Passive Radiative cooling
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1. Hierarchically Structured Passive Radiative Cooling Ceramic with High Solar Reflection, Science 382(6671), 691-697.
K. Lin, S. Chen, Y. Zeng, T.C. Ho, Y. Zhu, X. Wang, F Liu, B. Huang, C.Y. Chao, Z. Wang, C.Y. Tso, 2023
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https://www.science.org/doi/10.1126/science.adi4725
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Abstract: Passive radiative cooling using nanophotonic structures is limited by its high cost and poor compatibility with existing end uses, whereas polymeric photonic alternatives lack weather resistance and effective solar reflection. We developed a cellular ceramic that can achieve highly efficient light scattering and a near-perfect solar reflectivity of 99.6%. These qualities, coupled with high thermal emissivity, allow the ceramic to provide continuous subambient cooling in an outdoor setting with a cooling power of >130 watts per square meter at noon, demonstrating energy-saving potential on a worldwide scale. The color, weather resistance, mechanical robustness, and ability to depress the Leidenfrost effect are key features ensuring the durable and versatile nature of the cooling ceramic, thereby facilitating its commercialization in various applications, particularly building construction.
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2. Critical Sky Temperatures for Passive Radiative Cooling, Renewable Energy 211 214-226.
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Ross Y.M. Wong , C.Y. Tso , S. Jeong , S. Fu, Christopher Y. Chao , 2023.
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https://www.sciencedirect.com/science/article/pii/S0960148123006080
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Abstract: Passive radiative coolers can preserve the surface temperature below ambient by simultaneously reflecting incoming solar radiation and emitting thermal radiation to the sky. Apart from thermo-optical properties of the materials, radiative cooling performance is affected by various environmental factors which determine the atmospheric transmittance. As such, field investigations lack convergence and completion. And energy balance consideration, which aids in interpreting the field investigative results, is a deterministic cogitation on convective and radiative heat transfer by the radiative cooler that ignores the uncertainties abundant in field study. In this work, we examine the cooling performance of radiative cooling materials under different subtropical weather conditions in Hong Kong and approach the problem based on probabilistic regression modelling as an alternative. At nighttime, the response variable of surface temperature reduction can be correlated with a single predictor variable of sky temperature difference, which is a lumped parameter of ambient temperature, relative humidity, and cloud fraction. At daytime, it should be parametrized with an additional variable regarding solar intensity. The regression analysis reveals that, the higher the thermal emissivity, the larger is the temperature reduction at nighttime, especially obvious for large sky temperature difference. And heavy solar heat load is absorbed by the coolers at daytime even they feature reasonably high solar reflectivity. In this regard, further increment in solar reflectivity poses the top priority in improving daytime radiative cooling performance.
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3. Study of a Passive Radiative Cooling Coating on Chemical Storage Tanks for Evaporative Loss Control, Renewable Energy 211 326-335.
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S. Chen, K. Lin, A. Pan, T. C. Ho, Y. Zhu, C. Y. Tso, 2023.
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https://www.sciencedirect.com/science/article/pii/S0960148123006079
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Abstract: All-day passive radiative cooling is a promising cooling technique achieves cooling without any energy input, thereby attracting tremendous attention. Most of the studies focus on building applications, but there remains a lack of information on other potential applications of radiative cooling systems. Herein, we report an application of a polymeric passive radiative cooling coating (PRCC) on the exterior surface of chemical storage tanks, which is often overlooked but exhibits great potential to efficiently control the evaporative loss of the chemicals. The PRCC exhibits strong solar reflectivity (96.2%) and emissivity (95%) in the mid-infrared wavelength range. According to the results of an outdoor test on a small-scale chemical storage container, the PRCC achieves 81.4%, 54.1%, and 19.8% of chemicals saved compared with a raw container, container painted with commercial white paint (CWP), and the shaded raw container, respectively. A numerical simulation is also conducted to estimate the chemical standing loss from an industrial large-scale chemical storage tank. It is found that the PRCC can achieve a saving of over 50% of the chemical compared with commercial white paint for a chemical storage tank, showing a promising potential to be applied on chemical storage systems.
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4. Adsorption-based Atmospheric Water Harvesting by Passive Radiative Condensers for Continuous Decentralized Water Production, Applied Thermal Engineering, 225 120163.
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S. Bai, Y. Tian, A. Pan, Y. Zeng, S. Chen, T.C. Ho, J. Shang, C.Y. Tso*, 2023.
