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Drones will fly for days with new photovoltaic engine

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Re: Drones will fly for days with new photovoltaic engine

#81

Post by baking »

crusso wrote: post #77

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.

Eli Yablonovitch's company, Alta Devices, manufactures a thin film high efficiency PV cell but they can't compete right now against cheap Chinese silicon wafer cells in the solar panel market, so they are focusing on niche markets, primarily drones at first.

His 2017 talk to the MIT Energy Initiative (https://www.youtube.com/watch?v=u_K1URyarE0) goes into the economics of solar and why his company is using this strategy. Basically, they need to find niche markets until they can work their way down the learning curve and achieve economies of scale.

He also discusses thermophotovoltaics. Very efficient light-weight PV allows for longer drone flights but night is an obstacle to multi-day flights. Given the choice between batteries and fuel to keep aloft until dawn this does seem like a possible breakthrough, deserving of the article's title, although the text could be a lot more clear.

Re: Drones will fly for days with new photovoltaic engine

#82

Post by [deleted] »

walrus01 wrote: post #38
amirhirsch wrote: post #29

Earlier quoted context omitted.

I’ve seen reports of thin film organic solar cells that produce >1 watts per gram of material. A quadcopter motor can generate multiple grams per watt of power. You can today go and buy 100W from a 3.3lb panel, which is like half way there and not even trying. I’m guessing it’s probably possible to make a quadcopter that can hover indefinitely in the sun.

No, not really. The math for watts consumed in total just to hover, for a very large quad, hex or octocopter (with 24 to 28 inch size props, so optimal grams/watt thrust efficiency for something you can fit in a van), doesn't work out. The weight and size of a 400W solar panel made from the very best cells you can buy commercially today is considerably more than the wattage you would need to hover a 10+ kilogram coax…

[deleted]

Re: Drones will fly for days with new photovoltaic engine

#83

Post by flanbiscuit »

dwighttk wrote: post #57

"For the past 15 years, the efficiency of converting heat into electricity with thermovoltaics has been stalled at 23 percent. But a groundbreaking physical insight has allowed researchers to raise this efficiency to 29 percent. Using a novel design, the researchers are now aiming to reach 50 percent efficiency in the near future by applying well-established scientific concepts." ^interesting part, the drone stuff is…

left out the actual science part of this interesting part:

> 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 at the time and have continued to do so with subsequent research.

Re: Drones will fly for days with new photovoltaic engine

#84

Post by taneq »

marcosdumay wrote: post #65
taneq wrote: post #27

Earlier quoted context omitted.

Wouldn't you slap this on top of a steam boiler to get the best of both worlds, though?

If you make it harder to cool the turbine by putting those neat the radiator, efficiency will go down; if you make it harder to keep the heater hot, by putting those near the fire, efficiency will go down. As animats already said, turbine efficiency on power plants is very near the theoretical optimum. The only way left to improve it is by increasing the temperature.

The furnace burns at over 900C but the superheated steam (which is your ‘hot’ temperature for your heat engine) is only at ~400C, you might lose some power output but you won’t be losing efficiency.

Re: Drones will fly for days with new photovoltaic engine

#85

Post by skykooler »

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…

This also relies on a "cold side" - the thermophotovoltaic cell has to be colder than the IR source or it will emit photons faster than it can absorb them.

Re: Drones will fly for days with new photovoltaic engine

#86

Post by bufferoverflow »

dwighttk wrote: post #57

"For the past 15 years, the efficiency of converting heat into electricity with thermovoltaics has been stalled at 23 percent. But a groundbreaking physical insight has allowed researchers to raise this efficiency to 29 percent. Using a novel design, the researchers are now aiming to reach 50 percent efficiency in the near future by applying well-established scientific concepts." ^interesting part, the drone stuff is…

We have multijunction cells with 47.1% efficiency (as of June 20th).

Other than the price, what's the significance of single-junction 50% efficiency?

https://en.wikipedia.org/wiki/File:CellPVeff(rev190702).pdf

Re: Drones will fly for days with new photovoltaic engine

#87

Post by amirhirsch »

walrus01 wrote: post #38
amirhirsch wrote: post #29

Earlier quoted context omitted.

I’ve seen reports of thin film organic solar cells that produce >1 watts per gram of material. A quadcopter motor can generate multiple grams per watt of power. You can today go and buy 100W from a 3.3lb panel, which is like half way there and not even trying. I’m guessing it’s probably possible to make a quadcopter that can hover indefinitely in the sun.

