Power/Performance Bits: March 1

Low power Wi-Fi

Computer scientists and electrical engineers from the University of Washington came up with a way to generate Wi-Fi transmissions using 10,000 times less power than conventional methods and which consumes 1,000 times less power than existing energy-efficient wireless communication platforms such as Bluetooth Low Energy and Zigbee.

The system, Passive Wi-Fi, uses backscatter communication to directly generate 802.11b transmissions capable of being decoded by any Wi-Fi device. A passive Wi-Fi IC designed by the team shows 1 and 11 Mbps transmissions consuming 14.5 and 59.2 microwatts respectively.

Passive Wi-Fi architecture. The passive Wi-Fi devices perform digital baseband operations like coding, while the power-consuming RF functions are delegated to a plugged-in device in the network. (Source: University of Washington)

To cut the power consumed by Wi-Fi, the team decoupled the digital and analog operations involved in radio transmissions. The Passive Wi-Fi architecture assigns the analog, power-intensive functions to a single device in the network that is plugged into the wall.

An array of sensors perform digital baseband operations and produces Wi-Fi packets by reflecting and absorbing the signal. In real-world conditions on the UW campus, the team found the passive Wi-Fi sensors and a smartphone can communicate over distances of 30–100 feet in various line-of-sight and through-the-wall scenarios.

“All the networking, heavy-lifting and power-consuming pieces are done by the one plugged-in device,” said co-author Vamsi Talla, an electrical engineering doctoral student. “The passive devices are only reflecting to generate the Wi-Fi packets, which is a really energy-efficient way to communicate.”

Solar on a soap bubble

Researchers at MIT developed a new approach to solar cell manufacturing, resulting in the thinnest, lightest solar cells yet produced. Though it may take years to develop into a commercial product, the laboratory proof-of-concept shows a new approach to making solar cells that could help power the next generation of portable electronic devices.

The key to the new method, according to MIT professor Vladimir Bulović, is to make the solar cell, the substrate that supports it, and a protective overcoating to shield it from the environment, all in one process. The substrate is made in place and never needs to be handled, cleaned, or removed from the vacuum during fabrication, minimizing exposure to contaminants that could degrade the cell’s performance.

In the initial proof-of-concept experiment, the substrate and the overcoating were composed of the flexible polymer parylene, used widely to protect implanted biomedical devices and PCBs from environmental damage, while the organic material DBP acted as the primary light-absorbing layer. The entire process takes place in a vacuum chamber at room temperature and without the use of any solvents, unlike conventional solar cell manufacturing. In this case, both the substrate and the solar cell are created using established vapor deposition techniques.

The final ultra-thin, flexible solar cells, including substrate and overcoating, are just one-fiftieth of the thickness of a human hair and one-thousandth of the thickness of equivalent cells on glass substrates — about two micrometers thick — yet they convert sunlight into electricity just as efficiently as their glass-based counterparts.

To demonstrate just how thin and lightweight the cells are, the researchers draped a working cell on top of a soap bubble, without popping the bubble.

(Source: Joel Jean and Anna Osherov/MIT)

While the solar cell in the demonstration device is not especially efficient, because of its lightness the power-to-weight ratio is among the highest ever achieved, an important consideration for applications such as on spacecraft or on high-altitude helium balloons used for research, where weight is critical. Whereas a typical silicon-based solar module, whose weight is dominated by a glass cover, may produce about 15 watts of power per kilogram of weight, the new cells have already demonstrated an output of 6 watts per gram — about 400 times higher.

The demonstration cell may also be too thin to be practical. “If you breathe too hard, you might blow it away,” admitted doctoral student Joel Jean, but noted that parylene films of thicknesses of up to 80 microns can be deposited easily using commercial equipment, without losing the other benefits of in-line substrate formation.

The team emphasized that the particular choices of materials were just examples, and the in-line substrate manufacturing process is the key innovation. Different materials could be used for the substrate and encapsulation layers, and different types of thin-film solar cell materials, including quantum dots or perovskites, could be substituted for the organic layers used in initial tests.

Cadmium telluride solar breakthrough

Researchers from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) in collaboration with Washington State University and the University of Tennessee improved the maximum voltage available from a cadmium telluride (CdTe) solar cell, overcoming a practical limit that has been pursued for decades and is key to improving efficiency.

Silicon solar cells represent 90% of the solar cell market, but significantly reducing their manufacturing cost is proving difficult. CdTe solar cells offer a low-cost alternative, as well as having a low carbon footprint. But while performing better under adverse conditions, such as hot, humid weather and under low light, CdTe cells have lagged silicon cells in efficiency.

One key area where CdTe has underperformed is the maximum voltage available from the solar cell. Limited by the quality of CdTe materials, researchers for the past 60 years were not able to get more than 900 millivolts out of the material, which was considered its practical limit.

The research team improved cell voltage by shifting from a standard processing step using cadmium chloride. Instead, they placed a small number of phosphorus atoms on tellurium lattice sites and then carefully formed ideal interfaces between materials with different atomic spacing to complete the solar cell.

This approach improved both the CdTe conductivity and carrier lifetime by orders of magnitude, enabling fabrication of CdTe solar cells with an open-circuit voltage breaking the 1-volt barrier for the first time. Plus, the team says the material’s efficiency could be improved an additional 30% and be capable of providing electricity at a lower cost than fossil fuels.