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https://doi.org/10.1016/j.applthermaleng.2023.120163
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Abstract: Water scarcity is serious nowadays, due to the limited fresh water resources and growing world population. Transport of fresh water to remote areas is also challenging. It leads to the necessity of decentralized water production for mitigating water stressed conditions, which is especially beneficial to people having difficulty accessing fresh water in remote areas. Atmospheric water harvesting is a promising solution as atmospheric water is a huge renewable reservoir that can satisfy the needs of human beings. Adsorption-based atmospheric water harvesting (ABAWH) and passive radiative condensers are two possible methods already investigated, but it is of interest to apply passive radiative condensers to ABAWH systems, considering their merits and limitations. Herein, for the first time, we designed and demonstrated a novel dual single-bed ABAWH system with passive radiative condensers using MIL-101-Cr (HF) as the adsorbent and P(VdF-HFP) as the radiative condenser to capture and deliver water from air for decentralized fresh water production. Based on several outdoor experiments, the estimated water harvesting productivity is ~3.2 mL/g MIL-101-Cr (HF)/day or ~678 mL/m2 P(VdF-HFP) radiative cooler/day with the ratio of adsorbent weight to cooler area at ~214 g/m2 during clear days in Hong Kong, with no degradation in performance of the prototype after several outdoor experiments over 40 days. Water harvesting performance under different climatic conditions, i.e., mid-latitude winter, mid-latitude summer and tropical, was also mathematically predicted. This study provides a new path toward quasi-continuous daytime and nighttime decentralized water production to mitigate the current water stress.
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5. Potential Passive Cooling Methods based on Radiation Controls in Buildings, Energy Conversion and Management 272 116342.
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Y.H. Chan, Y. Zhang, T. Tennakoon, S.C. Fu, K.C. Chan, C.Y. Tso, K.M. Yu, M.P. Wan, B.L. Huang, S. Yao, H.H. Qiu, C.Y. Chao, 2022
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https://doi.org/10.1016/j.enconman.2022.116342
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Abstract: Buildings inevitably absorb solar (thermal) radiation through the envelope, i.e., window, roof and wall, whose characteristics, in terms of material, thickness, area etc., affects the performance of space cooling, thereby inducing energy wastage by air-conditioning system to maintain satisfied indoor thermal comfort. Metropolises with congested-built architectures and humid climates consume tremendous energy in space cooling, contributing to massive carbon emissions. Space cooling has been the most soaring electricity end-user and its increasing carbon emission amplifies global warming, which in turn increases occupants’ dependence on artificial cooling, especially in hot climates. The urgency of the climate crisis has put passive cooling technologies, which can efficiently manage heat transfer of buildings, to the forefront of research. Thermochromic smart windows, daytime radiative coolers and reflective paints are three prominent technologies that have drawn industries’ attention. These technologies respond to incident sunlight and thermal radiation differently, compared to conventional building envelope, and can passively mitigate solar radiation absorbed at the building envelope, reducing electricity consumption for air-conditioning. This review article comprehensively discusses the characteristics of the three passive energy-efficient techniques that can be integrated with building façade (window, roof and wall), and their feasibility in building cooling applications through thermal analyses. Through this review, the ideal spectral properties of these technologies will be distinguished. Although the properties of these technologies have been abundantly explored, the research on their long-term cooling performance under the influence of weatherability and aging remains scarce. Further, their cost-effectiveness is essential for realizing commercialization and reducing hesitancy in adopting these novel passive cooling technologies for taking a significant leap towards carbon neutrality.
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6. Nanoparticle-polymer Hybrid Dual-layer Coating with Broadband Solar Reflection for High-performance Daytime Passive Radiative Cooling, Energy and Buildings, 276 112507
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K. Lin, Y. Du, S. Chen, L.C. Chao, H.H. Lee, T.C. Ho, Y. Zhu, Y. Zeng, A. Pan, C.Y. Tso*
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https://doi.org/10.1016/j.enbuild.2022.112507
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Abstract: The passive cooling technology is refrigerant-free and energy-free, making it an appealing alternative to reduce the energy consumption of traditional cooling systems. Although effective in optical design, the materials involving optical structures and metal reflectors are rarely cost-effective or readily applicable. Radiative cooling materials with promising optical properties and good practical applicability remain an urgent research need. We present a simple and scalable method to fabricate a dual-layer nanoparticlepolymer hybrid coating which achieves a high diffused solar reflection of 92.2 % without using metal reflectors, and a strong mid-infrared emission of 95.3 % within the atmospheric window. The coating allows continuous sub-ambient cooling in both daytime and nighttime. Promisingly, the coating reaches about 4 _C lower temperature than ambient under intensive solar irradiation, along with a 78.9 Wm_2 cooling power generation. Moreover, the cooling coating exhibits great reliability and cooling effect when applied as an exterior coating on model houses and the roof of a real building. Applying the proposed material as a roofing material is capable to save cooling energy across various climate zones, especially in the hot climate, which confirms the material’s possibility for carbon reduction and tackling global climate change.
