An average person of 68 kg has an approximate air intake rate of 30 liters per minute [Morton, 1952]. However, available breath pressure is only 2% above atmospheric pressure [Donna Wren, 1995,Paul Picot, 1995]. Increasing the effort required for intake of breath may have adverse physiological effects [Paul Picot, 1995] so only exhalation will be considered for generation of energy. Thus, the available power is
During sleep the breathing rate, and therefore the available power, may drop in half, while increased activity increases the breathing rate. Forcing an elevated breath pressure with an aircraft-style pressure mask can increase the available power by a factor of 2.5, but it causes significant stress on the user [Gillies (ed.), 1965].
Harnessing the energy from breathing involves breath masks which encumber the user. For some professionals such as military aircraft pilots, astronauts, or handlers of hazardous materials, such masks are already in place. However, the efficiency of a turbine and generator combination is only about 40% [Halliday et al, 1992], and any attempt to tap this energy source would provide additional load on the user. Thus, the benefit of the estimated 0.40 W of recoverable power has to be weighed against the other, more convenient methods discussed in the following sections.
Another way to generate power from breathing is to fasten a tight band around the chest of the user. From empirical measurements, there is a 2.5 cm change in chest circumference when breathing normally and up to a 5 cm change when breathing deeply. A large amount of force can be maintained over this interval. Assuming a respiration rate of 10 breaths per minute and an ambitious 100 N force applied over the maximal 0.05 m distance, the total power that can be generated is
A ratchet and flywheel attached to an elastic band around the chest might be used to recover this energy. However, friction due to the small size of the parts may cause some energy loss. With careful design, a significant fraction of this power might be recovered, but the resulting 0.42 W is a relatively small amount of power for the inconvenience.