News

# California’s Piezoelectric Energy Harvesting Aims to Turn Traffic into Green Energy

June 19, 2017 by Robin Mitchell

## California wants to turn one of its weaknesses—its traffic—into an energy-harvesting strength.

Is high-volume traffic a good source for energy harvesting? The California Energy Commission intends to find out. Here's how piezoelectric devices could turn nightmare traffic into the dream of renewable energy.

Energy harvesting methods have been growing in popularity lately, especially with regards to renewable sources of energy. Renewable sources are not just limited to solar; there are others such as wind, geothermal, and tidal. The only problem with all these forms of energy is that they are either location-dependent or weather-dependent and are not always available.

But what about human activity as a source of energy? Could energy be harvested from our day to day activities to help power our modern society?

There's a particular form of human activity that California has in spades: traffic. California has decided to run an experiment to see if energy can be harvested from vehicle vibrations on the road.

### The Piezoelectric Idea

California has funded an experiment whereby roads which experience heavy traffic will be fitted with piezoelectric transducers to convert the vibration generated by vehicles (both stationary and in motion), into electricity. This electricity can then be fed into the grid to provide power for everything including buildings, homes, and street lights.

Piezoelectric energy harvesting is not “new” technology. It's actually been around for some time. In fact, energy harvesting from foot traffic has not only been proven but is in use in some places. For example, several train stations in Tokyo use piezoelectric energy to generate the power needed to run the ticket machines and electronic displays. Another example is a Dutch nightclub which uses piezoelectric tiles on the floor to power lights.

##### Tokyo uses piezoelectric tiles in some of its stations to power billboards. Image courtesy of the Japan Railways Group

However, will such a system work in traffic conditions and will enough electricity be generated to compensate for the cost?​

### Cost For Piezoelectric Devices and Energy Output

Piezoelectric devices are more commonly used for generating sounds and measuring vibrations as opposed to generating electricity for energy consumption. This makes finding energy data for piezoelectric energy harvesting rather difficult. Luckily, there is a transducer for sale that is specifically designed for generating electricity in energy harvesters. Here are some of the specs given:
• Open circuit voltage at rated deflection = 20.9V
• Closed circuit current at rated deflection = 57 microamps
• Power output at rated deflection = 7.1mW
• Operating temperature = -20°C to 90°C
• Dimension = 70mm x 31.8mm (height 1.5mm)
• 1 piece = $301 : 100 pieces =$132
Given this data, we can roughly project that the amount of energy generated by piezo devices (50Hz vibration) is 3.189 W / square meter.

The cost for a single transducer (if purchased in bulk) would be $132 per device which gives a total cost of$60,000 per square meter (450 devices in a square meter). While this may seem like an unusually hefty sum of money, this is for a square meter of material that can stretch a long way if made narrow. In the case of California, the idea is to create a 60 meter stretch of road and use 2 cm wide piezoelectric generators in stacks. However, this suggests that the piezo devices used in the project will be of a more common type found in everyday electronics as opposed to devices specifically designed for energy harvesting (such as the one produced by Piezo Systems Inc).

Ler's look at an example where the pavement that is to be fitted with energy harvesters will be a complete strip of piezo material. Taking a pavement width of 1.5 meters, the total area of such a strip would be 90 meters square which results in a piezo cost of $5,400,000 and a total energy generation (assuming that all paving is occupied by people jumping at 50Hz) of 287.01W. It's more likely, however, that piezo devices would be scattered so, if we change the example to require a 10% coverage of piezo devices, the cost would be$540,000 with an overall energy output of 28.7W.

Considering that a single 1KW solar panel can cost as little as \$1000, the price for harvesting electricity from walking seems extreme.

### A Place for Cars?

There are issues beyond the costs of such a system; there is the practical side to consider, too.

Piezo generators work well in night clubs and train stations because people are generally unlikely to destroy their pathways. Since piezo devices rely on deflection to generate electricity, the footpath needs to allow for movement and vibration to maximize efficiently of the energy harvesting devices. This is not a problem for pedestrian areas as people are not very destructive, but roads that handle vehicle traffic are a different matter.

##### Piezo discs: Great for buzzing, not so much for cars. Image courtesy of Adam Wysocki [CC-BY-SA-3.0]

Roads must be built to resist damage from tires and debris if they are to be safe and reliable. As a result, roads have a tendency to not move around and deform under impact (instead they break into pieces). This makes placing transducers under roads problematic on two fronts. Either the road is built to allow vibrational energy to reach the transducers which potentially makes the road weaker or the transducers do not reach anywhere near their potential making them a costly investment.

Also, the world is shifting from internal combustion engines to electric batteries. Combustion engines tend to vibrate a lot, even when stationary. This is what the Californian system would rely on during heavy traffic. However, electric cars do not vibrate nearly as much when stationary, thus rendering the energy harvesters less effective.

This raises the question of whether this piezo road idea will age well as more and more consumers turn to electric cars. As a matter of fact, California has arguably been the global leader in the electric vehicle movement, giving China a run for its money in electric car sales. This isn't to say that electric vehicles would render traffic energy harvesting moot, but it's a factor that should be considered when investing millions of state dollars into infrastructure.

### Summary

Perhaps the researchers working on this proposal will use a new type of device that can generate more electricity or maybe the stacks will be much larger. It's difficult to project how effective this system will be without knowing more details. It's difficult to imagine, however, how such a system could be successful enough to warrant expanding.

• AEKron June 19, 2017

I thought solar feakin’ roadways were bad enough, but piezo freakin’ roadways? why not stack the two together, and get double government grants?

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• I do not Consent June 22, 2017

Be aware though, that these piezo devices are also used as microphones.

Imagine the possibilities of microphones everywhere, matched with voice recognition systems…

Arrays of microphones, which can determine where a person is, where they are, and how fast they are going - imagine the applications…

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• O
ozjon69 June 22, 2017

Well, it doesn’t read like a practical concept - certainly not with Piezo generators at the prices suggested.
However, MUCH cheaper piezo generators can be made with pieces of solid-dielectric coaxial cable, made so that the dielectric has high residual stress.
Maybe, that could improve the economics enough to make the concept viable?
This is proven technology (at low power levels)  - stressed cable has been used for many years on intruder detecting security fences.

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