Space Based Solar Power

I would take this as an example that the point of academic research is to explore possibilities, it's very often not possible to guess where these might lead. 

The cycle tends to be that:

- the academic publishes a thought exercise in order to promote further research

- the press leap on it and publish it as "this is coming NOW!"

- the press and public together then ridicule it as "well they promised this but it never happened"

- the academic goes and gets very drunk and depressed and wished they'd never had to publish it

Incidentally I'm very aware that University press departments often don't help this cycle at all...they get pushed to promote early stage research in order to gain funding.

These days I regret to say that I read New Scientist rather than E&T, NS is very good at being clear whether research is "at the point of practical implementation" or is "here's an idea, what does it make the rest of you think of that might be a better idea?"

Back in the engineering world, please let's start putting solar panels on all the acres and acres of flat (and flattish) roofing...I cringe every time I pass an industrial estate or trading estate at the wasted roof space potential...

I'm inclined to agree - any article that says "harvested electricity is then converted into microwave or laser beams for transmission. Microwave frequencies (1–10 GHz), especially 2.45 or 5.8 GHz,²⁵ are commonly selected to balance transmission efficiency, atmospheric attenuation, and safety constraints."  is very much at the "idea bouncing" stage.

Given what I know of current state of play of lasers and microwaves, this is unlikely to be simultaneously  efficient and safe for folk on the ground. Consider a laser like dragonfire - one of the highest energy densities fir which figures are available - here we have 50kW or so (not sure if that is DC input or optical output mind you available for short bursts, in a spot perhaps 25mm wide at a km, and presumably diverges with distance, and that is before we worry about cloud cover, dust storms and so on,). The conversion efficiency is likely to be in the high tens of percent, so lets assume for now that it can be as good as 50% - to beam down a gigwatt - a sensible single power station sort of figure, then another gigawatt of waste heat needs to be sweated off at the source - hard to do in the vacuum of space.

At the receiving end, the sensible thing to do for a laser system is probably to boil water or melt salt or heat oil, to then boil water, and run turbines. The damage such a sytstem could do if beam tracking went awry, needs I hope no explanation.

A microwave system might use rectennas - antennas with RF rectifiers built in, but there are problems, as to rectify at thousands of MHz requires very small diode chips, and these are necessarily limited to a few volts peak, and have very awkward impedances. Of course one can stack lots of them, which is presumably where the references to kilometer scale receiving stations come in.

The efficiency is not good at either end for such systems - we'd probably need to shed a gigawatt on the ground, and then 2 more in space, per gigawatt delivered.

 doable of course, but bigger than the study suggests and probably never worth taking beyond a demo system - which would still be worthwhile as a training aid and to improve the modelling.

Mike

I'm inclined to agree - any article that says "harvested electricity is then converted into microwave or laser beams for transmission. Microwave frequencies (1–10 GHz), especially 2.45 or 5.8 GHz,²⁵ are commonly selected to balance transmission efficiency, atmospheric attenuation, and safety constraints."  is very much at the "idea bouncing" stage.

Given what I know of current state of play of lasers and microwaves, this is unlikely to be simultaneously  efficient and safe for folk on the ground. Consider a laser like dragonfire - one of the highest energy densities fir which figures are available - here we have 50kW or so (not sure if that is DC input or optical output mind you available for short bursts, in a spot perhaps 25mm wide at a km, and presumably diverges with distance, and that is before we worry about cloud cover, dust storms and so on,). The conversion efficiency is likely to be in the high tens of percent, so lets assume for now that it can be as good as 50% - to beam down a gigwatt - a sensible single power station sort of figure, then another gigawatt of waste heat needs to be sweated off at the source - hard to do in the vacuum of space.

At the receiving end, the sensible thing to do for a laser system is probably to boil water or melt salt or heat oil, to then boil water, and run turbines. The damage such a sytstem could do if beam tracking went awry, needs I hope no explanation.

A microwave system might use rectennas - antennas with RF rectifiers built in, but there are problems, as to rectify at thousands of MHz requires very small diode chips, and these are necessarily limited to a few volts peak, and have very awkward impedances. Of course one can stack lots of them, which is presumably where the references to kilometer scale receiving stations come in.

The efficiency is not good at either end for such systems - we'd probably need to shed a gigawatt on the ground, and then 2 more in space, per gigawatt delivered.

 doable of course, but bigger than the study suggests and probably never worth taking beyond a demo system - which would still be worthwhile as a training aid and to improve the modelling.

Mike