Power/Performance Bits: Oct. 1

Nighttime energy generation; smart pajamas; organic solar cell requirements.


Nighttime power
Researchers at UCLA and Stanford University created a low-cost device that harnesses radiative cooling to provide a small amount of renewable energy at night. While the device only provides a small amount of power, it could be useful for areas without reliable electricity or access to batteries.

Radiative cooling happens when a surface that faces the sky emits heat as thermal radiation. “This effect occurs naturally all the time, especially on clear nights,” said Aaswath Raman, an assistant professor of materials science and engineering at the UCLA Samueli School of Engineering. “The result is that the object ejecting the heat, whether it’s a car, the ground or a building, will be slightly cooler than the ambient temperature.”

To demonstrate the concept, the team built an experimental setup for less than $30 using parts sourced from hardware and electronics supply stores.

Their setup, arranged on the roof of a building, included an aluminum disk that was painted black on one side, which faced the sky. The researchers used the disk to radiate the heat being given off by the surrounding air. A thermoelectric generator then converted that heat into electricity.

An experimental setup of electricity-generating device that uses radiative sky cooling to harvest energy. (Source: Aaswath Raman/UCLA)

The device generated up to 25 milliwatts per square meter, enough to power a single LED light bulb. Although the device generates substantially less energy than a similarly sized solar cell, Raman said it could be used to generate power at night, in locations that are off of the electrical grid or for users who don’t have easy access to batteries.

“We think this is an intriguing demonstration of how the cold of space can be accessed as a renewable energy resource and result in modest yet usable amounts of electricity,” Raman said. “We think it also could form the basis of a complementary technology to solar energy. While the power output will always be substantially lower than that of solar devices, this new technology can operate at hours when solar cells cannot.”

Raman said the technology could be improved with better components, and that it could potentially generate as much as 0.5 watts per square meter — about 20 times more than the device the researchers demonstrated — especially in hot, dry climates where the radiative cooling effect is the strongest. That output could provide enough power overnight to charge a cell phone or to light a room with LED bulbs.

Smart pajamas
Researchers at the University of Massachusetts Amherst developed physiological-sensing textiles that are soft and flexible enough to be woven or stitched into sleep garments for unobtrusive health monitoring.

“The challenge we faced was how to obtain useful signals without changing the aesthetics or feel of the textile,” said Trisha L. Andrew, a materials chemist at UMass Amherst. “Generally, people assume that smart textiles refer to tightly worn clothing that has various sensors embedded in it for measuring physiological and physical signals, but this is clearly not a solution for everyday clothing and, in particular, sleepwear.”

The smart sleepwear, which the team has subbed “phyjamas,” relies on a fabric-based pressure sensor combined with a triboelectric sensor, creating a distributed sensor suite that could be integrated into loose-fitting clothing.

“Our insight was that even though sleepwear is worn loosely, there are several parts of such a textile that are pressed against the body due to our posture and contact with external surfaces. This includes pressure exerted by the torso against a chair or bed, pressure when the arm rests on the side of the body while sleeping, and light pressure from a blanket over the sleepwear,” said Deepak Ganesan, a computer scientist at UMass Amherst. “Such pressured regions of the textile are potential locations where we can measure ballistic movements caused by heartbeats and breathing, and these can be used to extract physiological variables.”

Fabric-based pressure sensor combined with a triboelectric sensor. (Photo courtesy of UMass Amherst/Andrew lab.)

While individual sensors may be unreliable, the researchers found that many sensors placed across different parts of the body can be intelligently combined to get a more accurate composite reading. Using data analytics, they could fuse signals from many points that took into account the quality of the signal coming in from each location.

The team said the combination of sensors allowed them to detect physiological signals across many different postures. They performed multiple user studies in both controlled and natural settings and showed that they can extract heartbeat peaks with high accuracy, breathing rate with less than one beat per minute error, and perfectly predict sleep posture.

“We expect that these advances can be particularly useful for monitoring elderly patients, many of whom suffer from sleep disorders,” said Andrew. “Current generation wearables, like smartwatches, are not ideal for this population since elderly individuals often forget to consistently wear or are resistant to wearing additional devices, while sleepwear is already a normal part of their daily life. More than that, your watch can’t tell you which position you sleep in, and whether your sleep posture is affecting your sleep quality; our Phyjama can.”

Organic solar cell requirements
Scientists at the University of Warwick found that creating organic solar cells could be easier than previously thought. Organic solar cells consist of a thin layer of organic semiconductors between two electrodes which extract charges generated in the semiconductor layer.

To maximize efficiency, it’s been assumed that 100% of the surface area of each electrode should be conductive. But the team found that isn’t the case: only about 1% of the surface area needs to be conductive, meaning a range of different composite materials could be used at the interface between the electrodes and semiconducting layer to reduce cost an improve performance.

“It’s widely assumed that if you want to optimize the performance of organic solar cells you need to maximize the area of the interface between the electrodes and the organic semiconductors,” said Ross Hatton of Warwick’s Department of Chemistry. “We asked whether that was really true.”

The researchers developed a model electrode that they could systematically change the surface area of, and found that when as much as 99% of its surface was electrically insulating the electrode still performs as well as if 100% of the surface was conducting, provided the conducting regions aren’t too far apart.

“This new finding means composites of insulators and conducting nano-particles such as carbon nanotubes, graphene fragments or metal nanoparticles, could have great potential for this purpose, offering enhanced device performance or lower cost,” said Dinesha Dabera, a post-doctoral researcher at Warwick. “Organic solar cells are very close to being commercialized but they’re not quite there yet, so anything that allows you to further reduce cost whilst also improving performance is going to help enable that.”

Organic solar cells don’t contain toxic elements and can be processed at low temperatures using roll-to-roll deposition, lowering the amount of energy required to fabricate them.

“There is a fast growing need for solar cells that can be supported on flexible substrates that are lightweight and colour-tuneable,” added Hatton. “Conventional silicon solar cells are fantastic for large scale electricity generation in solar farms and on the roofs of buildings, but they are poorly matched to the needs of electric vehicles and for integration into windows on buildings, which are no longer niche applications. Organic solar cells can sit on curved surfaces, and are very lightweight and low profile.

“This discovery may help enable these new types of flexible solar cells to become a commercial reality sooner because it will give the designers of this class of solar cells more choice in the materials they can use.”

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