Google Little Box Challenge: Winning inverter design claims 10x power density improvement
The Little Box Challenge was an open competition to design and build a small kW-scale inverter with a power density greater than 50 Watts per cubic inch while meeting a number of other specifications related to efficiency, electrical noise and thermal performance. More than 2,000 teams from across the world registered for the competition and more than 80 proposals qualified for review by IEEE Power Electronics Society and Google. In October 2015, 18 finalists were selected to bring their inverters to the National Renewable Energy Laboratory (NREL) for testing.
The Red Electrical Devils (named after Belgium’s national soccer team) were declared the winner by a consensus of judges from Google, IEEE Power Electronics Society and NREL. Honorable mentions went to finalist teams from Schneider Electric and Virginia Tech’s Future Energy Electronics Center.
Schneider, Virginia Tech and The Red Electrical Devils all built 2 kW inverters that passed 100 hours of testing at NREL, adhered to the technical specifications of the competition.
The Red Electric Devils’ winning inverter design had the highest power density and smallest volume of the three finalists,
When The Little Box Challenge organizers launched the Prize they dared the world’s engineers to: “Figure out how to shrink an inverter down to something smaller than a small laptop (a reduction of > 10× in volume) and smaller than everyone else, and you’ll win a million dollars (and help revolutionize electricity for the next century).”
The key goal of the challenge was to reach an inverter power density in excess of 50 W/cubic inch in a volume of under 40 cubic inches – a feat which had never been done before. The Red Electrical Devils presented their entry at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, and successfully passed exhaustive testing. The winning inverter design produced a power density of 143 W/cubic inch in 14 cubic inches, outperforming the Little Box Challenge power density goal by nearly a factor of three, which, according to Google “is 10 times more compact than commercially available inverters.”
The Energy Department’s National Renewable Energy Laboratory (NREL) provided critical analysis of the 18 finalists teams’ inverters to help determine the winner.
On October 21, 2015, each of the finalists brought their inverters to the Energy Systems Integration Facility (ESIF) on the NREL campus in Golden, Colorado, for testing. “The overall idea was to test these inverters in a similar fashion to how they would be used out in the field,” explained Blake Lundstrom, the NREL project lead.
The first step was to verify that the inverters met all critical safety-related specifications and then to simply turn the inverters on and see if they functioned. Next, was a three-hour procedure to operate the inverters at a number of different operating points and to verify that key specifications were met throughout the three hours. After these challenges, the field of eighteen finalists was narrowed to the remaining inverters that would proceed to the third round.
The final inverters were subjected to a 100-hour simulation of real-life conditions, including a direct-current source of electricity that emulated a solar power system, with rapid ramp-ups and ramp-downs in power typical of an intermittently cloudy day, as well as a realistic, changing load typical of a residence that the inverter needed to supply. Each inverter had to meet most of the same specifications required of commercially-available inverters.
“We were checking that all the specifications were met, under realistic conditions that a similar solar inverter in the field would experience, and evaluating their thermal performance over the long term-those are the key things that we were looking for over those 100 hours,” said Lundstrom.
Shrinking inverters by an order of magnitude and making them cheaper to produce and install will enable more solar-powered homes and more efficient distribution grids, while helping bring electricity to remote areas. A key factor in the winning inverters was the use of wide bandgap semiconductors, a technology that enables power electronics to operate at higher voltages and temperatures, allowing them to transmit more energy through a smaller volume.
“Wide bandgaps offer a lot of advantages over traditional silicon that enabled teams to hit some of the miniaturization and efficiency targets that were needed to be successful in the competition,” said Lundstrom. “Not every single team used wide-bandgap devices, but the vast majority did.”
Leading teams were also innovative in their cooling designs and packaging techniques, including efficient integration of their designs onto printed circuit boards, said Lundstrom.
“The Little Box Challenge actually forced people to try to optimize space, and a nice outcome of that is that some of the techniques to do that are going to be pretty helpful for other aspects of inverter development,” said Lundstrom. “For example, once you have a device that is almost entirely integrated onto a printed circuit board, it’s easier to manufacture. Plus, some of the teams were able to incorporate all this innovation without adding any additional cost to the inverter and in some cases these designs may result in reduced inverter cost when mass-produced.”
A key component in the winning team’s success was GS66508P gallium nitride power transistor from GaN Systems.
“The use of GaN technology enabled our team to reach a power density of ~145 W/in³ for the 2 kVA inverter designed for this project. The reduced gate drive and switching losses of GaN Systems’ GS66508P were critical to our thermal and power density goals. Additionally, we were highly impressed at how reliably the devices performed over the months of rigorous, real-world testing by the NREL team,” explained Olivier Bomboir, VP of Product Management and New Business at CE+T Power.
GaN Systems’ CEO, Jim Witham, said: “This achievement is added confirmation that gallium nitride semiconductors are instrumental in helping power design engineers respond to the ever increasing need to develop more efficient power conversion solutions. GaN technology clearly paves the way toward more powerful, compact and efficient inverter designs.”
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