Monday, August 3, 2009

Solar Power from SPAAACE ??!

The idea of beaming solar power down from orbital powersats is not exactly new. It was first proposed (that I know of) in 1968, and got a fair amount of attention during the energy-crisis years of the late 1970s and 1980s. I'll confess to having always regarded the idea as including an excessive coefficient of hype.

The theoretical advantages are straightforward enough. There's no night in space, no cloud cover, and sunlight is about 40 percent brighter than what reaches the earth's surface. All in all a solar installation in space delivers about two and a half times the power of the same installation on Earth, and delivers it steadily, requiring no storage system to keep the lights on at night. The disadvantage (apart from worries about cooking people or animals that stray into the surface rectenna that the power is beamed down to) is that you have to put the solar cells in geosynch orbit, An Expensive Thing To Do.

For this reason I've tended to think of space solar power as one of those things, like maglev tunnels under the oceans, that you shouldn't hold your breath waiting for.

But in my day gig I read the tech press, and was surprised to discover that my friendly local electric utility, PG&E, has signed an agreement with a company called Solaren to provide 200 megawatts of solar power from space. Not by some conveniently remote date like 2030, but starting in 2016, which according to my calendar is only seven years off.

The usual caveats apply. The whole thing has to jump through regulatory hoops, and PG&E is fronting no money. Perhaps it is just as well that they are not involved at the technical end, because they are not my favorite utility, and not just because they bill me. Back in the 1980s there was a big local stink about construction of the Diablo Canyon nuke plant, a few miles (upwind!) of where I live. I was a tad embarrassed when, in the middle of defending nuclear power to friends and neighbors, it turned out that PG&E reversed the blueprints and built a fair amount of the plant in mirror image to the intended design. (!) Yes, the error was fixed, and the plant has a fine safety record, but it did leave me wondering whether PG&E should be trusted with anything more complex than a dry cell.

For now, I'll assume that Solaren, at least, knows what they are doing.

In the meanwhile, let's play with some numbers. The current average cost of electric power is about a dime per kilowatt hour. Solar cells, so I am told, are now approaching a power density of 400 watts/kg. For a bit of margin let us say 300 watts/kg, or 7.2 kilowatt hours per day per kg, or a shade over 65,000 kWh over a service lifetime of 25 years. The current launch cost to LEO is about $10 million per ton, $10,000/kg, so the launch cost of our solar cell works out to 15 cents per kWh.

This is not great, but it is far from awful. LEO is not geosynch, where the powersat needs to be, but a solar powersat is uniquely well suited to using solar electric power to spiral out to its desired orbit. There's also the actual cost of the solar cells, and the rest of the setup, and the interest charge of paying up front for 25 years' worth of power. On the other hand, current electric power pricing reflect recession-level energy costs. Over the next few decades, juice is probably not getting cheaper, and barring catastrophe the world will be using more than the 15 TW it uses now (from all sources).

Let us suppose that, over the next 50 years, a terawatt of power supply is put in orbit, supplying a few percent of total world power. At 300 watts/kg that calls for 3.3 million tons of solar cells put into orbit - say 4 million tons for the whole shebang, including fuel for the spiral up to geosynch. This averages 10,000 tons per year for 40 years, on order of 100x current traffic volume. If anything can pull down launch cost it is lots of launches, with resulting economies of scale.

So. If average power cost near midcentury is 15 cents per kWh - probably an optimistic figure - and launch cost falls only by half, to $5 million/ton = 7.5 cents per kWh, launch cost becomes only half of what the installation earns in 25 years. I'm still ignoring the cost of the solar cells, but the ones in space provide more juice, and interest cost applies wherever you put them.

I'll be damned. This thing might actually work out.

Related links: Last year I discussed solar electric spacecraft.


agricola64 said...

"Solar cells, so I am told, are now approaching a power density of 400 watts/kg. For a bit of margin let us say 300 watts/kg."

There is a problem with this - while solar cells might approach this power density, solar cells alone do not make a SPS ... need structural mass, cabling, power conditioner, transmitter antenna, transmitter & power control system, communication & control system, attitude control system & fuel & tanks

once you have figured all this in,your power density is substancially lower ..

look up the power density for the ISS solar sails .. you will be dismayed


what does this mean?

1. solaren has created some breakthrough in power density, on a shoestring budget and without real world testing ... this would make investment incredible risky and the contract with the electricity company would likely be a PR maneuver on the electricity companies side ..

2. its a scam and somebody is looking for someone hapless enough to drop a few M$ down a black hole ..

Citizen Joe said...

I just saw some sort of laser launch system for putting small payloads into orbit. Maybe get a bunch up there, then assemble them in space. Or get them up there and use solar drive systems to get them into proper locations.

