Biomedical Engineering Reference
In-Depth Information
when droplets of urine (or other acid fluid, even apple juice) interacts
with the doped paper. This battery produced power as high as 1.5 mW,
with a voltage of 1.5 V. As the battery can be manufactured and inte-
grated with bioMEMS, the author suggests that applications involving
urine samples can routinely monitor patient
'
s physiological responses.
Potential additional applications related to home-base health kits and
biosensors. Interestingly enough, this battery is actually found in the
market in Japan in AA and AAA size presentations
1
.
3.2 THERMAL ENERGY
Thermal energy generation is limited by the Carnot efficiency equation
(see Eq. 1.4 in Chapter 1). Taking the body temperature as the
T
high
term at 37
C (310 K), and using outdoor temperatures of 27
C (300 K)
and 20
C (293 K) for the
T
low
term into Eq. (1.4) allows for efficiencies
ranging from 3.2% to 5.5%. Although theoretical numbers, they are
quite low for efficient energy generation. Today, thermoelectric materi-
als offer efficiencies on the order of 1% for temperature gradients
under 20
C (Starner and Paradiso, 2004).
The human body radiates around 300 W of power as heat. If all
body heat could be used for energy generation with efficiencies
between 3% and 5%, 9
17 W of power could be harvested. The extrac-
tion of all body heat would require wearing a special suit that could
not be comfortable to carry. Thus, using a smaller area for power gen-
eration, about 5% of body (similar to the neck area), a maximum of
0.4
0.8 W could be produced. Increasing the exposed area to include
the head from 0.6 to 1.0 W of electrical power could be recovered.
Commercial devices have been using this technology in a number of
products. The Citizen TEG wristwatch is capable of producing up to
13
W/cm
2
using a temperature difference of 1
C (Flipsen, 2005), while
the Applied Digital Solution
'
s Thermo Life Generator can produce up
to 60
μ
W/cm
2
with a temperature gradient of 5
C (Paradiso and
Starner, 2005). Researchers have also being able to show that a subcu-
taneous temperature gradient of 0.3
μ
1.7
C can generate more than
70
W of power (Watkins et al., 2005). This is about the same level as
the power requirements for some pacemakers. Mateu et al. (2007) pre-
sented an investigation where an externally mounted thermoelectric
μ
