Okay ya’ll, this is a long one. Feel free to skim to get the general idea. This concept is cool to me so I went pretty hard. Enjoy!
Piezoelectricity is a result of the piezoelectric effect. “Piezo” means to squeeze or to press. This phenomenon was discovered circa 1880 by brothers Pierre and Jacques Curie. In the simplest terms, the piezoelectric effect is when certain materials are applied mechanical pressure or vibration (kinetic energy), they produce an electric charge (electric energy), and piezoelectric generation is a form of vibrational energy harvesting (Jintanawan, et. al., 2020). The charge created has the potential to be transmitted as usable energy directed towards the grid or within a technology device, also known as piezoelectricity. Oppositely, when an electric charge is applied to a certain material the material changes form slightly, which is known as the reverse piezoelectric effect. The Curie brothers discovered this effect by the use of crystals, like quartz. Today, over 200 and materials and counting have been found to have piezoelectric properties, most of these belonging to the ferroelectric family (Sharma, et. al., 2022).
This phenomenon occurs because of the elemental structure of certain crystal lattices, and some other materials, which have a neutralized balance of negative and positive charges throughout their atomical structure. (Mishra, et. al., 2019). When a mechanical stress like tension or compression is applied, and their original arrangement of protons and electrons shift and polarize. All of the electrons move to one end, and the protons on the other, creating an imbalance of negative and positive charge, thus generating the conditions for a voltage and potential current. This imbalance is mainly due to the asymmetry of the material’s structure, and the polar axes throughout the material. If the formation of the material were symmetrical, the charges would exhibit a one-way direction, and would not cause an electric charge because there would be no division of protons and electrons on either face of the material (Caliò, et. al., 2014).
Piezoelectricity is the harnessing of this effect and voltage derived from the polarization of charges (otherwise known as a dipole) when the material is put under pressure. When the materials are integrated into a circuit (like with the use of wires on opposing charged ends of the material), the voltage drop across it can be used to create an electric current (Cramm, J., et. al., 2011). This process requires three main components: a rectifier for converting alternating current (AC) into direct current (DC), a voltage regulator for regulating the DC power provided to the storage device, and a storage device for storing the output energy from the piezoelectric energy harvesting device (Cramm, J., et. al., 2011).
While the piezoelectric effect is widely used around the world in all kinds of technology like lighters, watches, sonar, ultrasounds, and much more, the focus here is on using piezoelectricity as an energy source for the grid drawn from floor tiles. Using human movement and the pressure from people walking, tiles with piezoelectric capable material are placed on the floor or sidewalks to harness the energy created using the piezoelectric effect and turn it into usable electric energy. There are a variety of designs used for these floor tiles, including but not limited to cantilever-based, curved piezo element-based, and piezoelectric diaphragm-based. All revolve around the basics of the piezoelectric effect and this technology is in constant evolution as technology innovation and material extraction improves overtime. (Sharma, et. al., 2022)
There are a number of considerable advantages to this type of electricity production. To start, piezoelectric tiles are nonmodular, in that they can be made to fit a certain area, and no one size fits all. Additionally, they are not at the mercy of unpredictable weather conditions. This means they can be placed outside, or indoors, which is already a leg up over solar, wind and hydropower renewables. Like other renewables, though, piezoelectric tiles have virtually no emissions, no sound or air pollution, and can aid in reducing CO2 emissions by acting as an additional source of energy that can replace some of the production share that fossil fuels hold (Solban, & Moussa, R. R., 2019). These tiles overall can further ignite innovation towards finding and implementing green energy sources.
Another beneficial characteristic similar to other renewables that these tiles have is that they are decentralized, as numerous companies have the capability to implement. This means that if and when a major power source/plant ceases to produce, these tiles are still able to generate because they work independently of one centralized source. The autonomy of these tiles also means they are modular systems that can be altered and updated overtime, with the ability of being moved if necessary. The shape of the tiles can be unique to a certain area, so these tiles can fit in a variety of already existing infrastructure without looking too out of place. Locomotion is essential, because when positioned in prime, heavily trafficked areas, the amount of energy produced, coupled with storage, would be enough for local onsite powering of street signs, lights, and other facilities (Mraz, 2017). Some examples of applied tiles show that in a single second, 0.1 W were produced in a single second by a person weighing 60 kg with just two steps, and this potential is even greater when used in large areas with heavy, continuous traffic (Moussa, Ismaeel, & Solban, 2022).
