Drones will fly for days with new photovoltaic engine • Hacker News Board

Re: Drones will fly for days with new photovoltaic engine

#71

Post by k__ » Mon, Jul 29, 2019, 1:27 PM UTC

pjc50 wrote: post #35

IanCal wrote: post #23

People seem to be misinterpreting this a lot. This is not about solar cells. This is about thermophotovoltalics, and the application here is using a burning fuel source to generate electricity. The "drone" the paper talks about seems to be a reference to another paper on unmanned sea vehicles though. Edit - to be fair the flying drone part is from the corresponding author.

Exactly. The paper is here: https://www.pnas.org/content/early/2019/07/15/1903001116 The context of thermophotovoltalics is very interesting - conventional heat-to-electricity engines are limited by Carnot efficiency, which relies on a "cold side" and generally has low theoretical maximum efficiencies. This approach takes any hot emitter heated by any process - fuel burning, solar concentration, nuclear - and extract…

Isn't Carnot efficiency higher than 29%?

Re: Drones will fly for days with new photovoltaic engine

#72

Post by Retric » Mon, Jul 29, 2019, 1:30 PM UTC

pjc50 wrote: post #35

IanCal wrote: post #23

People seem to be misinterpreting this a lot. This is not about solar cells. This is about thermophotovoltalics, and the application here is using a burning fuel source to generate electricity. The "drone" the paper talks about seems to be a reference to another paper on unmanned sea vehicles though. Edit - to be fair the flying drone part is from the corresponding author.

Exactly. The paper is here: https://www.pnas.org/content/early/2019/07/15/1903001116 The context of thermophotovoltalics is very interesting - conventional heat-to-electricity engines are limited by Carnot efficiency, which relies on a "cold side" and generally has low theoretical maximum efficiencies. This approach takes any hot emitter heated by any process - fuel burning, solar concentration, nuclear - and extract…

Photovoltaic’s are also limited by Carnot efficiency.

The theory is you can increase the hot side more as you are less limited by mechanical stresses and moving parts. However, the sun’s surface is 5,505°C, so these are always going to have lower efficiency than traditional solar panels. Which are still less efficient than the bets thermal engines.

Re: Drones will fly for days with new photovoltaic engine

#73

Post by kmacdough » Mon, Jul 29, 2019, 1:34 PM UTC

For the record, to sustain a modern small drone at 50% conversion, you still need at least a 50cm x 50cm panel with optimistic estimates. This is completely ignoring the fact that such panels would need to be mounted without destroying the aerodynamics of the propellers

Re: Drones will fly for days with new photovoltaic engine

#74

Post by Retra » Mon, Jul 29, 2019, 1:50 PM UTC

mobilemidget wrote: post #31

IanCal wrote: post #14

Earlier quoted context omitted.

This isn't about sunlight, but about putting this with a furnace or something else generating heat.

Is this something you could use a laser for too? To bring heat to the flying drone? Or would that be super inefficient?

Beamed energy always has attenuation problems that make it inefficient.

Re: Drones will fly for days with new photovoltaic engine

#75

Post by IanCal » Mon, Jul 29, 2019, 1:56 PM UTC

spuz wrote: post #36

IanCal wrote: post #23

People seem to be misinterpreting this a lot. This is not about solar cells. This is about thermophotovoltalics, and the application here is using a burning fuel source to generate electricity. The "drone" the paper talks about seems to be a reference to another paper on unmanned sea vehicles though. Edit - to be fair the flying drone part is from the corresponding author.

> According to Yablonovitch, this finding builds on work that he and students published in 2011, which found that the key to boosting solar cell efficiency was not by absorbing more photons (light) but emitting them. By adding a highly reflective mirror on the back of a photovoltaic cell, they broke efficiency records > "What the mirror does is create a dense infrared luminescent photon gas within the solar cell, a p…

> Given these paragraphs and the name "thermophotovoltalics", that this is about solar power seems like a reasonable conclusion to draw. Also, googling around I am seeing figures for steam turbine efficiency of between 40 and 80%. How sure are you this is not about solar cells?

That it's not about the solar cells people in the comments mean? Very, the paper and citations talk about furnaces, and the temperature this operates at is 1200C. This isn't something you'd stick on your roof.

