Thermal Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 89

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Thermal En ergy Harvesting System
Thermal energy harvesting (TEH) is the process of converting thermal en-
ergy to electrical energy by using a thermoelectric generator (TEG) made
of thermocouples. Thermoelectric power generators have been successfully
developed for decades for kilowatt-scale power generation by using waste
heat from industrial processes such as vehicle exhaust, space travel, and so
on [84]. Such systems involve heat flows at the kilowatt scale and a tempera-
ture of hundreds of degrees Centigrade. However, small-scale TEGs [85-86]
for obtaining power on the order of milliwatts or lower from ambient ther-
mal energy sources with small temperature differences have only recently
been researched [37]. The challenges with utilizing TEG technology in small-
scale TEH are low energy conversion efficiency, inconsistency, and low output
power due to temperature fluctuation and high costs [85]. As such, there is a
significant need for an efficient power management circuit to maximize the
power transfer from the TEG source to its connected electronic load over a
wide range of operating conditions.
For decades, maximum power point tracking (MPPT) schemes [87-90] have
been proposed for large-scale power generation systems. However, these
MPPT schemes are not suitable for small-scale energy harvesting systems as
they consume a significant amount of power for their continuous operation.
At lower power levels of milliwatts of interest in this chapter, implementa-
tion of such accurate MPPT schemes for small-scale TEH, whereby the power
consumed by the complex MPPT circuitry could be higher than the harvested
power itself, is not desirable. It is thus important to ensure that the gain in
input energy is always higher than the additional losses that are caused by
the MPPT operation. Thus far, limited research can be found in the literature
that discusses a simple but yet compatible MPPT algorithm addressing the
issue of a small-scale TEH system. This chapter presents a resistor emulation
approach and its associated circuitry for harvesting near maximum energy
from the thermal energy source. The rationale behind the resistor emulation
approach [69-71] is that the effective load resistance is controlled to emulate
the source resistance of the thermoelectric generator to achieve impedance
matching between the source and load; hence, the harvested power is always
at its maximum at any operating temperature difference. A power-electronic-
based converter with minimal open-loop control overhead is realized to act
as a near-constant resistance at its input port to emulate the TEH source while
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