The How and Why of Energy Harvesting for Low-Power Applications

In this article, we'll go over the basics of energy harvesting and discuss what forms it can take when scavenging energy from different sources.

What Is Energy Harvesting

Energy harvesting is the capture and conversion of small amounts of readily available energy in the environment into usable electrical energy. The electrical energy is conditioned for either direct use or accumulated and stored for later use. This provides an alternative source of power for applications in locations where there is no grid power and it is inefficient to install wind turbines or solar panels.
Other than outdoor solar, no small energy sources provide a great deal of energy. However, the energy captured is adequate for most wireless applications, remote sensing, body implants, RFID, and other applications at the lower segments of the power spectrum. And even if the harvested energy is low and incapable of powering a device, it can still be used to extend the life of a battery.
Energy harvesting is also known as energy scavenging or micro energy harvesting.

Why Harvest Energy

Most low-power electronics, such as remote sensors and embedded devices, are powered by batteries. However, even long-lasting batteries have a limited lifespan and must be replaced every few years. The replacements become costly when there are hundreds of sensors in remote locations. Energy harvesting technologies, on the other hand, provide unlimited operating life of low-power equipment  and eliminate the need to replace batteries where it is costly, impractical, or dangerous.
Most energy harvesting applications are designed to be self-sustaining, cost-effective, and to require little or no servicing for many years. In addition, the power is used closest to the source, hence eliminating transmission losses and long cables. If the energy is enough to power the device directly, the application or device powered by the energy can operate batteryless.

The Building Blocks of an Energy Harvesting System

The process of energy harvesting takes different forms based on the source, amount, and type of energy being converted to electrical energy. In its simplest form, the energy harvesting system requires a source of energy such as heat, light, or vibration, and the following three key components.

            

   

Figure (1) Basic components of an energy harvesting system. Image courtesy of harvesting-energy.com.

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

 

Common Sources of Energy

• Light energy: From sunlight or artificial light.
• Kinetic energy: From vibration, mechanical stress or strain.
• Thermal energy: Waste energy from heaters, friction, engines, furnaces, etc.
• RF energy: From RF signals.

Energy Harvesting Technologies

Harvesting electrical power from non-traditional power sources using thermoelectric generators, piezoelectric transducers, and solar cells still remains a challenge. Each of these requires a form of power conversion circuit to efficiently collect, manage, and convert the energy from these sources into usable electrical energy for microcontrollers, sensors, wireless devices, and other low-power circuits.

Harvesting Kinetic Energy

Piezoelectric transducers produce electricity when subjected to kinetic energy from vibrations, movements, and sounds such as those from heat waves or motor bearing noise from aircraft wings and other sources. The transducer converts the kinetic energy from vibrations into an AC output voltage which is then rectified, regulated, and stored in a thin film battery or a super capacitor.

Figure (2)  Midé Volture Piezoelectric Energy Harvesting Circuit. Image courtesy of Mouser.

Potential sources of kinetic energy include motion generated by humans, acoustic noise, and low-frequency vibrations. Some practical examples are:

• A batteryless remote control unit: Power is harvested from the force that one uses in pressing the button. The harvested energy is enough to power the low-power circuit and transmit the infrared or wireless radio signal.
• Pressure sensors for car tires: Piezoelectric energy harvesting sensors are put inside the car tire where they monitor pressure and transmit the information to the dashboard for the driver to see.
• Piezoelectric floor tiles: Kinetic energy from people walking on the floor is converted to electrical power that can be used for essential services such as display systems, emergency lighting, powering ticket gates, and more.

 

Harvesting RF Energy

In this arrangement, an RF power receiving antenna collects the RF energy signal and feeds it to an RF transducer such as the Powercast’s P2110 RF Powerharvester.

A P2110 Powerharvester receiver evaluation board. Image courtesy of Nuts and Volts (PDF).

