Can we predict future optimal efficiency of the entire sunlight to DC conversion chain?

[Tim Cash]

If we use what we know about the future improvements in new materials of space solar power components, can the end to end efficiency of the entire sunlight to DC chain be modeled or predicted today?  I have received comments that some do not believe that 50% end to end efficiency can be achieved soon, I believe it can, but do not have the data to back up such a statement.  Do any of you?  Say we switch from photovoltaics to thermal engines. That should improve from 25 to 35 percent for that one system, and reduce system size considerably, as a single point example.  I do believe with all the research being done, improvements are on the way.  I also believe space 3.0 will come about through military funding research being done far quicker than commercial funding alone could achieve. The future is happening at a much faster pace  these days.

Tim Cash

cash...@gmail.com

[Sanjay Singh]

Years ago I read about multi-junction solar cells made with gallium indium phosphide that could have their band gap adjusted to convert a larger range of the Sun's wavelengths. I'm going only by memory but I remember something approaching 70% or so is theoretically possible. I don't know the extent of the progress since then, but it seems to me that this is probably the single biggest factor that affects the viability of SBSP systems.

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[Robert Poor]

Is it the case that even with ideal PV efficiency that the M^-2 cost dominates the total system cost?

If not, then $/W is the important metric: when the cost per M^-2 gets low enough, then the balance of system (deployment, maintenance, etc) becomes dominant.  And $/W continues to fall, even if efficiency is stuck under 30% or so.

And then there's http://www.novasolix.com -- any update on them?

- rdp

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[Keith Henson]

I think these cells have reached about 45%, but it does not matter.

The amount of indium available is not enough to build a significant
number of power satellites, as I recall, less than 10. It's ok if you
only need one or two, I worked this kind of cell in the Beamed Energy
Bootstrapping video.

Re what Tim Cash wrote, a two-stage Rankin system could hit 60%.
Optimizing power satellite design is tricky. What you are after is
low-cost power. For example, low mass might seem desirable but low
mass requires countering light pressure which requires reaction mass
resupply.


Keith

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[Roger Arnold]

I can't say much about the practical efficiency of the sunlight to DC conversion chain, but hasn't 50% been demonstrated? Well, no, I guess. 50% would be for DC to DC, with microwave transmission and reception in the middle. It wouldn't include the initial sunlight to DC conversion. But I do remember seeing 50% bandied about.

From a theoretical perspective, there are no hard limitations. For a heat engine, the source heat temperature is limited by the blackbody radiation temperature of the Sun. However, at ca. 5800K, that's not much of a limitation. The heat sink temperature is limited by the size of the radiator. If the radiator is large enough to maintain a heat sink temperature of 400K, then the Carnot efficiency of a heat engine would be 93%. Rather than a heat engine, however, it would probably be advantageous to use concentrated thermal photovoltaics. That involves a cavity capturing concentrated solar radiation, just as a heat engine would start with. But the walls of the cavity are a metamaterial with near-zero emissivity at most wavelengths, and a narrow window of high emissivity at wavelengths around the optimum for PV conversion. For silicon cells, that would be around one micron, in the near infrared. I could be wrong, but I believe conversion efficiencies, solar to DC, of 80% have already been demonstrated. The PV cells do have to be cooled, but with 80% of the input energy going out as DC electricity, the cooling load is, relatively speaking, modest.

If the DC power from the PV cells is going to be transmitted via a microwave beam, then a host of other considerations come into play. A high bandgap semiconductor class C amplifier, pumping a high Q resonator, can I believe achieve DC to microwave conversion efficiencies upward of 90%. The catch is the "high Q resonator". That calls for either superconductor circuits, or possibly MEMS devices. Or perhaps spitronics? None of which are areas about which I can speak with any authority. But I'll still say 90% is feasible.

Then there's the transmission beam. If you're willing to oversize both the transmitter and receiver apertures a bit, I'd say that 90% is feasible there as well. But more conservative estimates say 50%. 

Finally there's the microwave to DC conversion at the rectenna. Simple tuned dipole oscillator antennas coupled with Schottky diodes will deliver (I think) about 50%. But as with the DC to microwave transmitting elements, receiver elements with higher Q factors than simple dipole antennas can improve on that. Just an issue of cost.

--

[Narayanan Komerath]

Can estimate from the other side: "50% - no excuses" might be the minimum for anyone to buy into SSP. IOW until that happens, "don't call us v'll call u".

I want to say 80% but I am an SSP Quasi-Realistic Enthusiastic.

nk

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[Keith Lofstrom]

On Wed, Mar 15, 2023 at 03:33:57PM -0700, Keith Henson wrote:
> ...

> Optimizing power satellite design is tricky. What you are after is
> low-cost power. For example, low mass might seem desirable but low
> mass requires countering light pressure which requires reaction mass
> resupply.