To further exemplify the minimal pollution and waste potential of this type of generation, piezoelectric tiles are generally extremely easy to recycle. The tiles consist of mostly already recycled car tires that can be converted into goods like playground surfacing or athletic tracks, Aluminum that is easily recycled or sold as scrap, and quartz (which is the typical piezoelectric material used) that can be recycled similarly to glass (Cramm, J., et. al., 2011). Of course, quartz is not the only material that creates a piezoelectric effect. The wide array of materials, both natural and humanmade, that can generate energy in this manner is another benefit. To name only a few of the materials that could potentially serve the tiles’ purpose, there is crystal quartz, thin film and polymeric materials like polyvinylideneflouride, and piezoceramic materials like lead zirconate titanate (Maghsoudi, et. al., 2019). Lead zirconate titanate (PZT) is an artificial material and one of the most popular and effective materials for piezoelectric tiles because it can produce an average of 8.4 mW per tile over 20 years, only costs $36.10 a tile, and is has a great efficiency with the ability to convert up to 80% of the mechanical energy into electricity (Solban, & Moussa, 2019).
Looking away from PZT tiles only, when comparing piezoelectric technology companies, a tile ranges from around $30 up to $400, with a lifespan of up to 20 years and an energy production varying from just 1 W to 250 kW per tile; this is fairly affordable (Moussa, Ismaeel, Solban, 2022). In essence, the tiles have the potential to recover the costs of the initial purchase, transport, installation, and disposal over their lifespan in the amount of energy they generate (Cramm, J., et. al., 2011).
Piezoelectric tiles allude the impression of being an effectively innovative and futuristic form of harnessing energy passively. There are numerous drawbacks to this technology, which may attribute to why these tiles are not widespread or significant. The lack of macro-scale implementation of means that there is much uncertainty about the actual efficiency of energy harvesting due to varying voltage that depends on factors like temperature, pressure, the type of material used (Li & Strezov, 2014) and even factors like pedestrian weight, speed, and amount of use. The novelty and uncertainty in itself are a, perhaps temporary, drawback of this technology because of the misunderstanding of how to measure inputs and results. An overestimated result produced by laboratory studies could lead to inefficient energy generation in the real world, thus leaving the system useless (Yingyong, et. al., 2021). Thus, there is unreliable and somewhat uncontrollable capacity factor. This means that high traffic areas should be identified to maximize efficiency (Li & Strezov, 2014) and even then, these areas may become obsolete or vary dramatically. If the tiles are not being stepped on or applied pressure, they are theoretically useless. The materials used themselves also have various errors in measurement, as crystals have a high impedance, or resistance to converting to an alternating current, so they need to be connected to an amplifier and auxiliary circuit (Solban, & Moussa, R. R., 2019).
The durability of these tiles is also in question, as both the tile construction and piezoelectric material can be rigid, brittle, toxic, high density, low voltage, and inflexible – all limiting the ability to create, harness and supply energy (Mishra, et. al., 2019). There is a bit of a safety concern with these factors too, because if a pedestrian is walking and the tile breaks, they can be hurt or exposed to electrical and toxic hazards. Additionally, the weakness of the materials means the tiles will require regular maintenance and will not survive in harsh conditions, like roadways – making their existence somewhat costly and labor intensive. This is a shame because harnessing the energy from roadways would be an extraordinary feat in the increasing levels of energy generated from heavy machinery and speed. Furthermore, the tiles are temperature sensitive, fatigue sensitive, and difficult to manufacture (Visconti, et. al., 2022). This means the tiles may not work well in all outdoor climates.
