Space-Based Solar Power

A solar panel reaps only a small portion of its potential due to night, weather, and seasons, simultaneously introducing intermittency so that massive storage is required to make solar power work at a large scale. A perennial proposition for surmounting these impediments is that we launch solar collectors into space—where the sun always shines, clouds are impossible, and the tilt of the Earth’s axis is irrelevant. On Earth, a flat panel inclined toward the south averages about 5 full-sun-equivalent hours per day for typical locations, which is about a factor of five worse than what could be expected in space. More importantly, the constancy of solar flux in space reduces the need for storage—especially over seasonal timescales. I love solar power. And I am connected to the space enterprise. Surely putting the two together really floats my boat, no? No.

I’ll take a break from writing about behavioral adaptations and get back to Do the Math roots with an evaluation of solar power from space and the giant hurdles such a scheme would face. On balance, I don’t expect to see this technology escape the realm of fantasy and find a place in our world. The expense and difficulty are incommensurate with the gains.

How Much Better is Space?

First, let’s understand the ground-based alternative well enough to know what space buys us. But in comparing ground-based solar to space-based solar, I will depart from what I think may be the most practical/economic path for ground-based solar. I do this because space-based solar adds so much expense and complexity that we gain a large margin for upping the expense and complexity on the ground as well.

For example, transmission of power from space-based solar installations would likely be by microwave link to the ground. If we’re talking about sending power 36,000 km from geosynchronous orbit, I presume we would not balk about transporting it a few thousand kilometers across the surface of the Earth. This allows us to put solar collectors in  hotspots, like the Desert Southwest of the U.S. or Northern Africa to supply Europe. A flat panel tilted south at latitude in the Mojave Desert of California would gather an annual average of 6.6 full-sun-equivalent hours per day across the year, varying from 5.2 to 7.4 across the months of the year, according to the NREL redbook study.

Next, surely we would allow our fancy ground-based panels to articulate and track the sun through the sky. One-axis tracking about a north-south axis tilted to the site latitude improves our Mojave site to an annual average of 9.1 hours per day, ranging from 6.3 to 11.2 throughout the year. A step up in complexity, two-axis tracking moves the yearly average to 9.4 hours per day, ranging from 6.8 to 12.0 hours. We only gain a few percent in going from one to two axes, because the one-axis tracker is always pointing within 23.5° of the direction to the sun, and the cosine projection of this angle is never less than 92%. In other words, it is useful to know that a simple one-axis tracker does almost as well as a more sophisticated two-axis tracker. Nonetheless, we will use the full-up two-axis performance against which to benchmark the space gain.

On a yearly basis, then, getting continuous 24-hour solar illumination beats the California desert by a factor of 2.6 averaged over the year, ranging from 2.0 in the summer to 3.5 in the winter. One of my points will be that launching into space is a heck of a lot of work and expense to gain a factor of three in exposure. It seems a good bet that it’s cheaper to build three times as many panels and stick them on the ground. It’s not rocket science.

For technical accuracy, we would also want to correct for the atmosphere, which takes a 21% hit for the energy available to a silicon photovoltaic (PV) on the ground vs. space, using the 1.5 airmass standard.  Even though the 1347 W/m² solar constant in space is 35% larger than that on the ground, much of the atmospheric absorption is at infrared wavelengths, where silicon PV is ineffective.  But taking the 21% hit into account, we’ll just put the space gain at a factor of three and call it close enough.

What follows can apply to straight-up PV panels as collectors, or to concentrated reflectors so that less photovoltaic material is used. Once we are comparing to two-axis tracking on the ground, concentration is on the table.

Orbital Options

Are we indeed dealing with 24 hours of exposure in space? A common run-of-the mill low-earth-orbit (LEO) satellite orbits at a height of about 500 km. At this height, the earth-hugging satellite spends almost half its time blocked from the Sun by the Earth. The actual number for that altitude is 38% of the time, or 15 hours per day of sun exposure. It is possible to arrange a nearly polar “sun synchronous” orbit that rides the sunrise/sunset line on Earth so that the satellite is always bathed in sunlight, with no eclipsing by Earth.

But any LEO satellite will sweep past the ground at over 7 km/s, appearing for only 2 minutes above a 30° elevation even for a direct overhead pass (and only about 6 minutes from horizon to horizon). What’s worse, this particular satellite in a sun-synchronous orbit will not frequently generate overhead passes at the same point on the Earth, which rotates underneath the orbit.

In short, solar installations in LEO could at best provide intermittent power to any given site—which is the main rationale for leaving the ground in the first place. Possibly an armada of smaller installations could zip by, each squirting out energy as it passes by. But besides being a colossal headache to coordinate, the sun-synchronous full-sun satellites would necessarily only pass over sites experiencing sunrise or sunset. You would get all your energy in two doses per day, which is not a very smooth packaging, and seems to defeat a primary advantage of space-based solar power in avoiding the need for storage.

