Night-time radiative cooling: harvesting the darkness of the universe

Transcription

1 Night-time radiative cooling: harvesting the darkness of the universe Shanhui Fan Ginzton Laboratory and Department of Electrical Engineering Stanford University

2 Thermodynamic resources in the sky Sun (6000K) Outer Space (3K)

3 We do have radiative access to the outer space 300K Blackbody Spectrum

4 Previous work: night-time radiative cooling outer space (3K) Thermal radiation atmosphere emitter Insulating material earth surface (300K) Requires a good blackbody emitter. Cool to a temperature that is 13C below ambient. Limited use of night-time cooling. C. G. Granqvist and A. Hjortsberg, Journal of Applied Physics 52, 4205 (1981). A. R. Gentle and G. B. Smith, Nano Letters 10, 373 (2010).

5 Outline Radiative Cooling Solar Cell Cooling Night-time radiation: pushing the limit

6 Daytime radiative cooling emissivity 1 Daytime radiative cooler Wavelength (micron) No previous work has demonstrated daytime radiative cooling. E. Rephaeli, A. Raman and S. Fan, Nano Letters 13, 1451 (2013).

7 Stanford daytime radiative cooling experiment Sample: 8 inch wafer A. Raman, M. Anoma, L. Zhu, E. Rephaeli, and S. Fan, Nature 515, 540 (2014).

8 Emissivity of the photonic cooler Strong solar reflection Strong and selective thermal emission

9 Rooftop Setup

10 Daytime Cooling Experiment: Results A. Raman, M. Anoma, L. Zhu, E. Rephaeli, and S. Fan, Nature (2014).

11 Towards system demonstration

12 Outline Radiative Cooling Solar Cell Cooling Night-time radiation: pushing the limit

13 Cooling of silicon solar cells >50-55 o C Relative Efficiency -0.45%/ C Ingersoll, J. Sol. Energy Eng. (1986) Jones et al. Sol Energy (2001) Davis et al. J. Sol. Energy Eng. (2001) Skoplaki et al. Sol. Energy (2009)

14 Absorption properties of silicon Absorptivity 200 µm mirror Strong absorption in solar wavelength range Weak absorption/emissio n in thermal wavelength range

15 Radiative Cooling of Solar Cells: Operating Principle Absorptivity mirror 200 µm L. Zhu, A. Raman, K. X. Wang, M. Anoma and S. Fan, Optica 1, 32 (2014) Strong absorption in solar wavelength range Strong absorption/emissio n in thermal wavelength range

16 Experimental demonstration of radiative cooling of solar absorber 6 micron Bare absorber 500 µm silica Silica photonic crystal

17 Photonic crystal as a visibly transparent thermal blackbody Solar Thermal The three structures have very similar solar absorption properties The photonic crystal sample behaves as a visibly transparent blackbody.

18 13 C reduction of solar absorber temperature Rooftop setup Cooling of solar absorber L. Zhu, A. Raman and S. Fan, Proceedings of the National Academy of Sciences 112, (2015).

19 Combination of radiative and convective cooling Remove the polyethelyne cover

20 Outline Radiative Cooling Solar Cell Cooling Night-time radiation: pushing the limit

21 Previous work: night-time radiative cooling outer space (3K) Thermal radiation atmosphere emitter Insulating material earth surface (300K) Requires a good blackbody emitter. Cool to a temperature that is 13C below ambient. Limited use of night-time cooling. C. G. Granqvist and A. Hjortsberg, Journal of Applied Physics 52, 4205 (1981). A. R. Gentle and G. B. Smith, Nano Letters 10, 373 (2010).

22 Blackbody emitter is sub-optimal for night-time radiative cooling Transmission spectrum of the atmosphere at Stanford (-1 o C dew point) 1.0 Emissivity 0.0 Receiving heat Emit heat out Receiving heat

23 Selective emitter is essential for high-performance night time radiative cooling Transmission spectrum of the atmosphere at Stanford (-1 o C dew point) 1.0 Emissivity 0.0 Emit heat out

24 Very low temperature can be achieved with selective emitter Assumes no parasitic heat load 0 0 ΔT = T sample - T ambient [ o C] T -80 ambient = 20 o C Dew Point [ o C]

25 Suppressing parasitic heat leakage is essential for high performance BB: Blackbody; Sel: Selective Emitter In order to see the advantage for selective emitter, it is crucial to reduce parasitic heat transfer.

26 Preliminary Results (Aug. 14 Night) Temperature [ o C] T sample T ambient pm 10pm 12am 2am 4am 6am 8am Time 27.2 o C below ambient!

27 Radiative Cooling Summary Solar Cell Cooling Night-time radiation: pushing the limit Aaswath Raman Linxiao Zhu Eli Goldstein Marc Anoma Eden Rephaeli Linxiao Zhu Aaswath Raman Zhen Chen Linxiao Zhu Aaswath Raman