Sub-ambient Temperatures Research Articles

Knittable ultra-high molecular weight polyethylene microporous fibers for all-day personal passive radiative cooling in outdoors

Polyethylene (PE) has been recognized as a basic material with great potential for personal thermal management garments, expected to realize industrialization at a low cost. However, post-treatment or functional additives tend to be used to improve the wearability properties and cooling performance of PE textiles, which not only increases the production expense but also is unfavorable to recycling. Here, we fabricated an ultra-high molecular weight polyethylene (UPE) microporous fiber with high porosity (∼70.2 %) and nano pores (0.56 ± 0.43 μm) by gel spinning and multistage extraction. Without any modification, the woven fabrics can exhibit a low thermal conductivity (31–37 mW m−1 K−1), high mid-infrared transmittance (85.7 %) and solar reflectivity (96 %). The UPE fabric was demonstrated to reduce the simulated skin temperature by average 5.2 °C under direct sunshine of 800 W m−2 and allowed for a subambient temperature close to bare skin at night, realizing personal cooling all day in outdoors. Attributing to the weaving structure of the UPE fabric, its wearability and durability are superior to traditional textiles, such as cotton and Tyvek, and it can meet the requirements of most people for wearing comfort. UPE microporous fibers provide a possible opportunity for large-scale production and practical application of radiation cooling clothing.

Investigation of recycled materials for radiative cooling under tropical climate

Abstract As a sustainable alternative to using virgin polymer, we propose the use of recycled polymer for the fabrication of passive radiative cooling materials to tackle both the increasing demand for cooling systems and to upcycle plastic waste. Using recycled acrylic sheets as the binder for BaSO4 thorough sol–gel method, a sustained 1.2 °C sub-ambient temperature during daytime and 3 °C sub-ambient temperature during nighttime was achieved under the hot and humid conditions of the tropical climate. The coating achieved 97.7 % solar reflectance and 95 % infrared emittance. Separately, when porosity is introduced to recycled acrylic sheets through a phase inversion method, a near-ambient temperature during noontime and sustained sub-ambient temperature of 2.5 °C during nighttime was achieved (with 96.7 % solar reflectance). Comparable performances are also obtained using recycled polyvinyl chloride (PVC) pipe and expanded polystyrene (EPS) foam.

Experimental comparison on the performance of radiative, reflective and evaporative cooling in extremely hot climate: A case study in Chongqing, China

Conventional energy-intensive cooling methods are inadequate for addressing the growing cooling demand exacerbated by severe heat waves. Passive cooling technologies with no energy consumption hold substantial promise as favorable alternatives. Radiative, reflective and evaporative cooling are three emerging passive cooling techniques that operate through either spectrum modification or phase change to effectively reject or dissipate heat. In this study, we investigated the cooling performance of four roof models equipped with a bare surface, as well as radiative, reflective and evaporative cooling technologies on the exterior under extremely hot conditions. The results highlighted the remarkable superiority of radiative cooling, particularly during extremely hot periods, wherein it achieved subambient exterior surface temperature by 2.0 °C during working hours. On sunny days following a rain event, the evaporative cooling exhibited a distinctive cooling performance. However, during working hours from 09:00 to 18:00, the average exterior surface temperature of the evaporative cooling model remained 1.9 °C higher than the ambient temperature and 2.7 °C higher than that of the radiative cooling model. Therefore, radiative cooling demonstrated the most effective cooling performance, and its applications should be further investigated to address the growing demand for cooling under global warming.

