Energy Harvesting Technologies

- overview of the basics of energy harvesting technologies used as green sources of energy from the environment.

Energy harvesting is also known as power harvesting or energy scavenging. It is now a topic which is receiving a considerable level of interest in view of the requirement to have "green" sources of energy

Energy harvesting utilises naturally occurring sources of energy, and converts them into forms that can be utilised in the normal ways.

Traditionally energy has been taken from fuels that occur in the ground - coal, gas, etc. However to ensure that greenhouse gasses and other forms of pollution are not created, energy harvesting techniques can be used - these have a much smaller impact on the environment.

Although large wind generators could be termed energy harvesting, this technology typically looks more at the forms of energy harvesting that provide small amounts of power for powering low-energy electronics.


Energy harvesting sources

There are a number of sources of energy that can be used for energy harvesting:

  • Solar power
  • Thermal energy
  • Wind energy
  • Tide energy
  • Kinetic energy
  • ... etc...

There is plenty of energy available within the environment. The challenge is to harvest or capture the energy and convert it into a usable format.


Energy harvesting basics

Energy harvesting technology aims to convert ambient energy into electrical energy that can be used for a variety of purposes. It has attracted a considerable degree of 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 cellphones, mobile computers, radio communication equipment, etc. 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.


Energy harvesting techniques

There are many energy harvesting techniques that are available. The actual techniques to be employed will obviously vary according to the source and the form of energy to be harvested and also the load to be supplied - some will be very small (e.g. remote wireless sensors, etc.) others will be much larger (e.g. to provide energy for motors, etc.).

There are many technologies that can be used for energy harvesting.

  • RF energy harvesting:   This form of energy harvesting utilises RF energy in the environment and converts this into energy to power a small device. Obviously receiving antennas are needed to pick up the RF signals which are then rectified and used.
  • Piezo-electric energy harvesting:   The piezo-electric effect has been known and used for many years. When a piezo-electric crystal is distorted, a potential appears across the crystal. In this way movement can be used to create power. These devices would only be used to provide small amounts of power.
  • Thermo-electric energy harvesting:   This form of energy harvesting uses the same principle as that used in thermocouple temperature sensors. When dissimilar metals are joined, a potential is created. Although the current produced is small, it can nevertheless be utilised in some instances
  • Wind generators:   While large wind turbines can be used for the large scale harvesting of energy, small micro-generators can also be used. This form of energy harvesting is increasingly being used for powering small remote systems - some roadside signs and sensors use this form of energy harvesting.
  • Solar cells:   Collecting sunlight and converting it into electrical energy is a long known form of energy harvesting. Currently solar cells are expensive and offer a relatively low level of efficiency. As a result they are not widely used for large scale electricity generation. However they are very useful for small levels of electricity generation. Often they are seen with wind generators powering small roadside signs and sensors. The theory is that if there is no sunlight there could be wind.

Harvested power storage

Often harvested power sources may not produce an even supply of power. Solar energy or wind energy are two good examples of this. Unfortunately many loads need a constant supply of electricity otherwise they may cease to function when they are needed.

In order to overcome this, power needs to be stored. There are a number of alternatives that may be used for power storage dependent upon the application / power requirement.

  • Capacitor:   In some applications where current is very low and short interruptions of the power source are envisaged, it may be possible to use a capacitor.
  • Super capacitor:   Super capacitors or supercaps provide much greater levels of capacitance and can be sued to provide power over longer periods of at a higher current. However even these supercaps would not be able to store high levels of charge. Nevertheless they would be able to provide a "hold-up" voltage for low power applications.
  • Rechargeable batteries:   Rechargeable batteries are able to store much higher levels of charge. They could be used to provide higher levels of power and over longer periods of time. They could be recharged when the harvested power source becomes available again.

Summary

Energy harvesting is a growing industry. The need for harvesting naturally occurring energy is now acute and over the coming years the technology will grow in importance and also what it is able to deliver. Accordingly energy harvesting is a major development and growth area.

By Ian Poole

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