Introduction - Energy Harvesting Autonomous Sensor Systems - page 25

Another SEH system, known as AmbiMax, has been proposed by Park et al.
[35]. The AmbiMax system exploits a small photosensor to detect the ambient
light conditions and to force the solar panel to work in its maximum power
point (MPP) (see Figure 1.17c ) . Similar to AmbiMax, Dondi et al. presented
another circuit [36] that uses a miniaturized photovoltaic module as the pilot
panel instead of a photosensor to achieve maximum power point tracking
(MPPT) for the SEH system. Indeed, these SEH prototypes have successfully
demonstrated that solar energy is a realistic energy source for sensor nodes.
However, there is still room for improvement, including system form factor,
performance, and so on to suit the power requirement of embedded wireless
sensor nodes deployed in application areas such as indoors and in overcast
areas where access to direct sunlight is often weak or not available. In some
cases where solar energy source may not be a suitable choice, it is required to
search for alternative energy sources either to replace the solar energy source
as a whole or to supplement the solar energy source when the intensity of the
light is low.

1.3.3.2 Thermal Energy Harvesting System
Thermal energy is another example of alternative energy sources. Several ap-
proaches to convert thermal energy into electricity are currently under inves-
tigation (through Seebeck effect, thermocouples, piezo-thermal effect) [37].
The efficiency of these approaches is related to Carnot's law, expressed by
=

T max . According to Carnot's equation, for a thermal gradi-
ent of 5 K with respect to the normal ambient temperature of 300 K, the thermal
energy harvesting (TEH) efficiency is computed to be around 1.67%. Consider
a silicon device with thermal conductivity of 140 W/mK, as illustrated by Cot-
tone [38], the heat power that flows through conduction along a 1-cm length
for

( T max − T min ) /

T=5Kis7W/cm 2 . Hence, the electrical power obtained at Carnot's effi-
ciency is calculated to be 117 mW/cm 2 .Atfirst sight, this heat power density of
7 W/cm 2 seems to be an excellent result, but the TEH devices have efficiencies
well below the simple Carnot's rule, so the attainable electrical power den-
sity turns out to be a small fraction of that, only 117 mW/cm 2 . Many research
works on TEH devices have been discussed in the literature, and a thermo-
electric generator is one of the popular devices that has been developed to
harvest thermal energy based on Seebeck's effect. A summary of the imple-
mented TEGs capable of generating from 1 to 60

W/cm 2 at a 5 K temperature
differential is illustrated in a review paper [37] presented by Hudak et al.
A TEH system requires one or more TEGs, heat exchangers on the hot and
cold sides of the TEG, a mechanical structure for clamping the heat exchang-
ers to the module and ensuring good thermal contact, thermal insulation to
prevent heat losses through the sides, and power electronics for impedance
load matching [29]. One commercial application example of TEG is the Seiko
Thermic wristwatch, as shown in Figure 1.18a , which is powered by heat
generated from the human body.
In the Seiko wristwatch shown in Figure 1.18a , the TEH system consists of
a thermoelectric module, a lithium-ion battery, and a simple DC-DC step-up