The Powerharvester converts the low-frequency RF signal to a DC voltage of 5.25V, capable of delivering up to 50mA current. It is possible to make a completely battery-free wireless sensor node by combining sensors, the P2110, a radio module, and a low-power MCU.
Typical applications for these types of sensors include building automation, smart grid, defense, industrial monitoring, and more.

Figure (3) Powercast P2110 RF energy harvesting for a batteryless wireless sensor. Image courtesy of Powercast.

Harvesting Solar Energy

Small solar cells are used in industrial and consumer applications such as satellites, portable power supplies, street lights, toys, calculators, and more. These utilize a small photovoltaic cell which converts light to electrical energy. For indoor applications, light is usually not very strong and typical intensity is about 10 µW/cm².
The power from an indoor energy harvesting system thus depends on the size of the solar module as well as the intensity or spectral composition of the light. Due to the intermittent nature of light, power from solar cells is usually used to charge a battery or supercapacitor to ensure a stable supply to the application.

 

Harvesting Thermal Energy

Thermoelectric energy harvesters rely on the Seebeck effect in which voltage is produced by the temperature difference at the junction of two dissimilar conductors or semiconductors. The energy harvesting system consists of a thermoelectric generator (TEG) made up of an array of thermocouples that are connected in series to a common source of heat. Typical sources include water heaters, an engine, the back of a solar panel, the space between a power component such as a transistor and its heat sink, etc. The amount of energy depends on the temperature difference, as well as the physical size of the TEG.
The TEGs are useful in recycling energy that would otherwise have been lost as heat. Typical applications include powering wireless sensor nodes in industrial heating systems and other high-temperature environments.

 

Harvesting Energy from Multiple Sources

Manufacturers such as Maxim, Texas Instruments, and Ambient Micro have developed some integrated circuits with the ability to simultaneously capture different types of energy from multiple sources. Combining multiple sources has the benefit of maximizing the peak energy as well as providing energy even when some sources are unavailable.
An example of a circuit that harvests energy from multiple sources is as shown below:

Figure (4) Maxim Integrated MAX17710 multiple source circuit . Image courtesy of Maxim Integrated.

Benefits of Energy Harvesting

There is plenty of energy in the environment which can be converted into electrical energy to power a variety of circuits.
Energy harvesting is beneficial because it provides a means of powering electronics where there are no conventional power sources, eliminating the need for frequent battery replacements and running wires to end applications. By this same token, it opens up new applications in remote locations, underwater, and other difficult-to-access locations where batteries and conventional power are not realistic.
Energy harvesting is also largely maintenance free and is environmentally friendly.

 

Applications for Energy Harvesting Technologies

Alternative power sources provide a means of extending the battery life of remote sensors in industrial, commercial, and medical applications. This enables installation of standalone sensors in hard-to-reach or remote areas to provide a variety of information and warnings. These sensors can monitor and warn of air pollution, worn out bearings, bridge stresses, 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

 

Desirable Properties of Energy Harvesting Applications

Since the energy from harvested sources is intermittent and small, the systems must be carefully designed to efficiently capture, condition, and store the power. The systems should further incorporate circuits to control the charging process and regulate the power for the sensors, MCUs, and other low-power loads.

 

Harvesting Circuit

Energy management system components should have:

• High energy efficiency in capturing, accumulating, and storing small energy packets. Efficiency must be high enough to ensure that the energy consumed by the energy harvesting circuit is much smaller than the energy captured from the source.
• High energy retention with minimal leakage or losses in energy storage.
• Energy conditioning to ensure the output meets power requirements for the application or desired task.
• Tolerance of a wide range of voltages, currents, and other irregular input conditions.

Application Circuit

Circuits receiving harvested energy for application should:

• Consume the lowest amount of electrical power possible when active.
• Consume the lowest standby current.
• Be capable of turning on and off with minimal delay.
• Operate at the low-voltage range.

Conclusion

Harvesting energy from nonconventional sources in the environment has received increased interest over the past few years as designers look for alternative energy sources for low-power applications.

Even though energy harvested is small and in the order of milliwatts, it can provide enough power for wireless sensors, embedded systems, and other low-power applications.