Since the human body eliminates energy as heat, it
follows naturally to try to harness this energy. However, Carnot
efficiency puts an upper limit on how well this waste heat can be
recovered. Assuming normal body temperature and a relatively low room
temperature (
C), the Carnot efficiency is
In a warmer environment (
C) the Carnot efficiency drops to
Table 2 indicates that for sitting, a total of 116 W of power is available. Using a Carnot engine to model the recoverable energy yields 3.7-6.4 W of power. In more extreme temperature differences, higher efficiencies may be achieved; but, robbing the user of heat in adverse environmental temperatures is not practical.
However, even under the best of conditions (basal, non-sweating), evaporative heat loss accounts for 25% of the total heat dissipation. This ``insensible perspiration'' consists of water diffusing through the skin; sweat glands keeping the skin of the palms and soles pliable; and the expulsion of water-saturated air from the lungs [Gillies (ed.), 1965]. Thus, the maximum power available, without trying to reclaim heat expended by the latent heat of vaporization, drops to 2.8-4.8 W.
The above efficiences assume that all of the heat radiated by the body is captured and perfectly transformed into power. However, such a system would encapsulate the user in something similar to a wetsuit. The reduced temperature at the location of the heat exchanger would cause the body to restrict blood flow to that area [Gillies (ed.), 1965]. When the skin surface encounters cold air, a rapid constriction of the blood vessels in the skin allows the skin temperature to approach the temperature of the interface so that heat exchange is reduced. This self-regulation causes the location of the heat pump to become the coolest part of the body, further diminishing the returns of the Carnot engine unless a wetsuit is employed as part of the design.
While a full wetsuit or even a torso body suit is unsuitable for many applications, the neck offers a good location for a tight seal, access to major centers of blood flow, and easy removal by the user. The neck is approximately 1/15 of the surface area of the ``core'' region (those parts that the body tries to keep warm at all times). As a rough estimate, assuming even heat dissipation over the body, a maximum of 0.20-0.32 W could be recovered conveniently by such a neck brace. The head may also be a convenient heat source for some applications where protective hoods are already in place. The surface area of the head is approximately 3 times that of the neck and could provide 0.60-0.96 W of power given optimal conversion. Even so, the practicality, comfort, and efficacy of such a system are relatively limited.