Sound

Cars. Subways Church bells. Planes. Ambulances. Sirens. Jackhammers.

Sounds and noises are practically synonymous with urban environments. A 2015 study found that the average street noise level around New York City was 73.4 dbA, and the noisiest areas reached up to 95 dbA [1]! For comparison, sustained sounds above 85 dbA are dangerous to the health of your ear. Even here in Copenhagen, an extraordinarily quiet city, average sound levels in the inner city are above 60 dbA [2]. Clearly, there is no shortage of sound in the city. But where does it all come from? Well, most of it is produced as “waste energy” during other energy conversion processes. We usually think of “waste energy” as a lost form of energy, but, theoretically, the energy is still there and able to be collected.

In fact, physicists are currently researching a number of ways to harvest this energy. One of the most promising avenues relies on a property of some materials called piezoelectricity. Simply put, piezoelectric materials produce a voltage in response to a mechanical stress - essentially having pressure put on it. This stress causes a slight rearrangement of the atoms in the material, causing it to go from being electrically neutral to polarized. 

In 2017, a team from Malaysia used piezoelectric sensors to convert sound to electrical energy [3]. They were not the first team to do so, but they were one of the first to demonstrate effectiveness at ambient noise levels and frequency. Previous research required around 125 dbA of sound to produce a voltage [4], but these researchers were able to produce voltage from 60-85 dbA, peaking at 96 dBA. They produced a peak of 3.9V at 33.1 dBuW, which is the equivalent of roughly 2 mW, but using voltage multipliers were able to achieve a signal of over 9.5V. The 2mW power output corresponds to about a 0.5 W/m^2 power density.

0.5 W/m^2 is about the same energy density as biomass [5]. One 2mW output is good enough for small electronic systems, and a 5m high by 1 km long panel on the side of a busy road could produce up to 2.5 kW, or enough for 2.5 US households. Alternatively, this technology could have value in industrial and manufacturing centers that are exposed to much higher noise levels (110+ dbA), as power and voltage generated increases with stronger sounds [3,4]. In both cases, while production capacity is highly variable throughout the day based on the level of activity in the city, activity patterns can be fairly predictable and the aggregate intensity of the noise should remain relatively constant for short time scales. 

Of course, we must factor in the fact that there will be losses as you scale up the implementation. Additionally, individual piezoelectric sensors are quite expensive, and wouldn’t currently justify the costs needed for a larger-production facility. But, the potential for harvesting the energy is there! And this is energy that would be produced anyway by the normal activities of a city. It normally would have gone to waste, but in the future, cities will be able to harvest some of it back.

The major drawback to the use of piezoelectric materials - putting aside the low amounts of power - is that they are mostly lead-based compounds. PZT, for example, the most commonly-used piezoelectric and the one used in the Malaysian study, is 60% lead by weight. The use of such lead-heavy compounds poses significant health risks during the manufacturing process and environmental risks during lead mining. However, alternative lead-free piezoelectric materials do exist, and one in particular is especially promising. Sodium potassium niobate is a material that has been found to exhibit the same properties as PZT, while also being much more resistant to high frequencies that cause PZT devices to break down. Moreover, the 3 elements found in this compound are incredibly abundant, and research has shown promise in producing large quantities for low-cost [6]. So, while you probably won’t be able to shout into your phone to charge it anytime soon, we could see some of the unpleasant noise pollution in cities become just a little bit more useful.

Citations [Sound]

[1] McAlexander, Tara P., Gershon, Robyn RM., and Neitzel, Richard L. “Street-level noise in an urban setting: assessment and contribution to personal exposure,” Environ Health, 14(18), 2015.
[2] Martinelli, L. “Global Noise Pollution Map”, Lukas Martinelli.ch, 2016. Accessed 28 Nov, 2020. https://lukasmartinelli.ch/gis/2016/04/03/openstreetmap-noise-pollution-map.html.
[3] Fang, LH., Hassan, SIS., Abd Rahim, R., and bin Ismail, MIB. “Exploring Piezoelectric for Sound Wave as Energy Harvester,” The 8th International Conference on Applied Energy, 2017.
[4] Zhou, Z., Qin, W., and Zhu, P. “Harvesting bi-acoustic energy by coherence resonance of a bi-stable piezoelectric harvester,” Energy, Vol. 126, 2017.
[5] van Zalk, J. and Behrens, P. “The spatial extent of renewable and non-renewable power generation,” Energy Policy, Vol. 123, 2018. 
[6] Dolhen, M., Mahajan, A., Pinho, R., Costa, ME., Trolliard, G., and Vilarinho, P. “Sodium potassium niobate thick films by electrophoretic deposition,” RSC Advances, 2015.