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7. Maxwell-Garnett Permittivity Optimized Micro-porous PVDF/PMMA Blend for Near Unity Thermal Emission through the Atmospheric Window, Solar Energy Materials and Solar Cells 248 112003.
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Y.M. Wong, C.Y. Tso, S.C Fu, C.Y. Chao, 2022.
https://doi.org/10.1016/j.solmat.2022.112003
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Abstract: Owing to excellent solar reflectivity and sky window emissivity, disordered heterogenous materials, including filler-abundant matrices, paints, and coatings, as well as foam-like, fiber-stacked and composite porous structures, form a major class for efficient passive radiative cooling. Contrary to well-established empirical understanding, this work offers a generalized analytical overview of their macroscopic thermo-optical properties from the microscopic electromagnetic perspective of Maxwell-Garnett effective medium theory. With the family of micro-porous poly(vinylidene-fluoride)/poly(methyl-methacrylate) blends as a representative example, procedures for tailoring mid-infrared spectral emissivity via effective permittivity are outlined. Theoretical framework and design scheme are validated by finite difference time domain simulation and Fourier transform infrared spectrometry. It is shown that poly(vinylidene-fluoride) and poly(methyl-methacrylate) form a pair of complementary constitutive materials for near unity thermal emission through the atmospheric window. Optimized binary polymeric blend, prepared by spray-coating method, features a window emissivity of 98% and realizes nocturnal radiative cooling with a temperature reduction of 6.8 â—¦C and a cooling power of 94 W/m2 in an outdoor field investigation. It can serve as a promising bifunctional material for simultaneous radiative heat dissipation and capacitive energy storage, which meets the demand for nocturnal, radiative cooling aided thermoelectricity generation and storage potential.
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8. Radiative Cooling Nano-Fabric for Personal Thermal Management, ACS Applied Materials & Interfaces 14 23577-23587.
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M.I. Iqbal, K.X. Lin, F.X. Sun, S. Chen, A.Q. Pan, H.H Lee, C.W. Kan, Carol S.K. Lin, C.Y. Tso*, 2022.
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https://doi.org/10.1021/acsami.2c05115
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Abstract: A wearable textile that is engineered to reflect incoming sunlight and allow the transmission of mid-infrared radiation simultaneously would have a great impact on the human body’s thermal regulation in an outdoor environment. However, developing such a textile is a tough challenge. Using nanoparticledoped polymer (zinc oxide and polyethylene) materials and electrospinning technology, we have developed a nanofabric with the desired optical properties and good applicability. The nanofabric offers a cool fibrous structure with outstanding solar reflectivity (91%) and mid-infrared transmissivity (81%). In an outdoor field test under exposure of direct sunlight, the nanofabric was demonstrated to reduce the simulated skin temperature by 9°C when compared to skin covered by a cotton textile. A heattransfer model is also established to numerically assess the cooling performance of the nanofabric as a function of various climate factors, including solar intensity, ambient air temperature, atmospheric emission, wind speed, and parasitic heat loss rate. The results indicate that the nanofabric can completely release the human body from unwanted heat stress in most conditions, providing an additional cooling effect as well as demonstrating worldwide feasibility. Even in some extreme conditions, the nanofabric can also reduce the human body’s cooling demand compared with traditional cotton textile, proving this material as a feasible solution for better thermoregulation of the human body. The facile fabrication of such textiles paves the way for the mass adoption of energy-free personal cooling technology in daily life, which meets the growing demand for healthcare, climate change, and sustainability.
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9. A Solution-processed Inorganic Emitter with High Spectral Selectivity for Efficient Sub-ambient Radiative Cooling in Hot Humid Climates, Advanced Materials, 2109350.
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C. Lin, Y. Li, C. Chi, Y. S. Kwon, J. Huang, Z. Wu, J. Zheng, G. Liu, C. Y. Tso, Christopher Y. H. Chao and B. Huang, 2022.
https://doi.org/10.1002/adma.202109350.
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Abstract: Daytime radiative cooling provides an eco-friendly solution to space cooling with zero energy consumption. Despite significant advances, most state-ofthe-art radiative coolers show broadband infrared emission with low spectral selectivity, which limits their cooling temperatures, especially in hot humid regions. Here, an all-inorganic narrowband emitter comprising a solutionderived SiOxNy layer sandwiched between a reflective substrate and a selfassembly monolayer of SiO2 microspheres is reported. It shows a high and diffusive solar reflectance (96.4%) and strong infrared-selective emittance (94.6%) with superior spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer are achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all-inorganic hydrophobic structure, the emitter shows outstanding resistance to ultraviolet and water in long-term durability tests. The scalable-solution-based fabrication renders this stable high-performance emitter promising for large-scale deployment in various climates.
