Power/Performance Bits: March 8

Configurable analog chip

Researchers at Georgia Tech built a new configurable computing device, the Field-Programmable Analog Array (FPAA) SoC, that uses analog technology supported by digital components and can be built up to a hundred times smaller while using a thousand times less electrical power than comparable digital floating-gate configurable devices.

Professionals familiar with FPGAs will find the programming interface of the new analog chip surprisingly like the digital circuits in many ways, said Jennifer Hasler, a professor in the Georgia Tech School of Electrical and Computer Engineering. “But in other ways the FPAA is going to seem quite different,” she said. “In terms of the power needed, it’s extremely different because you need only milliwatts to run the analog device, while it’s hard to get an FPGA to work on less than a watt.”

Examples of FPAA chips and printed circuit boards. (Source: Fitrah Hamid/Georgia Tech)

To perform computation using an analog-style physical architecture, the team had to reliably position electrons in an FPAA’s connective structure. This approach stands in contrast to FPGAs, which process electrons through floating gates in ways similar to conventional digital semiconductors such as memory chips or central processing units.

One advantage of FPAAs is that they’re non-volatile, reducing power consumption compared to the higher power needs of volatile SRAM configurations typically used in FPGAs.

“In addition to being non-volatile, our analog architecture lets us do something fairly radical – we can compute using the routing fabric of the chip, exploiting areas that are usually considered just dead weight,” Hasler said. “To help do this, we’ve developed highly efficient switches that can be programmed on, off, or in-between – partially on and partially off. This flexibility provides both increased computation capabilities and reduced power consumption.”

Carbon recycling

Scientists from Vanderbilt University and George Washington University worked out a way to reduce the amount of atmospheric carbon dioxide and build battery components at the same time.

The project builds upon a solar thermal electrochemical process, or STEP, that uses solar energy to provide both the electrical and thermal energy necessary to break down ambient carbon dioxide into carbon and oxygen – and to produce carbon nanotubes which can be incorporated into lithium-ion batteries for use in electric vehicles and electronic devices as well as lower-cost sodium-ion batteries under development to support the electric grid.

In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly. In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned using STEP, can unlock stable storage performance over 3.5 times above that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon-nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

The Solar Thermal Electrochemical Process converts atmospheric carbon dioxide into carbon nanotubes that can be used in advanced batteries. (Source: Julie Turner / Vanderbilt University)

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40% of a battery could be made out of recycled CO2. This excludes the outer protective packaging, but the team suggested a process like theirs could eventually produce the packaging as well.

The researchers estimated that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the Department of Energy in 2013), a kilogram of carbon dioxide has a value of about $18 as a battery material – six times more than when it is converted to methanol. And unlike methanol, combining batteries with solar cells would provide renewable power with zero greenhouse emissions.