Space-Based Solar Power

a public discussion sponsored by the Space Frontier Foundation

Environmental Safety is Paramount

Posted by Coyote on June 28, 2007

The goal of space-based solar power is to provide clean energy whose collection and use is safer than the current carbon-based alternatives.  An interesting concern was raised by a person who blogged in response to the posting by my friend, Noah Shactman, in the “Danger Room.”  The blogger writes:

“If the conversion efficiency [at the rectenna on the ground] were 100%, you’d lose nothing to heat during the conversion. If it’s less than 100%, there will be a net warming of the planet as a consequence to collecting this energy. It doesn’t matter if [the power beamed from space is] infrared or microwave.

But the real losses come from *using* that energy. Few tasks that we use electricity for do not produce waste heat somehow. This is heat energy that did not previously exist in our atmosphere. I’d be surprised if anything less than 99.9% of this energy did not ultimately get converted to heat on the ground.”

I think it is safe to assume that people will continue to *use* energy in the future, but we are trying to provide a safer, more environmentally friendly alternative energy source. What efficiencies do we need on the gound to prevent this from becoming a curse rather than a blessing?


12 Responses to “Environmental Safety is Paramount”

  1. spacepolicy said


    This is a global systems issue, that I believe needs to be studied. We have the time to conduct this research, and it should be pretty straight-forward to conduct.

    Yes, SBSP will add energy into the Earth’s atmosphere in the form of waste heat. But the real questions are “Is the amount is significant?” and “Is this something to be worried about from a global warming perspective?”

    These systems question can be answered scientifically, and definitively. I can think of a couple approaches.

    It is quite possible that the amount of increased waste heat energy is small, and in the noise, compared to the change in the Earth’s heating from the random changes in the amount of energy that hits the Earth’s atmosphere from the Sun. The Sun does NOT beam power to Earth at a constant amount. It fluctuates. If the size of this fluctuation is orders of magnitude larger than the total waste heat (which is quite possible), then this additional waste heat will not be significant.

    Somebody here might even be able to supply us with a ROM order-of-magnitude on the variability in how much energy is hitting the Earth from the Sun. That would be useful to characterize whether this is a problem to be concerned about. If nobody supplies this, I suggest contacting somebody in NOAA, or NASA’s space weather center (which monitors and studies the Sun.)

    You could also look at this from a cost-benefit analysis. On one side, you add in the “waste heat” from outside the system (as the cost). On the other side, you don’t add carbon into the atmosphere (that is the benefit). QUESTION: Does the reduction in green house gasses in the atmosphere, which creates a reduction in thermal retention, overcome the increase in waste heat?

    This question that can (and should) be answered analytically and scientifically. We have invested huge amounts in environmental modelling. Those models could address this question. There will be error bars, and uncertainty, but this is better than uninformed public debate.

    Since we have time (we are not proposing to make a national decision to move all our power generation off planet for some time) then we have time to answer these scientific questions.

    – Charles

  2. shubber said

    What efficiencies do we need on the gound to prevent this from becoming a curse rather than a blessing?

    Coyote, I believe that this is another argument to be put into the red herring category.

    Regardless of what source energy is extracted from – be it solar, geothermal, nuclear, fusion, petrochemical, tidal, etc. – at some point it is employed to do work and in doing so a certain percentage of that energy is transformed into “waste” heat energy. Remember the law of thermodynamics – that energy can neither be created nor destroyed, it simply changes form.

    As long as humans continue to have devices which do “work” – be it microprocessors, blenders, or transport vehicles, waste energy will be created. Some energy production methods DO create other potentially harmful byproducts, depending on your viewpoint, including C02, NO2, radioactive byproducts, etc. THOSE must be assessed for their social economic cost to do a true comparison. But SBSP proponents argue that these byproducts are a primary rationale for WHY SBSP is a better (cleaner) solution.

    Perhaps they are right.

    But the efficiency of conversion argument is really one of economics, because the efficiency metric can then be used as part of the determination of cost/kwh for production, which can then be compared to alternative (existing) forms of energy production.

