Metamaterials for radiative sky cooling

From fundamental thermodynamic considerations, in order to convert heat to usable work, it is important to have a heat source with a temperature that is as high as possible, and to have a heat sink with a temperature that is as low as possible. The vast majority of energy conversion processes at the moment use our ambient surroundings on Earth itself, with a temperature of approximately 300 K, as the heat sink. On the other hand, the universe, with a temperature of approximately 3 K, represents a much better heat sink. The ability to harness the coldness of the universe could therefore have broad implications for energy technologies in general, and represents an important emerging frontier in renewable energy research. The ability of photonic structures such as metamaterials to control the behavior of electromagnetic waves is essential to effectively harness the coldness of the universe. Earth's atmosphere is largely transparent to electromagnetic waves in the wavelength range of 8–13 μm. This wavelength range coincides with the spectral peak for black body radiation at 300 K. Thus, any object, when exposed to the sky, can radiate its heat out in a process known as radiative cooling (Fig. 1a) and passively reach sub-ambient temperatures. This natural phenomenon has been implemented and studied at night for centuries. However, to improve the thermodynamic efficiency of energy technologies in general, and for cooling applications in particular, it would be far more useful to enable the same cooling effect during the day. The challenge here is that a sky-facing object faces the sun directly during daylight hours. For this purpose, then, one would need to create a structure that reflects the entire solar spectrum very well, while at the same time generating strong thermal radiation in the 8–13 μm wavelength range (Fig. 1b). (a) The concept of radiative sky cooling presented in the context of major thermodynamic resources around Earth. (b) To optimize radiative sky cooling, one needs to create a structure that achieves broad-band reflection of sunlight and strong thermal emission in the primary transparency window of the atmosphere between 8–13 μm. (c) A multilayer metamaterial structure deposited on a silicon wafer that functions as a daytime radiative cooler. (Taken from Ref. [2].) The four layers with nanoscale thickness are illustrated in the left panel. The dark and light regions correspond to HfO2 and SiO2 respectively. (d) A metamaterial film developed for radiative sky cooling. (Taken from Ref. [4].) (e) A fluid cooling panel incorporating a metamaterial radiative cooler, heat exchanger and circulating water loops. (Taken from Ref. [5].) In 2013, our group at Stanford University provided the first theoretical design of a photonic structure capable of achieving daytime radiative cooling [1]. The structure has a spectrum that resembles the ideal spectrum shown in Fig. 1b. The theory predicted [1] that under realistic atmospheric conditions, such a photonic structure would be able to reach a temperature 40–60°C below ambient air temperature passively, or alternatively to achieve a cooling power density exceeding 100 W/m2 at the ambient temperature of 300 K. In 2014, as the first experimental demonstration of daytime radiative cooling, we created a multilayer dielectric structure with a silver backing layer (Fig. 1c). The dielectric layers collectively enhanced solar reflection while also providing strong and selective thermal emission at the desired mid-infrared wavelengths through a metamaterial-like design. When placed in a roof-top measurement setup, we showed that the structure was able to reach a temperature that is 5°C below the ambient air temperature, in spite of having 900 W/m2 of sunlight directly incident upon it [2]. This experimental work also highlighted the importance of thermal design of the overall experimental setup, together with metamaterial design, for radiative cooling purposes. In a subsequent experiment, we demonstrated over 40°C reduction from the ambient air temperature with a selective thermal emitter [3], by placing the sample inside a sky-facing vacuum chamber that largely eliminated parasitic heat gain. Building upon these initial demonstrations, further experimental progress has been made in scaling up metamaterial designs, and in integrating metamaterial radiative cooling surfaces with air-conditioning and refrigeration systems. The groups of Xiaobo Yin and Ronggui Yang [4] at the University of Colorado demonstrated a random glass–polymer hybrid metamaterial (Fig. 1d) that is scalable over a large area, and observed a cooling power density of 93 W/m2. Our group has recently made an advance in the integration of radiative sky cooling with commercial-scale cooling systems, by demonstrating fluid cooling panels that incorporate scalable multilayer films. We demonstrated large-area (> 1 m2) fluid cooling panels (Fig. 1e) that passively cool fluids 3–5°C below air temperature at relevant flow-rates [5]. Such fluid cooling panels can be used to improve the overall efficiency of the cooling system by 20% or more, by cooling refrigerant at the condenser of a vapor-compression cooling system below what is possible today. Central to recent advances in radiative sky cooling has been the capability of photonic structures and metamaterials to control electromagnetic radiation over a very broad-band wavelength range, spanning ultraviolet to mid-wave infrared. This success in demonstrating radiative sky cooling, from the conceptual to pre-commercial stage, highlights the significant untapped potential of using metamaterials for renewable energy applications. The work was supported by the Advanced Project Research Agency—Energy (ARPAE) of the US Department of Energy (DE-AR0000316) and the Global Climate and Energy Project (GCEP) at Stanford University. The authors acknowledge the contributions of Dr E. Rephaeli, Mr M. Anoma, Dr L. Zhu, Dr Z. Chen and Dr E. Goldstein to the works carried out at Stanford University that are reviewed here.