Daytime Radiative Coolers Research Articles

Dynamic impact of climate on the performance of daytime radiative cooling materials

By strongly reflecting solar radiation and being highly emissive within the atmospheric window, daytime radiative coolers can achieve sub-ambient temperature under direct sunlight. Radiative cooling performance is strongly coupled to specific climatic conditions since cooling efficiency is strongly affected by ambient air temperature, wind speed, and solar and ambient radiation intensity. In this paper, using a well-validated thermal model, the cooling performance of three radiative cooling materials with varying optical properties was evaluated under three distinct and representative climates. This analysis permits us to better understand the sensitivity of daytime radiative cooling materials to different climatic conditions, present strategies for selecting the ideal spectral properties of materials and investigate how to enhance cooling performance under adverse climatic conditions. It is shown that radiative cooling materials have better performance in hot and arid climates. Most radiative cooling materials exhibit the greatest response to changes in ambient radiation. Higher ambient air temperatures correspond to larger sub-ambient temperature of the surfaces, but this change is lower than that of the corresponding air temperature. Furthermore, by coupling a special optical grating window onto the surface of a radiative cooler, cooling performance can be significantly enhanced by asymmetrically reflecting incoming radiation but permitting outgoing emission. While an ideal material that only emits in the atmospheric window wavelengths presents the best performance under a large range of solar radiation, ambient radiation, and air temperature, the broadband ideal emitter exhibits higher cooling potential when coupled with the optical grating window.

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.

Ultra-broadband all-dielectric metamaterial thermal emitter for passive radiative cooling.

Passive radiative cooling, which pumps heat to outer space via thermal radiation, has been a promising energy free technology to maintain the earth surface temperature. Nighttime radiative cooling technology is quite mature, while daytime radiative cooling still poses many challenges due to the requirement of minimization of incident solar absorption and maximization of the mid-infrared emissivity in the atmospheric transparency windows. However, the mid-infrared emissivity efficiency of natural materials is usually poor, providing a low cooling efficiency and the realization of a high performance daytime radiative cooler is still quite challenge. In this work, we design and numerically investigate a three dimensional (3D) all-dielectric pyramidal multilayer metamaterial (PMM), which not only avoids the problem of high absorptivity loss of metal materials to solar, but also provide extremely high infrared absorptivity due to the attenuation effect of moth-eye structure and the electromagnetic resonant absorption in the metamaterial, achieving the purpose of both extremely low solar spectrum absorption and strong infrared emissivity within the atmospheric windows under the direct sunlight. Eventually, our designed cooler presents the potential to achieve a net radiative cooling power exceeding 156 W/m2 at ambient temperature of 300 K under direct solar irradiation, leading to a temperature reduction of 42.4°C. At nighttime, the net cooling power is more than 199 W/m2 at ambient temperature, resulting in a temperature reduction of 58.5°C. Even considering the non-radiative heat exchange conditions, this metamaterial cooler can still cool down 9.6°C at the daytime and 12.3°C at the nighttime respectively. Therefore, this work further promotes the development of all-dielectric metamaterial based passive radiative coolers and is of great significance for energy conservation.

Silver ants-inspired flexible photonic architectures with improved transparency and heat radiation for photovoltaic devices

Silver ants forage at extremely high environmental temperatures, the hairs of which play a crucial role in reducing the body temperature via enhanced optical reflection and radiative heat dissipation as a passive daytime radiative cooler. Inspired by the hierarchical feature structure of the hairs, we fabricated the flexible photonic architectures (FPA) on polydimethylsiloxane (PDMS). With some modification to the feature structure, the FPA-PDMS exhibited not only a large emittance in the mid-infrared spectral range, comparable with that of state-of-the-art thermal emitters, but also the record-high transmittance and haze in the visible and near-infrared range, which would be particularly appealing to photovoltaic devices for maximizing sunlight absorption and minimizing parasitic heat. The ease of application of the FPA-PDMS was demonstrated in both emerging perovskite and mature commercial crystalline silicon solar cells, where the FPA-PDMS was electrostatically attached to their glass superstrates or covers and the improvements in photovoltaic characteristics as a result of enhanced light harvesting were confirmed through laboratory measurements. Though the synergetic effects of the improved transparency and heat radiation brought by the FPA-PDMS need to be characterized outdoors in the future, these findings pave the way for suppressing optical and thermal losses simultaneously in opto-electric-thermal coupled devices through learning sophistically evolved photonic architectures from nature.