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10. Potential Building Energy Savings by Passive Strategies combining Daytime Radiative Coolers and Thermochromic Smart Windows, Case Studies in Thermal Engineering, 28 101517.
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K. Lin, L. Chao, H. H. Lee, R. Xin, S. Liu, T. C. Ho, B. Huang, K. M. Yu, C. Y. Tso and C.Y. Tso, 2021.
https://doi.org/10.1016/j.csite.2021.101517
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Abstract: Reducing the energy consumed by space cooling and providing outstanding thermally insulated windows are essential requirements for a smart green building. In this study, a new building design concept for envelope/façade is proposed and demonstrated, providing a solution for thermal management in buildings. This new design for the envelope/façade of buildings comprises two major passive and energy-free technologies, a daytime radiative cooler and a thermochromic smart window. The PDMS-silica-silver daytime passive radiative cooler can provide a cooling effect to the indoor environment without energy input, meanwhile the thermochromic smart window using PNIPAm hydrogel can passively modulate the solar irradiance entering buildings through windows. Hence, the energy needed for indoor heating, cooling and lighting can be reduced significantly. To evaluate the energy-saving performance, model houses are built, one of which is assembled using the proposed building design. The orientation effect on the indoor air temperature of the model houses is investigated. Under intensive solar irradiance, a maximum reduction of 4.8 â—¦C of the indoor air temperature is achieved in the model house constructed using the proposed building design. An energy saving of about 17% of air-conditioning systems in buildings constructed with this new design is expected during daytime operation.
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11. Thermo-radiative Energy Conversion Efficiency of a Passive Radiative Fluid Cooling System, Renewable Energy 180 700-711.
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Y.M. Wong, C.Y. Tso and C.Y. Chao, 2021.
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https://doi.org/10.1016/j.renene.2021.08.109
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Abstract: In the passive radiative cooling process, a sky-facing surface emitting thermal radiation through the bandwidth coincident with the atmospheric window highly transparent to the radiation within 8e13 mm can preserve the temperature below ambient spontaneously. The cold surface can act as a fundamental building block for energy conversion, in which thermo-radiative energy conversion can be the simplest form and realized by a functionalized fluid-wall heat transfer interface. Energy conversion efficiency denotes the ratio of enthalpy converted by the working fluid to the cooling effect harvestable from the sky. In parallel with fluid cooling capacity, they are discussed by thermal and energy responses of a cooling system subjected to a perturbation in fluid flow, and demonstrated by measurement on a wafersized system acted by an equivalent heat current. According to interfacial heat transfer characteristics, cooling performance can be classified into inhibition, transition and saturation regimes, where the saturated performance is the most outstanding. However, fluid cooling and energy conversion capacities are always inversely correlated, where the reduction in fluid temperature decreases with increasing flow rate, but efficiency increases with increasing flow rate. Experimental results, in line with the theoretical prediction, show that 12.4 mL/s of water can be chilled by _4.1 _C at an overall efficiency of 14%, but 116 mL/s of water can be weakly chilled by _1.5 _C at an elevated efficiency of 49%. The dilemma in energy efficient collection of cooling fluid is an innate physical mechanism restricted by Newton's law of cooling and the 1st law of thermodynamics.
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12. Energy Consumption Modelling of a Passive Hybrid System for Office Buildings in Different Climates, Energy, 239 121914.
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Y. Zhang, T. Tennakoon, Y.H. Chan, K.C. Chan, S.C. Fu, C.Y. Tso, K.M. Yu, B. L. Huang, S.H. Yao, H.H. Qiu, C.Y. Chao, 2022.
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https://doi.org/10.1016/j.energy.2021.121914
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Abstract: Thermochromic smart windows and radiative coolers are two passive cooling technologies, whose adoption as windows and roofs, respectively, is feasible for building energy-saving. However, to the authors' knowledge, the investigation of annual energy performance incorporating both techniques is scarce at the time of writing. Therefore, a passive hybrid system involving both technologies is proposed in this study. A perovskite thermochromic smart window and three different radiative coolers were chosen based on their superior performance. The energy performance of the passive hybrid system in a prototypical medium-sized office building was simulated using EnergyPlus and the results were rigorously
analyzed. Both thermochromic smart window and radiative cooler could reduce total energy consumptions by up to 10.6% and 23.0%, respectively, regardless of building's year of completion, while the synergic system saved up to 32.0%. Among the chosen cities of various climates, thermochromic smart windows and radiative coolers perform better in cities where cooling demand dominates. The west- and east-facing thermochromic smart windows could mitigate more energy usage in contrast to the other orientations. If this passive hybrid system can be offered at a reasonable cost, the technology is likely to be a viable energy-efficient option for buildings.
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13. A Flexible and Scalable Solution for Daytime Passive Radiative Cooling Using Polymer Sheets, Energy and Buildings 252 111400.
