Page 53 - EE Times Europe Magazine | April2019

EE|Times EUROPE — Boards & Solutions Insert 53

INTERNET OF THINGS
Thermal and Vibration Energy

Powers IoT Devices

By Maurizio Di Paolo Emilio

chieving so-called zero-power enables output voltages of 0.2 mV/K per
sensors will require harvesting cell, while higher values are obtained if the
energy from sources in the environ- thermoelectric converter uses multiple p and
A ment. After narrowing down one’s n pairs (20 mV using 10 cells at ∆T = 10K). The
options to available sources, the next criteria equivalent model of the source is represented Figure 1: The S234-H5FR-1803XB piezo
will be the amount of energy available and by a Thévenin generator with an RT output crystal converts vibrations into electrical
the amount of energy needed. Solar and wind resistor, and the maximum power that can be energy. (Image: Piezo.com)
harvesting can provide a solid foundation for supplied to the load is obtained by resistive
high-power solutions. Heat is often readily impedance adaptation R load = RT.
available as waste by-product from engines, A temperature difference between two The total performance of the system
machines, and other sources. Thermal-gradient points results in a flow of thermal energy depends on many factors such as the input
harvesting is the process of capturing environ- from the highest temperature point to the vibrations, the geometry and material of the
mental heat and putting it to use. And among lowest temperature point. Heat will flow until transducer, the mass that causes the vibrations,
the many ways to tap environmental phenom- thermal equilibrium is reached and can be and the electronic interface. For this reason,
ena for energy, the use of piezoelectric devices used to collect reusable energy. The process of even during the early design phases, a rapid
to convert vibrations into electrical energy extracting energy from the heat exchange is and reliable quantitative estimate of the trans-
seems particularly effective, with the ability governed by the laws of thermodynamics. ducer and circuit junction behavior is strongly
to produce hundreds of microwatts (µW/cm ), Jean Charles Athanase Peltier discovered desired to optimize the system as a whole.
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depending on size and construction. that by passing an electric current through The analysis of the piezoelectric effect
the intersection of two conductors, heating or can be schematized with the circuit shown
THE THERMAL GRADIENT cooling would occur. The direction of the flow in Figure 2. The inductor L represents the
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Energy harvesting through temperature determines the direction of the temperature equivalent inertial mass, the capacity C
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gradients is done using pyroelectric and change, either upward or downward. The heat refers to the elasticity of the transducer, and
thermoelectric solutions. The use of pyroelec- produced or absorbed is relative to the electric the resistor R represents the mechanical
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trics is limited because it requires a variable current, and the proportionality constant is losses. The mechanical part is powered by the
temperature input, whereas other approaches called the Peltier coefficient. force generator F , opposite to the feedback
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can provide nonstop operation for hundreds force generator α-V , which is controlled by
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of thousands of hours but at low efficiency. THE PIEZO EFFECT the voltage that develops on the output of
Thermoelectric solutions are enabled by Mechanical vibration is another method the device on the capacity C (inverse effect
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Peltier cells. to provide a sufficient energy solution of the piezoelectric). At the same time, the
“Examples of thermoelectric materials are for electronic systems. Oscillations of the mechanical speed ż produces a current βż
bismuth telluride, lead telluride, cobalt trianti- piezoelectric transducer through the use of that supplies both capacitive outputs (direct
monide, and silicon germanium, [all of] which special masses and special systems that allow effect of the piezoelectric) and other possible
can provide good performance,” said Alfred movement have been widely used in electrical loads connected to the transducer.
Piggott, founder and CTO of Applied Thermo- energy-harvesting applications. Therefore, model identification involves the
electric Solutions. “Using these materials, a Piezoelectric converters exploit the independent parameters L , C , R , C , α, and
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thermoelectric generator can achieve up to 9% direct piezoelectric effect, i.e., the property β (Figure 1). α and β are thermal coefficients
to 11% efficiency in an ideal application with of some crystals to generate a potential related to the system.
a properly designed thermoelectric generator. difference when subjected to mechanical
Which material is best depends on many con- strain. This effect occurs at the nanoscale POWER MANAGEMENT IC
siderations, but mainly, the decision is based and is reversible. Recently, polymeric plastic Temperature differences can be used to create
on the application, the budget, and the design matrix piezoelectric materials (such as electricity. Waste heat from solar thermal and
of the thermoelectric generator.” polyvinylidene difluoride, or PVDF) have been geothermal systems and even discharge flows
Ideal thermoelectric materials should have developed, and efforts are under way to find from household appliances can be harvested.
low thermal conductivity, high electrical new materials and develop more advanced Suppose we use battery-powered wireless
conductivity, and a high Seebeck coefficient. manufacturing processes. IoT devices that operate in an environment
The thermoelectric effect leveraged for energy The piezoelectric effect converts kinetic that has thermal gradients generated by a
harvesting is attributed to Thomas Johann energy in the form of vibration or shock into human body, an oven, and a motor. With-
Seebeck. In a thermoelectric device, voltage is electrical energy. Piezoelectric generators out energy harvesting, the batteries of such
produced when the different temperatures are (energy harvesters) offer a robust and reliable devices need to be replaced because they dis-
combined. Likewise, a temperature difference solution by converting the vibrational energy charge energy. This generates operating costs.
occurs when voltage is applied. The ability of normally wasted in the environment into Depending on the available temperature
a material or device to generate voltage per usable electricity. They are ideal for appli- gradients, thermoelectric generators (TEGs)
unit temperature is called the Seebeck effect. cations that need to charge a battery, power can generate from 20 µW/ cm² to 10 mW/cm².
The material usually used to create the p a supercapacitor, or directly power remote TEGs and piezo transducers combined with
and n regions (bismuth telluride, or Bi Te ) sensor systems (Figure 1). PMICs can charge batteries in IoT gear.
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www.eetimes.eu | APRIL 2020