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Comment by unchocked | Hacker News Reader

Not true. Photovoltaics are ~20% efficient, so you're looking at 200 W/m2 in full sunlight. Plus typical solar capacity factors are about 25% so you're down to 50 W/m2 average power generation, with a subjective penalty for intermittency.

RF rectification is ~90% efficient, so that same 1000 W/m2 gives ~4.5x the power density of daylight PV, plus a capacity factor of nearly 100% gives 18x the energy density of terrestrial PV - and its baseload power.

Mankins will pitch you 100 W/m2 as a useful RF energy density to be competitive with terrestrial solar. You may also be able to include photovoltaics in your receiving antenna and have it both ways.

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If you replace microwaves with a laser, you can get much higher efficiencies out of a solar panel as solar panels really want to convert monochromatic light.

What really kills microwaves for power transfer is the minimum sizes of the transmitter/receiver thanks to diffraction. If your going from GEO to ground, I believe you need on the order of square kilometer sized arrays on both ends.

For ground to air applications, rectennas are too bulky and heavy to be practical. If you look at the last NASA power beaming challenge, all of the contestants went with optical power transfer because it was better in W/kg and in W/sq meter.

Disclaimer: I’m involved with free-space optical power transfer.

You're right about the size. The book I mentioned puts the cost of the ground antenna at 0.7 cents/kWh, so not terrible.

Lasers have definitely been considered for SPS. The main challenge is that clouds get in the way. (And you don't want the power density so high that it works as a weapon, but I imagine you can avoid that.)

Was the NASA challenge for SPS, or other applications?

The challenge was nominally for determining feasibility of a space elevator as there was a sister challenge for making high strength/weight cables (it had a bigger problem of the DOD snapping up the promising teams to work on armor every year).

If you keep the power down to the order of 10x sunlight, it doesn’t make a very good weapon.

I’m sorry but 10X the sun over a square kilometer 24/7 is a incredible weapon.

You mean 0.7 cents per kW, not kWh, right? Because hydroelectric energy costs 0.7 cents per kWh everything included, including profit.

No, I mean levelized cost per kWh (estimated retail, iirc).

Hydro is quite cheap, but it's not available everywhere and only scales to about 10% of global energy demand: https://dothemath.ucsd.edu/2011/12/how-much-dam-energy-can-w...

All other sources cost a good bit more than that. The US average is over 13 cents/kWh: https://www.electricchoice.com/electricity-prices-by-state/

Well you must be getting screwed over in the US. Québec hydro is 6 cents per kWh, at your house, and is profitable. State owned nuclear energy in Europe has comparable costs.

If only the transmitter and receiver for space solar is a very significant fraction of the cost per kWh shipped at your house with profits of nuclear or hydro it can't be viable.

13.3 cents (us)/kWh is the US national average for electricity cost, not any given region or source.

Canada's national average (this excludes the territories) is 10 cents (us)/kWh. Québec has the cheapest energy in Canada, primarily due to its proximity to hydroelectric sources, at 5.5 cents (us)/kWh. In parts of the US where hydroelectric is the dominant energy source, prices are comparable.

Including the territories Canada's average is 13 cents (us)/kWh.

Right, and total cost for SPS would be around 5 cents/kWh. The receiver is a small portion of that at 0.7 cents.

13 cents/kWh is the US national average including all sources and regions, according to the link I posted.

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