Power/Performance Bits: Aug. 15

Solar sunglasses; saliva-powered battery; thermally conductive plastic.


Solar sunglasses
Researchers at the Karlsruhe Institute of Technology (KIT) developed sunglasses with colored, semitransparent organic solar cells applied onto the lenses capable of supplying a microprocessor and two displays with electric power.

The solar cell lenses, perfectly fitted to a commercial frame, have a thickness of approx. 1.6 millimeters and weigh about six grams, just like the lenses of traditional sunglasses.

A signal-processing unit and two small displays are integrated into the temples of the solar glasses, which show the solar illumination intensity and the ambient temperature as bar graphs. The solar glasses were fully self-powered with the continuously generated photovoltaic energy being the only energy source.

These Solar Glasses with lens-fitted semitransparent organic solar cells supply two sensors and electronics in the temples with electric power. (Source: KIT)

Rather than use batteries to bridge low-light and hence low-power periods, the team capitalized on a particular strength of organic solar cells to perform under low-light conditions. The solar glasses work in indoor environments under illumination down to 500 Lux, which is the usual illumination of an office or a living area. Under these conditions, each of the “smart” lenses still generates 200 microwatts of electric power – enough to operate devices such as a hearing aid or a step counter.

The solar cells, which are based on hydrocarbons, show good mechanical flexibility and the ability to adapt their color, transparency, shape, and size to the desired application. “The Solar Glasses we developed are an example of how organic solar cells may be employed in applications that would not be feasible with conventional photovoltaics,” said Dominik Landerer, a PhD student at the Material Research Center for Energy Systems of KIT.

Beyond sunglasses, the team sees promise in coating large surfaces with these organic solar cells using roll-to-roll fabrication technology for integration into high-rise buildings.

Saliva-powered battery
Researchers at Binghamton University, State University of New York have developed a battery activated by spit that can be used in extreme conditions where normal batteries don’t function, or in places where access to electricity is limited.

The team created a high-performance, paper-based, bacteria-powered battery by building microbial fuel cells with inactive, freeze-dried exoelectrogenic cells which generates power within minutes of adding saliva. The proposed battery generated reliable power from with one drop of saliva, supplying on-board power that could be used in disposable, paper-based diagnostic medical testing.

For several years, the team has focused on developing micro-power sources for the use in resource-limited regions to power point-of-care diagnostic biosensors, and has created several paper-based bacteria-powered batteries.

“On-demand micro-power generation is required especially for point-of-care diagnostic applications in developing countries,” said Seokheun Choi, electrical and computer science assistant professor at Binghamton. “Typically, those applications require only several tens of microwatt-level power for several minutes, but commercial batteries or other energy harvesting technologies are too expensive and over-qualified. Also, they pose environmental pollution issues.”

This battery, activated by saliva, could be helpful in extreme conditions, where normal batteries are not readily available. (Source: Seokheun Choi/Binghamton)

“The proposed battery has competitive advantages over other conventional power solutions because the biological fluid for on-demand battery activation is readily available even in the most resource-constrained settings, and the freeze-drying technology enables long-term storage of cells without degradation or denaturation,” wrote the researchers.

Next, the team will work on improving the battery’s power density so that more applications can be powered.

“Now, our power density is about a few microwatts per centimeter square. Although 16 microbial fuel cells connected in a series on a single sheet of paper generated desired values of electrical current and voltage to power a light-emitting diode (LED), further power improvement is required for other electronic applications demanding hundreds of milliwatts of energy,” said Choi.

Thermally conductive plastic
Materials scientists at the University of Michigan developed a new technique to improve how plastics conduct heat, which the team hopes can lead to lighter, more energy-efficient product components for electronics and vehicles.

In preliminary tests, the process made a polymer about as thermally conductive as glass–still far less so than metals or ceramics, but six times better at dissipating heat than the same polymer without the treatment.

“Plastics are replacing metals and ceramics in many places, but they’re such poor heat conductors that nobody even considers them for applications that require heat to be dissipated efficiently,” said Jinsang Kim, U-M materials science and engineering professor. “We’re working to change that by applying thermal engineering to plastics in a way that hasn’t been done before.”

Previous approaches focused on adding metallic or ceramic fillers to plastics. However, a large amount of fillers must be added, which is expensive and can change the properties of the plastic in undesirable ways. Instead, the new technique uses a process that engineers the structure of the material itself.

A sample of heat-conducting polymer is tested for thickness in U-M’s Lurie Nanofabrication Facility. (Source: Joseph Xu, Michigan Engineering)

The process involves expanding and straightening the long, tangled coils of molecules that make up plastics to give heat energy a more direct route through the material. To accomplish this, they first dissolved a typical polymer in water, then added electrolytes to the solution to raise its pH, making it alkaline.

The individual links in the polymer chain, called monomers, take on a negative charge, which causes them to repel each other. As they spread apart, they unfurl the chain’s tight coils. Finally, the water and polymer solution is sprayed onto plates using a common industrial process called spin casting, which reconstitutes it into a solid plastic film.

The uncoiled molecule chains within the plastic make it easier for heat to travel through it. The team also found that the process has a secondary benefit: it stiffens the polymer chains and helps them pack together more tightly, making them even more thermally conductive.

“Researchers have long studied ways to modify the molecular structure of polymers to engineer their mechanical, optical or electronic properties, but very few studies have examined molecular design approaches to engineer their thermal properties,” said Kevin Pipe, U-M associate professor of mechanical engineering. “While heat flow in materials is often a complex process, even small improvements in the thermal conductivities of polymers can have a large technological impact.”

The team is now looking at making composites that combine the new technique with several other heat dissipating strategies to further increase thermal conductivity. They’re also working to apply the concept to other types of polymers beyond those used in this research. However, they say a commercial product is likely several years away.

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