Letter | Published:

Passive radiative cooling below ambient air temperature under direct sunlight

Nature volume 515, pages 540544 (27 November 2014) | Download Citation



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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  1. 1.

    (ed.) 2011 Buildings Energy Data Book (US Department of Energy, Office of Energy Efficiency and Renewable Energy, 2011)

  2. 2.

    Perspectives sur l’utilisation des rayonnements solaires et terrestres dans certaines régions du monde. Revue Générale Thermique 6, 1285–1314 (1967)

  3. 3.

    et al. The radiative cooling of selective surfaces. Sol. Energy 17, 83–89 (1975)

  4. 4.

    et al. Nocturnal and diurnal performances of selective radiators. Appl. Energy 3, 267–286 (1977)

  5. 5.

    & Surfaces for radiative cooling: silicon monoxide films on aluminum. Appl. Phys. Lett. 36, 139–141 (1980)

  6. 6.

    & Radiative cooling to low temperatures: general considerations and application to selectively emitting SiO films. J. Appl. Phys. 52, 4205–4220 (1981)

  7. 7.

    , & Thermal performance of radiative cooling panels. Int. J. Heat Mass Transf. 26, 871–880 (1983)

  8. 8.

    Radiative cooling with MgO and/or LiF layers. Appl. Opt. 23, 370–372 (1984)

  9. 9.

    , & Radiative cooling efficiency of white pigmented paints. Sol. Energy 50, 477–482 (1993)

  10. 10.

    & Radiative heat pumping from the earth using surface phonon resonant nanoparticles. Nano Lett. 10, 373–379 (2010)

  11. 11.

    , & Optimized cool roofs: integrating albedo and thermal emittance with R-value. Sol. Energy Mater. Sol. Cells 95, 3207–3215 (2011)

  12. 12.

    & Radiative cooling during the day: simulations and experiments on pigmented polyethylene cover foils. Sol. Energy Mater. Sol. Cells 37, 93–118 (1995)

  13. 13.

    , & A solar reflecting material for radiative cooling applications: ZnS pigmented polyethylene. Sol. Energy Mater. Sol. Cells 28, 175–193 (1992)

  14. 14.

    , & Ultrabroadband photonic structures to achieve high-performance daytime radiative cooling. Nano Lett. 13, 1457–1461 (2013)

  15. 15.

    et al. Enhancement and suppression of thermal emission by a three-dimensional photonic crystal. Phys. Rev. B 62, R2243–R2246 (2000)

  16. 16.

    et al. Coherent emission of light by thermal sources. Nature 416, 61–64 (2002)

  17. 17.

    & Thermal emission control with one-dimensional metallodielectric photonic crystals. Phys. Rev. B 70, 125101 (2004)

  18. 18.

    , , & Thermal radiation from photonic crystals: a direct calculation. Phys. Rev. Lett. 93, 213905 (2004)

  19. 19.

    , & Coherent thermal emission from one-dimensional photonic crystals. Appl. Phys. Lett. 87, 071904 (2005)

  20. 20.

    & Ab initio design of coherent thermal sources. J. Appl. Phys. 102, 114305 (2007)

  21. 21.

    , & Optical antenna thermal emitters. Nature Photon. 3, 658–661 (2009)

  22. 22.

    & Absorber and emitter for solar thermo-photovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit. Opt. Express 17, 15145–15159 (2009)

  23. 23.

    et al. Metamaterial-based integrated plasmonic absorber/emitter for solar thermo-photovoltaic systems. J. Opt. 14, 024005 (2012)

  24. 24.

    et al. Conversion of broadband to narrowband thermal emission through energy recycling. Nature Photon. 6, 535–539 (2012)

  25. 25.

    et al. A nanophotonic solar thermophotovoltaic device. Nature Nanotechnol. 9, 126–130 (2014)

  26. 26.

    et al. Modtran5: 2006 update. Proc. SPIE 6233, 62331F (2006)

  27. 27.

    & Characteristics of infrared sky radiation in the United States. Sol. Energy 33, 321–336 (1984)

  28. 28.

    , , , & Optical properties of HfO2 thin films deposited by magnetron sputtering: from the visible to the far-infrared. Thin Solid Films 520, 6793–6802 (2012)

  29. 29.

    et al. Engineering photonic density of states using metamaterials. Appl. Phys. B 100, 215–218 (2010)

  30. 30.

    & Modeling global residential sector energy demand for heating and air conditioning in the context of climate change. Energy Policy 37, 507–521 (2009)

  31. 31.

    , & Application of the needle optimization technique to the design of optical coatings. Appl. Opt. 35, 5493–5508 (1996)

  32. 32.

    Handbook of Optical Constants of Solids (Academic Press Handbook Series, Elsevier Science & Tech, 1985)

  33. 33.

    , , & Energyplus: Energy simulation program. ASHRAE J. 42, 49–56 (2000)

  34. 34.

    et al. US Department of Energy commercial reference building models of the national building stock. Tech. Rep. NREL/TP-5500-46861, (National Renewable Energy Laboratory, 2011)

  35. 35.

    & User’s manual for TMY2s. Tech. Rep. (National Renewable Energy Laboratory, 1995)

  36. 36.

    Charting the progress of PV power plant energy generating costs to unsubsidized levels, introducing the PV-LCOE framework. In Proc. 26th Eur. Photovoltaic Solar Energy Conf. (Hamburg) 4409–4419 (2011)

  37. 37.

    & Study of potential cost reductions resulting from super-large-scale manufacturing of PV modules. Tech. Rep., Final Subcontract Report NREL/SR-520–36846 (National Renewable Energy Laboratory, 2004)

  38. 38.

    et al. Re-considering the economics of photovoltaic power. Renew. Energy 53, 329–338 (2013)

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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.

Author information


  1. Ginzton Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA

    • Aaswath P. Raman
    • , Eden Rephaeli
    •  & Shanhui Fan
  2. Department of Mechanical Engineering, Stanford University, Stanford, California 94305, USA

    • Marc Abou Anoma
  3. Department of Applied Physics, Stanford University, Stanford, California 94305, USA

    • Linxiao Zhu


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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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Aaswath P. Raman or Shanhui Fan.

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