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K. Lin, L. Chao, T.C. Ho, C. Lin, S. Chen, Y. Du, B. Huang and C.Y. Tso, 2021
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https://doi.org/10.1016/j.enbuild.2021.111400
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Abstract: Daytime passive radiative cooling shows great potential in thermal management by highly reflecting the solar irradiation and intensively releasing mid-infrared thermal emission to the cold universe (~3 K). Although the desired optical property for radiative cooling can be achieved by optical-scale structures, a solution for mass production and cost reduction is still being sought. A flexible and scalable daytime passive radiative cooler in the form of a sheet that consists of a reflector layer and a polydimethylsiloxane (PDMS) layer is demonstrated to achieve promising sub-ambient cooling in Hong Kong’s hot and humid climate. The proposed cooling sheet exhibits an average solar reflection of 92.1% and mid-infrared emissivity
of 94.5%. A 48-hour continuous sub-ambient cooling, with an average ambient temperature drop of 2.4 _C, is experimentally demonstrated without any windshields or sun shields. Under a clear sky condition, cooling powers of 52.4 W/m2 and 84.7 W/m2 are measured at noontime and night, respectively. The impact of the sky view factor on cooling performance has also been numerically investigated for the first time. In the scenario of obstruction by a wall, a cooling power degradation of 29 W/m2 is assessed during the daytime when the wall temperature is 15 K above ambient air with a view factor of 0.2. To assess the feasibility for cooling as building exterior materials, a field test using a scaled-down model house has been conducted. The cooler-applied model house produced a better indoor thermal environment with lower indoor air temperature and inner wall temperature compared with one coated with commercial white paint. Last but not least, to investigate the heat exchange of the cooler with ventilated air, a passive radiative air-cooling system has been built and tested, and sub-ambient cooling with 1 _C air temperature reduction was achieved through a 24-hour operation. This work leads to a new solution for daytime passive radiative cooling, which can potentially be applied in various scenarios, such as buildings and vehicles, with its great flexibility and scalability.
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14. An Energy-free and Low-cost Daytime Passive Radiative Cooling Paint for Energy-Saving in Buildings and Decarbonization, HKIE Environmental Division, (Environmental Paper Award, Champion).
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Y. Du, S. Chen, L. Chao, K. Lin, H.H. Lee, T.C. Ho, M.K.H. Leung, C.Y. Tso
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Abstract: Air-conditioning consumes over 30% of the energy used in buildings in Hong Kong, driving peak electricity demand. Compressors of traditional air-conditioning systems consume enormous amounts of energy and the usage of refrigerants is a major cause of ozone layer depletion, contributing to global warming. Therefore, an energy-saving and environmentally friendly cooling technology is urgently needed to reduce the energy for space cooling, and in the long term, mitigate climate change. In this study, an energy-free and low-cost polymer-based daytime passive radiative cooling paint (DPRCP) with a promising cooling performance and wide applicability has been developed, that effectively reflects the sunlight and dissipates the heat to the cold universe (~ -270 oC). The DPRCP, consisting of a unique porous structure with 48% porosity, has been fabricated using a novel wet phase separation method. Excellent optical performance with 96% solar reflectivity (0.25-2.5 μm) and 95% mid-infrared (8-13 μm) thermal emissivity are achieved. A small-scale prototype field test revealed that the DPRCP can reduce the ambient air temperature by about 4 oC and 6 oC during the daytime and at night, respectively, corresponding to a cooling power of about 81 W/m2 and 84 W/m2. A scale-up demonstration was also conducted on a rooftop of a real building in Tung Chung, showing a remarkable reduction in the roof surface temperature of about 15 oC under direct sunlight has been achieved compared to the roof without the DPRCP coating. Last, an EnergyPlus simulation further proved that the DPRCP could save around 8% of cooling energy when applied on the one-story building roof. These results strongly demonstrate the potential and capability to perform as an effective and promising passive cooling strategy for buildings and broader applications in various fields.
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15. Corrected Radiative Cooling Power measured by Equivalent Dissipative Thermal Reservoir Method, International Journal of Heat and Mass Transfer 174 121341.
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Y.M. Wong, C.Y. Tso and C.Y. Chao, 2021.