Minimizing polyphenols and enzymes degradation using hydrogel packaging with combined evaporative and daytime radiative cooling functions during ambient transportation

Polyphenols and enzymes usually suffer from degradation during transportation due to the lack of a cold chain system in developing countries and regions. In this study, anthocyanin and trypsin were selected as examples of polyphenols and enzymes and investigated for minimizing their degradation during ambient transportation using hydrogel packaging with combined evaporative and daytime radiative cooling functions. A polyacrylamide/polyvinyl alcohol contained with nanoparticles (NPs@PAAm/PVA) hydrogel packaging was thus developed. The NPs@PAAm/PVA packaging exhibited desirable swelling behaviour, high solar reflectance, and strong atmospheric emissivity to synergistically achieve evaporative and daytime radiative cooling. The indoor experiments indicated that the vial with NPs@PAAm/PVA packaging realised sub-ambient temperatures under different working temperatures and humidities, and the field tests indicated that the vial with NPs@PAAm/PVA packaging could effectively preserve the anthocyanin and trypsin without degradation caused by strong sunlight and high temperature. Consequently, the NPs@PAAm/PVA packaging with evaporative and daytime radiative cooling effects has promising prospects for anthocyanin and trypsin transportation in an energy-saving and sustainable manner.

Critical Influence of Water on the Polymorphism of 1,3-Dimethylurea and Other Heterogeneous Equilibria.

It is shown that the presence of hundreds of ppm of water in 1,3-dimethylurea (DMU) powder led to the large depression of the transition temperature between the two enantiotropically related polymorphic forms of DMU (Form II → Form I) from 58 °C to 25 °C, thus explaining the reported discrepancies on this temperature of transition. Importantly, this case study shows that thermodynamics (through the construction of the DMU-water temperature-composition phase diagram) rather than kinetics is responsible for this significant temperature drop. Furthermore, this work also highlights the existence of a monohydrate of DMU that has never been reported before with a non-congruent fusion at 8 °C. Interestingly, its crystal structure, determined from X-ray powder diffraction data at sub-ambient temperature, consists of a DMU-water hydrogen bonded network totally excluding homo-molecular hydrogen bonds (whereas present in forms I and II of DMU).

Phosphate activated geopolymer-based coating with high temperature resistance for sub-ambient radiative cooling

Passive sub-ambient daytime radiative cooling (SDRC) is an energy-free method enabling heat dissipation away from buildings or infrastructures to cold outer space (∼3 K). However, although organic coatings (based on polymer matrices) have been proven to be a cost-effective means to achieve the SDRC effect by incorporating various functional fillers, organic coatings may have environmental concerns and aging problems. Herein, an inorganic phosphate activated geopolymer-based (PAGP) SDRC coating was synthesized using nano-silica (SiO2) particles and barium sulfate (BaSO4) sheets as the modifiers. The synthesized coating could achieve high infrared emissivity of 0.9634 and solar reflectance of 0.9471, which was also found to exhibit high-temperature resistance and optical stability up to 1000 °C due to its covalently bonded structure and electrically neutral system. The chemical composition, surface morphology, and elemental distribution of the coating were characterized by XRD, FTIR, SEM, BET, and EDS. The evolution of condensed structures and weight loss analysis during the high-temperature treatment was conducted by TGA and XRD to analyze the dehydration process and phase change during the heat process. The outdoor field tests showed a maximum sub-ambient temperature reduction of 3.8 °C by the developed coating without any energy input under direct sunlight in Hong Kong. Furthermore, the energy-saving performance of the PAGP-based SDRC coating is analyzed under different climatic conditions in China with the aid of EnergyPlus. Simulation results demonstrated that the PAGP-based SDRC coating can significantly save the cooling load in all five selected cities, which shows great potential in building energy conservation and carbon emission reduction.

Bioinspired Switchable Passive Daytime Radiative Cooling Coatings.

Passive daytime radiative cooling (PDRC) relies on simultaneous reflection of sunlight and radiation toward cold outer space. Current designs of PDRC coatings have demonstrated potential as eco-friendly alternatives to traditional electrical air conditioning (AC). While many features of PDRC have been individually optimized in different studies, for practical impact, it is essential for a system to demonstrate excellence in all essential aspects, like the materials that nature has created. We propose a bioinspired PDRC structure templated by bicontinuous interfacially jammed emulsion gels (bijels) that possesses excellent cooling, thinness, tunability, scalability, and mechanical robustness. The unique bicontinuous disordered structure captures key features of Cyphochilus beetle scales, enabling a thin (130 μm) bijel PDRC coating to achieve high solar reflectance (≳0.97) and high longwave-infrared (LWIR) emissivity (≳0.93), resulting in a subambient temperature drop of ∼5.6 °C under direct sunlight. We further demonstrate switchable cooling inspired by the exoskeleton of the Hercules beetle and mechanical robustness in analogy to spongy bone structures.