Light pressure can be "countered" with a somewhat elliptical
orbit, assuming that both the transmitter and the rectenna
can electronically adjust for east-west azimuth variations
over the course of a day, and somewhat smaller adjustments
for north/south elevation.

I characterize low-mass satellites by their "sail ratio":
the mass divided by the sun-facing area.

As the sail ratio decreases, the required eccentricity
increases. For the "thinsats" I contemplate, the sail
ratio can be characterized as an equivalent thickness
of aluminum.

100 micrometer thick aluminum is 270 grams per square meter,
compensated by an eccentricity of 0.04 or 4%. 50 μm Al
( 135 g/m² ) is compensated by an eccentricity of 0.08.

The elliptical orbit will pass through GEO twice per orbit,
but in different positions over the course of a year. For
that and other reasons, the SSPS orbit should be slightly
inclined, so that it is rarely collinear with comsats.

Even if the main beam of the SSPS is pointed elsewhere,
I imagine the off-axis sidelobes and harmonics of the
SSPS will saturate the LNA (low noise amplifier) front
end of a satellite receive dish on the Earth's surface.

See the math at:

http://server-sky.com/LightOrbit#Eccentric

... slightly modified from "Handbook of Geostationary Orbits"
by E.M. Soop.

One effect I leave out of the math on that web page is a
small gravitational effect from the "lumpiness" of the
Earth, which makes GEOsat orbits slowly migrate east or
west towards minima at two different longitudes.

Keeping a GEO-sat in a tight "latitude box" away from
those longitude valleys requires continuous (but small)
delta-V, which consumes propellant proportional to
satellite mass divided by Isp.

Another large effect is lunar tides; while those tend to
average out, they will add "noise" to SSPS longitude
station keeping.

These are not problems for the non-synchronous MEO orbits
I contemplated for Server Sky, whose intended role is now
performed by Starlink (with more mass and more astronomer-
vexing light pollution).

----

So, light pressure effects on a low-mass SSPS can be
compensated without reaction mass expenditure with orbit
eccentricity, but lumpy gravity field effects require
thrust proportional to mass. Hopefully, ultra-high ISP
electric thrust.

----

Keith L.

--
Keith Lofstrom kei...@keithl.com

[Keith Henson]

On Wed, Mar 15, 2023 at 8:09 PM Keith Lofstrom <kei...@keithl.com> wrote:
>
> On Wed, Mar 15, 2023 at 03:33:57PM -0700, Keith Henson wrote:

snip

>
> Light pressure can be "countered" with a somewhat elliptical
> orbit, assuming that both the transmitter and the rectenna
> can electronically adjust for east-west azimuth variations
> over the course of a day, and somewhat smaller adjustments
> for north/south elevation.

I have no idea how IP would apply in GEO, but if this works, and you
can get a patent on it, it should be worth a fortune.

I don't understand the proposal at all.

KeithH

[Paul Werbos]

This was within the scope of a research activity I ran at NSF, which then led the world in what we learned.

For SSP, cost per kWh delivered was the big bottom line issue.Mankin's book The Case for SSP was THE definitive source collecting best information on proven methods, with links to comparative cost studies. Efficiency is just one input to cost, sometimes overvalued, depending on HOW whole system efficiency is achieved.

"Sandwich" multilayer cells have a natural efficiency advantage. There were serious numbers out there from 40 percent to more like 90, but all more expensive than more homogeneous designs. But factors of 10 in launch cost dominate. And other cost factors matter a lot too.

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[Roger Arnold]

Not directly related to the topic of light pressure and geosynchronous orbits, but relevant to interference of power beams with communications equipment ... 

> Even if the main beam of the SSPS is pointed elsewhere,

> I imagine the off-axis sidelobes and harmonics of the
> SSPS will saturate the LNA (low noise amplifier) front
> end of a satellite receive dish on the Earth's surface. 

It probably would, if the LNA were exposed to them. But it's possible to hide the LNA behind a passive L-C notch filter with sharp zeroes at the harmonic frequencies. I don't think comm interference is a killer issue for SSPS. 

On Wed, Mar 15, 2023 at 8:09 PM Keith Lofstrom <kei...@keithl.com> wrote:

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[Tim Cash]

Let me elaborate on why I posted this message in the first place.

I have an opportunity to design and test a prototype WPT system, perhaps next year (2024) as time flies and it is already March 2023, and as you know it takes time to put together Bills of Materials, and all the support hardware that a prototype demonstration would require.  Unless any of you know where I can purchase a prior WPT demonstration system Used, "Off the Shelf", modify it, use it as my own test system?

The immediacy of my desire to move on this is my own mortality.  I am not getting any younger, and if I am to make ANY significant contributions to this planet before passing, it needs to happen real soon.

