Months in the stratosphere: Harvard names 500 Wh/kg as battery target for solar aircraft
Uncrewed solar aircraft could collect climate and weather data for weeks or months. According to a Harvard study, their batteries need roughly twice the energy density of today's EV batteries.
Graphic: TERRA WORLD NEWS (own graphic based on the sources listed)
Solar-powered, uncrewed aircraft are meant to circle for weeks or months in the stratosphere – about twice as high as the roughly 30,000-foot cruising altitude of airliners and above the weather. During the day, surplus solar power charges the batteries; at night, the aircraft fly on them alone. By comparison, NASA's ER-2 high-altitude research aircraft, which carries a pilot, stays aloft for only six to eight hours at a cost of tens of thousands of dollars per flight hour.
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), together with the University of Michigan and industry partners, have calculated performance targets for such batteries; the study appeared in "ACS Energy Letters" (DOI 10.1021/acsenergylett.6c01616), and Harvard presented it on 28 October. They identify an energy density of 500 watt-hours per kilogram as an important threshold. Today's EV batteries are at about 250, experimental or specialty cells at around 400 watt-hours per kilogram. Because the aircraft need little power but a lot of stored energy, developers should prioritise energy density over power density.
"We're almost there," says Craig Mascarenhas, who led the study. "Those cells are very expensive right now, but they're getting cheaper very quickly." The work was carried out in the lab of atmospheric chemist Jim Anderson, who heads the Harvard branch of the NASA-funded SACOS collaboration. According to Harvard, applications would include atmospheric chemistry, hurricane tracking and wildfire monitoring and prediction.