No, not really. The math for watts consumed in total just to hover, for a very large quad, hex or octocopter (with 24 to 28 inch size props, so optimal grams/watt thrust efficiency for something you can fit in a van), doesn't work out. The weight and size of a 400W solar panel made from the very best cells you can buy commercially today is considerably more than the wattage you would need to hover a 10+ kilogram coax…

Hey! I love this problem :) The motor you shared shows >20g/W efficiency at ~40W of power and net lift of 500grams with those 28x9.2 (100g CF props). This implies that we need to produce roughly 40 Watts with under 500grams -- hard to assume negligible structural/electronics mass so more conservatively 40 Watts under 250g or 320Watts under 2kg to make like an octocopter with some power to spare. this is roughly 1 to 4 sq meters of solar area at 32% to 8% efficiency, and the swept area of eight 28" props is about 12 square meters it's might not even look totally ridiculous!

I can find super-light 100Watt panes under 2kg on Amazon (probably actually 60 Watts) but it's definitely more like 2 - 4 times more mass efficiency than what is commercially available and not 18-30.

Silicon mass is under 4 grams per Watt of solar so I think it should be possible to get there with silicon with a focused engineering effort. Organic photo-voltaic materials have shown 6 watts per gram in the lab (imagine for space applications!) -- though they would need even larger area due to lower efficiency and act as more of a sail .... not saying it is a practical machine to make, but again I definitely think it is in the realm of possible.

Re: Drones will fly for days with new photovoltaic engine

#88

Post by macspoofing »

travisporter wrote: post #68
macspoofing wrote: post #49

Earlier quoted context omitted.

>You're not going to make toasts all day I bet you're doing something throughout the day. Maybe not making toast, but cooking, ironing, heating, cooling, watching TV etc. I'm not sure you can rely on any excess energy being available for storage if you're not connected to the grid. >The idea behind individual solar energy is not consuming it real time but associating it with a storage solution to allow bigger through…

“Forcing over building of” solar is still cheaper than building a new coal or gas plant in some areas. I am positive there are plenty of people who live off grid and do not always need gas or biofuel backup.

>there are plenty of people who live off grid and do not always need gas or biofuel backup.

I'm sure there are plenty of such people, but those people don't really factor when you take into account the society as a whole. The vast majority of people and businesses cannot live off grid.

Re: Drones will fly for days with new photovoltaic engine

#89

Post by walrus01 »

amirhirsch wrote: post #87
walrus01 wrote: post #38

Earlier quoted context omitted.

No, not really. The math for watts consumed in total just to hover, for a very large quad, hex or octocopter (with 24 to 28 inch size props, so optimal grams/watt thrust efficiency for something you can fit in a van), doesn't work out. The weight and size of a 400W solar panel made from the very best cells you can buy commercially today is considerably more than the wattage you would need to hover a 10+ kilogram coax…

Hey! I love this problem :) The motor you shared shows >20g/W efficiency at ~40W of power and net lift of 500grams with those 28x9.2 (100g CF props). This implies that we need to produce roughly 40 Watts with under 500grams -- hard to assume negligible structural/electronics mass so more conservatively 40 Watts under 250g or 320Watts under 2kg to make like an octocopter with some power to spare. this is roughly 1 to…

It doesn't scale down like that. Small motors and small props under 7 inch size have less than 5g/watt thrust efficiency. If you look at the motor and propeller setup that would go on an Armattan Chameleon Ti 7" for instance (a freestyle/gopro hero7 carrying sized FPV quad frame you add your own parts to).

On the large scale you would never actually be applying only 40W to a tmotor u8 with a 28" prop on it, because the total weight of the motors and the whole craft will mean it would just sit on the ground. You'd be moving air around like a fan and not lifting off. Ideally you would design a large octo for a 1:1 thrust ratio (hover) at about 48 to 50% throttle per motor, which is why you see the thrust tables starting from 50% and going upwards to 100% throttle.

Bigger and slower (lower kV) props and motors have higher efficiencies. If you google a bit about the few successful human powered helicopters that have been built in the past ten years you'll see the ultimate extent of using very big, very slow props to maximize lift per watt.

I would dearly love to be proven wrong on my above post. If you can find me a super efficiency triple junction GaAs panel to mount on top of a 20 kilogram sized coaxial filming octo, I'll find a way to get it into the hands of XM2 and see if they want to try flying it.

Re: Drones will fly for days with new photovoltaic engine

#90

Post by [deleted] »

blamestross wrote: post #70
dwighttk wrote: post #57

"For the past 15 years, the efficiency of converting heat into electricity with thermovoltaics has been stalled at 23 percent. But a groundbreaking physical insight has allowed researchers to raise this efficiency to 29 percent. Using a novel design, the researchers are now aiming to reach 50 percent efficiency in the near future by applying well-established scientific concepts." ^interesting part, the drone stuff is…

Does this mean we can make lighter/smaller RTGs?

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