Hot Bodies

July 22, 2010

Humans are warm-blooded animals. Thanks to the chemical reactions our bodies use to "burn" sugars and fats, and biological homeostasis that controls these reactions, we maintain our body temperature at about 98.6o Fahrenheit (37.0o C). This is just a median number, since a healthy human body temperature will vary about a degree Fahrenheit during a 24-hour cycle. Assuming you keep a normal sleeping schedule, your body temperature is lowest at about 4:00 AM and highest at about 5:00 PM. Much of the year, at least in the Northeastern part of the US where I reside, there is a large differential temperature between a human and his environment. If we're lucky enough to work in an air-conditioned indoor environment, a large differential temperature is maintained during most of the day for most of the year.

The Second Law of Thermodynamics tells us that where there's a temperature difference, extraction of energy is possible. This is because heat flows from a warmer body to a colder body, a principle discovered by Sadi Carnot. Quantitatively, the thermal energy output of a resting human is about a watt per kilogram. An exercising human emits three times that. Calculations for the quantity of air conditioning needed in a building usually estimate about 100-150 watts per person. We could harness this heat energy from our bodies if we festoon ourselves with pipes and rig a heat engine, but there's an easier way. The thermoelectric effect is a way to convert a temperature difference to electrical current directly, albeit with low efficiency.