance the value of our study. Based on the raised questions by the\nreviewer, we would like to clarify our intensions and application. The motivation behind\nadding the PDMS layer at the bottom is to increase the inner heat absorption. The PDMS\nlayer of 50 µm-thick acts as a broadband absorber capable of absorbing thermal radiation at\nhigh temperatures, especially in black-colored inner space. Such an idea has been\ndemonstrated in other studies as well particularly for cooling heated enclosed space [R3.1.1-\n3].\n[R3.1.1] Zhou, L. et al. A polydimethylsiloxane-coated metal structure for all-day radiative\ncooling. Nat. Sustain. 2(8), 718-724 (2019).\n[R3.1.2] Heo, S. Y. et al. A Janus emitter for passive heat release from enclosures. Sci. Adv.\n6(36), eabb1906 (2020).\n[R3.1.3] Kim, M. et al. Visibly transparent radiative cooler under direct sunlight. Adv. Opt.\nMater. 9(13), 2002226 (2021).\nNonetheless, to address the reviewer’s concern and emphasize the excellence of our proposed\nTeRC system, we have conducted additional experiments to compare the effect of the PDMS\nlayer, which functions as an IR absorber, on the radiative cooling performance.\nFigure R3.1.1. a Temperature variation during the rooftop measurement. T , T ,\nglass TENG\nT , and T represents inner temperature of the absorbing chamber on\nTeRC,with PDMS TeRC,without PDMS\nwhich the reference and the cooler sample are placed, respectively. T demotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box and T is the\namb\ntemperature outside the acrylic boxes. b Temperature difference between T and\nTeRC,without PDMS\nT .\nTeRC,with PDMS\nAs shown in Fig. R3.1.1a, the inner chamber heated by solar radiation reaches 84.6°C with\nconventional glass sample, whereas the TENG and TeRC samples demonstrate greater\ncooling effect of the inner temperature. A notable aspect of this additional experiment is the\ndifference in radiative cooling performance with and without the PDMS layer. Therefore, we\ncalculate the temperature difference in the TeRC samples with and without the PDMS layer\n(Fig. R3.1.1b). Consequently, we demonstrate that the TeRC sample with the PDMS layer\ncan reduce the inner temperature by up to 3.9 °C compared to the TeRC sample without the\nPDMS layer. This confirms that the PDMS layer positively functions as an IR absorber in\nterms of radiative cooling. Therefore, we have reflected the reviewer’s constructive comment\nand emphasized the validity of our proposed TeRC system’s design strategy by adding the\nfollowing statements to the revised manuscript and supplementary information.\n(Revised manuscript, page 17)\n“Additionally, we verify the strong validity and potential of TeRC for real-life applications\nby measuring its radiative cooling performance under various conditions: absence of IR\nabsorbing PDMS layer (Supplementary Fig. 26), low solar energy absorptive white indoor\nenvironment (Supplementary Fig. 27), and when tilted (Supplementary Fig. 28).”\n(Revised supplementary information, page 33)\n“In our proposed design of TeRC, the PDMS layer, functioning as an IR absorber, is applied\nto enhance radiative cooling performance by absorbing heat within the chamber, especially in\nhigh-temperature scenarios such as a black-colored enclosed chamber. To verify the effect of\nthe PDMS layer, we measure radiative cooling performance with and without PDMS. The\nexperimental results show that TeRC samples significantly surpass conventional glass and\nTENG sample in terms of radiative cooling, effectively lowering the chamber’s inner\ntemperature (Supplementary Fig. 26a). More specifically, comparing the TeRC samples with\nand without PDMS layer, the TeRC sample with PDMS reduces the inner temperature by up\nto 3.9°C more than the TeRC without PDMS layer (Supplementary Fig. 26b). The\nexperimental result indicates that the PDMS layer contributes to effective indoor radiative\ncooling acting as an IR absorber.”\nSupplementary Figure 26. a Temperature variation during the rooftop measurement. T ,\nglass\nT , T , and T represents inner temperature of the absorbing\nTENG TeRC,with PDMS TeRC,without PDMS\nchamber on which the reference and the cooler sample are placed, respectively. T\nchamber\ndemotes the temperature outside the absorbing chamber but inside the acrylic box and T is\namb\nthe temperature outside the acrylic boxes. b Temperature difference between T\nTeRC,without PDMS\nand T .\nTeRC,with PDMS\nHowever, as the reviewer has mentioned, this trend may only apply in situations where the\ninternal temperature rises significantly, particularly when the interior is black, resembling a\nperfect absorber. In cases where inner temperatures are assumed to be lower or similar to the\nambient such as windows in buildings, PDMS layers may not be necessary. In these\nenvironments, low-E coatings in the inner side can be preferable as they help block infrared\nlight from penetrating the glass, improving the cooling/heating power efficiency. For these\nreasons, to evaluate the effect of the PDMS layer in environments where inner temperatures\ndo not rise excessively, we additionally measure the radiative cooling performance with and\nwithout the PDMS layer of TeRC in a white chamber.\nFigure R3.1.2. a Schematic and b photograph of the rooftop measurement setup using white\nchamber. c Temperature variation during the rooftop measurement. T , T ,\nglass TeRC,with PDMS\nT represents inner temperature of the absorbing chamber on which the\nTeRC, without PDMS\nreference and the cooler sample are placed, respectively. T denotes the temperature\nchamber\noutside the absorbing chamber but inside the acrylic box. d Temperature difference between\nT and T .\nTeRC,without PDMS TeRC,with PDMS\nAs shown in Fig. R3.1.2a, we applied a white inner chamber to prevent the rapid increase of\ninner temperature. To simulate more realistic conditions and consider both radiative and non-\nradiative effects, we removed the PE film from the original setup and measured the cooling\nperformance. The results indicate that the inner temperature of the white chamber with the\nglass sample did not rise as sharply as in the original black chamber. Nevertheless, even with\nthis experimental setup, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Fig. R3.1.2c). Compared to the glass sample, the TeRC cooled down the inner\ntemperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS layer\neffectively radiates internal heat in this configuration, we compared the radiative cooling\nperformance with and without the PDMS layer (Fig. R3.1.2d). The TeRC sample with the\nPDMS layer lowered the inner temperature by up to 2.1°C more than the sample without the\nPDMS layer, confirming that the PDMS layer serves as an effective IR absorber and aids in\nthe dissipation of internal heat.\nIn summary, according to target applications and working environment, these optimal design\nspecifications may be subjected to change. However, we would like to stress our motivation\nbehind adding PDMS layer was to promote its usage in a broad range of applications not only\nas building windows but as windows in cars or other vehicles. Consequently, to emphasize\nthe broad applicability of TeRC and its suitability for various applications, we have included\nthe details of the additional measurements and analyses in the revised supplementary\ninformation.\n(Revised supplementary information, pages 34-35)\n“To demonstrate the efficacy of TeRC in practical applications and to explain the\nfunctionality of PDMS layer, we conduct additional cooling experiments on a clear day under\nvarious conditions resembling an environment of actual real-life use. First, we applied a\nwhite inner chamber to prevent the rapid increases in inner temperature. Second, to simulate\nmore realistic conditions and consider both radiative and non-radiative effects, we removed\nthe PE film from the original setup and measured the cooling performance. Compared to the\noriginal black chamber, the inner temperatures do not rise as high in the white chamber. Even\nin such conditions, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Supplementary Fig. 27c). Compared to the glass sample, the TeRC cooled down\nthe inner temperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS\nlayer effectively radiates internal heat in this configuration, we compared the radiative\ncooling performance with and without the PDMS layer (Supplementary Fig. 27d). The TeRC\nsample with the PDMS layer lowered the inner temperature by up to 2.1°C more than the\nsample without the PDMS layer, confirming that the PDMS layer serves as an effective IR\nabsorber and aids in the dissipation of internal heat.”\nSupplementary Figure 27. a Schematic and b photograph of the rooftop measurement setup\nusing white chamber. c Temperature variation during the rooftop measurement. T ,\nglass\nT , T represents inner temperature of the absorbing chamber on\nTeRC,with PDMS TeRC, without PDMS\nwhich the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box. d Temperature\ndifference between T and T .\nTeRC,without PDMS TeRC,with PDMS\n(Revised manuscript, page 3)\n“Specifically, transparent RC has significant potential for application as self-cooling\nwindows, by integrating transparent RC into vehicles or buildings for an energy saving\nstrategy that can replace the conventional glass.”\n(Revised manuscript, page 12)\n“To consider TeRC as an alternative to conventional glass windows, we evaluate radiative\ncooling potential by assuming the structure is placed on an absorbing chamber, mimicking\nwindows of vehicles or buildings where most of the non-reflected light is absorbed inside.”\n(Revised manuscript, pages 16-17)\n“Furthermore, TeRC can serve as an alternative to tinted films essential for heat insulation,\ncooling spaces where the inner temperature soars high, such as vehicles and container houses.\nUnlike tinted films, which have limited lifespans and generate volatile organic compounds\nand carcinogenic substances harmful to both human health and the environment, TeRC,\nprimarily composed of dielectric materials and conventional polymer films, offers a long-\nlasting and inexpensive solution. Given that the transmission regulation for tinted films\napplied to vehicles is approximately 70%, TeRC with high transmission can potentially\nreplace conventional tinted films, allowing it to lower indoor temperatures without fuel\nconsumption for air conditioning, making it a cost-effective alternative.”\n(Revised manuscript, page 12)\nFig. 4 Design and characterization of the radiative cooler (RC) part of the raindrop\ntriboelectric nanogenerator and radiative cooler (TeRC). a Schematic of an ideal\ntransparent RC which transmits visible light, reflects ultraviolet (UV) and near-infrared (NIR)\nlight, and emits thermal energy by blackbody radiation in the atmospheric window (AW).\nComment 2: The authors should provide the R, T and E values of their real design (line 244),\nand not just the simulations, as that is important to validate the performance of their designs.\nResponse 2: We fully agree with the reviewer's suggestion that exact values for reflectance,\ntransmission, and absorption of our proposed TeRC system should be provided, not only as\nsimulated results but also as measured results. While we believe that the measurement results\nwere presented in Figure 4c and 4d of the original manuscript, we acknowledge that stating\nthe exact values in the main text would provide readers with more precise information and\nbetter emphasize the excellence of the TeRC system. As a result, we have enhanced the\ncompleteness of the manuscript by adding the statement accordingly in the revised\nmanuscript.\n(Revised manuscript, page 11)\n“The optimized TeRC shows a high visible transparency maximum of 95.7% and an NIR\nsolar reflectance maximum of 88.3% (Fig. 4c). The fabricated TeRC exhibits slightly reduced\nvisible transparency maximum of 80.0%, and NIR solar reflectance maximum of 70.9%, and\noverall higher absorptance than the optimized structure (Fig. 4d). This discrepancy is\nattributed to inherent losses in the actual materials including TiO , Ag, and ITO\n2\n(Supplementary Fig. 16). Notably, considering that the TENG-only part in TeRC registers a\nmeasured visible transmittance below 84.7% (Supplementary Fig. 17), it is expected that the\nfabricated sample aligns with or falls below this threshold. However, despite the observed\ndiscrepancy, the fabricated TeRC maintains commendable level of visible transmittance,\nenabling clear visibility (Fig. 1c).”