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What is the feasibility of solar powered drones? From my understanding, solar panels are becoming more and more efficient, in terms of weight vs energy captured, and in terms of surface area vs energy captured.

What I am interested in is whether solar panels could be placed on different sized drones (small civilian quadcopter and MQ-9 Reaper), and provide enough solar power to keep the aircraft aloft indefinitely (under a theoretical condition of an "endless sunny day").

If this is not possible, are there are realistic changes that can be made to make it possible, while still keeping the core functionality of these two different drones intact?

For the purposes of this question, instead of focusing on a 100% solar powered drone, we can assume that fuel can be carried for takeoff/climb to loitering altitude, at which point is can switch to 100% loiter.

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    $\begingroup$ I am aware of the following similar question, but I'm not interested in all the "contributing factors", such as maintenance, fluids, etc - I'm interested in whether efficient solar power can provide enough energy. aviation.stackexchange.com/questions/13673/… $\endgroup$
    – M28
    Oct 19, 2018 at 14:05
  • $\begingroup$ Possible, but unwieldy, currently: newatlas.com/solar-powered-quadcopter/55993 --- Also, please alter your question to be clearly about quadcopter drones (if that is what you want to know) because solar crawlers have been real for years. $\endgroup$
    – bukwyrm
    Oct 19, 2018 at 14:11
  • $\begingroup$ @bukwyrm I'm interested in both fixed wing drones, and quadcopters. Presumably, fixed wing drones would be easier to implement this on, as they have more surface area, and less energy required to produce lift at loitering altitude. $\endgroup$
    – M28
    Oct 19, 2018 at 14:15
  • $\begingroup$ what are the 'three different drones' you are talking about in the question? civvy quadcopter and MQ-9 and ...? -- Please alter your question to reflect your general interest in flying drones, regardless of type. $\endgroup$
    – bukwyrm
    Oct 19, 2018 at 14:23
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    $\begingroup$ Without making the MQ9 much, much bigger it isn't possible and won't be. There just isn't enough surface area to generate the required energy. Certainly not possible for quad-copters because of how much energy they use to fly (it is all electric, whereas aircraft has lift from wings) $\endgroup$
    – Ron Beyer
    Oct 19, 2018 at 15:05

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Let's take the MQ-9 as an example...

The MQ-9 is powered by a 900 HP Honeywell TPE331-10GD engine. In order to generate 900 HP, it will take 2100 kVA (assuming 3-phase electric AC motor). 2100 kVA is about 1680 kW. That is 1.68 million watts.

Now, the sun delivers (if you could get 100% of the energy sent to Earth) about 4.8kW per square meter (depends on Latitude, which can range from about 6kW to less than 1kW). This would mean you need about 350 square meters of 100% efficient solar panels.

The problem is that the current "world record" for solar efficiency is 46%, let's round up to 50% for easy math sake. So we have to double our solar panel area to 700 square meters.

Now the wingspan of an MQ-9 is 66 feet (about 20 meters), and a length of 36 feet (11 meters). If the MQ-9 was box shaped, it has a surface area of 220 square meters. Lets assume that 20% of that is real "upward facing" surface area so in reality, if you covered the MQ-9 with the most efficient solar panels in the world, you would only generate (220 * .2 * (4.8 * .46)) about 97kW or about 6% of the power actually required to run the aircraft.

This is also assuming that the extra weight of the batteries/wiring/charge controllers don't make the aircraft heavier or require a larger power plant, and assuming that the 3-ph 2100kVA motor is the same weight as the TPE331-10GD.

This gets even worse for quad-copters because they just don't have the surface area, and don't benefit from aerodynamic lift to reduce the amount of power required to fly.

Sources:
USAF MQ-9 Facts
Wikipedia Solar Cell Efficiency

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  • $\begingroup$ Thanks for the answer, it's really interesting. Seeing as a Reaper doesn't operate at 100% thrust in any normal situation outside of takeoff, I wonder how much energy it takes to stay at cruising altitude... certainly not 6% though! Looks like a complete redesign would have to be in order to accomplish that. $\endgroup$
    – M28
    Oct 19, 2018 at 20:33
  • $\begingroup$ Just for another data point, the Solar Impulse has a wingspan of 236 feet (72m), surface area of 269.5sq-m and generates 66kW peak. It uses 57kW motors (4x13kW) and max Take-off weight of 4400lbs and cruises at 43mph. Reaper can take off at up to 10,000lbs and fly 230mph. The Solar impulse also has to carry almost 1400lbs of batteries for night/reduced lighting flying. $\endgroup$
    – Ron Beyer
    Oct 19, 2018 at 21:16
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Check out the solar powered drones used to provide cell coverage in Puerto Rico

https://www.npr.org/sections/thetwo-way/2017/11/17/564879549/faa-approves-drone-as-cell-phone-tower-in-the-sky-for-puerto-rico

The solar panels were land based and the drone tethered in place. Seems a great compromise - keep the heavy stuff on the ground, but get the antenna/electronics up in the air for line of sight & range.

Free flying solar powered drones are apparently quite feasible per this article https://www.iflscience.com/technology/solar-powered-drone-could-fly-nonstop-five-years/ enter image description here

Titan Aerospace is currently developing a drone named Solara 50 that is being hailed as an “atmospheric satellite” and has quite a lot to offer in terms of gathering scientific data. The drone will fly at an altitude of over 19,000 meters (65,000 feet) where there is little air traffic and above most weather that could impede its travels. Flying at that height will also give it unobstructed access to the sun, which will power the 3,000 solar cells that cover its 50-meter-long (164 feet) wings. The solar cells will also charge lithium ion batteries stored inside the wing so it can fly at night, which means that this drone can fly uninterrupted for up to 5 years all while producing zero emissions.

The body of the drone is durable carbon fiber, and the 5-kilowatt electric motor will allow the aircraft to cruise at about 96 km/h (60 mph). Though the aircraft itself weighs only 160 kg (350 lbs), it will be able to carry 32 kg (70 lbs) worth of payload up into the stratosphere.

Solara 50 is completely self-piloted; able to take off, cruise, and land on its own. It will function much the same way as a satellite, though it will cost much less to launch. From the air, it would be able to track developing storms, migrating wildlife, vegetation patterns, and it would also have data communications capabilities; a feature that has caught the eye of the social media giant, Facebook.

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  • $\begingroup$ Yes, I have seen that type of technology at drone conferences. While it's very interesting, it doesn't answer my question. $\endgroup$
    – M28
    Oct 19, 2018 at 14:14

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