Energy harvesting basics
Transducers that create electricity from readily available physical sources such as temperature differentials (thermoelectric generators or thermopiles), mechanical vibration or strain (piezoelectric or electromechanical devices) and light (photovoltaic devices) are viable sources of power for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near “zero” power devices using only harvested energy.
Even though the concept of energy harvesting has been around for a number of years, the implementation of a system in a real world environment has been cumbersome, complex and costly. Nevertheless, examples of markets where an energy harvesting approach has been used include transportation infrastructure, wireless medical devices, tire pressure sensing, and building automation.
A typical energy scavenging configuration or system (represented by the four main circuit system blocks shown in Figure 1
), usually consists of a free energy source. Examples of such sources include a thermoelectric generator (TEG) or thermopile attached to a heat-generating source such as an aircraft engine, or a piezoelectric transducer attached to a vibrating mechanical source such as an aircraft airframe or wing.
In the case of a heat source, a compact thermoelectric device can convert small temperature differences into electrical energy. And where vibration or strain is available, a piezoelectric device can convert these small vibrations or strain differences into electrical energy. In either case, the electrical energy produced can be converted by an energy harvesting circuit (the second block in Figure 1
) and modified into a usable form to power downstream circuits. These downstream electronics usually consist of some kind of sensor, an analog-to-digital converter and an ultralow power microcontroller (the third block in Figure 1
). These components can take this harvested energy, now in the form of an electric current, and wake up a sensor to take a reading or a measurement and then make this data available for transmission via an ultralow power wireless transceiver – represented by the fourth block in the circuit chain shown in Figure 1
Figure 1: The four main blocks of a typical energy-scavenging system
Each circuit system block in this chain, with the possible exception of the energy source itself, has had its own unique set of constraints that have impaired its economical viability until now. Low cost and low power sensors and microcontrollers have been available for a couple of years; however, it is only recently that ultralow power transceivers have become commercially available. Nevertheless, the laggard in this chain has been the energy harvester.
Existing implementations of the energy harvester block typically consist of low performing discrete configurations, usually comprising 30 or more components. Such designs have low conversion efficiency and high quiescent currents. Both of these deficiencies result in compromised performance in end-systems. The low conversion efficiency increases the amount of time required to power up a system, which in turn increases the time interval between taking a sensor reading and transmitting the data. A high quiescent current limits how low the output of the energy-harvesting source can be, since it must first overcome the current level needed for its own operation before it can supply any excess power to the output.