Energy harvester is an umbrella term for technologies that extract usable electric energy from the environment. Many different working principles exist, though they all provide limited and intermittent power. Energy harvesting is best suited for sensing applications. In this context, only limited amounts of power are used and intermittent operation should pose no problem. Such devices tend to spend most of their time sleeping, waiting for sufficient charge to build up in the system. Once the energy reservoir filled up, the device’s microcontroller powers up, performs measurements and transmits these wirelessly, and then goes back to sleep. Harvested energy can be stored in a battery, in a capacitor bank, or in an ultra-capacitor. This choice depends on capacity, discharge rate, and discharge frequency.
Solar panels are photovoltaic modules that rely on the photovoltaic effect to convert incident light into low-voltage DC power. The efficiency of solar panels is typically between 10% to 20%. Power output of solar panels is specified in Watt-peak (Wp), which is measured under precisely controlled test conditions. The actual output can be lower or even higher than the Wp rating of the panel. In solar panels, output voltage and output current are highly dependent on the environment conditions. For this reason, maximum power point tracking (MPPT) is often used in conjunction with solar panels. This technique continually adjusts the electric load of the solar panel in order to extract the maximum amount of power.
As of yet, the most common type of solar panels is crystalline silicon solar panels, though organic solar panels are currently under active research. Organic panels offer several advantages over traditional panels: they can be (semi-)transparent and they can be printed on flexible substrates. However, many challenges remain in the development of organic photovoltaics, mainly concerning panel efficiency and lifetime.
Certain materials have piezoelectric properties, meaning that they show a relation between mechanical deformation and electrical potential (voltage). One of the most common applications for this effect is in buzzers. Here, an electrical signal is used to make a crystal plate deform, creating sound in the process. However, the effect also works in the opposite direct: deforming a piezoelectric material generates a small amount of electric energy. As with solar panels, specialized circuitry (e.g. the LTC3588 chip) is needed to capture and convert this to a usable form. The picture to the right shows an application of piezoelectric energy harvesting, the Hue Tap. This device serves as a wireless remote for Philips Hue lamps. The system requires no batteries: a small amount of energy is extracted each time a button is pressed.
Thermoelectric generators (TEG) rely on the Seebeck effect to generate electricity from heat. It is important to note that energy is not extracted directly from heat, but from the temperature difference between the hot and cold side of the TEG. Similar to the piezoelectric effect, the thermoelectric effect is reversible: Peltier coolers use a DC current to generate a temperature differential. TEGs are often used in the oil, gas and mining industry, where they extract thermal energy from pipelines in order to power remote sensors.