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.
Mechanical stress ↔ electrical signal
Piezoelectricity literally translates as ‘electricity resulting from pressure’. It is the charge that accumulates in certain solid materials in response to applied mechanical strain.
The converse piezoelectric effect is where a mechanical strain is induced in the material by applying an electrical field.
Human motion, low-frequency vibrations, and acoustic noise are just some of the potential sources that could be harvested by piezoelectric materials.
Examples of piezoelectric EH:
- Battery-less remote control – the force used to press a button is sufficient to power a wireless radio or infrared signal
- Piezoelectric floor tiles – there is much interest in harvesting the kinetic energy generated by the footsteps of crowds to power ticket gates and display systems
- Car tyre pressure sensors – EH sensors attached inside the tyres continuously monitor the pressure and send the information to a display on the dashboard
Temperature differences across the material ↔ electric voltage
When there is a temperature across a thermoelectric crystal (ie one side is warmer/cooler than the other), it causes a voltage across the crystal.
If the temperature difference is kept constant this results in a steady voltage across the crystal.
Example of thermoelectric EH:
- Road transport – Cars and lorries equipped with a thermoelectric generators (TEG) would have significant fuel savings (especially with the increasing cost of petrol). In 2009, VW demonstrated this proof of concept. The thermoelectric generator of their prototype car gained about 600W from running on a highway, reducing fuel consumption by 5% – it is highly likely efficiency has improved significantly since then
Change in temperature ↔ electric charge
As the temperature of a pyroelectric crystal changes, it generates an electrical charge. A charge is only generated whilst the temperature of the material is changing.
Once the temperature is constant (even if it is higher or lower than initially), no charge is generated.
Example of pyroelectric EH:
- The pyroelectric effect is used in some sensors, but it is still some way from commercial energy harvesting applications