Passive radiative cooling below ambient air temperature under direct sunlight

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Abstract

Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States1. A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres2,3,4,5,6,7,8,9,10,11. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved3,4,12,13 because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach14,15,16,17,18,19,20,21,22,23,24,25, we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

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Figure 1: Rooftop apparatus and photonic radiative cooler.
Figure 2: Emissivity/absorptivity of the photonic radiative cooler from the ultraviolet to the mid-infrared.
Figure 3: Steady-state temperature of photonic radiative cooler.
Figure 4: Cooling power of photonic radiative cooler.

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Acknowledgements

This work is supported by the Advanced Research Projects Agency-Energy (ARPA-E), Department of Energy (contract number DE-AR0000316). We acknowledge discussions with J. Eaton and K. Goodson. Part of this work was performed at the Stanford Nanofabrication Facility, which is supported by the National Science Foundation through the NNIN under grant number ECS-9731293, and the Stanford Nano Center (SNC)/Stanford Nanocharacterization Laboratory (SNL), part of the Stanford Nano Shared Facilities.

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Contributions

A.P.R. and S.F. envisioned and implemented the experimental studies, and wrote the manuscript. A.P.R. and M.A.A. built and executed the rooftop experiments. A.P.R. designed and characterized the radiative cooler. L.Z. and E.R. provided technical support and conceptual advice.

Corresponding authors

Correspondence to Aaswath P. Raman or Shanhui Fan.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Angular emissivity of photonic radiative cooler.

a, Measured emissivity/absorptivity of the cooler for variable angles of incidence over near- and mid-infrared wavelengths with a realistic atmospheric transmittance model plotted for reference26. b, Average measured emissivity of the photonic radiative cooler between 8 µm and 13 µm (the atmospheric transparency window) plotted as a function of polar angle of incidence. The cooler’s emissivity within the window remains relatively constant between 5° and 60°, and remains high even at remarkably large angles of incidence, reminiscent of the behaviour of hyperbolic metamaterials.

Extended Data Figure 2 Numerical modelling of non-radiative heat exchange.

a, A numerical simulation of the apparatus and radiative cooler (layout shown at top) yields a temperature distribution (Tsample − Tambient) (bottom) within the geometry that takes into account non-radiative heat exchange in the apparatus. b, The simulation calculation for Pcond+conv as a function of Tsample − Tambient (blue line) yields a value of hc = 6.9 W m−2 K−1, which we use in our theoretical model to compare against experimental data in Fig. 4b. We also plot the theoretical model of net radiated power Pnet that is based on the absorption/emission of the photonic radiative cooler (grey line). The intersection represents the predicted value of Ts of the photonic radiative cooler in this apparatus, and matches well our observed experimental value of Ts = 4.3 °C below ambient in Fig. 4b. c, Theoretical model plotted for various values of hc. As hc → 0 the model predicts Ts of nearly 20 °C below ambient can be achieved by the photonic radiative cooler.

Extended Data Figure 3 Modelling steady-state temperature under daytime and night-time conditions.

a, The theoretical model is used to calculate the steady-state temperature of the photonic radiative cooler (see Methods) given observed air temperatures, solar irradiance and a range of values of hc from 4 W m−2 K−1 to 8 W m−2 K−1 (grey band). The model is able to predict well the observed performance of the photonic radiative cooler through the course of the day (blue line, extending the data from Fig. 2) under both direct solar irradiance (green line) and when the Sun sets behind nearby hills, reducing irradiance to near-zero values. b, Data from this latter time period (effectively night-time conditions). As expected, the photonic cooler performs even better under no solar irradiance, with Tsample − Tambient of approximately 7 °C below ambient achieved. c, The four terms on the right hand side of equation (1) are plotted, using the model at Ts as a function of time, to elucidate the impact of each component on performance.

Extended Data Figure 4 Energy savings for a medium commercial reference building with photonic radiative coolers.

We use a simple approach to estimate the energy savings for the National Renewable Energy Laboratory three-story commercial benchmark building31, assuming its 1,600 m2 roof is covered with the photonic radiative cooler, and heat from its interior is transferred to the radiative panels (see Methods). Using the model of our photonic radiative cooler, the thermal cooling power of the radiative cooler is set relative to ambient air temperature and solar irradiance for Phoenix, Arizona, over a typical year (TMY2 data), and with an interior air temperature of 24° C. EnergyPlus simulations determine the cooling load the building places on the air-conditioning units (which is assumed to have a coefficient of performance of 2.8). The calculated cooling power of the radiative cooler is then subtracted from the thermal load on an hourly basis for the year, and electricity savings are subsequently inferred and plotted. Although this is a rudimentary analysis, it does indicate that more kilowatt hours are saved in summer months where there is greater cooling demand, and that the scale of the potential annual savings (1.185 × 105 kWh) is substantial.

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Raman, A., Anoma, M., Zhu, L. et al. Passive radiative cooling below ambient air temperature under direct sunlight. Nature 515, 540–544 (2014). https://doi.org/10.1038/nature13883

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