Energy Harvesting

Solar cells can generate useful amounts of power by absorbing photons of light even at indoor locations. The open-circuit DC output voltage of a PV cell is around 500V–800mV at 100 Lux, but higher voltages can be generated by wiring several cells in series, using larger cells or by exposing the cells to stronger light. If heavily loaded, the output voltage drops significantly, so the optimum loading (maximum power point) should be constantly adjusted (tracked) to compensate for any changes in light intensity.

If there is a temperature gradient across two dissimilar conductors, then they will generate an electric current between them (this is known as the Seebeck effect). Thermoelectric generators (TEGs) use this effect to convert ambient heat into electricity using semiconductor junctions to generating usable amounts of DC power. The DC power increases in proportion with the temperature difference between the hot and cold junctions and the surface area of the generator.

The most common vibrational energy harvesters use a spring-loaded mass that moves a magnet within fixed coils to generate AC electric power. If the mass-spring system is tuned to be resonant with the main frequency of the vibration, then it can produce significant amounts of power.

Moving liquids or gases will spin a small turbine to produce electricity. Such micro-turbines can be placed in air-conditioning ducts, water pipes or on the external surface of vehicles to generate AC power from the flow of air or water generated by the movement of the vehicle. Vortex-shedding is an alternative mass-flow energy harvesting method that has no rotating parts (see piezoelectric).

Converts mechanical strain into a high voltage, low current output that can be used as an energy source for a harvester. For example, a piezoelectric base is often used with a vibrating vortex-shedding wand to convert the oscillations into an AC voltage.

A device that collects and uses electromagnetic radiation (electric fields, Wi-Fi signals, radio waves) using an antenna to generate very low power (typically µW). It is used indoors. However, high power can be generated if directed microwave beams are used as the source at outdoor locations.

Most ambient energy sources are capable of delivering output voltage that is too low to be used directly. So the first stage of a harvesting system is a DC/DC boost converter. The boost converter raises the low input voltage to a higher voltage that can then be used to charge a small battery or a supercapacitor. For example, the REH harvester accepts input voltages starting from 0.05 VDC and boosts the voltage up to either 4.12 VDC to charge a rechargeable battery or 4.50 VDC to charge a two-cell supercapacitor (pin selectable).

The system controller controls the charging and discharging of the energy storage elements to make sure that they are not over-charged or over-discharged. It also generates status signals and a warning signal of imminent power failure if the load drains the storage element completely. In the case of the REH harvester, the controller also contains a battery backup switch to alternately supply the load from a primary-cell battery if there is not enough ambient energy available (For example, for a photovoltaic cell source at night).

The voltage stored in the storage element (battery or supercapacitor) is variable and not short circuit protected. The buck converter efficiently drops this unregulated supply down to a stable, fixed-voltage output, which is short-circuit protected. The REH harvester contains two independent regulated buck converters to supply 3.3 VDC and 1.8 VDC to power the application.