That doesn't help people into space though, just small packages.

qwert said...

I am skeptical to the ida tha a new and untested, space based power generation system is going to be working earlier than 20-50from now on.

As far as i know the concept has never been tested, not even as a prototype, much less as a commercial attemp. I remeber hearing some years ago that the japanese space agecy had plans to launch a test version of a solar farm before 2010. But I don´t think they are going to make it.

I see it feasible to have some type of working prototype in 10-15 years. But even then it will take time until something like that will be practical, and the first busisnes attemp in something of such scale will most probably be a gigantic failure (as many predecesors in big projects).
So it will probably take time until the technology is ready and there at some serious attemps at making money with it.

The numbers you have given don´t give good impression. Individual solar panels are cheap, but they are expensive when compared to the amount of energy they give, add to that high mainteinace costs and solar energy is the most expensive form of generating electricity today,(if the numbers I have read are correct, although prices are sinking faster than anywhere else).

If launching it to space costs as much as half of its expected income in its entire lifetime, and you have to add a specialized ground crew, control systems and a way to send energy down and transform it back into electricty on earth, I think you have already eaten up all the profit.
To that you will have to add high insurance costs. If something goes bad on earth, you can send a technician and repair it. But if something goes wrong up there, you may have to launch a replacement.

High energy prices and a carbon market may help to make something like this profitably, however i think that for the time being earth based power may be a better bussines option.

Tere may be however areas where it might work. I remember reading that Dept. of Defense was interested in using this idea on small scale in order to be able to supply troops at the front with power without any need to be carriyng generators and fuel for them. I don´t know if you could use a sytem like this to supply addintional power to other satelites or spacecraft in orbit (and if it would be usefull at all).

Anonymous said...

On a related note, I did some quick work on a the reverse idea - Solar power to SPAAACE!

My numbers and their sources:

37.5 megawatt-hours to put one ton into orbit (1 ton at 30 g for 30 seconds, from Jerry Pournelle's A Step Further Out). Assuming 80% laser efficiency, that's 46.9 megawatt-hours per launch.

46.9 mWh per 1 ton ton per launch, assuming 90% storage and transmission efficiency, is 52.1 mWh per launch. (Source for 90% storage efficiency - None. But the transmission distance is negligible, so transmission losses are also negligible. And it's an industrial facility, so we can assume power-storage technologies that would be impractical for general use. Source for 80% laser efficiency - None. But it's the cut-off point for lasers as useful weapons, the happy dream of SF geeks and military-industrial researchers alike. If you want to stick with current laser efficiencies for this exercise, increase the needed power by x2.5 or x3, for 130 to 156.3 mWh)

The PS10/PS20 solar power plant near Seville is set to produce 300 mw on 700 hectares by 2013, or .43 mw per hectare. (Source - 2006 PS10 technical report)

To produce the needed 52.1 mWh in one hour at .43 mw per hectare, we need 121 hectares.

The standard block in Manhattan is 80 metres x 274 metres, or 21920 square metres. 1 hectare is 10000 square metres. 121 hectares is 1210000 square metres, or a bit over 55 Manhattan blocks. (Source for the standard size of Manhattan city blocks - Wikipedia. Yes, I'm citing Wikipedia. Y'wanna make something out of it?)

Even at 156.3 mWh (With modern-efficiency lasers) we only need 365 hectares, or 167 Manhattan blocks.

Feel free to check my numbers, because only a drooling lunatic would trust my math. I don't. But it looks like solar-powered laser launch could be a technological match for solar-electric drives. And when you're not firing capsules into space, you can sell the power here on Earth.

And on a real downer note - Has any university or research institute done simulations on what happens to local weather when you spot-heat part of the terrain? Could be some real cranky farmers if the winds off that beam-collector push the rain away...


Anonymous said...

Orbital Solar Power, one (though apparently very expensive) way of solving our little electrical problem in powering our cities.

I remember seeing a program on the Discovery Channel documentary series "Project Earth" that detailed the prototyping and testing on the basic principles of the Orbital Solar Satellite idea. I don't remember the exact wording, but the power efficiency of a photovoltaic panel that had a curved piece of plastic mounted over it was increased over the unaided solar panel.

From what I remember, the chief problem were the last sixty miles where the atmosphere might have the greatest effect on the power transmitting microwaves. It apparently splits the microwave beam into two beams midway or fifteen miles, a problem that should be addressed. The experiment is writtin on this web page:

Personally, I think that a Maser delivery system would be a better way of transmitting said power over regular microwaves through the atmosphere. However, this also increases the "Hand of God" accident of people, animal, and things getting burned.