I have knowledge of what efficiency advancement technologies all of you speak to, but taking any TRL 1-3 tech up to TRL level 6, then flying it into space and making it TRL 7, is a long path both in time and Dollars.  Unless you have venture capital lined up to support you the entire path, forget it, not going to happen.  If on the other hand you happen to convince others of your ideas with enthusiasm and smooth talking, it will still require a fair amount of luck, providence, to win.  The primary reason we do not have a 50% minimum efficiency working WPT system at present is because it is a difficult to achieve and expensive goal, and cable is far cheaper, and reliable enough.

Does anyone how how much it cost for Dickinson and Brown to put on the 1975 JPL Power Beaming experiment?

GROUND BASED WIRELESS AND WIRED POWER TRANSMISSION COST COMPARISON

https://sqreports.s3.ca-central-1.amazonaws.com/2022/Jaffe_11-10-21.pdf

Space propulsion and power beaming using millimeter systems
https://www.researchgate.net/publication/241571937_Space_propulsion_and_power_beaming_using_millimeter_systems

This paper lays out the current classes of  WPT applications other than space solar power very well.

You will see that Dickinson did not give up after 1975, and others came forward with ground based WPT applications. Little for many years until recently when Emrod entered the scene.

It is my desire to make a positive impact on WPT perhaps in one of these classes of power transmission, perhaps a new class.  If we consider ground applications of WPT, there are many potential use cases where WPT can improve the overall reliability of a system.  It is this type of application I am searching to enable, and soon.

Tim Cash

cash...@gmail.com

On 3/15/23 19:59, Sanjay Singh wrote:

Hello. This is for your information ... this was the original article I read ... I didn't realize it was 20 years ago. Also a slight error. I said Gallium Indium Phosphide in my reply. It's actually Gallium Indium Nitride. The lab is a credible one. Lawrence Berkeley Labs.

https://enews.lbl.gov/Science-Articles/Archive/MSD-perfect-solar-cell.html

https://enews.lbl.gov/Science-Articles/Archive/MSD-perfect-solar-cell-2.html

I don't know if it's still relevant today, but it sure sounded like the way to go back then.

Regards,

(S)

On Wed, Mar 15, 2023 at 4:50 PM Tim Cash <cash...@gmail.com> wrote:

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[James Salsman]

On Wednesday, March 15, 2023 at 4:06:00 PM UTC-7 silverthorn44 wrote:

...

Then there's the transmission beam. If you're willing to oversize both the transmitter and receiver apertures a bit, I'd say that 90% is feasible there as well. But more conservative estimates say 50%. 

I'd like to see empirical evidence that 50% microwave power transmission efficiency is feasible over 40 meters, let alone kilometers or thousands of kilometers.

For example, the best efficiency per distance in Shinohara (2013) "Beam Control Technologies With a High-Efficiency Phased Array for Microwave Power Transmission in

Japan" Proceedings of the IEEE 101(6):1448-63 -- https://ieeexplore.ieee.org/document/6495699 -- is 54.6% over 33 meters (the 2009 Kyoto University airship demonstration on pp. 1459-60 and https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/182170/3/rishsh_00500_012.pdf for the altitude omitted in the IEEE paper.)

If there are estimates of 50% for SPS microwave power transmission beam efficiency, they are either dishonest or involve rectennas larger than Puerto Rico with hundreds of billions of dipole elements, and even then would be by no means "conservative" relative to what has ever been demonstrated.

I'm sorry Tim, but it's a dead end. The only reputable way forward is lasers to high-altitude craft, as in Thales Alenia Space's Receiv'Air program.

Best regards,

Jim

[k.a.c...@sympatico.ca]

Keith;

You wrote:

> So, light pressure effects on a low-mass SSPS can be compensated without

> reaction mass expenditure with orbit eccentricity...

That's an interesting way to put it. Another way of looking at it is that solar radiation pressure forces any satellite in GEO to have an elliptical orbit, something that is well-known to operators of GEO commsats. Unless you decide to apply some form of thrusting to counter the SRP. The question becomes, can your mission tolerate the resulting elliptical orbit (whose ellipticity parameters vary of the course of the year)? For GEO commsats, the answer is yes --- the resulting east-west excursions are inside of the longitude box assigned to each satellite. The longitude (and altitude) excursions are much larger for an SPS, because of its lower mass/area ratio; for SPS, the question is, what (if anything) to do about that?

I'm part of a team that just now completing a little study on that topic, funded by ESA. We came to much the same results as you (basically by building on Soop's excellent book), with maybe a couple of new and useful insights. We also are playing (i.e., doing sims) with the idea of adding inclination into the mix, phased to get the SPS to cross the equatorial plane at an altitude other than GEO, which maybe could let early SPS's cohabit with GEO commsats. Of course, if and when *lots* of SPS's get flown into GEO, everything changes --- amongst them, GEO commsats will likely have to bolt onto SPS's, instead of being their own free-flyers.

- Kieran