Half a croissant, on a plate, with a sign in front of it saying '50c'
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Replace "light" with "sausages" and this may work...

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High-Altitude Thermosolar Aircraft

Use thermoelectric power for continuous powered flight
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Solar aircraft with photo-voltaic cells are baked. Recent increases in solar cell efficiency, battery & motor technology mean that they're moving from "only just possible" toward "could do something useful, maybe".

One thing against the photo-voltaic route is the focus on certain types of efficiency. W/m2 or W/$ are the goals, weight isn't a consideration for the most part, and if they could make them out of sand, they probably would.

Thermoelectric generators are fussy and horrendously inefficient... sort of. In terms of heat in, electricity out, 10%, maybe, dreadful. But, in terms of Watts out/kg, they're really quite good. I just weighed a 250W* unit at 60 something grams, and it's clearly not optimized for low weight, at least 10 fold better than PV panels. How does this concern aircraft? Well, they have a couple of favorable resources available, intense, cloud-free sun and cold air. So let's design our aircraft.

First, lots of low powered aircraft look like gliders, sensible as this is efficient. Gliders have a lot of wing area, and often a bit of dihedral... in some solar craft, the wing is flexible and bends into a convenient parabola <link>.

So, let's have a great big, mirrored parabolic wing focussed on a point somewhere above the central fuselage. At that point, the hot sides of two thermoelectric generators face each wing. The reflected sunlight, all of it, supplies about 200kW of total energy, assuming a 90% efficiency mirror and 200m2 wing. The two cold sides facing each other are cooled by flowing -40C air over a shrouded heatsink** between them.

It's possible that even with the weight saving, the thermoelectric generator will prove insufficient, which at 10% conversion is only 20kW peak, less than half that of Solar Impulse's 45kW photovoltaic setup. In that case, we move to 30% efficiency with a Stirling engine at the focal point. There should be enough power left over to make tea, which is important.

*we'll see

**with good design, a Meredith-type thrust/drag mitigation should be possible.

bs0u0155, Aug 27 2019

Parabolic wing https://upload.wiki...elios_in_flight.jpg
[bs0u0155, Aug 27 2019]

Body heat generator, uW/cm2 https://phys.org/ne...ctricity-human.html
[bs0u0155, Aug 28 2019]

[link]






       //I just weighed a 250W* unit at 60 something grams// Is that a Peltier? And is 250W the maximum power output (and, if so, with what temperature differential) or the power consumption for heat-pumping?
MaxwellBuchanan, Aug 27 2019
  

       Peltier, TEG, Seebeck device etc. 250W comes at a delta of 410C, which isn't trivial. I've no idea how it performs in pumping mode, presumeabley with dreadful efficiency. It was hanging around in a recently closed lab and I can make a reasonable case for it being useful in a temperature control project.
bs0u0155, Aug 27 2019
  

       Wow. 250W from 60 grams is HUGE! I mean, yes, I guess there's not much to a Peltier, but even so that's impressive.   

       Out of curiosity, do you know what the difference is between Peltiers intended for TEG and Peltiers intended for heat- pumping?
MaxwellBuchanan, Aug 27 2019
  

       Yes, the manufacturer's part numbers are different.
8th of 7, Aug 27 2019
  

       Thank you, [8th], that was very insightful.
MaxwellBuchanan, Aug 27 2019
  

       We wouldn't like to be accused of being unhelpful.
8th of 7, Aug 27 2019
  

       Try telling that to the Intercalary. He says the ocelots are still off their food.
MaxwellBuchanan, Aug 27 2019
  

       //difference is between Peltiers intended for TEG and Peltiers intended for heat- pumping?//   

       There's a bunch of chemistries, essentially optimized for the temperature range. Garden variety Peltier coolers are Bismuth Telluride I think, which is great at low temps. Generators, like Lead Telluride and whatever TAGS is, go up to the 400-800C range, which is obviously a richer source of energy.   

       //250W from 60 grams is HUGE!// There's more problems when you dig deeper, it turns out thermal cycling semiconductors through 500C routinely isn't good for reliability. I think there is potential for a huge jump if some new chemistry is found that can interconvert temperature and electricity at anything like the levels we can do for say, light. Thermal logic gates are possible, huge jumps in machine efficiency, slightly thinner fridges, all sorts.
bs0u0155, Aug 27 2019
  

       //thermal cycling semiconductors through 500C routinely isn't good for reliability// Yes, I can imagine. The Peltiers in PCR machines die young because of multiple temperature cycles of only 100°C or so.   

       But for your thermosolar application, that needn't be an issue since it needn't temperature-cycle quickly (or often).   

       As an aside, my skin temperature is currently close to 500°C thanks to DNP; I am wondering if I could be running my laptop off of me.
MaxwellBuchanan, Aug 27 2019
  

       // Try telling that to the Intercalary //   

       We don't deny that we are actually, by and large, unhelpful. It's just that we resent people pointing it out.
8th of 7, Aug 27 2019
  

       That would probably take a full body coating. But a watch would be trivial, and the kind of gimmick that would go over well with watch fans.
bs0u0155, Aug 28 2019
  

       Presumably the watch fan is needed to keep the cold side of the watch cool.
MaxwellBuchanan, Aug 28 2019
  
      
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