Power/Performance Bits: Mar. 13

Wireless charging
Engineers at the University of Washington developed a method to safely charge a smartphone wirelessly using a laser, potentially as quickly as a standard USB cable. Safety features of the system include a reflector-based mechanism to shut off the laser and heatsinks.

The charging beam is generated by a laser emitter that the team configured to produce a focused beam in the near-infrared spectrum, which charges the smartphone via a power cell mounted on the back of the phone. A narrow beam can deliver a steady 2W of power to 15 square-inch area from a distance of up to 4.3 meters, or about 14 feet. But the emitter can be modified to expand the charging beam’s radius to an area of up to 100 square centimeters from a distance of 12 meters, or nearly 40 feet. This extension means that the emitter could be aimed at a wider charging surface, such as a counter or tabletop, and charge a smartphone placed anywhere on that surface.

The researchers programmed the smartphone to signal its location by emitting high-frequency acoustic “chirps” that can are inaudible to our ears but can be picked up by small microphones on the laser emitter. When the emitter detects the smartphone on the desired charging surface, it switches on the laser to begin charging the battery.

The wireless charging system. The charging laser and guard lasers are normally invisible to the human eye, but red beams have been inserted in place of the guard beams for demonstration purposes. (Source: Mark Stone/University of Washington)

The safety system that shuts off the charging beam centers on low-power, harmless laser guard beams, which are emitted by another laser source co-located with the charging laser-beam and physically surround the charging beam. Custom 3-D printed retroreflectors placed around the power cell on the smartphone reflect the guard beams back to photodiodes on the laser emitter. The guard beams deliver no charge to the phone themselves, but their reflection from the smartphone back to the emitter allows them to serve as a sensor for when a person moves in the path of the guard beam.

The researchers designed the laser emitter to terminate the charging beam when any object – such as part of a person’s body – comes into contact with one of the guard beams. The blocking of the guard beams can be sensed quickly enough to detect the fastest motions of the human body, based on decades of physiological studies.

To ensure that the charging beam does not overheat the smartphone, the team placed thin aluminum strips on the back of the smartphone around the power cell to act as a heatsink for dissipating excess heat from the charging beam. They were able to harvest a small amount of this heat to help charge the smartphone by mounting a nearly-flat thermoelectric generator above the heatsink strips.

“The beam delivers charge as quickly as plugging in your smartphone to a USB port,” said Elyas Bayati, a UW doctoral student in electrical engineering. “But instead of plugging your phone in, you simply place it on a table.”

Smart window coating
Researchers from RMIT University developed a new ultra-thin coating for smart windows that responds to heat and cold, automatically letting in more heat when it’s cold and blocking the sun’s rays when it’s hot.

“We lose most of our energy in buildings through windows. This makes maintaining buildings at a certain temperature a very wasteful and unavoidable process,” said Madhu Bhaskaran, an associate professor at RMIT. “Our technology will potentially cut the rising costs of air-conditioning and heating, as well as dramatically reduce the carbon footprint of buildings of all sizes.”

Smart glass windows are about 70% more energy efficient during summer and 45% more efficient in the winter compared to standard dual-pane glass, according to the researchers.

New York’s Empire State Building reported energy savings of US$2.4 million and cut carbon emissions by 4,000 metric tonnes after installing smart glass windows. This was using a less effective form of smart window technology.

“The Empire State Building used glass that still required some energy to operate,” Bhaskaran said. “Our coating doesn’t require energy and responds directly to changes in temperature.”

Mohammad Taha holds a sample of the coating. (Source: RMIT University/James Giggacher)

The self-regulating coating is 50-150nm in thickness and created with vanadium dioxide. At 67 degrees Celsius, vanadium dioxide transforms from being an insulator into a metal, allowing the coating to turn into a versatile optoelectronic material controlled by and sensitive to light.

The coating stays transparent and clear to the human eye but goes opaque to infra-red solar radiation, which humans cannot see and is what causes sun-induced heating.

While the coating reacts to temperature it can also be overridden with a simple switch, said Mohammad Taha, a PhD student at RMIT. “This switch is similar to a dimmer and can be used to control the level of transparency on the window and therefore the intensity of lighting in a room,” Taha said. “This means users have total freedom to operate the smart windows on-demand.”

The technology can also be used to control non-harmful radiation that can penetrate plastics and fabrics. This could be applied to medical imaging and security scans.

The team has filed a patent on the underlying technology and says it is readily scalable to large area surfaces.

Renewables capacity
According to a study by scientists at the University of California, Irvine, the California Institute of Technology, and the Carnegie Institution for Science, the United States could reliably meet about 80% of its electricity demand with solar and wind power generation. Achiving this would not come cheap, however.

The team analyzed 36 years of hourly U.S. weather data (1980 to 2015) to understand the fundamental geophysical barriers to supplying electricity with only solar and wind energy.

“We looked at the variability of solar and wind energy over both time and space and compared that to U.S. electricity demand,” said Steven Davis, UCI associate professor of Earth system science. “What we found is that we could reliably get around 80 percent of our electricity from these sources by building either a continental-scale transmission network or facilities that could store 12 hours’ worth of the nation’s electricity demand.”

The researchers said that such expansion of transmission or storage capabilities would mean very substantial investments. They estimated that the cost of the new transmission lines required, for example, could be hundreds of billions of dollars. In comparison, storing that much electricity with today’s cheapest batteries would likely cost more than a trillion dollars, although prices are falling.

Other forms of energy stockpiling, such as pumping water uphill to later flow back down through hydropower generators, are attractive but limited in scope. The U.S. has a lot of water in the East but not much elevation, with the opposite arrangement in the West.

“The fact that we could get 80% of our power from wind and solar alone is really encouraging,” Davis said. “Five years ago, many people doubted that these resources could account for more than 20 or 30%.”

However, meeting 100% of electricity demand with only solar and wind energy would require storing several weeks’ worth of electricity to compensate for the natural variability of these two resources, the researchers said.