Any serious talk of solar power in space is based on geosynchronous orbits. The period of a satellite around the Earth can be computed from Kepler’s Law relating the square of the period, T, to the cube of the semi-major axis, a: T² = 4π²a³/GM, where GM ≈ 3.98×1014 m³/s² is Newton’s gravitational constant times the mass of the Earth. For a 500 km-high orbit (a ≈ 6878 km), we get a 94 minute period. The period becomes a day at a ≈ 42.2 thousand kilometers, or about 6.6 Earth radii. For a standard-sized Earth globe, this is about a meter from the center of the globe, if you want to visualize the geometry.

A geosynchronous satellite indeed orbits the Earth, but the Earth rotates underneath it at like rate, so that a given location on Earth always has a sight-line to the satellite, which seems to hover in the sky near the celestial equator. It is for this reason that satellite receivers are often seen tilted to the south (in the northern hemisphere) to point at the perched platform.

Being so far from the Earth, the satellite rarely enters eclipse. When it does, the duration will be something like 70 minutes. But this only happens once per day during periods when the Sun is near the equatorial plane, within about ±22 days of the equinox, twice per year. In sum, we can expect shading about 0.7% of the time. Not too bad.

Power Transmission

Now here’s the tricky part. Getting the power back to the ground is non-trivial. We are accustomed to using copper wire for power transmission. For the space-Earth interconnect, we must resort to electromagnetic means. Most discussions of electromagnetic power transmission centers on lasers or microwaves. I’ll immediately dismiss lasers as impractical for this purpose, because clouds block transmission, because converting the power into electricity is not as direct/efficient as it can be for microwaves, and because generation of laser power tends to be inefficient (my laser pointer is about 2%, for instance, though one can do far better).

So let’s go microwave! For reasons that will become clear later, we want the highest frequency (shortest wavelength) we can get without losing too much in the atmosphere. Below is a plot generated from an interactive tool associated with the Caltech Submillimeter Observatory (where I had my first Mauna Kea observing experience). This plot corresponds to a dry sky with only 2.0 mm of precipitable water vapor. Even so, water takes its toll, absorbing/scattering the high-frequency radiation so that the fraction transmitted through the atmosphere is tiny. Only at frequencies of 100 GHz and below does the atmosphere become nearly transparent.

But if we have 25 mm of precipitable water (and thick clouds have far more than this), we get the following picture, which is already down to 75% transmission at 100 GHz. Our system is not entirely immune to clouds and weather.

But we will go with 100 GHz and see what this gets us. Note that even though microwave ovens use a much lower frequency of 2.45 GHz (λ = 122 mm), the same dielectric heating mechanism operates at 100 GHz (peaking around 10 GHz). In order to evade both water absorption and dielectric heating, we would have to drop the frequency to the radio regime.

At 100 GHz, the wavelength is about λ ≈ 3 mm. In order to transmit a microwave beam to the ground, one must contend with the diffractive nature of electromagnetic radiation. If we formed a perfectly collimated (parallel) beam of microwave energy from a dish in space with diameter Ds—where the ‘s’ subscript represents the space segment—we might naively anticipate the perfectly-formed beam to arrive at Earth still fitting in a tidy diameter Ds. But no. Diffraction imposes an angular spread of about λ/Ds radians, so that the beam spreads to a diameter at the ground, Dg ≈ /Ds, where r is the distance between transmitter and receiver (about 36,000 km in our case). We can rearrange this to say that the product of the diameters of the transmitter and receiver dishes must approximately equal the product of the propagation distance and the wavelength: DsDg ≈ 

So? Well, let’s first say that Ds and Dg are the same. In this case, we would require the diameter of each dish to be 330 m. These are gigantic, especially in space. Note also that really we need Dg = Ds + /Ds to account for the original extent of the beam before diffraction spreads it further. So really, the one on Earth would be 660 m across.

Launching a microwave dish this large should strike anyone as prohibitively difficult, so let’s scale back to a more imaginable Ds = 30 m (still quite impressive), in which case our ground-based receiver must be 3.6 km in diameter!

Now you can see why I wanted to keep the frequency high, rather than dipping into the radio, where dishes would need only get bigger in proportion to the wavelength.

Converting Back to Electrical Power

At microwave frequencies, it is straightforward to directly rectify the oscillating electric field into direct current at something like 85% efficiency. The generation of beamed microwave energy in space, the capture of the energy at the ground, then conversion to electrical current all take their toll, so that the end-to-end process may be expected to have something in the neighborhood of 50% efficiency.

Beam Safety and Consequences

I don’t worry too much about keeping the beam from veering off the collection region. There are clever, fail-safe schemes for ensuring proper alignment/pointing. According to the Wikipedia page on the topic, the recommended transmission strength would be 230 W/m² in the center of the beam. This is about a quarter the strength of full sunlight, and is thought to be a safe level through which aircraft and birds can fly.

