Energy harvesting (EH), also called power harvesting, energy scavenging, or ambient power, is the process of taking energy from outside sources, such as sunlight, heat, wind, saltwater differences, and movement, then saving it for later use by small, wireless devices that work on their own, like those in wearable technology, monitoring systems, and sensor networks.

Energy harvesters usually provide very little power for devices that use small amounts of energy. While large-scale energy generation uses resources like oil and coal, the energy used by harvesters is already present in the environment. For example, temperature differences occur when a car engine runs, and in cities, there is a lot of electromagnetic energy from radio and TV signals.

One of the earliest examples of using ambient energy to create electricity was the successful use of electromagnetic radiation (EMR) to power a crystal radio.

The way energy is harvested from ambient EMR can be shown using simple parts.

Operation

Energy harvesting devices change energy from the environment, such as movement or heat, into electricity. These devices are important in the military and for commercial uses. Some systems use motion, like ocean waves, to create electricity for sensors that monitor the ocean and operate without needing outside power. In the future, large energy-harvesting systems may be placed in remote areas to provide power for big systems. Energy-harvesting devices can also be used in wearable electronics, such as powering cell phones, computers, and radios. These devices must be strong enough to last in harsh conditions and sensitive enough to use a wide range of movements. One recent method to create electricity from vibrations uses Auxetic Boosters. This method is part of a type of energy harvesting called piezoelectric-based vibration energy harvesting (PVEH). The electricity generated can power wireless sensors, cameras, and Internet of Things (IoT) devices.

Small sensors, like those made with MEMS technology, can also be powered by harvesting energy. These sensors are tiny and need little power, but they often rely on batteries, which limit their use. Collecting energy from vibrations, wind, heat, or light could allow these sensors to work without batteries forever.

The amount of power energy-harvesting devices can produce depends on their use and design. For example, devices powered by human movement typically generate a few microWatts per square centimeter, while those powered by machines can produce hundreds of microWatts per square centimeter. Most devices used in wearable electronics create very little power.

Energy can be stored in capacitors, super capacitors, or batteries. Capacitors are used when a device needs a quick burst of energy. Batteries lose less energy over time and are used for steady power. The type of battery used affects how well it works. Common batteries include lead-acid or lithium-ion batteries, though older types like nickel metal hydride are still used. Super capacitors can be charged and discharged many times and do not need maintenance, making them useful for IoT and sensor devices.

Today, interest in low-power energy harvesting focuses on independent sensor networks. These systems collect energy, store it in a capacitor, and then increase or regulate the power to a second capacitor or battery for use in a microprocessor or data transmission. The energy is usually used in sensors, and the data is stored or sent, often through wireless methods.

Motivation

One of the main reasons scientists are developing new energy harvesting devices is to power sensor networks and mobile devices without using batteries. These batteries have limits, such as a short lifespan, harm to the environment, large size, heavy weight, and high cost. Energy harvesting devices can offer a different or extra power source for uses that need little energy, like remote sensing, wearable electronics, monitoring conditions, and wireless sensor networks. These devices can also help batteries last longer or allow some devices to work without batteries at all.

Another reason for using energy harvesting is to help solve climate change by reducing greenhouse gas emissions and the use of fossil fuels. Energy harvesting devices can use clean and renewable energy sources that are widely available in nature, such as sunlight, heat, wind, and movement energy. These devices can also reduce the need for power systems that waste energy and harm the environment. This helps create a more sustainable and strong energy system.

Recent research has led to the creation of devices that can power themselves through user actions. Examples include battery-free game consoles and toys, which use energy from actions like pressing buttons or turning knobs. These studies show how energy from user interactions can power devices and increase their ability to operate without needing traditional batteries. This supports the use of renewable energy and reduces dependence on regular batteries.

Energy sources

There are many small energy sources that cannot be expanded to the size of large industrial power systems like solar, wind, or wave energy:

• Some wristwatches use kinetic energy (called automatic watches) to power themselves. When the arm moves while walking, the motion winds the watch's mainspring. Other designs, like Seiko's Kinetic, use a loose magnet inside the watch to create electricity.

