Solar-panel-powered unmanned aircraft capable of flying in the stratosphere for weeks - above clouds and weather - are gradually turning from experimental prototypes into promising technology. Such craft can monitor the climate, track forest fires and hurricanes. But for long flights they need a new level of batteries.
A group of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), together with partners from industry and the University of Michigan, published an article in the journal ACS Energy Letters about the main obstacle to long solar flights - battery characteristics. The work was led by Craig Mascarenhas, who leads engineering research on solar aviation in the lab of Jim Anderson, professor of atmospheric chemistry.
A solar aircraft must accumulate excess energy from its panels during the day and draw on it from batteries at night. This creates a vicious circle: the larger the battery capacity, the heavier it is, and therefore more power is needed for flight. Mascarenhas himself, now an engineer for wireless power transfer systems for NASA lunar missions but still collaborating with Anderson's lab, described this.
According to the authors' estimates, for sustained flights in different regions of the world the specific energy density of batteries must reach 500 watt-hours per kilogram (Wh/kg). This is a measure of how much energy a unit of battery mass stores. For comparison: modern electric-car batteries provide about 250 Wh/kg, while experimental or special cells already reach about 400 Wh/kg. "We are almost there," Mascarenhas says. "Right now such cells are very expensive, but they are getting cheaper fast."
The scientists emphasize that for stratospheric aircraft it is not power but energy density that matters most. Such craft have very low power requirements, but they need a lot of energy to sustain flight. Battery developers should therefore focus on increasing energy storage rather than optimizing both parameters equally, as is usually done in engineering.
The work also involved battery specialists Venkatasubramanian Viswanathan and Shashank Sripad, aerospace entrepreneur John Langford, whose companies built and tested experimental solar aircraft, and others. The study draws on years of experience: Anderson has worked on solar aviation concepts for about thirty years and leads the Harvard branch of the SACOS (Stratospheric Airborne Climate Observing System) project.
The authors note that if the technology is optimized, it could provide continuous and detailed observations of the atmosphere that are today expensive and hard to obtain. For example, NASA's piloted ER-2 aircraft collects scientific data for only a few hours per flight, and an hour of flight costs tens of thousands of dollars. An unmanned aircraft, by contrast, can stay aloft for months, requires fewer people and costs less. Possible applications include monitoring atmospheric chemistry and tracking hurricanes and fires.
Mascarenhas is especially hopeful about use in fighting forest fires: "I want to reduce the damage from fires around the world. The opportunities this project opens up are very inspiring."