Saturday, August 1, 2009

Keep it Cool

This is a (part) solution to climate change that couldn't be more appropriate to India. In fact the idea is so simple that one wonders what took it so long to catch on.

Most Indians know, to their cost, that having a flat on the top floor means hot, stifling interiors. So what's the solution - a simple physics principle that a white or light coloured roof reflects more radiation than a dark one. In fact a white shiny plasticized cover is even better and this is what is being attempted from California to Dubai to New Delhi. White roofs reduce air conditioning costs by 20% and the consequent lower energy consumption has a direct bearing on global warming.

The numbers are almost unbelievable. Apparently turning all of the world’s roofs “light” over the next 20 years (easier said than done) could save the equivalent of 24 billion metric tons in carbon dioxide emissions. That's the whole world's total emission in a year.

The only downside is that in cold climates, correspondingly heating bills would rise slightly. However, this is an irrelevant issue for tropical countries like India.

Of course there have been white tiled roofs in hot climates for a long time. The idea is to make this catch on all over the world.

Some more information is available here along with some links.


AmOK said...

Thanks OLO! Amazing. Of course you meant reflects, not emits. Working in a closed system, what is the carbon footprint of manufacturing the light-coloured materials? I remember that nuclear energy was promoted as "too cheap to meter" long ago.

It's a good idea though and could go into the building codes of the future.

Niket said...

Steven Chu was on The Daily Show last week. He said that he had mentioned light-coloured roof just from the point of view of air conditioning cost.

On the show, he mentioned that light coloured roofs would absorb less heat and that itself might reduce warming at least locally.

Rahul Basu said...

Indeed, even if it reduces the air conditioning load, that too might help considering more and more people in India are going in for air conditioning. However, in India where most people in the cities live in apartment buildings, this light-coloured-roof principle is useful only for those in the top floor. I wonder if much difference will be made by colouring the side walls of the building white...perhaps just a little.

Indeed AmOK I meant 'reflect' not 'emit' -- this has now been corrected, thanks to the ever watchful commentators (commentors?)

AmOK said...

Well if you paint the sides of the building white you would need MORE air conditioning. The radiation losses would go down and trap the heat within the buildings. So the walls should be painted black. Roofs white, walls black. What do you think OLO? (It is left as an exercise for the reader to derive the color of a roof sloping at an angle of forty-five degrees.).

Anant said...

This humble commentator once mentioned to the LO that we in Bangalore claim that it is the air-conditioned city, whereupon there was an immediate repartee that they in Chennai have central heating! Thanks indeed for the learned article here.

Regarding the emits vs. reflects, you may wish to consider using the html command "strike" to indicate the eagle eyed nature of your eager readers and commentators.

Sourendu said...

Since I'm trying to work out the cost of air-conditioning for various machines, I ran my Mathematica code on your problem.

It turns out that the "best" AC we can buy needs 2.51 watts of power to remove 1 watt of dissipated power (in terms that the AC engineers use, you need 1400 Watt hours to remove 12000 BTU of heat). Since the solar constant is 1366 W/m^2, a 100 m^2 roof will receive at most 136.6 KW of solar power as heat. So with a perfectly black roof and the sun exactly overhead, you would need 343 KW of cooling power to remove all the heat coming in from the roof (more, if you mind heating from the walls).

Correcting for the albedo and the elevation of the sun, and the length of the day, you might bring your cooling power requirements down to about 250 KW hours a day. The reason you never spend so much is that (a) you don't cool all day, and (b) you don't remove all the heat that comes in from the roof. Nevertheless, painting the roof white seems to be a greener (?) way to go.

Rahul Basu said...

Yes but what of us who don't have roofs as I said in my previous comment. Then it's a matter of the side walls, angle of the sun and other factors so there seems little 'green' alternative for those living on lower floors.

Venkataraman said...

How about rooftop gardens? Won't they be doubly beneficial?

AmOK said...

Perhaps they should also paint the LHC white. I am waiting for the Guralnik particle to be discovered.

AmOK said...

ti si a doog aedi ot tsop ylkeew. Ew teg derob, su sredaer.