Energy harvesting

Energy harvesting (EH) – also known as power harvesting, energy scavenging, or ambient power – is the process by which energy is derived from external sources (e.g., solar power, thermal energy, wind energy, salinity gradients, and kinetic energy, also known as ambient energy), then stored for use by small, wireless autonomous devices, like those used in wearable electronics, condition monitoring, and wireless sensor networks.

Energy harvesters usually provide a very small amount of power for low-energy electronics. While the input fuel to some large-scale energy generation costs resources (oil, coal, etc.), the energy source for energy harvesters is present as ambient background. For example, temperature gradients exist from the operation of a combustion engine and in urban areas, there is a large amount of electromagnetic energy in the environment due to radio and television broadcasting.

One of the first examples of ambient energy being used to produce electricity was the successful use of electromagnetic radiation (EMR) to generate the crystal radio.

The principles of energy harvesting from ambient EMR can be demonstrated with basic components.

Energy harvesting devices converting ambient energy into electrical energy have attracted much interest in both the military and commercial sectors. Some systems convert motion, such as that of ocean waves, into electricity to be used by oceanographic monitoring sensors for autonomous operation. Future applications may include high-power output devices (or arrays of such devices) deployed at remote locations to serve as reliable power stations for large systems. Another application is in wearable electronics, where energy-harvesting devices can power or recharge cell phones, mobile computers, and radio communication equipment. All of these devices must be sufficiently robust to endure long-term exposure to hostile environments and have a broad range of dynamic sensitivity to exploit the entire spectrum of wave motions. In addition, one of the latest techniques to generate electric power from vibration waves is the utilization of Auxetic Boosters. This method falls under the category of piezoelectric-based vibration energy harvesting (PVEH), where the harvested electric energy can be directly used to power wireless sensors, monitoring cameras, and other Internet of Things (IoT) devices.

Energy can also be harvested to power small autonomous sensors such as those developed using MEMS technology. These systems are often very small and require little power, but their applications are limited by the reliance on battery power. Scavenging energy from ambient vibrations, wind, heat, or light could enable smart sensors to function indefinitely.

Typical power densities available from energy harvesting devices are highly dependent upon the specific application (affecting the generator's size) and the design itself of the harvesting generator. In general, for motion-powered devices, typical values are a few μW/cm³ for human body-powered applications and hundreds of μW/cm³ for generators powered by machinery. Most energy-scavenging devices for wearable electronics generate very little power.

In general, energy can be stored in a capacitor, super capacitor, or battery. Capacitors are used when the application needs to provide huge energy spikes. Batteries leak less energy and are therefore used when the device needs to provide a steady flow of energy. These aspects of the battery depend on the type that is used. A common type of battery that is used for this purpose is the lead acid or lithium-ion battery although older types such as nickel metal hydride are still widely used today. Compared to batteries, super capacitors have virtually unlimited charge-discharge cycles and can therefore operate forever, enabling a maintenance-free operation in IoT and wireless sensor devices.