[vansig]

probably not, unless we can force a situation where secondary electrons have exactly the right energy to be captured most-efficiently; eg: if the PV cell lases,
…which might be the case if, for a 215 nm band gap, for example, the semiconductor is, say, 215 nm thick, or forced to occupy a channel 215 nm wide.
perhaps something similar to this photonic crystal defect cavity : http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA471262

[vansig]

vansig wrote:
tightening up the interlayer spacing helps a lot, but reduces breakdown voltage. it’s doubtful that spacing could be less than .003mm; yielding 214 m³ volume. still kinda large.

it turns out that dielectric strength is easier to control on thin films than it is on thicker areas, (go figure?)
and i read some claims that thin films of silicon dioxide end up with a dielectric strength near 1000 kV/mm. These end up being used in chip fabrication. If true, this could enable shrinking the cells to a tolerable size, (eg: 25,000 layers, total thickness ~38 cm, 20m x 20m surface area, or a sphere 5.6 m radius).

[vansig]

vansig wrote:
Someone please prove me wrong

Doh! it turns out i did make a silly calculation error, after all. can you tell where it is?

MTd2 wrote: Don’t you think this onion would waste a lot of energy by thermal loses? Why not just heating a liquid?

Just as a point of comparison, in fact the only point to doing it this way is IF it can achieve better efficiency than a heat engine.

“The maximum theoretical efficiency of a heat engine (which no engine ever obtains) is equal to the temperature difference between the hot and cold ends divided by the temperature at the hot end, all expressed in absolute temperature or kelvins.”

this would be, about:
(900 – 300)/900 = 66.6% if operating at 627 °C, cooling to 27 °C; or,
(1273 – 300)/1273=76.4% if operating at 1000 °C, cooling to 27 °C.
both of these are challenging because they exceed the creep limit of several candidate structural materials at the hot side.

so if the 80% claim for the onion carries any weight, it’s well-worth investigating.

[vansig]

vansig wrote: But if the photo-voltaic system consists of thousands of layers of nearly x-ray transparent cells, (eg: 0.008% absorption), then we can limit the saturation in individual cells.

if we want to get really fancy, then each layer could also be tuned to receive different wavelengths, from longer to shorter as you move outward.

any idea about the fabrication costs?

[vansig]

Rezwan wrote: Fascinating posts. Can someone do us a favor and summarize this (and other posts) – extracting the information and explaining to the broader readership what this is about and how it connects with the project?

The onion was proposed some years ago, as a way to capture otherwise wasted x-ray radiation (Bremsstrahlung radiation) at high efficiency (perhaps 80%). And it really captured my imagination, when i saw drawings of it for the first time, shortly after Eric’s Google tech talks video was published.

Such a system is as yet undeveloped, as no existing photo-voltaic power systems consider x-ray energy as a source, probably because Earth’s atmosphere blocks such light of cosmic origin.

However, in principle, x-ray photo-voltaics work much the same way as regular ones. it’s a matter of choosing the right semi-conductor combinations to catch the photon energy.

Some x-ray photodetectors are using wide band-gap semiconductors, but they saturate under the kind of intense pulses that FF-1 will produce, which would reduce efficiency and could harm the detector. But if the photo-voltaic system consists of thousands of layers of nearly x-ray transparent cells, (eg: 0.008% absorption), then we can limit the saturation in individual cells.

As this is fresh science, the fabrication methods and cost, and ultimate efficiency, of this is unknown presently. Our discussion, here, should try to pin down the constraints.

[vansig]

some kind of geodesic should enable flat panels, yes.

intensity drops with the cosine of the angle from the normal, leading to large currents along layers, if they receive much different amounts of light. but if panels are kept to with ~ ±10% it should not cause huge problems.

