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Radiative Cooling New Opportunities & Enabling Technologies

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Radiative Cooling New Opportunities & Enabling Technologies 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. Rather than designing FigureFigure 1(a), 1(a),a cover represents represents filter which a a strong strong is spatially departureFigure departure 1(a), separated from from represents previously previously from a a strong black published published emitter departure systems. systems. we from introduce Rather Rather previously than an than integrated designing published designing thermally systems. Rather than designing aa cover cover filterselective filter which which emitter is is spatially spatially atop separated aa separated broadbandcover filter from from which mirror. a a black black is emitter Doingspatially emitter weso we separated enables introduce introduce usfrom an to an integrated a exploit integrated black emitter near-field thermally thermally we introduce coupling an integrated thermally selectiveselectivebetween emitter emitter atop material atop a a broadband broadband layers,selective leading mirror. mirror. emitter to Doing Doingstronger atop so so a enables control enables broadband us over us to to mirror. emission, exploit exploit Doing near-field near-field absorption so enables coupling coupling and us reflection. to exploit near-field coupling betweenbetweenUsing material material nanophotonic layers, layers, leading leading concepts,between to to stronger stronger our material photonic control control layers, structure over over leading emission, emission, is able to to stronger absorption strongly absorption control suppress and and over reflection. reflection. solar emission, absorption absorption and reflection. UsingUsing nanophotonic nanophotonicwhile enhancing concepts, concepts, thermal ourUsing our emission photonic photonic nanophotonic structure in structure the atmospheric concepts, is is able able to to ourstrongly transparency strongly photonic suppress suppress structure window: solar solar is absorption an able absorptionultrabroadband to strongly suppress solar absorption whilewhile enhancing enhancingperformance, thermal thermal shown emission emission inwhile Fig. in in the enhancing Figure the atmospheric atmospheric 1(b), thermal capable transparency transparency emission of achieving in window: window: the atmospheric a net an an coolingultrabroadbandultrabroadband transparency power exceeding window: an ultrabroadband 2 performance,performance,100W shown/ shownm at in ambient in Fig. Fig. Figure temperature. Figureperformance, 1(b), 1(b), capable capable shown of of in achieving achieving Fig. Figure a a net net 1(b), cooling cooling capable power power of exceeding achieving exceeding a net cooling power exceeding 100100WW/m/m2 2atatRadiative ambient ambient temperature. cooling temperature. devices100W/ operatem2 at ambient at near-ambient temperature. temperatures and therefore do not suffer from RadiativeRadiativemany cooling cooling of the devices difficulties devices operate operate associatedRadiative at at near-ambient near-ambient with cooling other temperatures devices temperatures thermal operate applications and and at therefore near-ambienttherefore of do photonic do not not temperatures suffer suffer structures from from and which therefore do not suffer from manymany of ofhave the the difficulties typically difficulties involved associated associated highmany with temperature with of other the other difficulties thermal thermal operation. applications associatedapplications These difficulties with of of photonic photonic other include thermal structures structures numerical applications which which uncertainty of photonic structures which havehave typically typicallyassociated involved involved with high thehigh temperature temperature-dependence temperaturehave typically operation. operation. involved These These of high optical difficulties difficulties temperature properties, include include operation. material numerical numerical cohesion,These uncertainty uncertainty difficulties small-feature include numerical uncertainty 30 associatedassociatedevaporation, with with the the temperature-dependence temperature-dependence and durabilityassociated that affect with of of optical otherthe optical temperature-dependence thermal properties, properties, applications. material material cohesion, cohesion, ofOur optical approach small-feature small-feature properties, and design material is cohesion, small-feature evaporation,evaporation,thus and a and departure durability durability from that that previousevaporation, affect affect other other work thermal and thermal in durability using applications. applications. nano- that and affect30 micro-photonic30OurOur other approach approach thermal and structures and applications. design design for is is thermal30 Our approach and design is thusthus a a departure departureapplications. from from previous previousthus work work a in departure in using using nano- nano- from and previous and micro-photonic micro-photonic work in using structures structures nano- for and for thermal micro-photonic thermal structures for thermal applications.applications.To begin our analysis,applications. we consider a photonic structure at temperature T whose radiative ToTo begin beginproperties our our analysis, are analysis, described we we consider by considerTo a spectral begin a a photonic photonic our and analysis, angular structure structure emissivity we at consider at temperature temperaturee(l, aq photonic). TheTTwhose structurewhose structure radiative radiative is exposed at temperature to a T whose radiative propertiespropertiesclear are are described sky described subject by by to a solar a spectral spectralproperties irradiance, and and angular are angular and described emissivity also emissivity atmospheric by ae spectral(el(l,q,)q irradiance.) The. and The structure angular structure corresponding emissivity is is exposed exposede to to(l anto a,q a ambient). The structure is exposed to a clearclear sky skytemperature subject subject to to solar solarTamb irradiance,. irradiance, The netclear cooling and sky and also subject also power atmospheric atmospheric toPnet solar(T) irradiance, irradianceof irradiance a structure andcorresponding corresponding with also area atmosphericA tois to givenan an ambient irradiance ambient by corresponding to an ambient Power balance equation temperaturetemperatureTambTamb. The. 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,
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