A major drawback to solar energy that it does not work in the shade. Enter the shadow-effect energy generator.

The device sounds like a futuristic gadget from Star Trek, but is in fact an invention of researchers at the National University of Singapore. The radical new invention takes advantage of the contrast between illuminated areas and shadows to generate electricity.

The contrast in illumination creates a difference in voltage between the two areas, which generates an electric current. Or as the researchers put it in a newly published study, the shadow-effect energy generator (SEG) “scavenges the illumination contrast that arises on the device from shadow castings, and generates a direct current, simply by placing a part of the generator in shadow.”

Needless to say, harvesting shadows for energy generation could prove revolutionary as shadows are everywhere. “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,” the scientists explain.

Not so with their device, however. It comprises a set of so-called SEG cells arranged on a flexible and transparent plastic film with each cell covered in a thin film of gold deposited on a silicon wafer. “When the whole SEG cell is under illumination or in shadow, the amount of electricity generated is very low or none at all,” explains Prof. Andrew Wee, who is a leader of the research team.

“When a part of the SEG cell is illuminated, a significant electrical output is detected,” Wee says. “We 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.”

Mobile electronic devices such as smart phones and e-watches could benefit from the technology because they are worn both indoors and outdoors. Wearable power sources could keep these devices charged for prolonged use, but current technologies are seeking to harness solar power. The new invention can improve the versatility of wearable power sources by working in the shade as well.

The scientists say their four-cell device is twice as efficient as commercially available silicon solar cells under the effect of shifting shadows. The energy that can be harvested in the presence of shadows in indoor lighting conditions can power a digital watch. In other words, sunlight will not be required and the device could generate electricity even indoors by harvesting the differences in illumination.

The device can also “serve as a self-powered sensor for monitoring moving objects,” they say. “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.”

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