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PostPosted: Jul 22, 2015 1:39 pm 
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I really like this concept a lot better than space solar power, with the space cost and transmission issues.

But I worry about a bunch of stuff. One thing is a 20 km tether. Even if its Dyneema, there are large forces on it, an enormous balloon in high wind 20 km up.
What tether thickness are we talking about? Even a slender 2 cm line made of Dyneema would weigh >5 tonnes if 20 km long.

The other thing is the power cable. Aluminium cable would simply rupture on its own weight alone. (2.7 kPa/meter * 20000 m) = 540 MPa tensile! So you'd have a conductive power cable core with a Dyneema reinforcement skin. But even a quite small core of 2 cm would weigh 17 tonnes!

So with 4 tethers and a power cable we're already in the tens of tonnes ranges, we don't even have a balloon or solar panels yet!

Then there is the question of balloon life. NASA is having major problems making stratospheric helium balloons last half a year. A much bigger balloon, filled with hydrogen, and power cables and whatnot, would be orders of magnitude harder.

This is not proven technology nor easy. When all is said and done it looks like a lot of expensive NASA class engineering for a hundred kilowatt for a year. Hard to see the economics of this going right.


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PostPosted: Jul 22, 2015 2:09 pm 
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I would probably be better to simply have bare ACCR (aluminium conductor composite reinforced, from 3M) cables with a platform suspended every few kilometres (a lightweight lattice structure spanning between the support tethers I imagine) where the cable is coupled to the tether with the required insulator strings.
That way a clashing conductors won't damage the dyneema strings.

ACCR has a breaking length about ten kilometres, and can be very heavily loaded in current terms since it is rated for temperatures up to 210C continuous. (500kg/km cable is rated for 864 continuous amps).

As an alternative you could simply taper the conductors so the conductor higher up runs cooler and supports the weight of the hot running conductor at the bottom.


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PostPosted: Jul 22, 2015 2:19 pm 
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But wind force in 20km altitude is very soft and thin. The troposphere is very unstable and stormy but the stratosphere is very stable because there is an inversion layer(caused by ozon) that prevents that all the bad weather could come up. A natural barrier.


I have asked Edmund Kelly about the life time of weather balloons because i found that also very strange. He told me that they were designed not too last and that it would be an easy thing to make them durable. He actually worked in the aerospace industry.
The solar panels wouldn't have any glass on them because up there there is no humidity.
Monocrystaline cells have a thickness of 150µm right now but they will get much thinner in future, up to 4 micron, the bare minimum. (raytonsolar, accelerated particle beam ingot peeling )
On the website http://www.stratosolar.com you can even see that they are planning to integrate a 500 ton gravity storage what i find a bit complicated.
It would make the concept a lot cheaper but also a lot more complicated IMO for the beginning.


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PostPosted: Jul 22, 2015 2:34 pm 
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The simple solution is to connect all the solar panels in the array in series and connect that to the power tethers, one conductor bundle up and one down. You would get very large voltages across the array and use a modified Voltage Source Converter station at the ground to produce useful power.

An auxiliary voltage source converter on the platform would provide the auxiliary power required to run it - at night the array could be powered by transmitting power up the VSC line, after all VSC technology allows power flow to reverse without reversing line polarity.
Lightweight contactors would allow parts of the array to be isolated during the night when its off load, or when the VSC system forces it to effective open circuit.

Corona Discharge supression at high altitudes will require something like 4 conductors.
4 ACCR Ostrich 300 conductors 25km long would have a rated current of 3456A continuous.
It would mass roughly 50t per bundle not counting spacers.
So 100t, but at +/- 500kV the system would support 3.4GWe at the generator end.
Bundle resistance would be about 2 ohm, so we would expect to lose 1.4% to resistance (48MWe).


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PostPosted: Jul 22, 2015 5:35 pm 
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Long conductors from earth to 20 KM straight up? As anyone considered lightning? To transmit power over that distance and be light weight, would mean a low current VERY high voltage conductor to pass
a large KW load/ source. This would be a prime target for lightning, both Cloud to Cloud and Cloud to Ground.


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PostPosted: Jul 22, 2015 7:22 pm 
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Indeed, it would be a +/- 500kV system or potentially even higher (we have essentially arbitrary voltages available and the only limit is we need a VSC type converter on the ground and that is the highest voltage one in existence that I know of).

Lightning Arrestor equipment is rather good now - so it should be able to handle the shock pulses without major issues.
I am not sure.
What sort of surge voltages would be expecting?


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PostPosted: Jul 22, 2015 8:37 pm 
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Right. A 500 kV system on the ground for a few hundred lousy solar kWs. That will make the whole scheme totally uneconomic (if it isn't already, which I believe is the case).

The problem with lighning will the amperage. Yahoo answers gives 30000 to 120000 amps. That's toasty! It should fry your high voltage low amp design conductive core.


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PostPosted: Jul 22, 2015 8:41 pm 
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I also found a lot of references that make it pretty clear Dyneema (while strong and light and awesome stuff) is very temperature sensitive and should not be operated above 100C or so. That further suggests that lightning will be a major issue (assuming we are talking here of a conductive core with Dyneema doing the load carrying).


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PostPosted: Jul 23, 2015 8:20 am 
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Cyril R wrote:
Right. A 500 kV system on the ground for a few hundred lousy solar kWs. That will make the whole scheme totally uneconomic (if it isn't already, which I believe is the case).

Uh.... I think your missing a few orders of magnitude there. 3450A at +- 500kV is 3.4GWe
Quote:
The problem with lighning will the amperage. Yahoo answers gives 30000 to 120000 amps. That's toasty! It should fry your high voltage low amp design conductive core.

