Shadows are often associated with darkness
and uncertainty.
Now, researchers from National University
of Singapore (NUS) are giving shadows a positive
spin by demonstrating a way to harness this
common but often overlooked optical effect
to generate electricity.
This novel concept opens up new approaches
in generating green energy under indoor lighting
conditions to power electronics.
The research team created a device called
a shadow-effect energy generator (SEG), which
makes use of the contrast in illumination
between lit and shadowed areas to generate
electricity.
Their research breakthrough was reported in
the scientific journal Energy & Environmental
Science.
Shadows are omnipresent, and we often take
them for granted.
In conventional photovoltaic or optoelectronic
applications where a steady source of light
is used to power devices, the presence of
shadows is undesirable, since it degrades
the performance of devices.
In this work, the NUS researchers capitalised
on the illumination contrast caused by shadows
as an indirect source of power.
The contrast in illumination induces a voltage
difference between the shadow and illuminated
sections, resulting in an electric current.
This novel concept of harvesting energy in
the presence of shadows is unprecedented.
Mobile electronic devices such as smart phones,
smart glasses and e-watches require efficient
and continuous power supply.
As these devices are worn both indoors and
outdoors, wearable power sources that could
harness ambient light can potentially improve
the versatility of these devices.
While commercially available solar cells can
perform this role in an outdoor environment,
their energy harvesting efficiency drops significantly
under indoor conditions where shadows are
persistent.
This new approach to scavenge energy from
both illumination and shadows associated with
low light intensities to maximise the efficiency
of energy harvesting is both exciting and
timely.
To address this technological challenge, the
NUS team developed a low-cost, easy-to-fabricate
SEG to perform two functions: (1) to convert
illumination contrast from partial shadows
castings into electricity, and (2) to serve
as a self-powered proximity sensor to monitor
passing objects.
The SEG comprises a set of SEG cells arranged
on a flexible and transparent plastic film.
Each SEG cell is a thin film of gold deposited
on a silicon wafer.
Carefully designed, the SEG can be fabricated
at a lower cost compared to commercial silicon
solar cells.
The team then conducted experiments to test
the performance of the SEG in generating electricity
and as a self-powered sensor.
When the whole SEG cell is under illumination
or in shadow, the amount of electricity generated
is very low or none at all.
When a part of the SEG cell is illuminated,
a significant electrical output is detected.
The researchers also found that the optimum
surface area for electricity generation is
when half of the SEG cell is illuminated and
the other half in shadow, as this gives enough
area for charge generation and collection
respectively.
Based on laboratory experiments, the team’s
four-cell SEG is twice as efficient when compared
with commercial silicon solar cells, under
the effect of shifting shadows.
The harvested energy from the SEG in the presence
of shadows created under indoor lighting conditions
is sufficient to power a digital watch (i.e.
1.2 V).
In addition, the team also showed that the
SEG can serve as a self-powered sensor for
monitoring moving objects.
When an object passes by the SEG, it casts
an intermittent shadow on the device and triggers
the sensor to record the presence and movement
of the object.