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https://doi.org/10.1016/j.ijheatmasstransfer.2021.121341
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Abstract: A radiative cooler is an optical device which can spontaneously preserve its temperature below ambient by simultaneously reflecting solar radiation and emitting thermal radiation through the atmospheric win- dow lying within 8–13 μm of the electromagnetic spectrum. In the radiative cooling process, the universe acts as the ultimate heat sink for heat dissipation. Cooling power is a key indicator of the radiative cool- ing performance, which can reach a typical value of 100 W/m 2 under a clear sky. However, atmospheric transparency, which is usually hard to determine, affects the cooling power heavily. To fairly evaluate the cooling power under prescribed and controlled environmental conditions, we built an in-lab testing facility based on a hybrid refrigerative thermoelectric cooling system, which can artificially simulate the equivalent radiative cooling effect. We characterized the system performance and preserved the tempera- tures of the equivalent thermal reservoir at -19.9 °C and -12.7 °C steadily at ambient temperatures of 25 °C and 45 °C respectively. Under a reference atmospheric transmission spectrum featuring a high 8–13 μm transparency, we measure the cooling powers of a representative radiative cooler and compare them with the theoretical values for ambient temperatures between 25 °C and 45 °C. Measured cooling powers range from 108 W/m 2 to 141 W/m 2 , which exceed the theoretical values by 17 - 33% due to excessive heat flow through the testing cavity and heat lost to the surroundings. The method can be extended to examine thermal and energy associated performances of derivative radiative cooling devices and systems.
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16. Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100-727 ºC, Advanced Materials, 33(1) 2005074.
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Y. Li, C. Lin, Z. Wu, Z. Chen, C. Chi, F. Cao, D. Mei, H. Yan, C.Y. Tso, C. Chao and B. Huang, 2020.
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https://doi.org/10.1002/adma.202005074
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Abstract: Low-cost and large-area solar–thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large-scale solar–thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concentrating solar power. Few state-of-the-art selective absorbers are qualified for both low- (<200 °C) and high-temperature (>600 °C) applications due to insufficient spectral selectivity or thermal stability over a wide temperature range. Here, a high-performance plasmonic metamaterial selective absorber is developed by facile solutionbased processes via assembling an ultrathin (≈120 nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in-plane plasmon and out-of-plane Fabry–Perot resonances, the all-ceramic plasmonic metamaterial simultaneously achieves high, full-spectrum solar absorption (95%), low mid-IR emission (3% at 100 °C), and excellent stability over a temperature range of 100–727 °C, even outperforming most vacuum-deposited absorbers at their specific operating temperatures. The competitive performance of the solution-processed absorber is accompanied by a significant cost reduction compared with vacuum-deposited absorbers. All these merits render
it a cost-effective, universal solution to offering high efficiency (89–93%) for both low- and high-temperature solar–thermal applications.
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17. Daytime Passive Radiative Cooling by Ultra Emissive Bio-inspired Polymeric Surface. Solar Energy Materials and Solar Cells, 206 110296.
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S. Jeong, C.Y. Tso, Y. Wong, C.Y. Chao, and B. Huang, 2020.
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https://doi.org/10.1016/j.solmat.2019.110296
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Abstract: Saharan silver ants can maintain their body temperature below ambient air due to unique triangular shaped hairs that enhance solar reflection and thermal emission through a transparent window that lies in the atmosphere. Applying this thermoregulatory prismatic structure to polydimethylsiloxane (PDMS), highly emsissive in the 8–13 μm spectrum, we present a geometrically modified polymer-based daytime passive radiative cooler. The selective thermal emitter was fabricated based on the optimized prismatic structure from Finite Difference Time Domain (FDTD) simulations. The average emissivity within the 8–13 μm spectrum was enhanced to 0.98 by the gradient refractive index effect, while the average solar reflectivity in the visible and near-infrared spectrum was measured to be 0.95. The net radiative cooling power is estimated to reach 144 W/m2, exceeding records of previously reported radiative coolers. Last, in Hong Kong’s hot and humid climate, a field test successfully demonstrated cooling by 6.2 �C below the temperature of ambient air corresponding to a net cooling power of 19.7 W/m2 in a non-vacuum setup during the peak daytime with shading. This is the largest temperature reduction observed in a tropical region for daytime passive radiative cooling. Our work presents an alternative method to enhance passive thermal emission and may facilitate its world wide application in eco-friendly space cooling
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18. Field Investigation of a Photonic Multi-layered TiO2 Passive Radiative Cooler in Sub-Tropical Climate. Renewable Energy, 146 44-55
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S.Y. Jeong, C.Y. Tso J. Ha, Y.M. Wong, B. Huang, C.Y. Chao and H. Qiu, 2020.
