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What is Energy Harvesting?

Energy harvesting can be described as a process of capturing and amassing byproduct energy when it is readily available and can be converted into usable electrical energy – such as operating a microprocessor within its limits. Energy harvesting is of great importance for both low-voltage and low-power applications in a wide range of portable or mobile markets such as consumer devices, medical equipment, transportation, industrial controls and military. It is also crucial for applications that need a backup battery, especially if the battery is located in a remote location that is difficult to reach. Perhaps, the biggest hope is that energy harvesting would help new market applications and products that are currently not possible or even thought of yet.

The energy can be derived from a lot of sources that would otherwise get wasted or remain unusable for any practical purpose. This process, also called energy scavenging, is about capturing residual energy as a byproduct of a natural environmental phenomenon or industrial process and can be also considered to be “free energy.” Quite often, this residual energy gets released into the environment as waste. Examples of this scenario would be mechanical energy because of vibration, thermal energy produced by heat escaped from furnaces, stress and strain, combustion engines and other heating sources. Other important sources are biological, solar energy coming from all types of light sources; coils and transformers; electromagnetic energy captured via inductors, wind and fluid energy coming from air and liquid flow; chemical energy from naturally recurring or biological processes; and the mammoth amount of RF energy in the environment due to the ubiquitous radio transmitters and television broadcasting.

In majority cases, these sources are responsible for creating energy in very small packets that have been previously difficult to obtain for use. All the energy-harvesting opportunities get enabled by new circuits that can capture and store these small energy packets and convert them into useful output. The energy management that these circuits provide requires high energy efficiency for capturing and accumulating these small energy packets. Note that even high energy retention is required to store the energy for long periods of time and proper energy conditioning is needed to perform the desired task. The energy management needs to be well-defined — and put up with a broad range of voltage, waveform inputs and current, including over-voltage, overcharge and other kinds of irregular input conditions.

A lot of materials, well-known devices or sensors are classically used for converting wasted energy into electrical voltages and currents, which can then be harvested, stored and conditioned for many types of low-voltage wearable electronics and wireless sensor applications that previously needed AC power supplies or batteries. Examples of energy generators would be materials such as solar photovoltaic cells, piezoelectric (PZT) crystals or fiber composites, thermoelectric generators (TEGs) and electromagnetic inductor coils. They are responsible for generating a wide range of output voltage and currents. But none of them can be directly used as power sources for operating low-energy electronics without energy-harvesting devices designed to capture the available power, manage it and communicate handshake instructions to compatible wireless sensor systems.

In many situations, these sources offer energy as bogus, random and otherwise irregular spikes or extremely low-level amounts. Thanks to the recent developments in MOSFET “zero-threshold” transistor designs, energy-harvesting electronics have managed to reach new heights, helping capture, store (in a capacitor, super-capacitor or battery) and manage high-retention efficiency.

Why Harvest Energy?

As we all know, the majority low-power electronics, such as embedded devices and remote sensors, are powered by batteries. However, even when it comes to long-lasting batteries, they have a limited existence and need to be replaced every few years. The replacements can turn out to be extremely costly as there are hundreds of sensors in remote locations. Whereas, energy harvesting technologies supply unlimited operating life of low-power equipment and even remove the need to replace batteries where it is costly, unfeasible, or unsafe.

Most of the energy harvesting applications are made to be self-sustaining, cost-effective, and also require minimum or no servicing for many years. Additionally, the power is used closest to the source, which completely eliminates transmission losses and long cables. When the energy is enough to power the device directly, the application or device powered by the energy can operate without batteries.

The Building Blocks of an Energy Harvesting System

The entire process of energy harvesting can be classified in various forms depending on their source, amount, and type of energy being converted to electrical energy. In its simplest form, the energy harvesting system needs a source of energy such as heat, light, or vibration, and these three key components.

  • Transducer/harvester: This is the energy harvester that is responsible for collecting and converting the energy from the source into electrical energy. Typical transducers comprise of photovoltaic for light, inductive for magnetic, thermoelectric for heat, RF for radio frequency, and piezoelectric for vibrations/kinetic energy.
  • Energy storage: Such as a super-capacitor or battery.
  • Power management: This conditions the electrical energy into a suitable form for the application. Common conditioners are regulators and complex control circuits that can run the power, based on power needs and the available power.

Energy Harvesting Technologies

When it comes to harvesting electrical power from non-traditional power sources with the help of thermoelectric generators, piezoelectric transducers, and solar cells always remain a challenge. All of these need a form of power conversion circuit for efficiently collecting, managing, and converting the energy from these sources into usable electrical energy for wireless devices, microcontrollers, sensors, and other low-power circuits.

Benefits of Energy Harvesting

There is abundant energy in the environment, which can be converted into electrical energy for powering a variety of circuits.

Energy harvesting is useful as it offers a means of powering electronics where there are no conventional power sources. It also eliminates the need for replacing batteries frequently and running wires to end applications. It also opens a lot of new applications in many remote locations, difficult-to-access locations and also underwater where batteries and conventional power are not practical to use.

Applications for Energy Harvesting Technologies

Alternative power sources help increase the battery life of remote sensors in industrial, commercial, and medical applications. It also helps installation of standalone sensors in hard-to-reach or remote areas to provide a variety of information and warnings. These sensors can effectively supervise and warn when it comes to air pollution, bridge stresses, worn out bearings, forest fires, and more.

Other applications include:

  • Remote corrosion monitoring systems
  • Implantable devices and remote patient monitoring
  • Structural monitoring
  • RFID
  • Internet of Things (IoT)
  • Equipment monitoring

Therefore, the whole concept of harvesting energy from nonconventional sources in the environment has gained popularity over the last few years as many designers are looking for alternative energy sources for low-power applications.

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