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•Cicada wing-inspired nanostructure is fabricated to enhance solar transmittance.•This proposed approach is at low heating temperature and atmospheric pressure.•Hydrophobicity and ...underwater oleophobicity are also designed and introduced.•Achieving stable evaporation rate near 1.40 kg/m2h in an oily seawater environment.
The severe light reflection phenomenon has become a critical challenge for the sufficient utilization of solar energy. For preventing the predation risk, cicada’s wings are gradually evolved into transparent morphology to preferably blend into the natural environment. The high solar transmittance in cicada wings is due to the presence of surface micro/nanostructures which avoids the large change of refractive index between air and wings. Herein, inspired by the high solar transmittance of cicada’s wings, an extremely-simple gaseous phase deposition with low heating temperature is developed to cover graphene film with methyl-modified silicon dioxide (CH3-SiO2) nanospheres that decelerate the refractive index reduction. The destructive interference effect of CH3-SiO2 nanospheres effectively increases the solar transmittance to promote the absorption and conversion of solar energy into heat by graphene nanosheets. As a result, compared to pure graphene film, CH3-SiO2 nanospheres covered graphene (SiO2-graphene) film presents a temperature increase of ∼6.3 °C with solar illumination of 1.0 kW/m2. After being combined with zwitterion-modified cotton fabric, a novel solar steam generation system mainly composed of SiO2-graphene film is prepared. Due to the hydrophobicity of CH3-SiO2 nanosphere and the hydration effect of zwitterionic polymer, the salt precipitation and oil fouling problems regarded as major challenges in solar desalination are simultaneously solved, thus achieving water evaporation rate near 1.40 kg/m2h at oily seawater environment. Most importantly, the solar transmittance enhancement and anti-reflection design inspired by cicada wings provide a valuable reference to promote the solar absorption capacity of other photo-thermal conversion materials.
This paper presents the results of an experimental study to measure and verify on-site global solar transmittance as a function of varying angles of incidence for glazing samples under natural ...clear-sky conditions, including: single-pane clear, double-pane clear, and double-pane low-e glazing. Field measurements were implemented using a silicon photodiode sensor and two thermopile-type pyranometers based on an easily-assembled test box with sample glazing. Measurement results were then compared to the published data (i.e., Tsol values in the WINDOW libraries). The results indicate that silicon photodiode sensors can be used to measure and verify direct solar transmittance within an acceptable range of accuracy. However, the global (i.e., direct and diffuse) solar transmittance measured by the thermopile-type sensors was significantly higher than the Tsol values from the WINDOW program. As a result, it is recommended that such field measurements could be used to verify the on-site direct and global solar transmittance of as-built glazing since the WINDOW program currently only accounts for direct (i.e., beam) solar radiation in the reference solar spectrum (i.e., ASTM E891).
•A switchable photovoltaic window based on thermotropic hydrogel is developed.•Selected hydrogel can reversibly switch between highly transmissive and scattering states.•The window performance is ...evaluated by indoor and outdoor experiments.•This window can passively control daylighting and heat gain during hot sunny days.•This window offers better electrical performance than conventional PV glazing.
Daylighting control technologies have become an essential part of sustainable building design to reduce overheating, glare and energy consumption in buildings. In this paper, a smart glazing system where a Building Integrated Photovoltaic (BIPV) glazing is coupled with an optically switchable thermotropic hydrogel layer is proposed to improve the daylighting control and electricity generation performance of traditional BIPV glazings. Thermotropic (TT) hydrogels made of various weight percentage combinations of hydroxypropyl cellulose (HPC) polymer, gellan gum and sodium chloride (NaCl) salt were synthesised and first evaluated by visible-near-infrared spectroscopy. Subsequently, small-scale prototypes of the proposed BIPV thermotropic (BIPV-TT) laminated glazing based on these TT hydrogels were fabricated and characterised experimentally under controlled laboratory conditions. The TT hydrogel, which was synthesised of 6 wt% HPC, 0.5 wt% gellan gum and 4.5 wt% NaCl, was selected for further experimental characterisations in a dynamic outdoor environment, due to its appropriate transition temperature of 30.7 °C for use in mild climates with a wide modulation range of solar transmittance from 85.8% (in the transparent state) to 9.6% (in the light-scattering state). The outdoor tests were conducted in Nottingham, the UK, on typical summer days with sunny and partial cloudy conditions. The results showed that using the prototype BIPV-TT laminated glazing can reduce up to 80% of the solar radiation transmitted into the outdoor test cell, while providing up to 12% higher electrical power outputs, compared to its counterpart system with no thermotropic hydrogel applied.
