Putting piezoelectrical energy to use
Mónica creates a power plant under your feet
It really doesn’t look that impressive. Just a couple of red bricks in the sidewalk leading from the railway crossing to the bus stop in Zuidhorn, no different than the others. The only thing betraying the fact that something is off here, is that there’s hardly any dirt between them. And there’s a tiny LED light that would normally illuminate the location, but is dim today.
Mechatronic engineer Mónica Acuautla Meneses frowns a little and looks up at the single solar panel among the tiles. ‘It should be working’, she says. ‘The LED light is powered by the solar panel. Maybe there’s just not enough sun.’
Fortunately, the computer that’s also powered by the panel is working. Data is still flowing to the computer back at Nijenborg 4, telling the researcher about the number of people that walk here, when they walk here and the speed with which they walk.
These loose bricks may not look like much, but underneath them are two little miracles of scientific engineering. Two piezoelectric tiles that create electrical energy solely by the pressure of the people walking on top of them. Acuautla is the one that created them.
The idea behind the tiles is quite simple. They use a scientific principle that was discovered in the nineteenth century that turns mechanical energy into electricity: the piezoelectric effect. ‘Certain materials, like quartz or other crystals, are easily deformed when you put pressure on them. When you release that pressure, they turn back into their original form. But during that process a very weak energy field is created.’
What if you could power a pacemaker through the pumping of the heart?
In a sense, we’ve been using the effect for decades already. Who has never used a disposable lighter? When you push the button, you’re squeezing a tiny crystal inside, creating an electrical spark that lights the gas. Piezoelectric elements are also present in the push-buttons of electrical equipment, or the pick-up element of your record player.
‘The energy is easily created and cleaner than that of solar panels and windmills. But the problem is that the field is weak. We’re talking micro- and milliwatts here, so we could only use it for sensors that required very low power’, Acuautla says. ‘The possibilities to put the power to use are limited.’
That is changing rapidly, however. Take the Internet of Things, which is quickly entangling itself in our day-to-day lives. All these electrical devices have tiny sensors monitoring everything. These sensors need to be powered all the time, but we do not want to see them. What if you could have your own little power plant at home? In your clothes, using the energy of your movements? In a thin layer under the floor, creating energy every time you walk from your fridge to your couch?
‘There are so many possibilities’, Acuautla says. ‘We’re working on biomedical devices that require flexible electronics. What if you could power a pacemaker by adding a film of piezoelectric material that is powered by the pumping of the heart or the vibration of your footsteps?’ Surgeries to change the battery, as they are performed now, would become unnecessary.
‘We’re even working on morphing materials’, Acuautla explains. ‘Imagine a surgeon using a biomedical device or tool that can change shape once it’s inside the patient.’ It’s possible. People are already working on them, but they need smart energy sources that piezoelectricity can provide.
Acuautla and her research group are putting all their energy, dedication and creativity into creating new materials and devices to optimise the effect, so she can put it to practical use. The tiles in Zuidhorn are only the beginning.
You cannot ask people to walk with this-or-that frequency
But putting theory into practice isn’t always easy. Take the tiles in Zuidhorn that – at this point – are just gathering data about the number of people passing. The first hurdle she had to take was maximising the output of her devices. ‘Most piezoelectric materials work best at a very high frequency’, Acuautla says. ‘When people walk, the vibrating frequency is easily too low. But you cannot ask people to walk with a particular frequency!’
She had to make sure the tiles produced their maximum energy at a lower frequency. And the material has to be very robust: when people walk on it all the time, it cannot break. The fatigue response has to be high – the material has to keep popping back to its original form, even after being compressed thousands and thousands of times.
Acuautla and her colleagues from other UG institutes like ZIAM, ENTEG and CogniGron found that the common PZT – lead zirconate titanate – had many of the properties she was looking for. ‘However, PZT contains lead, which is of course very toxic.’
She had to make sure she encapsulated the lead, so her tiles wouldn’t pollute the environment. She added zirconium oxide particles to the PZT so she got a good strong performance when somebody stepped on it. And finally, she ‘doped’ the material with niobium, a crystalline and ductile transition metal, to ensure a high energy output.
It took her three years and the help of bachelor and master students from different departments endlessly trying different combinations of different materials to come up with the prototype that is now buried below the bricks in Zuidhorn. Even now, she’s still working on perfecting the material. Circumstances in the lab are never like those in the real world. ‘The soil is loose in Zuidhorn’, she found. ‘The tiles aren’t as straight as they should be.’ Then there’s the rain that is affecting them more than she had anticipated. ‘We have to find a way to solve that.’
At this moment, another tile is being constructed. One that works a bit differently: instead of just having elements that are being pushed down, Acuautla installed a tiny cantilever that will vibrate and create up to 50 percent more energy. The success of the Zuidhorn experiment also triggered plans for more real-life tests. ‘There’s plans to install them in a kindergarten in town’, she says. ‘That is of course a completely different situation. I’m so excited to make things that are working in the real world, to make something that is useful for people.’
I’m so excited to make something that works in the real world
Next step will be harvesting the energy and saving it in a battery so it can power the lights in the area, or even systems that are used to monitor the crossing. ‘But before you can do that, the devices have to be absolutely reliable, of course.’
For now, she makes do with small steps and studying the data that is flowing in from Zuidhorn. That’s already showing interesting things. There’s been a significant drop in the number of steps on the tiles, for example. There used to be an average of eighteen steps during rush hour, but that has dwindled down to just three. ‘A clear change of behaviour due to the coronavirus.’