: Comparison of rechargeable battery technologies. Derived from data
released by CPSI [CPSI data sheet]. On-time refers to the running time of
a 100 cm
battery with a 5W drain.
Figure 4 shows a summary of the body-centered generation methods discussed so far. Every power generation system proposed, with the possible exception of heat conversion, would require some power storage device for periods between power generation cycles. Thus, some attention is necessary regarding the efficiency of storage.
Electrical storage may be preferable due to its prevalence and miniaturization. First however, the power must be converted to a useable form. For the piezoelectric method, a step down transformer and regulator would be needed. Current strategies for converting the high voltages generated by piezoelectric materials to computer voltage levels can attain over 90% efficiency [Hoffman, 1995]. Care is needed to match the high impedance of the piezo generator properly, and, due to the low currents involved, the actual efficiency may be lower. For the other generation methods, power regulators would be needed as well, and aggressive strategies can attain 93% efficiency.
The most direct solution to the problem of electrical storage is to
charge capacitors that can be drained for power during periods of no
generation. However, simply charging the capacitor
results in the loss of half the available power [Gershenfeld, 1995].
Unfortunately, a purely capacitive solution to the problem is also
restricted by size. Current small (less than 16 cm
) 5 V
supercapacitors are rated for approximately 3 Farads. Thus, only
of energy can be stored. Correspondingly, for non-generative cycles of a minute,
can be provided from a fully charged capacitor. This is acceptable
as an energy reservoir for breathing, blood pressure, and body
heat. Capacitive storage is not suitable
for upper limb motion, walking, or typing, except for domains in which the
particular body action is continuously performed, since power supplied from
the capacitor over an hour drops to 0.01 W. In order to
provide even 1 W of power over this time interval, 100 such capacitors
would be necessary. In such cases,
rechargeable batteries may be employed. Table 4
compares the energy densities, both by weight and volume, of currently
available batteries. The last line is of particular interest since
it shows the maximum amount of time a 5 W computer could be run from
a battery contained in the heel of a shoe (assuming 100cm
of volume).
Note that the zinc-air battery would mass around 0.12 kg if it could be
manufactured in this form factor.
Mechanical energy storage may be more attractive for some of the generation mechanisms described above. For example, with walking, flywheels, pneumatic pumps, and clock springs may prove more fruitful in storing power. However, the possibilities are numerous and coverage of the field is beyond the scope of this paper.