  3. Raymond Neil Cox said

    All activities produce some waste heat, but typically the amount is unimportant compared to planet Earth’s heat budget. It is sort of like worrying that costal cities will flood if too many people spit in the ocean.
    In my opinion, the Earth will warm less than 1/2 degree c = 0.9 degrees f, as the result of 1/2 of all energy used on planet Earth coming from solar power satelites. Neil

  4. Des Emery said

    Oh, Coyote — I think I am out of the conversation because it’s turning to statistics, but then something comes up, and I’m back in. The Toronto Star of Saturday, June 30, this year, in the Ideas section, has a full page on “A New Breed Of Engineer” which features new approaches to Solar Power. If you want to go to and then look up the Ideas section, Green Solutions, you could read it there. I don’t know if this is a general idea, or specific future solutions, but it does seem to tie in with this discussion. If your computer doesn’t bring it up, perhaps you know of a news vendor who could supply the Saturday edition.

    Generally speaking, “The Market Place” supposedly ruling the economy is more often supplanted by two insidious upstarts, “User-Pay” and “What The Market Will Bear.” Those two do indeed bring to the top of the pyramid more billionaires and millionaires, but they also shove down more and more people to the bottom layers of the same pyramid, spreading it out over a wider area to support the lifestyles above them. That is the best reason to involve the government, not a political party, in as large a project as this. Real costs must be amortized in a realistic manner, but government intervention is required in an effort to avoid modern “robber barons” taking control for only “profit” motives. The ideal of science is the fulfillment of dreams.

  5. Lee Valentine said

    Two points. First,this is a quantitative question. The Earth intercepts 150,000 Terawatts of solar energy (and re radiates the same amount). We are hoping to provide maybe 50 Terawatts of electrical energy to the entire world by the end of the century. This is an amount so small that it will be devilishly difficult to discern its effect.
    Second, the SSP should yield less waste heat than any conceivable ground based alternative. SSP avoids the ~50% thermal conversion inefficiency and,if rectennas are sited close to demand,they avoid 10-20+% long distance powerline transmission losses, all of which show up as heat.

  6. Sam Dinkin said

    Right now we are using energy equivalent to about 1/10,000 of what’s intercepted from the Sun. Typical solar cells have an albedo that is lower than the rest of the Earth. That is, they’re dark. That increases environmental heating, but less than fossil fuels because without a solar cell, the Earth still intercepts the solar at a slightly higher albedo and no heat trapping gases are created. Further, locally generated electricity incurs all of the waste heat from inefficiency of generation.

    Space solar increases waste heat a little from the power broadcast and the work, but decreases it a little from when the satellite passes in front of the Earth and again with avoided fossil fuel greenhouse gases and local generation.

    Since 99.99% of our heat is coming from the Sun, we should (and do) care a lot more about waste methane and CO2 than waste heat and albedo. That said, we can increase the Earth’s albedo (concrete instead of asphalt roads, white instead of blue tarps, steam up the sky) by enough to offset greenhouse gases emitted. But it would require covering major parts of the Earth’s surface.

    Waste heat will become an important issue before long. In 400 years at present growth rates, our waste heat will equal 100% of the Sun’s rays and we will have to reflect all of the Sun’s radiation (or increase emission or burial of waste heat through exotic high temperature radiators) in order to keep the Earth’s temperature from rising substantially. Or emigrate!

  7. Uh Sam

    Look at Lee Valentines numbers. Today the total electrical power generation on the planet is about 7-8 terawatts. Lets make the conservative assumption that all of this is at carnot level efficiencies of a crappy 25%. That means that we are generating about 28-32 terawatts of heat that is radiated to space. Lets say the rest of that power generated is turned into heat just to be conservative. That means about 35-42 terawatts. Lets say that we bring the rest of Earth’s population up to American standards of energy consumption and that will increase to about 1000 terawatts. The incident radiation on the Earth is 150,000 terawatts. Therefore if we have a planetary civilization with our level of comfort that is 6.67 x 10^-6 of the total heat budget of the earth. That is less than the total heat flow from the interior of the earth to space.