At this level, our 3.6 km diameter collecting area would generate about 40 GWh of energy in a day, at an assumed reception/conversion efficiency of 70%. By comparison, a flat array of 15%-efficient PV panels occupying the same area in the Mojave Desert would generate about a fourth as much energy averaged over the year. So these beaming hotspots are not terribly more concentrated than what the sunlight provides already. Again, I find myself scratching my head as to why we should go to so much trouble.

Launch Costs

This brings us to the tremendous cost of launching stuff into space. Today’s cost for putting stuff into geosynchronous orbit is about $20,000 per kilogram of launched material. We have a history of hope and optimism that launch costs will plummet in the future. So far, that has not really happened, and rising energy prices are not going to help drive costs ever-lower.  Meanwhile, the U.S. space program appears to be scaling back.

In 1999, NASA initiated a $22 million study investigating the feasibility of space-based solar power. Among their conclusions was that launch costs would need to come down to $100–200 per kg to make space-based solar power economically competitive. It is hard to imagine accomplishing a factor-of-100 reduction in launch costs.

Let’s do our own quick analysis. A standard rooftop panel delivers about 10 W per kilogram of mass (slightly better than this, but I will stick to round numbers). Let’s say a light-weighted version for space achieves an impressive factor-of-100 improvement: same power for 1% the mass. This gives 1 kW/kg. I might be grossly over-optimistic in this estimate, but we’ll see where it takes us. Ignoring other infrastructure overhead (wiring, propulsion systems, structural support, microwave transmission antenna, communications, etc.), we end up with a launch cost of $40 per delivered Watt, accounting for 50% delivery efficiency—and this is just the launch cost. I’ll bet the space-qualified ultralight PV panels are not as cheap as the knock-about panels we put on our roofs for $2/W. So maybe the cost of the space hardware, launch of all systems, and build-out of expansive ground systems will cost upwards of $60/W—becoming $400/W if we don’t manage the 100× weight reduction per Watt, settling for 10× instead. Granted, the constant illumination provides a factor of three in favor of space, so we can give it a 3× discount for its full-time contribution. But still, compared to typical ground installation costs at $5/W, we find that the solar approach is at least four times more expensive. You can even throw in batteries in the ground system without exceeding the space cost, and all the reasons for going to space have melted away.

Energy Return on Energy Invested

My initial reaction to the notion of flinging solar panels in space was that the energy needed to launch panels to geosynchronous orbit might totally undermine the energy delivered by such a system. Let’s take a quick look with approximate numbers.

First, today’s silicon solar panels return their investment of energy after 3–4 years of deployment. Stick them in the sun for 30–40 years, and you have an EROEI of 10:1. Specially light-weighted space panels will likely require more energy to make per kilowatt, but will spend a much greater fraction of their time in space soaking up energy. Let’s just guess that the payback would be 5 years if the space panel were deployed on the ground. But in space, the panel works five times longer per day than the panels for which the 3–4 year payback is calculated. So let’s call it an even one year for manufacture payback in space. Panels in space will be subjected to a much harsher cosmic ray (and damaging debris) environment than those on the ground, so we should reduce the lifetime to, say, 20 years. Still, that’s a 20:1 EROEI for the manufacturing piece alone. But then there’s the launch.

A study of gross weight of rockets compared to payload delivered to geosynchronous orbit reveals a roughly 100:1 ratio. This intuitively makes sense to me given the logarithmic rocket equation: much of the fuel is spent lifting the fuel that must be spent to lift more fuel, etc. (see the appendix of the stranded resources post for my explanation of this).

There is a nice rule of thumb—highly approximate—that the embodied energy in products is about the same as that of the equivalent mass of gasoline, at about 40 MJ/kg. Aluminum production requires more, at 220 MJ/kg, but many materials are surprisingly close to this value (and fuel will be right on the mark). A rocket will use a lot of aluminum, but much more fuel. So we might go with a round number like 50 MJ per kg.

If I take my ultra-lightweight panel producing 1 kW/kg, I must launch 100 kg of rocket, at a cost of 5 GJ. A 1 kW panel will deliver 0.5 kW to the end-user, after transmission/conversion losses are considered. The 5 GJ launch price tag is then paid off in 107 seconds, or about one third of a year. Add the embodied energy of the other components in space and on the ground, and I could easily believe we get to a year payback—now bringing the total (manufacture plus launch) to two years and an EROEI around 10:1. If my 100× light-weighting proves to be unrealistic, and we can only realize a factor of ten improvement over our rooftop panels, the solar panel launch cost climbs to three years, so that adding other components results in perhaps a 4:1 EROEI.

In the end, the EROEI is not as prohibitive as I imagined: it’s not a net energy drain as I might have feared. But it’s not obviously better than conventional solar either.

