Types and basics of energy harvesting techniques or methods | thermoelectric, piezoelectric, photovoltaic, pyroelectric, electromagnetic, wind, vibration

This page covers types and basics of energy harvesting techniques and energy harvesting materials. The energy harvesting techniques include thermoelectric energy harvesting, photovoltaic energy harvesting, piezoelectric energy harvesting, pyroelectric energy harvesting, rf or wireless or electromagnetic energy harvesting, wind energy harvesting, vibration energy harvesting etc.

Definition: The method of powering an embedded system by harvesting energy from environmental sources such as solar, wind, thermal and electomagnetic waves is known as energy harvesting. The purpose of energy harvesting is to increase lifetime of field nodes and devices. It helps in on-site charging of rechargeable batteries, traditional and super capacitors.

Common energy harvesting systems include car tire pressure monitors, wireless weather stations, implantable medical devices, traffic alert signs, Mars rover, WSN (Wireless Sensor Network) nodes, IoT (Internet of Things) devices etc.

There are two design goal in energy harvesting device.
• Select very low power electronics
• Harvest energy and store energy efficiently

Energy Harvesting Device

Following are the three main elements in energy harvesting device.
• Energy Source: There are various energy sources. Any one can be used for energy harvesting system. The sources such as wind energy, solar energy, electromagnetic waves, thermal energy etc. are used.
• Energy Harvesting IC: This IC converts ambient energy into some of energy which can be stored in energy storage devices. For example mechanical stress into electrical signal, temperature difference into electric voltage etc.
• Energy Storage: There are different ways energy can be stored which include rechargeable batteries, traditional capacitors, super capacitors etc. Super capacitors have characteristics viz. smaller size, higher efficiency, higher capacity etc.
• The other components are memory, actuators, communication interface, sensors, processing and control device such as microcontroller etc.

Thermoelectric energy harvesting | Photovoltaic energy harvesting

Thermoelectric energy harvesting

➨Function: It uses ambient sun energy to convert temperature differences across the material into equivalent electric voltage or electric current. The temperature difference on two sides of crystal (i.e. one warmer and the other cooler) causes voltage across crystal device. Steady voltage is available when difference in temperature remains unchanged.

Any light source can also be used to generate energy at small invisible amounts. For example, artificial lights produce 60Hz signal along with photons. Sun generates energy due to solar radiations.

Photovoltaic devices usually are made using silicon. They convert solar radiation into electric current.

Example of thermoelectric energy harvesting/harvester:
• Cars, trucks and Lorries which are equipped with such energy harvesting systems will have good amount of fuel savings.

Piezoelectric energy harvesting

Piezoelectric energy harvesting

➨Function: Convert mechanical stress into electrical Signal.
The charge gets accumulated in solid materials due to application of mechanical strain. The reverse of this i.e. mechanical strain gets induced due to subject of electric field on such solid materials will also occur. The common sources which can be exploited in piezoelectric energy harvesting are low frequency vibrations, acoustic noise, human motion etc. These sources can be harvested by piezoelectric materials.

Example of thermoelectric energy harvesting/harvester:
• Piezoelectric floor tiles
• Car tyre pressure sensors or monitors
• Battery less remote control

Pyroelectric energy harvesting

➨Function: Convert change in temperature into equivalent electric charge.
When the temperature of crystal material changes it induces electric charge. No charge is generated when temperature remains constant. This method is used in certain sensors.

RF/Wireless/Electromagnetic energy harvesting

RF energy harvesting system

➨Function: Convert energy available in electromagnetic waves transmitted from various radiating sources (e.g. TV, radio, cellular towers) into equivalent electric current. This method utlizes ambient EM waves available due to radiations from cellular base stations, satellites, TV and radio broadcasting stations. This RF harvesters converts RF energy into DC energy for storage. It does this using matching and rectifier circuits.
Refer RF Energy Harvesting System for more information.

Wind energy harvesting | Vibration energy harvesting

wind energy harvesting

➨Function: Convert vibration energy into electric signal. It can include wind power energy also. Energy of vibration depends on amplitude and frequency. Mass of harvesting device is equivalent to vibrating mass.

We know that vibration is typically made up of number of fundamental frequencies and their harmonics. This phenomenon is utilized in different ways to convert vibration energy into electrical form. The methods include piezoelectric conversion (commonly used materials are PZT, BaTiO3, PVDF etc.) , electrostatic conversion (using parallel plate capacitor) and electromagnetic conversion (using magnetic fields and coil).

RF and Wireless Terminologies