Piezoelectric GeneratorssuPPlemental Power suPPly without chanGinG social Behaviour

introduction In 1675, a French apothecary named Pierre Seignette prepared the compound Potassium Sodium Tartrate, a double salt that has become known as “Rochelle Salt,” named for Pierre’s home town of La Rochelle, France. This synthetic mineral possessed several properties which made it popular as a commercial laxative, but it was in 1880 that French physicists Jacques and Pierre Curie discovered that the large salt crystals responded to mechanical force with an electrical pulse, and vice-versa. The first practical use for piezoelectric crystals wasn’t employed until World War One, 1917, for a sonar submarine detection system. As their applications have increased in the past few years, piezoelectric generators have been put to use in a wide range of small scale devices such as pacemakers, flashing shoe soles, and audio pick-ups. The goal of this study was to assess whether piezoelectric crystals could be used to power entire population centers if applied at a mass scale. New York City metropolitan area was chosen as the potential application site where such crystals would be applied on sidewalks, and calculations were carried out by taking into account multiple factors such as efficiency of piezoelectric crystals available, estimated application area, number of pedestrians, cost, etc.

Potential The most efficient individual piezoelectric yet manufactured has a maximum output of 8.4 Watts per square inch. Several assumptions were necessary to estimate the pedestrian traffic on New York City sidewalks, and thus electricity generation potential of applied piezoelectric crystals. On average, there were estimated to be 350,000 pedestrians in Times Square, 97,000 on Main Street in Queens, and 80,000 in the Bronx. Such sites were chosen for the analysis due to their high pedestrian traffic, and so they provide reasonably good starting points for large-scale application of piezoelectric crystal generators. The maximum volume for comfortable pedestrian movement was assumed to be 12 people per minute per yard of sidewalk width. Assuming an average human stride-length of 31 inches, each square yard of sidewalk can be estimated to receive 18 to 25 steps per minute, or one step per second per 3.33 square yards.

18-25 steps/minute per sq. yd. or 1-2 steps/second per 3.3 square yards A sheet of 1,296 square inches of piezoelectric generators could be positioned beneath one square yard of sidewalk. Given this data, along with the potential output of each individual generator, each 3.33 square yards of sidewalk would produce a 36 kW output pulse each second.

8.4 W per sq. inch x 1,296 sq. inches = 10.9 kW per sq. yard Multiplied by the 12,750 miles of sidewalk in the city, the pedestrians of New York could produce 490 MW of power during peak hours.

Production & testinG A functional piezoelectric crystal was produced as part of the study by using easily accessible commercial products that may be found in a supermarket, and its electricity generation was demonstrated by tests. The piezoelectric crystal with the chemical formulation KNaC4H4O6·4H2O (Potassium Sodium Tartrate) was produced by mixing Sodium Carbonate (soda ash) with Potassium Bitartrate (cream of tartar) in water at 180º F, in settings that can easily be achieved in a common laboratory. The components, process, and result are shown in the images below. Following the production of piezoelectric crystals, they were tested for their electrical generation potential by using an oscilloscope. The waveform produced as a response of crystals to mechanical force is demonstrated in the figure above.

Cited ResouRCes1. Newnham, R.E.; Cross, L. Eric (November 2005). “Ferroelectricity: The Foundation of a Field from Form to Function”. MRS Bulletin 30: pp. 845–846.2. Gautschi, G, 2002. Piezoelectric Sensorics: Force, Strain, Pressure, Acceleration and Acoustic Emission Sensors, Materials and Amplifiers. Springer.3. Sodano, H.A., Inman, D.J., and Park, G. 2004. “Estimation of Electric Charge Output for Piezoelectric Energy Harvesting,” Strain Journal 40(2), pp. 49-58.4. Pavegen Systems Ltd., 2014. Accessed 1/2014. http://www.pavegen.com/technolohy5. Innowattech Ltd., 2014. Accessed 1/2014. http://www.innowattech.co.il/technology.aspx 6. Karpelson, M., Wei, G-Y., and Wood, R.J., 2012. “Driving high voltage piezoelectric actuators in microrobotic applications,” Sensors and Actuators A: Physical 176, pp. 78-89.7. Roberts, Sam, 2011. “You like walking in the city? So do plenty of others,” New York Times Online http://www.nytimes.com/archives

summary of the PrinciPle The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect, which is the internal generation of electrical charge resulting from an applied mechanical force, also exhibit the reverse piezoelectric effect which is the internal generation of a mechanical strain resulting from an applied electrical field. For example, lead zirconate titanate crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1% of the original dimension. Conversely, those same crystals will change about 0.1% of their static dimension when an external electric field is applied to the material.

imPlementation A company called “PaveGen” has developed a floor tile that flexes a specialized piezoelectric generator when it is stepped on to light a lamp nestled at its center, and a track segment was covered with such tiles during recent marathon competitions to generate small scale electricity as well as to raise awareness towards energy efficiency. Another company, “Innowattech”, is paving a full 100 meters of pavement with piezoelectric generators which can produce power from weight, motion, vibration, and temperature changes. Domestically, keyboard-mounted piezoelectric generators are being used to continuously recharge the machine’s battery while the user types, and the inverse reaction of energy to mechanical pressure is being utilized in military micro-robotics. The technology is available and ready for large-scale implementation, but such an improvement is not so easily applied; there are several mitigating factors such as cost and logistics which will effect whether energy generation of this type can be installed.

conclusions Results suggest that if piezoelectric generators were applied to all sidewalks of New York City, they would produce enough electricity to power approximately 10% of the city’s consumption. The project was not found to be feasible with current market prices, but further improvements in output efficiency coupled with declines in production cost may allow piezoelectric generators to become a viable option to provide a portion of electricity demand of major metropolitan centers around the globe.

TimoThy AAron DAnforTh, UnDergrADUATe STUDenT, inTerior DeSign ProgrAm, UniverSiTy of new hAven, CT tdanf1@unh.newhaven.edu

Dr. CAn B. AkTAS, ASSiSTAnT Prof., DePT. of meCh., Civil, AnD environmenTAl eng. UniverSiTy of new hAven, CT caktas@newhaven.edu