Achieving so-called zero-power sensors will require harvesting energy from sources in the environment. After narrowing down one’s options to available sources, the next criteria will be the amount of energy available and the amount of energy needed. Solar and wind harvesting can provide a solid foundation for high-power solutions. Heat is often readily available as waste by-product from engines, machines, and other sources. Thermal-gradient harvesting is the process of capturing environmental heat and putting it to use. And among the many ways to tap environmental phenomena for energy, the use of piezoelectric devices to convert vibrations into electrical energy seems particularly effective, with the ability to produce hundreds of microwatts (µW/cm2), depending on size and construction.
Energy harvesting through temperature gradients is done using pyroelectric and thermoelectric solutions. The use of pyroelectrics is limited because it requires a variable temperature input, whereas other approaches can provide nonstop operation for hundreds of thousands of hours but at low efficiency. Thermoelectric solutions are enabled by Peltier cells.
“Examples of thermoelectric materials are bismuth telluride, lead telluride, cobalt triantimonide, and silicon germanium, [all of] which can provide good performance,” said Alfred Piggott, founder and CTO of Applied Thermoelectric Solutions. “Using these materials, a thermoelectric generator can achieve up to 9% to 11% efficiency in an ideal application with a properly designed thermoelectric generator. Which material is best depends on many considerations, but mainly, the decision is based on the application, the budget, and the design of the thermoelectric generator.”
Ideal thermoelectric materials should have low thermal conductivity, high electrical conductivity, and a high Seebeck coefficient. The thermoelectric effect leveraged for energy harvesting is attributed to Thomas Johann Seebeck. In a thermoelectric device, voltage is produced when the different temperatures are combined. Likewise, a temperature difference occurs when voltage is applied. The ability of a material or device to generate voltage per unit temperature is called the Seebeck effect.
The material usually used to create the p and n regions (bismuth telluride, or Bi2Te3) enables output voltages of 0.2 mV/K per cell, while higher values are obtained if the thermoelectric converter uses multiple p and n pairs (20 mV using 10 cells at ∆T = 10K). The equivalent model of the source is represented by a Thévenin generator with an RT output resistor, and the maximum power that can be supplied to the load is obtained by resistive impedance adaptation Rload = RT.
A temperature difference between two points results in a flow of thermal energy from the highest temperature point to the lowest temperature point. Heat will flow until thermal equilibrium is reached and can be used to collect reusable energy. The process of extracting energy from the heat exchange is governed by the laws of thermodynamics.
Jean Charles Athanase Peltier discovered that by passing an electric current through the intersection of two conductors, heating or cooling would occur. The direction of the flow determines the direction of the temperature change, either upward or downward. The heat produced or absorbed is relative to the electric current, and the proportionality constant is called the Peltier coefficient.
Mechanical vibration is another method to provide a sufficient energy solution for electronic systems. Oscillations of the piezoelectric transducer through the use of special masses and special systems that allow movement have been widely used in energy-harvesting applications.
Piezoelectric converters exploit the direct piezoelectric effect, i.e., the property of some crystals to generate a potential difference when subjected to mechanical strain. This effect occurs at the nanoscale and is reversible. Recently, polymeric plastic matrix piezoelectric materials (such as polyvinylidene difluoride, or PVDF) have been developed, and efforts are under way to find new materials and develop more advanced manufacturing processes.
The piezoelectric effect converts kinetic energy in the form of vibration or shock into electrical energy. Piezoelectric generators (energy harvesters) offer a robust and reliable solution by converting the vibrational energy normally wasted in the environment into usable electricity. They are ideal for applications that need to charge a battery, power a supercapacitor, or directly power remote sensor systems (Figure 1).
The total performance of the system depends on many factors such as the input vibrations, the geometry and material of the transducer, the mass that causes the vibrations, and the electronic interface. For this reason, even during the early design phases, a rapid and reliable quantitative estimate of the transducer and circuit junction behavior is strongly desired to optimize the system as a whole.