Heinlein:Robert:Have Space Suit Will Travel:23:Heat Production of Human Body (Neal Stephenson)

From the Quicksilver Metaweb.

Heinlein here gives a good explanation of the problem of heat rejection, which is a problem that has bedeviled designers (and wearers) of space suits for a long time. In the early days, engineers tended to underestimate the amount of heat that astronauts' bodies would generate when they were moving around in their suits, and so did not make the heat rejection systems as robust as some of the astronauts might have liked.

Heinlein says that the body produces enough heat to melt upwards of fifty pounds (about 23 kg) of ice per day. This might sound like a funny unit of measurement, but it is commonly employed in the air-conditioning industry.

The heat of fusion of water (i.e. the amount of energy required to melt it) is about 6000 Joules per mole, where one mole of water weighs about 18 grams. The fifty pounds or 23 kg of ice referred to by Heinlein therefore works out to 23000/18 = 1300 (approx.) moles of water. If 6000 Joules are required to melt each of those moles of water, then the total heat required is 6000 * 1300 = 7.8 million Joules.

Remember, this is the energy output for an entire day. To convert it into a power level in Watts, we simply divide it by the number of seconds in a day. The result is about 90 Watts.

So what Heinlein is saying here is that the minimum heat output of a human body is about 90 Watts. This figure is consistent with rules of thumb commonly used by architects, etc. to estimate the cooling load imposed on a building's air conditioners by the body heat of the occupants.

However, Heinlein is careful to specify that this is only a minimum figure. As he must have known, it can be several times this amount in the case of a large or physically active person.

The amount of heat generated increases heavily when a human beeing thinks heavily, thus increasing information density in it's closed system A.A.