• Photovoltaics is a way to make electricity by turning sunlight into direct current using special materials that react to light. These materials are used in solar panels. Solar panels have been made large enough for industrial use, and big solar farms now exist.

• Thermoelectric generators (TEGs) use two different materials joined together and a temperature difference. By connecting many of these materials in series and parallel, they can create small amounts of electricity. These generators can collect energy from machines, buildings, or even the human body. They often use heat sinks to increase the temperature difference.

• Micro wind turbines capture wind energy to power small devices like wireless sensors. When wind flows over the turbine blades, it creates a pressure difference that causes the blades to spin. Like solar panels, large wind farms are used to make significant amounts of electricity.

• Piezoelectric materials, such as crystals or fibers, create a small electrical charge when they are bent or pressed. Vibrations from engines, footsteps, or button presses can cause these materials to generate electricity.

• Special antennas can collect energy from radio waves. This can also be done with a Rectenna or a Nantenna, which may work with higher frequency radiation.

• Pressing keys on a remote control or portable device can generate electricity using magnets, coils, or piezoelectric materials to help power the device.

• Vibration energy harvesting uses electromagnetic induction, which involves a magnet and a copper coil to create electricity. This method is used in some devices to convert motion into power.

• A device called Air-gen, developed by a team at the University of Massachusetts, uses tiny pores to generate electricity from humidity in the air.

A possible energy source is radio waves from common transmitters. Historically, large areas or close proximity to these transmitters were needed to collect usable power. A Nantenna is a proposed technology that could use natural radiation, like sunlight, to improve this process.

One idea is to send radio waves intentionally to power remote devices. This is already used in RFID systems, but regulations limit the power levels for safety. This method has been used to power sensors in wireless networks.

Turbine and non-turbine technologies can capture airflow. Towered wind turbines and airborne wind systems use wind energy. These technologies can work in low-light areas, such as inside HVAC ducts, and can be adjusted for specific energy needs.

Blood flow can also be used to power devices. For example, a pacemaker developed at the University of Bern uses blood flow to wind up a spring, which then drives a small generator.

Water energy harvesting has advanced with designs like transistor-like generators that convert water movement into electricity efficiently.

Photovoltaic (PV) technology offers advantages over wired or battery-powered systems by using nearly endless sunlight with little environmental harm. Indoor PV systems have used amorphous silicon, but newer technologies like Dye-Sensitized Solar Cells (DSSC) now produce more power. These cells use dyes that absorb light, similar to how plants use chlorophyll, to generate electricity.

The piezoelectric effect turns mechanical movement into electricity. This can come from human motion, vibrations, or sound. Most piezoelectric sources produce small amounts of power, enough for devices like self-winding watches but not for larger systems. One example uses pressurized fluid to drive a piston connected to piezoelectric elements, creating electricity.

Piezoelectric energy harvesting is still a developing technology. However, some advancements include a battery-free wireless doorbell and sensors powered by vibrations. These systems are used in wearable devices and clothing. Research has also explored using body movements, like walking or breathing, to generate power.

Piezoelectric materials are being placed in walkways to capture energy from footsteps.

Future directions

Electroactive polymers (EAPs) have been suggested as a way to collect energy. These materials can stretch a lot, store energy efficiently, and convert energy with high effectiveness. Systems using EAPs are expected to weigh much less than systems using piezoelectric materials.

Nanogenerators, like the one developed by Georgia Tech, might offer a new method to power devices without batteries. As of 2008, this generator produces only a small amount of power, measured in nanowatts, which is too weak for practical use.

Noise has been studied by the NiPS Laboratory in Italy as a way to collect small vibrations across a wide range of frequencies. Their method uses a special mechanism that can improve energy collection efficiency by up to four times compared to traditional methods.

Uneven features in certain energy harvesters can greatly influence how well they work and how much energy they collect. Studies show that while these uneven features may reduce performance in some situations, they can also be adjusted—by changing angles and selecting strong parameters—to improve energy conversion and outperform symmetrical designs.

Combining different types of energy harvesters can reduce reliance on batteries, especially in places where the types of available energy change over time. This method, called complementary balanced energy harvesting, may improve the reliability of wireless sensor systems used for monitoring the health of structures.