  8. Mike Stewart said

    Unfortunately, a simple point which makes this whole subject moot has been overlooked.

    The parent states “This is heat energy that did not previously exist in our atmosphere. I’d be surprised if anything less than 99.9% of this energy did not ultimately get converted to heat on the ground.”

    Roughly speaking, 100% of all electrical power winds up as waste heat. This is regardless of the source of the electrical power, fossil or solar. In either case, extra atmospheric heat is created, of equal quantities.
    In the fossil fuel case, the energy was previously locked up chemically, underground. Over the process of burning the fuels for power, transmission, and finally end work, all of that energy becomes waste atmospheric heat, that was not there before.

    Hence the solar method does not actually create any more heat influx than the fossil fuels; the only difference is that the extra energy comes from outside the planet in the solar case, and deep inside in the fossil case. Either way, the heat influx is the same, even though the energy influx is different.

  9. Coyote said


    Great point! I struggled to explain that.

  10. Coyote,

    It is certainly true that beaming energy to earth increases our heat load. However, by utilizing solar power, whether surface or space, allows us to generate less energy from fossil fuels and thus less carbon dioxide.

    The rectenna for power conversion of microwaves does not need to be dense; in point of fact the energy density of the microwave beam must be very low to prevent damage to aircraft, birds, bats, etc. However, there is a saving grace. Rectennas are cheap, I can buy 10,000 diodes for $5, new in box. So the rectenna needs to be large. Unlike surface solar power, the rectenna can be made of mesh, nearly completely transparent in visible and UV wavelengths. This means that agriculture can be carried on beneath these large antennas. With the number of countries who subsidize farming in every way possible, it seems that this would provide another opportunity for subsidies, and perhaps get a nice return.

    Insolation at the surface works out to be less than 2 kw/m^2. At this power density, a microwave beam of just 5% of the solar load ( 100 w/m^2 ) would not need nearly the focus requirements of microwave power at high densities. A one megawatt beam would only require an active rectenna 100 m per side. A more reasonable power beam, of 10 gigawatts, would require an antenna only 10 kilometers on a side (yes, presuming square, etc). This is not a large area for a wheat farm in, say, Nebraska.

    It’s doable, which it seems is an important precursor question to, “Is it practical.”

  11. Coyote said

    Norman L Reitzel,

    I agree with you completely. Perhaps I left myself open for misinterpretation. I do not favor proposals of using mirrors to reflect greater intensity of white sunlight back into the atmosphere to boost the potential of ground-based solar power farms. Our concept thins the sunlight into microwave energy that is broadcast to large rectenna fields as you suggest.

    What did I say that confused you???


  12. Who has published a definitive Environmental Impact Assessment of extra planetary power beamed to Earth at terawatt scales over shorter and shorter intervals of time? Is such an EIA possible without massive new experimentation, massively cautious, beforehand? As always, the military have taken the lead on an energy technology that risks enormous environmental penalties once worked up to scale.

    I refer you to petroleum and atomic energy technologies, both of which military-industrialists developed by trivializing their environmental downsides until it was too late to do anything significantly remedial (runaway global warming and tripling natural radiation background counts; so far, so bad). Given this dismal record, you should tread lightly where angels fear to go, and carpet your chosen path with meticulous EIAs, preferably drafted by your worst detractors.

    Why not use this collected energy where it is captured, in orbital factories that would drop finished products, manufactured from space-mined raw materials, to Earth’s surface (thus its end-use transport would be virtually free)? This arrangement would minimize energy wastage via conversion and transmittal, and eliminate potential damage to the biosphere, the ozone layer, the Van Allen belt and whatever other subtle structures science reveals in the future, just prior to wrecking them permanently.

    If you are operating on the presumption that Earth’s atmosphere is transparent to your beams and cannot be disturbed by them, ever, I must wish you every possible failure in advance. If you are developing this technology as a beamed weapon, ditto. No ad hominem attack intended; merely twice burned, thrice shy. Your idea has an enormous negative bias to live down; it is your first responsibility to put it to rest, in all scientific honesty, before you proceed.

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