Future Energy Paints as a Scalable and Effective Radiative Cooling Technology for Buildings
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Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
Paints as a Scalable co-founder and chief scientific officer to outer space, so the surface sponta-
of SkyCool Systems, a startup commer- neously cools, even under strong sun-
and Effective cializing radiative cooling technolo- light. The passive operation and net
Radiative gies. Dr. Raman’s research interests cooling effect overcome the disadvan-
include nanophotonics, metamaterials, tages of active cooling methods. Since
Cooling Technology radiative heat transfer, and energy ap- the surfaces of buildings exchange
for Buildings plications, including radiative cooling. large amounts of heat with their envi-
His works have been published in lead- ronment as radiation, this makes
Jyotirmoy Mandal,1,*
ing journals such as Nature, Nature PDRC attractive for cooling buildings.
Yuan Yang,2 Nanfang Yu,2
Energy, Physical Review Letters, and
and Aaswath P. Raman1,*
Joule. Research on radiative cooling has a
rich history, with materials like poly-
Jyotirmoy Mandal completed his PhD
mers (e.g., poly(4-methyl-1-pentene)
in Applied Physics at Columbia Univer- Introduction
and poly(vinyl fluoride)), dielectrics
sity in the City of New York and is As climate change and global energy
(e.g., SiOX, ZnSe), polymer compos-
currently a Schmidt Science Fellow consumption manifest in rising global
ites, and paints investigated for their
and a postdoctoral researcher at Uni- temperatures and heat-islands, cool-
cooling properties since the 1960s.2,3
versity of California, Los Angeles. His ing living environments has become
In the last decade, the field has seen
research interests include low-cost op- an urgent challenge. In developed set-
a revival, with reported enhancements
tical designs for radiative cooling and tings, air conditioning of buildings
of earlier designs,4–6 and new pho-
solar heating, with a focus on applica- consumes energy, generates heat,
tonic7 and polymeric1 ones. While
tions in developing countries. and releases greenhouse gases, exac-
these are efficient at cooling, their util-
erbating cooling needs. In regions of
ity depends on the application. For
Yuan Yang is an associate professor in the world such as South Asia and
instance, photonic multilayer films,
the Department of Applied Physics sub-Saharan Africa, inadequate power
which can be tailored to have a high
and Applied Mathematics at Columbia infrastructure for cooling buildings
Rsolar and selective єLWIR, attain deep
University in the City of New York. His has led to rising casualties during sum-
sub-ambient temperatures useful for
research interests include electrochem- mers. Passive cooling technologies,
water-cooled HVAC systems, refrigera-
ical energy storage and thermal energy which are sustainable alternatives or
tors, and thermoelectric devices.7,8
management. Dr. Yang has published complements to active cooling
While these emerging applications
over 70 peer-reviewed journal articles, methods, can address these issues.
hold promise, cooling buildings re-
including in leading journals such as Here, we consider passive daytime
mains the largest application of radia-
Science, Proceedings of the National radiative cooling of building envelopes
tive cooling technologies. And
Academy of Sciences, Advanced Mate- and propose that white paints, which
although photonic designs have made
rials, and Joule. are well adapted for application on
strides in the area,7 white ‘‘cool-roof’’
buildings and moderately good at
paints and materials, which have a
Nanfang Yu is an associate professor radiative cooling, could be developed
modest Rsolar (0.8) and high єLWIR
in the Department of Applied Physics into highly efficient radiative coolers
(0.95), are currently the most widely
and Applied Mathematics at Columbia for buildings on a global scale.
used cooling approach for building en-
University in the City of New York. His
velopes. Given their inherent scalabil-
research interests include mid-infrared Passive Daytime Radiative Cooling
ity, with enhancements in Rsolar, paints
and far-infrared optics, metamaterials, Passive daytime radiative cooling
thus have the potential to become an
biophotonics, and biologically inspired (PDRC) involves the reflection of sun-
optimal solution for the radiative cool-
flat optics. Dr. Yu’s research has been light (wavelengths l 0.3–2.5 mm)
ing of buildings.
published in leading journals like and radiation of long-wave infrared
Science, Nature Materials, Nature Nano- (LWIR, l 8–13 mm) heat through the
technology, and Joule. respective atmospheric transmission Radiative Cooling Requirements
windows into outer space (Figure 1A). While reflective coatings on buildings
Aaswath P. Raman is an assistant When a surface under the sky has a suf- are known to reduce solar heating,
professor of Materials Science and En- ficiently high solar reflectance (Rsolar) PDRC technologies go further to
gineering at the University of Califor- and LWIR emittance (єLWIR),1 solar heat- achieve heat loss even under sunlight,
nia, Los Angeles (UCLA). He is also ing is outweighed by radiative heat loss potentially doubling the cooling energy
Joule 4, 1–7, July 15, 2020 ª 2020 Elsevier Inc. 1
Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
Figure 1. Radiative Cooling by a White Paint
(A) Schematic showing passive daytime radiative cooling by solar reflection and LWIR thermal emission through the atmospheric transmission windows.
