Due to the growing demand for comfort in housing and the increase in temperature due to global warming, energy consumption for air conditioning is increasing, greatly impacting the environment. Limiting expenses for air conditioning is becoming a real issue. Among the various options to achieve this goal, the design of an innovative coating allowing radiative cooling could be a solution.
Very often in thermodynamics, in order to have a good conversion efficiency, it is necessary to have a high temperature heat source and a low temperature heat sink. Now the space which has a temperature around 3 K is a very interesting heat sink ... if we are able to interact with it. However, the Earth's atmosphere has a window of transparency for electromagnetic waves between 8 and 13 µm. This window of transparency coincides with the wavelengths of thermal radiation at typical ambient temperatures. Using this phenomenon, a body can be cooled simply because its heat is radiated away to space. This is called passive radiative cooling. The use of this kind of mechanisms will make it possible to design a perfect passive cooling system for terrestrial applications, that is to say without input of energy.<br /><br />The objective of this project is to design and optimize diurnal passive radiative coolers, first from a numerical point of view, then to experimentally prove the feasibility of the concept. The final goal is to design demonstrators. Nighttime radiative cooling systems have been widely studied. However, the cooling demand is much more during the day, and if the radiative cooling occurs naturally at night because the ambient temperature is low, it becomes very complicated during the day because the radiative cooler is heated by the sun. The radiative cooler must therefore both perfectly reflect solar radiation and be an almost perfect emitter in the transparency window of the Earth's atmosphere.
Design of the radiative properties of metamaterials for passive radiative cooling
The first stage focused on the theoretical and numerical design of metamaterials (i.e. artificial materials with innovative radiative properties) using numericall optimization tools developed by the coordinator. The second part concerned the manufacture of these metamaterials. Samples composed of stacks of thin layers and / or etching of micrometric gratings on the surface were manufactured at the Pprime Institute as well as at ESIEE Paris, service provider identified upstream of the project for its expertise in terms of of nanostructuring. The third part concerned the measurements of the optical and radiative properties of the samples. Numerical simulations and experimental measurements were then compared in order to validate the scientific approach of this project. The optical indices of each material have in particular been measured at the Pprime institute in order to have the exact characteristics of each layer constituting the system: the risk of having differences between the optical properties of materials from the literature and the real properties. thin synthesized layers is in fact not negligible. These measurements have thus made it possible to carry out new numerical simulations with input parameters closer to reality and to considerably improve the design of the radiative properties of these supercoolers. Larger samples were then manufactured at the Pprime Institute and at ESIEE Paris to enable the cooling capacity of these systems to be measured in real conditions.
Design of the radiative properties of metamaterials for passive radiative cooling
The first year of this project was devoted to the digital design of multilayer structures + surface network whose radiative properties will allow optimal cooling power to be obtained. The first production campaign enabled the realization of the stack of thin layers without a surface network. Thin layers of each material were also fabricated and optically and thermally characterized by ellispometry and by FTIR. The optical indices of each material obtained by ellipsometry over the entire spectrum considered were used as new input parameters for the calculation codes. The new numerical simulations were then compared with the measured reflectance spectra and a great convergence between numerical and experimental results was observed.
In parallel, a theoretical model to calculate the cooling capacity of these structures was carried out. These preliminary results being very promising, an experimental measurement bench was also set up in order to be able to evaluate in real conditions the cooling power of various samples as well as the temperature variations of the various elements of the system.
The use of radiative cooling in thermophotovoltaic (TPV) systems has also been studied. New structures have also been fabricated and characterized optically and thermally.
In parallel, a study on the use of silica nanofibers as a supercooler was undertaken. Silica, a natural element which makes it possible to manufacture glass, emits very well in the infrared, and allows radiation to pass in the visible region and therefore does not absorb it. However, it was also necessary that the material considered could reject heat or reflect radiation: if we take the example of water, we know that it is transparent in the visible, but if we transform it into droplets, light is returned in other directions. It is said that the radiation is diffused. This is what we can see on an airplane above the clouds: the droplets composing them are very diffusing and in the end the clouds are very reflective. The principle is the same for silica with a coating composed of entangled fibers that can maintain themselves in the air, forming a mattress. This principle is well known as it is used in the design of glass wool. To obtain optimized cooling powers, this glass wool must be made of extremely fine silica fibers (200 nanometers in diameter).
The year 2020 was mainly devoted to new studies on the design of colored radiative coolers. The problem becomes even more complex: obtaining a color means absorbing part of the solar radiation and therefore heating the coating and degrading the cooling power. Three different types of coatings have been proposed.
