The race to find renewable energy sources has been on for decades, with its importance constantly increasing as the effects of climate change wreak havoc on this Earth. Unlike traditional finite energy sources like crude oil, renewable energy sources are non-finite. They are resources that can be replenished and do not run out. Commonly seen renewable energy sources include solar, wind, hydro, etc with developments in technology introducing a variety of smart materials to this list.
Smart materials are materials which produce an output in response to a stimulus. The stimuli can range from temperature to light intensity to pressure. These technologies are already part of our everyday lives. For example, colour-change thermometers use a thermochromic pigment, meaning the colour of the pigment changes when the stimulus temperature is applied. One of the most useful smart materials in the context of renewables and the focus of this article is piezoelectric materials.
What does piezoelectric mean?
The term piezoelectric refers to a phenomenon called the piezoelectric effect. This effect was discovered by the Curie brothers through experimental results based on Lippmann’s theory of electrocapillarity. Lippmann’s theory describes the ability of a material to generate charge when pressure is applied. From this, the Curie brothers formulated the piezoelectric effect, which can be described as follows:
‘the production of electrical potential energy in a substance as the pressure on it changes’ – Bera, Das Sarker (2016)
The production of an electric current occurs in piezoelectric substances such as ZnO or GaN when stress is applied as either a stretch or squeeze. This is due to the asymmetric polarisation in the material’s molecules. Polarisation occurs in all molecules where one side is slightly positive and the other slightly negative. This creates a dipole. When compressed, a piezoelectric material produces a voltage of the same polarity as the dipole, and when stretched, it produces the opposite polarity. The type of material may depend on whether heat needs to be added to adjust the position of the molecules and therefore the direction of dipoles to allow for asymmetric polarisation.
Figure 1: Whitford, Malachi & Klimley, A. (2019). An overview of behavioural, physiological, and environmental sensors used in animal biotelemetry and biologging studies. Animal Biotelemetry. 7. 10.1186/s40317-019-0189-z.
How does this affect renewables?
Piezoelectric materials respond to stress, also known as kinetic energy, which can require little to no fossil fuels to produce, e.g., a person walking: their footstep applies a stress. This can be utilised in energy production and has already been implemented in our daily lives. Some examples are as follows.
- East Japan Railway Company – Has implemented piezoelectric tiles beneath railway ticket gates that take the kinetic energy produced from people walking across tiles and convert it into electrical energy to help power the station.
- Innowattech – A piezoelectric road system that generates electricity through kinetic energy produced when cars drive on roads.
- University of California, Berkeley – Has researched and produced piezoelectric floor panels that can be implemented in public. This also takes kinetic energy from human interaction.
Piezoelectric technologies offer amazing opportunities to create a greener society, as their source can be created endlessly. That being said, it does come with its limitations. This kind of technology is expensive not just in initial setup but in maintenance as well. The energy output from technologies such as the ones used by the East Japan Railway Company is more varied and inconsistent, as it relies on human activity on floor pads. In other words, the input of kinetic energy is not constant. Some would argue that the cost outweighs the benefits, while others would say that the technology requires development and integration.
Now, having a bit about what piezoelectricity can offer, what do you think? Do the costs outweigh the benefits, and do you see this being a definitive way forward to a greener society?
This article was written by Sofia Steadman.