\nComment 3: The authors should explain the mechanism of the triboelectricity generation in\nmore detail, especially since this paper will be read by people from various disciplines. For\ninstance, why do the polymer films in Line 103 have a negative charge? Is it true for all\npolymers? If so, why? How is the charge regenerated so quickly after the water drops leave?\nResponse 3: We would like to express our gratitude to the reviewer for providing critical\ncomments that can enhance the professionality of our study. We fully agree with the\nreviewer’s suggestion that, considering our work covers various fields such as triboelectricity\nand radiative cooling, we should provide detailed explanations for each discipline to cater to\na broad range of readers. In particular, we have actively addressed the reviewer’s feedback\nregarding the insufficient background information on triboelectricity generation and made\nefforts to provide detailed information to enhance understanding.\nTo actively accommodate the reviewer’s comment, we will first explain the\ntriboelectrification effect occurring in the liquid–solid interface. The mechanism behind\ntriboelectrification has long been a subject of inquiry, with various hypotheses proposed over\ntime. In examining the working mechanism of triboelectrification, it is important to pay close\nattention to what entity initiates the movement of charge. However, recent research results by\nProfessor Wang’s group have revealed that both electron transfer and ion adsorption\n(chemical interaction) are involved in triboelectrification [R3.3.1-2].\n[R3.3.1] Wang, ZL. et al. On the origin of contact-electrification. Mater. Today 30, 34-51\n(2019).\n[R3.3.2] Lin, S. et al. Contact electrification at the liquid–solid interface. Chem. Rev. 122(5),\n5209-5232 (2022).\nThis hybrid electric double layer (EDL) model consists of a “two-step” process. In the initial\nphase, molecules and ions within the liquid collide on the solid surface through thermal\nmotion and pressure. Consequently, the electron clouds of solid atoms and water molecules\noverlap, facilitating electron transfer between them. Notably, materials possessing a high\nelectron-capturing capability can directly acquire electrons from water molecules, including\nions. This electron transfer involves a transition from high-energy states to low-energy states,\nensuring that if the energy variation of electrons is lower than the potential energy barrier,\nthe transferred electrons can be retained on the surface. In the second step, the electrified solid\nsurface attracts free ions in the next droplet through electrostatic interaction, resulting in the\nformation of an EDL. This constitutes the mechanism of triboelectrification occurring at the\nliquid–solid interface. In our original manuscript, we omitted a detailed explanation of this\nprocess and inadvertently failed to distinguish between triboelectrification and the\nmechanism of droplet-based triboelectric nanogenerators, potentially causing confusion for\nthe reviewer. Therefore, in the revised manuscript and supplementary information, we have\nprovided detailed explanations of triboelectrification occurring at the liquid–solid interface.\nAdditionally, we have modified the description of electron behavior in the droplet sequence\nof droplet-based triboelectric nanogenerators as follows.\nMoreover, we will explain the reason why the polymer film becomes negatively charged, as\nraised by the reviewer. When a droplet collides with a virgin solid surface, water molecules\nand surface atoms come into close proximity. During this process, electron transfer occurs\ndue to the overlap of electron clouds in the repulsive force region. Electron transfer occurs\nfrom water molecules to the solid surface. This is because polymers, especially those with a\nlarge number of fluorine groups such as fluorinated ethylene propylene (FEP) or\nperfluoroalkoxy (PFA), possess a strong electron-withdrawing ability, leading to a significant\nelectron transfer effect during the contact electrification process with liquids. For this reason,\nwe evaluated the triboelectricity generation capability of various polymers (FEP, PFA,\npolyvinyl chloride (PVC), and polyester (PET)) and confirmed that all polymers generate\ntriboelectricity, but polymers with fluorine groups exhibit outstanding output performance.\nTherefore, we have modified statements in the revised manuscript to ensure that readers can\neasily understand the basic principles of triboelectricity generation. Additionally, we have\nadded the following contents to the supplementary information.\n(Revised manuscript, page 5)\n“To understand the principle by which TeRC generates electricity through raindrops, it is\nessential to investigate charge transfer, the predominant mechanism of liquid–solid contact\nelectrification (i.e., triboelectrification). According to the hybrid electric double layer (EDL)\nmodel, during which the first droplet collides with an electrically pristine solid surface, the\ninteratomic potential barrier decreases due to the overlap of the electron cloud when the\ninteratomic distance is within the repulsive region, resulting in electron transfer\n(Supplementary Fig. 1). The transferred electrons and the chemically absorbed ions on the\nsolid surface form an electrostatic field, resulting in the formation of an electrostatically\ninduced opposite charge of equal magnitude on the lower electrode. When the next droplet\ncollides with an accumulated negatively charged solid surface, the positively charged ions\ncontained within the droplet are attracted to the solid surface due to the electrostatic induction\neffect, forming an EDL at the liquid–solid interface (Fig. 2a).”\n(Revised manuscript, page 6)\nFig. 2 Design and mechanism of triboelectric nanogenerator (TENG) part in the\nproposed raindrop triboelectric nanogenerator and radiative cooler (TeRC) system. …\ne Schematic of the electricity generation process during the droplet flow sequence.\n(Revised manuscript, page 7)\n“After the droplet collides with the FEP surface, the triboelectricity generation is categorized\ninto five stages based on the droplet movement: (i) When the droplet contacts with an\nelectron-accumulated FEP surface, the cations in the droplet bind with the electrons on the\nFEP surface, forming a pseudo capacitor 𝐶 at the contact interface. (ii) While the droplet\nF\nflows along the surface, charge transfer to the external circuit does not occur until it contacts\nthe top electrode, maintaining a switch-off state. (iii) When the droplet contacts the top\nelectrode, new pseudo capacitors are formed between the triboelectric layer and droplet 𝐶 ,\nE1\nas well as between the top electrode and droplet 𝐶 . Consequently, a closed-loop is\nE2\nestablished with the droplet resistance 𝑅 , transitioning to a switch-on state. Then, electrons\nD\npreviously stored in 𝐶 rapidly flow to the lower electrode (Supplementary Fig. 4). (iv)\nF\nDuring droplet shrinkage, electrons return to the top electrode, and 𝐶 is recharged by 𝐶\nF E1\nand 𝐶 . The spreading and shrinking of droplets represent the charging and discharging\nE2\nphenomena of the pseudo capacitors. (v) As the droplet continues to flow away from the top\nelectrode, the electricity generation cycle ends.”\n(Revised supplementary information, pages 4-5)\n“For a liquid–solid interface, contact electrification (i.e., triboelectrification) has been the\nsubject of much speculation and debate over its mechanism, dating back as far as 2600 years\nago to the present day. Key to understanding the mechanism of liquid–solid\ntriboelectrification is identifying the charge carrier, with hypotheses ranging from electron\ntransfer to ion transfer and species transfer of material. However, recently, according to the\nhybrid electric double layer model proposed by Professor Wang’s group, in the process of\nliquid–solid contact electrification, electron transfer exists. It elucidates that electron transfer\noccurs from water molecules to solid atoms as part of a ‘two-step’ model, followed by the\noccurrence of ion transfer due to the electrostatic induction effect.\nWhen the electrically neutral droplet collides with a pristine solid surface, the distance\nbetween water molecules and solid surface atoms decreases. When the interatomic distance 𝑙\nis greater than the equilibrium distance 𝑙 , they tend to attract each other (Supplementary Fig.\n0\n1a). However, when 𝑙 becomes smaller than 𝑙 , the electron clouds formed by the two\n0\natoms overlap, repel each other (Supplementary Fig. 1c). In this scenario, electron transfer\ncan occur due to the overlap of electron clouds. The highest occupied energy level belonging\nto atom A 𝐸 is greater than of atom B 𝐸 . Therefore, when the two atoms are in the\nA B\nrepulsive region, the potential barrier between them decreases, leading to electron transfer\nfrom atom A to atom B. Additionally, since 𝐸 is greater than 𝐸 , photon energy equivalent\nA B\nto the difference in the two potential energies is emitted during the electron transfer process.\nConsequently, electron transfer occurs from water molecules to the solid surface due to\nliquid–solid contact electrification, resulting in the solid surface being negatively charged. As\nthis process repeats, electron transfer continues to occur and accumulate, eventually reaching\nsaturation depending on the capacity of the solid surface material. As the next step, when a\ndroplet collides with the negatively charged solid surface, the positive ions contained within\nthe droplet are attracted to the negatively charged solid surface due to the electrostatic\ninduction effect. Consequently, this leads to the formation of an electric double layer (EDL)\nat the liquid–solid interface.\nIn this study, electron transfer occurs from the droplet to the solid surface. This phenomenon\narises due to the strong electron-capturing capacity of solid materials, enabling them to\ndirectly acquire electrons from water molecules. Especially, polymers, including fluorinated\nethylene propylene (FEP) and perfluoroalkoxy (PFA) containing numerous fluorine groups,\nexhibit notable electron transfer effects during liquid–solid contact electrification.\nConsequently, the formation of EDL may also be influenced by the electron-donating or\nelectron-withdrawing characteristics of solid materials.”\nSupplementary Figure 1. Hybrid electric double layer model: Electron transfer occurs as a\nresult of the overlap of electron clouds due to interatomic distance within the repulsive region.\na Attractive region, b equilibrium region, and c repulsive region.\nComment 4: The authors are requested to add details about the optical properties of the\nTENG in the main text, as that is probably more critical for cooling. Would the authors please\nclarify if the properties account for all components, like the conductive tape, which covers a\nfraction of the area of their design? Is it solar absorptive?\nResponse 4: We greatly appreciate the reviewer’s valuable comments, which has enhanced\nthe quality of our manuscript. As the reviewer pointed out, the optical properties of the\nmaterials composing the TeRC play a crucial role in determining its radiative cooling\nperformance. Therefore, we fully agree with the reviewer’s suggestion that the optical\nproperties of the materials used in designing the TeRC should be provided to potential\nreaders.\nIn our original manuscript, we provided optical properties for the glass substrate, TENG-only\nstructure, and various polymer films. However, the optical properties of the conductive tape\nattached to the FEP film as the top electrode were omitted. We subsequently measured the\noptical properties with the conductive tape. Consequently, transmission of the conductive\ntape converged to zero in most optical ranges. However, given its thread-like attachment in a\nvery narrow area (10 × 0.1 cm2) compared to the TeRC system’s area (10 × 10 cm2), its\nimpact on the transparency of the TeRC system is minimal. Moreover, the conductive tape\nexhibited predominance of reflectance rather than solar absorption in most optical ranges. If\nabsorption were high, it could have negative affected radiative cooling performance, but with\nhigh reflectance, its impact is minimal or even potentially positive from the perspective of\nradiative cooling. As a result, we have included these details in the revised manuscript and\nsupplementary information.