Low Solar Absorptance Research Articles

Influence of enclosure filled with phase change material on photo-thermal regulation of direct absorption anaerobic reactor: Numerical and experimental study

Direct absorption methane reactor can maintain the fermentation process of microorganism by utilizing solar absorption and scattering of media in biogas reactor to improve the slurry temperature. However, direct absorption heating alone can save the corresponding electric energy and ensure the normal fermentation process of the biogas slurry in the reactor, but there are still problems of big temperature fluctuation and low solar heat absorption efficiency. In order to improve the stability of the fermentation process, it is proposed to optimize the design of this kind of reactor by filling paraffin PCM (phase change material). In this paper, a novel transient model for simulating the photo-thermal transfer process of the reactor with PCM structure was proposed. Experimental and numerical methods were utilized to analyze the effect of PCM parameters on photo-biochemical transformation process and solar heat absorption efficiency. The results show that filling paraffin phase change material in the enclosure of direct absorption reactor can effectively improve its thermal insulation and thermal storage performance, so as to improve the rate of volatile fatty acids (VFA) and gas production by anaerobic fermentation by maximum 31.1%. The average solar heat absorption efficiency of reactor equipped with PCM is bigger than 50%. Thicknesses of 4 cm and 2 cm for side and top structure were selected, respectively.

Design of highly reflective film for smart radiation device

Smart radiation device (SRD) based on the asymmetrical Fabry-Perot cavity automatically tune its IR emittance depending on the ambient temperature, making it an ideal choice for thermal control system of spacecrafts. The low solar absorption is desirable for SRD to prevent the spacecraft from overheating by sun light. In this paper, a multilayer highly reflective film with LHk stacking layers is designed to reduce the solar absorptance (As) of the Ag/Al2O3/VO2 structured SRD. The reflective film achieves a high reflection band in ~220 nm bandwidth from 460 nm to 680 nm, which results in a reduction of the solar absorptance by 25.56 % for SRD working at low temperature and 24.27 % at high temperature as the stacking factor k= 5. The simulation results indicate that an economic reflective film with k= 3 can achieve effective suppression of As of SRD, demonstrating the promising potential of the proposed reflective film in thermal control application of spacecrafts.

Optical properties and radiation stability of SiO2/ZnO composite pigment prepared by co-sintering method

Thermal control coatings (TCCs) are an essential part of the thermal control systems in the spacecraft. Solar absorptance and emittance are the key performance parameters of TCCs. To develop an ultra-low solar absorption and stable inorganic TCCs for surface radiator, different TCCs were prepared by co-sintering ZnO and SiO2 nanoparticles to form Zn2SiO4/SiO2 pigment in this work, and the optical properties and radiation stability were systematically studied. It is found that the coating based on composite pigment has high reflectivity in the ultraviolet band and excellent optical performance possessing the low solar absorption of 0.06. In addition, the Zn2SiO4/SiO2 coating demonstrates the highest proton and electron radiation stability because that SiO2 between Zn2SiO4 particles acts as the relaxation center of the defects caused by radiation.

Blackened calcium‐based composite particles and their apparent kinetics features for solar thermochemical energy storage

AbstractThe calcium looping (CaL) thermochemical thermal energy storage is one of the best high‐temperature heat storage schemes for 3th concentrating solar power (CSP) photothermal power. However, the application of this technology is greatly hindered by the low solar absorption capability and the poor cyclic stability of CaCO3/CaO‐based material. In this article, the solar absorbing capability of CaCO3 particles is enhanced by doping Mn‐Fe oxides, meanwhile, awns of setaria faberis (ASF) and microcrystalline cellulose (MCC) are used as bio‐templates to generate pores inside the particles. The pore‐making process promotes the cyclic stability and carbonation kinetic features of the composite particles simultaneously. The test results show that the proposed particles possess adequate anti‐crushing strength, high cyclic stability, high solar absorption, and high carbonation rate. In addition, the apparent carbonation kinetic features of the composite porous particles are studied with the influencing factors such as CO2 partial pressure, reaction temperature, and particle morphology taken into consideration. A kinetic equation involving these parameters is developed with the thermogravimetry data of the prepared samples. By employing this kinetic function, the carbonation reaction of the prepared particles inside the carbonator becomes predictable, which is of great significance for the design and regulation of the carbonator achieving highly stable thermal output from the CaL thermochemical heat storage system.

