Hacking the planet: white pavement and roofs for hot cities | Hacker News Reader

Hacking the planet: white pavement and roofs for hot cities

solveclimatenews.com

36 pointsiwwr31 comments

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

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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.

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

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?

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

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.

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.

White pavement is a real issue on a human scale, more than a white roof. It's true, we don't spend a lot of time looking at roofs, and where people congregate in large numbers, roofs tend to be flatter and more out of view.

Pavement, on the other hand, is visible everywhere people are and you do look at it all the time when you're outside in a city. Regardless of the heat, a dark road/footpath is easier on the eyes than a light one. Concrete roads on hot, sunny days require either squinting or sunglasses as all that lovely light gets reflected into your eyes.

Not to mention that concrete roads and footpaths are less pleasant to use in general.

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?)).

Done and done http://www.inventables.com/technologies/heat-reactive-paint

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

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

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.

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.)

Except it won't absorb anything if the snow reflects all of the light.

It makes sense to absorb heat when there is little or no snow, but it doesn't at all when snow covers all of the surface.

When there is snow and ice on the road the surface will is light colored (white or grey).

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.

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

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.

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

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

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.

  *stain

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

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.

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.

Now you can get snow blindness in summer.

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.

-Running A/C vents inside room

What does this mean?

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?

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

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