Radiative cooling for energy sustainability: Materials, systems, and applications (original) (raw)
Related papers
Radiative Cooling: Principles, Progress, and Potentials
Advanced Science, 2016
decades ago and practical cooling during the nighttime operation was also demonstrated. [ 7-23 ] The use of bulk materials comprising intrinsic infrared (IR) emissions for considerable radiative cooling were discussed earlier. [ 10,18,21,23,24 ] However, radiative cooling was mostly limited during the nighttime as suitable materials with high IR emission within the atmospheric window and yet delivering strong solar refl ection during the day was not achieved. Although some solar refl ecting materials were reported, daytime cooling below the ambient temperature was not achieved as the absorbed solar energy exceeded the emitted energy by thermal radiation. [ 25,26 ] It was only the recent demonstrations where the use of advanced nanophotonics led to daytime radiative cooling well below the ambient temperature. [ 3,27 ] The radiative coolers with photonic designs can simultaneously possess a high solar refl ection up to 97% and strong IR emission within the atmospheric window. [ 1,3,27 ] On the other hand, microstructure based thermal emitters can also offer highly effi cient cooling power with their strictly selective and strong IR emission within the atmospheric window. [ 2 ] These emerging photonic devices, offering substantial passive cooling during the day and night, have triggered signifi cant research interest. In this article, we seek to review the detailed characteristics, recent advancements, and the scope for further performance improvements of radiative coolers. In Section 2, we present a basic overview of operating principles of radiative cooling. The material and structural design requirements for nocturnal and daytime applications are identifi ed and the potential cooling effi ciency is investigated. The performances for selective and broadband radiators are compared. The impact of atmospheric conditions and nonradiative heat exchange processes on cooling effi ciency is briefl y discussed. In Section 3, we review the materials and device designs used for nocturnal radiative cooling in earlier studies. We analyze their performances and discuss their limitations for use in daytime applications. Section 4 discusses the recent developments of daytime radiative coolers based on photonic approach and microstructure based thermal emitters for daytime and highly effi cient cooling applications. We also highlight the technical challenges for these devices for further performance improvement. In Section 5, the achievable cooling effi ciency for optimized photonic and device characteristics are analyzed. Section 6 presents the conclusion of The recent progress on radiative cooling reveals its potential for applications in highly effi cient passive cooling. This approach utilizes the maximized emission of infrared thermal radiation through the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation. These simultaneous processes can lead to a device temperature substantially below the ambient temperature. Although the application of radiative cooling for nighttime cooling was demonstrated a few decades ago, signifi cant cooling under direct sunlight has been achieved only recently, indicating its potential as a practical passive cooler during the day. In this article, the basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed. The recent advancements over the traditional approaches and their material and structural characteristics are outlined. The key characteristics of the thermal radiators and solar refl ectors of the current state-of-the-art radiative coolers are evaluated and their benchmarks are remarked for the peak cooling ability. The scopes for further improvements on radiative cooling effi ciency for optimized device characteristics are also theoretically estimated. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Radiative cooling as low-grade energy source: A literature review
Renewable and Sustainable Energy Reviews
Radiative cooling is a technology intended to provide cooling using the sky as a heat sink. This technology has been widely studied since 20 th century but its research is scattered all over the literature, requiring of a review to gather all information and a state-of-the-art. In the present article, the research has been classified in: (1) radiative cooling background, (2) selective radiative cooling, (3) theoretical approach and numerical simulations, and (4) radiative cooling prototypes. Even though this is a low-grade technology it can dramatically reduce the energy consumption, since it is renewable and requires low energy for its operation. However, new functionalities of the device, apart from radiative cooling, are required for profitable reasons.
