Sensors and Transducers - Introduction to Biomechatronics - page 20

Capturing all of the work of exhalation based on the differential pressure that can be
exerted without interfering with normal breathing is another potential source of power. It
is shown in Chapter 9 that the average breathing rate at rest is about 15 breaths per minute
for a tidal volume of 400 ml, making the total flow Q = 6 lit/min (0.1 lit/s). For a pressure
differential, P ,of10cmH 2 O (980 Pa) the power is

P = Q P
= 10 − 3 0 . 1 × × 980

= 98 mW

A face mask housing a small turbine driven generator with a good overall efficiency would
be required to capture this energy.
A tight band around the chest can also be used to capture energy from breathing.
For normal breathing an increase in circumference, x , of about 10 mm occurs during
inspiration. For a restraining force, F , of 100 N the total energy per breath is

E = F x
= 10 − 3 100 × 10 ×

= 1J

For a breathing rate of 15 breaths per minute, or 0.25 breaths per second, the total power
output is only 250 mW. Once again, a harvesting efficiency of about 50% for ratchet and
flywheel or a piezoelectric material could be expected. The use of internal piezoelectric
materials is discussed in the next section.
The power required for arm motion can easily be calculated from its mass and the
change in height of the center of gravity per unit time. For example, to lift an arm weighing
3 kg through a height of 0.6 m once per second requires about 18 W. This is obviously
unnatural and would be unsustainable over long periods. However, normal arm move-
ment could still provide a reasonable amount of energy without loading the limb or joint
significantly.
Activities specifically designed to generate electricity such as shake-driven torches
or wind-up radios and cell phone chargers can be reasonably efficient. For example, the
power supply of the Freeplay radio stores about 500 J of energy from 60 s of winding
(Starner and Paradiso, 2004).
Of course, running uses the most energy of any human activity, so capturing even a
fraction of that could produce significant amounts of power. A good example of one of the
larger devices is a knee brace designed and built by researchers at Simon Fraser University
in British Columbia led by Max Donelan. The 1.5 kg device consists of a flexible joint
that converts knee flexion into rotary motion that drives a generator. When used at end
swing (when the leg is decelerating), it is capable of producing5Wofelectrical energy
for a measured expenditure of only 5 W. This should be considered in comparison with
the 6 W of energy required to produce each watt of electrical power for a hand-cranked
generator.
Unfortunately, just carrying the prototype device, even when it is not generating any
energy, uses about 60 W. This is a significant percentage of the 300 W used for walking.
The researchers suggest that this overhead could be reduced to about 15 W by moving
the device higher up the leg and reducing its mass. When these overheads are taken into
account, it is not any more efficient than other techniques such as shoes with power
generation mechanisms in the soles or oscillating backpacks (Johnston, 2008).