The winners of the Little Box Challenge were announced on the last month at the ARPA‑E Energy Innovation Summit, where the Red Electrical Devils were presented with the one million dollar prize. The volume of the winning design was 14in3 and demonstrated a ridiculous inverter power density of 143W/in3. This is approximately 100 times smaller in volume than other leading commercially-available transformerless inverters with the same capacity.
The use of gallium nitride (GaN) semiconductor in the winning design enabled very fast switching and miniaturisation of the inverter components but also required careful design of control circuitry, due to the inherent difficulties of controlling GaN transistors. Other notable design features which allowed for a more compact structure include specially-designed ‘MLC’ capacitors, a honeycomb structure for the surrounding copper heat sink and a geometric arrangement of thermal interfaces to enable lower temperatures during operation.
The Red Electrical Devils simulated and optimised the structure of the copper heat sink to improve thermal management [Image Credit: Little Box Challenge]
The runner up and third place teams also met the minimum power density requirement of 50 W/in3 with their 21in3 and 29in3 inverter prototypes, along with other performance criteria which included a DC‑to‑AC efficiency of greater than 95%, the ability to maintain a maximum external temperature below 60 °C during operation, a restriction to air‑only cooling schemes, an input DC voltage of 450V and an output AC voltage of 240V at 60 Hz.
The success of the Little Box Challenge contestants greatly exceeded the expectations of the competition organisers. Ross Koningstein, Engineering Director Emeritus at Google Research, stated: “Impressively, the winning team exceeded the power density goal for the competition by a factor of three… When we initially brainstormed technical targets for the Little Box Challenge, some of us at Google didn’t think such audacious goals could be achieved. Three teams from around the world proved decisively that it could be done.”
As stated in an earlier post, smaller inverters like these winning units can significantly reduce pricing by minimising materials, transportation and installation costs. Miniature string inverters can enable the deployment of solar energy and battery storage in scenarios previously thought of as off‑limits due to weight and space restrictions. For example, miniature kW‑scale inverters may also add another level of functionality to electric vehicles (EVs) by providing transportable off‑grid power or back‑up power in emergencies.
Top & Bottom Image Credit: CE+T Power
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