22-11-2012, 03:36 PM
Energy Harvesting for Microsystems
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INTRODUCTION
We would like to harvest energy from the environment and store that
energy in a battery or super capacitor and thus provide a long lasting
power supply for wireless Microsystems. There are many sources of
energy in the environment that we can use including, light,
temperature difference and mechanical vibration. We also have
many devices that can convert these sources of energy into voltage
and current such as photovoltaic cells, thermopiles, thermoelectric
generators (TEG), piezoelectric generators and electromagnetic
Rochester Institute of Technology
Microelectronic Engineering
induction devices.
This document will investigate energy conversion devices and the
power conditioning electronic circuits used between the energy
conversion devices and the storage element.
SEEBECK EFFECT (THERMOCOUPLE)
When two dissimilar conductors are connected together a voltage
may be generated if the junction is at a temperature different from
the temperature at the other end of the conductors (cold junction)
This is the principal behind the thermocouple and is called the
Seebeck effect.
PELTIER EFFECT (TEG)
Heat pump device that works on the gain in electron energy for
materials with low work function and the loss in energy for materials
with higher work function. Electrons are at higher energy (lower
work function) in n-type silicon.
PIEZOELECTRIC ENERGY HARVESTING DEVICES
Microelectronic Engineering
A piezoelectric material will exhibit a change in length in response to
an applied voltage. The reverse is also possible where an applied
force causes the generation of a voltage. Single crystal quartz has
been used for piezeoelectric devices such as gas grill igniters and
piezoelectric linear motors. Thin films of various materials (organic
and inorganic) exhibit piezoelectric properties. ZnO films 0.2 μm
thick are sputtered and annealed 25 min, 950C giving piezoelectric
properties. Many piezoelectric materials also exhibit pyroelectric
properties (voltage out – heat in).
POWER CONDITIONING ELECTRONICS
Transformers can increase the voltage for ac signals. Voltage
multiplier circuits exist for AC signals. DC signals can be
switched on and off and thus create changing signals in inductors
and transformers that can increase the voltage. These higher AC
voltages can be rectified using diodes or switches and charge a
battery or super capacitor.