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https://doi.org/10.1016/j.renene.2019.06.119
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Abstract: Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiativecooling due to its attractive cooling strategy that does not require any energy input. Its potential application varies widely from air conditioning systems for buildings, photovoltaic cells, electronic device cooling and automobiles. However, recently proposed daytime passive radiative coolers are based on photonic structures which are high in cost. A relatively cheap metal oxide material, TiO2, which lowers the cost but is highly emissive in the mid-infrared range has been used, also improving the cooling performance of the photonic daytime passive radiative cooler. An optimized TiO2eSiO2 alternating multilayered
photonic daytime radiative cooler with average emissivity of 0.84 within 8e13 mm while reflecting 94% of incident solar energy is developed. Its net cooling power is estimated to be 136.3 W/m2 at ambient air temperature of 27 _C which shows an improvement of 90 W/m2 compared to that of the HfO2-SiO2 photonic radiative cooler. Last, a field test has been conducted in Hong Kong's subtropical climate (i.e. relative humidity ? 60e70%) to investigate its feasibility, and with the help of solar shading, successfully demonstrated temperature reduction of 7.2 _C with a net cooling power of 14.3 W/m2 under direct sunlight.
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19. Scalable All-ceramic Nanofilms as Highly Efficient and Thermally Stable Selective Solar Absorbers. Nano Energy 64 103947.
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Y. Li, D. Zhou, C. Lin, D. Li, C. Chi, H. Huang, S. Yang, C.Y. Tso,C.Y. Chao and B. Huang, 2019.
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https://doi.org/10.1016/j.nanoen.2019.103947
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Abstract: The pressing demands for next-generation concentrating solar power drive the pursuit of high-efficiency, thermally stable, and scalable spectrally selective absorbers. Multilayer metal/ceramic nanofilms are promising candidates owing to their strong sunlight absorption provided by extremely simple configurations and facile fabrication. However, the commercial success of such absorbers is still hindered by their unsatisfactory spectral selectivity and high-temperature stability associated with metal/ceramic interfaces. Here we first propose an allceramic TiN/TiNO/ZrO2/SiO2 absorber with highly selective absorption, i.e., a high solar absorptance (92.2%) yet an ultralow thermal emittance (17.0% at 1000 K), producing an unprecedented solar-thermal conversion efficiency (82.6% under 100 suns). Remarkably, the absorber shows great thermal stability even after long-term (227 annealing at 1000 K, boosting the operating temperature of conventional multilayer absorbers by at least 227 K. The efficient and stable all-ceramic absorber can be readily produced in quantity via low-cost processes, rendering it attractive for high-temperature solar-thermal technologies.
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20.A Numerical Study of Daytime Passive Radiative Coolers for Space Cooling in Buildings. Building Simulation, 11(5) 1011-1028.
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S.Y. Jeong, C.Y. Tso, M. Zouagui, Y.M. Wong and C.Y. Chao, 2018.
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https://doi.org/10.1007/s12273-018-0474-4
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Abstract: A passive daytime radiative cooler is made of a sky facing surface which can preserve the indoor air temperature below ambient without energy consumption by simultaneously reflecting solar radiation and emitting thermal radiation to the universe through the atmospheric window located between 8–13 μm of the electromagnetic spectrum. After the first demonstration of radiative cooling under direct sunlight, a solar mirror coated with a mid-infrared (MIR) emissive thin film has become the standard device architecture. This study firstly reviews recent developments in daytime passive radiative cooling, followed by describing the development of an energy balance mathematical model to study the potential application of passive radiative coolers in HVAC systems of buildings. Some micro-channels are fabricated on the back side of the passive radiative cooler, allowing fluid to flow in an isolated loop such that the coolant can be chilled and transported to the demand side for spacing cooling. This leads to the partial replacement of conventional vapor compression refrigeration by the radiative cooling panel. Considering the steady state energy balance within the radiative cooling panel integrated HVAC systems, the cooling performance and indoor air temperature are evaluated by numerical analysis. A 100 m2 passive radiative cooling panel could chill water for the cooling of air, reducing indoor air temperature by 10 °C, equivalent to a net cooling power of 1600 W. This study suggests that the proposed passive radiative cooling system should be used to pre-cool the ambient hot air such that the overall energy consumption of a traditional air-conditioning system can be reduced. The findings promise the application of passive daytime radiative cooling in building HVAC systems.
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21. Ultra-broadband Asymmetric Transmission Metallic Gratings for Subtropical Passive Daytime Radiative Cooling. Solar Energy Materials and Solar Cells, 186 330-339.
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Y.M. Wong, C.Y. Tso,C.Y. Chao, B. Huang and M.P. Wan, 2018.
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https://doi.org/10.1016/j.solmat.2018.07.002
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Abstract: By simultaneously reflecting solar irradiation and emitting thermal radiation to the cold universe through an atmospheric window lying within 8–13 μm of the electromagnetic spectrum, surfaces could be cooled below ambient temperature under direct sunlight. However, both humidity and cloud coverage can raise the sky emissivity, significantly intensifying thermal emission within the 8–13 μm spectrum. A radiative cooler is strongly absorptive of the additional heat load, and eventually, the cooling capacity drops in a humid environment.
