Biomedical Engineering Reference
In-Depth Information
Linear electromagnetic generators have also been optimized for
energy harvesting while walking, as performed by von Buren and
Troster (2007). The generator consisted of an air-core tubular structure
having a flexure bearing and a free-sliding magnet stack surrounded
by coils. Energy harvesters having a volume of 0.25 cm
3
were analyzed
with different quantities of magnets (6
10). The power
output varied according to the body location but on average 2
9) and coils (6
25
μ
W
was recorded. A comparison was offered with a lithium
ion battery
having an energy density of 0.3 Wh/cm
3
. The battery would be
depleted in a 4-year period if 2
W is drawn, or its energy would be
completely consumed in 4 months if the power drain is 25
μ
W. A
prototype with a volume of 0.5 cm
3
(15 mm long, 6 mm diameter) hav-
ing 6 magnets and 5 coils was tested below the knee while walking for
an average output power of 35
μ
μ
W and a peak power of 1 mW (electri-
cal efficiency of 66% on a 10
Ω
load).
Another study employing a linear electromagnetic generator
(55 mm long, 17 mm diameter) using a free-sliding magnet surrounded
by coils was presented by Saha et al. (2008). Two different configura-
tions using magnetic springs (magnets located at the ends to repel the
free-sliding magnet) were presented. A first configuration having fixed
magnets at the ends (top and bottom of the tubular structure) was
placed in a backpack. It provided 0.3 mW when walking and 2.46 mW
when slowly running. A second configuration with the top fixed mag-
net removed produced 0.95 mW when walking and 2.46 mW when
slowly running. The second arrangement had a higher sliding magnet
displacement for a 300% increase in power output while walking and
32% increase while slowly running. The energy stored in a Li
MnO
2
coin coil cell battery reached 3.5 J after 1 h of walking. The energy
generated exceeded the power consumption of 700
W (2.5 J in 1 h)
for a wearable system composed of a light sensor, microphone, acceler-
ometer, microprocessor, and RF transceiver.
μ
Impact forces have also been studied for energy harvesting using pie-
zoelectric materials. For example, a linear impact-based generator was
proposed by Renaud et al. (2005) for harnessing limb motion. This
design consisted on a free-sliding mass (750 mg) with piezoelectric canti-
lever beams at the ends for 10 mm displacement. When the sliding-mass
impacts the cantilever beams, they resonate generating energy for an
estimated power output of 40
W. Further work (Renaud et al., 2009)
tested a prototype (25 cm
3
, 60 g sliding mass) that produced 47
μ
μ
W