Thin lamellar films with enhanced mechanical properties for durable radiative cooling

Passive daytime radiative cooling is a promising path to tackle energy, environment and security issues originated from global warming. However, the contradiction between desired high solar reflectivity and necessary applicable performance is a major limitation at this stage. Herein, we demonstrate a “Solvent exchange-Reprotonation” processing strategy to fabricate a lamellar structure integrating aramid nanofibers with core-shell TiO2-coated Mica microplatelets for enhanced strength and durability without compromising optical performance. Such approach enables a slow but complete two-step protonation transition and the formation of three-dimensional dendritic networks with strong fibrillar joints, where overloaded scatterers are stably grasped and anchored in alignment, thereby resulting in a high strength of ~112 MPa as well as excellent environmental durability including ultraviolet aging, high temperature, scratches, etc. Notably, the strong backward scattering excited by multiple core-shell and shell-air interfaces guarantees a balanced reflectivity (~92%) and thickness (~25 μm), which is further revealed by outdoor tests where attainable subambient temperature drops are ~3.35 °C for daytime and ~6.11 °C for nighttime. Consequently, both the cooling capacity and comprehensive outdoor-services performance, greatly push radiative cooling towards real-world applications.

Scalable wet-spinning of wearable chitosan-silica textile for all-day radiative cooling

Passive radiative cooling for cost-effective personal thermal management (PTM) has received tremendous attention as it permits continuous subambient temperatures without energy consumption. Enhancing emission in the mid-infrared wavelength range and minimizing solar heat absorption are two independent processes that impart materials with effective radiative cooling performance. However, scalable all-day subambient cooling for PTM has been limited for wide applications due to the compromise between complex manufacturing and effective broadband radiative cooling. Herein, this work reports spectrally selective fabrics consisting of chitosan (CS) fibers and silica microparticles (SiO2) for all-day PTM using a scabble and easy wet-spinning technique. Along with the porosity of the fiber, woven CS/SiO2 textiles provide high reflectivity (82.3 %) in the solar window (wavelength λ, 0.3–2.5 µm) and high infrared emissivity (95.6 %) within the atmospheric transparency window (wavelength λ, 8–13 µm). Additionally, the textiles exhibit great cloth comfort due to the physical properties of fibers including flexibility, breathability, and strength. The cooling power of wearable textiles during daytime and nighttime is 95.7 W m−2 and 103.3 W m−2. Comparing the outdoor thermal test results of nylon and cotton all day, the temperature difference of 11.2 °C and 5.4 °C proved the passive radiation cooling ability of the material. This effective radiative property arises from the chemical bond vibrations of the CS fabric matrix and phonon polarization resonance of SiO2 microparticles. Our scalable and inexpensive radiative cooler could be an excellent wearable textile for practical all-day PTM.

"Warm in Winter and Cool in Summer": Scalable Biochameleon Inspired Temperature-Adaptive Coating with Easy Preparation and Construction.

The highly reflective solar radiation of passive daytime radiative cooling (PDRC) increases heating energy consumption in the cold winter. Inspired by the temperature-adaptive skin color of chameleon, we efficiently combine temperature-adaptive solar absorption and PDRC technology to achieve "warm in winter and cool in summer". The temperature-adaptive radiative cooling coating (TARCC) with color variability is designed and fabricated, achieving 41% visible light regulation capability. Comprehensive seasonal outdoor tests confirm the reliability of the TARCC: in summer, the TARCC exhibits high solar reflectance (∼93%) and atmospheric transmission window emittance (∼94%), resulting in a 6.5 K subambient temperature. In the winter, the TARCC's dark color strongly absorbs solar radiation, resulting in a 4.3 K temperature rise. Compared with PDRC coatings, the TARCC can save up to 20% of annual energy in midlatitude regions and increase suitable human hours by 55%. With its low cost, easy preparation, and simple construction, the TARCC shows promise for achieving sustainable and comfortable indoor environments.

