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A Japanese paper on beaming energy from solar installation on the moon to earth via laser and microwave recently made the rounds in the media. Would this, or any space-based energy system cause a net increase of atmospheric temperatures? To this layperson, it would seem like any such plan would pump energy into the atmosphere that otherwise would not have been there. Would this be a real concern, or would the "clean" nature of the source offset it?

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Yes, since they put it on the Moon, it would increase the total energy or "heat" as you call it being deposited on Earth. By a small amount. 17 toe is about $7\times 10^{20}\,\mathrm{J}$. The total solar insolation on the Earth from the Sun in a year is about $5\times 10^{24}\,\mathrm{J}$. So the (incredible) 400 km wide swath of solar collectors on the Moon would add 0.01% energy to Earth's input.

That does not necessarily mean that the average temperature of the Earth would be increased, depending on what other activities it offsets and how the added energy is dissipated.

Note that nuclear energy also adds to the solar insolation.

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    $\begingroup$ Of course every form of fossil fuel power generation also adds heat to the atmosphere. Before burning coal it's chilling out under the ground. Post mining and burning you have the same amount of matter, but at a lower energy state (CO2, ash etc). The chemical energy unleashed by burning mostly ends up as heat. $\endgroup$ – Blake Walsh Nov 30 '14 at 5:44
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Yes. Any system beaming power from space to the earth will add heat to the earth. Most current plans call for solar power satellites ("powersats") located in geostationary orbit, about 36,000 km up. The sunlight they intercept would have otherwise missed earth, so the energy they beam down would not otherwise have been part of earth's heat balance.

If you are evaluating electrical generation systems, then the question becomes one of scale and comparison with other energy systems, both of the heat delivered and also the overall effect on the environment. In spite of the heat they add, powersats compare very favorably with other power generation systems.

I'll have to veer into speculation and opinions about energy policy, but I can pull in some specific backing details if they are requested. Since no powersat systems have been tested, everything about them is speculation to some degree.

As noted earlier, JAXA's beamed power demonstration is projected to have an end-to-end efficiency of only 22%, but researchers hope to achieve 80% at each end eventually. So somewhere between 20% and 50% of the microwave energy reaching earth will not generate electrical power but instead will create waste heat, primarily at the rectenna. The atmosphere is very transparent to the microwave frequencies being used, so very little of the energy will be lost directly to the air as it traverses the atmosphere.

This compares favorably with most power generation concepts currently in use. Coal, natural gas, and nuclear plants all generate more waste heat than electricity. A good reference is this: http://www.mpoweruk.com/energy_efficiency.htm

More importantly, the fossil-fueled plants also release carbon dioxide, a greenhouse gas that causes earth's atmosphere to retain heat rather than radiating it back into space.

So in decreasing order of environmental damage, you have coal (roughly 50% waste heat, plus CO2 release), natural gas (roughly 60% waste heat but less CO2), Nuclear (roughly 60% waste heat but zero CO2), Space Solar Power (20% - 50% waste heat and zero CO2), Wind (no waste heat) and terrestrial solar (converts roughly 25% of incoming sunlight to electricity, not heat). Hydropower is its own special case because it does not generate waste heat or CO2 in the conventional sense but does cause serious environmental damage through other mechanisms. Wind and terrestrial solar are great from a waste heat perspective but it will be impossible to meet our energy needs with just those in the coming decades. (That's digressing from your question and into energy policy. I can discuss further in comments or PM if you like.)

Summing up, a utility-scale powersat system would add less waste heat per megawatt-hour of electricity delivered to the grid than most current grid-scale generation systems and zero greenhouse gases. If we can overcome the engineering challenges and get the cost comparable to nuclear, then it would be the hands-down winner for "green" energy on large scales.

Solar Power Satellites "SPS" or "Powersats", remains an active area of research and development. Check out Wikipedia https://en.wikipedia.org/wiki/Space-based_solar_power, recent work by JAXA http://spectrum.ieee.org/green-tech/solar/how-japan-plans-to-build-an-orbital-solar-farm, and many other sources. The best configuration for a powersat system remains under evaluation and debate. Most proposed systems use microwaves to beam power from a powersat located in geostationary orbit. System proposals also exist which beam the power from a lower orbit, decreasing transmitter size and improving efficiency but with other system trade-offs.

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  • $\begingroup$ Excellent, detailed, and informed answer. I guess the thing I am driving at in my question is "Which would result in a greater net gain of average temperature on the planet? Unlocking rocks' energy (coal and nuclear) and releasing 50-60% into the atmosphere or beaming energy that would otherwise miss us down and releasing 20-50% of it into the atmosphere? Obviously tricky to answer given the unknowns. One big unknown I think would be "will it really offset terrestrial generation or would both continue"? Hard to answer, but thanks for the details! $\endgroup$ – Daniel Jun 6 '16 at 15:15
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In principle, where is that 'heat' coming from?

Any solar area that collects energy intercepts photons, which excite electrons, which get converted to microwaves to be beamed to earth. Incidental molecules that get excited by the beam vibrate and generate heat.

Each step along the way has efficiency losses.

All those photons would probably have hit earth anyway. Instead, heat is being radiated at each stage, in high orbit, before a small portion of the energy is lost as heat in the atmosphere.

So I would say, not even worth worrying about.

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    $\begingroup$ Those photons hitting the Moon would not have hit Earth anyway. $\endgroup$ – Mark Adler Jan 10 '14 at 16:24
  • $\begingroup$ I did not see the moon reference, in the question. I guess it was in the link. $\endgroup$ – geoffc Jan 10 '14 at 18:14
  • $\begingroup$ Um, read the question again. $\endgroup$ – Mark Adler Jan 10 '14 at 18:15
  • $\begingroup$ Mark, well gee I should have read more carefully then. :) $\endgroup$ – geoffc Jan 10 '14 at 18:32

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