One major concern over these materials is level of toxicity, as the most popular and efficient piezoelectric material, lead zirconate titanate (PZT) is a toxic health risk. PZT is valued for its high cost-effectiveness, but contains a significant level of lead toxicity, so suggestions have been made to use lead-free alternatives, but these materials are not established nor or the cost benefits clear (Moussa, Ismaeel, & Solban, 2022). PZT is 100 times more efficient than quarts and produces a fairly high voltage (Solban, & Moussa, 2019) so it is the most common piezoelectric tile material, yet its usage is directly putting toxic materials into the ground right beneath our feet.
The piezoelectric effect is used widely, in propane grills, lighters, watches, record players, microphones, ultrasounds, sonar, and more common items used on a daily basis. This effect has only started to be used to harness energy into the grid, though. When it comes to piezoelectric tiles specifically, there exists a variety of companies utilizing this concept including but not limited to, Waynergy from Portugal, Soundpower from Japan, and most notably Pavegen from the UK (Li, & Strezov, 2014). While these companies are not yet mainstream, Pavegen is dominating the niche market, and according to their website they are currently installed in over 36 countries. They have developed a paving unit that is 600 mm x 450 mm x 82 mm and is able to generate up to 7 W electricity per footstep, which is totally competitive against other companies with only a 0.1 W per step generation (Li, & Strezov, 2014). The competitive nature of these novel companies will allow for surprising innovation and competitive prices, making the tiles more affordable and available overtime.
One place that successfully exemplifies Pavegen tiles is in a nightclub, The Club Watt in Rotterdam, where the tiles produced over 30% of the energy consumed, and the overall savings over a ten-year period are about $824,503 (Solban, & Moussa, R. R., 2019). A dance floor is an excellent example of the creative areas where these tiles can thrive. Another example is in Sydney at Macquarie University. Finding a the optimal high-traffic area was essential for this project because the building floor area was 16,000 m squared, and at the time the cost of tiles were $3850 per tile from Pavegen (Li, & Strezov, 2014). This example is exemplary of the importance of choosing a location due to the cost benefit factor and to maximize efficiency. In Tokyo, a transportation station, the Taesu North Exit, only became a success after a few rounds of implementation. The tiles in the station quickly deteriorated after only three weeks, which is not totally surprising because over two million steps are taken over that area each day. The next set of tiles were made much larger and thicker (Solban, & Moussa, R. R., 2019). With this new design, the payback for installation will be returned after only three years.
As for the future, these tiles can provide more than just electricity, but they can enhance the availability of data. For example, in London, there are various incorporations of Pavegen used around the city and in some experimental spaces each tile is equipped with the ability to transmit real-time movement data analytics by connecting to mobile devices and building management systems (www.pavegen.com). This information could be used to improve smart cities while simultaneously harvesting energy that feeds into nearby structures like interactive billboards or other signage.
Eventually, these tiles could also be built durable enough to withstand the tough conditions of roadways. When this happens, the tiles can sense and distribute information about roadway conditions, temperature, or traffic to vehicles (Mraz, 2017). Additionally, the integration of such sensors can provide safety monitoring both indoors and outdoors. For example, if there were an intruder in a building, the tiles can aid in finding the exact location. On the other hand, these sensors and data analytics can be viewed as a breach of privacy. These tiles might one day be ubiquitous on the streets, buildings, even in our shoes and chairs, and therefore basically documenting our every move.
These tiles can also be coupled with photovoltaic technology in the future, otherwise known as hybrid energy (HEF) tiles. This tile and panel combination is uses Copper Indium Selenide solar technology, which is beneficial because of the ability to perform well in shady areas. (Mousa, Ismaeel & Solban, 2022). This setup maximizes energy production while minimizing input, increasing efficiency further. These tiles are already in experimental use in some places, typically installed in commercial streets, public squares, and parks. Perhaps, eventually, this technology will replace streets, sidewalks and floorings as we know them today.
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Alrighty. That’s it!
In Soil We Trust,