\n(Revised manuscript, page 18)\n“Subsequently, thread-like conductive fabric tape (GYT12-5020, GT-Wave), consisting of\nnickel and copper, was attached to the FEP film surface as the top electrode (10 × 0.1 cm2).”\n(Revised supplementary information, pages 21-22)\nThe reflectance (R), transmittance (T) and absorption (A) of conventional bare glass, TENG,\nRC, and the conductive tape sample are measured. The glass sample shows high\ntransmittance and almost no absorption in the solar spectrum (Supplementary Fig. 17a). The\nabsorption is only strong in 𝜆 > 10 μm, which falls short of the requirement for effective\nradiative cooling (Supplementary Fig. 17b). On the other hand, TENG-only sample shows\nhigh transmittance in the visible but also have notable amount of absorption in the solar\nspectrum (Supplementary Fig. 17c). This is attributed to inherent loss of ITO layer which\npotentially caused the mismatch between the optimized design and fabrication sample\nproperties. However, the absorption of TENG is notably high in the AW suggesting TENG\nitself is a favorable device for radiative cooling (Supplementary Fig. 17d). Moreover, RTA\nmeasurements on the conductive tape attached to the FEP film, serving as the top electrode of\nTENG, reveal that transmission converges to zero in all regions (Supplementary Fig. 17e and\n17f). It is noteworthy that the conductive tape occupies very thin, thread-like areas, thereby\nhaving minimal impact on the transparency of the TeRC system. Out of all samples, RC has\nthe highest NIR reflectance and lowest solar absorption (Supplementary Fig. 17g).\nSupplementary Figure 17. Measured optical properties of a, b bare glass, c, d triboelectric\nnanogenerator (TENG), and e, f the conductive tape, and calculated optical properties of g, h\nradiative cooler (RC). The black, blue and red lines represent reflectance (R), transmittance\n(T), and absorption (A) respectively.\nMoreover, we additionally provided the values for optical refractive index and extinction\ncoefficient, which were included in the process of using evolutionary algorithms to optimally\ndesign the thickness of TeRC for both radiative cooling performance and transparency,\nshould be provided. Accordingly, we have added this data to the revised supplementary\ninformation.\n(Revised supplementary information, page 19)\nSupplementary Figure 15. The measured optical refractive index and extinction coefficient\nof the materials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick\n2 2 2\nnm), and f ITO layer (150 nm).\nComment 5: While I do not doubt the functionality of their design, it is unclear if the authors\nhave done the best experiments to show their design’s benefits. For instance, why is there a\nPE film above their design in Fig. 5? Windows don’t have PE film above them? If the\nexperiments represent car front windows, then they are fine, but if they represent building\nwindows, should they not be vertical? Is it representative to have a setup that absorbs all\ntransmitted sunlight to become very hot (not true for buildings)? How are the indoor\ntemperatures measured – are the thermocouples shaded from the sun?\nResponse 5: We are grateful to the reviewer for providing insightful comments that\ncontribute to enhancing the completeness of our work. Regarding the concern raised by the\nreviewer, our original experimental setup for measuring radiative cooling performance\nincludes the use of polyethylene (PE) film. This is because attaching the PE film helps to\nblock convection effects, which allows us to evaluate the cooling performance driven only by\nradiative heat transfer between the system, the sun and the atmosphere of the Earth. There are\nmany cases in various literatures where such experimental setups have been commonly used\nto verify the effects of radiative heat transfer alone, excluding the influence of conduction\nand convection effects [R3.5.1-4]. Indeed, in real-life scenarios, both radiative and non-\nradiative heat transfer affects the total system. However, we would like the point out our\nintention was to make a fair evaluation of our system’s pure radiative cooling performance.\na c\nb\nd\nFigure R3.5.1. Examples of previous studies using polyethylene (PE) film for measuring\nradiative cooling performance. a [R3.5.1], b [R3.5.2], c [R3.5.3], and d [R3.5.4].\n[R3.5.1] Raman, AP. et al. Passive radiative cooling below ambient air temperature under\ndirect sunlight. Nature 515(7528), 540-544 (2014).\n[R3.5.2] Li, T. et al. A radiative cooling structural material. Science 364(6442), 790-763\n(2019).\n[R3.5.3] Zhu, B. et al. Subambient daytime radiative cooling textile based on nanoprocessed\nsilk. Nat. Nanotechnol. 16(12), 1342-1348 (2021).\n[R3.5.4] Wang, T. et al. A structural polymer for highly efficient all-day passive radiative\ncooling. Nat. Commun. 12(1), 365 (2021).\nHowever, in line with the reviewer’s suggestion, to demonstrate that our TeRC system also\nperforms well in real environments, we conducted additional experiments to measure\nradiative cooling performance without the PE film. These additional experiments are carried\nout in a white body chamber to address another concern raised by the reviewer regarding the\nsetup absorbing all transmitted sunlight and becoming excessively hot. Furthermore, to\naddress the reviewer’s inquiry about the cooling performance at different angles of the TeRC\nsystem, we conducted additional experiments with all samples tilted at a 45°. The reason for\nsetting this angle is that the functionality of TENG in the TeRC performs best at 45°, and we\nwanted to assess whether radiative cooling also works well under this condition. In all the\npast and newly conducted experiments, the thermocouples which measured the inner\ntemperatures were placed on top of the inner surface of the chamber, without being shaded\nfrom the sun. As all the sample case were measured the same way, we assume it is a fair\nevaluation of comparing their cooling performance. The results of the additional experiments\nmentioned above are as follows.\nFigure R3.5.2. a Schematic and b photograph of the rooftop measurement setup using white\nchamber. c Temperature variation during the rooftop measurement. T , T ,\nglass TeRC,with PDMS\nT represents inner temperature of the absorbing chamber on which the\nTeRC, without PDMS\nreference and the cooler sample are placed, respectively. T denotes the temperature\nchamber\noutside the absorbing chamber but inside the acrylic box. d Temperature difference between\nT and T .\nTeRC,without PDMS TeRC,with PDMS\nAs shown in Fig. R3.5.2a, we applied a white inner chamber to prevent the rapid increases in\ninner temperature. To simulate more realistic conditions and consider both radiative and non-\nradiative effects, we removed the PE film from the original setup and measured the cooling\nperformance. The results indicate that the inner temperature of the white chamber with the\nglass sample did not rise as sharply as in the original black chamber. Nevertheless, even with\nthis experimental setup, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Fig. R3.5.2c). Compared to the glass sample, the TeRC cooled down the inner\ntemperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS layer\neffectively radiates internal heat in this configuration, we compared the radiative cooling\nperformance with and without the PDMS layer (Fig. R3.5.2d). The TeRC sample with the\nPDMS layer lowered the inner temperature by up to 2.1°C more than the sample without the\nPDMS layer, confirming that the PDMS layer serves as an effective IR absorber and aids in\nthe dissipation of internal heat.\nFigure R3.5.3. a Schematic and b photograph of the rooftop measurement setup using 45°\ntilted white chamber. c Temperature variation during the rooftop measurement. T ,\nglass\nT , T represents inner temperature of the absorbing chamber\nTeRC,with PDMS TeRC, without PDMS\nwhich the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box. d Temperature\ndifference between T and T .\nTeRC,without PDMS TeRC,with PDMS\nFurthermore, to address the reviewer’s concern about situations where not all sunlight is\nabsorbed, we investigated the effect of the TeRC’s tilted angle. We measured the inner\ntemperature with the samples inclined at a 45° (Fig. R3.5.3a). As a result, although the inner\ntemperature did not rise significantly for any of the samples compared to the black chamber,\nwe demonstrated that the TeRC maintains excellent cooling performance even under these\nconditions (Fig. R3.5.3c). The TeRC cooled the inner temperature by up to 10.0°C compared\nto the glass sample, and by up to 4.4°C compared to the TeRC sample without the PDMS\nlayer, confirming the PDMS layer’s effectiveness as an IR absorber and heat regulator even\nin tilted samples. These results indicate that the TeRC has the potential for a wide range of\napplications, including vehicles, in addition to buildings. Consequently, we have added\nstatements highlighting the TeRC’s excellence in the revised supplementary information, and\nwe would like to thank the reviewer for the insightful comments that have helped enhance the\nquality and completeness of our study.\n(Revised manuscript, page 17)\n“Additionally, we verify the strong validity and potential of TeRC for real-life applications\nby measuring its radiative cooling performance under various conditions: absence of IR\nabsorbing PDMS layer (Supplementary Fig. 26), low solar energy absorptive white indoor\nenvironment (Supplementary Fig. 27), and when tilted (Supplementary Fig. 28).”\n(Revised supplementary information, pages 34-35)\n“To demonstrate the efficacy of TeRC in practical applications and to explain the\nfunctionality of PDMS layer, we conduct additional cooling experiments on a clear day under\nvarious conditions resembling an environment of actual real-life use. First, we applied a\nwhite inner chamber to prevent the rapid increases in inner temperature. Second, to simulate\nmore realistic conditions and consider both radiative and non-radiative effects, we removed\nthe PE film from the original setup and measured the cooling performance. Compared to the\noriginal black chamber, the inner temperatures do not rise as high in the white chamber. Even\nin such conditions, we demonstrated that the TeRC possesses excellent radiative cooling\ncapabilities (Supplementary Fig. 27c). Compared to the glass sample, the TeRC cooled down\nthe inner temperature by up to 8.5°C. Furthermore, to verify whether the IR absorbing PDMS\nlayer effectively radiates internal heat in this configuration, we compared the radiative\ncooling performance with and without the PDMS layer (Supplementary Fig. 27d). The TeRC\nsample with the PDMS layer lowered the inner temperature by up to 2.1°C more than the\nsample without the PDMS layer, confirming that the PDMS layer serves as an effective IR\nabsorber and aids in the dissipation of internal heat.”\nSupplementary Figure 27. a Schematic and b photograph of the rooftop measurement setup\nusing white chamber. c Temperature variation during the rooftop measurement. T ,\nglass\nT , T represents inner temperature of the absorbing chamber\nTeRC,with PDMS TeRC, without PDMS\nwhich the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box. d Temperature\ndifference between T and T .\nTeRC,without PDMS TeRC,with PDMS\n(Revised supplementary information, pages 36-37)\n“We investigated the effect of the TeRC’s tilted angle. We measured the inner temperature\nwith the samples inclined at 45° on a clear day (Supplementary Fig. 28a and 28b) with the\nsame condition applied in Supplementary Note 28. As a result, although the inner\ntemperature did not rise significantly for any of the samples compared to the black chamber,\nwe demonstrated that the TeRC maintains excellent cooling performance even under these\nconditions (Supplementary Fig. 28c and 28d). The TeRC cooled the inner temperature by up\nto 10.0°C compared to the glass sample, and by up to 4.4°C compared to the TeRC sample\nwithout the PDMS layer, confirming the PDMS layer’s effectiveness as an IR absorber and\nheat regulator even in tilted samples. These results indicate that the TeRC has the potential\nfor a wide range of applications, including vehicles, in addition to buildings.”