Energy Harvesting Techniques: Types and Basics

This article explores the various types of energy harvesting techniques, focusing on how they work and their potential applications. Energy harvesting involves capturing and converting ambient energy sources into usable electrical power, offering a way to power devices and systems in a sustainable manner.

What is Energy Harvesting?

Definition: Energy harvesting is the process of powering an embedded system by collecting energy from environmental sources such as solar, wind, thermal gradients, and electromagnetic waves.

The primary goal of energy harvesting is to extend the operational lifetime of field nodes and devices. It enables on-site charging of rechargeable batteries, as well as traditional and supercapacitors.

Common applications of energy harvesting systems include:

• Car tire pressure monitors
• Wireless weather stations
• Implantable medical devices
• Traffic alert signs
• Mars rovers
• Wireless Sensor Network (WSN) nodes
• Internet of Things (IoT) devices

There are two key design considerations for energy harvesting devices:

• Selecting ultra-low-power electronics
• Efficiently harvesting and storing energy

The main components of an energy harvesting device are:

• Energy Source: Various ambient energy sources can be utilized, including wind, solar, electromagnetic waves, and thermal energy.
• Energy Harvesting IC: This integrated circuit converts the ambient energy into a form that can be stored. For example, it might convert mechanical stress into an electrical signal or a temperature difference into an electrical voltage.
• Energy Storage: Energy can be stored in rechargeable batteries, traditional capacitors, or supercapacitors. Supercapacitors offer advantages such as smaller size, higher efficiency, and higher capacity.
• Other Components: These may include memory, actuators, a communication interface, sensors, and a processing/control device like a microcontroller.

Types of Energy Harvesting Techniques

Let’s delve into the different types of energy harvesting techniques:

Thermoelectric Energy Harvesting

• Function: Thermoelectric energy harvesting converts temperature differences across a material into an equivalent electric voltage or current.

  A temperature gradient across the thermoelectric material (one side warmer, the other cooler) generates a voltage. A stable voltage is maintained as long as the temperature difference remains constant. While sunlight is a primary source, even artificial light can generate small amounts of energy. Photovoltaic devices, often made of silicon, convert solar radiation into electrical current.

• Example:

  • Cars, trucks, and lorries equipped with thermoelectric energy harvesting systems can achieve significant fuel savings by capturing waste heat.

Piezoelectric Energy Harvesting

• Function: Piezoelectric energy harvesting converts mechanical stress into an electrical signal.

  When mechanical strain is applied to certain solid materials, electrical charge accumulates. Conversely, applying an electric field to these materials induces mechanical strain. Sources like low-frequency vibrations, acoustic noise, and human motion can be exploited.

• Examples:

  • Piezoelectric floor tiles (generating electricity from footsteps)
  • Car tire pressure sensors or monitors
  • Battery-less remote controls

Pyroelectric Energy Harvesting

• Function: Pyroelectric energy harvesting converts changes in temperature into an electric charge.

  When the temperature of a pyroelectric crystal material changes, an electric charge is induced. No charge is generated if the temperature remains constant. This method is often employed in specific types of sensors.

RF/Wireless/Electromagnetic Energy Harvesting

• Function: RF energy harvesting converts the energy present in electromagnetic waves from various sources (e.g., TV, radio, cellular towers) into an electrical current.

  This method leverages ambient EM waves radiated from cellular base stations, satellites, and TV/radio broadcasting stations. RF harvesters convert RF energy into DC energy for storage using matching and rectifier circuits.

Wind and Vibration Energy Harvesting

• Function: Wind and vibration energy harvesting converts vibration energy (including wind power) into an electric signal.

  Vibration energy depends on both amplitude and frequency. The mass of the harvesting device corresponds to the vibrating mass. Vibration typically comprises fundamental frequencies and their harmonics. This phenomenon is utilized to convert vibration energy into electrical form through various methods:

  • Piezoelectric conversion (using materials like PZT, BaTiO3, PVDF)
  • Electrostatic conversion (using parallel plate capacitors)
  • Electromagnetic conversion (using magnetic fields and coils)