Corresponding solar and thermal spectra are shown below.
(B) Schematic showing how high solar reflectance and thermal emittance enable PDRC. It should be noted that at near-ambient temperatures, radiative
transfer ( = radiated heat from emitter – downwelling heat from sky) is small outside the LWIR window, making broadband and selective LWIR emitters
similarly effective at cooling.
(C) Schematic showing cooling powers ( = thermal emission – solar absorption) 1 of emissive coatings (єLWIR 0.95) as a function of R solar (or solar
reflectance index [SRI], which varies linearly with R solar ). R solar > 0.95 usually yields sub-ambient radiative cooling.
2 Joule 4, 1–7, July 15, 2020
Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
savings in buildings.9 From a physical ings, paints readily fulfil these practical stricts Rsolar toPlease cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
Figure 2. Optical Properties of Paints and Other Radiative Coolers
(A) Measured spectral reflectances of paints based on TiO2, Al 2 O3 , BaSO 4 , porous P(VdF-HFP) and PTFE, and silvered plastics (from top left to bottom
right). The paint films are all 1 mm thick.
(B) R solar of paints and silvered plastics. APOC 256X paint is rated by the Cool Roof Rating Council as the most reflective on the market. The dotted lines
show the reflectances of TiO 2 powder, spectralon (porous PTFE), and silver and represent the likely upper limits of R solar of TiO 2 -based, polymer-based,
and silvered radiative coolers, respectively. Note that the reflectances were weighted using ASTM G-173 solar spectrum, which assumes a solar zenith
angle of 48.19 . During summertime or in the tropics, the noon-time zenith angle is smaller. Consequently, UV light is stronger and further lowers R solar
of TiO 2 and silver-based designs.
(C) є LWIR of common building materials, paints, and silvered emitters.
In that case, the emittance є arises solely fewer C-H or O-H bonds, which absorb paint. Lastly, because fluoropolymers
from the porous polymer itself. sunlight at l 1.2, 1.4, 1.7, and 2.3 mm, have lower refractive indices (1.38–
and more C-F bonds, which weakly 1.43) than acrylics (1.495), they
The second is achievable by using fluo- absorb at 2.1 mm. Moreover, fluoro- enhance scattering by pigments and,
ropolymers such as P(VdF-HFP) or polymers absorb less UV than acrylic, consequently, Rsolar.
commercially available aqueous P(VdF) further enhancing Rsolar. The absorp-
variants. Compared to acrylic or sili- tance can be further lowered by The above alterations are compatible
cone, fluoropolymer variants have reducing the amount of polymer in the with paint design and can significantly
4 Joule 4, 1–7, July 15, 2020Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
improve Rsolar. Figures 2A and 2B show highly viable platform for the radiative Characterizing the weathering and
the results relative to reflectances of cooling of buildings at large scales. failure modes of paints with such mod-
TiO2-based white paints, a super-white ifications will be essential for adoption
PTFE-based reflectance standard Challenges and Opportunities by the construction industry.
(Spectralon SRM-99), and silvered emit- While paints hold the potential to
ters. Evidently, in the absence of achieve optimal optical parameters for (3) Reducing glare. While reflection
intrinsic UV absorption, scattering by radiatively cooling building envelopes, off white paints is diffuse and less
pigments results in high UV-blue reflec- challenges and questions remain. The intense than those off silvered de-
tance. Reducing polymer content yields major technical ones in our view are signs, it may harm eyesight and
similar results in the NIR wavelengths. listed below, with potential solutions heat dark structures in view. Coating
For the BaSO4 and porous P(VdF-HFP) (Figure 3). super-white paints with commercial
paint coatings, Rsolar reaches 0.98, high-index (n 1.9) retroreflective
and for the Al2O3 and the PTFE-based (1) Maximizing Rsolar and єLWIR with spheres may address the issue.
paint coatings, it exceeds 0.94. These minimal use of material. Cost re- However, their impact on Rsolar and
reflectance values match or even mains a central challenge for any єLWIR remains unexplored.
