Power/Performance Bits: Feb. 7

Infrared links for data centers

Researchers at Penn State, Stony Brook University and Carnegie Mellon University developed a free space optical link for communication in data centers using infrared lasers and receivers mounted on top of data center racks.

According to Mohsen Kavehrad, professor of electrical engineering at Penn State, “It uses a very inexpensive lens, we get a very narrow infrared beam with zero interference and no limit to the number of connections with high throughput.”

The architecture, dubbed Firefly, uses MEMS with tiny mirrors for rapid targeting and reconfiguring, Kavehrad said. These MEMs use tiny amounts of electricity from four directions to reposition the mirror that targets the receiver. The laser beam can also be rapidly moved to target a different receiver.

The system is not yet implemented, but the researchers created a simplified, proof-of-concept system to show that their infrared laser can carry the signal and target the receiver. They transmitted wavelength division multiplexed bi-directional data streams each carrying data at a transmission rate of 10 Gigabits per second from a Bit Error Rate (BER) test set. BER testing determines the number of errors in a signal caused by interference, noise, distortion or synchronization problems.

An infrared laser beam is going into the receiver of the signaling system. (Source: Patrick Mansell/Penn State)

The proof of concept setup has the bidirectional signal wavelength division multiplexed with a one-way cable television signal. The total data stream goes from fiber-optic cable to the infrared laser, across the room to the receiver and shows the results on a TV and the BER test set. A hand breaking the laser beam shuts off the system, but when the hand is removed, the signal is rapidly reacquired.

Accurately targeting and sending a signal via infrared laser are only two of the hurdles the researchers need to pass before Firefly is operational. Once the signal arrives at the target it must seamlessly enter the fiber-optic cable. Controlling and managing the data distribution system in an unwired environment is also important.

“We are trying to come up with something reconfigurable using light instead of millimeter waves (radio frequency),” said Kavehrad. “We need to avoid overprovisioning and supply sufficient capacity to do the interconnect with minimal switches. We would like to get rid of the fiber optics altogether.”

Investigating cathode particles

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory observed what happens inside a cathode particle as lithium-ion batteries are charged and discharged, including the discovery of particle cracking as the cathode is charged, which can reduce battery capacity and life.

Using transmission microscopy imaging combined with X-ray absorption analysis, the team looked at lithium manganese nickel oxide cathode samples, chosen for its potential as a next-generation battery material. But while the material has high charge and discharge voltage, it also leads to a less stable battery.

While mapping out the chemical and phase distribution on their particles at a very high spatial resolution, the researchers found evidence of phase transformation, which occurs when lithium comes out of the particles as the battery is being charged or goes back in when discharged.

What they saw was a unique nucleation and growth process involving multiple phases simultaneously on the same particle. The impact of the volume differences between the phases, a reduction of more than 6% in total, caused the particles to crack. This became more significant as the particle approached the fully delithiated state.

According to Berkeley Lab materials chemist Guoying Chen, the cracking is likely one of the leading causes of the fade in long-term battery cycling that researchers have seen with this cathode. “If you have cracking, it means fresh surface keeps getting exposed, thus causing more reactions with the electrolyte, which consumes the electrolyte and reduces the lifetime of the battery,” Chen said. “If we can minimize or eliminate the cracking issue, we probably will see much improved stability.”

The researchers are examining two ideas to minimize the cracking, using smaller particles and avoiding fully charging the particles. “Reducing particle size can be tricky as it also increases the surface area,” Chen said. “Higher surface area means more side reactions to begin with, so it is important to find the optimal size.”

Meanwhile, Chen said her group is also looking for other approaches for high-energy batteries, such as materials that can provide a high capacity. “The approach we developed here is broadly applicable for designing and optimizing both new and existing electrode materials,” she said.

Storing solar not so green

There is a lot of focus on improved batteries for storing the excess energy generated by home solar panels, but researchers at the University of Texas at Austin and the U.S. Department of Energy warn that home storage may be counterproductive: storing solar energy for nighttime use actually increases both energy consumption and emissions compared with sending excess solar energy directly to the utility grid.

“The good news is that storage isn’t required to make solar panels useful or cost-effective,” said co-author Michael Webber, a professor in the Department of Mechanical Engineering and deputy director of UT Austin’s Energy Institute. “This also counters the prevailing myth that storage is needed to integrate distributed solar power just because it doesn’t produce energy at night.”

The team analyzed the impact of home energy storage using electricity data from almost 100 Texas households that are part of a smart grid test bed managed by Pecan Street Inc., a renewable energy and smart technology company housed at UT Austin.

They found that storing solar energy for nighttime use increases a household’s annual energy consumption — in comparison with using solar panels without storage — because storage consumes some energy every time it charges and discharges. The researchers estimated that adding energy storage to a household with solar panels increases its annual energy consumption by about 324 to 591 kilowatt-hours.

“I expected that storage would lead to an increase in energy consumption,” said Robert Fares of the U.S. Department of Energy. “But I was surprised that the increase could be so significant — about an 8 to 14% increase on average over the year.”

The researchers also found that adding storage indirectly increases overall emissions of carbon dioxide, sulfur dioxide and nitrogen dioxide based on the current Texas grid mix, which is primarily made up of fossil fuels. The increase in emissions is primarily due to the increase in energy consumption required to account for storage inefficiencies. Because storage affects what time of day a household draws electricity from the grid, it also influences emissions in that way.

If a homeowner is seeking to reduce his or her environmental footprint, adding storage would not make the household more green, but it shouldn’t be dismissed either, the researchers said. “Solar combined with storage is still a lot cleaner than having no solar at all,” said Fares.