

Hacking the planet: white pavement and roofs for hot cities - iwwr
http://solveclimatenews.com/news/20100721/doe-buildings-get-cool-roofs-reflect-heat-back-space

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bobds
People in hot climates have been painting their roofs white for many decades.

[http://www.pictureninja.com/pages/israel/coastline-of-
israel...](http://www.pictureninja.com/pages/israel/coastline-of-israel.JPG)

[http://travelsplendid.com/wp-
content/uploads/2010/10/Santori...](http://travelsplendid.com/wp-
content/uploads/2010/10/Santorini-11-1024x768.jpg)

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jws
In typical northern zones, the daily solar energy delivered in July is about
twice that delivered in December. Winter suffers from a combination of oblique
angles, shorter days, and more clouds.

If painting white[1] saves you from moving _X_ joules with your air
conditioner in July, that will cost you an extra _X/2_ joules from your heater
in December. This is a win until you have many more heating days (without snow
cover) than cooling days.

[1] for perfect white and black, reality will be closer together.

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leot
The lack of imagination of the unsympathetic knee-jerk skeptic who reads only
the headline never ceases to amaze:

"... I'd never see anything [with white pavement] ..." [right, how many roofs
do you spend time scrutinizing?]

"... now you can get snow blindness in the summer ..." [have you been to
Greece??]

"... it will take snow longer to melt off of them ..." [except when the snow
is thick enough for this to matter, the coating ceases to be irrelevant
anyway]

"... what happens when it's cold and the extra heat is lost ..." [again, snow
reflects plenty of light anyway. and this is at least as much about
atmospheric effects as it is about effects on the actual building]

It seems: Obama admin + recommendations for US public = immediate skepticism

~~~
burgerbrain
I don't know. Having lived somewhere with a concrete driveway, and another
place with a pavement/tar driveway (in the same area), I can definitely see
the melting concern. It's not the thick piles of snow that cause problems,
plows take care of that. It's the thin layers of slush, and the pools of water
that refreeze into thin layers of ice. The difference between the almost
straight white driveway and the black driveway was dramatic enough in my case
that I had to resort to going outside and busting up the ice by hand with a
piece of rebar, several days after snowstorms.

Now, I'm fairly certain that using reflective surfaces on roofs of buildings
in warm climates and using absorbing surfaces on roofs of buildings in cool
climates has been standard practice for... decades?

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eof
Well since this is for 'hot cities' the snow thing is pretty irrelevant.

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burgerbrain
How is that defined? Even cities south of the mason-dixon line like DC can get
some pretty crippling snowstorms, while being brutally hot during the summer.
Reflective roofs make sense just about everywhere of course, but I really only
see light roads being acceptable in places like Florida. And the glare would
make it pretty unworkable in hot cities without decent cloud-cover.

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Locke1689
If you've lived in Maryland or DC you know that the reason that the winter
weather is crippling in those areas is crippling incompetence, not
incapability. The behavior in MD/DC after a _light_ snowstorm is pathetic.

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pragmatic
A real innovation would be material that turned white/reflective in hot
weather and black in cold weather.

Heat is only a concern half the year in temperate climates. The other half is
severe cold. (Think 90F and 90% humidity in summer and 0F in Winter (makes you
wonder why we live here, doesn't it?)).

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rgoddard
But given that dark colors also radiate heat, wouldn't the white be better
year round anyways?

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jarek
Dark coloured surfaces will tend to absorb more heat. You want that in the
winter to help melt snow and ice.

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iwwr
Dark surfaces radiate the minimum amount of heat, though. A perfectly black
body will absorb all incoming radiation and re-emit it back by a Boltzmann
distribution (according to its temperature). Less than perfect surfaces would
reflect some the incident light/heat; if they reflect at the same constant or
regular angle, we recognize it as 'shiny', while if it's a random angle, our
eyes recognize it as matte/solid white.

~~~
jarek
While you are correct, in the case of dark pavement with patches of snow lying
on top, conduction will be a far more prominent transfer mode than radiation.

(Although neither will be really large, and I think the blinding effect from a
light pavement would be a more important issue here.)

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dp7531
One thing to keep in mind about the white pavement - it will take longer for
snow and ice to melt off of them, which means more equipment/salt/chemicals to
clear them. Granted, in warmer climates it probably isn't as much of a
concern, but after the mess we've had in Atlanta for the last week it is still
a factor.

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gte910h
We have storms that leave snow for several days once every 17 years or so. I
wouldn't worry about it here so much.

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dp7531
Agreed about the large accumulations. I was more thinking about the smaller
ones that leave a thin coating - we seem to get those once or twice a year.
With the asphalt, we can just wait until the sun has been up for an hour or
two and melted them off.

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PostOnce
If the pavement were white, I'd never be able to see anything. Bright
reflections = low visibility.

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leot
FTA: "lighter colored pavement". In any case, it's not like any "white"
(walked-on) pavement would stay that way very long.

~~~
PostOnce
The sidewalks and driveways in the suburb where I live are pretty light gray.
If the roads were the same color, it'd be hard to see.

Also, walking/driving doesn't stay pavement very much, save for skidmarks and
oil.

~~~
PostOnce

      *stain

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danpker
What happens when it's cold and the extra heat is lost? Requiring extra
heating and therefore more energy.

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brudgers
For office buildings, cooling loads are often significant even in winter due
to the heat produced by equipment and occupants. Occupants present a
surprisingly high load because of the latent heat introduced via the water
vapor in their breath.

~~~
mturmon
The estimate I've read is 100W per person.

Sometimes, in a crowded room, it's an interesting thought experiment to
imagine how hot it would be if each person except you was replaced by a 100W
incandescent light bulb.

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kirubakaran
Now you can get snow blindness in summer.

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lhnn
White roofs OR garden roofs are good for insulation.

-Black metal pipes for heating water in the summer

-Running A/C vents inside room

-Setting refrigerators to take in outside air during winter

-Dehumidifying and reusing dryer exhaust for heating in winter

There are many hacks to buildings (homes in the list above) that can save a
lot of energy per year.

A little more on topic: I think white pavement would be a little too bright
for comfort while driving, and oil would instantly stain it and make it look
much worse than equivalent blacktop.

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iwwr
_-Running A/C vents inside room_

What does this mean?

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pyre
I assume it's about running the ducts in the room rather than in the walls so
that the cold air cools the room as it travels through the ducts and not just
when it exits the vents?

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lhnn
It keeps the hot attic air in homes from heating the air. Yes, it's insulated,
but this would be even more effective.