In Summary

I sense that people have a tendency to think space is easy. We have lots of satellites, we’ve gone to the Moon (remember that?!), we used to have a space shuttle program, and we have seen many movies and television shows set in space. But space is a very challenging environment, and it is extremely costly and difficult to deliver things there. If you go to the Fed-Ex site to get delivery costs, you immediately get hung up on not knowing the postal-code for space. Once in space, failures cannot be serviced. The usual mitigation strategy is redundancy, adding weight and cost. A space-based solar power system might sound very cool and futuristic, and it may seem at first blush an obvious answer to intermittency, but this comes at a big cost. Among the possibly unanticipated challenges:

  • The gain over the a good location on the ground is only a factor of 3 (2.4× in summer, 4.2× in winter at 35° latitude).
  • It’s almost as hard to get energy back to the ground as it is to get the equipment into space in the first place.
  • The microwave link faces problems with transmission through the atmosphere, and also flirts with roasting ducks on the wing.
  • Diffraction of the downlink beam, together with energy density limits, means that very large areas of the ground still need to be dedicated to energy collection.

Traditional solar photovoltaics in good locations can accomplish much the same for much reduced cost, and with only a few times more land than the microwave link approach would demand. The installations will be serviceable and will last longer. Batteries seem an easier way to cover storage shortcomings than launching stuff to space. I did not even address solar thermal schemes in this post, which competes well with photovoltaics and can very naturally build in storage capability.

I am left puzzled as to why we would want to take a harder, more expensive road to solar power. I think it is just not intuitive to most how difficult and expensive space is. And perhaps they think it’s very futuristic and cool to push our power generation out to space: it fits the preferred narrative about where we’re going. I don’t know—I’m just guessing.

Astronomers frequently face this issue: should we build a telescope/observatory on the ground, or launch something into space? The prevailing wisdom is that if the science can be accomplished on the ground, then by golly you’d best do it that way. You’ll have the result sooner, at less expense, and with a greater chance of success. The lion’s share of astronomical advance is carried out from the ground. Space is reserved for those places where there is no other way. The atmosphere blocks many interesting wavelengths, creates turbulence that makes high-resolution imaging difficult, and produces variations in transmission that make it impossible to measure fluxes to high precision. The rotating Earth gets in the way of continuous observation of a single target for long periods. Some of the more exciting (an well-publicized) discoveries come from space missions, because these avenues are not generally available to us, increasing discovery potential.

Space-based solar power contains little intrinsic advantage that we can get “only from space.” It looks like a wash at best, and the astronomers would say “don’t bother.”

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137 thoughts on “Space-Based Solar Power

  1. On the other hand, NASA already has an $18 billion annual budget and wouldn’t it be nice to tap into some of that? 😉

    • Having spent much of my career trying to capture 0.001% of this flow, I can tell you that those buckets don’t fly freely. And again, we could take a fourth of the money needed for space solar and have the equivalent ground capacity. And we have the technology to do this today. Space-based solar would take a decade of development (at least) at great cost that I have not yet factored in. Money/willpower is part of the problem, but the solution is not to throw what money we can find into space for a low return.

      • Your 0.001% doesn’t include the cost of putting those retro-reflectors on the moon in the first place! Although my comment was meant mostly in jest, I do get the sense that serious scientists (Sam Ting?) have used that kind of reasoning–occasionally with success. And I see that another commenter has already made a similar argument.

      • A question raised by throwing money into space for a low return is looking at what Geosynchronous power stations compete with: communications satellites.

        If your going to use the limited number of geosynchronous transmitting ‘slots’ for something, use them for something that offers a good return on investment and can’t be replaced by terrestrial solutions.

        • “Geosynchronous power stations compete with: communications satellites.”

          They don’t.

          The reason for the slots is the uplink. In order to reuse the uplink frequencies, the satellites have to be far enough apart for the uplink microwave beams not to overlap. There is no reason a communication satellite could not be attached to a power sat.

  2. Great post Tom. I really appreciate your taking the time to build a ‘database’ of responses to the common refrain that “technology will save us”. In a sense these posts are both fascinating – finally a ‘first principles’ review of these options. But also tiring, as I’m sure others will chime in with more high-tech solutions to some of the problems you’ve outlined (space elevator!).

    I recently attended a presentation on the Fukushima disaster by David Novog, nuclear engineering professor at McMaster University. It was very instructive to learn about the multiple redundancies built into the plants and how they failed. In short, there was no single fault and no obvious remedy that they overlooked. The problem was how various paths to preventing the meltdown were closed or limited given the circumstances. Yet, the audience couldn’t help it and hounded the speaker with suggestions “why didn’t the roof have air vents”, “why didn’t they bring generators in by helicopter” etc. I don’t think it’s a bad thing, people are inherently optimistic and want to help solve our problems. Car won’t start? You don’t need to know how a battery works, just to replace the battery. But this energy problem we are facing, it seems like offering helpful suggestions (electric car!) just won’t cut it.

