Author Topic: Space-based solar power for Earth  (Read 119462 times)

Offline Robotbeat

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Re: Space-based solar power for Earth
« Reply #520 on: 06/09/2016 05:34 PM »
I worked on a concept at Uni back in the day and my own idea which I still find very attractive is to just put the sats in LEO and have two GEO satellites with individual mirrors per LEO sat. Instead of microwave beaming you use laser to transfer the energy from the LEO sat to the GEO sat to the groundstation on Earth. From LEO to GEO the airy disk (which contains 86% of the energy of the laser beam) only widens to something like 5 meter. If the laser would have a pointing accuracy of an order of magnitude worse than the hubble telescope it would still be very spot on.
Lasers are much less efficient than microwave generators.  Photovoltaics, even designed for a single wavelength, can't beat a rectina for efficiency.  Lasers don't deal well with overcast, while microwaves of a well chosen wavelength have little loss through the atmosphere even with heavy clouds.  On the other hand, you can get a much tighter, even dangerous, power beam.
ACTUAL very high frequency (i.e. ~1mm) microwave generators and rectennas are actually pretty close to the round-trip efficiency of direct-diode lasers and concentrating, single-wavelength photovoltaics.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline SICA Design

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Re: Space-based solar power for Earth
« Reply #521 on: 06/10/2016 07:19 AM »
I worked on a concept at Uni back in the day and my own idea which I still find very attractive is to just put the sats in LEO and have two GEO satellites with individual mirrors per LEO sat. Instead of microwave beaming you use laser to transfer the energy from the LEO sat to the GEO sat to the groundstation on Earth. From LEO to GEO the airy disk (which contains 86% of the energy of the laser beam) only widens to something like 5 meter. If the laser would have a pointing accuracy of an order of magnitude worse than the hubble telescope it would still be very spot on.
Lasers are much less efficient than microwave generators.  Photovoltaics, even designed for a single wavelength, can't beat a rectina for efficiency.  Lasers don't deal well with overcast, while microwaves of a well chosen wavelength have little loss through the atmosphere even with heavy clouds.  On the other hand, you can get a much tighter, even dangerous, power beam.
ACTUAL very high frequency (i.e. ~1mm) microwave generators and rectennas are actually pretty close to the round-trip efficiency of direct-diode lasers and concentrating, single-wavelength photovoltaics.

Is that not another way of saying 1mm (300 GHz) microwave conversion and atmospheric propagation is far less efficient than "well chosen" 52mm (5.8 GHz) or 122mm (2.45 GHz) wavelengths?

Offline Hanelyp

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Re: Space-based solar power for Earth
« Reply #522 on: 06/10/2016 03:24 PM »
The chief advantage of space based solar over ground based is the potential for near 24/7/365 operation.  A power link that has trouble with overcast defeats that advantage.

Uptime might be improved against the solstice outage if multiple solar satellites can be coordinated to deliver coherent microwaves to a receiving station.  Instead of one HUGE satellite, a swarm of smaller ones.

Offline Robotbeat

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Re: Space-based solar power for Earth
« Reply #523 on: 06/10/2016 03:45 PM »
I worked on a concept at Uni back in the day and my own idea which I still find very attractive is to just put the sats in LEO and have two GEO satellites with individual mirrors per LEO sat. Instead of microwave beaming you use laser to transfer the energy from the LEO sat to the GEO sat to the groundstation on Earth. From LEO to GEO the airy disk (which contains 86% of the energy of the laser beam) only widens to something like 5 meter. If the laser would have a pointing accuracy of an order of magnitude worse than the hubble telescope it would still be very spot on.
Lasers are much less efficient than microwave generators.  Photovoltaics, even designed for a single wavelength, can't beat a rectina for efficiency.  Lasers don't deal well with overcast, while microwaves of a well chosen wavelength have little loss through the atmosphere even with heavy clouds.  On the other hand, you can get a much tighter, even dangerous, power beam.
ACTUAL very high frequency (i.e. ~1mm) microwave generators and rectennas are actually pretty close to the round-trip efficiency of direct-diode lasers and concentrating, single-wavelength photovoltaics.

