Tiny solar-powered drones could stay in the air forever

A drone weighing just 4 grams is the smallest solar-powered aerial vehicle to fly yet, thanks to its unusual electrostatic motor and tiny solar panels that produce extremely high voltages. Although the hummingbird-sized prototype only operated for an hour, its makers say their approach could result in insect-sized drones that can stay in the air indefinitely.

Tiny drones are an attractive solution to a range of communications, spying and search-and-rescue problems, but they are hampered by poor battery life, while solar-powered versions struggle to generate enough power to sustain themselves.

As you miniaturise solar-powered drones, their solar panels shrink, reducing the amount of energy available, says Mingjing Qi at Beihang University in China. The efficiency of electric motors also declines as more energy is lost to heat, he says.

To avoid this diminishing cycle, Qi and his colleagues developed a simple circuit that scales up the voltage produced by solar panels to between 6000 and 9000 volts. Rather than using an electromagnetic motor like those in electric cars, quadcopters and various robots, they used an electrostatic propulsion system to power a 10-centimetre rotor.

This motor works by attracting and repelling alternating components with electrical charges arranged in a ring, creating torque, which spins a single rotor blade like a helicopter. The lightweight components are made with wafer-thin slivers of carbon fibre covered in extremely delicate aluminium foil. Their high voltage demands are actually a bonus, as current is reduced, leading to very low losses to heat.

“The operating current is extremely low for the same power output, resulting in almost no heat being generated by the motor. The high efficiency and low power consumption of the motor allow us to power the vehicle with a very small solar panel,” says Qi. “We have managed to get a micro-aerial vehicle to fly using natural sunlight for the first time. Before this, only very large, ultralight aircraft could achieve this.”

The researchers’ machine, which they call CoulombFly, weighs just 4.21 grams and managed a 1-hour flight before it failed mechanically. Qi says these weak points can be designed out, and future versions will effectively be able to fly indefinitely by using solar panels in the daytime and harvesting radio signals, such as 4G and Wi-Fi, for energy at night.

CoulombFly is capable of carrying a payload of 1.59 grams, which could allow for small sensors, computers or cameras. But with refined designs, the researchers think this could be increased to 4 grams, and fixed-wing versions could even carry up to 30 grams. Work is also under way to create an even smaller version of CoulombFly that has a rotor less than 1 centimetre in diameter.