•A novel model of window with transparent insulation slats (WTIS) was developed.•The numerical model of WTIS was validated with experiments.•The impact of key factors on the WTIS was revealed by ...parameter analysis.•The optical and thermal performance of WTIS was evaluated.
In order to achieve the sustainability of the built environment, extensive efforts have been made to improve the performance of glazed facade systems, thereby increasing indoor comfort and building energy conservation. A kind of window with transparent insulation slats (WTIS) is promising to achieve improved optical and thermal performance. In this study, an optical and thermal model of WTIS was established and the validity of the model was verified by experimental data. The impact of various influential factors on optical and thermal performance of WTIS was analyzed. The results show that the total transmittance decreases and the solar absorptance of transparent insulation slats (TIS) increases with the absorptivity of slat surface (αsl) and the ratio of slat width to the distance between two adjacent slats (W/L ratio). When the slat spacing increases from 10 mm to 25 mm and the slat width is 20 mm, the peak instantaneous heat flux and k-value of WTIS increase by 30.57 % and 17.54 %. When the transmissivity of slat surface increases from 0.1 to 0.7, the peak instantaneous heat flux and k-value of WTIS increase by 25.13 % and 5.31 %. Besides, the variation of shining factor has little effect on the total transmittance and the solar absorptance of TIS. The total transmittance and the solar absorptance of TIS have linear correlation with the proportion of diffuse radiation.
•First infrared transmittance and reflectance data for droplet-covered glass windows.•The droplet contact angle varied from 26 to 76o and surface area coverage up to 60%.•In most cases, the droplets ...reduce the transmittance due to absorption and/or back-scattering.•Measurements agree well with predictions from Monte Carlo ray-tracing simulations.•Solar transmittance of water droplet-covered glass windows can decrease significantly.
Water droplets are commonly observed on the inner or outer surface of solar energy conversion systems due to rain or condensation. These droplets affect the systems’ efficiency and thermal load. This study experimentally and numerically investigates infrared radiation transfer through semitransparent windows covered with droplets on their front or back sides. In order to validate our previously developed numerical code and to facilitate the systematic characterization of the samples, acrylic droplets (instead of water) were deposited onto glass windows with contact angle ranging from 26 to 76° and projected surface area coverage from 0% to 60%. The measured transmittance of glass windows with slightly absorbing droplets on the front increased while the reflectance decreased with increasing contact angle and surface coverage due to antireflection effects. For slightly absorbing droplets on the back with contact angles larger than the critical angle for total internal reflection at the droplet/air interface, the transmittance decreased by up to a factor 2 with increasing contact angle and surface area coverage. In the infrared spectral range when droplets were strongly absorbing, the window transmittance decreased by up to a factor 2.5 with increasing surface coverage for droplets either on the front or back sides. Experimental measurements were in excellent agreement with numerical predictions obtained using the Monte Carlo ray-tracing method. Then, the experimentally-validated simulation tool was used to predict the solar transmittance and emittance of glass windows covered with water droplets. The solar transmittance was found to decrease significantly with water droplets on either side depending on the time of day while the emittance remained unchanged.
Photovoltaic performance can be efficiently enhanced by improving the transmittance of solar cell cover, but the heating from unavailable solar irradiation has significantly adverse impact on both ...efficiency and reliability of solar cells. Radiative cooling can passively dissipate heat to the cold space (∼3 K) through the atmospheric window (8–13 μm). In this work, we reported a high-performance and scalable radiative cooler for solar cells using nanoporous anodic aluminum oxide (AAO). More specifically, the fabricated AAO cooler exhibited a near-unity infrared (IR) emissivity of ∼0.97 (weighted with the IR emission intensity of 298 K blackbody) in the atmosphere window, while maintaining an excellent solar transmittance over 0.94. Moreover, radiative cooling experiments showed that the porous cooler can cool bare silicon wafer by ∼7.5 K under a solar irradiation of ∼850 W/m2 in humid areas (humidity ∼70%). More importantly, concentrating the solar irradiation of ∼554 W/m2 with 50 times Fresnel lens, the cooler can achieve a maximum temperature drop of ∼35.6 K without wind cover even in a cloudy day. To further reveal the practicality of proposed AAO cooler, performance modeling of single-junction crystalline silicon (c-Si) solar cell encapsulated with the AAO cooler has been developed. Compared with commercial solar cells, the improved annual output electricity of the solar cell with AAO cooler can reach ∼16.5 kW h/(a∙m2).
•The AAO film exhibits a near-unity IR emissivity of ∼0.97 in the atmosphere window and a solar transmittance over 0.94.•The low–cost chemical fabrication process of cooler exhibits the scalable property of the AAO film.•A temperature drop of ∼35.6 K can be efficiently realized for the concentrator solar cell with the AAO film.•The improved output electricity of the AAO-cooler solar cell can be ∼16.5 kW h/(a∙m2) compared with commercial solar cells.