As mentioned in previous replies and the post itself, the big challenge and bank breaker is in how to get those things in orbit. Citizen Joe suggested the Laser Launch alternative, but the amount of power necessary to lug that much kilograms into orbit via laser power might be counter productive. However, this doesn't mean that said kilograms doesn't have to account for a single, large solar satellite. Rather it would probably be more economical to launch a constellation of smaller, more economical solar satellites in a series of launches than a single one.

Planetary Mass Drivers might also solve this problem also, but there's the little hurdles of high power and technological innovation neccessary just to launch satellites and other such payloads into LEO. Something tells me that a better alternative for surface-to-orbit delivery systems would have been perfected and very economical by then.

As for that Anon comment on the 50 year tech thumb rule, yeah that was me. I just forgot to add in my name.

- Sabersonic

Rodney said...

I’m picturing one of the disasters in Sim City 2000 which involved the microwave power station (basically solar power from space beamed down to an antenna) having a targeting oopsie and melting a bunch of buildings in the city. I would hope the engineering team would keep that sort of thing from happening, but that leads to what might be a silly question from me. I admit that I don’t know much about these things, but how would the energy get sent to the surface? I can’t see a long cable the equivalent of a high tension wire stretching from GEO down to a station, which leads to a transmission system.

Would microwave transmission work? If so, is it possible to catch a megawatt beam of energy without melting the receiver?

Anonymous said...

Power would be transmitted as a low-density microwave beam. Keeping it on-target is relatively simple (Optics should do the trick, but you can use laser targeting if you want to get fancy), and most of the designs I've heard about have included safeties to shut the beam down if it does drift.

I can't remember the numbers and I'm too drunk to google them right now, but the beam is low-density. You should be able to fly an airplane through the beam without damage. The problem I see is that there's no such thing as 100% efficiency, and this beam is on all the time. A very narrow column of air is going to be under constant heating, and there will be some loss at the ground as well. Given that solar power satellite enthusiasts regularly endorse covering entire deserts with receivers, I'd really like to see some work done on what that constant heating does to wind patterns.


Rick said...

Tough crowd, tough crowd!

This is good: The space community is generally notorious for cockeyed techno-optimism and (especially) lo-balled cost estimates.

And welcome to new posters here!

agricola64 - My google fu came up short on ISS wing power density, but I did come up with this, saying that the (2002) state of the art is 50-110 watts/kg, while predicting 400.

Another couple of relevant factoids - the support girder for the latest solar wing has a mass of 14 tons (16 'short' tons), and those wings will provide 36 kw. So that's about 0.5 kg/kw for the spar, supporting a wing that with the design period state of the art is probably no more than 0.1 kg/kw - so the spar is about 20 percent of wing mass.

I can't imagine the cabling would be too massive, but I haven't a clue about the mass/power ratio of the microwave (or whatever) gizmo that beams the power down.

Rodney - As a metric, noon solar power density on Earth is about 1 kw/square meter, so a beam density at Earth of 100 watts/m2 won't cook you, and a rectenna area of 1 km2 is good for 100 Mw, 10 km2 good for a gigawatt.

More comments-on-comments after dinner! :-)

Rick said...

A few more general notes. Business floats on a sea of hype, but the coefficient is still vastly lower than you usually get from the space community. I don't think PG&E would have spent lawyer hours on a contract unless Solaren passed some minimum threshold smell test.

That said, I assume this is a purely pilot project, a technology demonstrator. By my own rule of thumb from last post, if the thing pans out it might mean a space solar boom around the 2060s.

In terms of space launch, the actual energy is an insignificant fraction of current cost. The current average cost of juice, 10 cents/kWh, means that a dollar buys 36 megajoules. (A gallon of gas has 132 MJ.) The kinetic energy you need to impart for LEO is about the same, ~35 MJ, not accounting for lift losses. Even you only get 10 percent energy efficiency - because most of it lifts fuel, and the spacecraft structure, that's $10/kg energy cost to orbit, 1 percent of launch cost.

Sabersonic - based on the above, Pournelle is arguing for a laser launch energy efficiency of 27 percent, not counting losses in the laser itself (as you did). It sounds reasonable ... with the warning that it's late, and I may be completely screwing up my numbers.

agricola64 said...

Rick ..

.. i do not disagree that 300 to 400 W per kg will be possible at some time ..

but i do question that this weight includes all the overhead - it might include structural mass, but it does not include other weighty components like power conditioning, control and communications, the transmitter system or attitude control ..

and i have not even mentioned the apogee kick engine needed for GEO positioning (or the fuel if you plan to use the attitude control system for this)

all this will reduce power density .. and increase costs

i would LOVE to see a SPS in the time horizon planed, i am not optimistic

agricola64 said...

small addition:

ISS solar wing mass: 15900 kg ..
power 38400 W (38,4 kW)

power density is therefore about 2.2 W / kg .. two orders of magnitude off from the goal for SPS (i think you slipped a decimal point in your post, rick)

this DOES include structure and pointing, some power conditioning, radiators for those - it does not contain communication systems, attitude control and the power transmitter.

so i would say it is a good example for the current state of the art ..