    • But it’s sort of like a game of whack-a-mole. It’s impossible to hit all the topics, and if it is in someone’s nature to be a techno-optimist, they will find ways to believe we’ll solve our problems and be fine. I can’t claim to hope they are wrong, and it should not be my mission to beat all ideas into submission. Then why do I keep swinging the club? Because I feel that the technofix mindset could be dangerous for our species. How ’bout we act like this is a real problem and shift attention to it like there’s no tomorrow. Waiting for the white night to gallop in just in time leaves me a little nervous.

      • Not only that, but I’m not sure that a white knight energy source would really be greeted as a savior. (I just wrote up a little thought experiment on this question: what might happen if a cheap and plentiful form of cold fusion were invented?)

        • I tried commenting over there, not sure if it worked. Suffice to say that any inventor worried about the regulatory capture by extractive interests of the US and the IP piracy of China should consider operation in some *other* First World country without its own oil industry. France, Germany, Sweden, Israel…

      • Tom,

        Here’s another way to look at technical optimism:

        Technical pessimism is an effective weapon for the status quo. If solutions are difficult and risky, why not “drill, baby drill”??

        I think it’s extremely useful for the cause of progress to show that the alternatives to oil and fossil fuels are actually better.

        The danger lies in waiting, and staying with the status quo, not in moving to new things.

        • I have a different take on techno-optimism/pessimism. I worry that techno-optimism leads to a sense of non-urgency that actually delays real effort. I perceive that techno-optimisim currently dominates our culture. “If it can be imagined, it can be done”—is the attitude. This attitude has not actually led to much progress in developing non-fossil solutions,and not for lack of warning/time. Jimmy Carter advocated serious attention to energy out of a position of pessimism more so than optimism in the 1970’s. He was derided for his well-meaning actions (sweater, solar panels on the White House roof). Had we embraced his recommendation that we take energy seriously, perhaps we would have completely revamped our energy infrastructure in the last three decades. Instead, our techno-optimistic nature said there was no real crunch: no need to prepare for a storm that will never come. We may come to rue that attitude.

          Meanwhile, starry-eyed pursuit of technically disadvantaged paths can be a colossal waste of effort, energy, and time. Let’s not do things just because they sound cool, or fit into some narrative of an imagined future. There are technologically feasible paths for transitioning from fossil fuels. They’re hard, and expensive. My main point is that we should not pick hard-squared and expensive-squared “solutions” just because enough people are optimistic that we can accomplish anything we set our minds to.

          Meanwhile, I feel like such the wet blanket in pointing out how hard things are. It takes guts and thick skin, in this culture.

          • Not only that, Tom, but I think the “space cadet” ideas are unnecessary because human civilization’s exergy efficiency is abysmally low (e.g., ~20% in Japan). In other words, we’re barely taking advantage of the exergy content of the fuels we’re using right now. It blows my mind that serious people the world over are coming up with all sorts of complicated and/or destructive ways (e.g., drilling in ANWR) of increasing energy supply without tackling the waste problem. (But if you’re in the business of selling oil/nat gas/coal, waste is good!)

            For example, laws in the U.S. make it impossible or extremely difficult for industrial facilities to generate electricity from waste heat and sell the surplus without going through the utility monopolies (which results in unfavorable prices for the producer and/or final consumer). This company in Louisiana abandoned plans for a 30 MW waste heat to electricity facility because the utility wouldn’t play ball, so now the heat is (presumably) being dumped into the environment instead of performing useful work for the economy. “Simply” revising the law would remove these destructive and artificial barriers to recapturing waste heat and help create a more efficient economy.

          • “Meanwhile, I feel like such the wet blanket in pointing out how hard things are. It takes guts and thick skin, in this culture.”

            Until very recently there was not a solution that passed the “do the math” physics. It’s still going to be a huge amount of very hard engineering, but at least the physics of beamed energy propulsion makes sense and the recent development of high efficiency lasers makes it thinkable.

            If you can find a hole in what I have been working on, please do. I just want to solve the energy problem with something that actually scales to the need.

          • Tom,

            I feel like we’re not really communicating here, and repeating ourselves. Nevertheless, I’ll try again.

            If you tell people that their future is dark, they will be skeptical. Fortunately, in this case they’re correct.

            Jimmy Carter saw the status quo as unsustainable, and was a techno-optimist (for good reason). He didn’t say “drill, baby, drill”, he said: let’s develop alternatives.

            Now, in good part due to his work, the alternatives are here: EVs (and their cousins HEVs, PHEVs and EREVs) work and are very affordable; wind, solar and nuclear also work and are very affordable.

