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Space Power Technology

NSSO on Space Based Solar Power 198

apsmith writes "About a year ago some of the people at the US National Security Space Office began looking into space-based solar power (SBSP) as a technology in the near-term strategic interests of the United States. At first the participants were skeptical, and the "phase 0 study" went along with no official funding. In a rather innovative move, they organized the study as a series of internet-based (bulletin-board and email) discussions, with the wordpress site open to the public, and a closed experts-only discussion using Google Groups. Initially expecting only a dozen or so interested parties, the discussion grew to include over 170 people with past expertise and interest in the issues. The final report was released Wednesday morning; it provides an excellent broad-brush review of the status of SBSP, showing immense potential, but also a number of challenges that appear only surmountable with a strong government commitment to the project. The big question is where it goes from here — NASA? DARPA? The new ARPA-E? Or something new? I was able to attend the press conference, which included Buzz Aldrin in an announcement of a new alliance to push for implementing the recommendations of the report."
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NSSO on Space Based Solar Power

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  • by Goonie ( 8651 ) <robert DOT merkel AT benambra DOT org> on Wednesday October 10, 2007 @08:37PM (#20934635) Homepage
    There are several advantages space solar power has:
    • higher intensity sunlight than even a cloudless day, 24 hours a day
    • you've always got direct sunlight, so you can use cheap mirrors to focus the light on a very expensive but efficient solar cell (you can do this on Earth as well, but it doesn't work as soon as you get clouds)
    • No need for backup power. That's worth a lot of money.
    • The ground based gear is much smaller and lighter than equivalently-powerful terrestrial solar panels. This is a big advantage for the military, who are the proposed initial customers.

    I'm skeptical too, but it's not quite as crazy as it sounds.

  • by Jubedgy ( 319420 ) on Wednesday October 10, 2007 @08:49PM (#20934719)
    Portability and extensibility. The sun provides, about 1367 W/m^2 in space (courtesy, Space Mission Analysis and Design third edition, page 432) and about 250 W/m^2 on the Earth's surface (FTFA). In any case...


    By using an orbital energy collection system, you can simply re-route the beam to any place on the planet within the system's FOV...done right, you can get full 4*pi sr coverage of the Earth 24/7. Design a portable ground station, and you can provide power to a disaster area that has been removed from the rest of the power grid (paraphrased directly FTA).


    If, once in place and a standard orbital collection platform design has been established, more power is required, simply launch the spare unit. Proper formation flying techniques (something currently at about the cutting edge of orbital design) should allow the new unit to 'hook in' to the system to boost the amount of available power. This may be in the article, I have not finished reading it yet.

    The LISA mission provides a pretty good overview of how I see the entire system distributing power from the collectors to the emitters (the things that will transmit the power down to the surface), though I may be totally off base from what the authors have in mind. The LISA mission will consist of three satellites forming an equilateral triangle with leg lengths of 5 million Km shooting lasers at each other. Last time I checked, anyway.

    It is currently not economical, nor is it really achievable yet. I encourage you to at least browse through the article as it does discuss some of your questions in a more cogent manner than I have.
  • by Anonymous Coward on Wednesday October 10, 2007 @09:24PM (#20934981)
    Well, it may not be economical yet, but it is certainly worth investigating.

    First, such a plant is unlikely to use photovotaic cells at the space end. A good old fashioned steam turbine plant can give an efficiency of around 50-60 % for the initial conversion of solar heat to electricity. The hardest part of this is actually the radiator to get rid of the waste heat.

    Next, the conversion to microwave energy is pretty efficient...I don't have any up to date figures for magnetrons but something approaching 90% should be attainable.

    Now, the bit that always mystifies people....the path loss. Provided the frequencies chosen are in a suitable window with no atmospheric absorbtion, there is virtually no path loss. For those familiar with the usual sorts of path loss seen in radio systems, that is very counterintuitive. The reason for it is that the sending and especially the receiving antennae are both large enough that all the power sent is intercepted by the ground antennae. To put it another way, the antennae are operating in the near field. That means that the inverse square law does not apply.

    There will of course be resistive losses in both antennae, and power conversion losses in the ground equipment.

    Finally, just to deal with the "microwave death beam" worry. The size of the sending antennae determines how tight the beam delivered to the ground can be. It will be immediately obvious to a ground based scope if the system has been built with the capability of delivering a dangerous power intensity to the ground. Since in doing so it is radiating a microwave signal at high power, it is also effectively saying "here I am, shoot me down" such that a missile could home on it without needing to turn on its own radar. This would make it a single use weapon, which, considering the capital cost, would make such use unattractive.