Re: Drones will fly for days with new photovoltaic engine

#76

Post by asteli » Mon, Jul 29, 2019, 2:04 PM UTC

numlock86 wrote: post #18

asteli wrote: post #7

Earlier quoted context omitted.

To elaborate, solar energy hitting the Earth’s surface is about a kilowatt/m^2 if conditions are perfect. If you were able to convert solar power at the maximum theoretical PV efficiency (the Shockley-Queisser limit, 33.7%), you’d need about 3 square meters to just to make toast.

Is this supposed to sound bad or good? Well, 3m² to make toast intuitively sounds bad I guess. Well, consider this: It's enough to run 2 small office PCs with all peripherals. Considering that it sounds much more positive. Really just anything producing heat would sound bad because of power dissipation. A toaster is probably one of the most extreme ... (Imagine putting 1kW into a slice of bread without power dissipat…

Someone else wrote the comment I meant to, essentially doing the math to show that one would need an awkwardly large solar panel to power something like a quadrotor drone.

Re: Drones will fly for days with new photovoltaic engine

#77

Post by crusso » Mon, Jul 29, 2019, 2:09 PM UTC

I really dislike headlines like this on HN that imply that lab research mixed with theory are somehow practical. It would be great if there were a "Theoretically" in there along with removing the hype-included "drone" reference.

Re: Drones will fly for days with new photovoltaic engine

#78

Post by lightedman » Mon, Jul 29, 2019, 2:30 PM UTC

duckqlz wrote: post #58

I was unclear how this differed from what I would traditionally consider as solar power (photovoltaic power) as I had never heard of thermophotovoltaic power. After a quick search I found the following helpful: “The basic principle [of thermophotovoltaics] is similar to that of traditional photovoltaics (PV) where a p-n junction is used to absorb optical energy, generate and separate electron/hole pairs, and in doing…

Basically a highly-inefficient method of converting energy. I've looked at the article and the pictures, and what I see is basically slapping a solar cell with a gold mirror backing right near a hot ribbon of graphite - eg we're trying to use highly inefficient incandescence to generate light.

Want to know what works better? Just the simple mix of gold into the p-n junction matrix and finger lines. Takes a silicon solar cell up to about 35% efficiency when properly doped into the junction material.

This technology? Well, given incandescence isn't going to break ~30% efficiency (excepting in the IR-glass-sandwiched tungsten filament) you're literally just wasting energy. Best we've seen in general incan is ~12% efficiency, so enjoy your 30-ish percent of that (4%).

You're better off with other technologies. This is neat, but you're looking at pretty much over-simulating the conditions of a solar cell welding line, which means that solar cell is going to likely bust over time under the heat stress. I foresee tons of problems with this tech, from the incan ribbon breaking to the cell harvesting energy breaking from the heat/cool cycling.

Re: Drones will fly for days with new photovoltaic engine

#79

Post by lightedman » Mon, Jul 29, 2019, 2:31 PM UTC

Retric wrote: post #72

pjc50 wrote: post #35

Earlier quoted context omitted.

Exactly. The paper is here: https://www.pnas.org/content/early/2019/07/15/1903001116 The context of thermophotovoltalics is very interesting - conventional heat-to-electricity engines are limited by Carnot efficiency, which relies on a "cold side" and generally has low theoretical maximum efficiencies. This approach takes any hot emitter heated by any process - fuel burning, solar concentration, nuclear - and extract…

Photovoltaic’s are also limited by Carnot efficiency. The theory is you can increase the hot side more as you are less limited by mechanical stresses and moving parts. However, the sun’s surface is 5,505°C, so these are always going to have lower efficiency than traditional solar panels. Which are still less efficient than the bets thermal engines.

THERMOphotovoltaics are limited by Carnot's efficiency. This does not apply to basic pure-photon non-thermal photovoltaics.

Re: Drones will fly for days with new photovoltaic engine

#80

Post by rodolphoarruda » Mon, Jul 29, 2019, 2:32 PM UTC

I've got an immediate idea reference from the movie Interstellar, which shows a drone that supposed to be flying non stop around the globe for X number of years until it had its navigation system hacked by people and then landed.