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Paraphrasing the article

Paraphrasing the article
Fossil fuels are finite and environmentally costly. Sustainable, environmentally benign energy can be derived from nuclear fission or captured from ambient sources. Large-scale ambient energy (eg solar, wind and tide), is widely available and large-scale technologies are being developed to efficiently capture it.
At the other end of the scale, there are small amounts of ‘wasted’ energy that could be useful if captured. Recovering even a fraction of this energy would have a significant economic and environmental impact. This is where energy harvesting (EH) comes in.
Energy harvesting, or energy scavenging, is a process that captures small amounts of energy that would otherwise be lost as heat, light, sound, vibration or movement. It uses this captured energy to:
• Improve efficiency – eg computing costs would be cut significantly if waste heat were harvested and used to help power the computer
• Enable new technology – eg wireless sensor networks
EH also has the potential to replace batteries for small, low power electronic devices. This has several benefits:
• Maintenance free – no need to replace batteries
• Environmentally friendly – disposal of batteries is tightly regulated because they contain chemicals and metals that are harmful to the environment and hazardous to human health
• Opens up new applications – such as deploying EH sensors to monitor remote or underwater locations
Successfully developing EH technology requires expertise from all aspects of physics, including:
• Energy capture (sporadic, irregular energy rather than sinusoidal)
• Energy storage
• Metrology
• Material science
• Systems engineering
Different types of waste energy can be captured using different EH materials. The most promising microscale EH technologies in development include:
• Vibration, movement and sound can be captured and transformed into electrical power using piezoelectric materials
• Heat can be captured and transformed into electrical power using thermoelectric and pyroelectric materials
Types of energy harvesting materials
There are several promising microscale energy harvesting materials (including ceramics, single crystals, polymers and composites) and technologies currently being developed.
The aim of these is not to generate large-scale power, but to capture small amounts of energy that is ‘wasted’ during industrial and everyday processes.
• Piezoelectric materials
• Thermoelectric materials
• Pyroelectric materials
Piezoelectric materials
Mechanical stress ? electrical signal
Piezoelectric materials convert electrical energy into a strain (or the reverse). The best known use of piezoelectricity is for medical ultrasound. It is also used for many everyday technologies including gas cooker ignition switches, electric guitar pick-ups, inkjet printers, and mobile phone speakers.
Performance
Piezoelectric energy harvesting applications are still largely at the development stage, although some devices are commercially available. There are many claims in the scientific literature (and popular press) about efficiency and expected performance.
It is difficult to validate the claims for these devices, because there is no internationally-recognised way to characterise and compare their efficiency and performance. Each researcher or company highlights the conditions that show the optimum performance of their device. These sometimes have little regard to performance under realistic conditions – for example the amplitude and frequency of vibration on a motorway bridge.
Application
Traditionally, the power industry has used electromagnetic methods to harvest mechanical energy and converts it into electrical form. However, electromagnetic devices require bulky magnets and coils, making miniaturisation difficult.
This is where piezoelectric materials come into their own, as they provide a compact, efficient solution for applications where size and weight are an issue. There are numerous potential applications, such as on-body and wireless sensors.
Thermoelectric materials
Thermoelectric materials convert wasted heat into electrical energy. Thermoelectricity is regarded as one of the most promising technologies for increasing energy efficiency in industrial processes and automotive applications, which produce a large amount of waste heat.
Performance
The performance of the energy conversion scales with the thermoelectric figure of merit of the active material. This is defined as ZT = S2sT/?
Where: S – Seebeck coefficient; s – electrical conductivity; ? – thermal conductivity; T – absolute temperature
There is fierce international competition to improve the figure of merit. However, this competition may be distorted as there is no undisputed reference material with known thermoelectric properties. This is necessary to validate testing methods and allow a reliable benchmarking of thermoelectric materials.
Fossil fuels are finite and environmentally costly. Sustainable, environmentally benign energy can be derived from nuclear fission or captured from ambient sources. Large-scale ambient energy (eg solar, wind and tide), is widely available and large-scale technologies are being developed to efficiently capture it.
At the other end of the scale, there are small amounts of ‘wasted’ energy that could be useful if captured. Recovering even a fraction of this energy would have a significant economic and environmental impact. This is where energy harvesting (EH) comes in.
Energy harvesting, or energy scavenging, is a process that captures small amounts of energy that would otherwise be lost as heat, light, sound, vibration or movement. It uses this captured energy to:
•    Improve efficiency – eg computing costs would be cut significantly if waste heat were harvested and used to help power the computer
•    Enable new technology – eg wireless sensor networks
EH also has the potential to replace batteries for small, low power electronic devices. This has several benefits:
•    Maintenance free – no need to replace batteries
•    Environmentally friendly – disposal of batteries is tightly regulated because they contain chemicals and metals that are harmful to the environment and hazardous to human health
•    Opens up new applications – such as deploying EH sensors to monitor remote or underwater locations
Successfully developing EH technology requires expertise from all aspects of physics, including:
•    Energy capture (sporadic, irregular energy rather than sinusoidal)
•    Energy storage
•    Metrology
•    Material science
•    Systems engineering