In this work, we suggest integrating the cooler with an asymmetric electromagnetic transmission (AEMT) window, which permits outgoing radiative transmission, but reflects incoming radiation of the same wavelengths, so as to recover the cooling performance under a humid climate. This study aims at discussing the working principle of an AEMT enhanced radiative cooler quantitatively as well as demonstrating a feasible design of an AEMT device for radiative cooling applications. First, a theoretical model on the basis of conservation
of energy is developed for the prediction of cooling performance of the AEMT enhanced radiative cooling systems. Cooling power is solved for a humid semi-transparent sky condition numerically, which shows that an AEMT window with forward and backward transmittances of 0.8 and 0.4 respectively could restore the cooling power of the passive radiative cooler by 57%. Second, validated finite difference time domain (FDTD) simulations reveal that an AEMT window implemented by near-wavelength tapered metallic gratings could meet the desired transmission ratio (i.e. contrast ratio of 2 within 8–13 μm spectrum) for radiative cooling.
22. A novel radiative sky cooler system with enhanced daytime cooling performance to reduce building roof heat gains in subtropical climate. Renewable Energy, 119686.
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Y. Zhang, C.Y. Tso, C. F. N. Tse, A. M. F. Fong, K. Lin, & Y. Sun (2023).
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https://doi.org/10.1016/j.renene.2023.119686
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Abstract: Radiative sky coolers (RSCs) can reduce building roof heat gains by radiating heat to outer space. However, their performance during daytime is limited, with substantial roof heat gains still occurring due to high ambient temperatures. Additionally, much of the cooling produced at night is wasted since air conditioners in non-residential buildings are often not operating. To address these limitations, we propose a novel thermal storage-heat pipe-integrated radiative sky cooler system (TS-HP-RSC). It utilizes water thermal storage to capture nighttime sky cooling for use during the day. A gravity-assisted heat pipe unidirectionally transports this stored cooling to the indoor space, preventing losses to the environment. An experimental platform is established integrating the proposed system, a baseline case, and measurement instrumentation. Compared to the baseline, the TS-HP-RSC system not only eliminated daytime cumulative heat gains (0.55–1.27 kJ) but also provided supplemental cooling (1.57–2.75 kJ). This yielded substantial daytime heat gain reductions of 223.62 %–600 % versus the baseline. Similar reductions occurred in peak heat gains. By enhancing daytime cooling, the TS-HP-RSC system can substantially curb roof heat gains in subtropical climates, enabling significant energy savings
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23. Investigations on Asymmetric Transmittivity of Optical Devices and Different Diode-like Behaviors. iScience.
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A. Pan, K. Lin, S. Chen & C. Y. Tso. (2023).
https://doi.org/10.1016/j.isci.2023.107032
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Summary: This study theoretically proved that although reciprocal optical devices can show asymmetric transmittivity (AT) under controlled incident modes (i.e., conditional AT), they cannot guarantee AT with arbitrary incident light modes, whereas only nonreciprocal optical devices can possibly guarantee AT. Besides, the thermodynamics of both reciprocal and nonreciprocal optical devices were discussed to show that the second law of thermodynamics is valid anyway. Furthermore, the diode-like behaviors of optical and electronic devices were compared. Electrons are identical to electronic devices, so electronic devices could have asymmetric conductance regardless of electrons. In contrast, electromagnetic waves are different from optical devices as transmittivity of different modes can be different, so reciprocal optical devices showing conditional AT cannot guarantee AT when incident modes are arbitrary. The mathematical proof and characteristic comparisons between electronic and optical diodes, which are firstly presented here, should help clarifying the necessary nonreciprocity required for being optical diodes.
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24. A field investigation of passive radiative cooling under Hong Kong’s climate. Renewable Energy, 106, 52-61.
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C.Y. Tso, K.C. Chan, Christopher Y.H. Chao.(2017)
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https://doi.org/10.1016/j.renene.2017.01.018
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Abstract: This paper discusses the feasibility of cooling using radiation under Hong Kong’s hot and humid climate. Three different designs of a passive radiative cooler were studied in this work. The three designs include non-vacuum, and vacuum with seven potassium chloride (KCl) IR-Pass windows as well as one system with a single KCl IR-Pass window. The coolers were examined during daytime and night time operation as well as under different sky conditions, such as clear, cloudy and partly cloudy. Investigation was mainly based on the temperature difference between the radiative cooler and ambient air. The experimental results showed that the passive radiative cooler with seven KCl windows and the cooler design without vacuum provided a satisfactory cooling effect at night (i.e. the ambient air temperature was reduced by about 6–7 °C), but the coolers could not produce a cooling effect during daytime under any of Hong Kong’s weather conditions. The same results were obtained for the passive radiative cooler with the single KCl window during daytime operation. However, the cooling capacity of the passive radiative cooler design without vacuum under a clear night sky achieved 38 W/m2