Highly suppressed solar absorption in a daytime radiative cooler designed by genetic algorithm

Abstract Here, we report a selective multilayer emitter for eco-friendly daytime passive radiative cooling. The types of materials and thickness of up to 10 layers of the multilayer structure are optimized by a genetic algorithm. The passive radiative cooler is designed to mainly target low solar absorption, which allows sub-ambient cooling under direct sunlight. We used a custom objective function in the solar region to achieve high-performance daytime radiative cooling to minimize solar absorption. The designed structure minimizes solar absorption with an average absorptivity of 5.0% in the solar region (0.3–2.5 μm) while strongly emitting thermal radiation with an average emissivity of 86.0% in the atmospheric transparency window (8–13 μm). The designed and fabricated structure achieves daytime net cooling flux of 84.8 W m−2 and 70.6 W m−2, respectively, under the direct AM 1.5 solar irradiation (SI) (total heat flux of 892 W m−2 in the 0.3–2.5 μm wavelength region). Finally, we experimentally demonstrate a passive radiative cooling of the fabricated selective emitter through a 72-hour day-night cycle, showing an average and maximum temperature reduction of 3.1 °C and 6.0 °C, respectively. Our approach provides additional degrees of freedom by designing both materials and thickness and thereby is expected to allow high-performance daytime radiative cooling.

Cellulose-based porous polymer film with auto-deposited TiO2 as spectrally selective materials for passive daytime radiative cooling

Passive daytime radiative cooling materials significantly alleviate the current energy shortage and global warming without consuming any energy. However, conventional inorganic nanoparticles can hardly achieve excellent cooling effect during daytime due to their low reflectivity. Broadband super white nanoparticle-based radiators demonstrate excellent solar reflectivity under direct sunlight and low absorption. Accordingly, we introduced TiO2 nanoparticles with the advantages of high refractive index, high infrared emissivity and low solar absorption into the porous cellulose-based film via a facile phase separation strategy. The cavities were distributed around 5 μm in size due to the limited evaporation of solvents, while the introduction of TiO2 nanoparticles effectively enhanced the solar reflectance to 0.97. Moreover, the scattering model demonstrated that the high solar reflectivity was mainly produced by the randomly agglomerate effect of nanoparticles and these nano-clusters also contribute to a high mid-infrared emissivity. As a result, the photonic film presented about 10 °C cooling capacity even under the high-density solar radiation (~897 W/m2) according to the out-door measurement. The random nano-clusters filling strategy avoids the complicate process of specific size particles and has a significant potential for propel the polymer-based cooling technology to satisfy the need of large-scale thermal management.

Spectrally Selective Nanoparticle Mixture Coating for Passive Daytime Radiative Cooling

Passive daytime radiative cooling, which is a process that removes excess heat to cold space as an infinite heat sink, is an emerging technology for applications that require thermal control. Among the different structures of radiative coolers, multilayer- and photonic-structured radiative coolers that are composed of inorganic layers still need to be simple to fabricate. Herein, we describe the fabrication of a nanoparticle-mixture-based radiative cooler that exhibits highly selective infrared emission and low solar absorption. Al2O3, SiO2, and Si3N4 nanoparticles exhibit intrinsic absorption in parts of the atmospheric transparency window; facile one-step spin coating of a mixture of these nanoparticles generates a surface with selective infrared emission, which can provide a more powerful cooling effect compared to broadband emitters. The nanoparticle-based radiative cooler exhibits an extremely low solar absorption of 4% and a highly selective emissivity of 88.7% within the atmospheric transparency window owing to the synergy of the optical properties of the material. The nanoparticle mixture radiative cooler produces subambient cooling of 2.8 °C for surface cooling and 1.0 °C for space cooling, whereas the Ag film exhibits an above-ambient cooling of 1.1 °C for surface cooling and 3.4 °C for space cooling under direct sunlight.

