What is Energy Harvesting?

How exciting would it be not to have to use (or change) batteries anymore? How great would it be to have maintenance-free, autonomous applications?

This is possible when a low-power technology is supported by an energy harvester that transforms, captures, and stores energy from external renewable sources. The specifications commonly required for energy harvesters for IoT device applications are small-sized with high power output. Energy harvesting is then specifically used to power a product that would otherwise need batteries or mains electricity available.

Other harvesting technologies are now emerging next to the widely known photovoltaic energy harvesting system (solar panels). These technologies exploit movement, vibration, pressure, heat, and magnetic fields to generate electricity.

Photovoltaics harvesters exploit natural light by using semiconducting materials to generate electricity.

In commercial setups, solar modules are made of many solar cells, and many solar modules together form a photovoltaic system that produces electrical energy.

Performances of the solar photovoltaic system depend on many parameters: the amount of solar irradiance, geographical site of installation, period of the year and time of the day, weather, and temperature.

It is typically a good option for many applications. However, it is limited to a location where it can be freely installed to collect light without the risk of damage.

Kinetic energy harvesting converts movement (or vibrations) into electrical energy. Energy harvesting solutions that harness kinetic energy to produce electricity for small devices can be divided into Electromagnetic Induction and Piezoelectricity.

Electricity is produced by a changing magnetic field that causes electromagnetic induction as a consequence of the voltage produced by moving a conductor in a stationary magnetic field (or by pushing a magnetic field around the conductor).

The changing magnetic fields that trigger the production of an electromotive force can be produced through rotation or linear movement. Such systems allow battery-free operation of wireless monitoring systems and autonomous devices and facilitate their placement and maintenance in locations where replacing a battery is not feasible or practical.

These systems can be scaled and customized to meet the application’s requirements. As a downside, development costs can run high. Kinetic systems are also a good solution for applications where you want to generate the exact energy you need at a specific moment.

Mechanical stress caused by low-intensity natural vibration or human motion can be harnessed and turned into current (or voltage) thanks to the piezoelectric effect.

Piezoelectric energy is harvested in small quantities, in the order of milliwatts. Because of that, piezo is suitable for miniature devices but too small for system applications, and it is sometimes perceived as unreliable because of the limited power output levels.

When there is a thermal gradient between two different electrical conductors, it is possible to observe the generation of a voltage difference between the two materials (Seebeck or Thermoelectric effect).

Materials used to harvest thermoelectric energy should be good electrical conductors and have low thermal conductivity, which keeps the thermal gradient at the junction between the two regions high.

Thermoelectric energy harvesters have a long shelf life and don't need maintenance. On the other hand, however, with current technologies, the energy conversion efficiency rate is lower than 10%. This limit could be overcome by developing electrical conductors and materials capable of working at high-temperature gradients.

The pyroelectric effect is the process that leads to a temporary voltage generation when an object is heated or cooled. The temperature difference changes the atomic position in the structure of these materials, causing a consequent change in polarization.

This energy harvesting method can help gather energy at way higher temperatures than thermoelectric methods because of the higher heating stability of the materials. However, Pyroelectricity requires inconsistent heat sources to work, and the power output is relatively small due to consequently short working periods.