In this illustration a
panel coated with a multilayered material designed by Stanford engineers
helps cool buildings without air conditioning. The material works in
two ways. It reflects incoming sunlight [yellow] that would otherwise
heat the panel. More importantly, it sends heat from inside the
structure directly into space as infrared radiation of a particular
wavelength [red]. The result is a cooler [blue] roof.
Conventional cooling is all about moving heat from a place
where you don’t want it to a place that you care about slightly
less. Your refrigerator, for example, cools itself by pumping heat into
your house. Your house cools itself by pumping heat into the outdoors.
It takes a significant amount of energy to keep this up—15 percent of
the energy consumption of most buildings is spent just on air
conditioning—meaning that the work put into transferring the heat
generates even more heat. And then it’s not like the heat just
vanishes when it gets outside: in urban areas, all of this waste heat
builds up to increase local temperatures as part of the urban heat
island effect.
Radiative cooling is a way of passively moving heat from one place to
another through thermal radiation, without the need for any additional
energy (like electricity). If you have a hot thing, it will radiate its
heat into whatever cooler thing is most convenient. In your house, this
is probably the air outside, and in your car, it’s also the air outside,
by way of the water in your radiator.
Since the general approach here is to use the atmosphere as the
final heat sink, radiative cooling doesn’t work if you’re trying to end
up at a temperature lower than the ambient temperature outside, which is why completely passive air conditioning isn’t a thing.
The clever thing about the passive radiative cooling system that
Stanford came with is that it skips the atmosphere completely, and uses the entire Universe
as a place to dump heat. The entire Universe, being mostly empty space,
has an average temperature of just under three Kelvin, meaning that
it’ll happily absorb just about as much heat as you can possibly throw
at it, making it a heat sink that’s nearly, you know, universal.
Photo: Norbert von der Groeben/Stanford EngineeringStanford electrical engineering professor Shanhui Fan
[center] gazes into the pizza-sized prototype with colleagues Linxiao
Zhu [left] and Aaswath Raman [right]. The high-tech mirror reflecting
their faces beams heat directly into space.
To use outer space as a heat sink, you need to have access to outer
space, which sounds like it’s probably a difficult thing to achieve. But
fundamentally, it just means being able to transfer heat straight
through Earth’s atmosphere. Stanford’s cooling system emits thermal
radiation in a very specific infrared wavelength that the Earth’s
atmosphere is completely transparent to, between 8 and 13 micrometers.
So, this is great, but the other part of the problem with radiative
cooling is that we really need it to work during the day, when the sun
is out and it’s hot. But if the sun is warming the radiator more than
the radiator can cool itself, the system isn’t going to accomplish much.
Stanford’s radiator also functions as a mirror that can reflect 97
percent of incident sunlight, enabling the radiator to cool itself (or
something underneath it) by up to five degrees Celsius even during the
heat of the day. In a three-story commercial building with a 1600 square meter roof, using the radiative cooler would save an estimated 118,500 kWh annually, the engineers calculate.
The radiator itself is composed of seven layers of silicon dioxide
and hafnium oxide on top of a thin layer of silver. The structure has
been tuned to only radiate at the specific infrared wavelengths that can
pass through the atmosphere. It’s just 1.8 microns thick in total, and
the researchers say that it can be fabricated at production scales in
existing facilities. Otherwise, the only remaining issue is to figure
out how to conduct the heat from inside a building through to the
exterior walls, to where the radiator could do its job.
These problems both seem surmountable, and even surmountable in the
near future, as opposed to the “five to ten years” void that many
technologies like this fall into. If this radiative cooler material can
in fact be produced inexpensively and efficiently, it could have a
significant impact on energy usage, especially in the developing world
where off-grid cooling is often the only option in rural areas.
Green roofs offer a lot of environmental benefits – they provide
additional insulation, reduce rainwater runoff, and can lower your
electricity bill. However a new study suggests that roofs painted white might actually be more effective at fighting climate change. A study published in the Energy and Buildings Journal
compared three types of roofs – green, black and white – and came to
the conclusion that white roofs have great economic benefits, and they
are also three times more effective than the other two at fighting
climate change.
Researchers at the Lawrence Berkeley National Laboratory
conducted an economic analysis of the costs and benefits of white,
black and green roofs and found that white roofs are far superior in
fighting climate change than the other two. While roofs painted black
absorb heat and contribute to the urban heat island effect,
white roofs reflect the sunlight back into the atmosphere and help cool
down its lower parts. The study advises those concerned with global climate change
to choose white roofs, adding to a host of other studies in the past
decade that have allowed the “white roof movement” to gain momentum
across the United States. However, things are not as simple as they
seem.
A series of climate simulations
carried out by Mark Z. Jacobson and Ten Hoeve of Stanford University
showed some unexpected results. Despite their beneficial effects on the
lower parts of the atmosphere, white roofs decrease the temperature
difference half a mile above ground-a difference which drives cloud
formation and less clouds means more sunlight reaching the Earth’s
surface. This, among other issues like the impact on fossil fuel consumption
and summer cooling vs. winter heating gains, is still subject of
scientific debates. Meanwhile, it should also be noted that vegetated
roofs offer built-in storm water management mechanisms in addition to
some cooling benefits.
Although we are excited to find out how different roofing strategies
may affect climate change, one should be aware of the fact that these
investigations involve a wide spectrum of factors and potential
consequences far too complex for a hotheaded (pun intended) thumbs-up
verdict. + Energy and Buildings Journal + GATOR-GCMOM Environmental Model
Via Fast Co.Design, Huffington Post