exceed those of previously reported radiative cooling technology,
radiative coolers and, along with the including paints, where higher mate- (4) Color as an aesthetic requirement
high, broadband є (Figure 2C), puts rial costs could potentially be a and solution to glare. The industry
paints on par with state-of-the-art roadblock. To mitigate this, and has used selectively visible-absorp-
PDRC designs. reduce material usage, we note tive colorants to create NIR-reflective
that high єLWIR could be achieved paints. Recent innovations like fluo-
by intrinsically emissive pigments rescent pigments that convert visible
Complementing PDRC: Paints as a
with specific microscale sizes, or absorption to NIR emission,11 and
Mature Technology
The high PDRC potential of white paints coating paints on emissive sub- the bilayer design discussed above,
is complemented by their generally low strates. A high Rsolar could be where a selectively visible absorbing
cost and ease of application on a broad achieved by incorporating air voids colorant is painted atop a broadband
range of surfaces. Furthermore, PDRC (n 1) in paints to increase optical solar scattering layer, could be used
applications of paints can leverage ad- scattering. Another possibility is to maximize the cooling performance
vancements made by the coatings in- bilayer designs that exploit the shal- while achieving color.
dustry in chemical engineering for lower penetration by shorter solar
higher durability. Examples include wavelengths in paint coatings (Fig- (5) In view of large-scale applica-
coatings based on silicone, fluoropoly- ure 3). A thin layer of UV-reflective tions, reducing the environmental
mer, and cross-linkable binders that paint (Figure 2A) could be coated impact of paints. Currently, paints
are resistant to UV damage and weath- on a TiO2 paint film, affording the often use environmentally hazard-
ering and remain stable under the sky high scattering efficiency of TiO2 ous pigments that eventually ‘‘run
for years. pigments while reflecting UV light off’’ into the environment or, for
from the top. porous polymeric paints, may use
Due to their long-standing usage, toxic solvents like acetone.1 Substi-
paints also have a significant advantage (2) Durability and resistance to soiling. tution of such materials with eco-
of being a part of buildings-related en- Many conventional white paints, while friendly ones (e.g., water-based flu-
ergy policies worldwide. In the U.S., cit- engineered for durability, experience oropolymer variants), as well as
ies like New York and states like Califor- drops in solar reflectance over time. enhancing the durability of paints
nia have implemented policies favoring Materials such as fluoropolymer- to reduce their usage and release
reflective coatings for buildings. Similar based binders could enhance reflec- into the environment, would make
policies exist in global hotspots such as tance lifetime and thereby lower paints more sustainable.
West and South Asia. Such policies may year-averaged costs. Soiling poses a
account for PDRC-capable paints as a challenge for all PDRC technologies Additionally, we propose three broader
natural extension of existing cool-roof as it reduces solar reflectance. Thus, challenges:
standards and immediately expand designs that are resistant to soiling,
their reach. Given these attributes and such as hydrophobic, biofouling-resis- (1) Mapping the global geograph-
the optical performance achievable, tant topcoats that can withstand phys- ical scope of radiative cooling
paints emerge as a compelling and ical cleaning, could maintain cooling paints, beyond which cold climates
performance and lengthen lifetime. cause PDRC to increase annual
Joule 4, 1–7, July 15, 2020 5Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Figure 3. Some Issues Concerning Paints as a Radiative Cooling Technology for Use on a Global Scale
Addressing these issues requires collaboration across a wide range of disciplines, including optics, materials science, meteorology, and policy. The
heatmap on the bottom right was obtained from the Climate Impact Lab website.
energy usage in buildings.9 Among tool,12 where a fraction of roofs across radiative coolers. Many regions
factors to be considered are clouds the world are painted to raise the that stand to greatly benefit from
and anthropogenic particulates earth’s albedo and reduce the climate radiative cooling technologies also
(see below), which can be transient impact of air conditioning by cooling see high airborne particulate and
and hinder both solar and LWIR the local environment,9 while pre- pollutant levels (e.g., South Asia;
transmission, as well as future varia- venting weather disruptions that may Figure 3). With the growing use of
tions of meteorological variables arise from large-scale, centralized PDRC technologies, evaluating the
with climate change. This can aid geoengineering. On a smaller scale, relation between PDRC perfor-
resource allocation by private and such an approach may also mitigate mance and pollution and dust
public sectors in the field. urban heat-island effects. (both airborne and settled on
PDRC designs), and potential miti-
(2) Exploration of super-white paints (3) Studying the effect of pollution gation strategies (e.g., covering
as a ‘‘distributed geoengineering’’ and dust on the performance of loose soil with vegetation, filtering
6 Joule 4, 1–7, July 15, 2020Please cite this article in press as: Mandal et al., Paints as a Scalable and Effective Radiative Cooling Technology for Buildings, Joule (2020),
https://doi.org/10.1016/j.joule.2020.04.010
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Future Energy
industrial emissions), can inform DECLARATION OF INTERESTS hybrid metamaterial for daytime radiative
cooling. Science 355, 1062–1066.
policymaking in those regions. A patent (PCT/US2016/038190) has
Related studies for photovoltaic been granted, and a provisional patent 7. Raman, A.P., Anoma, M.A., Zhu, L., Rephaeli,
E., and Fan, S. (2014). Passive
panels could be a useful guide in (U.S. 62/596,145) filed related to prior radiative cooling below ambient air
this regard. works cited in the paper. A patent temperature under direct sunlight. Nature
515, 540–544.