Ultrawhite BaSO4 Paints and Films for Remarkable Daytime Subambient Radiative Cooling.

Radiative cooling is a passive cooling technology that offers great promises to reduce space cooling cost, combat the urban island effect, and alleviate the global warming. To achieve passive daytime radiative cooling, current state-of-the-art solutions often utilize complicated multilayer structures or a reflective metal layer, limiting their applications in many fields. Attempts have been made to achieve passive daytime radiative cooling with single-layer paints, but they often require a thick coating or show partial daytime cooling. In this work, we experimentally demonstrate remarkable full-daytime subambient cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite paints. BaSO4 has a high electron band gap for low solar absorptance and phonon resonance at 9 μm for high sky window emissivity. With an appropriate particle size and a broad particle size distribution, the BaSO4 nanoparticle film reaches an ultrahigh solar reflectance of 97.6% and a high sky window emissivity of 0.96. During field tests, the BaSO4 film stays more than 4.5 °C below ambient temperature or achieves an average cooling power of 117 W/m2. The BaSO4-acrylic paint is developed with a 60% volume concentration to enhance the reliability in outdoor applications, achieving a solar reflectance of 98.1% and a sky window emissivity of 0.95. Field tests indicate similar cooling performance to the BaSO4 films. Overall, our BaSO4-acrylic paint shows a standard figure of merit of 0.77, which is among the highest of radiative cooling solutions while providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process.

Investigating the energy saving potential of thermochromic coatings on building envelopes

Thermochromic (TC) materials can switch solar absorptance (α) based on temperature stimuli. When coatings with TC properties are applied on building envelope surfaces, the amount of solar heat gains can be controlled to reduce the heating and cooling demand of buildings. To date, limited research has been conducted in investigating optimal TC coating properties for application on opaque building envelopes in various scenarios.In this research, a method to model TC coatings using building performance simulation (BPS) tools has been developed and coupled with python to optimize solar absorption states (α) and switching temperatures and reduce the annual heating and cooling demand. The simulation-based approach has been employed to perform early-stage exploration studies on multiple building types and climates to support material R&D in developing optimized coatings for target applications and assess the potential energy savings.The results indicate that the optimum TC properties are unique to climate and building types. TC coatings with high switching temperatures result in larger energy savings for scenarios with high heating demands, while TC coatings with low switching temperatures produce larger energy savings in scenarios with high cooling demands. Similarly, increasing the high solar absorption (αhigh) to 1 increases the heating savings, while reducing the low solar absorption (αlow) to 0 results in higher cooling savings. Furthermore, it was found that solar irradiance causes temperature spikes triggering the TC coatings to unnecessarily switch from high to low absorptance state in winters leading to heating penalties. Replacing optimal static with TC coatings on terraced houses in the Spanish climate with a 2:3 heating to cooling demand ratio results in 2 to 13% energy savings.

Scalable and paint-format microparticle–polymer composite enabling high-performance daytime radiative cooling

Radiative cooling, a process in which the Earth maintains a near-constant temperature despite significant solar flux, is a promising technology with no requirement of any input energy for sub-ambient cooling as well as no emission of greenhouse gas. Here, we demonstrate a paint-format microparticle–polymer composite with a low fabrication cost, simple structure, and high radiative cooling performance. Harnessing intrinsic optical properties of aluminum oxide and silicon oxide particles with large energy bandgaps and complementary IR emission properties within the atmospheric transparency window, the proposed radiative cooling paint has an extremely low solar absorptivity of 0.032 and strong IR emission of 0.935 in the atmospheric transparency window. Furthermore, many outdoor measurements proved the RC paint provided high-performance radiative cooling effect for comparison with commercial white paint even in hot summer season detrimental for daytime raditaive cooling, enabling the practical use of radiative cooling.

