energy harvester

Ultrathin energy harvester could power colour-changing clothes

An ultrathin energy harvesting system could be sown into clothes and used to power smartphones and other portable gadgets by extracting energy from the user’s movements.

The system is based on battery technology and made from layers of black phosphorus that are only a few atoms thick.

The device generates small amounts of electricity when it is bent or pressed even at the extremely low frequencies characteristic of human motion.

“In the future, I expect that we will all become charging depots for our personal devices by pulling energy directly from our motions and the environment,” said Cary Pint, who directed the research at Vanderbilt University’s Nanomaterials and Energy Devices Laboratory.

“Compared to the other approaches designed to harvest energy from human motion, our method has two fundamental advantages,” said Pint. “The materials are atomically thin and small enough to be impregnated into textiles without affecting the fabric’s look or feel and it can extract energy from movements that are slower than 10Hz over the whole low-frequency window of movements corresponding to human motion.”

But successfully extracting usable energy from such low frequency motion has proved to be challenging. For example, a number of research groups are developing energy harvesters based on piezoelectric materials that convert mechanical strain into electricity.

However, these materials often work best at frequencies of more than 100Hz. This means that they don’t work for more than a tiny fraction of any human movement so they achieve limited efficiencies of less than 5-10 per cent even under optimal conditions.

“Our harvester is calculated to operate at over 25 per cent efficiency in an ideal device configuration, and most importantly harvest energy through the whole duration of even slow human motions, such as sitting or standing,” Pint said.

Over the past three years, the team has explored the fundamental response of battery materials to bending and stretching. They were the first to demonstrate experimentally that the operating voltage changes when battery materials are placed under stress. Under tension, the voltage rises and under compression, it drops.

Although the device cannot store energy, it can fully exploit the voltage changes caused by bending and twisting and produce significant amounts of electrical current in response to human motions.

The team decided to go as thin as possible by using nanosheets of black phosphorus: a material that has become the latest darling of the 2D materials research community because of its attractive electrical, optical and electrochemical properties.

Because the basic building blocks of the harvester are about 1/5000th the thickness of a human hair, the engineers can make their devices as thin or as thick as needed for specific applications. They have found that bending their prototype devices produces as much as 400 microwatts per square metre and can sustain current generation over the full duration of movements as slow as 0.01Hz, one cycle every 100 seconds.

The researchers acknowledge that one of the challenges they face is the relatively low voltage that their device produces. It’s in the millivolt range. However, they are applying their fundamental insights of the process to step up the voltage. They are also exploring the design of electrical components, like LCD displays, that operate at lower than normal voltages.

One of the more futuristic applications of this technology might be electrified clothing. It could power clothes impregnated with liquid crystal displays that allow wearers to change colors and patterns with a swipe on their smartphone.

“We are already measuring performance within the ballpark for the power requirement for a medium-sized low-power LCD display when scaling the performance to thickness and areas of the clothes we wear.” Pint said.

Pint also believes there are potential applications for their device beyond power systems. “When incorporated into clothing, our device can translate human motion into an electrical signal with high sensitivity that could provide a historical record of our movements. Or clothes that track our motions in three dimensions could be integrated with virtual reality technology. There are many directions that this could go.”

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