A radiative cooling structural material
Science
Reducing human reliance on energy-inefficient cooling methods such as air conditioning would have a large impact on the global energy landscape. By a process of complete delignification and densification of wood, we developed a structural material with a mechanical strength of 404.3 megapascals, more than eight times that of natural wood. The cellulose nanofibers in our engineered material backscatter solar radiation and emit strongly in mid-infrared wavelengths, resulting in continuous subambient cooling during both day and night. We model the potential impact of our cooling wood and find energy savings between 20 and 60%, which is most pronounced in hot and dry climates.
A B S T R A C T Although nocturnal radiative cooling has been known for centuries, providing sub-ambient radiative cooling during daytime was a challenge until recent years. Recent advances in nano-fabrication technologies, have made it possible to manufacture structures with tailored radiative properties for various energy applications like daytime clear sky radiative cooling. It has been shown that photonic and plasmonic selective emitters can be tuned efficiently to emit heat through clear sky to the outer space and cool terrestrial objects providing passive cooling. There is a renewed interest in clear sky radiative cooling among researchers. Providing continuous day and night sub-ambient cooling and dissipation of low grade heat from renewable power systems without use of water or external energy under direct sunlight and other applications have made clear sky radiative cooling a hot research topic. This paper reviews relevant publications on clear-sky radiative cooling methods. An overview of radiative cooling fundamentals and a detailed literature survey of published studies on selective emitter structures for daytime and nighttime cooling purposes is presented. Furthermore, a detailed energy analysis is performed identifying key performance indicators and evaluating the cooling performance under various conditions. Findings from studies that have used empirical equations for numerical energy analysis and selective emitter structure designs for daytime and nighttime applications are summarized in tables for easy comparison.
Combined Radiative Cooling and Solar Thermal Collection: Experimental Proof of Concept
Energies
Climate change is becoming more important day after day. The main actor to decarbonize the economy is the building stock, especially in the energy used for Domestic Hot Water (DHW), heating and cooling. The use of renewable energy sources to cover space conditioning and DHW demands is growing every year. While solar thermal energy can cover building heating and DHW demands, there is no technology with such potential and development for space cooling. In this paper, a new concept of combining radiative cooling and solar thermal collection, the Radiative Collector and Emitter (RCE), through the idea of an adaptive cover, which uses different material properties for each functionality, is for the first time experimentally tested and proved. The RCE relies on an adaptive cover that uses different material properties for each functionality: high spectral transmittance in the solar radiation band and very low spectral transmittance in the infrared band during solar collection mode, and hi...
Analysis of Sustainable Materials for Radiative Cooling Potential of Building Surfaces
Sustainability
The main goal of this paper is to explore the radiative cooling and solar heating potential of several materials for the built environment, based on their spectrally-selective properties. A material for solar heating, should have high spectral emissivity/absorptivity in the solar radiation band (within the wavelength range of 0.2–2 μm), and low emissivity/absorptivity at longer wavelengths. Radiative cooling applications require high spectral emissivity/absorptivity, within the atmospheric window band (8–13 μm), and a low emissivity/absorptivity in other bands. UV-Vis spectrophotometer and FTIR spectroscopy, are used to measure, the spectral absorption/emission spectra of six different types of materials. To evaluate the radiative cooling potential of the samples, the power of cooling is calculated. Heat transfer through most materials is not just a surface phenomenon, but it also needs a volumetric analysis. Therefore, a coupled radiation and conduction heat transfer analysis is us...