  • by TripMaster Monkey ( 862126 ) on Wednesday October 10, 2007 @09:31PM (#20935041)
    Actually, carbon nanotubes ("buckytubes") are quite good conductors of electricity [azom.com].

    So that problem's solved...leaving only the original problem of manufacturing enough defect-free tubes in enough industrially-significant quantities to make the skyhook in the first place...
  • by QuantumG ( 50515 ) <qg@biodome.org> on Wednesday October 10, 2007 @09:51PM (#20935203) Homepage Journal

    The sun provides, about 1367 W/m^2 in space (courtesy, Space Mission Analysis and Design third edition, page 432) and about 250 W/m^2 on the Earth's surface (FTFA).
    Hmm.. the first number is correct, but we don't have 100% efficient solar panels and won't any time soon (if ever) so you've gotta down rate that.

    The second number, however is totally wrong. If you're going to talk about what "the sun provides", i.e., the theoretical 100% efficiency solar panel, then you get a figure of about 1000W/m^2 on the Earth's surface. You could say it is more like 800W/m^2 when you take cloud cover into consideration.. and then there's the fact that you only get that during daylight hours, so halve it to get 400W/m^2 but that's still a lot more than 250W/m^2. It *feels* like someone is downplaying the possible efficiency of solar panels on the Earth's surface vs the same solar panel in space in order to make their argument stronger. As you took that figure straight from the article, I'll give you the benefit of the doubt, but that's what it sounds like.

    It's still a heck of a lot of difference though. You're talking nearly 3.5 advantage to putting your solar panels in space over leaving them on the ground.. but there *are* losses to transmitting the power as microwaves through the atmosphere, and there is the astronomical cost of launching anything into space.

    Whenever I hear people talk about solar power satellites I'm reminded of the episode of Seinfeld where they stock the mail truck with bottles to collect the 5c deposit in the adjacent state. If you can get a free ride you might be able to make solar satellites work, but you've still gotta crunch a lot of numbers first, and no-one has done that successfully.

  • by melstav ( 174456 ) on Wednesday October 10, 2007 @11:03PM (#20935671)
    First off, putting them somewhere other than Earth orbit is silly - yes, you can get more energy from the Sun, but how do you transmit it to Earth? The microwave (or whatever) beam will also fall of with the square of the distance.

    Actually, no.

    Light intensity from the sun drops off at the square of the distance because the sun radiates as a point source in all directions.

    If you put your collector array closer to the sun, you collect significantly more sunlight. Then you use that energy to power a laser. If you can keep that beam tightly focused, you won't have much loss in the beam at all.
  • by CrimsonAvenger ( 580665 ) on Wednesday October 10, 2007 @11:36PM (#20935895)

    Putting them in equatorial geostationary orbits is *much* simpler. You'll lose a small amount of generating time each day (while the station is in Earth's shadow), but if you schedule as much of your maintenance as possible during this time, the effect is minimal.

    No. An equatorial orbit only goes into Earth's shadow during two short periods a year, near the Equinoxes. Off the cuff, I think the sats will be able to enter Earth's shadow once daily for about three weeks every Equinox.

    The rest of the time, the axial tilt of the Earth is enough to keep the sats in light 24 hours a day.

  • by alizard ( 107678 ) <alizard@NOspAM.ecis.com> on Wednesday October 10, 2007 @11:43PM (#20935943) Homepage
    meaning mainly, lower launch to orbit costs. Doing this not only gets us power, it gives us a platform for space industrialization.
  • by QuantumG ( 50515 ) <qg@biodome.org> on Thursday October 11, 2007 @12:13AM (#20936113) Homepage Journal
    Ya know, I think you should probably put down the crack pipe.

    It's space, dude.

    The orbit is 264,869 km around.

  • by AJWM ( 19027 ) on Thursday October 11, 2007 @01:41PM (#20942527) Homepage
    Brilliant idea!

    Thank you, but it's not original.

    Let's put the receivers and transmitters right next to the gigawatt microwave antenna! That will surely fix all those problems!

    Yep. Unless you're trying to watch satellite TV in the middle of a rectenna farm, of course -- although even there they'd likely have the transponders for that particular region mounted on a different satellite. It's not like everything on the powersat has to point in the same direction, you know, or operate at the same frequency.

If I have seen farther than others, it is because I was standing on the shoulders of giants. -- Isaac Newton