Power/Performance Bits: Oct. 25

Energy-harvesting floor; ultralow power transistors; perovskite solar cell reaches 20.3% efficiency.

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Energy-harvesting floor

Engineers at the University of Wisconsin-Madison developed a flooring material which can be used as a triboelectric nanogenerator to convert footsteps into electricity.

The method uses wood pulp, a common waste material already often used in flooring. The pulp is partly make of cellulose nanofibers, which when chemically treated produce an electrical charge when they come into contact with untreated nanofibers.

Because wood pulp is a cheap, abundant and renewable waste product of several industries, flooring that incorporates the new technology could be as affordable as conventional materials. While there are existing similar materials for harnessing footstep energy, they’re costly, nonrecyclable, and impractical at a large scale.

The team’s work falls under the green energy research field called “roadside energy harvesting.”

“Roadside energy harvesting requires thinking about the places where there is abundant energy we could be harvesting,” said Xudong Wang, an associate professor of materials science and engineering at UW-Madison. “We’ve been working a lot on harvesting energy from human activities. One way is to build something to put on people, and another way is to build something that has constant access to people. The ground is the most-used place.” The team thinks the approach could, in some settings, rival solar power.

Associate Professor Xudong Wang holds a prototype of the researchers’ energy harvesting technology, which uses wood pulp and harnesses nanofibers. The technology could be incorporated into flooring and convert footsteps on the flooring into usable electricity. (Source: Stephanie Precourt/UW-Madison)

Associate Professor Xudong Wang holds a prototype of the researchers’ energy harvesting technology, which uses wood pulp and harnesses nanofibers. The technology could be incorporated into flooring and convert footsteps on the flooring into usable electricity. (Source: Stephanie Precourt/UW-Madison)

Heavy traffic floors in hallways and places like stadiums and malls that incorporate the technology could produce significant amounts of energy, according to Wang. Each functional portion inside such flooring has two differently charged materials, including the cellulose nanofibers, and would be a millimeter or less thick. The floor could include several layers of the functional unit for higher energy output.

“Our initial test in our lab shows that it works for millions of cycles without any problem,” said Wang. “We haven’t converted those numbers into year of life for a floor yet, but I think with appropriate design it can definitely outlast the floor itself.”

The team says the technology could be easily incorporated into all kinds of flooring once it’s ready for the market. Their next step is building a prototype a high-profile spot on the UW-Madison campus to demonstrate the concept.

Ultralow power transistors

Engineers at the University of Cambridge developed an ultralow power transistor which could function for months or even years without a battery by scavenging energy from its environment.

The thin-film transistor, constructed of indium-gallium-zinc-oxide, was able to operate at ultralow power (less than 1 nanowatt) and at switching voltages of less than 1 volt with very high intrinsic gain.

By changing the design of the transistors, the researchers were able to use Schottky barriers to keep the electrodes independent from one another, so that the transistors can be scaled down to very small geometries.

“We’re challenging conventional perception of how a transistor should be,” said Professor Arokia Nathan of Cambridge’s Department of Engineering. “We’ve found that these Schottky barriers, which most engineers try to avoid, actually have the ideal characteristics for the type of ultralow power applications we’re looking at, such as wearable or implantable electronics for health monitoring.”

(Source: University of Cambridge)

(Source: University of Cambridge)

“If we were to draw energy from a typical AA battery based on this design, it would last for a billion years,” said Dr Sungsik Lee of Cambridge. “Using the Schottky barrier allows us to keep the electrodes from interfering with each other in order to amplify the amplitude of the signal even at the state where the transistor is almost switched off.”

According to Professor Gehan Amaratunga of Cambridge, “This type of ultra-low power operation is a pre-requisite for many of the new ubiquitous electronics applications, where what matters is function without the demand for speed. In such applications the possibility of having totally autonomous electronics now becomes a possibility. The system can rely on harvesting background energy from the environment for very long term operation, which is akin to organisms such as bacteria in biology.”

Perovskite solar cell reaches 20.3% efficiency

Researchers from Stanford and Oxford built an all-perovskite solar cell that converts sunlight into electricity with an efficiency of 20.3%, comparable to silicon solar cells currently on the market. The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic.

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build, the team said.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an energy gap and create an electric current. A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

An efficient tandem device would consist of two ideally matched cells, said co-lead author Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington.

“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” Eperon said. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”

“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8% conversion efficiency,” Eperon said. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”

Cross-section of a new tandem solar cell designed by Stanford and Oxford scientists. The brown upper layer of perovskite captures low-energy lightwaves, and the red perovskite layer captures high-energy waves. (Source: Scanning electron microscopy image by Rebecca Belisle and Giles Eperon)

Cross-section of a new tandem solar cell designed by Stanford and Oxford scientists. The brown upper layer of perovskite captures low-energy lightwaves, and the red perovskite layer captures high-energy waves. (Source: Scanning electron microscopy image by Rebecca Belisle and Giles Eperon)

One other concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days. “Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors wrote.

The team’s next step is to optimize the composition of the materials to absorb more light and generate an even higher current.

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