Bountiful power harvesting

A wide range of low-power industrial sensors and controllers are turning to alternative sources of energy as their primary or supplemental source of power. Ideally, such harvested energy will eliminate the need for wired power or batteries altogether. 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 becoming viable for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near “zero” power devices using just harvested energy (commonly referred to as “nanoPower” by some).

Although energy harvesting has been emerging since early 2000 (its embryonic phase), recent technology developments have pushed it to the point of commercial viability. In short, in 2010 we are at an inflection point and are poised for the commencement of the “growth” phase. Energy harvesting sensors for building-automation applications have already been deployed in Europe. Thus the growth stage may already be underway.

Commercial acceptance

The typical energy-scavenging system can be divided into four main blocks.
Select figure to enlarge.

Though the concept of energy harvesting has been around for a number of years, the implementation of a system in the real world 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 the largest so far, building automation. In the case of building automation, systems such as occupancy sensors, thermostats and light switches can eliminate the power or control wiring normally required and use a mechanical or energy harvesting system instead.

Similarly, a wireless network utilizing an energy harvesting technique can link any number of sensors together in a building to reduce heating, ventilation & air conditioning (HVAC) and lighting costs by turning off power to non-essential areas when the building has no occupants. Furthermore, the cost of energy harvesting electronics is often lower than running supply wires, so there is clearly an economic gain to be had by adopting a harvested power technique.

A typical application for an LTC3108 is in a wireless remote sensor application powered from a TEG (Peltier Cell).
Select figure to enlarge.

A typical energy scavenging configuration or system, 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 a HVAC duct, or a piezoelectric transducer attached to a vibrating mechanical source such as a windowpane.

In the case of a heat source, a compact thermoelectric device can convert small temperature differences into electrical energy. And in the case where vibration or strain is available, a piezoelectric device can also convert these small vibrations or strain differences into electrical energy. In either case, the electrical energy produced can then be converted by an energy harvesting circuit and modified into a usable form to power downstream circuits.