Humans have known how to harness energy from water for at least 2,000 years; the first known water wheel was Mesopotamian, a 4th century B.C.E. device that used running water to turn millstones. In the centuries that followed, enterprising people figured out how to spin an electricity-generating turbine with the water from a dammed river, but converting much smaller quantities of moving water into a useful amount of energy remains impractical, even today.

Still, individual water droplets do bring kinetic energy with them as they fall, and a rainstorm, in theory, represents a lot of wasted energy. Scientists at the School of Mechanical and Aerospace Engineering, Nanyang Technological University Singapore are hoping their device — a “droplet energy nanogenerator” made from recycled DVDs — will help make it possible for people to finally collect and make use of the energy of falling water.

“Our work highlights two strengths,” said Yifan Wang, who co-authored the study published in Advanced Energy Materials. “The first is the cost-efficiency since our device can be fabricated from recycled DVDs. The second is the high output. Even with simple and straightforward fabrications, we still achieve super high performance.”

Making use of waste

As more people turn to streaming and cloud data storage, DVDs and CDs are becoming antiquated. According to the U.S. Environmental Protection Agency, 100,000 pounds of CDs become obsolete every month, and each year 5.5 million boxes of software are thrown into landfills. In a landfill, it may take up to a million years for a CD or DVD to decompose. New technologies, like this one, that give new purpose to old DVDs could help reduce this problem.

A DVD has bumps and pits on its surface that cause electrons to oscillate in response to a droplet’s impact. Wang said this “generates a large amount of hot electron-hole pairs on the surface, which can significantly improve the electricity generation during the droplet impact”. An electron-hole pair is a basic unit of generation produced when an electron moves to a new position, leaving a “hole” or unoccupied position behind.

In their paper, the authors describe some potential real-world applications for their nanogenerator, which could be mounted on a rooftop or a window for energy collection on a rainy day.

The researchers say their nanogenerator could have a few different rainy-day applications, for example, the device could be used to power “smart window” functions. Smart windows are made with a layer of liquid crystal that realigns in response to an electric field, allowing the window to change from opaque to transparent and back again.

During their study, the researchers demonstrated that just five simultaneous water droplets could produce enough voltage to change the liquid crystal film from opaque to transparent.

Smart windows powered by these nanogenerators could automatically change the opacity of the window when it rains; alternately, the energy could be collected and stored so the user could control the changes on demand. For this, the energy harvesters could be connected to energy storage modules so the window could be switched from opaque to transparent regardless of the weather.

Beyond just buildings

Since each device weighs less than a gram, the nanogenerators could even — in theory — be mounted on raincoats, umbrellas, or the leaves of plants. A nanogenerator mounted on a plant leaf could be used to power smart monitoring systems, although additional components would be needed.

“We need two wires for signal transmission,” said Wang, “and also we need an energy storage module to store energy.” The device, he explained, would make traditional soil monitoring systems more sustainable. “Commercial soil sensors need to replace batteries,” he said. “We can power it with the energy from the raining water to render a self-sustainable platform.”

Wang said the technology hasn’t yet been studied for use in larger applications like powering an entire home. “If we want to power the home we need to fabricate a large-area generators array to [generate a] larger amount of electricity,” he said. Wang said his team hasn’t calculated just how much space this would occupy, but he estimated it could be “several square meters”.

Though larger applications for the technology are possible, the researchers wrote that their nanogenerator’s more immediate value is in its high output performance and inexpensive cost.

“We believe that it’s a complementary clean energy source apart from solar or wind energies, as our devices can generate power during rainy weather,” Wang said. “In future, we could include systems that are large and powerful enough to power a home during the rain, but a more immediate use for it is for powering smaller devices.”

Reference: Hai Lu Wang, Yifan Wang, et al., High-Efficiency Single-Droplet Energy Harvester for Self-Sustainable Environmental Intelligent Networks, Advanced Energy Materials (2023). DOI: 10.1002/aenm.202302858