\nSupplementary Figure 28. a Schematic and b photograph of the rooftop measurement setup\nusing 45° tilted white chamber. c Temperature variation during the rooftop measurement.\nT , T , T represents inner temperature of the absorbing chamber\nglass TeRC,with PDMS TeRC, without PDMS\non which the reference and the cooler sample are placed, respectively. T denotes the\nchamber\ntemperature outside the absorbing chamber but inside the acrylic box. d Temperature\ndifference between T and T .\nTeRC,without PDMS TeRC,with PDMS\nComment 6: Also, their cooling power modelling seems to be incorrect. For instance,\nAntarctica has a 0 cooling power in Fig. 6, while Greenland has high cooling power. That\ndoes not seem right. Also, using the earth skin temperature for calculations is not correct,\nthey should use ambient air temperatures as a reference. There are ample examples of this. I\nalso would like to suggest that to make calculations simpler, the authors could just do the\ncalculations for a few major cities – a global map is not necessary, and in their case,\ninaccurate.\nResponse 6: We extend our gratitude to the reviewer for providing valuable comments,\nwhich have helped enhance the completeness of our manuscript and ensure the delivery of\naccurate information to readers. We agree with the reviewer’s insightful comment that using\nearth skin temperature as reference data for calculating radiative cooling power and potential\nmay lead to inaccurate results. As suggested by the reviewer, calculations based on ambient\nair temperature provide more reliable information. Therefore, we have recalculated the\nradiative cooling power using ambient air temperature as reference data.\nFigure R3.6.1. Global climate database and cooling potential. World-wide data for a solar\nirradiance and b ambient air temperature in July. Cooling power 𝑃 for c TeRC, d glass,\ncool\nand e TENG sample. Cooling power difference between f TeRC and glass, and g TeRC and\nTENG.\nNote that the cooling power potential is defined as ∆𝑃 = 𝑃 −𝑃 , not 𝑃 itself\ncool TeRC glass TeRC\nwhich means in regions where the calculated cooling power value of TeRC and conventional\nglass is similar, the cooling power potential can reach zero. Furthermore, even in several\npreviously reported literatures, we find that the radiative cooling potential also reaches to\nzero in Antarctica, while Greenland exhibits relatively higher cooling power potential than\nAntarctica [R3.6.1-3]. The reasons for these calculation results can be attributed to two\nfactors. First, as shown in Fig. R3.6.1a, while the solar irradiance in Antarctica approaches\nzero, Greenland exhibits relatively higher values of solar irradiance. Secondly, as illustrated\nin Fig. R3.6.1b, the ambient air temperature in Antarctica falls below -60°C, indicating it as\nthe coldest region on Earth, whereas the ambient air temperature near Greenland is relatively\nhigher in comparison. Consequently, since radiative cooling power is calculated based on\nsolar irradiance and ambient air temperature, the indicators of radiative cooling potential\nshown in Figure R3.6.1 are reasonable. As a result, we have confirmed the validity of\ncalculated radiative cooling potential and world map plots presented in our manuscript.\nFigure R3.6.2. Examples of world map plots representing potential regions for applying\nradiative cooler. a [R3.6.1], b [R3.6.2], and c [R3.6.3].\n[R3.6.1] Yin, X. et al. Terrestrial radiative cooling: Using the cold universe as a renewable\nand sustainable energy source. Science 370(6518), 786-791 (2020).\n[R3.6.2] Kim, M. et al. Visibly transparent radiative cooler under direct sunlight. Adv. Opt.\nMater. 9(13), 2002226 (2021).\n[R3.6.3] Lee, D. et al. ViBA Rad: Visualization and basic analysis tools for radiative cooling.\nSoftwareX 24, 101562 (2023).\n(Revised manuscript, pages 15-16)\n“To evaluate the cooling effect compared to conventional glass windows, the cooling flux\ndifference ∆𝑃 = 𝑃 −𝑃 is calculated based on the Earth ambient air temperature\ncool TeRC glass\nand solar irradiance in July. The sample temperature is set equal to the Earth ambient air\ntemperature (Supplementary Fig. 24).”\n(Revised Supplementary information, page 31)\n“The Earth ambient air temperature and solar irradiance in July (Supplementary Fig. 24a and\n24b) are used to calculate the cooling power difference, ∆𝑃 = 𝑃 −𝑃 , where 𝑃\ncool TeRC ref ref\nis the reference cooling power which we set as a comparison. Ambient temperature is set\nequal to the Earth ambient air temperature. The solar irradiance is the average value of the\nwhole day including day and night. The calculated cooling power 𝑃 for TeRC, glass and\ncool\nTENG sample are plotted (Supplementary Fig. 24c-e). The color map of the three figures is\nreserved to represent the positive cooling region by blue and the negative cooling region by\nred, respectively. ∆𝑃 is calculated by subtracting the two cooling power for each\ncool\nreference case, 𝑃 −𝑃 and 𝑃 −𝑃 (Supplementary Fig. 24f and 24g).”\nTeRC glass TeRC TENG\nSupplementary Figure 24. Global climate database and cooling potential. World-wide data\nfor a solar irradiance and b ambient air temperature in July. Cooling power P for c TeRC,\ncool\nd glass, and e TENG sample. Cooling power difference between f TeRC and glass, and g\nTeRC and TENG.\nFurthermore, we have actively incorporated the reviewer’s helpful comments and plotted\ndetailed data on the potential applicability of triboelectric nanogenerators and/or radiative\ncoolers in major cities around the world. We are confident that this comparison can serve as a\nguideline, demonstrating how the dual functionality of TeRC can be optimally utilized\naccording to regional characteristics.\n(Revised Supplementary information, page 32)\n“We investigate the functional performance of TeRC in local regions. TeRC can be utilized\nfor energy harvesting in regions with high precipitation or dense tropical rainforests through\nraindrop TENG in TeRC. Additionally, the radiative cooling potential demonstrated by TeRC\nindicates high cooling power in regions with high solar irradiation and ambient air\ntemperature. Consequently, we have shown that while traditional TENG and RC may have\nlimitations in specific regions, TeRC has the potential for application covering wide range of\nareas, from near the equator to mid-latitude regions on Earth.”\nSupplementary Figure 25. Global potential of TeRC in selected cities.\nComment 7: It was not clear to us whether the authors have included a comparison of their\nTENG, and an outdoor radiative cooler (which would be just the optical part of the TENG),\nthat would help readers understand the benefit of the triboelectric part.\nResponse 7: We are grateful to the reviewer for suggesting constructive comments that allow\nus to convey richer information to readers about the originality and significance of our work.\nWe fully agree with the reviewer’s valuable comments that comparing the radiative cooling\nperformance of TENG-only system and RC-only system can highlight the advantages of our\nproposed TeRC structure. As TeRC is carefully designed to achieve cooling effect by\ncombining the two systems, we address this comment by comparing each independent system\nand then explain the collective effect of the total TeRC structure. We compare the reflectance,\ntransmittance, and absorptance (RTA) spectra of RC and TENG and discuss the\ncorresponding radiative cooling performance of three systems, RC, TENG, and TeRC. Note\nthat the RTA spectra of RC sample are obtained through a rigorous coupled-wave analysis\n(RCWA) simulation, and for all other samples, we used measured values.\nFigure R3.7.1. a, b Simulated reflectance, transmittance and absorptance (RTA) spectra of\nRC-only system, and c, d measured RTA spectra of TENG-only system.\nFigure R3.7.2. a Numerically calculated thermal fluxes and equilibrium temperatures for the\nambient temperature of 303 K. Thermal flux terms comparison of the proposed TeRC (solid),\nTENG-only (dashed), and RC-only (dotted). b P of TeRC (pink), TENG (green), and RC\ncool\n(black). c Equilibrium temperatures for different effective conduction and convection heat\ntransfer coefficient h .\ncc\nAs shown in Fig. R3.7.1a and R3.7.1c, in the solar spectrum regime, both systems show\nexcellent visible transmittance. However, the RC sample exhibits greater NIR reflectance\nthan TENG or any other samples, including TeRC or bare glass, resulting in RC having the\nlowest solar absorption (𝑃 ) (Fig. R3.7.2a). On the other hand, in the AW regime, TENG\nsun\nshows greater AW emittance than RC (Fig. R3.7.1b and R3.7.1d), resulting in RC having the\nlowest 𝑃 and 𝑃 . Compared to TENG, RC has favorable attribute of very low solar\nrad atm\nabsorption due to the absence of the ITO layer. However, this advantage is slightly\ncounteracted by the absence of the FEP layer, which act as a broadband emitter responsible\nfor effective heat release. Comparing the two-independent system of RC and TENG, RC\nachieves better cooling performance by having higher cooling power and lower equilibrium\ntemperatures (Fig. R3.7.2b and R3.7.2c). The high solar absorption and insufficient AW\nemittance in each system are complemented by combining the two. Consequently, TeRC\nachieves the highest cooling performance by adding the FEP layer on top and the NIR\nreflector at the bottom. Consequently, we have actively incorporated the reviewer’s insightful\ncomment, which emphasizes the advantages of the TeRC, and added the aforementioned\ninformation to the revised supplementary information. This comparison clearly demonstrates\nthe individual contributions of each component, which will provide readers with a clearer\nunderstanding of the benefits of our proposed TeRC.\n(Revised Supplementary information, pages 21-22)\n“The reflectance (R), transmittance (T) and absorption (A) of conventional bare glass, TENG,\nand the conductive tape sample are measured. The glass sample shows high transmittance and\nalmost no absorption in the solar spectrum (Supplementary Fig. 17a). The absorption is only\nstrong in 𝜆 > 10 μm, which falls short of the requirement for effective radiative cooling\n(Supplementary Fig. 17b). On the other hand, TENG sample shows high transmittance in the\nvisible but also have notable amount of absorption in the solar spectrum (Supplementary Fig.\n17c). This is attributed to inherent loss of ITO layer which potentially caused the mismatch\nbetween the optimized design and fabrication sample properties. However, the absorption of\nTENG is notably high in the AW suggesting TENG itself is a favorable device for radiative\ncooling (Supplementary Fig. 17d). Moreover, RTA measurements on the conductive tape\nattached to the FEP film, serving as the top electrode of TENG, reveal that transmission\nconverges to zero in all regions (Supplementary Fig. 17e and 17f). It is noteworthy that the\nconductive tape occupies very thin, thread-like areas, thereby having minimal impact on the\ntransparency of the TeRC system. Out of all samples, RC has the highest NIR reflectance and\nlowest solar absorption (Supplementary Fig. 17g).”\nSupplementary Figure 17. Measured optical properties of a, b bare glass, c, d triboelectric\nnanogenerator (TENG), and e, f the conductive tape, and calculated optical properties of g, h\nradiative cooler (RC). The black, blue and red lines represent reflectance (R), transmittance\n(T), and absorption (A) respectively.