Thursday, 26 January 2012

Black bodies and getting my head around emissivity

There's something about emmissivity that doesn't seem to have made sense, and probably should have done much sooner.

My attention to thermodynamics regarding the house has mostly been concentrating on conduction. Of the three ways heat gets around, this is probably the most significant. Obviously convection is important, but I think if you're dealing with a wall with inside on one side and outside on the other, and you treat the air as being at a constant temperature, then you won't be far wrong. What is actually happening, if the wall is hotter than the air next to it is that it is heating up that air, then that air is moving away and is replaced by a new bit of air which needs heating up again.

The resulting heat flow is more or less the same. 


Anyway, I haven't really been thinking about radiation. I can remember a physics teacher telling us that radiators don't really radiate, they transfer heat by convection. And that seems to have stuck in my mind. I have, of course, been thinking about solar radiation, as that is how the sun gets its energy to us. We worry about nuclear power and radiation in the hands of humans, but in the case of the sun, it's an important part of life. If you want to see how much radiation nuclear power produces, you just have to look at the sun, as that is basically a big nuclear power station. In fact it's probably not a good idea to look at the sun as it will make you go blind. 

So the solar heat coming into the house is coming in by radiation. The windows are low-e, which everyone who knows about windows will tell you is important, so that you keep the heat in the house. The "e" is for emissivity, and it doesn't really fit into my idea of common sense. As far as I can tell, "low e" just means that it reflects heat. 

Emissivity, on the other hand, is a measure of how much a body radiates. It is a number between 0 and 1, or zero and a hundred percent. For a black body, which I'll get to in a moment, the amount of radiation is proportional to the fourth power of the absolute temperature. This corresponds to emissivity of 100%. You can read more about that here, if you're really interested.

The black body is a theoretical object that will absorb all the radiation that falls on it. What confused me for a while is what exactly this has to do with windows reflecting heat. It seems that three things are going on when radiation hits glass. First, some of the radiation is reflected by the glass, second some it it goes through the glass, and third, the rest of it is absorbed by the glass and warms it up. These seem to be independent ideas. Obviously all the parts are going to add up to 100% otherwise we'd be breaking the law of the conservation of energy, but what does radiation have to do with absorption?

I've given in to what seems a much simpler reality. It just comes down to one number - the emissivity. Bodies are like mirrors. If they absorb a lot of heat, they can also radiate a lot of heat. If they reflect heat well, they radiate heat badly. 

Polished metals have very low emissivity, which is why the lunar module was wrapped in tin foil. Not necessarily what you'd want around your house as it would conduct the heat away, but in space there is no atmosphere, so conduction is not an issue, and heat is lost, or gained if you're in the sun, through radiation. Radiation and emissivity are issues in buildings on earth, as we shall see in the next post.