Radiative Cooling New Opportunities & Enabling Technologies Aaswath P. Raman, Ph.D. [email protected] Research Associate, Fan Group, Ginzton Laboratory ARPA-E Alternative Power Plant Cooling Workshop, May 12, 2014 1 An opportunity to tap an underutilized resource Use the cold of outer space to cold outer space! (-80°C ! -270°C)! radiatively pump heat from the ground through sky access New: Possible at all hours of the day through photonic design of thermally emissive layers Heat Atmosphere Thermal Meaningful cooling power that EM Waves scales with area: analogies to PV Radiative Cooling Surface! 2 I. INTRODUCTION Radiative cooling is a technique that exploits a natural transparency window for electro- magnetic waves in the Earths atmosphere to transport heat from terrestrial objects into cold space. As a result, objects with the appropriate radiative properties can passively cool them- selves down to temperatures well below the ambient. The atmospheric transparency window is found in the 8-13µm wavelength range, as shown in Fig. 1, and fortuitously overlaps with the blackbody spectralAtmospheric radiance corresponding to typical terrestrialtransmittance temperatures (0-50C), thus enabling objects at these temperatures to emit more power than they absorb. 1 Atmospheric Transmission Radiative cooling is enabled by 0◦C blackbody 50◦C blackbody an atmospheric transparency 0.5 window between 8 – 13 μm 0 7 9 11 13 15 Blackbody spectrum of typical λ [µm] Earth temperature objects overlap with window FIG. 1. Atmopheric Transmissioncold in the outer zenith space direction! vs. wavelength; normalized blackbody spectral radiance of a 0◦C and a(upper 50◦C blackbody atmosphere) emitter ! Varies with cloud cover, Prior work in radiative cooling has almost entirely focused on nighttime cooling,geographic where location and one aims to maximize emission in the atmospheric transparency window, without having to contend with solar radiation. In turn, nighttime cooling is ofHeat limited practical relevanceozone since pollution solar radiation is by far the greatest source of daytime heating. Early work on nighttime cooling mainly focused on choosing the right materials [3] and exploiting simple interference Radiative Cooling Surface! e↵ects [4, 5] to maximize emissivity in the transparency window. Granqvist et. al [4, 5] 3 theoretically investigated and characterized the properties of the ideal nighttime radiator, finding that such radiator could reach nighttime temperatures which were 50oC lower ⇡ than the ambient, with a cooling power of 100W/m2 when the radiator temperature was ⇡ 2 Figure 1(a), represents a strong departure from previously published systems. 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The net net cooling coolingtemperature power powerPnetPnetT(ambT(T) .of) Theof a a structure net structure cooling with with power area areaAPAnetisis given(T given) of by a by structure with area A is given by P (T)=P (T) P (T ) P , (1) net rad − atm amb − sun Cooling PnetP (T(T)=)=PP (T(T) ) PatmP (T(T ) )PPsunP(T,)=, P (T) P (T ) (1)P(1) , (1) where net radrad −− atm ambamb−−net sunPower rad − atm amb − sun wherewhere where • Radiative Cooling Surface! Prad(T)=A dWcosq dlIBB(T,l)e(l,q), (2) 0 Z PumpZ Heat in at Pnet • • to maintain at T • 4 PradPrad(T(T)=)=AA dWdWcoscosqq dldlIBBPIBBrad(T((,TTl,)=l)e)(elA(l,q,)qd,)W,