(U.S. 62/980,998) has been filed related
Interdisciplinary Science to Address to this work. A.P.R. is a founder of Sky- 8. Raman, A.P., Li, W., and Fan, S. (2019).
the Urgent Global Cooling Need Generating Light from Darkness. Joule 3,
Cool Systems Inc., its chief scientific of- 2679–2686.
Holistically addressing the above chal-
ficer, and a member of its board. N.Y. is
lenges requires a convergence of 9. Baniassadi, A., Sailor, D.J., and Ban-Weiss,
a founder of MetaRe. G.A. (2019). Potential energy and climate
expertise in fields like optics, materials benefits of super-cool materials as a rooftop
science, and meteorology. Given the strategy. Urban Climate 29, 100495.
intensifying global need for cooling hu-
1. Mandal, J., Fu, Y., Overvig, A.C., Jia, M., Sun, 10. Levinson, R., and Akbari, H. (2010). Potential
man environments and associated K., Shi, N.N., Zhou, H., Xiao, X., Yu, N., and benefits of cool roofs on commercial
climate issues, an interdisciplinary Yang, Y. (2018). Hierarchically porous buildings: conserving energy, saving
polymer coatings for highly efficient passive money, and reducing emission of greenhouse
approach to improve the already daytime radiative cooling. Science 362, gases and air pollutants. Energy Efficiency 3,
deployable solution in paints is perhaps 315–319. 53–109.
the most practical way forward. While 2. Sun, X., Sun, Y., Zhou, Z., Alam, M.A., and 11. Berdahl, P., Boocock, S.K., Chan, G.C.-Y.,
this piece highlights the exciting poten- Bermel, P. (2017). Radiative sky cooling: Chen, S.S., Levinson, R.M., and Zalich, M.A.
fundamental physics, materials, structures, (2018). High quantum yield of the Egyptian
tial of paint coatings as passive daytime and applications. Nanophotonics 6, 997– blue family of infrared phosphors
radiative coolers, we hope that it will 1015. (MCuSi4O10, M = Ca, Sr, Ba). J. Appl. Phys.
123, 193103.
spur further research that will establish 3. Zhao, D., Aili, A., Zhai, Y., Xu, S., Tan, G.,
Yin, X., and Yang, R. (2019). Radiative sky
super-white paints as a standard cooling: Fundamental principles, materials,
12. Munday, J.N. (2019). Tackling Climate
Change through Radiative Cooling. Joule 3,
approach for radiative cooling of build- and applications. Appl. Phys. Rev. 6, 2057–2060.
021306.
ings worldwide. 1Department of Materials Science and
4. Yu, N., Mandal, J., Overvig, A., and Shi, N. Engineering, University of California, 410
(2016). Systems and Methods for Radiative Westwood Plaza, Engineering V, Los Angeles, CA
ACKNOWLEDGMENTS Cooling and Heating. https://patentscope. 90095, USA
wipo.int/search/en/detail.jsf?
J.M. acknowledges support from docId=WO2016205717. 2Department of Applied Physics and Applied
Schmidt Science Fellows, in partnership Mathematics, Columbia University in the City of
5. Gentle, A.R., and Smith, G.B. (2010). New York, 500 West 120th St, Mudd 200, New
with the Rhodes Trust, and thanks Profes- Radiative heat pumping from the Earth using York, NY 10027, USA
sor Sir Keith Burnett for his valuable feed- surface phonon resonant nanoparticles.
Nano Lett. 10, 373–379. *Correspondence:
back on the work. A.P.R. acknowledges jyotirmoymandal@ucla.edu (J.M.),
6. Zhai, Y., Ma, Y., David, S.N., Zhao, D., Lou, R., aaswath@ucla.edu (A.P.R.)
support from the Sloan Research Fellow- Tan, G., Yang, R., and Yin, X. (2017). Scalable-
https://doi.org/10.1016/j.joule.2020.04.010
ship (Alfred P. Sloan Foundation). manufactured randomized glass-polymer
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