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Energy harvesting

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Energy harvesting (also known as Power harvesting or energy scavenging) is the process by which energy is captured and stored. Frequently this term is applied when speaking about small autonomous devices, like those used in sensor networks. A variety of different methods exist for harvesting energy, such as solar power, ocean tides, piezoelectricity, thermoelectricity, and physical motion.

Traditionally electrical power has been generated from fossil fuels in large, centralized plants. Large-scale ambient energy, such as sun, wind and tides, is widely available but trickier to harvest. In urban areas, there is a surprising amount of electromagnetic energy in the environment as a result of radio and television broadcasting.

Harvesting

Energy harvesting devices which convert mechanical energy into electrical energy have attracted much interest in both the military and commercial sectors. Some systems convert random 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. 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.

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, heat or light could enable smart sensors to be functional indefinitely. Several academic 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, MIT, UC Berkeley and Southampton University. Southampton has spun out a company on energy harvesting called Perpetuum Ltd.

Typical power densities available from energy harvesting devices are highly dependent upon the specific application and design of the harvesting generator. For motion powered devices, typical values are a few W/cc for human body powered applications and hundreds of W/cc for generators powered from machinery [1]

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.

Devices

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

  • Piezoelectric crystals generate a small voltage whenever they are mechanically deformed. Vibration from engines can stimulate piezoelectric crystals, as can the heels of shoes
  • Some wristwatches are already powered by kinetics, in this case movement of the arm
  • Thermoelectric generators produce energy from the heat difference between two objects. This is also used to power a type of wristwatch, as heat energy from the human body is radiated through the watch into the environment.
  • Special antennae can collect energy from stray radio waves or theoretically even light (EM radiation). [citation 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 devices 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.

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 to for low level power to implantable or wearable sensors. These energy harvesting sources by association have an impact on the body. DARPA's effort to harness 1-2 Watts from continuous shoe impact while walking were abandoned due to the impracticality and the discomfort from the additional energy expended by a person wearing the shoes. An international Workshop is organized by UT Arlington on Piezoelectric Energy Harvesting every year which reviews the past developments and current state of the technology.

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 [2] [3] to recover the "people energy" of footsteps. They can also be embedded in shoes [4] to recover "walking energy".

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 to the low-temperature region 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, act as a heater or coolant. 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, thermocouples exist as both heaters and coolers.

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.

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.

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.

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.

See also

References

  1. ^ "Architectures for Vibration-Driven Micropower Generators, P. D. Mitcheson, T. C. Green, E. M. Yeatman, A. S. Holmes"
  2. ^ "Japan: Producing Electricity from Train Station Ticket Gates"
  3. ^ "Commuter-generated electricity"
  4. ^ "Energy Scavenging with Shoe-Mounted Piezoelectrics"

General review

Piezoelectric

Photovoltaic

Thermocouple

Future directions

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