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 2017-06-19

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

• I do not Consent 2017-06-22

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…

• ozjon69 2017-06-22

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.

• pmd34 2017-06-23

So energy from nowhere huh?! Oh but wait, the deflection causes added road resistance, so the car fuel consumption has to go up to compensate. Still im sure they will get lots of research funding from the usual committees! Maybe you could make them out of graphine! Thats a sure fire application winner!

• viki2000 2017-06-23

I find this idea not very smart and without a real future.
The piezo ceramic membranes or buzzers used to produce a sound when a voltage is applied, the known application, suffers of low lifetime due to one reason: vibration. Most of the time the contact between membrane and the wire is cracked, the wire are broken right in that point. We speak here about thousands of hour’s lifetime under continuous vibration.
Here is the opposite effect, with vibration due to mechanical tension in the membrane to get the voltage, but the connection is the same, the continuous vibration will reduce the lifetime of the transducer. It is not going to work for long time under continuous vibrations.

• viki2000 2017-06-23

Sorry for the wrong place to place the comment.

• viki2000 2017-06-23

I find this idea not very smart and without a real future.
The piezo ceramic membranes or buzzers used to produce a sound when a voltage is applied, the known application, suffers of low lifetime due to one reason: vibration. Most of the time the contact between membrane and the wire is cracked, the wire are broken right in that point. We speak here about thousands of hour’s lifetime under continuous vibration.
Here is the opposite effect, with vibration due to mechanical tension in the membrane to get the voltage, but the connection is the same, the continuous vibration will reduce the lifetime of the transducer. It is not going to work for long time under continuous vibrations.

• theace 2017-06-23

This idea is so stupid that it cannot be described as it should using words (and yes, it’s on the same level as solar freakin’ roadways), but I’ll just make two remarks:
1) you want your roads to be as stable and as long lasting as possible. Promoting something as vibration is extraordinary counter productive. You don’t want your roads to vibrate at all. Because vibration goes to potholes and in order to fix them you’ll spend more money than what you get on the energy you produce.
2) where would that energy come from? you guessed it: from the cars. So, you’ll get the energy from burning fuel to the car to the road to the power grid. How stupid is that? So my car has to bounce up and down to give energy to the road. So I have to spend money on fuel to power the road with losses instead of producing more energy from the same fuel in a power plant.
Yes, yes, the vibration could be collected from static cars (engine vibration), but wouldn’t be efficient to put this vibration generators on the cars themselves and power the battery instead of using fuel to spin the alternator?
An the discussion can continue, but is pointless (see solar roadways).
Now I’m asking the reporters on this site to stop posting this kind of bullshit on an otherwise interesting site. Just because it contains electronics, doesn’t mean it actually has a place here. You can do all sort of things with electronics, but this doesn’t mean that they belong here.

• pmd34 2017-06-23

I wouldn’t be too hard on the site, its important to know what people are up to, but more a brief “look at the crazy idea” angle, or debunking it would be better!

• pmd34 2017-06-23

How do they get 7.1mW from Open circuit voltage 20.9V & Closed circuit current 57 microamps? Surely that’s an absolute maximum 1.1mW?!