Solar selective reflector materials: Another option for enhancing the efficiency of the high-temperature solar receivers/reactors

The cavity wall is an important part of a cavity receiver in determining the receiver efficiency. Using solar selective reflector (SSR) materials with low solar absorptivity and high thermal emissivity for the cavity wall design is one efficient way to improve the receiver efficiency. In this work, we present a systematic study of the optical and high-temperature stability performances of six different SSR materials: one refractory ceramic fiber based substrate material (Fiberfrax 140) and five metallic oxide coatings which are prepared by mixing metallic oxide powders of alumina, magnesium oxide and titanium dioxide with commercial inorganic adhesives. The thermal stability was studied by heating up and keeping the six candidate materials in atmospheric conditions at a temperature of 1250 °C for 200 h. The spectrum of hemispherical reflectance in the spectrum band 0.25–25 μm was measured for analyzing the optical performance of the candidate materials. The obtained results show that all the six materials studied have good solar selective reflection characteristics, i.e, low solar absorptivity and relatively high thermal emissivity. Especially, the alumina coated substrate material shows excellent performances both for thermal stability and solar selective reflection. The solar reflectivity can reach 94.6%.

Thermo-energetic performance of wooden dwellings: Benefits of cross-laminated timber in Brazilian climates

This research focused on studies of the cross-laminated timber (CLT) construction system and, in particular, on an innovative panel composition of eucalyptus (Eucalyptus grandis) heartwood, which is considered in Brazil as a low-added-value material that is often discarded or used for less noble purposes. The aim of this study was to investigate the thermo-energetic performance of CLT panels for use in low-income housing under several different climate conditions. The research was based on the simulation method of the Brazilian Technical Regulation of Quality for the Energy Efficiency Level of Residential Buildings (RTQ-R). The results, based on 72 parametric simulations, proved that CLT panels made of eucalyptus heartwood have significant potential for thermo-energetic improvement in various housing types under diverse climatic conditions. The simulated envelope combinations showed that in Curitiba (bioclimatic zone 1), it is important to associate 5-layered CLT panels with 10 cm thermal insulation; in São Paulo (bioclimatic zone 3), the envelope should be provided with low solar absorptance and 5-layered CLT panels without thermal insulation; and in Belém (bioclimatic zone 8), in addition to the low solar absorptance in the envelope, the CLT panels should be composed of only three layers and without thermal insulation. It was concluded that the innovative CLT construction system with low-added-value eucalyptus heartwood is a very promising technology for Brazilian dwellings that are more energy efficient, with further studies on mechanical behavior being necessary for its consolidation in the country.

Biomimetic MXene‐Polyvinyl Alcohol Composite Hydrogel with Vertically Aligned Channels for Highly Efficient Solar Steam Generation

AbstractSolar steam generation, which utilizes sustainable solar energy to produce fresh water, is regarded as a facile and effective way to solve water scarcity issues. However, challenges remain in terms of inefficient water transfer and vapor release, low solar absorption and conversion efficiency, and hydrophobic photothermal materials. Herein, enlightened by water transport through internal microchannels in trees, a tree‐inspired hydrogel (TIH) with vertically aligned channels incorporating MXene as the light absorber is fabricated. Due to the vertically aligned channels, the rapid transfer of water and unobstructed release of steam can be achieved. Furthermore, water state in molecular meshes can be changed by polymer−water interaction, and partial water can be activated to promote water evaporation. Excellent internal photothermal conversion efficiency and hydrophilicity of MXene are also conducive to evaporation. As a result, TIH has achieved an evaporation rate of 2.71 kg m−2 h−1 and energy efficiency of 90.7% under one sun irradiation, which is higher than membrane material and structure‐disordered hydrogel. This design principle, expandable manufacturing route, and excellent performance provide a potential way and design concept for water purification and desalination.

