Ronggui YANG, et al.
A team of University of Colorado Boulder engineers has developed a
scalable manufactured metamaterial — an engineered material with
extraordinary properties not found in nature — to act as a kind of
air conditioning system for structures. It has the ability to cool
objects even under direct sunlight with zero energy and water
When applied to a surface, the metamaterial film cools the object
underneath by efficiently reflecting incoming solar energy back
into space while simultaneously allowing the surface to shed its
own heat in the form of infrared thermal radiation.
The new material, which is described today in the journal Science,
could provide an eco-friendly means of supplementary cooling for
thermoelectric power plants, which currently require large amounts
of water and electricity to maintain the operating temperatures of
The researchers’ glass-polymer hybrid material measures just 50
micrometers thick — slightly thicker than the aluminum foil found
in a kitchen — and can be manufactured economically on rolls,
making it a potentially viable large-scale technology for both
residential and commercial applications.
“We feel that this low-cost manufacturing process will be
transformative for real-world applications of this radiative
cooling technology,” said Xiaobo Yin, co-director of the research
and an assistant professor who holds dual appointments in CU
Boulder’s Department of Mechanical Engineering and the Materials
Science and Engineering Program. Yin received DARPA’s Young
Faculty Award in 2015.
The material takes advantage of passive radiative cooling, the
process by which objects naturally shed heat in the form of
infrared radiation, without consuming energy. Thermal radiation
provides some natural nighttime cooling and is used for
residential cooling in some areas, but daytime cooling has
historically been more of a challenge. For a structure exposed to
sunlight, even a small amount of directly-absorbed solar energy is
enough to negate passive radiation.
The challenge for the CU Boulder researchers, then, was to create
a material that could provide a one-two punch: reflect any
incoming solar rays back into the atmosphere while still providing
a means of escape for infrared radiation. To solve this, the
researchers embedded visibly-scattering but infrared-radiant glass
microspheres into a polymer film. They then added a thin silver
coating underneath in order to achieve maximum spectral
“Both the glass-polymer metamaterial formation and the silver
coating are manufactured at scale on roll-to-roll processes,”
added Ronggui Yang, also a professor of mechanical engineering and
a Fellow of the American Society of Mechanical Engineers.
“Just 10 to 20 square meters of this material on the rooftop
could nicely cool down a single-family house in summer,” said Gang
Tan, an associate professor in the University of Wyoming’s
Department of Civil and Architectural Engineering and a co-author
of the paper.
In addition to being useful for cooling of buildings and power
plants, the material could also help improve the efficiency and
lifetime of solar panels. In direct sunlight, panels can overheat
to temperatures that hamper their ability to convert solar rays
“Just by applying this material to the surface of a solar panel,
we can cool the panel and recover an additional one to two percent
of solar efficiency,” said Yin. “That makes a big difference at
The engineers have applied for a patent for the technology and are
working with CU Boulder’s Technology Transfer Office to explore
potential commercial applications. They plan to create a
200-square-meter “cooling farm” prototype in Boulder in 2017.
The invention is the result of a $3 million grant awarded in 2015
to Yang, Yin and Tang by the Energy Department’s Advanced Research
Projects Agency-Energy (ARPA-E).
“The key advantage of this technology is that it works 24/7 with
no electricity or water usage,” said Yang “We’re excited about the
opportunity to explore potential uses in the power industry,
aerospace, agriculture and more.”
Co-authors of the new research include Yao Zhai, Yaoguang Ma and
Dongliang Zhao of CU Boulder’s Department of Mechanical
Engineering; Sabrina David of CU’s Materials Science and
Engineering Program; and Runnan Lou of the Ann and H.J. Smead
Department of Aerospace Engineering Sciences.
The research team, led by Principal Investigator Ronggui Yang
(right) and Co-Principle Investigator Xiaobo Yin (left), will
develop a system called RadiCold that if successful will enable
efficient, low-cost cooling for thermoelectric power generation.
Science ( 09 Feb 2017 )
glass-polymer hybrid metamaterial for daytime radiative
Yao Zhai, Yaoguang Ma, Sabrina N. David, Dongliang Zhao,
Runnan Lou, Gang Tan, Ronggui Yang, Xiaobo Yin
Passive radiative cooling draws heat from surfaces and radiates it
into space as infrared radiation to which the atmosphere is
transparent. However, the energy density mismatch between solar
irradiance and the low infrared radiation flux from a
near-ambient-temperature surface require materials that strongly
emit thermal energy and barely absorb sunlight. We embedded
resonant polar dielectric microspheres randomly in a polymeric
matrix, resulting in a metamaterial that is fully transparent to
the solar spectrum while having an infrared emissivity greater
than 0.93 across the atmospheric window. When backed with silver
coating, the metamaterial shows a noon-time radiative cooling
power of 93 W/m2 under direct sunshine. More critically, we
demonstrated high-throughput, economical roll-to-roll
manufacturing of the metamaterial, vital for promoting radiative
cooling as a viable energy technology.
