Sunday, October 25, 2009
Energy harvesting, according to Wikipedia, "is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy), captured, and stored." So any solar, wind, etc. system that stores energy in batteries or capacitors would be considered an energy harvester. What I am going to focus on are much smaller energy harvesters that can be used to power microControllers, sensors and wireless radios, like the ZigBee types of transceivers.
Assembled energy harvesters, in my opinion, are prohibitively expensive. If you click on any of the Google ads that occasionally appear on this site you will find they run $300-$500, and to purchase a development kit they are over $700. The military is keenly interested in energy harvesters and it seems they are priced for a military budget. For the rest of us, however, there are alternatives.
Basically an energy harvester is nothing more than a source that can collect energy (i.e. solar, wind) and store it for continued use. Both batteries and super capacitors can be used as a storage medium and certain types of DC-DC converters are perfect for this type of use.
Looking at figure 1, the energy collector is a piezoelectric vibration sensor that picks up vibrations and outputs a small AC signal. In its place one could a solar cell, a Peltier Cell or basically anything that can produce an AC or DC voltage, perferably higher than .7 Volts. The four diodes rectify the signal and turn it from AC to DC. In this circuit a Super Capacitor used, with the BZX85-C2V7 diode
in place to prevent voltage across the super-cap from increasing beyond the maximum voltage rating.
The controller of the circuit is a Max1675 compact, high-efficiency, step-up DC-DC converters. The important features of the converter is that it will continue to charge a battery or super-cap as long as the voltage is .7 Volts or above and it can be easily configured to output 5 Volts. In the figure 1 circuit, the FB pin is connected to the OUT pin. By connecting the FB pin to GND the output is 5 Volts.
In a 5 Volt circuit, the Zener diode would need to be replaced with one that is 5 Volts (TZX5V1B-TAP) and the super-cap would need to be replaced with a higher voltage one or with an appropriate battery.
The Max1675 converter is primarily designed for cell phones, PDAs, pagers, etc. so running at 3.3V & 5V is typical. The package only comes in SMT type so for those of you, like me, that have bifocals it will be tricky to solder.