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Keywords Electrical engineering, Human power, Energy conversion, Energy conservation, Microtechnology, Energy production, Energy efficiency
SDGs Sustainable Development Goals SDG07 Affordable and clean energy
Published by KVDP
Published 2009
License CC BY-SA 4.0
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Human energy harvesting is a term used to describe the using of systems that utilizes the human body as the primary source of energy to generate and store energy (often in the form of electricity).[1] Human energy harvesting systems are often small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. At present, human energy harvesters do not produce sufficient energy to perform mechanical work, but instead provide a very small amount of power for powering low-energy electronics. While the input fuel to large scale generation costs money (oil, coal, etc.), the "fuel" for energy harvesters is naturally present and is therefore considered free. For example, temperature gradients exist from the operation of a combustion engine and in urban areas, there is also a large amount electromagnetic energy in the environment because of radio and television broadcasting.

## Operation

Energy harvesting devices converting ambient energy into electrical energy have attracted much interest in both the military and commercial sectors. Some systems convert motion, such as that of ocean waves, into electricity to be used by oceanographic monitoring sensors for autonomous operation. Future applications may include high power output devices (or arrays of such devices) deployed at remote locations to serve as reliable power stations for large systems. Another application is in wearable electronics, where energy harvesting devices can power or recharge cellphones, mobile computers, radio communication equipment, etc. All of these devices must be sufficiently robust to endure long-term exposure to hostile environments and have a broad range of dynamic sensitivity to exploit the entire spectrum of wave motions.

### Accumulating energy

Energy can also be harvested to power small autonomous sensors such as those developed using MEMS technology. These systems are often very small and require little power, but their applications are limited by the reliance on battery power. Scavenging energy from ambient vibrations, wind, heat or light could enable smart sensors to be functional indefinitely. Several academic and commercial groups have been involved in the analysis and development of vibration-powered energy harvesting technology, including the Control and Power Group and Optical and Semiconductor Devices Group at Imperial College London, IMEC and the partnering Holst Centre,[2] MIT Boston, Georgia Tech, UC Berkeley, Southampton University, PMG Perpetuum, National University of Singapore and Columbia University.[3]

Typical power densities available from energy harvesting devices are highly dependent upon the specific application (affecting the generator's size) and the design itself of the harvesting generator. In general, for motion powered devices, typical values are a few $\displaystyle{ \mu }$W/cc for human body powered applications and hundreds of $\displaystyle{ \mu }$W/cc for generators powered from machinery.[4]

In practice, for energy scavenging devices for wearable electronics, most devices generate just a few milliWatts of power.[5]

### Storage of Energy

In general, energy can be stored in a capacitor, super capacitor, or battery. Batteries use stored chemical energy to provide energy for users. Capacitors are used when the application needs to provide huge energy spikes. Batteries leak less energy and are therefore used when the device needs to provide a steady flow of energy.

### Use of the power

In small applications (wearable electronics), the power follows the following circuit: after being transformed (by e.g. AC/DC-to-DC/DC-inverter) and stored in an energy buffer (e.g., a battery, condenser, capacitor, etc.), the power travels through a microprocessor (fitted with optional sensors) and is transmitted (usually wirelessly).

## Motivation

The history of energy harvesting dates back to the windmill and the waterwheel. People have searched for ways to store the energy from heat and vibrations for many decades. One driving force behind the search for new energy harvesting devices is the desire to power sensor networks and mobile devices without batteries. Energy harvesting is also motivated by a desire to address the issue of climate change and global warming.