CU-Boulder awarded $3 million for
transformational power plant cooling technology
The University of Colorado Boulder has received a $3 million
federal grant to develop cooling technology that will enable
efficient, low-cost supplementary cooling for thermoelectric power
The grant spans three years and is from the U.S. Department of
Energy’s Advanced Research Projects Agency-Energy (ARPA-E).
The CU-Boulder research team, led by Ronggui Yang, associate
professor of mechanical engineering in the College of Engineering
and Applied Science, will develop cold storage modules and a
system called RadiCold that cools by infrared thermal emission to
enable efficient, low-cost supplementary cooling for
thermoelectric power generation.
If successful, CU-Boulder’s design could provide power plant
operators a low-cost way to supplement cooling without using as
much water as they do now.
“I am confident that we will be successful in developing this
novel cooling technology that could be useful for both power
plants and buildings,” said Yang.
In thermoelectric power generation, only 40 percent of the energy
in the fuel is used for power generation. The remaining 60 percent
becomes low-grade heat that needs to be carried away by cooling
There are two types of cooling systems: wet and dry. Wet-cooling
systems use water resources such as a river, lake or ocean and
pass it directly over tubes containing condenser water, and then
return it, warmer, to the original source. Dry-cooling systems use
air to cool condenser water.
Most U.S. power plants use wet-cooling technologies because water
can cool better than air, which allows power plants to operate
In fact, thermo-electric power plants are among the biggest
consumers of fresh water in the world. Forty-one percent of total
fresh water withdrawal - about 139 billion gallons per day - is
used to cool condenser water. Three percent of the cooling water
is evaporated and lost. This has an enormous environmental impact,
especially in areas already suffering from fresh water shortages.
These systems also release heat waste into the environment, which
adversely affects wildlife, said Marta Zgagacz, of the University
of Colorado’s Office of Technology Transfer and part of the team
that will evaluate the commercialization potential of this
Researchers say dry cooling has the potential to significantly
reduce water consumption, but the high cost and low efficiency of
current technologies discourage their use.
Improved air-cooled heat exchangers can help overcome these
challenges. Since air-cooled heat exchangers can only cool water
temperatures as low as the surrounding temperature, supplemental
cooling technologies - such as RadiCold - are needed to further
decrease water temperatures in certain conditions.
Methods to cool a building roof by sending long-wavelength
infrared light into the dark night sky have been known for a long
time. However, cooling under direct sunshine, and more critically,
manufacturing these cooling systems in a scalable and
cost-effective way are areas ripe for research, said Co-Principle
Investigator Xiaobo Yin, an assistant professor in both mechanical
engineering and in the materials science and engineering program.
A RadiCold surface, which is a metal-coated micro-structured
polymer, reflects sunlight and allows radiative cooling through
infrared thermal emission for both day- and night-time power plant
Using the new system, a passive zero-energy consumption thermal
syphon will collect cold water in a local storage unit beneath the
RadiCold surface while a low power consumption pipe network
collects the cold water from local storage modules into a central
storage system that can be used to cool power plant condensers.
Roll-to-roll manufacturing technology will enable effective
radiative cooling at a low cost.
“I am excited to work with my colleagues at CU-Boulder to
transform innovative materials and component research into
engineering systems,” said Gang Tan, assistant professor in the
Department of Civil and Architectural Engineering at the
University of Wyoming. “I also foresee great potential in building
energy savings by developing cooling roof and ceiling systems
using RadiCold surfaces.”
In addition to these senior researchers, the team will include
three post-doctoral research associates, three doctoral students
and a few undergraduate students. Two MBA students from the
CU-Boulder Leeds School of Business will work closely with the
team on technology to market analysis.
Associate Professor of Strategy and Entrepreneurship Tony Tong
from the CU-Boulder Leeds School of Business is also part of the
team that will evaluate the commercialization potential of this
ARPA-E is an agency within the U.S. Department of Energy that
invests in disruptive ideas to create America’s future energy
technologies. For more information on ARPA-E and its innovative
project portfolio, please visit http://www.arpa-e.energy.gov/.
