Biomedical Engineering Reference
In-Depth Information
generator delivered 2 mW of power for a temperature difference of
7
C (dimensions of 2.5 cm
3
2.5 cm with an added heat sink).
3.3 KINETIC ENERGY
Harnessing daily activities such as walking for passive energy generation
is a well-documented topic. For instance, backpacks, the footfall, the
swinging of the legs have been studied and devices have been designed.
Items vary from large devices to those that can be carried in a pocket.
Backpacks have been engineered for energy generation. Rome et al.
(2005) employed the up-and-down movement of backpack loads to
generate energy. This backpack design was divided in two frames: a
vertical-moving structure where the load was placed and a fixed frame
attached to the individual. A toothed rack on the moving frame was
connected to a gear box on the fixed structure, which was attached to
a DC generator. When the movable structure traveled 4.5 cm, the gen-
erator could rotate up to 5,000 rpm due to the gear box. Power genera-
tion up to 7.4 W was reported when carrying heavy loads (38 kg). In
addition, the load peak force from the movable design decreased up to
12% when compared to a fixed cargo. This decrease in the metabolic
cost increases the efficiency of the overall energy generation process.
A study of backpack straps as locations for piezoelectric generators
was undertaken by Feenstra et al. (2008). The tension force on the
straps, with the stacks placed in series, was mechanically amplified and
converted into compressive load. It was reported a power generation
of 176
W when walking on a treadmill with a 40 lb load, while the
maximum power output is expected to be on the order of 400
μ
W.
Although this number seems small, it also requires minimal backpack
modification, enough for sensing capabilities.
μ
Li et al. (2008) presented a knee-mounted brace for biomechanical
energy harvesting during walking. A gear train and a small permanent
magnet generator were fitted on a custom knee brace. This generator
was designed to harness the energy from leg deceleration rather than
for continuous generation, similar to the generative braking process of
hybrid cars. The gait process is divided into two stages: swing and
stance. At the swing phase when the leg is moved forward, the body
uses energy to decelerate the limb. This generator, when operated
under the right conditions, was able to produce a power peak close to