            I think the problem of intermittency and storage of energy is your main concern: did you read my discussion of storage on your latest post on TOD? It’s here:

  3. Re $40/W v $5/W – I’m a little fuzzy here, probably due to a lack of coffee this a.m., … does the ground-based cost of $5/W account for the required storage systems to run at night?

    Does the $40/W orbital cost account for transmission lines from the ground receiver to the grid + transmission losses?

    • A typical PV installation these days runs about $5/W (or a little less), without batteries. It roughly doubles for batteries. The $40/W for space was just to launch the ultra-lightweighted collectors/panels. I upped this to $60/W to take care of all the other infrastructure, so sure—the ground transmission could be lumped into that as well. Then again, giant desert installations of PV on the ground will incur a similar cost (though higher because the demand is farther from the source).

      • According to you’re using 2000 prices, when they’ve fallen a fair bit since.

        “The average retail price of solar cells as monitored by the Solarbuzz group fell from $3.50/watt to $2.43/watt over the course of the year, and a decline to prices below $2.00/watt seems inevitable. For large-scale installations, prices below $1.00/watt are now common. In some locations, PV has reached grid parity, the cost at which it is competitive with coal or gas-fired generation”

        Though I expect the cost of solar+storage has not fallen so fast.

        • I’m basing my $5/W figure on recent residential installations, with panels themselves < $2/W. To the extent this is a high-ball estimate, it only enhances my main case.

          • You may want to use a high estimate to be conservative, but residential costs really aren’t appropriate.

            Industrial/commercial rooftop installlations are much cheaper, and account for many more MWs of capacity than residential.

  4. I was always under the impression that the MAIN advantage of space based solar was that the collection locations (earth based receivers) could be “close” to the locations using the power. i.e. for Europe to get all its power from the Sahara or the USA to get all its power from the Mojave you run into serious issues of electrical transmission issues. Using a proposal for Europe-Sahara and wiki notes that power loss at 10k km is around 25% and that you also need to install a super grid negating the ability to use solar for 100% replacement in remote locations were a super grid would be impractical. Add into the fact that the new super grid spanning 1000’s of miles are difficult to protect from terrorists and the advantage to solar isn’t solely efficiency.

    I’d also imagine that being able to add space based replacements piecemeal approach of one collection location at a time as ~100% replacement for current power would be a serious political advantage as after each new collection system is added power is instantly usable. Were as installation of a super grid, storage and ground based panels would require years of implementing without a visible benefit.

    • Thanks for pointing out these additional factors. So are people willing to pay at least four times more for these perks? Hint: people are not willing to pay twice as much for electricity right now to sweep fossil fuels out of electricity production, even though the perks of easing back on global warming are obvious.

      • Well politically it seems more reasonable becasue a single space based system once up would effectively operate like a power station does right now and be able to tap into the current power grid. You wouldn’t be building a system to power all the east coast for example but only a single power plant that can operate independently and commercially.

        I also doubt that the ground based system 4 times the cost if you consider the constant security that politicians would put on a single transmission system that runs 2000+ miles and provides power to tens of millions of people. As well security when u factor in a NEW transmission system and backup power (look at your post on pumped power how viable was that) how politically viable is that. When has any country spent the money and time say 10-20+ years to install something at that scale while incurring ZERO net gain in the mean time.

        Those 2 points are even more relevant when u look at Europe where they would need to get northern Africa to secure the transmission lines (and we all know how stable that region of the world is).

        I’m not saying that ground based solar isn’t a BETTER option look at the alternative power matrix you hit nuclear for politically viable option even though it seemed like a reasonable method. When you look at the fundamental organization of how the US power generation is set up, adding a single space based power station one at a time seems more viable (its the model currently used and ensures that the government isn’t the power company). I just find it unlikely that a super grid large enough to supply power to entire swatches of the usa would ever be deemed economically viable for any power company to implement simply due to the initial costs required. That is unless the government steps up and says the transmission and backup storage are fundamental national security issues a full ground based systems wont happen.

        • Speaking of security, I’m guessing it’s much easier to zap a satellite with a LASER or a missile than to fly to another country to bomb a power plant.

          • I’m not so sure. Hard to get *anything* into space, and a laser that could damage something at geosynchronous orbit (consider diffraction and atmospheric turbulence) is no snap of the fingers. By comparison, traveling across the Earth’s surface to deliver explosives is so 20th century. Low tech and easy by comparison.

        • Remember that space was at least 4x as expensive *with* an assumption of panels 100x lighter than the ones we use today *and* that such light-weighting wouldn’t notably lower the lifespan of the panels in the hostile space environment. (And it’s not just the panels, need the framework and transmission equipment too. Alternately, lower launch costs by 100.)