The prospects for this project are numerous. Many additional studies will be necessary to consider a large-scale development of this cold production technology: manufacture of such coatings for large surfaces, durability in the face of external conditions (rain, dust, etc.). The coordinator has already initiated these studies which go beyond the initial objectives of this project. A test bench in real conditions of samples of intermediate size has been developed and makes it possible to measure the variations in temperature of the samples over periods of 24 to 48 hours outdoors. The coordinator is also working on the possible association of such radiative coolers with photovoltaic solar panels in order to limit their heating and therefore guarantee a high photoelectric conversion efficiency. Many studies initiated during the project are also still in progress: Design of colored radiative coolers, better consideration of the interactions of radiation with the atmospheric layer, ...
6 articles in international peer-reviewed journals (ACL) have been published to date: 1 in 2018, 2 in 2019, 2 in 2020 and 1 in 2021. The results obtained during this project have been presented in numerous national and international conferences: 15 presentations in total including 3 invited presentations. An infographic on this project appeared in the journal «Le Monde« in 2020 and the coordinator shared his expertise for publication in the scientific journal «science & futur« in May 2020. Finally, an article in the journal Microscoop, edited by the CNRS Center-Limousin Poitou-Charentes, appeared in the July 2021 issue.
1.Hervé A., Drévillon J., Ezzahri Y., Joulain K., J. Quant. Spectrosc. Radiat. Transf., 221, 155, 2018.
2. Blandre E., Vaillon R., Drévillon J., Optics Express, 27, 25, 2019.
3.Yalcin R. A., Blandre E., Joulain K., Drévillon J., Solar Energy Materials And Solar Cells, 110320, 2019.
4. Yalcin R. A., Blandre E., Joulain K., Drévillon J., ACS Photonics, 7, 2020.
5. Blandre E., Ali Yalçin R., Joulain K., Drévillon J., Optics Express, 28, 20, 2020.
6. Yalcin R. A., Blandre E., Joulain K., Drévillon J., Journal of Photonics for Energy, 11(3), 2021.
It is well known that the earth’s atmosphere has a transparency window for electromagnetic waves between 8 and 13 µm. This transparency window coincides with thermal radiation wavelengths at typical ambient temperatures. Using this phenomenon, a body can be cooled just because its heat is radiated into cold outer space. It is the so-called passive radiative cooling. This mechanism is very interesting in the current context where we look for improve energy efficiency. This passive radiative cooling can, for example, be used in air-conditioning or to chill photovoltaic cells. The goal of this project is to design and optimize passive daytime radiative cooling systems based on nano/microstructured materials with specifics radiative properties. This project will be divided in three parts. A first part will concern the design and optimization of radiative coolers. Based on the expertise of the coordinator in the field of nano/micro structured systems thermal emission control and on the optimization numerical tools that he has developed, highly reflective systems for solar radiation and emitting only between 8 and 13 µm will be designed. The coordinator has already developed first numerical models based on systems coupling multilayer structures and surface gratings to obtain the desired radiative properties.
The second part of this project will deal with the fabrication of the system. Based on the results obtained in the first part, samples will be fabricated. Firstly, the coordinator will rely on the technology available in his laboratory. P' Institute has indeed a research bench composed of a dual-beam FIB (focused ion beam) that can etch the surface grating. The coordinator plan also to use the DRIE technique (Deep Reactive Ion Etching) available at ESIEE Paris. He will also collaborate with the team PPNa "Physics and Properties of Nanostructures" of the P' Institute. This team has extensive experience in the development of nanostructured materials. Two techniques will be used: the thin films deposits will be first done by sputtering and then by evaporation
The third part will concern the measurements of the radiative properties of the samples and the refractive index of each material composing the whole structure. To characterize the samples, the team TNR has an optical measurements bench using FTIR (Fourier Transform Infrared Spectroscopy). This experimental bench has already helped characterizing the selective thermal emitters that the coordinator has developed for thermophotovoltaics applications. The samples reflectivity spectra will be measured and compared with those obtained numerically. Spectral measurements can also be made at the LTEN (Nantes Thermal and Energy Laboratory). This team has recently acquired advanced equipment. This is an FTIR coupled to an IR microscope. To calculate the radiative properties of the system, each material is defined by its refractive index and at a first step, the coordinator will use the values available in the literature. However, the risk of having differences between these literature data and the actual properties of the synthesized thin layers is not negligible. Therefore, in order to carry out numerical simulations with input parameters closest to reality, measurements of refractive indexes over the entire range of wavelength under consideration will be systematically carried out. A great opportunity is the recent acquisition by the P' Institute of an IR spectroscopic ellipsometer. This experimental bench will be coupled with a visible ellipsometer. It is important to say that only two French laboratories have such an experimental bench.
Through these three scientific work packages, the final objective is the realization of one or more prototype of passive radiative coolers with high efficiency.
Monsieur Jérémie DREVILLON (Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique
Help of the ANR 236,520 euros
Beginning and duration of the scientific project: December 2017 - 36 Months