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.