\n(Revised Supplementary information, page 25)\n“RC sample exhibits greater NIR reflectance than TENG or any other samples, including\nTeRC or bare glass (Supplementary Fig. 16), which results in RC having the lowest solar\nabsorption (𝑃 ) (Supplementary Fig. 20a). On the other hand, in the AW regime TENG\nsun\nshows greater AW emittance than RC (Supplementary Fig. 16) resulting in RC having the\nlowest 𝑃 and 𝑃 . Compared to TENG, RC have favorable attribute of having very low\nrad atm\nsolar absorption due to the absence of ITO layer. However, this advantage is slightly\ncounteracted by the absence of FEP layer which act as a broadband emitter responsible for\neffective heat release. Comparing the two-independent system of RC and TENG, RC\nachieves better cooling performance of having higher cooling power and lower equilibrium\ntemperatures (Supplementary Fig. 20b and 20c). The high solar absorption and insufficient AW\nemittance in each system is complemented by combining the two. Consequently, TeRC\nachieves highest cooling performance attributing to adding FEP layer on top and NIR\nreflector at the bottom.”\nSupplementary Figure 20. a Numerically calculated thermal fluxes and equilibrium\ntemperatures for the ambient temperature of 303 K. Thermal flux terms comparison of the\nproposed TeRC (solid), TENG-only (dashed), and RC-only (dotted). b P of TeRC (pink),\ncool\nTENG (green), and RC (black). c Equilibrium temperatures for different effective conduction\nand convection heat transfer coefficient h .\ncc\nResponse to Reviewer #4’s comments\nGeneral Comment: In this manuscript, by rationally integrating transparent triboelectric\nlayer and radiative cooler on a glass substrate, the authors demonstrate the combination of\ntriboelectric nanogenerator and radiative cooling in a single window, enabling all-weather\nenergy harvesting and saving. The design is novel and inspirational, but I don’t think the\nmanuscript meet with the high requirement of Nature Communications.\nResponse: We extend our heartfelt gratitude to the reviewer for dedicating the reviewer’s\nvaluable time to providing constructive comments, which have greatly enhanced the quality\nof our manuscript and deepened our understanding of the study findings. We have carefully\nconsidered and actively incorporated the reviewer’s insightful comments, addressing each\npoint raised and resolving the concerns expressed. We have provided a point-by-point\nresponse to each comment and have reflected these changes in the revised manuscript. We\nhave made efforts to enhance the quality and professionalism of the manuscript based on the\ncomments provided by the reviewer, and through this revision process, we have strived to\nbring our manuscript to a level worthy of publication in Nature Communications. We would\nlike to express our gratitude to the reviewer for once again actively acknowledging the\nnovelty and originality of our manuscript.\nComment 1: The discrepancy of real RTA from the simulated results, as explained by the\nauthors, arises from inherent losses in FEP and ITO. I wonder how the FEP film was attached\nonto the ITO side of the glass. If there is an air gap left between them, the optical\nperformance would be deteriorated.\nResponse 1: We appreciate the constructive comment provided by the reviewer, which can\nenhance the completeness of our work. We acknowledge the discrepancy between the\nsimulated reflectance, transmittance, and absorptance (RTA) spectra of the designed TeRC\nand the experimentally measured results featured in Figure 4c and 4d, respectively. In\nresponse to the reviewer’s concern, we have conducted the following analysis.\nFor the simulated results of RTA spectra in the original manuscript, we have utilized optical\nproperties obtained from various references and literatures. Among these, the extinction\ncoefficient of materials used in our designed TeRC, especially TiO , approached zero.\nreflection of actual extinction coefficient values, and hence, we additionally measured the\noptical properties of the actual deposited layers.\nFigure R4.1.1. The measured optical refractive index and extinction coefficient of the\nmaterials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick SiO\n2 2\nlayer (100 nm), c thin TiO layer (35 nm), d thick TiO layer (100 nm), e Ag layer (38 nm),\n2 2\nand f ITO layer (150 nm).\nAs a result, upon examining the refractive index and extinction coefficient of the materials\nused in the designed TeRC, especially TiO , Ag, and Indium Tin Oxide (ITO), we confirmed\n2\nthe discrepancies were mainly caused by the actual material losses which we have overlooked\ninitially. Based on this finding, we have conducted additional simulations of TeRC to obtain\nRTA spectra considering the material losses. Consequently, we confirmed that the RTA\nspectra calculated with updated material losses matched better with the measured RTA\nspectra of the TeRC compared to those calculated without considering the material losses.\nTherefore, we have included the contents mentioned above in the revised supplementary\ninformation to enhance the completeness of our manuscript.\nFigure R4.1.2. a Simulated reflectance, transmittance, and absorptance (RTA) spectra of the\ndesigned TeRC, b simulated RTA spectra of the designed TeRC considering material losses,\nand c measured RTA spectra of the designed TeRC.\n(Revised supplementary information, page 19)\nSupplementary Figure 15. The measured optical refractive index and extinction coefficient\nof the materials used to design the radiative cooler (RC). a Thin SiO layer (37 nm), b thick\n2 2 2\nnm), and f ITO layer (150 nm).\n(Revised supplementary information, page 20)\n“The optical refractive index and extinction coefficient of the various materials used to\ndesign the TeRC are highly dispersive and sensitive to different fabrication conditions\n(Supplementary Fig. 15). Especially, the discrepancy of extinction coefficients can cause\nadditional material losses, negatively affecting the TeRC designed for high transparency in\nthe visible range and high reflectance in the near-infrared (NIR) range. Therefore, we\ncalculate the reflectance, transmittance, and absorptance (RTA) spectra of TeRC based on the\nupdated, measured optical refractive index and extinction coefficient (Supplementary Fig. 16)\nto improve our experiment-simulation consistency. As a result, we achieve RTA spectra with\na maximum transmittance of 81.0% in the visible range and a maximum reflectance of 80.3%\nin the NIR range, confirming that the results align with our desired optical responses.”\nSupplementary Figure 16. Simulated reflectance, transmittance, and absorptance (RTA)\nspectra of the designed TeRC considering material losses.\nFurthermore, in response to the reviewer’s concern, the FEP film and ITO side of the glass\nwere attached using conventionally used tape. This strategy was applied to minimize any\nunexpected optical performance that might arise from using adhesive layers to attach the FEP\nfilm and ITO side of the glass. To this end, we used tape only around the edges of the sample\nto attach the FEP film with the ITO glass substrate to prevent delamination. Note that this\nstrategy was inevitable to ensure free contact between the two layers for sufficient\ntriboelectrification. Nevertheless, by updating the material properties, we have verified that\nthe recalculated simulation results and measured ones align well, showing the experiment-\nsimulation consistency.\nWe have actively reflected on the constructive comment from the reviewer by adding a\nstatement on the attachment process for the FEP film on the ITO glass substrate in the revised\nmanuscript. This effort aims to provide readers with more accurate information regarding the\nexperimental procedures conducted in our work.\n(Revised manuscript, page 18)\n“To develop the TENG part of the proposed TeRC system, a 250-μm thick fluorinated\nethylene propylene (FEP) film (FEP0250/004, ALPHAFLON) was attached to all edges of\nthe ITO glass substrate using commercial tape.”\nComment 2: The authors did not give demonstration of the durability and real-environment\nperformance of TENG, evaluation of possibility of scaling up the sustainable window, and\ncomparison of energy saving and harvesting amount of RC and TENG part. Prospective\napplications should be enriched to better support the authors’ outlook.\nResponse 2: We would like to express our gratitude to the reviewer for providing\nconstructive comments that can enhance the value of our study. We wholeheartedly agree that\nconducting demonstrations along with the validation of TeRC’s TENG functionality would\neffectively convey the significance and impact of our work to the readers. Hence, to evaluate\nits potential for practical applications, we have included information on the durability of\nTeRC’s electricity generation capability. Consequently, we have confirmed that stable\nelectricity of over 300 V was measured under the same conditions for 10 seconds using the\nFEP film.\n(Revised manuscript, page 9)\n“Consequently, we verify that higher hydrophobicity generates a more stable and higher\ntriboelectric performance (Supplementary Fig. 8).”\n(Revised supplementary information, page 12)\n“To evaluable the stability of our TeRC system’s electricity generation capability, we\nmeasure the output voltage signal over an extended period for a single droplet. The total\nperiod was 10 seconds, which is sufficiently long to assess the TeRC’s stability, considering\nthat the electricity generation cycle for a single droplet is 40 ms. To assess this, we set load\nresistance, film thickness, drop height, drop angle, drop frequency, droplet volume, and water\ntype fixed at 10 MΩ, 250 μm, 45 cm, 45°, 2.5 Hz, 40 μL, and tap water. Over a period of 10\nseconds, we detect a total of 25 peaks, with maximum, minimum, and average values\nmeasured at 334.37 V, 310.25 V, and 317.57 V, respectively. Consequently, we verify that\nour TeRC system possesses stable electricity generation capability.”\nSupplementary Figure 8. Output voltage signal for a long period.\nMoreover, to investigate the influence of water type dropped in real environments, we\ncollected actual rain water and compared its performance with tap water commonly used in\nthe previous experiments. Compared to tap water, rain water contains numerous impurities,\nwhich induce an electric screening effect and result in performance degradation. However,\neven when compared to tap water, we confirmed that rain water still exhibits excellent\nperformance, reading 75% efficiency. This validates the superiority and potential of our\nproposed TeRC system. Consequently, we have added statements regarding this in both the\nrevised manuscript and supplementary information.\n(Revised manuscript, page 10)\n“Additionally, we evaluate the potential of TeRC system in real environments by verifying its\ntriboelectric performance using rain water, and confirmed its ability to maintain excellent\nelectricity generation capabilities (Supplementary Fig. 11).”\n(Revised supplementary information, page 15)\n“To assess the potential applicability of the TeRC system, which operates based on raindrop\ncollisions, in real-world environments, we collect rain water and conduct a comparison of its\nelectricity generation capability with tap water. To assess this, we set load resistance, film\nthickness, drop height, drop angle, drop frequency, and droplet volume fixed at 10 MΩ, 250\nμm, 45 cm, 45°, 2.5 Hz, and 40 μL, respectively. Consequently, during a 10-second\nmeasurement from rain water, the maximum, minimum, and average values of the peak\nvoltage and corresponding current are measured at 255.78 V, 231.66 V, 239.98 V,\nrespectively. The decrease in electricity generation performance from rain water can be\nattributed to the potential influence of impurities and high ion concentration between the\ndroplet and the triboelectric layer, resulting in deterioration compared to tap water.\nNevertheless, considering that the average value for tap water is 317.57 V, we confirm that\nrain water exhibits a respectable performance of 75.6% compared to tap water.”\nSupplementary Figure 11. Comparison of peak voltage and current according to the water\ntype.\nFurthermore, we believe that verifying the electricity generation capability of the TeRC\nsystem in a scaled-up scenario is crucial for its practical application. Therefore, we arrange\nthe TeRCs in a 3 × 3 array and integrate an electric circuit to form a TeRC array\n(Supplementary Fig. 23a). Through this experimental setup, we confirm that a stable voltage\noutput above a certain level could be consistently extracted from each TeRC over an\nextended period (Supplementary Fig. 23b). Consequently, we demonstrate that the electric\nsignal measured from the TeRC array can produce electric energy at a much higher frequency\ncompared to a single TeRC system.