Bulk material based selective infrared emitter for sub-ambient daytime radiative cooling

Through passively emitting excess heat to the outer space, radiative cooling has been demonstrated as an efficient way for energy saving applications. Instead of well-designed photonic structures, we rediscovered the potential of a bulk material as a selective emitter for sub-ambient daytime radiative cooling. The proposed cooler, consisting of a 1-mm-thick lithium fluoride crystal coated with silver backing, exhibits a low solar absorptance of 4.7% and nearly ideal infrared selectivity with high emission exactly within the atmospheric transmission band (i.e., 8–13 μm). Potentially, the cooler could achieve an ultra-low stagnation temperature up to 57 °C below the ambient. Sub-ambient daytime cooling performance was also demonstrated in an outdoor test with cooler stagnation temperature below the ambient one by 5 °C under solar irradiance above 900 W/m2 and a net cooling power of about 60 W/m2 in thermal equilibrium with the ambient. The proposed simple selective infrared emitter could efficiently and passively cool optical systems due to the wide applications of lithium fluoride crystal as optical windows and mirrors.

Theoretical investigation of broadband absorption enhancement in a-Si thin-film solar cell with nanoparticles

Thin-film solar cells have attracted increasing attention due to its low material cost and large flexibility, but they also face the challenge of low solar absorption due to reduced active layer thickness. Through exciting surface plasmon resonance, plasmonic metal nanoparticles are usually placed on the cell front surface to enhance solar absorption. However, if eliminating the unuseful intrinsic absorption in nanoparticles, we find that dielectric ones are better choices to enhance a-Si thin-film solar cell absorption efficiency. Moreover, a composite light trapping structure with dielectric nanoparticles on the front surface and metal hemispheres on the rear surface is proposed to achieve broadband absorption enhancement in both short and long wavelengths, with the aim to get a higher conversion efficiency. The finite-difference-time-domain simulation results show that, compared with bare 100-nm-thick amorphous silicon solar cell, the short-circuit current density and photoelectric conversion efficiency could be respectively improved by 21% and 18% with addition of optimized composite light trapping structure. The general method proposed in this study could provide valuable guidance to light trapping structure design for various kinds of thin-film solar cells.

Doped Graphitic Carbon Nitride: Insights from Spectroscopy and Electrochemistry

Photocatalytic use of the neat graphitic carbon nitride, g-C3N4, is usually restricted by unsatisfactory efficiency due to low solar light absorption and rapid recombination of photogenerated charge carriers. The introduction of metal ions into g-C3N4 structure can effectively reduce the recombination rate of photogenerated charges, expand the light absorption range and in consequence improve overall photocatalytic efficiency. The series of graphitic carbon nitride based materials, doped with metals cations (iron, bismuth, cobalt, zinc), has been prepared. The doping-caused changes of band gap energy, flat band potential, photocurrent generation, and photoactivity range have been studied using spectroscopic and electrochemical techniques. Moreover, based on the photocatalytic activity tests the efficiency of change recombination has been estimated. Presented studies proved multifarious positive effects of doping on g-C3N4, such as extended light absorption, an improved yield of electron–hole pairs separation, suppressed recombination of charge carriers, reduced electron transport resistance, and improved photocatalytic efficiency of dye decomposition.

Reduced Graphene Oxide-Based Spectrally Selective Absorber with an Extremely Low Thermal Emittance and High Solar Absorptance.

Carbon‐based black materials exhibit strong solar absorptance (αsolar >0.90), which play key roles in transforming solar energy into available power for solar‐thermal, thermophotovoltaic, thermoelectric, and many other systems. However, because of high thermal emittance (>95%), these carbon‐based materials always cause huge energy loss that hinders the solar‐thermal conversion efficiency tremendously. In this study, a reduced graphene oxide–based spectrally selective absorber (rGO‐SSA) is demonstrated, which possesses a recorded low thermal emittance (≈4%) and high solar absorptance (αsolar ≈ 0.92) by easily regulating the reduction level of inner 2D graphene sheets. Compared to conventional carbon‐based black materials, thermal emittance of this rGO‐SSA is largely reduced by ≈95.8% and the cutoff wavelength of rGO‐SSA is broadband‐tunable that can range from 1.1 to 3.2 µm. More importantly, this simply sol‐gel coated rGO‐SSA has high temperature tolerance at 800 °C for 96 h that is hardly achieved by other cermet‐based or photonic‐based SSAs. Based on this rGO‐SSA, ultrafast solar steam escape (0.94 mg cm−2 s−1) under concentrated solar irradiance is achieved directly. The insight from this study will provide a new strategy for constructing thermally stable carbon‐based SSAs and greatly facilitate the solar‐thermal practical significance.