Different types of waste energy can be captured using different EH materials. The most promising microscale EH technologies in development include:
•    Vibration, movement and sound can be captured and transformed into electrical power using piezoelectric materials
•    Heat can be captured and transformed into electrical power using thermoelectric and pyroelectric materials
Types of energy harvesting materials
There are several promising microscale energy harvesting materials (including ceramics, single crystals, polymers and composites) and technologies currently being developed.
The aim of these is not to generate large-scale power, but to capture small amounts of energy that is ‘wasted’ during industrial and everyday processes.
•    Piezoelectric materials
•    Thermoelectric materials
•    Pyroelectric materials
Piezoelectric materials
Mechanical stress ? electrical signal
Piezoelectric materials convert electrical energy into a strain (or the reverse). The best known use of piezoelectricity is for medical ultrasound. It is also used for many everyday technologies including gas cooker ignition switches, electric guitar pick-ups, inkjet printers, and mobile phone speakers.
Performance
Piezoelectric energy harvesting applications are still largely at the development stage, although some devices are commercially available. There are many claims in the scientific literature (and popular press) about efficiency and expected performance.
It is difficult to validate the claims for these devices, because there is no internationally-recognised way to characterise and compare their efficiency and performance. Each researcher or company highlights the conditions that show the optimum performance of their device. These sometimes have little regard to performance under realistic conditions – for example the amplitude and frequency of vibration on a motorway bridge.
Application
Traditionally, the power industry has used electromagnetic methods to harvest mechanical energy and converts it into electrical form. However, electromagnetic devices require bulky magnets and coils, making miniaturisation difficult.
This is where piezoelectric materials come into their own, as they provide a compact, efficient solution for applications where size and weight are an issue. There are numerous potential applications, such as on-body and wireless sensors.
Thermoelectric materials
Thermoelectric materials convert wasted heat into electrical energy. Thermoelectricity is regarded as one of the most promising technologies for increasing energy efficiency in industrial processes and automotive applications, which produce a large amount of waste heat.
Performance
The performance of the energy conversion scales with the thermoelectric figure of merit of the active material. This is defined as ZT = S2sT/?
Where: S – Seebeck coefficient; s – electrical conductivity; ? – thermal conductivity; T – absolute temperature
There is fierce international competition to improve the figure of merit. However, this competition may be distorted as there is no undisputed reference material with known thermoelectric properties. This is necessary to validate testing methods and allow a reliable benchmarking of thermoelectric materials.
Application
Widespread application of thermoelectric materials was previously limited due to their small conversion efficiency. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties.
Using nanostructured materials, the efficiency of thermoelectric materials has been improved by an order of magnitude.
There has been a rapid increase in thermoelectric materials R&D. Between 2000 and 2010 the number of papers published each year in this area increased 2.5-fold. This is a consequence of the growing need to increase energy efficiency through waste heat recovery.
Pyroelectric materials
Pyroelectric materials convert changes in absolute temperature into electrical energy. Unlike thermoelectrics which need a gradient of temperature across the material (spatial gradient), pyroelectric materials require temporal temperature changes (time vs spatial variation).
Performance
Like piezoelectricity, pyroelectricity requires energy input to be in a constant state of flux and suffers from small power outputs in energy harvesting applications. However, even at this early stage of development it seems to be more efficient than other energy harvesting techniques – but only under optimal conditions. Generating a useful amount of energy requires a temporal variation in temperature of a few K every few seconds, which almost never occurs outside the laboratory.
If efficiency is defined as the maximum ratio of the electrical energy that can be produced in a ‘real’ environment (such as a building, car, computer, street, or road) divided by the amount of (thermal, mechanical, etc) energy available then pyroelectric materials are not efficient at all, whilst thermoelectric materials are.
Both of these definitions of efficiency are equally hard to measure and compare.
Compared with thermoelectricity, pyroelectricity is also easier to get working using limited surface heat exchanges. The two technologies may be complementary.
Application
The pyroelectric effect is used in some sensors, but it is still some way from commercial energy harvesting applications.

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Opportunities
Companies in a wide variety of industrial sectors are already exploring the potential of energy harvesting (EH) technologies to reduce costs, increase energy efficiency, and power autonomous embedded systems.
The expected growth market is huge. An IDTechEx report, Energy Harvesting and Storage for Electronic Devices 2010-2020 (published in Oct 2010) states the market for energy harvesting devices will rise to $4.4bn by 2020, from $605m in 2010. Examples of industry interest in EH technologies:
Automotive – Leading auto manufacturers are interested in thermoelectricity to capture heat from exhaust fumes, and piezoelectricity to harvest energy from engine vibration
•    Mobile telecommunication – Leading mobile phone manufacturers have active R&D programmes to minimise or eliminate batteries
•    Sensors and instrumentation – A single battery change for an offshore oil or gas system costs over €1m and requires personnel intervention in hostile environments. EH-powered wireless sensor networks will be quick and cost-effective to deploy and maintain. A recent study has shown that over a typical 10-year lifecycle the cost of traditional battery energy supply exceeds that of an EH device


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