
Electricity-free air conditioning - lcuff
http://www.economist.com/news/science-and-technology/21634992-new-materials-may-change-way-temperatures-are-regulated-cool-idea
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r0s
Existing passive solar air conditioning is underutilized.

Some ideas are very old, for example:
[http://en.wikipedia.org/wiki/Solar_chimney](http://en.wikipedia.org/wiki/Solar_chimney)

~~~
tomohawk
I can see how something like that would work in arid conditions, but in areas
where it is humid, it seems like you would get a lot of problems with mold and
dampness.

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kijin
None of the A/C alternatives I've seen on HN in the last couple of years even
try to tackle the problem of humidity. They only focus on lowering the
temperature, and sometimes even increase the humidity while doing so.

Maybe this has something to do with the fact that a lot of U.S. tech startups
are located in relatively arid parts of the country, such as the Bay Area. I'd
like to see what they can come up with if they were all relocated to Florida,
or even better, Southern China.

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mchannon
Not sure why everyone is trying to couple this to solar panels. The TL;DR for
the article is a material that is 97% reflective to incoming light and also a
heckuva radiative source. As such, it bounces away all the sunlight and
radiates additional energy out into the void. The international space station
uses a similar setup to achieve this- there's nowhere for waste heat in the
space station's solar panels to go (there's no air to accept it) so radiation
is the only method of dumping it. Nobody does this on Earth, because radiators
are heavy and take up a lot of space. It'd be far cheaper to just install more
solar panels.

Solar panels try to absorb all the light they can; that 10-20% is sunlight to
electricity, and most of the rest is absorbed as heat.

If you tried to combine the two by stacking, a reflector/radiator with an
absorber, you'd render one or both useless. The radiator would block the solar
panel's light, or the solar panel would block and reflect the radiated light
from the radiator. You could point the radiator down, which would cool the
solar panel a bit (making it more efficient), but you'd be heating up whatever
was beneath the solar panel. That'd be great in an open field but awful on a
rooftop.

This invention's biggest application is in the existing situations where you
want to dump heat. Air conditioners have large radiator sections (not sure why
they call them radiators because they do not radiate so much as conduct and
convect the heat away) which would be able to dump a lot more heat with a
properly oriented coating using this novel material (also potentially
eliminating blowers and fans).

I'd really like to see a graph of heat flux versus ambient temperature versus
area. If it took 1 square meter versus 100 square meters to dump, say, 1000
watts, that would change this from a curiosity to a breakthrough.

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aaron695
Very cool. I love the way computers are smashing technology forward.

But the article didn't answer the big question, why this over solar panels.

Grab the energy and turn it into electricity seems to make more sense, cool
the building while creating $.

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upofadown
Solar panels are only 10-20% efficient. The rest of the sunlight ends up as
heat which presumably might end up in the building. It would be difficult to
cover an entire sunward side of a building with solar panels anyway which is
what you would want to do with this technology should it ever be made
practical.

My question would be: why this over insulation?

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logfromblammo
You wouldn't just cover the sunward side of a building with the new material.
You would cover every surface that has an unobstructed view of the sky, from
any angle.

Furthermore, the surface could be corrugated instead of flat, to increase the
radiating surface area. The cooling would work even when radiating at a
reflection of the sky.

This would likely be used in addition to insulation. Your radiators would be
thermally connected to the building through a managed, thermally conductive
channel, so that the cooling effect would not continue throughout the winter,
for instance.

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upofadown
The outside surface would also be fairly strongly coupled to the ambient
environment due to convection. I suspect that coupling the outside surface
past the insulation would be a loss in any case where it was hot enough
outside to make people care about air conditioning.

~~~
logfromblammo
Couldn't you just put the whole thing behind a layer that does not absorb IR
or visible light, but is also a poor thermal conductor?

The article mentioned that they did that using an enclosed box to test the
material. It wouldn't be much additional effort to add a heat pipe that
transfers heat from the object to be cooled, through the box, to the radiator-
reflector. That way, you are cooling only what you want to cool rather than
the whole environment surrounding the device.

~~~
upofadown
I was thinking about that too. You would need one or more sheets of something
transparent to infrared 8-13 microns. Such materials apparently do exist:

[http://www.infraredtraininginstitute.com/infrared-
transparen...](http://www.infraredtraininginstitute.com/infrared-transparent-
materials/)

Such a material would also have to not absorb too much sunlight.

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jeroen94704
I am somewhat skeptical about this.

It sounds like they tried to apply thin-film optics for this material, given
their talk of layers that are both extremely thin and precisely defined. Thin-
film optics relies on interference effects between transmitted and reflected
light, and enables very specific optical behavior. For example, using thin
films it is possible to make a stack of layers that reflects a very specific,
narrow band of wavelenghts, while transmitting everything else. In real-life,
rewritable optical media such as CD-RW and DVD-RW make use of thin-film
optics.

The point though is that thin-film optics only works for coherent light, such
as laser light. Sunlight is not coherent, and you will not get the desired
interference effects, because the light waves are not "synchronized", as it
were.

So while I could be wrong, I wouldn't be surprised if the performance of this
reflective sheet is very low, and nowhere near what would be required to
replace air-con.

