Was there a time when you were introduced to the
piezoelectric effect in one of your Physics classes and wondered, “If
piezoelectric crystals generate voltage when subjected to vibration, can’t we
harness this voltage to power our electronics?” It was a pretty interesting
afterthought. What about the voltage developed from the Seebeck effect? There
are a lot of naturally occurring temperature gradients in our environment such
as the thermal gradient between our body, the engines we use, even our gadgets
and ambient temperature. It would feel wasteful to watch all the energy from
these potential sources dissipate to the empty void.
Apparently, such sources would only yield power just enough
for the mobile phone of an ant. But recent developments in materials science as
well as improvements in power consumption of modern electronics have aroused
interest anew. Thus, in 2006 the first nanogenerator emerged drawing energy
through the piezoelectric and semiconductor characteristics of a vertically
grown ZnO nanowire.
Nanogenerators, why not Microgenerators?
I’m just kidding. The answer to that should be trivial, and
so should the term’s definition (microgeneration is a thing by the way). Nanogenerators
(NGs) are generators that harvest energy from the environment for nanosystems.
This scope has extended towards much larger systems with the advent of new
discoveries and innovations. There are generally 3 known environmental sources
from which nanogenerators can draw energy: from friction (triboelectric – where
research seems to be most active today), shaking and vibrating (piezoelectric),
and temperature gradients (thermoelectric/pyroelectric). I say 3 known sources
just in case we find possible sources in the future (maybe cosmic/background
radiation or that annoying EMI through teeny weenie antennas). Then again,
drawing from a single source can be troublesome when the environment changes
(the source stopped moving or the temperature of both sides reached equilibrium).
To solve this, NGs have to draw on multiple sources. These NGs are now hybrid
cells that can be composed of a dye-sensitized solar cell and a piezoelectric
nanogenerator in series.
What about storage? Aside from the traditional full-wave
bridge rectifier, which demands a wasteful 2-step conversion process
(mechanical to electrical and electrical to chemical); a more efficient
approach would be to use a self-charging power cell (cut down to a mechanical
to chemical process). Without the drops on the diodes, a self-charging power
cell would deliver more power to the storage unit. The only limitation I see is
that the cell would always be charged (you can’t place a switch to cut off the
charge path). Or am I oblivious to an alternative?
Let’s put nanogenerators everywhere!
NGs look like the next big thing in renewable energy. But
why can’t we see any of them on our Galaxy Tabs or Iphones or at the very
least, digital watches? Perhaps it is still too uneconomical to manufacture
them, the technology for mass production of the materials is still unavailable,
many are still unwilling to invest on this kind of enterprise, the tech is too
“new” and is still earning publicity, the overall power density delivered is
insufficient for most mobile devices, and so on. I can personally attest to
some of the latter reasons though.
Specific nanostructures are hard to produce on particular
compounds. They require an exacting chemical process under unique conditions
that may be too alien for some manufacturing companies to quickly adapt to.
This effectuates the hurdle on mass production. However, I’ve come upon
journals lately with titles and abstracts that promise improvements in these
areas. Simply do a search on nanogenerator efficiency and some are bound to
come up.
The current capacity of NGs and even hybrid cells are too
small, unless you want to walk around with a humongous bulk on your pants (on
your pocket sides). The maximum achieved ratings of NGs are quite far from the
demands of our modern gadgets. I’ve worked on special low-power charging applications
for certain phones and tabs and they require a charging current of around 300mA
to 400mA at an input voltage of 4.2V and above. Based on the figures, they’re
not even enough to support battery charging at trickle or top-off. Not to be so
pessimistic, NGs do have a place in sensors (they are for micro/nano systems
after all). A popular field of interest is on integrating NGs with biosensors
and bioelectronics.
“Hello… Hello?” Oh, battery is dead. *shake* *shake* *shake*
In spite of all these obstacles, the future isn’t hopeless
for NGs and mobile devices. If Moore’s law does not cease (some say until
2020), then mobile power consumption is bound to decrease continuously. The
same goes for other low power applications (depends on how creative you can get
I guess). Mr. Zhong Lin Wang, a pioneer in the field of nanogenerators, and his
team at Georgia Institute of Technology introduced a smart self-powered self-cleaning
keyboard early February. They designed the keyboard in a way that would attract
electrons when the user pushes down on a key, inducing current. Nanowires help
amplify this effect by increasing surface area. With enough current, the simple
typing action of the user would be sufficient to power the entire keyboard. I
find this ingenious because the typing pattern of the user can be extracted as
well from the current waveform, giving a bonus security feature.
There may be a plethora of other applications for NGs and
hybrid cells that you can think of. Where else would you want to see NGs being
used?
References:
This ‘smart’ self-cleaning keyboard is powered by
you. (2015, February 17). Retrieved
September 2015, from Student Science: A Resource of the Society for Science
& the Public: https://student.societyforscience.org/article/%E2%80%98smart%E2%80%99-self-cleaning-keyboard-powered-you
Harvesting the World's Mechanical Energy. (n.d.). Retrieved September 2015, from Georgia
Tech: http://www.news.gatech.edu/features/harvesting-worlds-mechanical-energy
Wang, Z. L., Chen, & Lin. (2015). Progress in
triboelectric nanogenerators as a new energy technology and self-powered
sensors. Energy Environ. Sci., 2250-2282.
Yoon, L. (2014, May 15). KAIST made great
improvements of nanogenerator power efficiency. Retrieved September 2015,
from EurekAlert: The Global Source for Science News:
http://www.eurekalert.org/pub_releases/2014-05/tkai-kmg051514.php
Zhu, G., Chen, Zhang, Jing, & Wang. (2014).
Radial-arrayed rotary electrification for high performance triboelectric
generator. Nature Communications, 1-9.
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