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Most hobby and commercial drones can fly for only about 20-30 minutes max. That’s because batteries eventually die. And the larger and more powerful the battery, the heavier the craft, which in turn needs more battery power to fly.

But there are a number of projects taking off now aiming to keep drones in the air much longer by harnessing solar energy, as well as a technology known as wireless energy transfer.

If drones don’t need to recharge, they won’t have to land. And that means drones will be able to travel longer distances and potentially never land, which will be especially important for drones beaming internet access or performing long-term surveillance operations.

Last month, researchers from the Imperial College of London released a paper detailing how long-range wireless charging for drones might work. The technology is only at an early stage: Right now the researchers are only able to fly a quadcopter wirelessly within five inches of a charging base that transmits power to a receiver made of copper foil attached to the drone. The team had to engineer a drone especially for wireless charging so as not to fry the drone’s components.

Earlier this summer, a company called Global Energy Transmissions demonstrated how it was able to fly a tethered drone for one hour, which it claimed was charging wirelessly while it flew. Aerial wireless charging without docking would require some infrastructure upgrades. The Global Energy Transmissions team has invented a power cord that would wrap around buildings and charge a passing drone equipped with wireless charging receiver hardware.

Aerial charging is something Amazon has been thinking about with its drone program, too. The company was issued a patent this summer for a network of docking stations on cell towers or other tall structures where their drones could recharge and share navigational information.

Solar power is also an option for drone charging, but that’s mostly being explored for winged drones, which require fewer motors and may handle smaller payloads than quadcopters.

Facebook’s Aquila drone, which the company hopes to eventually power with solar energy, completed a successful test earlier this summer. Google also has a solar-powered drone project called SkyBender that the company has been testing in New Mexico.

Last year the first solar-powered manned aircraft, the Solar Impulse 2, completed a flight around the world without refueling. Monday, the Swiss aviators of the Solar Impulse team said their next project is a solar-powered drone.