Electrical engineering Professor Shanhui Fan (center) and graduate students Aaswath Raman (left) and Eden Rephaeli (right) have developed a solar cooling device that may be able to supply air conditioning without using electricity to poor and off-the-grid areas.
Such a
structure could vastly improve the daylight cooling of buildings, cars
and other structures by reflecting sunlight back into the chilly vacuum
of space.
"People usually see space as a source of heat from the sun, but away
from the sun outer space is really a cold, cold place," explained Shanhui Fan,
a professor of electrical engineering and the paper's senior author.
"We've developed a new type of structure that reflects the vast majority
of sunlight, while at the same time it sends heat into that coldness,
which cools manmade structures even in the daytime."
Before Moving into the Solar Structure part of the Topic, let's know some basics. Why do Buildings heat up? Most buildings are poor reflectors and absorb too much sunlight, heating up in the process. Also much of the structures including Buildings do not efficiently radiate heat back into space. To be Scientific, the structure
must emit thermal radiation very efficiently within a specific
wavelength range in which the atmosphere is nearly transparent. Outside
this range, the thermal radiation interacts with Earth's atmosphere.
Most people are familiar with this phenomenon. It's better known as the
greenhouse effect – the cause of global climate change.
Two goals in one
The new structure accomplishes both goals. It is an
effective broadband mirror for solar light – it reflects most of the
sunlight. It also emits thermal radiation very efficiently within the
crucial wavelength range needed to escape Earth's atmosphere. Radiative cooling at nighttime has been studied extensively as a
mitigation strategy for climate change, yet peak demand for cooling
occurs in the daytime. "No one had yet been able to surmount the challenges of daytime
radiative cooling –of cooling when the sun is shining," said Eden
Rephaeli, a doctoral candidate in Fan's lab and a co-first-author of the
paper. "It's a big hurdle."
The Stanford team has succeeded where others have come up short by
turning to nanostructured photonic materials. These materials can be
engineered to enhance or suppress light reflection in certain
wavelengths. "We've taken a very different approach compared to previous efforts
in this field," said Aaswath Raman, a doctoral candidate in Fan's lab
and a co-first-author of the paper. "We combine the thermal emitter and
solar reflector into one device, making it both higher performance and
much more robust and practically relevant. In particular, we're very
excited because this design makes viable both industrial-scale and
off-grid applications."
Using engineered nanophotonic materials, the team was able to
strongly suppress how much heat-inducing sunlight the panel absorbs,
while it radiates heat very efficiently in the key frequency range
necessary to escape Earth's atmosphere. The material is made of quartz
and silicon carbide, both very weak absorbers of sunlight.
Net cooling power
The new device, which the scientists had developed is capable of achieving a net cooling
power in excess of 100 watts per square meter. By comparison, today's
standard 10-percent-efficient solar panels generate about the same
amount of power. That means radiative cooling panels could
theoretically be substituted on rooftops where existing solar panels
feed electricity to air conditioning systems needed to cool the
building.
To put it a different way, a typical one-story, single-family house
with just 10 percent of its roof covered by radiative cooling panels
could offset 35 percent its entire air conditioning needs during the
hottest hours of the summer.
Radiative cooling has another profound advantage over other cooling
equipment, such as air conditioners. It is a passive technology. It
requires no energy. It has no moving parts. It is easy to maintain. You
put it on the roof or the sides of buildings and it starts working
immediately.
A changing vision of cooling
Beyond the commercial implications, Fan and his
collaborators foresee a broad potential social impact. Much of the human
population on Earth lives in sun-drenched regions huddled around the
equator. Electrical demand to drive air conditioners is skyrocketing in
these places, presenting an economic and environmental challenge. These
areas tend to be poor and the power necessary to drive cooling usually
means fossil-fuel power plants that compound the greenhouse gas problem.
"In addition to these regions, we can foresee applications for
radiative cooling in off-the-grid areas of the developing world where
air conditioning is not even possible at this time. There are large
numbers of people who could benefit from such systems," Fan said.