## Devices

There are many small-scale energy sources that generally cannot be scaled up to industrial size:

• Piezoelectric crystals or fibers generate a small voltage whenever they are mechanically deformed. Vibration from engines can stimulate piezoelectric materials, as can the heel of a shoe.
• Some wristwatches are already powered by kinetic energy (called kinetic watches), in this case movement of the arm. The arm movement causes the magnet in the electromagnetic generator to move. The motion provides a rate of change of flux, which results in some induced emf on the coils. The concept is simply related to Faraday's Law.
• Thermoelectric generators (TEGs) consist of the junction of two dissimilar materials and the presence of a thermal gradient. Large voltage outputs are possible by connecting many junctions electrically in series and thermally in parallel. Typical performance is 100-200 uV/degreeC per junction. These can be utilized to capture mW of energy from industrial equipment, structures, and even the human body. They are typically coupled with heat sinks to improve temperature gradient.
• Micro wind turbine are used to harvest wind energy readily available in the environment in the form of kinetic energy to power the low power electronic devices such as wireless sensor nodes. When air flows across the blades of the turbine, a net pressure difference is developed between the wind speeds above and below the blades. This will result in a lift force generated which in turn rotate the blades. This is known as the aerodynamic effect.
• Special antennae can collect energy from stray radio waves or theoretically even light (EM radiation). [verification needed]

### Ambient-radiation sources

A possible source of energy comes from ubiquitous radio transmitters. Unfortunately, either a large collection area or close proximity to the radiating source is needed to get useful power levels from this source.

One idea is to deliberately broadcast RF energy to power remote devices: This is now commonplace in passive Radio Frequency Identification (RFID) systems, but the Safety and US Federal Communications Commission (and equivalent bodies worldwide) limit the maximum power that can be transmitted this way.

### Biomechanical harvesting

Biomechanical energy harvesters are also being created. One current model is the biomechanical energy harvester of Max Donelan which straps around the knee.[6] Devices as this allow the generation of 2.5 watts of power per knee. This is enough to power some 5 cell phones.

### Piezoelectric energy harvesting

The piezoelectric effect converts mechanical strain into electrical current or voltage. This strain can come from many different sources. Human motion, low-frequency seismic vibrations, and acoustic noise are everyday examples. Except in rare instances the piezoelectric effect operates in AC requiring time-varying inputs at mechanical resonance to be efficient.

Most piezoelectric electricity sources produce power on the order of milliwatts, too small for system application, but enough for hand-held devices such as some commercially-available self-winding wristwatches. One proposal is that they are used for micro-scale devices, such as in a device harvesting micro-hydraulic energy. In this device, the flow of pressurized hydraulic fluid drives a reciprocating piston supported by three piezoelectric elements which convert the pressure fluctuations into an alternating current.

Piezoelectric systems can convert motion from the human body into electrical power. DARPA has funded efforts to harness energy from leg and arm motion, shoe impacts, and blood pressure for low level power to implantable or wearable sensors. The nanobrushes of Dr. Zhong Lin Wang are another example of a piezoelectric energy harvester.[7] They can be integrated into clothing. Careful design is needed to minimise user discomfort. These energy harvesting sources by association have an impact on the body. the Vibration Energy Scavenging Project[8] is another project that is set up to try to scavenge electrical energy from environmental vibrations and movements.

The use of piezoelectric materials to harvest power has already become popular. Piezoelectric materials have the ability to transform mechanical strain energy into electrical charge. Piezo elements are being embedded in walkways [9] [10] to recover the "people energy" of footsteps. They can also be embedded in backpacks [11]and shoes [12] to recover "walking energy".

### Pyroelectric energy harvesting

The pyroelectric effect converts a temperature change into electrical current or voltage. It is analogous to the piezoelectric effect, which is another type of ferroelectric behavior. Like piezoelectricity, pyroelectricity requires time-varying inputs and suffers from small power outputs in energy harvesting applications. One key advantage of pyroelectrics over thermoelectrics is that many pyroelectric materials are stable up to 1200 C or more, enabling energy harvesting from high temperature sources and thus increasing thermodynamic efficiency. There is a pyroelectric scavenging device that was recently introduced, however, that doesn't require time-varying inputs. The energy-harvesting device uses the edge-depolarizing electric field of a heated pyroelectric to convert heat energy into mechanical energy instead of drawing electric current off two plates attached to the crystal-faces. Moreover, stages of the novel pyroelectric heat engine can be cascaded in order to improve the Carnot efficiency.[13]

