The Space Elevator and Solar Power Satellites (SPS)

For me, the “killer app” of Space Elevator Development has always been the economically viable deployment of Solar Power Satellites.  It promises unlimited, clean energy from the sun, captured by SPS and beamed to earth-based rectennas and from there, channeled into the earth’s power grids.  The problem has always been, as I understood it, getting these SPS into orbit cheaply enough so they could be economically viable, and this is still true.

But there’s a BIG difference between making SPS a commercially viable enterprise and making SPS able to generate the lion’s share of earth’s energy usage.  In my naiveté, I had hoped they would be one in the same, that we could replace most or all of our polluting energy sources with SPS.  Sadly, unless my calculations are way wrong, this turns out to be not the case.

Here’s the basic numbers (I’ll explain where I got them later on in the posting):

  • The performance numbers of SPS are projected to be on the order of 2kg/kW (Henson / URSI / Wikipedia)
  • A 200T ribbon (140T payload) can handle 125 trips per year (Edwards-Westling)
  • This equals a lift capacity of 17,500T per year, per ribbon
  • This equals ~8.75 GW per year, per ribbon
  • Planet-wide energy usage is project to increase ~.5TW per year (EIA)
  • This means it will take ~60 of these ribbons to handle just the INCREASE in planet-wide energy usage, not making a dent at all in currently generated power

I hope my numbers are wrong because they mean that, unless SPS efficiency can be increased by at least an order of magnitude (or two), a SE-SPS solution may be commercially viable, but will not seriously decrease the release of CO2 and other pollutants into our atmosphere.

Another interesting number I came up with is that a space-based solar power array sufficient to power the planet in the year 2030 would be approximately equal in area to the country of India and the rectennas would need to be approximately equal in area to the country of Italy.  Well, I thought it was interesting…

I invite everyone to try and prove my numbers wrong – I’ll be very happy if you can show that I’ve overstated the problem somewhere…

Where did I get my numbers?

I got the SPS performance number from a presentation by Keith Henson.  This presentation (PowerPoint format here, pdf format here) is quite interesting.  He presents the 2kg/kW number as an aside; his proposal is for a “direct drive” Space Elevator (as opposed to the Edwards-Westling “laser-drive” version).  I went to a couple of other sources to try and verify his number.  Wikipedia gives a range of .5kg – 10kg per kW.  A White paper by the International Union of Radio Science (URSI) gives a number of 1kg/kW for their proposal.  So 2kg/kW seemed reasonable to me.  Incidentally, I had blogged about his “direct-drive” proposal earlier, here.

The 140T / 125 trips per year number comes directly from the Edwards-Westling book, The Space Elevator.

The projected, yearly, planet-wide energy increase comes from the US Governments Energy Information Administration website.  This website contains all sort of fascinating information and I had a lot of fun poking around in it.

This information, in part, led me to the conclusion that only the profit-motive is strong enough to drive the development of the Space Elevator, not the ability to solve our global energy and/or pollution problems.  I presented this at the recently completed SESI conference and blogged about it here.

One final note; Keith Henson was recently interviewed by RUSirius.  You can find the podcast here.  It’s a long podcast; they actually begin talking about at 10:03 into it.  I strongly recommend you review Keith’s proposal before you listen to this rather rambling podcast – otherwise it may not make much sense to you.

(The picture is from – click on it for a larger version)

20 thoughts on “The Space Elevator and Solar Power Satellites (SPS)

  1. Andrew Price

    I think it unlikely that we would stop at 200T when the larger the SE the safer it is. If energy production were a priority then 10kT per trip would be more like it.

    Of course the surface area of the solar panels matters little — it would not block significant light from the earth on its rare occlusions of the sun.

    Rectenna area is something more of an issue, but since it is entirely possible to grow crops etc. beneath it, I can’t see it being a huge problem. Sea based rectenna might also be a possibility.

    All that said, regular such doses of reality are essential. 🙂

  2. Jasper

    Well, your numbers are correct…unfortunately.
    But we have alternatives!

    I am in favor of space solar thermal power reactors.
    The principle:
    A large parabolic mirror concentrates the solar energy on a block of graphite. The graphite will heat up. A (closed cycle) stirling engine converts the thermal ernergy into electricity with an efficiency of about 30% (about the same as the best solar cells). The waste heat is fed back to the mirror to be dissipated to deep space.
    If the mirror is large enough, the mass will be mostly determined by the mirror itself. With specific weights of less than 20g/m^2, a incoming flux of 1400W/m^2 and a conversion efficiency of about 30%, the specific energy can become 20kW/kg (or 0.05kg/kW).

    Another alternative can be to use this parabolic mirror to concentrate on a unhabited area on Earth. But I don’t have the numbers if the diffraction an mirror curvature precission provide feasible numbers for such a sollution.