Energy Savings Potential of Radiative Cooling Technologies
2015
EnergyPlus does not support the specification of rooftop radiative heat exchangers, so custom heat transfer modeling was applied to simulate the flows of heat between the heat exchanger, building, and sky, and the anticipated hydronic loop conditions were passed back into the EnergyPlus model. This custom modeling was performed in EnergyPlus's energy management system (EMS) framework, which allows the user to build equations that overwrite certain predetermined points of intervention in the EnergyPlus model. For the nighttime radiative cooler, first-principle thermodynamic equations were used with some simplifying assumptions to model heat flows for conventional heat exchanger surfaces. PNNL partnered with researchers at Stanford who developed the photonic surfaces being investigated for radiative cooling. The researchers provided PNNL with detailed spectral characterizations of the thermodynamic properties of their material. Calculation of radiative heat transfer from photonic materials, however, required mathematical integration functions that are not supported by the EMS. To get around this problem, PNNL used a regression equation for radiative heat exchange based on an integration performed in MATLAB, developed by the Stanford researchers. Results Relative to the VAV system, the proposed photonic radiative cooling system saves 103 MWh electricity in Miami, 55 MWh in Las Vegas, 50 MWh in Los Angeles, 24 MWh in San Francisco and 43 MWh in Chicago, per year. The saved electricity represents 50%, 45%, 65%, 68%, and 55% of the VAV system cooling electricity, respectively in the above five cities. Relative to the high-end nighttime radiative cooling products available in the market, the photonic radiative cooler saves 10 MWh electricity in Miami, 13 MWh in Las Vegas, 8 MWh in Los Angeles, 3 MWh in San Francisco and 6 MWh in Chicago, per year, which represents 9%, 16%, 23%, 22%, and 14% of cooling electricity savings, respectively in the above five cities. Market Assessment and Conclusions Radiative cooling in buildings is best harnessed with hydronic distribution systems. Because achievable chilled water temperatures from radiative cooling are typically well above chilled water temperatures required for forced-air-based delivery systems, this necessitates the simultaneous specification of radiant zone cooling. Both radiative cooling and radiant zone cooling are investigated for market benefits and barriers. There are a wide variety of mechanisms by which radiant cooling and its required set of technologies can produce benefits to building owners and occupants. These include energy savings (and associated energy cost savings), other cost savings from elimination of alternative heating, ventilation and air conditioning (HVAC) infrastructure and downsizing of equipment as well as improved comfort. Besides providing additional electric energy savings, a system that integrates the radiative cooling heat exchanger to a building cooling loop via a cold water storage tank may be a very favorable participant in demand response. Several market barriers exist for which recommended mitigation strategies are provided. The barriers are that radiative cooling solutions are not well suited for existing buildings/retrofits, additional installation costs, complexity and the need for holistic design, as well as limitations imposed by climate, by certain building shapes, and by space available for new equipment. A simple economic analysis shows that for upgrading the new construction design from a VAV systems for HVAC delivery to photonic radiative cooling with radiant zone cooling, the maximum incremental cost for a 5-year simple payback ranges from 8.25to8.25 to 8.25to11.50 per square meter of total building floor area, based on climate. For an upgrade from nighttime radiative cooling using conventional materials to photonic radiative cooling, the maximum incremental cost for a 5-year simple payback
A Self‐Assembled 2D Thermofunctional Material for Radiative Cooling
Small, 2019
The regulation of temperature is a major energy‐consuming process of humankind. Today, around 15% of the global‐energy consumption is dedicated to refrigeration and this figure is predicted to triple by 2050, thus linking global warming and cooling needs in a worrying negative feedback‐loop. Here, an inexpensive solution is proposed to this challenge based on a single layer of silica microspheres self‐assembled on a soda‐lime glass. This 2D crystal acts as a visibly translucent thermal‐blackbody for above‐ambient radiative cooling and can be used to improve the thermal performance of devices that undergo critical heating during operation. The temperature of a silicon wafer is found to be 14 K lower during daytime when covered with the thermal emitter, reaching an average temperature difference of 19 K when the structure is backed with a silver layer. In comparison, the soda‐lime glass reference used in the measurements lowers the temperature of the silicon by just 5 K. The cooling p...