~~~
gallamine
Layers of films can be stacked to produce a broader absorption band. Since
they aren't concerned with transmission then essentially they can build as
many layers as they want to block the spectrum of interest (ignoring cost, of
course). Perhaps a larger concern is the dependence on thin-film interference
to the incident angle of the light.

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Mithaldu
> enabled the siesta-free working habits of the temperate regions

Frankly, that is an absolute downside.

I live in a very temperate region and having switched to freelancing i find
that the daily siesta makes me much more productive.

~~~
polshaw
Enabling is not enforcing. No downside.

~~~
Mithaldu
In practice it is enforced in my country to forego siesta, as long as you are
not self-employed.

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kwhitefoot
It would probably make more sense to use it to concentrate solar energy on to
solar panels. That plus a combination of better insulation, triple glazing,
and increased thermal mass would make better use of the energy. Many of the
buildings that have window air-conditioners like the one used as an
illustration in the Economist article are so badly designed from a thermal
perspective that they would not benefit much from such mirrors anyway. A lot
of the heat they suffer from is generated internally. Another way of dealing
with the problem is to use the heat pumps to dump the heat into water to use
for washing and process heat instead of just throwing it away. As far as I can
see the mirror is exactly the same idea as a window air-conditioner except
more efficient, that is, it simply moves the problem elsewhere. Still, as a
component in a system of measures to control temperature it sounds like a good
idea.

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qwerta
Solar panels are very expensive and can not be used on windows.

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VLM
"Solar panels are very expensive"

Times are a changing, $1/watt stereotypical 100 watt panel is about 2x4 feet.

A window of decent quality , somewhat energy efficient, of 2x4 foot size is
about $300.

TV's sell for about twice what a comparable surface area window costs. A 50
inch TV has about the same surface area as a 100 watt solar panel.

Aquariums cost about the same as windows (2ft by 4 ft front would probably be
about a 60 gallon, which is about $300)

Another way to put it, is if you're looking at glass, per square inch, the
cheapest (aside from picture frames) is solar panels, then windows and fish
tanks cost about 3 times as much as solar panels, then TVs cost about six
times as much as solar panels, per sq inch.

Back on topic you'd be far better off installing perhaps 3x to 10x the square
footage of solar panels than installing very expensive windows with this film.

At least theoretically you could apply the film to siding, maybe even roofs,
and have slightly cooler surfaces.

~~~
qwerta
> 2x4 foot size is about $300

2 micron thick coating probably costs $0.1 for the same size.

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nfriedly
My dad did a simpler version of this a few years ago: he used to live in a
trailer with a black roof; one summer he painted the roof silver and the
inside got noticeably cooler.

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swimfar
That's half of what is going on with this material. A silver/shiny roof will
reduce the amount of solar energy absorbed into the home. But it will also
reduce the heat which is radiated out of the house. So this method won't cool
the house it will just insulate it better from the sun. (Which will result in
cooler temperatures, though)

Something (e.g. a color, surface finish) which absorbs heat efficiently also
gives off heat efficiently and vice versa. That's why steel/iron motorcycle
cylinders are often painted black. The amount of heat absorbed by the sun is
much less than the amount of heat which can be radiated away.

What these researches are trying to do is optimize this reflection/absorption
ratio for the various light/infra-red frequencies. Pretty cool idea, I think.

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p1mrx
This reminds me of the technology that generates electricity by emitting
photons at night. You might think it were a joke if it weren't from Harvard:

[http://www.seas.harvard.edu/news/2014/03/infrared-new-
renewa...](http://www.seas.harvard.edu/news/2014/03/infrared-new-renewable-
energy-source)

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awjr
So this material reflects light at a specific frequency. Could this type of
reflection be used to make solar panels more efficient?

I can see this material being useful on roof terraces but in high rise
buildings less so.

However I still don't understand why people would use this over a solar panel
setup (cost I would assume).

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tomohawk
I wonder how it compares to this:

[http://www.jetsongreen.com/2009/06/desert-modern-rimrock-
ran...](http://www.jetsongreen.com/2009/06/desert-modern-rimrock-ranch-
house.html)

They actually had to get special approval to downsize the AC units.

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hasenj
I don't get it. How is this different from going under a shade?

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upofadown
A shade heats up and radiates long wave radiation from the back side. This
would keep the shade material cool.

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frownie
what if there is water condensating on the panel ? what if the panel is not
perfectly oriented towards the sun ? (dunno, just asking)

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icegreentea
Don't know what happens if there's condensation. As for orientation, here's
the paper.

[http://www.nature.com/nature/journal/v515/n7528/full/nature1...](http://www.nature.com/nature/journal/v515/n7528/full/nature13883.html)

You're looking for Extended Data Figure 1b). It shows relatively constant
emissivitiy from 5 through 60 degrees of incident light.

Speculating on water condensation. This would probably lower its efficiency.
That said, they only claim cooling of ~40Wm^-2, corresponding to ~5 degrees
below ambient. Realistically, this stuff is most useful when ambient is above
~20-25 degrees (this is when most people want cooling), where the drop in
temperature by ~5 degrees shouldn't cause too much of a condensation problem.
Of course, that's just one application.