[vansig]

vansig wrote:
a 15 kJ pulse would then raise 1.2 J on each cell

i’m starting to think that, to keep PV cells from saturating, it will be necessary to bring the onion’s inner radius to ~4.4m

back-of-the-envelope calculation follows.

radius of 50 cm and 12,500 layers leads to 1.2 joule per layer.

Assuming saturation has something to do with capacitance, and breakdown voltage,
1.2 joule = .5 C V^2; and the cell is ~ 4.4 volts, therefore C needs to be 124 millifarad.

to meet this at spacing of .01 mm, (breakdown ~30 V), a single pair of plates needs overlap of 378 x 378 m (in air).

instead, bringing the onion’s radius out to 4.4 m, and using glass as the insulator, only 1/77 th of the energy needs to be absorbed by each plate, and C becomes 1610 uF per pair, yielding overlap of 15.5 x 15.5 m, which is about the surface area of a sphere at 4.4m radius.

Someone please prove me wrong

[vansig]

i just realized that because each PV cell is itself a diode, it would be better to have ~12,500 of these 4 to 5V cells arranged in series, and ~.01 mm interlayer spacing, to bring it to the 50 kV range needed for the next pinch.

with total thickness 25cm, and average overlap of 1570 x 2000 mm, total capacitance would drop to a few dozen pF.

easiest way to manufacture is probably to grow the PN junction epitaxially onto one side of a continuous, 2 m wide by 20km length of foil, as you roll it up into a cylinder. a continuous process would yield an even thickness.

you’d want to pull out gases, and bake, to anneal it nicely, and finally cut through and insulate along its width, on one side, changing it from a coil into the final cylindrical PV battery.

a 15 kJ pulse would then raise 1.2 J on each cell

[vansig]

seems like we want all electrical conductors to be transparent to x-ray, then.

every layer of the onion will be a sandwich, of: a thin semiconductor, that catches a small fraction of x-rays; and two plates of a thicker foil, that carries the current, but is otherwise transparent. there is a maximum thickness, due to skin effect, as well.

if i’m looking at this correctly, then
within each uniformly distributed pulse, upon each layer of the onion, there’d be ~100 nm lateral separation between collisions.

each x-ray absorption imparts an electron with a lot of momentum, like striking a pool cue ball, during the break, causing maybe 10,000 secondary collisions, and lots of electrons rise up about 4 or 5V to the conduction band. the respective plates are thus charged, and we want this current to be collected.

the onion is looking like a big capacitor. the capacitance calculator at http://deepfriedneon.com/tesla_f_calccap.html
gives me ~500 µF, if we choose 1000 of each +ve and -ve plates, with 1mm spacing, assuming average 5317×5317 mm overlap.

unlike an ordinary capacitor, every other layer of insulation is the semiconductor.

will a greater charge than the breakdown voltage develop, from a single pinch?

[vansig]

yes, a 3 m x 5 m, double sided radiator could dissipate about 5 MW, if it ran at that temperature

[vansig]

Are the citizens of DPRK allowed to own property? Are they allowed to run businesses? or is everything under state control?

[vansig]

jamesr wrote:
A rough calculation of 15kJ of X-rays at average of 50keV emitted over 50ns onto a 50cm radius sphere puts the flux at ~10^21 photons/cm^2/s. Which is many orders of magnitude greater than a normal detector would saturate at.

something’s not right. Google calculator tells me 50 keV x 10^21 = 8.0 MJ.

if we were emitting 5MW of xrays, that’d be 160 W/cm² at a distance of 50 cm.

==Edit==
. o Oh! you mean flux as the peak intensity. that makes much more sense.

this really is new territory

[vansig]

i’m guessing that instead of a 2mm thick slice, we have closer to 2µm thick slices, and that we have many thousands more of them. also, something tells me this will be thick enough that the greater surface area of outer layers does, indeed, matter.

this is a job for transparent aluminum

[vansig]

that was a typo in the url. remove the final ‘l’, you’ll get the article.

[vansig]

no, a satanic death cult is about killing in the name of Satan.

[Author]

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