Ronggui Yang, 303-735-1003
Xiaobo Yin, 303-492-9689
Julie Poppen, CU-Boulder media relations, 303-492-4007
Radiative Cooling and Cold Storage
Radiative Cooled-Cold Storage Modules and Systems (RadiCold)
Critical Need: In thermoelectric power generation, only about 40%
of the energy in the fuel is converted into electricity. In other
words, the power plant operates at about 40% efficiency. The
remainder of the energy is converted to low-grade waste heat that
must be removed to maintain the power plant's efficiency. Most
power plants use water from nearby rivers, lakes, or the ocean for
cooling. The water may pass directly over tubes containing the
plant's heated condenser water, and then be returned, warmer, to
the original source, or it may be evaporated to carry off the heat
in water vapor. In areas with limited water or under drought
conditions, dry-cooling systems use air to remove heat from the
plant's condenser water. However, present dry-cooling technology
reduces the power plant's efficiency and requires costly
equipment. With water supplies becoming increasingly strained in
many areas, economical dry-cooling approaches that do not reduce
the efficiency of power plans are critically needed. Innovative
methods to allow cooling below the daytime ambient air temperature
and improve heat exchange between air and the plant's
recirculating condenser water will provide the keys to ensuring
the continued efficiency of power generation while decreasing the
burden on water supplies.
Project Innovation + Advantages: Researchers from the University
of Colorado at Boulder (CU-Boulder) will develop Radicold, a
radiative cooling and cold water storage system to enable
supplemental cooling for thermoelectric power plants. In the
Radicold system, condenser water circulates through a series of
pipes and passes under a number of cooling modules before it is
sent to the central water storage unit. Each cooling module
consists of a novel radiative-cooling surface integrated on top of
a thermosiphon, thereby simultaneously cooling the water and
eliminating the need for a pump to circulate it. The
microstructured polymer film discharges heat from the water by
radiating in the infrared through the Earth's atmosphere into the
heat sink of cold, deep space. Below the film, a metal film
reflects all incoming sunlight. This results in cooling with a
heat flux of more than 100 W/m2 during both day and nighttime
operation. CU-Boulder will use roll-to-roll manufacturing, a
low-cost manufacturing technique that is capable of high-volume
production, to fabricate the microstructured RadiCold film.
Potential Impact: If successful, CU-Boulder's design could provide
power plant operators a low-cost way to supplement cooling without
consuming additional water.
Security: Power plants can maintain energy efficiency by using the
team's dry-cooling technology instead of water cooling when water
use is restricted.
Environment: The team's system enables more efficient radiative
cooling - eliminating the need for additional water or power
inputs to cool power plant condenser water.
Economy: By applying low-cost manufacturing techniques, CU-Boulder
estimates the structure will be an economical option for dry
THERMAL GROUND PLANE
[ PDF ]
Inventor(s): YANG RONGGUI, et al.
Methods, apparatuses, and systems are disclosed for flexible
thermal ground planes. A flexible thermal ground plane may include
a support member. The flexible thermal ground plane may include an
evaporator region or multiple evaporator regions configured to
couple with the support member. The flexible thermal ground plane
may include a condenser region or multiple condenser regions
configured to couple with the support member. The evaporator and
condenser region may include a microwicking structure. The
evaporator and condenser region may include a nanowicking
structure coupled with the micro-wicking structure, where the
nanowicking structure includes nanorods. The evaporator and
condenser region may include a nanomesh coupled with the nanorods
and/or the microwicking structure. Some embodiments may include a
micromesh coupled with the nanorods and/or the microwicking
This application relates generally to thermal ground planes. More
specifically, this application relates to methods, apparatuses,
and systems for flexible thermal ground planes.
The complexity and size of integrated circuits may been limited by
the heat generated. Heat pipes have been used to transfer heat
efficiently from one location to another. They have also been used
to cool integrated circuits. The existing heat pipes for these
purposes may consist of a rigid structure composed of copper,
silicon, etc. Some modern electrical devices and systems demand a
flexible circuit board along with a high capacity for heat
There is thus a need for methods, systems, and devices that may
also be flexible while transferring heat efficiently from one
location to another or spread high flux heat from a small area to
low heat flux over a larger area.
Certain embodiments thus provide methods, systems, and devices
that may include a flexible thermal ground plane. Embodiments of
flexible thermal ground planes may provide extremely high thermal
performance with high evaporation/condensation heat transfer and
effective liquid supply. Flexible configurations may be enabled by
using polymer casing laminated and covered by moisture barrier
coatings, enabled by atomic layer deposition, chemical vapor
deposition, physical vapor deposition, or thin metal laminate,
merely by way of example. Embodiments of flexible thermal ground
planes may also involve low cost construction resulting from large
size manufacturing, e.g. 3 ft wide and 1000 ft long, merely by way
of example. For example, flexible thermal ground plane
construction may take advantage of flexible circuit board
manufacturing technology. Large size flexible thermal ground
planes may thus be constructed for some embodiments, e.g. 20 cm by
40 cm by 1 mm, merely by way of example...
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