M. D. Van Norman said...

Or for far less than $10,000 per kilogram, we could install solar panels on rooftops everywhere. Of course, then PG&E and other utilities wouldn’t be able to sell us the electricity.

Anonymous said...

agricola64, that power density in the same range as the higher-end home solar panels, although it's higher than the midrange and cheap kits. I'd guess NASA went for durability and cost over 'state of the art'.

Van Norman, solar power is intermittent, you need a good battery system to store power for times when the sun isn't shining. And the really good battery systems tend to be dangerous. Also, solar power is low-density. The only way people can go off the grid with solar is by getting rid of their AC, buying high-efficiency appliances or doing without large appliances, and rationing their power us. These are not popular lifestyle choices.

Unless people make a serious effort to switch to a low-energy society, some form of centralized power generation will be needed. And if society does transition to low-energy, the result will be very different than our current vision of suburban houses with solar panels on the roof.

Personally I don't think we have a choice in the matter. The Great Divergence seems to be running out of steam.


Citizen Joe said...

The trick is to get more than your peak need in solar panels and pump the rest back into the grid so that the power company doesn't need to generate the surplus. The Grid has the battery banks to store the excess. The big problem with that is that the service personnel could get electrocuted because they don't have access to the solar side of the grid. And if you simply cut the grid end, the solar system could burn itself out without the power dump.

Solar panels on roofs really only work for about 6 hours a day. And if it is cloudy, you lose that as well. The space based solar therefore has 4 times the time in addition to more effective sunlight during those times.

M. D. Van Norman said...

Beamed power from space is exciting, but it’s just not really necessary. Big, centralized generation facilities have always been the wrong approach for solar power. As ground-based solar panels spread, space-based systems will become increasingly less attractive.

However, the grid will always be important. It will distribute the daily surpluses and make up for the inevitable shortfalls. Residential batteries may not be required at all.

Solar power appears to be viable even in colder, cloudier environments. Obviously, though, additional architectural consideration will be required to gain the best advantage.

Anonymous said...

"Solar power appears to be viable even in colder, cloudier environments."

Your link connects to a site about solar heating in Maine, which is not at all a cold region. In cold regions going without heat for three days in winter (As the people at that site did during the December 2008 ice storm) would kill you. We get 3-5 dead homeless people a year in this city, and 1 dead homeowner every 3-5 years.

You'll take my gas heating when you pry it out of my cold dead hands.


Citizen Joe said...

That's another topic. Why turn sunlight into electricity and then convert that into heat (inefficiently) when you can just turn the sunlight directly into heat?

Rick said...

agricola64 - I slipped three orders of magnitude, mixing up watts and Kw. (Just shows to go that using SI units doesn't protect against ALL stupid blunders!)

Ah, but here is another benchmark, the solar-electric ion drive Dawn probe to Ceres and Vesta. Its solar wings generate 10.3 kW [PDF; html link here], and each is 63 kg, 126 kg for the pair, thus 80 watts/kg, a 37x improvement over the ISS wings, and only four times my target mass. (!)

Apparently that includes the whole wing structure with cabling, etc., though not the other equipment the SPS must have. Still it gets us close to the ballpark, and since Dawn launched in 2007 its technology is already a few years behind the current state of the art.

Dawn has also flight demonstrated solar electric propulsion, which hugely reduces fuel mass for the climb from LEO to geosynch. Dawn's ion engines have an Isp of 3100 sec. You'd probably want some other electric drive for the much bigger powersat, but even though a spiral is an inefficient orbit, an Isp in that range pulls the fuel fraction down to the 20-25 percent range.

(If anyone has mass specific power benchmarks for the microwave beam system, I'd love to know.) The control system shouldn't be a big fraction of total mass.

MD - I'd suggest that space solar is really in competition not against earth-based solar cells but power storage - whether jumbo storage batteries or biofuels - and long distance transmission lines. People will pay some modest premium to have lights and power at night, especially through the evening, and in higher latitudes in winter there is a lot of night to go around. :-)

The grid, as you say, is not gonna disappear, even if much of the supply put into it is decentralized. Space solar will not go anywhere if batteries or biofuel can store power cheaper than beaming it down. But if beaming it down is competitive, some power will be beamed down.

Citizen Joe - Direct solar heating is a whole 'nother issue. The advantage is you can store heat directly, e.g. with water. The disadvantage is that you need another investment in the system.