If you sacrifice a bit of efficiency to put a bunch of SiC diodes in the circuit you can protect the solar cells and VSC converters are built to handle lightning strikes as it is.


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PostPosted: Jul 23, 2015 10:11 am 
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E Ireland wrote:
Cyril R wrote:
Right. A 500 kV system on the ground for a few hundred lousy solar kWs. That will make the whole scheme totally uneconomic (if it isn't already, which I believe is the case).
Uh.... I think your missing a few orders of magnitude there. 3450A at +- 500kV is 3.4GWe


Whoa! You wanna make a 3400000000 Watt solar blimp!

What have you had for breakfast Ed?


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PostPosted: Jul 23, 2015 10:29 am 
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cerebral wrote:
But wind force in 20km altitude is very soft and thin.


Uhm, no. Interestingly there has been a lot of work on high altitude airships.

https://ams.confex.com/ams/pdfpapers/128256.pdf

1 m/s one day, 50 m/s the other. That is not so regular and 50 m/s is not soft and thin even at 0.12 kg/m3.

We're probably looking at over 50 N/m2 peak for an airship shape.

This is not a soft and thin wind, its a ginormous sail at 20 km up. It will need a stout Dyneema tether weighing many tonnes. Note the airship in the above link cannot even lift that tether. In fact it can't even stay in place during the highest wind speeds - suggesting the wind simply overcomes the solar panel output, it gives you an idea of the relative forces involved...


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PostPosted: Jul 23, 2015 10:32 am 
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Cyril R wrote:
E Ireland wrote:
Cyril R wrote:
Right. A 500 kV system on the ground for a few hundred lousy solar kWs. That will make the whole scheme totally uneconomic (if it isn't already, which I believe is the case).
Uh.... I think your missing a few orders of magnitude there. 3450A at +- 500kV is 3.4GWe


Whoa! You wanna make a 3400000000 Watt solar blimp!

What have you had for breakfast Ed?

Wannabe Sci Fi author remember :P

If we assume a 20% efficiency in 1kW/square metre - 17 million square metres.
I make that be a circular array with 4.6km in diameter.
Which sounds huge but at 20-25km we can build such an array, after all it could rotate to vertical and the lower edge would still be 18km above ground.

You would spread gas cells under the entire array and then use ballast tanks to get it to rotate in the vertical axis - the hard part is stopping the whole system twisting, so you would have to splay the tethers out.

Assuming we can get it to fly.


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PostPosted: Jul 23, 2015 12:57 pm 
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cerebral wrote:
I have asked Edmund Kelly about the life time of weather balloons because i found that also very strange. He told me that they were designed not too last and that it would be an easy thing to make them durable.


I find that very difficult to believe. NASA classifies 100 day balloon missions as "ultra long duration balloons". That does NOT suggest that making balloons last long is easy. It suggests some pretty obvious physics; thin membranes being porous to helium; helium having escape velocity above gravity field of the earth; ultra low pressures at these altitudes sucking out the helium.

If you recall party balloons, being filled with air, with a 101kPa of pressure around it, being filled with low mobility air, losing their air pretty quickly through diffusion... helium party balloons being much faster loss rates.

Now try 20 km altitude... 6 kPa vacuum sucking out your helium. Sun shining on your balloon heating the helium further as well as the membrane. It makes the membrane porous, short term thermal effect and long term UV and other ageing effects.

Now try hydrogen to cut cost... containing hydrogen is even worse than helium. It also has chemical effects unlike helium.

I do not work in aerospace but I do not have difficulty seeing why balloon life is so short. If it were easy to make balloons last longer, that would really cut cost. You could use 1 balloon in say 10 years rather than 30 balloons. That would be much cheaper, so if it were easy it would be done already. 100 days is not very long to amortize expensive scientific instruments, space age balloon fabric, not to mention 60000 m3 of helium.


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PostPosted: Jul 23, 2015 1:22 pm 
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cerebral wrote:
The solar panels wouldn't have any glass on them because up there there is no humidity.


They are highly reactive materials that you are overheating with solar energy to quite high surface temperatures, and you are operating in a blanket of >1000 Pascals of oxygen. That is a chemical reactor. They need encapsulation. They need protection. They will need a heat sink and/or back support plate. The real weight of panels is in making them survive and operate properly, not in getting a photovoltaic effect. They are not disposable items. They are long duration electronic products in full exposed outdoor environments.

There is wind in the stratosphere, so there is dust in the stratosphere. There is moisture, not much but its there. There is ozone and UV radiation that loves to tear apart balloon material as well as degrade electronics, solar cell material.

If the humidity is low, then there is no option of onboard electrolysers to regenerate hydrogen losses, which will be enormous. You can forget long balloon life altogether.

You can get very low specific weight panels, very cheap panels, or very long duration panels. Pick any 2 of 3.


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PostPosted: Jul 24, 2015 6:38 pm 
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Cyril R wrote:
Now try hydrogen to cut cost... containing hydrogen is even worse than helium.
Containing the lift gas should not be a problem at all. If we use hydrogen, because its low cost, we just put a small tube in the tether, and let the amount of hydrogen up the tube as needed. Gravity does all of the work.
I would not consider Helium do to its cost, and availability. Hydrogen was used for almost all zeppelins safely carrying hundreds of thousands of passengers. Only the Hindenburg accident caught on film in 1936 caused the death of hydrogen as a lift gas for airships.

The above arguments aside, I think airship hoisted solar power plants are a, “bridge to far”. They make LFTR look easy.

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Although environmental groups say we must reduce CO2 to prevent global warming they can never mention the “N” word as part of the solution.


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