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Commercial drones have come a long way in the last decade. By making the skies more accessible, modern drone technology has practically transformed the landscape of every sector imaginable. From filming to firefighting, making deliveries to monitoring the weather, not a day goes by that we don't hear about another use for these unmanned aerial vehicles (UAVs).
But almost all drones regardless of their purpose or size must face the same issue: overcoming limited flight endurance. It turns out that finding a power source which can last along time, be based on a resource that's widely available and affordable, have a good energy-to-weight ratio, be easy on the environment, create little noise or vibrations, and that can be quickly and easily replenished... ain't so easy.
Below is a rundown of 6 different drone power sources. While none of them may be perfect in every measure, they each have very distinct benefits that are suitable for certain situations.
1. Batteries
Many smaller drones rely on lithium-ion (Li-Ion) and lithium-polymer (Li-Po) battery technology to power their flight. While there are other battery options currently available or in the works, such as Lithium-Thionyl-chloride (Li-SOCl2) and Lithium-Air (Li-air), perhaps the most promising are the Lithium-Sulfur (Li-S) batteries which are relatively low cost and offer a higher energy density (i.e. the amount of energy held in the battery compared to its weight).
Pros: Though batteries may have their limitations, they are still a popular option particularly among smaller UAVs. The best thing about batteries is their availability and ease of use. They required little effort to store and transport and can be recharged from virtually anywhere, and refueling a drone is as simple as changing the battery packs.
Plus, while batteries may not have a particularly high energy density as compared to other fuel options, they are nevertheless extremely energy efficient especially when compared to gas fueled combustible engines mentioned below.
Cons: There are several potential ways battery life can be extended, and researchers throughout the world are racing to make these breakthroughs. But those breakthroughs they are still far out in the future. Till that time, battery-powered drones can only be in the air for a very limited amount of time (usually no more than a half an hour) before they start to sputter. This is a problem if a drone is required for a long distance job or needs to be airborne for more than a few minutes.
2. Gas-Powered Engines and Gas-Electric Hybrids
Combustion engines, are robust, small, light-weight and have a decent fuel consumption. They can also be used with various types of fuel, such as petrol, kerosene, methanol, ethanol, and propane. Drones can either be fully powered by a combustion engine or by a combination of a small combustion engine that produces power to charge an on-board battery pack or act as a generator to an electric motor.
Like hybrid electrical cars, these hybrid drone systems combine the operation of an electric motor with all the benefits of a gas-powered engine. They tend to be more fuel efficient, and gas can be conserved during shorter trips that remain within the battery's energy capacity.
Pros: Gas-powered drones have incredible endurance, with many well established gas-powered hybrid UAVs available offering several hours of flight time with one full tank of gas. Another endurance-related advantage is that a gas-powered drone will lose weight during the course of the flight, thereby increasing its flight range. Finally, the required fuel is easy to acquire and store, and refueling takes just a few minutes.
Cons: Most gas-powered engines are actually pretty inefficient, with much of the energy in gas wasted in some way. Combustion engines also create a lot of noise, heavy vibrations and in some cases harmful emissions. Though modern technology can counteract these effects, the added equipment can increase the drone's weight, thus effecting flight performance.
3. Hydrogen Fuel Cells
Hydrogen fuel cells use hydrogen (H2) combined with oxygen to create electricity. Water, the byproduct of this chemical reaction, is removed, and refueling consists of refilling the depleted hydrogen.
Pros: Hydrogen fuel cells offer several benefits. Namely there's no direct pollution (the only “exhaust” is water and heat), they produce no sound, and they are powered by an energy-dense source that happens to be the most abundant chemical on Earth.
Compared to other drone power sources, hydrogen fuel cells are also more energy efficient (up to 60 percent in the case of gas-power systems), can power up to four hours of flight time in many models, and they do well in low temperatures. Plus, refueling a hydrogen fuel cell drone takes one minute.
Cons: While all of these benefits may sound impressive, there's a catch. Using hydrogen fuel cells can get very expensive. The production, storage and distribution of the hydrogen itself is a complex and costly process. Another issue with hydrogen fuel cells is that they emit a significant amount of heat. Given that plastic is widely used in many drones, the production of heat could end up melting some of the drone's components.
In short, drones powered by hydrogen fuel cells are primarily meant for those organizations that care more about performance than price.
4. Solar Cells and Solar-Battery Hybrids
Solar power technology has seen a lot of advancement in recent years, with solar cells reaching a power ratio of about 175W/m2. To power a drone exclusively via solar energy requires a large span of solar cells along the surface of the drone, like the top side of a drone's wing. Once aloft, a solar-powered drone can technically fly as long as the sun is shining. But, once the sun goes down, it's also lights out for the drone's power source.
Lately, solar-battery hybrids have been gaining attention due to their surprising endurance. Just last year, a solar powered hybrid recorded a flight time of over 25 days. With a solar hybrid system, the sun’s light is used to power the drone by day, while at night, the UAV switches to the solar power that was simultaneously stored in its onboard batteries.
Pros: The biggest benefit to relying on solar energy, especially when it is combined with a rechargeable battery, is the idea that a drone can fly for extending periods (not just days, but literally years) without needing to be charged or refueled. Solar powered drones can also fly at higher than average altitudes due to their stable, yet light weight frame. Finally, solar-powered cells represent a clean energy source with no harmful emissions.
Cons: In order to generate enough power, there needs to be enough surface area dedicated to holding the solar cells. This can lead to design limitations. Most solar powered drones as well as solar hybrids are also heavily dependent on the weather. Overcast skies could cause problems if the onboard batteries aren't big enough to wait for the sun to shine. But bigger batteries can also be a limitation since it will add to the drone's weight.
The last two drone power sources are still in development, but they each offer a tremendous amount of potential...
Super/Ultra-Capacitor Hybrids
Batteries can hold large amounts of power, but they can take several hours to charge up. Capacitors, on the other hand, charge almost instantly, but hold only small amounts of power. Super or Ultra-capacitor hybrid drones significantly increase the power and efficiency of Li batteries by combining the qualities of a lithium battery with the super fast charging of a capacitor. They promise to save a lot of weight while significantly increasing a drone's range and flight time.
Wireless Charging Technology
Imagine being able to recharge a drone's battery pack without having the UAV touch the ground. Ground-to-air recharging systems allow drone operators to experience almost unlimited flight times.
There are currently two different types of technology in the experimental stage. The first is a laser-based recharging system. In this system, drone operators shine a laser beam at specially equipped solar panels that then allow the drone to recharge. Recharging a UAV with lasers has some limitations, though, such as the fact that the laser beams can be hindered by distance, rain, fog, smoke and haze. The second option uses an electromagnetic field to keep drones charged. A ground-based wire frame reminiscent of a spider's web provides an electromagnetic field that can charge a drone equipped with special antennas.
In short, all commercial drone power sources have their own advantages and limitations making them useful for certain jobs, but not for others. Each organization should decide what their priorities are before choosing one form of drone technology over another.