Passive radiative cooling to sub-ambient temperatures inside naturally ventilated buildings

Passive radiative cooling to sub-ambient temperatures inside naturally ventilated buildings

Thermal hazards assessment of three azo nitrile compounds

Commercially available azo radical compounds provide efficient initiation of many chemical reactions. However, such energetic azo group initiators have thermal stability issues at ambient or even sub-ambient temperatures. Azo decomposition initiated by heat and/or light can generate significant amounts of nitrogen gas with rapid pressure increases, presenting a safety challenge for shipping, storage and usage. In this study, three azo nitrile compounds were examined using various thermal calorimetry techniques with only 5 mg to 1 g scale samples. We obtained calorimetric data on exothermic activity and gas generation. The testing results were subsequently analyzed by using differential iso-conversional methods to obtain key decomposition kinetics parameters, including activation energy and pre-exponential factor. The model was then validated and utilized for prediction of several key safety parameters for scale-up applications, including Self-Accelerating Decomposition Temperature (SADT), Time to Maximum Rate (TMR), temperature profiles at iso- or non-isothermal conditions for shelf-life evaluation, adiabatic temperature rise, and rate of pressure increase. For compounds such as 2,2’-azobis-(2,4-dimethylvaleronitrile) and 1,1’-azobis(cyclohexane-1-carbonitrile) which went through phase transitions associated with thermal decomposition, the thermal decomposition behavior of liquid and solid state was found to differ in characteristic. This methodology for thermal decomposition kinetics assessment enables safe storage, handling, and scale-up process preparation.

Hypothermal opto-thermophoretic tweezers

Optical tweezers have profound importance across fields ranging from manufacturing to biotechnology. However, the requirement of refractive index contrast and high laser power results in potential photon and thermal damage to the trapped objects, such as nanoparticles and biological cells. Optothermal tweezers have been developed to trap particles and biological cells via opto-thermophoresis with much lower laser powers. However, the intense laser heating and stringent requirement of the solution environment prevent their use for general biological applications. Here, we propose hypothermal opto-thermophoretic tweezers (HOTTs) to achieve low-power trapping of diverse colloids and biological cells in their native fluids. HOTTs exploit an environmental cooling strategy to simultaneously enhance the thermophoretic trapping force at sub-ambient temperatures and suppress the thermal damage to target objects. We further apply HOTTs to demonstrate the three-dimensional manipulation of functional plasmonic vesicles for controlled cargo delivery. With their noninvasiveness and versatile capabilities, HOTTs present a promising tool for fundamental studies and practical applications in materials science and biotechnology.

Comprehensive performance analysis of cold storage Rankine Carnot batteries: Energy, exergy, economic, and environmental perspectives

Rankine Carnot batteries have demonstrated promise as a viable solution for electricity storage due to their high energy density at low temperatures. A specific variant of these batteries, known as the Cold Storage Rankine Carnot Battery (CSRCB), utilizes a vapor compression refrigeration (VCR) unit to store cold energy at sub-ambient temperatures. In this paper, three different configurations of CSRCB are constructed: the Basic CSRCB (B-CSRCB), CSRCB with a recuperator in VCR (R-CSRCB), and CSRCB with a recuperator in VCR and a preheater in ORC (RP-CSRCB). The system is analyzed from four perspectives: energy, exergy, economic, and environmental (4E). The impact of key parameters, such as heat source temperature, ambient temperature, and pinch point temperature, on the system's performance is evaluated. The results indicate that the RP-CSCBR configuration outperforms others in terms of energy, exergy, and economic analysis. The evaporator in ORC-subsystem is the component with the largest exergy loss. When the three configurations of CSRCB achieve the maximum exergy efficiency under different conditions, the ORC-subsystem evaporator accounts for 41.9 %, 41.9 % and 17.6 % of the exergy loss of B-CSRCB, R-CSRCB and RP-CSRCB, respectively. The lowest levelized cost of storage (LCOS) can be obtained when the system is operating at high heat source temperature, low ambient temperature and small pinch temperature difference. The environment assessment results show that the changes of the key parameters of the system have different effects on the annual emission reduction (AER) and total equivalent warming impact (TEWI) of CSRCB with different configurations.

