Five years ago, energy harvesting transducers only existed in research labs. What was once used as a sensor is now commonly thought of as an energy source. Manufacturers have reversed their original thinking and introduced products targeted at the energy harvesting market. Although the transducers are starting to catch up to the power needs, they still face market challenges that are hindering their widespread adoption-namely, successful field trials, high-volume applications to reduce the piece cost and a convenient method to mix the conventional battery use model with energy harvesting. Ideally the energy harvesting source will extend the battery life to the useful life of the product, in which case the battery is no longer a concern and becomes just another component in the solution. For the wireless sensor market, typical transducers deliver between 50µW and 1mW.
Harnessing power from ultralow power transducers has its own challenges. The most common thermal electric generators have output voltages as low as 20mV with 1 degree across them and their output resistance is only 3 Ohms. Newer thermal electric generators have output voltages 10 times greater with 50 times the output resistance. Piezoelectric transducers have 4 to 200 volts peak-to-peak AC voltage swings with output resistance as high at 1 million Ohms. Solar panels, just like the piezoelectric sources, have an optimum operating voltage. If the power circuit can maintain the optimal input voltage, by varying its input source impedance, then the transducer will deliver the maximum output power. The market is benefiting from some recently released products in the industrial control space and the momentum continues to build.