Ian - I imagine we will get more efficient about energy consumption, and there will be some lifestyle changes. Energy from whatever sustainable source probably won't be as cheap as it has been. But postindustrial societies are unlikely to sit in the dark at night if affordable power sources are available.

In general, I imagine that by midcentury we will be getting power from a variety of sources, but it at least seems plausible that space solar could be one part of that mix - something I had not really expected. With all the caveats mentioned here, the thing does not look grotesquely impractical given fairly modest tech advances.

Anonymous said...

As far as I recall, the microwave generating equipment would only be a small percentage of the total; megawatt klystrons only weigh a couple of hundred pounds and they're not the most efficent type of microwave generators...most, if not all, of the power conditioning would be done at the ground far as the experiment goes, we really won't know until they try.


Rick said...

That matches my intuition that the microwave end of the SPS would not be all that heavy.

We certainly won't know how this whole thing plays out till it is tried. What motivated me to write this post was my surprise that an electric utility - a business that usually has a pretty conservative institutional culture - was even willing to sign onto a deal. PG&E may be based in San Francisco, but it is hardly part of tech-startup culture, though Solaren doubtless is.

It's cool that someone is actually planning to try this, and it would be way cool if it works!

Anonymous said...

A thought had just crossed my mind.

Now correct me if I'm wrong, but a geostationary orbit of a Solar Power Transmission Satellite would experience periods of darkness once the satellite orbits to the night side of the Earth, though due to its distance from the surface its dusk would be some time (hours maybe?) after the terrestrial sunset. A period of time when it's unable to collect and transmit solar energy to the receiving station on the ground.

This might necessitate some form of energy capacitor storage that electricity could be drawn from until the solar satellite returns from the night side of the planet to the day side. Even possible Polar Orbits would have periods in which the Solar Satellite is unable to transmit the energy down to the receiver station due to the rest of the planet being in the way.

Another problem would be Orbital Stationkeeping in the proposed Geostationary Orbit for the Solar Satellite due to the orbital plane not being parallel to not only the Sun-Earth orbital plane, but also the Earth-Moon orbital plane as shown in this image:

Naturally a solar satellite isn't going to stay up there forever and the orbital maneuvers to keep the orbit stable is going to use up propellant and electricity. The same electricity that's suppose to be sent planet-side. Granted, this gives plausible excuse to replace an older solar satellite with a new one that features more economical and efficient solar collection and transmission technologies, among other things.

There's also the idea of some that Solar Power Satellites can be built in space using orbital resources such as asteroids as claimed by some proponents. Good luck trying to convince the general public that the asteroid heading towards earth isn't going to collide with it and that you've done the math and timing.

- Sabersonic

agricola64 said...

rick ..

many thanks for that article on Dawn .. i read through it diagonally this morning and i believe that while the mass you state does include structure, it does not include the gimballing mechanism ... and a SPs needs to gimbal either the PV wings or the transmitter antenna ..

but it is interesting to see something that is a lot closer to current state of the art then the - probably deliberate - very conservative ISS design ..

and dont worry about slipping decimal points .. worse things have happened to me ..

the question what is better: classic direct injection into GTO with apogee kick or spiraling up will be a difficult one .. while spiraling up will probably be better in terms of delivered mass to GEO, it will also maximise the time the PV cells will be in the van allen belts, which will reduce their power ..

and while the magnetrons or other microwave generators will not be to heavy per se, they will need some cooling system - and a antenna ..

what are the thoughts on the antenna, parabolic or phased array .. ?

agricola64 said...

MD - the grid is not going to disappear .. it is going to get larger and more complex, as it will have to handle power input from many small sources (individual wind mills, PV sets, biogas / biomass power generators, wave and tidal generation, etc. etc.) as well as really long connection via HVDC - for instance importing solar power from states, where the sun has not yet set (or has already risen)

agricola64 said...

ferrel ..

its not the magnetrons alone - its all their ancilliary equipment .. and i would be really astonished if the PV cells could give their output directly to the magnetrons ..

and - of course it will be necessary to really try it out .

agricola64 said...

rick ..

electric utilities are VERY conservative - which is why it was such a big surprise a few weeks ago when the major electric utilities in germany formed a cooperation with major equipment producers and financial institutions to plan major building of large solar power stations in the sahara (with HVDC lines back up to europe ..)

which is why the engagement of PG&E in such a experimental thing is a little puzzling .. i still thing they did it for future PR value .

agricola64 said...


afaik commo sats at GEO loose power only during equinox - this is the only time of the year where the earths shadow will be in plane with GEO orbital plane .. the other time the earths shadow will pass "over" or "under" the orbital plane

.. and due to the distance earths shadow is already pretty small and the time of power loss is about 30 minutes ..

if you have a (not too close) chain of SPS in GEO only one will loose power at any time,so reserve power capacity to cover this out-time

Anonymous said...