  3. Ted Semon Post author

    Andy, you’re right of course, but it doesn’t change the meaning of this problem in my mind. We’re still many years away from building a 20T Space Elevator, let alone a 200T or 10KT one. I think by the time a 10KT one is built (if ever), we will have solved our energy and pollution situation, one way or the other…

    Jasper, I’ve not heard of this possible solution, so can’t really comment on it. But if you’re right, and the efficiency is increased by a factor of 40 (2kg/kW to .05 kg/kW), then two 140T (payload) SE’s would be able to haul up enough material to handle both the increase in the energy consumption rate plus starting to make a real dent in already existing capacity.

    Can you point me to a URL (or two) that has more details about this? Thanks…

  4. Jasper Bouwmeester

    A good starting point:

    The numbers they show are not so great, but they are based on ‘smaller’ systems based on current technology. With the use of nanotube reinforcements the reflective/radiative material can be very light. With decreased transportation costs the system can become very large, increasing efficiency.

    There are even more possibilities you can think off, like the use of flat mirrors to point to Earth. We can then locally increase the amount of sunlight by a factor 3 in an unhabited area. This is not immediatly dangerous to people and wildlife, but will make Earth based solar panels more efficient. The possibilities are endless and stops at our own creativity.
    Off course you must always ask yourself if it can compete with a fully Earth based sollution…

  5. Ted Semon Post author

    Jasper, thanks for the URL’s. I don’t have a subscription to sciencedirect, so am unable to view that aritcle. I did look at the NASA article and, as you say, “the numbers they show are not so great.” Their most efficient system shows 600W/kg, only a 20% improvement over the number I was quoting (500W/kg).

    Yes, future developments may alter this, and we certainly should keep working towards them, but I think my basic premise is still correct; the replacement of even just the energy delta between today and (pick a date) is just too massive a problem to be solved by SPS, at least with any technology we know of.

  6. Darnell Clayton

    Hey Ted!

    I read this article earlier, and I unfortunately have to agree with you.

    SPS may not bring about the energy we need, although I was wondering whether (theoretical) fusion power plant powered by Helium-3 on the Moon could.

    Since we would have to bring the stuff back to Earth anyways, a space elevator could serve as a useful tool to transport this stuff back to Earth (not to mention future metals from asteroids).

  7. Ted Semon Post author

    Hey Darnell!

    Oh yes, I still think a Space Elevator is a great idea (and a huge money maker for someone). But I was really disappointed that the SPS numbers just don’t make it. If they did, I think it would impart tremendous momentum to the project. Oh well.

    I don’t know much about fusion power (other than what it is) and still less about Helium-3 so can’t really comment on that. But I hope something works…

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  10. Catfish N. Cod

    Your numbers fail to account for the possibility of building the SPS’s, or components thereof, in places other than the Earth. Earth has the deepest energy well of any non-gaseous body in the Solar System — significantly deeper than any other body except Venus. Any economically competent space system should be designed to lift as little as possible from the homeworld.

    However, your analysis highlights the onworld:offworld mass ratios that would have to be achieved to make SPS a significant portion of Earth’s generating capacity… and they are impressive!

  11. Ted Semon Post author

    CNC – you are correct, I’m only considering stuff lifted from earth. I don’t think anything else is going to be feasible for a long time…

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  15. Royce

    I aggree with Jasper. The solution is solar thermal not solar photovoltaic. Solar thermal requires much less mass to orbit. Solar thermal satellites placed in LEO could be 75% smaller = 75% less mass to take up due to transmission distance reduction verse GEO concepts. They could also be serviced in orbit as needed.


  16. Hiro Hara

    Your calculation should be changed! The new concepts (I have two variations) will dramatically reduce the weight of SPS. Probably in the order of magnitude or two. When you published this in 2006, 2007, it was correct. We were more inclined to put SPS up in the sky from moon manufacturing plant. But the situation has been changing. Two possibilities.

    1. Thin Film Technology
    High performance thin film solar arrays has been reducing the cost of ground solar. Roll-up of these modules can be stretched in the space miles long for solar collection. The cost of solar films has come down and will continue to come down. On the ground, installation takes more cost than modules. Remember, SPS is inherently more efficient than ground, about ten times, because of its independence to the angle of the sun, weather, time of the day. The strength of sun.

    2. Foil type thin film mirrors and high efficiency central cells.
    Light weight thin film parabolic concentrator is the key.

    In space, there is no need for structural support. Only thing is that each rather large modules has to be able to avoid inter-celestial debris. Or it may be such that the thin film replacement and repair is going to be much simpler than repairing the structural body.

    It is becoming doable.

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  18. kornu hemanthkumar

    the transmition of energy from satellite to earth is done by microwaves , inspite of using microwaves we use any other electro magnetic waves what is the problem caused by that

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