## Thermoelectrics

In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two dissimilar conductors produces a voltage. At the heart of the thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference. In 1834, Jean Charles Athanase Peltier discovered that running an electric current through the junction of two dissimilar conductors could, depending on the direction of current flow, cause it to act as a heater or cooler. The heat absorbed or produced is proportional to the current, and the proportionality constant is known as the Peltier coefficient. Today, due to knowledge of the Seebeck and Peltier effects, thermoelectric materials can be used as heaters, coolers and generators (TEGs).

Ideal thermoelectric materials have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Low thermal conductivity is necessary to maintain a high thermal gradient at the junction. Standard thermoelectric modules manufactured today consist of P- and N-doped bismuth-telluride semiconductors sandwiched between two metallized ceramic plates. The ceramic plates add rigidity and electrical insulation to the system. The semiconductors are connected electrically in series and thermally in parallel.

Miniature thermocouples have been developed that convert body heat into electricity and generate 40μW at 3V with a 5 degree temperature gradient, while on the other end of the scale, large thermocouples are used in nuclear RTG batteries.

Practical examples are the finger-heartratemeter by the Holst Centre and the thermogenerators by the Fraunhofer Gesellschaft.[14][15]

Advantages to thermoelectrics:

1. No moving parts allow continuous operation for many years. Tellurex (a thermoelectric production company) claims that thermoelectrics are capable of over 100,000 hours of steady state operation.
2. Thermoelectrics contain no materials that must be replenished.
3. Heating and cooling can be reversed.

One downside to thermoelectric energy conversion is low efficiency (currently less than 10%). The development of materials that are able to operate in higher temperature gradients, and that can conduct electricity well without also conducting heat (something that was until recently thought impossible), will result in increased efficiency.

Future work in thermoelectrics could be to convert wasted heat, such as in automobile engine combustion, into electricity.

### Electromagnetic energy harvesting

This technique gathers power as vibrating magnets move past a coil. An example are the systems of PMG Perpetuum and which are used in the petrochemical industry.[16][17]

### Electrostatic (capacitive) energy harvesting

This type of harvesting is based on the changing capacitance of vibration-dependent varactors. Vibrations separate the plates of an initially charged varactor (variable capacitor), and mechanical energy is converted into electrical energy. An example of a electrostatic energy harvester with embedded energy storage is the M2E Power Kinetic Battery. Another example is CSIRO's Flexible Integrated Energy Device (FIED) [18]

## Blood sugar energy harvesting

Another way of energy harvesting is through the oxidation of blood sugars. These energy harvesters are called biofuelcells. They could be used to power implanted electronic devices (e.g., pacemakers, implanted biosensors for diabetics, implanted active RFID devices, etc.). At present, the Minteer Group of Saint Louis University has created enzymes that could be used to generate power from blood sugars. However, the enzymes would still need to be replaced after a few years.[19]

### Tree metabolic energy harvesting

Voltree has developed a method for harvesting energy from trees. These energy harvesters are being used to power remote sensors and mesh networks as the basis for a long term deployment system to monitor forest fires and weather in the forest. Their website says that the useful life of such a device should be limited only by the lifetime of the tree to which it is attached. They recently deployed a small test network in a US National Park forest.[20]

### Future directions

Electroactive polymers (EAPs) have been proposed for harvesting energy. These polymers have a large strain, elastic energy density, and high energy conversion efficiency. The total weight of systems based on EAPs is proposed to be significantly lower than those based on piezoelectric materials.

Nanogenerators, such as the one made by Georgia Tech, could provide a new way for powering devices without batteries.[21] Although at present (2008) it only generates some dozen nanowatts, which is too low for any application.