Spectrally Selective Inorganic-Based Multilayer Emitter for Daytime Radiative Cooling.

Daytime radiative coolers are used to pump excess heat from a target object into a cold exterior space without energy consumption. Radiative coolers have become attractive cooling options. In this study, a daytime radiative cooler was designed to have a selective emissive property of electromagnetic waves in the atmospheric transparency window of 8-13 μm and preserve low solar absorption for enhancing radiative cooling performance. The proposed daytime radiative cooler has a simple multilayer structure of inorganic materials, namely, Al2O3, Si3N4, and SiO2, and exhibits high emission in the 8-13 μm region. Through a particle swarm optimization method, which is based on an evolutionary algorithm, the stacking sequence and thickness of each layer were optimized to maximize emissions in the 8-13 μm region and minimize the cooling temperature. The average value of emissivity of the fabricated inorganic radiative cooler in the 8-13 μm range was 87%, and its average absorptivity in the solar spectral region (0.3-2.5 μm) was 5.2%. The fabricated inorganic radiative cooler was experimentally applied for daytime radiative cooling. The inorganic radiative cooler can reduce the temperature by up to 8.2 °C compared to the inner ambient temperature during the daytime under direct sunlight.

Scalable dual-layer film with broadband infrared emission for sub-ambient daytime radiative cooling

As a passive cooling method with no energy input, radiative cooling has great potential in applications like building energy saving. The key to effective radiative cooling lies in selectively controlling the radiative properties in both solar and mid-infrared spectrums, and simple structures based on polymer materials attract growing attentions due to their overwhelming advantages of low cost and large-scalable fabrication. In this study, a dual-layer film consisting of polyvinyl fluoride (PVF) layer and Ag coating is proposed to act as an efficient daytime radiative cooler with low solar absorption and broadband infrared emission. An outdoor test demonstrates that sub-ambient daytime cooling is successfully achieved by the proposed cooler with an equilibrium temperature 2 °C below the ambient under solar irradiation of 950 W/m2. Due to better durability and better performance in anti-staining and corrosion protection than other polymer materials, the demonstrated cooling performance based on the PVF film would facilitate radiative cooling principle in much broader applications.

Synergy of dopants and defects in ultrathin 2D carbon nitride sheets to significantly boost the photocatalytic hydrogen evolution

Atomically thin 2D carbon nitride sheets (CNs) can improve the photocatalytic performance due to their high specific surface area, exposed active sites, and nano-scaled diffusion path of the charge transferred to surface. However, in practical application, 2D CNs photocatalysts always suffer the low solar absorption. Here, we present a novel approach to achieve the ultrathin 2D CNs photocatalysts with wide spectrum absorption in visible-light range. The new method, i.e. thermal annealing of atomically thin CNs in Se vapor, can dramatically extend the absorption edge of the ultrathin CNs from 418 to 574 nm. Using structural characterization and spin-polarized density functional theory, we identify the loss of the nitrogen atoms at the edges and in situ Se doping into the network of ultrathin CNs, which could change the electronic band structure of the 2D CNs. As a consequence, the ultrathin 2D CNs photocatalyst via Se vapor annealing (Se-CNs) modified by Pt cocatalyst shows a superior photocatalytic hydrogen evolution activity under visible light irradiation. More significantly, the ultrathin Se-CNs photocatalyst also exhibits an impressive photocatalytic hydrogen evolution activity under visible-light irradiation in the absence of Pt cocatalyst. We believe that our study provide a facile and environmentally friendly strategy to realize the utilization of 2D CNs photocatalyst under wide spectrum range of solar radiation to harvest solar energy efficiently, and it can be further applied in various advanced fields, including CO2 photoreduction, organic photosynthesis, and pollutant controls.