\n(Revised manuscript, page 15)\n“Furthermore, to demonstrate the potential real-world applicability of TeRC, we arrange\nmultiple TeRCs in an array and measure their electricity generation, proving that the TeRC\narray produces electrical energy at a much higher frequency than a single TeRC\n(Supplementary Fig. 23).”\n(Revised supplementary information, pages 29-30)\nSupplementary Figure 23. a Picture of the experimental setup for TeRC array measurement,\nand b the measured electric signal from TeRC array.\n“TeRC has the drawback of limited electric energy generation within a given time due to its\nlow operating frequency. To overcome this limitation and further emphasize the practicality\nof the TeRC system, we create a scaled-up structure composed of a 3 × 3 TeRC array\n(Supplementary Fig. 23a). The entire TeRC array is assembled by cutting 10 × 10 cm2 holes,\nthe size of the TeRCs, into a 40 × 40 cm2 acrylic plate. Then, the TeRC array, tilted at 45°,\nallows droplets to collide from drop heights of 45 cm, 36 cm, and 27 cm. The drop frequency,\ncontrolled by a flow regulator, is set to 2.5 Hz, and droplets with a volume of 40 μL are\nreleased. Consequently, the electric circuit, connected with conductive tape, is routed out to\nmeasure the electric output signal through a load resistance set at 10 MΩ (Supplementary Fig.\n23b). Examining the extracted output voltage signal reveals that electric energy is generated\nat a much higher frequency compared to the voltage signal from a single TeRC at the same\ndrop frequency, with various voltage levels depending on the drop heights. Furthermore, by\nextracting a consistent output signal over 100 seconds, we demonstrate that the TeRC array\ncan produce stable electric energy.”\nAdditionally, in our proposed TeRC system, the TENG part performs energy harvesting,\nwhile the RC part functions in energy saving. We fully agree with reviewer’s valuable\nsuggestion for comparing the energy harvesting and saving amounts of the TENG and RC\nparts. However, energy harvesting in the TENG part is based on the dynamic response of\npulse-like signals, whereas energy saving in the RC part is passive. Therefore, we believe that\ndirectly quantitatively comparing the two amounts of energy is not appropriate. Nonetheless,\nwe conducted a potential comparison through the power per unit area for both functionalities.\nIn the TENG part, we harvest 248.28 Wm-2 of electrical power from a single droplet at the\noptimal load resistance. On the other hand, in the RC part, we demonstrate energy savings by\nconfirming that TeRC’s cooling power is 410.96 Wm-2 higher than the cooling power of a\nconventional glass sample at an inner temperature of 65.1°, thus proving energy savings.\nConsequently, summarizing these points, we emphasize the significant energy harvesting and\nsaving capabilities of each part of TeRC by adding following statements to the Discussion\nsection.\n(Revised manuscript, page 16)\n“Systematically optimized TeRC generates 248.28 Wm-2 from a single droplet, enabling us to\npower a hundred LEDs, on the other hand, in terms of radiative cooling, we demonstrate\nenergy savings by confirming that TeRC’s cooling power is 410.96 Wm-2 higher than that of\na conventional glass at an absorbing chamber.”\nComment 3: For TENG, peak power density is measured, but total energy output of a single\nraindrop is not given. As the resistance of the load increases, it is expected that the release of\ncharges takes up a long period, and so the total energy output may increase. It is suggested\nthat the total energy output should be measured for different load resistance. The discussion\npart gives an unrealistically high value of 0.446 kWh in an hour. This value should be\nrecalculated based upon total energy output instead of upon peak power density to reveal real\noutput capability.\nResponse 3: We would like to express our gratitude to the reviewer for providing critical\ncomments that can enhance the value of our study. We fully agree with the reviewer’s\nconstructive suggestion that, to evaluate the performance of our proposed TeRC system in\ngenerating electrical energy, it is essential to provide not only power density but also total\nenergy output. Therefore, we have derived the total energy output of a single droplet using\nthe following relationship:\n𝐸 = 𝑃𝑜𝑤𝑒𝑟×𝑇𝑖𝑚𝑒 = ∫\n𝑡 discharging𝑉2\n𝑑𝑡, (R4.3.1)\ntotal 𝑡0 𝑅L\nwhere 𝑡 and 𝑡 represent the moment when the droplet contacts the top electrode\n0 discharging\nand when the pseudo capacitor 𝐶 discharge phenomenon ends, respectively. The total\nF\nenergy output 𝐸 obtained by varying the load resistance 𝑅 according to Equation\ntotal L\nR4.3.1 yields the following results.\nFigure R4.3.1 a Total energy output according to load resistance for a single droplet and b\nthe corresponding discharging time.\nUpon examining Fig. R4.3.1a, it is evident that the total energy output, influenced by the load\nresistance, follows a similar trend as the power density depicted in Fig. 3b. It increases with\nthe increasing load resistance, reaching its peak at 0.5 MΩ, where the optimal power density\nis achieved, before starting to decline. However, while power density exhibits a continuous\ndecrease with the ongoing increase in load resistance, the total energy output appears to\nconverge at a certain level. This trend is closely related to the discharging time of the pseudo\ncapacitor, as mentioned by the reviewer. According to Equation R4.3.1, the total energy\noutput is associated with the output voltage, load resistance, and discharging time. As the\nload resistance increases, the discharging time lengthens, compensating for the decrease in\npower density, thus maintaining the total energy output at the certain level. Consequently,\nrecognizing the significance of this phenomenon in evaluating the performance of electrical\nenergy harvested from TeRC, we have included this information in the supplementary\ninformation.\nFurthermore, we express our gratitude to the reviewer for pointing out an error in the\ndescription of electrical energy in our original manuscript, and we apologize for any\nconfusion caused by this mistake. We fully agree with the reviewer’s insight that calculating\nenergy output over an extended period based on peak output power is not appropriate. Instead,\nwe should calculate electric energy based on the total energy output generated from a single\ndroplet, as advised by the reviewer. Following this constructive advice, we have calculated\nthe electric energy that can be produced over 1-hour using a single droplet as follows:\n𝐸 = 𝐸 ×𝐷𝑟𝑜𝑝 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦×1 ℎ𝑜𝑢𝑟\ntotal,hour total\n= 2.95 μJ×2.5 Hz×3600 s\n= 0.0265 J (R4.3.2)\nAs a result, we have added the contents in the revised manuscript and supplementary\ninformation based on the calculated electric energy. We express our gratitude once again to\nthe insightful comments provided by the reviewer, which have helped enhance the\nprofessionalism and quality of our manuscript.\n(Revised manuscript, page 9)\n“Furthermore, TeRC produce 2.95 μJ of energy from a single droplet at identical 𝑅 ,\nL\nserving as an indicator for evaluating harvested energy (Supplementary Fig. 6).”\n(Revised supplementary information, page 10)\n“A quantitative comparison of the total energy output of the TeRC system for a single droplet\nis an important evaluation metric in assessing the energy harvesting capability of TeRC\nsystem. To evaluate the total energy output, we fix the FEP film thickness, drop height, drop\nangle, drop frequency, droplet volume, and water type at 250 μm, 45 cm, 45°, 2.5 Hz, 40 μL,\nand tap water, respectively. Then, the total energy output 𝐸 is calculated based on the\ntotal\nelectric signal obtained by varying the load resistance from 0.1 MΩ to 10 MΩ as follows:\n𝐸 = 𝑃𝑜𝑤𝑒𝑟×𝑇𝑖𝑚𝑒 = ∫\n𝑡 discharging𝑉2\n𝑑𝑡, (S2)\ntotal 𝑡0 𝑅L\nwhere 𝑡 , 𝑡 , 𝑉, and 𝑅 represent the moment when the droplet contacts the top\n0 discharging L\nelectrode, the moment when pseudo capacitor 𝐶 discharging phenomenon ends, output\nF\nvoltage, and load resistance, respectively. Consequently, 𝐸 is obtained by varying 𝑅\ntotal L\naccording to Equation S2 (Supplementary Fig. 6a). It is evident that 𝐸 , influenced by 𝑅 ,\ntotal L\nfollows a similar trend as the power density (Fig. 3b), which increases with the increasing 𝑅 ,\nL\nreaching its peak at the optimal 𝑅 of 0.5 MΩ. However, while power density exhibits a\nL\ncontinuous decrease with the ongoing increase in 𝑅 , 𝐸 appears to converge at a certain\nL total\nlevel. This trend is closely related to 𝑡 . According to Equation S2, 𝐸 is\ndischarging total\nassociated with 𝑉 , 𝑅 , and 𝑡 . As 𝑅 increases, 𝑡 lengthens,\nL discharging L discharging\ncompensating for the decrease in power density, thus maintaining 𝐸 at the certain level\ntotal\n(Supplementary Fig. 6b).”\nSupplementary Figure 6. a Total energy output according to load resistance for a single\ndroplet and b the corresponding discharging time.\nComment 4: At the beginning of Result section. The description on the electrification\nmechanism of triboelectric layer is confusing. Triboelectrification (or contact electrification)\nrefers to the microscopic separation or generation of positive and negative charges caused by\ncontact or friction, which originates from the overlap of electron wave function of different\nmaterials (refer to: 10.1016/j.mattod.2019.05.016), instead of due to electrical discharge as\nimplied in your description. Triboelectrification produces the negative charges on the\npolymer film, not the opposite. According to literature (10.1038/s41586-020-1985-6), the\ncontinuous impinging of droplets generates the negative charges. The electric discharge is\noccurred at the stage that the droplet contacts the top electrode, which forms the electric\nsignal in external circuit. This is the working mechanism of droplet-based TENG, not the\ntriboelectrification process.\nResponse 4: We would like to express our gratitude to the reviewer for providing critical\ncomments that have helped enhance the completeness and professionalism of our manuscript.\nAdditionally, we apologize for any confusion caused to the reviewer regarding the\nexplanation of the working mechanism of triboelectrification (i.e., contact electrification) in\nour proposed TeRC system.\nTo actively accommodate the reviewer’s comment, we will first explain the\ntriboelectrification effect occurring in the liquid–solid interface. The mechanism behind\ntriboelectrification has long been a subject of inquiry, with various hypotheses proposed over\ntime. In examining the working mechanism of triboelectrification, it is important to pay close\nattention to what entity initiates the movement of charge. However, recent research results by\nProfessor Wang’s group have revealed that both electron transfer and ion adsorption\n(chemical interaction) are involved in triboelectrification [R4.4.1-2].\n[R4.4.1] Wang, ZL. et al. On the origin of contact-electrification. Mater. Today 30, 34-51\n(2019).\n[R4.4.2] Lin, S. et al. Contact electrification at the liquid–solid interface. Chem. Rev. 122(5),\n5209-5232 (2022).\nThis hybrid electric double layer (EDL) model consists of a “two-step” process. In the initial\nphase, molecules and ions within the liquid collide on the solid surface through thermal\nmotion and pressure. Consequently, the electron clouds of solid atoms and water molecules\noverlap, facilitating electron transfer between them. Notably, materials possessing a high\nelectron-capturing capability can directly acquire electrons from water molecules, including\nions. This electron transfer involves a transition from high-energy states to low-energy states,\nensuring that if the energy variation of electrons is lower than the potential energy barrier,\nthe transferred electrons can be retained on the surface. In the second step, the electrified solid\nsurface attracts free ions in the next droplet through electrostatic interaction, resulting in the\nformation of an EDL. This constitutes the mechanism of triboelectrification occurring at the\nliquid–solid interface. In our original manuscript, we omitted a detailed explanation of this\nprocess and inadvertently failed to distinguish between triboelectrification and the\nmechanism of droplet-based triboelectric nanogenerators, potentially causing confusion for\nthe reviewer. Therefore, in the revised manuscript and supplementary information, we have\nprovided detailed explanations of triboelectrification occurring at the liquid–solid interface.