Field investigation of a photonic multi-layered TiO2 passive radiative cooler in sub-tropical climate

Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiative cooling 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 TiO2–SiO2 alternating multi-layered photonic daytime radiative cooler with average emissivity of 0.84 within 8–13 μm 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 = 60–70%) 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.

Colored Passive Radiative Cooler: Colored, Daytime Radiative Coolers with Thin‐Film Resonators for Aesthetic Purposes (Advanced Optical Materials 22/2018)

Colored Passive Radiative Cooler: Colored, Daytime Radiative Coolers with Thin‐Film Resonators for Aesthetic Purposes (Advanced Optical Materials 22/2018)

A numerical study of daytime passive radiative coolers for space cooling in buildings

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.

Colored, Daytime Radiative Coolers with Thin‐Film Resonators for Aesthetic Purposes

AbstractRecently developed approaches in passive radiative cooling enable daytime cooling via engineered photonic structure layouts. However, the use of these daytime radiative coolers is restricted owing to their nonaesthetic appearance resulted from strong solar reflection. Therefore, this article introduces a colored passive radiative cooler (CPRC) capable of generating potential cooling power, based on a thin‐film optical resonator embedded in an efficient thermal emission structure. This CPRC not only selectively emits infrared wave through the atmospheric transparency window but also displays subtractive primary colors to exhibit the desired appearance. Theoretical analysis and systematic experiments prove the possibility of subambient cooling via CPRC by lowering the temperature to 3.9 °C below the ambient air in the daylight. This is the first example of coloring radiative cooler by photonic structures. Successful demonstration of cooling/coloring behavior with wearable electronic devices under solar irradiation represents a major step forward in the field of temperature‐sensitive, flexible, wearable electronic/optoelectronic devices.

The design of ultra-broadband selective near-perfect absorber based on photonic structures to achieve near-ideal daytime radiative cooling

Passive cooling, which cools without any electricity input, has had a great impact on global energy consumption. The recent progress on radiative cooling has many potential applications in efficient passive cooling. During the day, this strategy uses the maximized infrared emissivity via the atmospheric transparency windows for radiating heat and minimizing solar absorption. However, the realization of daytime radiative coolers with ideal selective mid-infrared emissivity is still a great challenge. Here, we firstly design and numerically demonstrate a near-ideal radiative cooler operating below the ambient temperature, achieving both broadband selective emissivity in the infrared atmospheric window and extremely low absorption in the entire solar spectrum, realizing a net cooling power exceeding 122W/m2 at ambient temperature. The cooling effect can still persist under significant nonradiative heat exchange conditions. The design of multi-layer all-dielectric micropyramid structure in this work not only solves the shortcoming of poor mid-infrared selectivity in planar photonics device, but also overcomes the disadvantage of high solar absorption in metal/dielectric metamaterials. The comparisons of physics mechanism between this multi-layer all-dielectric structure and previously reported multi-layer metal/dielectric structure also are investigated clearly. Thus, this study can help pave the way for designing ideal daytime radiative coolers.

Thermal Emission Control via Bandgap Engineering in Aperiodically Designed Nanophotonic Devices.

Aperiodic photonic crystals can open up novel routes for more efficient photon management due to increased degrees of freedom in their design along with the unique properties brought about by the long-range aperiodic order as compared to their periodic counterparts. In this work we first describe the fundamental notions underlying the idea of thermal emission/absorption control on the basis of the systematic use of aperiodic multilayer designs in photonic quasicrystals. Then, we illustrate the potential applications of this approach in order to enhance the performance of daytime radiative coolers and solar thermoelectric energy generators.