Agricola64 - "which is why the engagement of PG&E in such a experimental thing is a little puzzling .. i still thing they did it for future PR value ."

I think something similar is going on with ZENN Cars and EEStor. EEStor claims to have a revolutionary new capacitor technology ready to ship for 'next year'. They've claimed to be ready to ship 'next year' for several years now. ZENN has licensed that capacitor for its electric cars, and issues press releases promising a revolution in transportation real soon now.

The people at EEStor seem to truly believe they have achieved a breakthrough that will be ready for market any day now. They remind me of Blondiot and his N-rays. The people at ZENN seem to be using EEStor's publicity for what looks suspiciously like a pump-and-dump scheme. They remind me of the cynics who think the next bubble will be in green technologies.


Rick said...

Direct injection to GEO versus spiraling depends on how badly the Van Allen belts fry the cells.

Direct injection requires chemfuels, and with a required delta v of about 4 km/s your payload fraction (from LEO) can't be better than about 35 percent. The spiral orbit requires more delta v, perhaps close to 7 km/s, but with electric drive the payload fraction could be maybe 75 percent.

I imagine that PG&E's involvement has a hefty dose of PR involved, and I wouldn't be a bit surprised if Solaren is hoping to hustle up a subsidy.

Still rather coolific, though!

agricola64 said...


the final decision what is better can only be done after a very careful analysis

we also forgot to give the midpoint position .. injection into GTO by the chemical launcher (this is the standard mode for most launchers and would not need any mods) , but doing the apogee burn with advanced engines (and you probably would use electric engines for attitude control anyways in order to reduce fuel mass)

Jim Bowery said...

Who is talking about earth launched solar power satellites that don't rely on mirrors for their primary light gathering surface?

The numbers are a lot more realistic if you, quite rationally, concentrate with light weight mirrors. Moreover, it isn't clear to me you use solar cells at all. Solar thermal may make more sense.

1kW/kg is quite achievable.

agricola64 said...


even IF 1 kW/kg is achievable for the PV part (and i would love to see a demonstration for this) .. PV cells alone do not a SPS make

we have been discussing the other neccessities of SPS that will add mass and reduce power density

in your blog you were talking about huge clusters of smaller satellites ... 2 questions:

1. how do you intend to prevent destructive heterodyning at the receiver antenna?

2. slots in GEO are a very limited resource - so rare that they often are sold in huge auctions .. finding a slot for one huge SPS is a lot easier then for a whole costellation

while it is possible to park a whole constellation in halo orbits around a single GEO slot (some TV sat companies do this - for instance Astra) the satellites in such clusters create additional traffic control loads on the ground station and do also use more station holding fuel)

Rick said...

Jim - I don't have any information on Solaren's intended cells, but 'multijunction' cells that concentrate light from mirrors or Fresnel lenses are getting a lot of buzz, and may be what they have in mind.

Traditional solar thermal, concentrating light on a boiler, would be plumbing intensive. I just recently read about work with nanosmaterials that could enable efficient thermoelectric cells. I doubt that's what Solaren is thinking of, given the time frame, but it might be a big deal down the road.

agricola64 - Presumably smaller payloads could be assembled into one powersat. In fact would have to be, because we certainly won't be launching these things in one piece. OTOH, the smaller the individual payloads the more complicated the on-orbit assembly process.

My instinct is that the other parts of the powersat would be no more than a third of total mass and probably less ... but that is sheerest guesswork.

But the gimbals you mentioned upthread might not be needed. The cell structure has to face the sun, and the microwave part has to face the Earth, but they could be connected by cable. (Yes, that leaves some tricky stationkeeping subtleties, which are pretty much above my pay grade.)

Jim Bowery said...

Agricola64, I am specifically making the case for using concentrating mirrors rather than PV cells as the primary light gathering surface. How massive do mirrors have to be in a zero-g, zero-weather environment?

As for destructive heterodyning at the receiver, see section 2.3.5 of Supporting Document for the URSI White Paper on Solar Power Satellite Systems.

Only one slot is required in geostationary orbit for all of the satellites in a 3GW constellation -- not because they are in a halo orbit, but because they are all part of the same virtual phased array antenna. Station keeping is also less necessary since all satellites are co-located and subject to the same purturbations -- simply electronically steering the phased array to compensate. Moreover what stationkeeping is required can be a lot more efficient in its use of reaction mass because of the large amount of energy available.

Its rather funny because I ran into a similar problem when putting through the first Ka band satellite license (Norstar for Norris Satellite Communications) back in 1992. The "orbital slots" argument came up without regard for the fact that Ka band is a lot higher frequency. Each new generation of technology presents legacy objections that are sometimes made moot by the new regime.