\nAdditionally, we have modified the description of electron behavior in the droplet sequence\nof droplet-based triboelectric nanogenerators as follows.\nMoreover, we will explain the reason why the polymer film becomes negatively charged, as\nraised by the reviewer. When a droplet collides with a virgin solid surface, water molecules\nand surface atoms come into close proximity. During this process, electron transfer occurs\ndue to the overlap of electron clouds in the repulsive force region. Electron transfer occurs\nfrom water molecules to the solid surface. This is because polymers, especially those with a\nlarge number of fluorine groups such as fluorinated ethylene propylene (FEP) or\nperfluoroalkoxy (PFA), possess a strong electron-withdrawing ability, leading to a significant\nelectron transfer effect during the contact electrification process with liquids. For this reason,\nwe evaluated the triboelectricity generation capability of various polymers (FEP, PFA,\npolyvinyl chloride (PVC), and polyester (PET)) and confirmed that all polymers generate\ntriboelectricity, but polymers with fluorine groups exhibit outstanding output performance.\nTherefore, we have modified statements in the revised manuscript to ensure that readers can\neasily understand the basic principles of triboelectricity generation. Additionally, we have\nadded the following contents to the supplementary information.\n(Revised manuscript, page 5)\n“To understand the principle by which TeRC generates electricity through raindrops, it is\nessential to investigate charge transfer, the predominant mechanism of liquid–solid contact\nelectrification (i.e., triboelectrification). According to the hybrid electric double layer (EDL)\nmodel, during which the first droplet collides with an electrically pristine solid surface, the\ninteratomic potential barrier decreases due to the overlap of the electron cloud when the\ninteratomic distance is within the repulsive region, resulting in electron transfer\n(Supplementary Fig. 1). The transferred electrons and the chemically absorbed ions on the\nsolid surface form an electrostatic field, resulting in the formation of an electrostatically\ninduced opposite charge of equal magnitude on the lower electrode. When the next droplet\ncollides with an accumulated negatively charged solid surface, the positively charged ions\ncontained within the droplet are attracted to the solid surface due to the electrostatic induction\neffect, forming an EDL at the liquid–solid interface (Fig. 2a).”\n(Revised manuscript, page 6)\nFig. 2 Design and mechanism of triboelectric nanogenerator (TENG) part in the\nproposed raindrop triboelectric nanogenerator and radiative cooler (TeRC) system. …\ne Schematic of the electricity generation process during the droplet flow sequence.\n(Revised manuscript, page 7)\n“After the droplet collides with the FEP surface, the triboelectricity generation is categorized\ninto five stages based on the droplet movement: (i) When the droplet contacts with an\nelectron-accumulated FEP surface, the cations in the droplet bind with the electrons on the\nFEP surface, forming a pseudo capacitor 𝐶 at the contact interface. (ii) While the droplet\nF\nflows along the surface, charge transfer to the external circuit does not occur until it contacts\nthe top electrode, maintaining a switch-off state. (iii) When the droplet contacts the top\nelectrode, new pseudo capacitors are formed between the triboelectric layer and droplet 𝐶 ,\nE1\nas well as between the top electrode and droplet 𝐶 . Consequently, a closed-loop is\nE2\nestablished with the droplet resistance 𝑅 , transitioning to a switch-on state. Then, electrons\nD\npreviously stored in 𝐶 rapidly flow to the lower electrode (Supplementary Fig. 4). (iv)\nF\nDuring droplet shrinkage, electrons return to the top electrode, and 𝐶 is recharged by 𝐶\nF E1\nand 𝐶 . The spreading and shrinking of droplets represent the charging and discharging\nE2\nphenomena of the pseudo capacitors. (v) As the droplet continues to flow away from the top\nelectrode, the electricity generation cycle ends.”\n(Revised supplementary information, pages 4-5)\n“For a liquid–solid interface, contact electrification (i.e., triboelectrification) has been the\nsubject of much speculation and debate over its mechanism, dating back as far as 2600 years\nago to the present day. Key to understanding the mechanism of liquid–solid\ntriboelectrification is identifying the charge carrier, with hypotheses ranging from electron\ntransfer to ion transfer and species transfer of material. However, recently, according to the\nhybrid electric double layer model proposed by Professor Wang’s group, in the process of\nliquid–solid contact electrification, electron transfer exists. It elucidates that electron transfer\noccurs from water molecules to solid atoms as part of a ‘two-step’ model, followed by the\noccurrence of ion transfer due to the electrostatic induction effect.\nWhen the electrically neutral droplet collides with a pristine solid surface, the distance\nbetween water molecules and solid surface atoms decreases. When the interatomic distance 𝑙\nis greater than the equilibrium distance 𝑙 , they tend to attract each other (Supplementary Fig.\n0\n1a). However, when 𝑙 becomes smaller than 𝑙 , the electron clouds formed by the two\n0\natoms overlap, repel each other (Supplementary Fig. 1c). In this scenario, electron transfer\ncan occur due to the overlap of electron clouds. The highest occupied energy level belonging\nto atom A 𝐸 is greater than of atom B 𝐸 . Therefore, when the two atoms are in the\nA B\nrepulsive region, the potential barrier between them decreases, leading to electron transfer\nfrom atom A to atom B. Additionally, since 𝐸 is greater than 𝐸 , photon energy equivalent\nA B\nto the difference in the two potential energies is emitted during the electron transfer process.\nConsequently, electron transfer occurs from water molecules to the solid surface due to\nliquid–solid contact electrification, resulting in the solid surface being negatively charged. As\nthis process repeats, electron transfer continues to occur and accumulate, eventually reaching\nsaturation depending on the capacity of the solid surface material. As the next step, when a\ndroplet collides with the negatively charged solid surface, the positive ions contained within\nthe droplet are attracted to the negatively charged solid surface due to the electrostatic\ninduction effect. Consequently, this leads to the formation of an electric double layer (EDL)\nat the liquid–solid interface.\nIn this study, electron transfer occurs from the droplet to the solid surface. This phenomenon\narises due to the strong electron-capturing capacity of solid materials, enabling them to\ndirectly acquire electrons from water molecules. Especially, polymers, including fluorinated\nethylene propylene (FEP) and perfluoroalkoxy (PFA) containing numerous fluorine groups,\nexhibit notable electron transfer effects during liquid–solid contact electrification.\nConsequently, the formation of EDL may also be influenced by the electron-donating or\nelectron-withdrawing characteristics of solid materials.”\nSupplementary Figure 1. Hybrid electric double layer model: Electron transfer occurs as a\nresult of the overlap of electron clouds due to interatomic distance within the repulsive region.\na Attractive region, b equilibrium region, and c repulsive region.\nComment 5: You mentioned raindrops carry positive charges before reaching the polymer.\nThat’s true. However, even if the raindrops are strictly electrostatically neutral, they become\npositively charged after impinging the polymer, during which triboelectrification leads to\ncharge transfer from the droplet to the polymer. This process takes place not due to electrical\ndischarge between raindrop and polymer, but due to microscopic solid-liquid interaction and\ncharge transfer. As raindrops continuously collide, triboelectric negative charges on the\npolymer accumulate till saturation. As a result, the original positive charges on the raindrops\nbarely contribute to this process. It is safe to conclude that the effect of the positive charges\non the raindrops due to colliding with air particles do not contribute to triboelectrification.\nThe triboelectrification process should be rewritten.\nResponse 5: We extend our gratitude to the reviewer who provided constructive and helpful\ncomments that have contributed to improving the completeness of our manuscript. We fully\nagree with the reviewer’s comment that the statement regarding the cause of droplet\npositively charging is not universally applicable. One of the key aspects of triboelectrification\nis the phenomenon of droplet surfaces becoming positively charged due to the electrostatic\ninduction effect of the electric double layer (EDL) formed as a result of liquid–solid contact\nelectrification. When a droplet collides with a negatively charged solid surface due to\nelectron transfer, the positive ions contained within the droplet are attracted to the solid\nsurface through electrostatic induction, forming an EDL. Therefore, to provide a more\ncomprehensive explanation of the triboelectrification effect, we have revised the manuscript\nto reflect the triboelectrification mechanism. Indeed, the content aligns closely with the\nresponse provided to Comment 4 by the reviewer. Therefore, we have comprehensively\nrevised the processes of the triboelectrification and droplet-based triboelectric nanogenerator\nbased on the constructive comments provided by the reviewer in Response 4.\nComment 6: “the geographical dependency of the two climate conditions exhibits a\ncomplementary effect,” the complementary effect of the two climate conditions also seems\nthat the “multifunctional” of TeRC is unnecessary, because there is only one part (TENG or\nRC) workable in some places.\nResponse 6: We express our gratitude to the reviewer for providing valuable comments that\nhave contributed to enhancing the completeness of our manuscript. We utilized the statement\nemphasizing the potential universality of our proposed TeRC system, incorporating both\ntriboelectric nanogenerator and radiative cooler functionalities, considering various\ngeographical and climatic factors worldwide, aiming to highlight its multifunctionality.\nFigure 6d and 6e respectively illustrate regions with potential for functioning as TENG and\nRC components of the TeRC system. TENG covers the regions of Central America, Asia, and\nAfrica, while RC covers North America, Europe, and Southwest Asia. This indicates that our\nTeRC system can exhibit multifunctionality according to weather conditions and can be\nappropriately applied based on geographical conditions. However, we agree with the\nreviewer’s perspective that “multifunctionality” may entail a somewhat conflicting notion\nwith the concept of “complementary effects”. Hence, we have revised the statement to\nconvey the accurate meaning that our TeRC system can be applied across a wide range of\nconditions worldwide, considering various climate and geographical factors.\n(Revised manuscript, page 16)\n“Consequently, we have confirmed that TeRC, with its functionality as both a TENG and RC,\ncan perform multifunctionality in the same region according to climate conditions.\nAdditionally, we have identified its potential to be more appropriately applied, considering\ngeographical conditions.”\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI am impressed with the comprehensive response by the authors. As a researcher in glazing\ntechnology, I believe that the authors have addressed questions from the reviewers with the\ninformation provided in the revised manuscript, supporting information, and rebuttal letter. I\nrecommend to publish this revised manuscript in its current form.