A solar reflecting material for radiative cooling applications: ZnS pigmented polyethylene
Solar Energy Materials and Solar Cells, 1992
Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material during the night and avoiding heating in direct sunlight during the day. The foils could be used for condensing water or cooling food, buildings, etc. We have prepared ZnS pigmented polyethylene foils with various thicknesses and volume fractions of pigments. The optical properties of the foils were compared with theory, and good agreement was achieved for pigment volume fractions up to 0.1. The optimum solar reflectance of the foil is 0.825 for the available pigment powder; it should be 0.88 if heating were to be avoided at noon with the sun in its zenith. The computed cooling power for a radiator covered with the best sample was 52 W m-2 at night, and the equilibrium temperature of the radiator was predicted to be 12 K below the ambient temperature. Computations showed that heating of the radiator could be avoided 18 hours per day, and the radiator temperature at noon was 8 K above the ambience.
Determining the Effectiveness of Radiative Cooler‐Integrated Solar Cells
Advanced Energy Materials, 2021
achieve this efficiency due to its dependency on the operating temperature. It is stated that the high efficiency in various SCs can be achieved at an illumination of AM 1.5G and a temperature of 25 °C. However, the temperature of the SC typically exceeds this value in outdoor conditions, where it heats up by tens of degrees above the ambient temperature, which decreases the lifespan and efficiency of the SC. [14,15] The passive radiative cooling method can potentially resolve the heating issue of the SC owing to its compact and costeffective approach. It involves spontaneously cooling objects by emitting heat to the outer space without consuming energy through the transparent atmospheric transmittance window (λ ≈ 8-13 µm). [16-18] Recent studies have presented various types of radiative coolers (RCs) [19-27] which have been demonstrated to successfully lower the temperature of SCs. [28-30] Research has also been conducted to theoretically analyze the effectiveness of RCs in compensating for the reduced conversion efficiency of SCs due to elevated temperatures. [31-34] However, these studies have evaluated or tested the potential of an RC on specific target SCs, such as silicon, [35-39] concentrated perovskite, [40-43] or low-bandgap concentrated perovskite, [41] which are limited to single-junction SCs. Further research is required to determine the type of SC that can most retain its original efficiency even at high environmental temperatures, when adapting the RC technique. The efficiency of SCs can be significantly improved through a comprehensive understanding of the practical operation of the RC on diverse SCs since the SC industry encompasses various types of cells. This study theoretically proves that the multi-junction SC (MJSC) is the most effective type of SC when an RC is applied. It also presents the limitations of the radiative cooling technique when sub-bandgap (sub-BG) absorption is considered. Consequently, the proposed MJSC is demonstrated to be immune to heating by sub-BG photons, which can lead to the development of novel SCs by reducing the burden of designing additional sub-BG filters [44] or reflectors. [45-47] A structure is then fabricated which performs both light trapping and radiative cooling based on pioneering SC research, and is applied to the InGaP/GaAs/Ge MJSC. Multiple outdoor experiments are conducted to demonstrate that radiative cooling can contribute to a temperature drop of ≈6 °C. The reduced temperature also results in an absolute increase of the open-circuit The power-conversion efficiency of solar cells (SCs) is reduced at high temperatures. A radiative cooling process can be implemented to overcome this issue. The radiative cooler (RC) presents considerable potential in the design of an ideal broadband emitter, which emits heat through the entire atmospheric transmittance window for devices with operating temperatures that significantly exceed the ambient temperature. However, the performance of these devices varies based on the type of SCs. This study aims to determine the dependency of the radiative cooling power for various types of SCs and proposes the multi-junction SC (MJSC), which is the SC that benefits the most from RCs. The integrated cooler is designed with a micro-grating which can enhance the emissivity within entire atmospheric transmittance window and can also lead to the light-trapping aspect in the solar spectrum. Outdoor field tests demonstrate both the enhanced cooling performance and the power conversion efficiency of the proposed MJSC when compared to a conventional glass-mounted MJSC under direct sunlight of ≈900 Wm −2 including a temperature drop of ≈6 °C and minimization of the variation of the opencircuit voltage to ≈6%. Future research is expected to develop a theoretical bridge between the field of SCs and radiative cooling.