Rick, the Fresnel lens merely reduces the cost of terrestrial cells by reducing the amount of PV area. They don't really target mass the way weightless mirrors do.

As for the 'plumbing' of solar thermal, the main requirement for mass is in the high temperature and pressure focus. The moment the fluid leaves that chamber it is adiabatically cooled to the maximum degree possible to create a high velocity, low temperature, low pressure gas stream. Then you are dealing with power to mass ratios of turbines.

In any event, high flux PV can also be used at the mirror focus to achieve the mass economy that the Fresnel lens approach doesn't (unless there is a new approach to low mass Fresnel lenses with which I'm unfamiliar).

Rick said...

I'm certainly not committed to any particular technology! Mainly I'm rather enchanted that something I always regarded as jive may have some credibility to it.

agricola64 said...


how do you untwist that cable ?

something will have to rotate in relation to the other part - and thats by definition a gimbal (no matter if its on a rigid structure or a cable) and gimbals - especially large ones - always were a headache in space

as for launching the SPS in one piece - we could do that .. one word: SeaDragon 8-)

and your guesstimate on the mass of those "other parts" is a good as mine .. 8-) .. but i do notice that in the history of spaceflight such guesstimates have a long and distinguished history of being too small

agricola64 said...


thanks for that document .. i will have to work through that carefully .. lots of material for thought there ..

in regards to stationkeeping: i dont get to play with satellites 8-( ... and i based my question on years old memories of comments made by an Astra engineer (european TV sat company that maintains a fleet of tv sats in one orbital slot) who said that they have a small, but noticeable increase in fuel use due to the fact that they have to be much more careful in station keeping to avoid interference between their sats .

Jim Bowery said...

agricola64: The situation with a phased array cloud is very different. Interference is necessary -- its what makes the phased array work.

A brief calculation of the orbital slot requirement:

tan(.1degrees)*22kmi;140m/satellite;5MW/satellite ? GW
= 2.20694 GW

What this means is that for each tenth of a degree of geostationary orbit, you can fit a string of satellites, each with a 140m diameter mirror, that can produce 2GW on the ground.

This calculation is simplistic but you get the idea.

Rick said...

agricola64 - Untwisting the cable ... um, yeah, a gimbal will be required, but a much less massive one!

On SeaDragon, Jim Bowery had a link on his own blog to an article about the virtues of frequent launches. I'm not sure it can be reduced to a formula, but for a given annual tonnage to orbit, I suspect that frequent smaller launches will be cheaper than occasional big ones. It spreads out the development cost across more vehicles, and - even more important, I suspect - means that the launch team is actually doing its job on a daily basis, rather than spending most of their time training to do it a few times a year.

agricola64 said...


the gimbal for the cable might be lighter - otoh, so far nobody has managed to manage cables properly for longer periods in zero-g ..

yes - i knoe about "a rocket a day" .. and i agree with its premise to a point .. otoh making the payloads smaller and smaller will probably reduce launch cost, but will also increase the "overhead" mass (and associated costs)

and finally - who , who has seen the power and glory of a Saturn V, would not immediately fall in love with a SeaDragon launch .. can you already see the fleet of cruise ships at the safe perimeter 8-)

agricola64 said...

Jim: i follow your math ..

but having many small units flying around in close vincinity will carry a much higher collision risk then a few larger units ..

Jim Baerg said...

I still don't believe is solar power satellites to generate power for use on earth.

& go down the page a bit to the post by Kirk Sorenson at Nov 30, 2006 3:30 pm.

The basic problem is that microwave transmission from Clarke orbit means the antennas have to be several km across (for any power level), which kills the economics.

On earth intermittency & the low power density of solar greatly reduce the usefulness of solar power. In space solar is continuous and concentrating mirrors of aluminum foil can be really cheap & flimsy, unlike on earth where gravity & winds have to be dealt with.

So barring some other much cheaper power beaming tech, I think it will be solar power in space, & nuclear power on planetary surfaces.

Rick said...

I have no earthly idea (so to speak!) of how much rectennas cost - though surely much less than a BIG satellite at geosynch!

There is also cutting edge work being done on big, cheap storage batteries, which might make fully Earth-based solar competitive. (Biofuels are of course another way to store solar power!)

agricola64 said...

the space under a receiving wire rectenna is perfectly suited for a solar-thermal power station (of the mirror trough type) or a pv cell power station


Rick said...

True. I guess the question is how much the rectenna itself costs. Wire is cheap, but presumably a rectenna multiple km on a side has LOTS of wire.

agricola64 said...

does anyone has some data on rectenna design? how far away from each other do the wires have to be, etc ..