Development and degradation analysis of novel three-layered sustainable composite coating for daytime radiative cooling

Structural stability, surface cleaning and wind speed effect, along with degradation behavior against sun exposure, dust deposition and water (rain) are important technical issues of daytime radiative cool coating for real-world application. Hence, we develop a three-layered coating consisting of an adhesive bottom layer (better stability), a middle layer of novel barium fluoride particle-based composite in PMMA polymer and a top protective cleanable PMMA layer for efficient daytime radiative cooling. Cooling performance is evaluated in terms of cooling flux and sub-ambient temperature drop. Performance degradation behavior of developed coating against sun exposure, water immersion and dust immersion is investigated. The effect of wind velocity on the radiative cooler performance is also evaluated experimentally. A temperature drop of nearly 3–5 °C is observed at the peak solar irradiance and a maximum temperature drop of approximately 7.2 °C is observed during afternoon hours. A maximum cooling power of 65 W/m2 is recorded experimentally in the afternoon. The negligible cooling performance degradation has been observed against sun, dust and water exposures, which confirms long-term sustainability. However, the significant effect of wind speed on cooling performance is observed for developed radiative coating (sub-ambient temperature drop is found to be inversely proportional to wind speed).

Design and scalable fabrication of core-shell nanospheres embedded spectrally selective single-layer coatings for durable daytime radiative cooling

Daytime radiative cooling (DRC) can spontaneously cool a surface without consuming energy by reflecting sunlight and emitting thermal radiation to the outer space through atmospheric transmission windows. However, the manufacturing of efficient DRC designs with low cost, high scalability, strong applicability, and along with achieving great weather resistance for practical applications remains a challenge. Here, we report a facile strategy to fabricate spectrally selective single-layer DRC coatings by facilely embedding hydrophobically modified TiO2@SiO2 nanospheres in commercially available fluorocarbon resin matrix for improving daytime cooling. The TiO2 shell thickness, pigments volume fraction, and coating thickness are optimized using finite difference time-domain (FDTD) simulation to maximize the sunlight scattering efficiency with minimal material usage. The coating prepared with a spaying time of 40 s reflects above 93% of solar irradiance and exhibits an infrared emissivity of ∼94% at atmospheric transmittance window wavelength, leading to a desirable daytime sub-ambient temperature drop of ∼10.9 °C. Building energy simulations demonstrates that 32.6% of cooling energy can be saved per year in China when the coating is used as building envelopes. The coatings also show improved scalability, peel strength, self-cleaning, and weather resistance, which makes them attractive candidates for long period outdoor DRC applications. This work paves a new way to design radiative cooling coatings with low cost and ease of application for the development of highly energy-efficient cooling technology.

Two morphology distinct fillers with chemical similarity incorporated into Tröger’ base polymers towards enhanced gas separation

Mixed matrix membranes (MMMs) with one class of fillers always require high loadings to maximize gas permeability. The incompatibility between polymers and high content fillers causes non-selective interfacial voids in the MMMs and poor mechanical properties of MMMs. This work adopts two morphology distinct fillers with chemical similarity to achieve improved gas separation performance of Tröger’ base membranes without sacrificing their mechanical properties. The three-dimensional ZIF-8 nanoparticles are beneficial for membrane gas permeability. The addition of low content ZIF-8 nanosheets can greatly improve H2/CH4 and CO2/CH4 selectivities of the membranes. ZIF-8 particles and ZIF-8 nanosheets with similar chemical properties can mix more homogeneously and disperse well in the polymer matrix with expected hydrogen bonding interaction compared to mixed ZIF-8 and CuBDC nanosheets. Eventually, the resulting dual filler MMMs doped with 4 wt% ZIF-8 nanosheets and 10 wt% ZIF-8 nanoparticles have H2/CH4 ideal selectivity of 60 with H2 permeability of 426 Barrer, which is an increase of 18% and 102% over pure membranes, respectively. Moreover, the membrane has a significantly enhanced CO2/CH4 mixed gas selectivity of 68 at sub-ambient temperature, accompanied by temperature-dependent CO2-induced plasticization behavior. Mixing morphology distinct fillers with chemical similarity is proved to be an effective method to prepare a robust MMM at low filler amounts, and the synergistic effect on gas permeability and selectivity endows these dual filler MMMs with great H2/CH4 and CO2/CH4 separation performance.

