Passive Radiative Cooling Development for Use in Built Environment

Christopher ChaoHead & Chair Professor of Mechanical and Aerospace Engineering

The Hong Kong University of Science and Technology

2

Thermal comfort is currently well-controlled by HVAC, but this ACTIVE approach has high energy

consumption. Any innovative and PASSIVE approach?

Radiative cooling is the process of heat removal from a sky-facing surface to the universe through radiation, cooling the surface below ambient temperature.

Natural Phenomenon/Bio-inspired Frost on leaves

Frost on vehicles

Funding:• CRF (C6022-16GF): Study of Cooling Effect by Surface Treatment and its Application to Smart Green Buildings (~7M)

Led by CYH Chao (HKUST)• ITSP (ITS/013/16): Development of a high-performance passive radiative cooler for buildings (~1.4M)

Sahara Silver Ants

To achieve daytime cooling:High solar reflection;Eliminating heat via radiative emissionthrough the atmospheric window.

0.4~2 μm

8~13 μm

Ideal Passive Radiative Cooler Reflectivity & Emissivity

e.g. SiO2 + HfO2 or SiO2 + PDMS

Metals: Ag or Au or Cu Photonic/Polymer Structure

Photonic & Polymer-based Passive Radiative Coolers

Plasmonic Passive Radiative CoolersSurface plasmon confined at metal-dielectricinterfaces, a collective resonance oscillation ofcharges induced by light, can lead to extremelyhigh local photon density of states (up to 10 –1,000 times enhancement) at certain energylevel, leading to much improvedemissivity/absorptivity within these resonancebands in a small regime with minor influence onlights in other bands.

Size of particles < visible λ Plasmonics phenomenon

Photonic Plasmonic PolymerMechanism Selective thermal

emission executed by interatomic vibration of constituent atoms

Selective thermal emission executed by

surface plasmonpolaritons

Selective thermalemission executed by interatomic vibration of constituent atoms

Materials SiO2 +HfO2/TiO2/Si3N4

Al, Ge, Ag, SiNx, Al2O3 PDMS + SiO2 orPMP + SiO2

Zhai et al. Science, 355 (2017)

Raman et al. Nature, 515 (2014)

Raman, A. P., et al. (2014). Nature, 515(7528), 540-4.

C.Y. Tso, K.C. Chan and C.Y.H. Chao, (2017) Renewable Energy, 106 52-61.

Tested under hot and humid climate (Hong Kong)

Vacuum Design (Revised) Rooftop of the CYT building at the HKUST

Results:Cooling Effects during the daytime, but poor in cloudy days

C. G. Granqvist, A. Hjortsberg, J. Appl. Phys. 52, 4205, 1981

Atmospheric thermal emission

Atmospheric window

High emission/Low transmission

Low emission/High transmission

Ideal

In a highly humid environment,transparency of the atmosphericwindow is lost thermalemission within 8-13μmincreases.

Can we block the 8-13μm thermal radiation from the water vapour in the atmosphere to the radiative cooler?

Major Problem: Reduced cooling power under a humid climate (i.e. Hong Kong)

Before After

Permitting outgoing mid-infraredtransmission emitted by the radiativecooler (8-13 μm)

Reflecting incoming radiation of the 8-13 μm from the atmosphere

Forward transmission

Backward transmission

Over 80%

Below 40%

50W/m2

(+140% improvement)

Y.M. Wong, C.Y. Tso, B.L. Huang, M.P. Wan, C.Y.H. Chao, Solar Energy Materials and Solar Cells, (Under Review)

Nighttimeradiative cooling only (1975)

Daytime applicable (2014), but expensive

Daytime with low-cost cooler, but it is only successful under clear sky and low humid climate

Future direction: promising radiative coolers + asymmetric electro-magnetic transmission windows

Low Cost Multi-layer Photonic Radiative Cooler

Number of sub-layers: 8

Sub-layer thickness: 500 nm SiO2 + 500 nm TiO2

Total Thickness: 4 μm

Backing reflector material: Ag

Multi-layer SiO2-TiO2 Photonic Radiative CoolersMulti-layer SiO

Fabricated plasmonic structure

Advantages of plasmonics: Very strong selective absorption/emission in a smallregime, so that the structure can be quite thin and simple, which can reducethe fabrication cost.

The plasmonic structure can work as scattering elementsto couple and trap freely propagating incident photons infree space into the photon absorption regime. Thesecombined effects can lead to much stronger photonabsorption in a target spectral range.

Thtofrecoab

Our Preliminary FDTD (Finite-difference time-domain) Simulation Result

Shi NN, et al., 2015. Keeping cool: Enhanced optical reflection and radiative heat dissipation in Saharan silver ants. Science, 349(6245), 298-301.

Saharan Silver Ant (Cataglyphis Bombycina)Inhabit Saharan Desert;Endures strong sunlight (i.e. high temperature);Forages during the hottest time of the day;Covered by the triangular prism shaped hair.

Thermoregulatory effect of the hair• Enhanced reflectivity in visible & NIR

With Hair: reflects 67% of the solar radiationWithout Hair: only 41%

• Enhanced emissivity in MIRHair cover enhances by about 15%

Triangular structure shows a big difference from the other two geometrical structures

FDTD Simulation to optimize emissivity of biomimetic polymer-basedradiative cooler

Field Test

Parameters for optimization:• Thickness of substrate• Geometry (i.e. triangle, circular, flat

plane, etc.)• Size of geometrical structure• No. of surface layers

Photonic Plasmonic Polymer with Rectangular

Shape

Polymer with Triangular Shape

(Biomimetic thermal selective radiative cooler)

Performance Acceptable Excellent Good Very Good

ManufacturingDifficulty

Medium Medium Easy Easy

Material Cost Medium High Low Low

Fabrication Cost Highest High Low Medium

Five Unknown Temperatures:: Air temp. inside the room

: Cold air temp.: Hot water temp.: Cold water temp.: Radiative cooler temp.

2.4 kW @310 K

1.6 kW @300 K