...at least if they want to compete with insects for energy management. 

A University of Maryland professor's experiments with solar-powered microdrones finds that sunlight is still an order of magnitude too low in power to keep his aircraft aloft. That's why insects don't use sunlight, but rather food: chemical energy (like digesting meat) is much higher density than solar. IEEE Spectrum explains more:

UMD professor S.K. Gupta explains how well Robo Raven works with the solar wings:

So how good is the performance? Solar cells currently cover less than half the wing area in Robo Raven III. These solar cells produce 3.6 W of power during a sunny day. The efficiency of these solar cells appears to be around 6 percent, and the combined efficiency of batteries and motors is somewhere between 25 to 50 percent. We hope that the performance will go up significantly as more efficient solar cells become available and we cover more of Robo Raven III’s wing and body area with solar cells in future versions.

Robo Raven's motors currently draw about 30 watts, meaning that the solar power output is an order of magnitude below what would be required to keep the robot aloft indefinitely. Real ravens, of course, rely on foods like meat, which has a crazy high power density:

Nature has a significant edge over engineered system in other areas. For example, 1 gram of meat stores 20 times more energy than 1 gram of the current battery technology. So in terms of the energy density, we engineers have a lot of catching up to do. In nature, solar energy collection devices (e.g., trees) are not on-board ravens. Hence, ravens ultimately utilize a large collection area to gather energy into a highly dense storage source (e.g., meat), giving them a much longer range and better endurance than Robo Raven III.

I think what Dr. Gupta is saying here is that we should just forget solar power entirely and start feeding our robots meat. But until that happens, continuing advances in solar cell technology should allow Robo Raven to at least significantly extend its flying time, if not power itself from the sun completely

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Comment by Dave Wicks on October 15, 2013 at 5:06pm
I can just picture a DIYer making some sort of gastro fuelled steam punk robo bird that eats meat and turns the gasses into energy.
Comment by HeliStorm on October 15, 2013 at 7:05pm

I for one welcome our new robot overlords...and hope they don't eat me!


I have often wondered if a small UAS (hummingbird size or smaller) could be built which would feed off of flowers. With the rapid disappearance of bees, a drone drone could become a necessity someday. 


Harvard is working on a robo-bee project, but they offer some high energy density power system as its supply, without really going in depth on the power system. It seems real bees have figured out a good power supply, and maybe we should work to model our technology after nature.

Comment by Harry on October 15, 2013 at 8:40pm

I can see some good scifi inspired by this.  A lab genetically modifies some lower form of life to consume food and make electricity.  They put it in a research drone that has a fly away and crashes in a swamp.  The genetically engineered blob recombines with a gator that eats it and when it lays its eggs hundreds of monsters are unleashed.  The new lifeform is voracious and multiplies super fast as they consume everything in their path, including the resarchers searching for their drone.  The Army wants to nuke the swamp as the only solution but the one remaining green minded researcher saves the day when she discovers the monsters have a weakness - junk food.

Comment by Gary McCray on October 15, 2013 at 8:59pm

These solar / battery / motor (not to mention propeller) efficiencies seem excessively low. 6% efficiency is about as bad as you can do for solar cell efficiency and if you want to spend the bucks you can actually get cells in the 20 to 30 percent efficiency range (even up to 37 percent) and somewhat affordably in the 15 percent range.

And if you took the robo ravens efficiency and simply doubled it to 12% and covered all of the wing instead of half of it you'd have 15 or so watts and proper brushless motors and ESCs can operate (in narrow speed ranges) at efficiencies of 70 to 80% which would seem to put the whole project on the plus side.

Possibly with a bit more sophisticated than normal ESC and a purpose designed brushless motor you could kick it up a bit more.

In fact, there are other solar UAVs out there working at doing just that including some with the lofty goal of being able to climb high enough each day so that they can glide each night and then recover the altitude the next day, in effect becoming low flying perpetual drones.

Also clearly for a pure solar plane, battery technology could be considered to be irrelevant, since you are, in fact flying on solar not battery and it is questionable if you even need one.

Maybe a small super cap for short term storage and smoothing voltage levels for the electronics.

In fact, optimal propeller design not mentioned in the article is more critical than motor or speed control efficiency let alone somewhat pointless battery.

Comment by HeliStorm on October 15, 2013 at 9:03pm

Harry. Nope, the drone crashes in the ocean, where it recombines with a shark, which it kills. Yet through nano-technology built into the drone, the shark reanimates, AND can fly, leading to THIS!

Comment by HeliStorm on October 15, 2013 at 9:07pm

Actually, excuse me everyone, I have a pitch to make to the SyFy Channel.

Comment by Morli on October 15, 2013 at 11:33pm

Drone that bites :)),  make sure it's belly is full before doing ground/flight tests.

Comment by Ruwan on October 16, 2013 at 7:49am

@Morli, lol. Definitely one item to the safety page once these things develop. 

Comment by Jared Reabow on October 16, 2013 at 1:56pm

i thnk the only way to do this is to have some kind of genetically engineered bird that has no brain and instead has a computer implanted


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