State of the art
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.
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.
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 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.
The performance of the energy conversion scales with the thermoelectric figure of merit of the active material. This is defined as ZT = S2σT/κ
Where: S – Seebeck coefficient; σ – electrical conductivity; κ – 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.
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 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).
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.
The pyroelectric effect is used in some sensors, but it is still some way from commercial energy harvesting applications.
Although each of these materials show great potential for EH, we need to develop ways to directly compare their performance and efficiency.