Is that not another way of saying 1mm (300 GHz) microwave conversion and atmospheric propagation is far less efficient than "well chosen" 52mm (5.8 GHz) or 122mm (2.45 GHz) wavelengths?
Calculate the size of satellite antenna and ground station for 2.45GHz. You're talking a dish 1km wide for transmitting and 4km wide for receiving.

Additionally, please show me microwave conversion efficiencies for specific components in that range, as well as rough operating temperatures.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline SICA Design

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Re: Space-based solar power for Earth
« Reply #524 on: 06/10/2016 05:52 PM »
Calculate the size of satellite antenna and ground station for 2.45GHz. You're talking a dish 1km wide for transmitting and 4km wide for receiving.

Additionally, please show me microwave conversion efficiencies for specific components in that range, as well as rough operating temperatures.

Actually the multiple of the diameters at 2.45 GHz is more like 10.8E6 m^2 (i.e. 11km rectenna for 1km dish*), or 4.5E6 m^2 at 5.8 GHz. HESPeruS has an effective transmitter aperture of 4.4 km; which is of the right order to match its (non rotating) solar-collecting area (5 - 10 GW sunlight) for delivering 1 GW to the grid.

This could focus down to a 1km rectenna, BUT:

(a) a 1km Airy disc (for 1 GW to grid) would have a central beam intensity of 1.4 kW/m^2 which would be deemed unacceptable for overflying aircraft or wildlife - it is deliberately defocused (by means of the 40.68 MHz pilot beam emitted from the rectenna site) for a minimum 4.6km Airy disc. This gives a central beam intensity of 300 W/m^2, or under one-third equatorial noon sunlight intensity.

(b) HESPeruS is optimised for latitudes above 45deg, where the incoming beam angle may be as low as 20 degrees above the horizon, stretching the Airy disc by nearly 200%. For this reason, the rectenna diameter is chosen as 12 km, which also increases the 20deg beam interception to 88.5% - or for a zenith beam from 84% (just for the Airy disc) to 94.7%.

Uniquely for HESPeruS, a 12km rectenna would allow a one-tenth scale prototype to deliver 100 MW to the grid from its target Molniya orbit. This could later be expanded to full scale.

This one-tenth scaling, reduced mass from essentially flat film construction, coupled with the fact that a Molniya orbit requires less than half the orbital energy of GSO, allows a fully working prototype to be demonstrated at far lower cost than other GSO/GEO SBSP systems.

As requested, I've included examples of actual demonstrated rectenna efficiencies - which is mostly a function of schottky diode efficiency:  http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020091852.pdf

Just recently a group based at Manchester University has announced a graphene-based Ballistic Rectifier which has zero band-gap (a function which normally limits conversion efficiency at lower field strengths): http://www.nature.com/ncomms/2016/160531/ncomms11670/abs/ncomms11670.html

 * A parabolic dish is a bit too 70's for me - try google "planar phased array"
« Last Edit: 06/10/2016 05:55 PM by SICA Design »

Offline TrevorMonty

Re: Space-based solar power for Earth
« Reply #525 on: 03/23/2018 01:54 AM »
The development of SBSP as always is financial hurdle. SpaceX BFR and Blue NG by lowering cost of access to space may help lower cost of SBSP but it will need a large space base ISRU infrastructure to really bring price down to 10cUnit.

What SBSP really needs is someone willing spend a lot more per Unit to help start ball rolling. Yet again it could be internet companies to rescue with their huge cash reserves and enviroment conscious.
Google, Amazon and Microsoft are all committed to reducing their carbon foot print.

https://www.geekwire.com/2018/microsoft-makes-largest-corporate-solar-deal-u-s-history-buying-power-750000-solar-panels/

I'm picking Jeff Bezo will be one of first, whether it is via Blue, Amazon or consortium I don't know.

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