An Introduction to Energy Harvesting

Solar cells can generate useful amounts of power by absorbing photons of light even in indoor locations. The open-circuit DC output voltage of a PV cell is around 500mV–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 load (maximum power point) must be constantly adjusted (tracked) to compensate for 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 generate usable 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 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 generate 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).

This method 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 a very low level of power (typically µW). This method is primarily 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 an output voltage that is often 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 0.05VDC and boosts the voltage up to either 4.12VDC to charge a rechargeable battery or 4.50VDC to charge a two-cell supercapacitor (pin selectable).

The system controller controls the charging and discharging of the energy storage elements, which prevents them from being 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 lacks short-circuit protection. The buck converter efficiently steps this unregulated supply down to a stable, fixed-voltage output, which includes short-circuit protection. The REH harvester contains two independent regulated buck converters to supply 3.3VDC and 1.8VDC to power the application.