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Use of Piezoelectrics (Human Powered Energy)
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Use of Piezoelectrics (Human Powered Energy)
Team Members: Reed Williston, Kevin Meredith, Grant Martens, Eric Gibson
1. Why cant you simply set something really heavy on top of a piezoelectric material and keep harvesting energy from it?
2. Are piezoelectric materials currently a realistic way to produce energy? Why or why not?
The piezoelectric effect is the electric charge that is created as a result of stressing certain materials. In this project we will investigate different uses for harvesting energy off of human movement using piezoelectric materials. We will analyze various situations in which humans move in their every day lives. This project will investigate these situations on potential to harvest energy, the monetary cost and the practicality. These things will be taken into account to determine whether or not piezoelectric energy harvesting is a realistic pursuit.
Piezoelectric materials were first theorized in the mid 1700's. However, it wasn't until 1880 that the piezoelectric effect was demonstrated. This is largely due to the fact that measurement devices were not sensitive enough to detect the small voltage created in experiments. It took 40 years for someone to find a purpose for these interesting materials when they were first put to use in WWI in sonar devices.
Piezoelectric materials create a net voltage across the material when they are stressed. The mechanism for this lies in the molecular structure of the material. When a stress is applied to a piezoelectric there is a realignment of the dipole moments within the material. Initially oriented randomly, under stress the dipoles create a net alignment across the material. With a large enough stress, the alignment of these dipoles sums up into a net voltage across the material. As soon as this net voltage is created, the electrons within the material attempt to redistribute themselves to eliminate the voltage. Consequently, piezoelectric materials are more useful in the presence of a dynamic stress such as an oscillation or vibration rather than a static stress such as weight. With most materials, stress and strain generally occur together. Therefore, it can be stated that a voltage is produced in response to a mechanical deformation of a piezoelectric material. In many of these materials, the reverse piezoelectric effect can also be demonstrated. In response to an applied voltage, a mechanical strain will be induced in the material. This effect is very important in many applications of piezoelectric.
There are many modern applications for piezoelectric materials. One of the most prevalent is in acoustics. Many acoustic instruments such as guitar pickups and ultrasound equipment utilize the piezoelectric effect. The reason is that piezoelectric make it easy to transform a high frequency mechanical vibration into a corresponding electric oscillation. Similarly, high frequency AC currents can create rapid mechanical deformations. This effect is utilized in inkjet printers. For our project we are going to investigate how a stress on a piezoelectric material can be harvested as electrical power.
Recently, a company called Pavegen has released a product that attempts to use piezoelectric materials to harness energy from humans walking. The idea is that walking in a necessary part of our daily lives and the impact of our feet on the ground is wasted energy. We are going to do an energy and cost analysis of using these blocks to harvest energy on a large scale. The Pavegen website claims that one of their tiles can generate 7 watts of power for the duration of an average human step. The tiles measure 0.6 m x 0.45 m and are made from approximately 90% recycled materials. Although the tiles are being used in a few places, they are not out for consumer purchase at this time. No official price has been announced, though Pavegen employees have been reported as saying the tiles cost as much as $4000 in 2011, $200 in 2013, and as little as $76 in the future. For the purposed of our analysis, we are going to assume that each tile costs $100 to purchase and permanently install. This estimate could be grossly inaccurate, however we do not have much information to work with in this aspect. Considerations need to be taken for connection to the power grid, permanent installation costs and the cost of the tiles themselves. Temporary installation is also a possibility, though it is equally difficult to measure the cost of this. Additionally, we have no way of knowing how long a tile will last. The durability is a huge factor in determining whether these tiles could be a financially realistic source of energy.
The idea is to place pavegen tiles throughout New York City's Time Square on the sidewalks. Since Time square gets roughly 350,000 pedestrians every day, it would make sense to put these on the sidewalks to generate a little electricity, but how much? On average a pedestrian in New York City will walk approximately one mile a day or around 20 blocks. If we assume that each person who goes through Time Square walks one mile, the number of total steps taken is equal to around 650 million steps per day. This is calculated by multiplying the number of pedestrians, 350,000, by 5280 feet per mile and dividing that by 2.5ft per step. Pavegen tells us that each of their blocks will produce 7 watts per impulse of a step, which is 0.68 seconds when measured. This produces 4.76 Joules per step of energy. Multiplying this by the number of steps total for one day and then for one year, the amount of energy produced for one year is 1,105GJ. This is equal to about 307,000 kWh per year of electricity produced. If we assume that electricity costs $0.15 per kWh, the total savings of installing the tiles is $46,042 per year. Unfortunately, each tile costs well over $100 at the moment, and to cover all of Time Square's sidewalks with these tiles, needing around 236,000 of them, it would cost upwards of $23 million dollars. The number of tiles needed was calculated by knowing the area of each tile from Pavegen's website, and calculating the total area of sidewalk within Time's Square. This was done by assuming that each sidewalk is roughly 12 feet in width and then using Google Maps to determine how much sidewalk is in Time Square. The square footage of sidewalk estimated to be in Times Square is roughly 686,500 ft squared.
In conclusion, the feasibility of covering the sidewalks in Time Square with Pavegen Tiles is uneconomical and will not produce enough power to be remotely worth it. This is because so many tiles are required and they do not produce enough power. What if the number of people was increased, increase the density of people in one area to reduce the number of tiles required and produce much more electricity?
Lets look at Time Square during the New Years Eve Celebration. The number of people that come to Time Square for the celebration is approximately one million. Plus, the area in which they are all moving around is much smaller than all of Times Square. Assuming that the area in which the one million people are is equal to a right triangle with lengths of 1500ft and 250ft, roughly half the area of 2 city blocks, and that each person again walks an estimated one mile total, the number of total steps taken for that night is 2.1 billion. The dimensions for the area of the celebration was gotten by using Google Maps and estimating the area as accurately as possible. With that many steps, the amount of power generated is equal to 14.8 million kW. Lets also assume that the celebration goes on from 7pm to 1am, or six hours, and therefore the number of kWh generated is equal to 2792 kWh. Even in a very congested area, the amount produced is not that significant.
Conclusion and recommendations:
Currently, piezoelectric materials are not a realistic method for generating energy on a large scale. The amount of money spent to install the tiles far outweighs any potential money to be made by the electricity generation. This type of technology may be well suited to other applications though. One potential application would be to connect one of these tiles directly to a low consumptions device. A few few potential candidates are a vending machine or a parking pass dispenser. A customer would walk or drive up the the machine and depress the pad. This may be able to generate enough power to run the device for the very short period of time that it is in use. It would take very little power for a vending machine to push a candy bar forward or a machine to push out a parking pass. This way, many devices could be disconnected from the grid completely.
Another potential use of pavegen tiles would be education and awareness. Being "carbon neutral" or "energy neutral" is very much in style these days. These tiles could be implemented temporarily for major sporting events, festivals, concerts or similar things. The goal here would not be to save money. Instead, the event could advertise that it is not using any electricity from the grid. These days, that is a rather cool thing for an event to brag about. This could be a very effective way to create dialog and awareness of related energy issues.
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