Large scalable, ultrathin and self-cleaning cellulose aerogel film for daytime radiative cooling

Passive cooling strategy shows great potential in mitigating global warming and reducing energy consumption. Because of the high emissivity in the atmospheric transparency window (λ ≈ 8–13 µm), cellulose is considered as a good candidate for radiative cooling. However, traditional cellulose coolers generally show poor solar reflection and can be polluted by dust outside, thereby resulting in poor daytime cooling efficiency. To address these drawbacks, we developed sustainable cellulose nanowhiskers (CNWs)/ZnO composite aerogel films with favorable optical performance, mechanical robustness, and self-cleaning function for efficient daytime radiative cooling, which can be achieved via freeze casting and hot-pressing process. Due to formation of multi-level porous structure and chemical bonds (Si-O-C/Si-O-Si), such aerogel film exhibited high solar reflectance (97%) and high infrared emittance (92.5%). It achieved a sub-ambient temperature drop of 6.9 °C under direct sunlight in hot weather. Most importantly, the surface roughness and low surface energy enable cellulose aerogel film hydrophobicity (contact angle = 133°), thereby resulting in an anti-dust function. This work provides insight into the design of sustainable thermal regulating materials to realize carbon neutrality.

Self-cleaning energy-free PDMS@KL film for daytime radiative cooling

Passive daytime radiative cooling (PDRC) technique is desirable for outdoor application as it can cool objects without energy consumption. In this work, a polydimethylsiloxane (PDMS) film incorporated with Kaolin particles was fabricated by a simple and low-cost method. Integrating with high sunlight reflectance and great thermal emissivity, the sample displayed a remarkable PDRC property by yielding an average temperature drop of 12.8℃ compared to the sub-ambient temperature. Additionally, the superhydrophobicity of the sample enabled the surface of the sample free-contamination after water rinse. The superhydrophobic properties can be maintained even after 60 days of UV light continuous exposure.

Influence of surface preparation and polymer backing properties on the quasi-static and impact response of ceramic faced 1D armour systems

Ballistic impact is a highly complex environment, the understanding of which is compounded by the advanced materials used for ceramic-polymer composite armours. In this study, the influence of surface preparation and polymer backing properties on the energy absorptive capabilities and ballistic performance of simplified model armours was approached via a methodical investigation using simplified materials and geometries. Quasi-1D (beam) specimens consisting of polyurethane-backed alumina were loaded under three-point bend quasi-statically and at impact speeds. The specimen geometries allowed for the fracture pathways to be observed. To produce the specimens, two polyurethanes with different glass transition temperatures were cured onto alumina strips, eliminating the requirement for a dedicated adhesive and thus simplifying the interfacial dynamics. Further modifications were made by applying a primer surface treatment, by using unbonded polyurethane and adhered polyurethane, and by replacing the polyurethane with glass-reinforced polycarbonate. The beam specimens were loaded quasi-statically at 0.05 mm s-1 at ambient and sub-ambient temperatures, and at 50–300 m s-1 at ambient temperatures. The quasi-static specimens were found to fail by one of two failure modes with approximately a 3x energy absorption difference; the failure was highly dependant upon the interface and polymer characteristics. Under impact, a significant proportion of the energy absorption was provided by the kinetic energy of the fragments. Although, the polymer backing and interfacial properties were found to influence fracture paths during loading. FEA simulations were used to model the behaviours using characterisation data from a previous study; these were found to predict the experimental response well.