A single performance indicator, the solar transmittance factor (STF), has been proposed in previous works, together with the derived solar transmittance index (STI). It is aimed at evaluating the ...summer performance of the roofing system and allowing the selection of the most effective mix of surface and mass properties. It is easily calculated from low-level indicators such as U-value, module of periodic thermal transmittance, and solar reflectance. In the present work, the correlation between STF and the cooling energy demand, integrated over a reference period, was studied, as well as the peak of ceiling temperature increase with respect to the indoor temperature, relevant for thermal comfort. In particular, the thermal behavior of different roof types with variable insulation was calculated numerically by TRNSYS 17 for a wide set of locations and environmental conditions. Unlike other commonly used indicators, to which the analysis has been extended, a strong correlation with STF was found for both cooling energy demand and ceiling temperature rise.
Translucent concrete (TC) is promising to transmit daylight into buildings with high thermal performance and energy-saving potential. However, most of the existing studies on TC omit its dynamic ...transmittance. More research is required to analyze the impact of dynamic transmittance on the optical and thermal performance of TC walls, and to investigate how to take advantage of this distinctive feature so that the building energy-saving potential of TC walls can be fully exploited. In this study, an optical and thermal model of TC embedded with optical fibers (OFs) was developed considering the dynamic transmittance of TC under the influence of solar incidence angle. The model was validated with experimental data. By using the model, the effects of numerical aperture (NA), fiber volumetric ratio (FVR) and solar incident angle on TC were revealed. In addition, a kind of novel TC with inclined OFs was proposed to achieve improved climate-responsive transmittance, which can reduce the dependence on high NA OFs. It was found that in Changsha the direct radiation transmittance of an OF (NA = 0.51, tilt angle = 60°) embedded in TC walls can be controlled within 0.20 in summer, while it can increase up to 0.90 in winter. Though the inclusion of OFs imposes an adverse effect on the wall insulation, TC walls can reduce daily heat loss by about 9.14% and increase daily heat gain by about 51.66% in winter, contributing to energy conservation in buildings.
Abstract Passive radiative cooling materials can efficiently reflect solar radiation and spontaneously dissipate heat through the long‐wave infrared (LWIR) transmission window to the coldouter space. ...They are highly attractive for application in walls and roofs of sustainable energy‐efficient buildings, but it is a challenging task to develop switchable radiative coolers for transparent windows. Herein, free‐standing bacterial cellulose‐templated radiative cooling liquid crystal films with high LWIR emissivity and self‐adaptive solar transmittance are reported. Biosynthetic silanized bacterial cellulose with 3D interconnected porous nanostructure is harnessed as a robust template for confining novel liquid crystals with smectic A to chiral nematic phase transition, thereby endowing the film with high solar transmittance modulation ability. The resulting film is found to not only exhibit a remarkable LWIR emissivity, but also adaptively change its solar transmittance between a transparent state and an opaque state according to environmental temperature variation. Outdoor radiative cooling experiments are performed, and the energy‐saving performance is evaluated through a simulation of a 12‐story reference office building with the films applied as radiative cooling windows. This research can shine light on the development of advanced radiative cooling materials with switchable transmittance and their widespread applications in buildings, vehicles, and transparent photovoltaics.
Green façades have long been used to reduce the overheating of buildings by excessive solar gain, but little research attention has been paid to the plant traits that reduce the surface temperature ...of urban structures. This study aimed to identify the key traits contributing to the cooling effects of green façades by comparing five vine plant species. The vines were trained to climb a plastic net that was attached on the freestanding walls under outdoor conditions. One of the walls was not covered with plants to serve as control. We measured the longest vine length, total number of leaves, percentage coverage, leaf transpiration rate, leaf solar transmittance, wall surface temperatures, global solar radiation on a vertical south surface (GSR) and ambient temperature. We identified the percentage coverage as the key trait determining the overall cooling effect: a statistically positive relationship (P < 0.001) was identified between the percentage coverage and the wall surface temperature reduction (WTR; the wall surface temperature differences between the non-covered wall and the covered wall) under GSR of more than 0.1 kW m−2. The percentage coverage was in turn found to be determined by the vine length. We also found that genotypic differences in WTRs that were not explained by those in the percentage coverage were related to those in leaf solar transmittance, indicating that this trait also contributed to the cooling effects.
•We identified the plant traits contributing to the cooling effects of green façades.•Percentage coverage was the key trait reducing the wall surface temperatures.•The developmental trait of vine length determined the percentage coverage.•Leaf solar transmittance also contributed to the cooling effects.
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