Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Thermal matching of a thermoelectric energy
harvester with the environment and its application in
wearable self-powered wireless medical sensors
V. Leonov 1 , P. Fiorini 1 , T. Torfs 1 , R. J. M. Vullers 2 , C. Van Hoof 1
1
IMEC
Kapeldreef 75
3010 Leuven, Belgium
2
Holst Centre / IMEC
High Tech Campus 31
Eindhoven 5656 AE, The Netherlands
Abstract-In this work, we discuss why classical thermoelectric
theory is not enough to design an optimized energy harvester.
Then, the general conditions are defined, which are required to
make a thermoelectric converter effective in such application.
The necessity of the work has been prompted by the fact that
while modeling the harvesters neither the constant temperature
difference, nor the heat flow cannot be assumed. We show that
simple equations obtained using electro-thermal analogy allow
optimization of energy harvesters to reach their top
performance characteristics. Thermal matching in MEMS
thermopiles is discussed then. The examples of application
thermally matched thermopiles for powering state-of-the-art
wearable wireless sensors are discussed in the end.
I. INTRODUCTION
Powering wireless autonomous sensors by using energy
harvesters could move such devices into mass production.
Batteries and wiring are excluded as unrealistic ways. One
thousand wireless sensors per each person is a current vision
of the future wireless sensor network called “ambient
intelligence”. These should be self-powered, preferably
using photovoltaic cells. The thermoelectric conversion of
wasted heat from low temperature sources however is the
best way to provide power autonomy to the devices in
locations, where no daylight and indoor illumination take
place. However, because of energy saving reasons, wasted
heat flows are usually minimized with the use of thermally
isolating materials. Thermoelectric conversion of waste heat
is complicated by the high thermal resistance of the heat sink
(air) and, frequently, of the heat source (e.g., walls of
buildings, plastic pipes, or living beings). The remaining
heat flows and temperature differences available for energy
harvesting are therefore relatively small. However, these
wasted heat flows can be used for eliminating the need of
primary batteries in most of autonomous devices placed
inside buildings, machinery, or in closed compartments, and
forming smart self-organizing autonomous networks. This
paper discusses the principles of designing thermoelectric
generators optimized for energy harvesting on lowtemperature
sources of waste heat. As a proof of concept,
the examples of fully self-powered wearable medical devices
are designed, fabricated and briefly described.
II. THERMOELECTRIC THEORY AND OPTIMIZED
THERMOPILE
In the thermoelectric theory, the optimization of a
thermopile in power generation mode is discussed for two
basic regimes of operation: (i) the heat flow through the
thermopile, W, is constant, i.e., independent of its thermal
resistance R tp
and (ii) the temperature drop on the thermopile,
ΔT, is constant, solid lines in Figs. 1a and 1b, respectively.
There are also the second-order effects that discussed in the
theory. These are Joule heating of thermopiles due to
generated current and Peltier effect. However, these effects
are minor and become important only in case of hightemperature
operation (more than 100°C) and low serial
thermal resistance of the environment, R env , therefore in the
following discussion of energy harvesters we omit these
effects for the sake of simplicity. The power generated by a
thermopile on the matched load in this case is
P = V 2 /4r el = α 2 ΔT 2 /4r el , (1)
Fig. 1. Two regimes of a thermopile operation in power generation mode:
(a) constant heat flow; R tp > R env .
©EDA Publishing/THERMINIC 2009 95
ISBN: 978-2-35500-010-2