\nReviewer #2 (Remarks to the Author):\nI believe that all my comments were well addressed, and the manuscript is now suitable for\npublication.\nReviewer #3 (Remarks to the Author):\nI would like to thank the authors for their extensive efforts in response to my comments – this\nrepresents a tremendous effort. In particular, I appreciated the explanation of the triboelectric\nphenomenon.\nThere are, however, two fundamental issues that they have not addressed, which can be done\nrelatively easily, after which I can recommend their work for publication:\n1. The authors overlooked my comment that buildings in hot weather usually have cooler indoors\nthan the outdoors. This is because they are either actively cooled, or thermal inertia of the building\ncarrying over from the night. The authors simply need to consider an average building and the\nweather in Southeast asia in the summer. In that case, having an indoor PDMS radiative cooler\nclearly does not help, just from basic thermal considerations. Doing experiments with a white\ninterior does not change that fact. Acknowledging such limitations of their design is a good think,\nand I would ask the authors to *comment* on that front.\n2. It is not clear to me why the authors did a 45° tilt experiment for their windows – I had\nsuggested an experiment where the windows are vertical, as they normally are. If their design is\nnot suitable for vertical windows, or if they did not do such an experiment for practical reasons,\nthey should acknowledge that fact as a comment.\nReviewer #4 (Remarks to the Author):\nThe authors have addressed all the questions in detail.\nResponse Letter\nAll-weather sustainable windows for energy harvesting and saving\nGeon Lee1,10, Hyunjung Kang1,10, Jooyeong Yun1,10, Dongwoo Chae2, Minsu Jeong1, Minseo\nJeong3, Dasol Lee3, Miso Kim4,5, Heon Lee2, and Junsuk Rho1,6,7,8,9,*\n1Department of Mechanical Engineering, Pohang University of Science and Technology\n(POSTECH), Pohang 37673, Republic of Korea\n2Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic\nof Korea\n3Department of Biomedical Engineering, Yonsei University, Wonju 26493, Republic of\nKorea\n4School of Advanced Materials Science and Engineering, Sungkyunkwan University\n(SKKU), Suwon 16419, Republic of Korea\n5SKKU Institute of Energy Science and Engineering (SIEST), Sungkyunkwan University\n(SKKU), Suwon 16519, Republic of Korea\n6Department of Chemical Engineering, Pohang University of Science and Technology\n(POSTECH), Pohang 37673, Republic of Korea\n7Department of Electrical Engineering, Pohang University of Science and Technology\n(POSTECH), Pohang 37673, Republic of Korea\n8POSCO-POSTECH-RIST Convergence Research Center for Flat Optics and Metaphotonics,\nPohang 37673, Republic of Korea\n9National Institute of Nanomaterials Technology (NINT), Pohang 37673, Republic of Korea\n10These authors contributed equally: Geon Lee, Hyunjung Kang, Jooyeong Yun\n*Corresponding author: Junsuk Rho (jsrho@postech.ac.kr)\nOverall response to Editor and reviewers\nWe appreciate the Editor and reviewer’s invaluable time and effort in offering positive\nsuggestions for strengthening our manuscript. Thus, it is with great pleasure that we submit our\nrevised manuscript for further consideration. We have made substantial changes based on the\nsuggestions you so kindly provided. Additionally, we believe that our edits and responses\nadequately address the Editor’s and reviewer’s suggestions. The revision’s \"responses\" and\n\"modifications\" are highlighted in \"blue\" and \"green\" respectively.\nResponse to Reviewer #1’s comments\nGeneral Comment: I am impressed with the comprehensive response by the authors. As a\nresearcher in glazing technology, I believe that the authors have addressed questions from the\nreviewers with the information provided in the revised manuscript, supporting information, and\nrebuttal letter. I recommend to publish this revised manuscript in its current form.\nResponse: We appreciate your valuable and constructive comments for improving the quality\nof our work, and your recommendation to publish the revised manuscript in Nature\nCommunications.\nResponse to Reviewer #2’s comments\nGeneral Comment: I believe that all my comments were well addressed, and the manuscript\nis now suitable for publication.\nResponse: We appreciate your valuable and constructive comments for improving the quality\nof our work, and your recommendation to publish the revised manuscript in Nature\nCommunications.\nResponse to Reviewer #4’s comments\nGeneral Comment: The authors have addressed all the questions in detail.\nResponse: We appreciate your valuable and constructive comments for improving the quality\nof our work, and your recommendation to publish the revised manuscript in Nature\nCommunications.\nResponse to Reviewer #3’s comments\nGeneral Comment: I would like to thank the authors for their extensive efforts in response to\nmy comments – this represents a tremendous effort. In particular, I appreciated the explanation\nof the triboelectric phenomenon. There are, however, two fundamental issues that they have\nnot addressed, which can be done relatively easily, after which I can recommend their work for\npublication.\nResponse: We are grateful to the reviewer for the constructive comments, which have enabled\nus to enhance the quality of our manuscript and provide more detailed information to the\nreaders. Also, we are pleased that these revisions have been helpful to the reviewer’s\nunderstanding. Nonetheless, to more clarify the strengths and limitations of our work, we have\naddressed the two fundamental issues raised by the reviewer in the following responses and\nrevised the manuscript accordingly. We have made every effort to ensure that our work meets\nthe standards for a recommendation for publication in Nature Communications.\nComment 1: The authors overlooked my comment that buildings in hot weather usually have\ncooler indoors than the outdoors. This is because they are either actively cooled, or thermal\ninertia of the building carrying over from the night. The authors simply need to consider an\naverage building and the weather in Southeast Asia in the summer. In that case, having an\nindoor PDMS radiative cooler clearly does not help, just from basic thermal considerations.\nDoing experiments with a white interior does not change that fact. Acknowledging such\nlimitations of their design is a good think, and I would ask the authors to *comment* on that\nfront.\nResponse 1: We thank the reviewer for providing insightful comment which has helped us\nclarify the usage and limitation of our proposed structure. We fully acknowledge the point\nraised that in situations where indoor temperatures are lower than outdoors due to active\nthermal regulation or air conditioning, the PDMS layer is not necessary as it is meant for\neffective inner heat absorption.\nThe primary motivation behind including low-E layers in other heat reflective windows is to\nmaintain stable indoor temperatures by providing better insulation and reducing heating,\nventilating, and air conditioning (HVAC) load. Our TeRC structure, with the PDMS layer, is\ndesigned differently and consequently does not aid in scenarios involving active thermal\nregulation. Moreover, we agree that our additional white chamber experiment does not\naccurately represent such scenarios since the temperature inside the white chamber is higher\nthan the ambient temperature. Our intention was to demonstrate radiative cooling in a more\nrealistic environment without an extensive heat absorber. However, due to the lack of\nexperimental apparatus, we could not fully simulate a thermally-regulated environment as\nhighlighted by the reviewer. We apologize for any misrepresentation that may have caused an\noverestimation of our work. To better explain the performance limitations of TeRC, we have\nadded the following discussion to the revised manuscript.\n(Revised manuscript, page 17)\n“Additionally, we characterize the performance of TeRC for real-life applications by\nmeasuring its radiative cooling performance under various conditions: absence of IR absorbing\nPDMS layer (Supplementary Fig. 26), low solar energy absorptive white indoor environment\n(Supplementary Fig. 27), and when tilted (Supplementary Fig. 28). In regions such as Southeast\nAsia, where active thermal regulation through air conditioning is common and indoor\ntemperatures are lower than outdoors, the application of a PDMS layer for internal heat\nabsorption may not be necessary. In such cases, TeRC without the PDMS layer could be more\ndesirable, as the ITO layer functions as a low-E layer in other heat-reflective windows [R1-2].”\n[R1] Wang, S. et al. Thermochromic smart windows with highly regulated radiative cooling\nand solar transmission. Nano Energy 89, 106440 (2021).\n[R2] Wang, S. et al. Scalable thermochromic smart windows with passive radiative cooling\nregulation. Science 374, 1501-1504 (2021).\nComment 2: It is not clear to me why the authors did a 45° tilt experiment for their windows\n– I had suggested an experiment where the windows are vertical, as they normally are. If their\ndesign is not suitable for vertical windows, or if they did not do such an experiment for practical\nreasons, they should acknowledge that fact as a comment.\nResponse 2: We are grateful to the reviewer for providing critical comment on the\ncharacterization needed to verify the practical aspects of TeRC, which has greatly contributed\nto enhancing the quality of our work. Additionally, we apologize for not sufficiently addressing\nthe reviewer’s concerns in the previous revision process.\nWe would like to explain the rationale for measuring the cooling performance of TeRC tilted\nto 45°, rather than the vertically, as suggested by the reviewer. In our study, TeRC is proposed\nas an alternative for conventional windows, which, as the reviewer mentioned, are typically\ninstalled vertically. To address the reviewer’s concern, we would like to highlight that the core\nproposition of our work is the multifunctionality achieved by combining the triboelectric\nnanogenerator (TENG) and the radiative cooler (RC) in TeRC. While the cooling performance\nof RC in a vertical position is indeed a practical consideration, as the reviewer noted, the\nvertical orientation poses a limitation for the TENG functionality. Specifically, if we assume\nthat rain droplets fall vertically, they would not effectively collide with the surface of TeRC,\nthus hindering the TENG’s ability to generate electricity. Nonetheless, we have verified the\ncooling performance under conditions that maintain the functionality of both RC and TENG,\nspecifically in a tilted position. So, we verify the cooling performance by selecting 45°, which\nwas able to obtain the highest electrical output in terms of TENG. Therefore, we have\nconducted experiments to demonstrate that the radiative cooler performs excellently even when\ntilted and confirmed that it effectively lowers the inner temperature at a tilted configuration.\nNonetheless, we acknowledge the limitation pointed out by the reviewer: the TeRC’s\nfunctionality as TENG is challenging to achieve in a vertical configuration. We agree it is\nnecessary to comment on this limitation explicitly for better understanding of the practicality\nof our device. However, we believe this limitation can be adequately addressed through future\nadvanced configuration. To guide future research in related fields, we have added a perspective\non this issue in the Discussion section, highlighting potential improvements and directions for\nsubsequent work. To address the reviewer’s concern, we have added the following statements\nto the revised manuscript.\n(Revised manuscript, page 17)\n“Moreover, while conventional windows are typically installed vertically, we have measured\nthe RC performance of TeRC at a 45° tilt to evaluate if the RC performance is maintained under\noptimal conditions for TENG. TeRC may face challenges in functioning as a TENG in vertical\norientations, but modifying the design of TeRC array appropriately could enable TeRC to\nperform both as an RC and TeRC even in vertical orientations.”"}]

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