Jim Baerg said...

I don't know many details, but IINM putting the wires farther apart than the wavelength of the EM radiation will let a lot of it through rather than converting it to electricity.

Rick said...

Jim's point makes sense, so we're probably talking about rectenna wires on order of a foot apart (0.3 m). Which probably does not come to all that much wire, as wire goes - a few thousand wires, each a few km long. I imagine the equipment to 'clean up' the power and feed it into the grid will cost more than the rectenna itself does.

Anonymous said...

"does anyone has some data on rectenna design? how far away from each other do the wires have to be, etc ..
...don't know many details, but IINM putting the wires farther apart than the wavelength of the EM radiation will let a lot of it through rather than converting it to electricity."
Yes, to have an effective rectenna the spacing between elemens (the wires) must be no further apart than one wave length...a quarter wave length would be better, but one wave length will do. The effecency of the rectenna is a function of the conductivity and the 'coupling' (the amount of power the conductor absorbs), for example, an aluminum wire (a poor conductor) is set up in as a full wave rectenna (each wire is one wave length away from the next) would be more effective than a steel wire rectenna whose wires are just a bit over one wave length apart. So, the cost of the wire material, the size of the rectenna, and the frequency of the microwave beam would be some of the primary concerns in building a solar powersat system. Actually, since the microwave beam will be optimized to pass through the atmoshere with minimum loss, then the rectenna will be built to maximize coupling efficency to the beam. I guess we'll see how competent Solaren's engineers are...


Rick said...

Thanks for the fill-in!

To return to the start point, the cool thing is that we may well find out how good Solaren's engineers are, and whether they can build a credible tech demonstrator. To paraphrase a familiar saying, engineering talks, BS walks.

markus said...

ok ..

so lets say 30 cm wavelength and a quarter wave antenna - this gives us a wire every 7 cm and a (very substancial) 0.5 cm wire diameter ..

this means the power input to a solar power station sitting under the rectenna is reduced by 1/15 or about 7% by shadowing

if we can reduce the wire diameter (which i think is possible) to 0.25 cm the power loss will be 3.5%

7% power loss hurts ..

otoh - the land for either the rectenna or the solar thermal power station will essentially be "free" - you only have to purchase it once

.. you only need one connection to the electric grid, you need only one transformer station, only one administration and control building and you have to jump through the legal and regulatory hoops only once

i think the last point will be especially interesting to the company building the thing .. 8-)

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Paul D. said...

Power beaming with microwaves has the problem of the large scale needed due to the rather long wavelength. You end up with power plants in the gigawatt range. It's hard to do pilot projects.

This has led people to look at laser power beaming. The issues here are lower efficiency of producing the photons, and then of converting them back to electricity.

My suggestion: look at terrestrial applications of photons, so you can skip one step (conversion of laser light back to electricity) and so the terrestrial competition has to pay for the other step (conversion of electricity to photons).

An example is the photochemical process for the production of caprolactam, the precursor of Nylon-6. This process mixes cyclohexane and nitrosyl chloride, then uses light to dissociate the NOCl into NO and Cl radicals. The cyclohexane is converted to an oxime, which can then be converted to caprolactam by a Beckman rearrangement.

Another possible application would be pump radiation for the dye lasers for laser isotope separation.
The threshold for dissociation of NOCl is in the near IR, so any visible frequency laser should work (although the dissociation cross section is much higher at shorter wavelengths.)

leridan said...

I've discovered your blog a few weeks ago, by way of atomic rockets, and I'm going through all your posts from day one.

Somthing that strikes me is how pessimistic you are re launch costs.

SpaceX is already selling their falcon heavy for $100m, or $2,000/kg to leo. We can reasonably expect their prices to get cut in half when they start recovering their first stages, so we're looking at launch costs dropping below $1,000/kg before the end of this decade.

A great many unaffordable projects are going to suddenly become realistic.

agricola64 said...

i strongly suspect that the cost to LEO for F9H is already including stage recovery ..

Rick said...

Welcome to the discussion threads!

I wouldn't call myself so much a pessimist as conservative about costs. This is largely a reaction to the notorious history of cost lo-balling by space advocates, going back to von Braun himself.

If SpaceX can actually hit that cost target, more power to them! But Falcon Heavy hasn't actually flown yet, let alone entered regular commercial service. So it may turn out more expensive than they are hoping (or hyping).

agricola64 said...

yes - of course the proof of the pudding is in the eating ..

but - otoh & afaik - spacex is the only launching company that is actually willing to write a price per launch onto its homepage ... which speakes of a certain confidence

Rick said...

Fair point!

A cost of $1 million/ton is not out of line if all the 'ifs' work out, especially traffic demand. That has always been the frustrating circular bootstrapping problem.