          You need a large degree of magic for space solar to become even vaguely competitive with concerns about batteries or having grid power in Chad. *Actual* space solar…

          • Space solar would almost surely use concentration, the DoD space solar study assumed this, so that a large part of the collection array is the equivalent of polished aluminum foil. Nor does a space collector have to withstand wind or rain loads, which does away with much of the structural mass required of roof panels. 1kW/kg seems more likely than not with this in mind.

            As others have pointed out, launch costs are $13k/kg to GEO with the Falcon 9.

      • Maybe crossing wires here, but could you remind us how it’s possible to rely on solar alone for electricity, given the Nation-Sized Battery problem? And doesn’t space-solar also suffer from that NSB issue? You can’t beam energy from space when the planet is between the collector and the Sun, right?

        • It’s still true that building a battery to power the U.S. at today’s level for a week is prohibitive. I’m still a fan of solar, because it can work extremely well. In conjunction with a substantial decrease in our energy demand, we don’t need to build the nation-sized battery I described. For example, the average American household uses 30 kWh/day of electricity. Supplying enough battery to power all homes for a week would be a serious challenge. But cut back to < 5 kWh/day, and the problem opens up.

          As for space being effected by Earth eclipse: in low-earth orbit this is a big problem. In geosynchronous, not so much—details in post.

          • My girlfriend and I used about 110 kWh in February. Equalling 4 kWh/day for a two-person- and 8 kWh/day for a four-person-household.

            I’d consider our standard of living to be about the same as the average german household (electric: dishwasher, washing machine and dryer, stove and oven, microwave oven,…). Except we don’t have a TV, but our two laptops all day long.

            Natural gas for heating, AC is not really required for our climate.

            If we’d use the washing line instead of the dryer, we’d probably get down to about 2.5-3 kWh/day.

            Saving electricity can be done NOW and it saves money…

          • You can also substitute additional generation for batteries. If, instead of sizing your generation for annual average production, you sized it to provide enough power for high demand on a cloudy day, you’d only ever need enough batteries to last overnight.

            Which option is better is an economic one. Is it cheaper to double, triple, quadruple the number of panels you’ve got, or to add enough batteries to last two, three, several days with no input?

            I don’t know…I’d have to do the math….



          • A valid and good point. Overbuilding on the panel end relieves much of storage concern. But the worst days deliver maybe 10% of the average day, so you’re looking at a 10× overbuild, which is likely prohibitive. And no matter how much overbuilding you do, you still need an overnight storage solution—as you say, which is approximately a tenth the size of a full-scale battery that could last a week.

          • Fair enough, but introducing reduced consumption in response to storage viability questions is changing the goal posts in a comparison between space based and ground based solar power.

        • NSB via lead acid was prohibitive due to lack of lead. A comment noted sodium-sulfur materials are a lot more abundant.

          Iron phosphate would probably work too. OTOH this doesn’t consider cost. OTOH again, if we combine a grid and overbuilding of solar input with 16 hours of storage, numbers like $1 trillion a year become $100 billion a year, at least on the battery side.
          Also while here Tom looked at PV, there’s solar thermal, with easy storage though more sensitivity to clouds.

      • “Hint: people are not willing to pay twice as much for electricity . . .”

        That’s why my models start with the assumption that SBSP has to be half he price of coal, currently the low cost leader.

        Turns out to be hard to get started, but once over the investment hump it’s easy. At least when considering the physics of the rocket equation . . . .

    • Part of the problem with this approach is that the best locations for space-based power are also the best locations for comms satellites.

      A comms sat over the middle of the Atlantic has little demand, equally a power-sat over the middle fo the Atlantic is sub-optimal. Whereas, a sat over Europe is gravy.

      • “A comms sat over the middle of the Atlantic has little demand”

        I don’t know now with all the optical fiber but it used to be the highest demand. But power sats and communication sats are compatible, the power sats providing station keeping and unlimited power for attached com sats.

  5. I view this topic from another angle. We have things to gain from going into space other than power (science, exploration, and perhaps ultimately colonisation) – so we should find a way to do it that won’t impose a big net drain on our resources/industrial capacity back on Earth.

    As you point out, space based solar has a positive EROEI, even by your calculations which assume that material has to be launched from the Earth and can’t be retrieved from the Moon or an asteroid via some sort of catapult/tether/elevator system which isn’t as wasteful as chemical rockets.

    So long as you can make net energy from putting solar panels in space, you can do that and then using the launch capacity you’ve created for other things, expand our presence in space, so long as this doesn’t push your EROEI too low. The sheer scale of space operations required to implement space based solar would mean that even ambitious exploration projects tagged onto it shouldn’t have too much of an impact.

    This isn’t going to solve any of our problems here on Earth; but I think it does actually form an argument for *not* abandoning space travel in order to address our terrestrial problems; in the very long term we would want to move out into space as a species, and what you’ve done here shows it may be possible to lay the foundations for that in a way that can be energetically self sufficient, and some.

    • See the post called Why Not Space for why I think fantasies of colonizing space are a waste of our time—especially given near-term (this century) challenges.

