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Vibration Energy To Power Cell Phones
Can I Power my Smart Phone with
Vibration Energy Harvesting?
Chris Ludlow
I get this question quite a
lot... unfortunately the short answer is no.
However, there is hope (see below), just not from vibration
energy harvesting.
Good
Vibrations Needed for Good Energy Harvesting
The energy potential from vibrations is directly tied to the
mass that is being vibrated and the frequency (how many times per second) with
which the mass is moving.
Generally the higher the mass
and the higher the frequency the more energy there is to harvest from.
Harvesting vibrations with
most harvesting technology such as piezoelectrics is best suited for
consistent high frequency vibrations, like those emanating from a
piece of machinery as it is operating.
The human body is active
sometimes and other times completely dormant.
Also, some of us are carrying
a little extra dampening material if you know what I mean. Not much there to
harvest from.
You could possibly squeeze
out some more juice by having a very large piezoelectric element with a big
mass on it to get more strain into and thus power out of the device from these
low frequency events.
But who wants to carry
around a brick of piezoelectric material with a small phone attached to it. Oh
yeah and it would make the phone cost an amazing amount of money.
The Math
Powering the iPhone
An iPhone has around 1900 mA-hr at 1.2V, that works out to
roughly 8200 Joules of energy.
A typical piezoelectric
vibration energy harvester can output in IDEAL circumstances (ideal being high
frequency machine vibrations) 10 mW of power.
So to fully charge an iPhone
in these IDEAL conditions you would need to harvest energy for 9.5 days.
You might be saying, "ok that is not great but still not
horrible."
Here is the rub, vibration
harvesting from human motion is not nearly ideal and you won't get anywhere
near 10 mW of harvested power out of a reasonably sized vibration energy
harvester, be it electromagnetic or piezoelectric.
Some people have tried to embed the piezoelectric elements into
a shoe or other object that will bend and deflect during human motion.
It's not a bad idea, but it
still won't work too well unless you have a very large amount of piezoelectric
material.
To get good power you need to
be straining the piezo material at as high of a frequency as possible, Usain
Bolt could not run fast enough to make this type of approach work.
Some Good
News - There is Hope!
Other forms of energy harvesting are much more appealing for charging
phones.
Embedded solar cells should
be able to charge a phone in about 3 hours using a 15% efficient solar cell.
Doesn't do us much good if
the phone is in our pocket, but still a decent option if you are lounging
around outdoors on a sunny day.
There are a host of
aftermarket solar phone charging products on the market, but none of the major
phones have designed this capability in yet.
Thermal
energy harvesting from thermoelectric generators (TEG) provide
similar low power output to piezo's or eletromagnetic harvester, on the order
of low mW's, but at least their harvesting mode is matched nicely to the human
body which is constantly generating heat.
Also, since the phone is
often in our pockets there is ample time to trickle charge the battery.
Lastly, there is RF
harvesting which would work nicely but again the power levels are very low. Check out www.powercastco.com if you are
interested in that.
There is hope!
As electronics become lower
power and as the efficiency of solar (check out this small 44% solar cell),
thermal and RF harvesters improve I'm hopeful that someday we won't be
slaves to recharging our phones. Till then, keep plugging!
Chris
Ludlow is the Vice President of Engineering at Mide Technology Corporation.
Chris has spent the past 15 years developing technologies and products to help
customers solve their problems. If you are in need of innovative solutions for
your applications please visit www.mide.com.
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