Is a dark coloured building better than a lgiht coloured one for energy usage?

[The Other Mike]

But is radiation from those black walls a significant factor in say midwinter at night?

[The Other Mike]

Assume 20 deg C !

Not heard of supercooling in that context before, why would white be worse than another colour such as grey or black after dark?

[Fredxx]

No, most energy falling on buildings comes from sunlight which is visible light which actually peaks at yellow/green. Hence black paint is an efficient absorber of sunlight.

Any radiative loss will be at much longer wavelength and pretty much independent of any visible pigment in a paint. It will be far more dependent on surface texture.

[harry]

It will make very little difference but no one can say. There are other more important factors. A black building will collect more energy in theory but it will radiate more energy away too.

[harry]

That is how a passive house functions in ALL climates.

[harry]

Radiation is not effected by air temperature or day or night, only by the temperature of the radiating body. As soon as any body is above absolute zero, it starts to radiate energy.

[harry]

He is talking drivel. The thing you need to grasp is here.

[harry]

All radiation falling on a surface results in a heating effect. Only a percentage of sunlight is visible more than half is in the infra-red part of the specrtrum

[Martin Brown]

No. Anything that isn't a shiny polished metallic surface to a very good approximation is black at the characteristic wavelengths of 300K thermal infra red. Almost all paint resins are black for long wave IR.

Only the military have paints that have modified thermal IR properties.

[Martin Brown]

The dome in effectively black in longwave IR no matter what colour it is painted. Adding the metallic component makes it a less efficient radiator after dark and doesn't compromise daytime heating too much.

The daytime dome surface temperature is also slightly higher with the new paints so it starts from a different state. They have greatly improved the resolution of dome based instruments by preventing dome slit turbulence using these paints and more aggressive internal airconditioning to hold the scope at working temperature during daytime.

[Brian Gaff]

Well heat sinks are usually black to radiate more infra red.

Brian

[Martin Brown]

[Martin Brown]

They don't have to be black. Black is just cheaper. Almost any colour that is not shiny metallic will do.

[The Natural Philosopher]

conversely when I was in Africa I measured an unpowered heat sink, in the sun at 75C...

[The Natural Philosopher]

since 85% of heat loss is via convection anyway, even shiny metallic works.

[Martin Brown]

Depends on the shape. I have seen the odd thing made of highly polished mirror finish aluminium sheet run mad hot until it was painted black.

It was pretty much a rectangular box containing a PSU with linear regulator and no ventilation. Without the coat of paint the linreg went into thermal shutdown. Painted black it was just pleasantly warm.

[GB]

So, I don't really understand whether there is a simple answer to this question?

In summer, it's clearly undesirable to have a black building that absorbs a lot more heat from the sun. If there is lots of ventilation, it may be possible to mitigate that. Nevertheless, for summer, white seems an obviously superior colour in most circumstances.

In winter, in this country, will a black building absorb enough extra heat from the sun during the short daylight hours to compensate for the extra radiation at night time? Nobody seems to know.

[newshound]

Not in this case. Warm object inside cool container.

[newshound]

Let's take a very simplified "winter" case. Suppose your building is a black metal box without any insulation (a bit like a van, say). Even in brightest winter sun you are unlikely to get uncomfortably warm. But under clear night skies the metal temperature will readily fall well below the air temperature, and you will get cold. For the reasons I explained above, being white won't make all that much difference. But make it polished metal and you might not be comfortable, but you will be less uncomfortable. Which I guess is why spacecraft are built like this.

[Martin Brown]

There is no simple answer.

If you wanted to be cunning you would have system whereby the external walls were black until the interior reaches target temperature and silver mirror finish at all other times and at night. A clever scheme includes very high thermal inertia and good insulation so that daytime heating doesn't get through until late at night.

Not a chance. The sun barely gets above the horizon here and it mostly foggy. Latitudes of 40 or less you stand a chance with passive solar heating in winter provided the climate is sunny and not too extreme.