      • I read that, but I don’t think it applies. I’m not saying that space exploration will solve any problems on Earth in the near term, merely that it can be done without making them worse.

        A society that has a division of labour is capable of tackling multiple challenges at once, and I don’t think that having some people focus on space exploration is going to necessarily prevent us from overhauling our energy infrastructure and reforming our society to be less energy intensive.

        Different fields of technological endeavour rarely impede each other, and the often help each other by the cross-pollination of ideas. I think it would be damaging in the long term if every qualified scientist and engineer suddenly downed tools to come and work on one (admittedly very serious) problem.

  6. While I share your skepticism of space solar, I will mention that launch costs may be on a dramatic downward trajectory. SpaceX is saying they’ll charge $1800/kg to LEO with the Falcon Heavy, about an order of magnitude improvement over the Delta. How they’re achieving that is described here:

    That’s with an expendable rocket, and Musk says the fuel is less than one percent of they cost. They’re working on full reusability, which would drop the price a lot more, depending on how often the can reuse the rocket.

      • True. The (more expensive) Falcon 9 gets 10,500 kg to LEO and 4,500 kg to geosynchronous, same launch price either way:

        So roughly a factor of two difference, which is still not far off.

        On the other hand, Kirk Sorenson (who’s advocating LFTR) has said in presentations that when he was at NASA, they analyzed the economics of solar power satellites and couldn’t make it work with a launch price of zero.

        • I read Sorensen’s critique; can’t remember offhand which of the technical problems were most intractable. Do you need some way of dumping heat from the satellite, or is that only if you go solar thermal? Brian Wang at NextBigFuture has an interesting post on the nuclear cannon, which would blast a multi thousand ton payload into orbit a few orders of magnitude cheaper than a rocket, and without the fallout problems of Project Orion ( the shaft collapses behind the launch vehicle, trapping the radiation) but the cargo would have to be designed to take a few hundred Gees. Nuclear plants near demand avoid the transmission problems of both space and desert solar, and the storage required by the latter.

    • I recall $10,000/kg to LEO for US or EU launch, $3000/kg for Russian. So SpaceX wouldn’t be that huge an improvement on the bargain bin. OTOH, maybe Tom should redo with Russian launch costs.

  7. I was going to say we could re-use the Arecibo dish to collect (since you said we would not be in Radio bands) but it turns out that is only 305m across and you wanted a 3500m diameter. Aww shucks.

    Thanks for the part about the microwave beam not cooking people. I always thought that would be a problem/risk but no one ever could explain that well enough.

    Also with the money that this would cost, we could probably install 2 square meters on the roof of every single family home, plus the device to push it back on the grid. This would load balance the energy over the Distribution grid and not have to worry about the Transmission loss over long distances. Of course this assumes that business are on the same Distribution grid as the homes. But in my visions, there are not gotchas 😉

    • Hold on there, buckeroos! What if we sent down the power with targeted LASERS then collected it with a giant PV array…oh wait…nevermind.

    • If we ignored safety considerations (ie re-routed aircraft, and just cooked the birds who strayed through), what is the maximum power density we could transmit?

      Does this significantly improve the numbers?

      (Although, if you made the beam steerable… nah, that would probably breach several treaties regarding weapons in space.)

    • That was suppose to be 2 square meters of PV on every single family home. Or some calculated value, does not have to be 2.

      I always wondered the potential for all the sun facing roof tops and some kind of CIGS based (or something cheap to make and lighter that pure Si) shingles, over the entire sun facing side. Sure they might only get 10-12% efficiency, but they are covering a large area. $ per square meter is the driving force. You would also have to run multiple serial zones in parallel so that one bad shingle does not take out the entire roof system.

  8. Second tier Chinese silicon PV manufacturers have been offering product at well below $1/W for months. The US response? Not purchase and installation on any real scale. No – we file WTO complaints. This country has no real official interest in mass deployment of renewable energy – all programs and plans to date are just eyewash. Tom repeatedly points out why – RE is simply not a substitute for fossil energy at the scale required to support civilization that evolved at the scale possible with fossil energy density.

  9. [shortened by moderator]

    The big advantage you don’t mention to space solar power is what you do with the land under those receivers.
    Solar panels are opaque. Big solid things. You can’t do much under a solar panel.
    Rectenna arrays are not; the old press material always has them placed overtop farmland near cities.
    That’s a pretty big advantage in land use costs, I should think. Worth it? Not at current launch costs. Real estate doesn’t cost that much, yet. But launch costs are going down– and if you cut the cost to LEO in half, look at the rocket equation. You should lose MORE than half in your cost to GEO.

    • Lowest end of cheap US land seems to be 5 cents a square meter. ($200/acre.) Granted, that probably includes having to build your own road, grid connection, and water supply for workers. The solar power itself is probably costing $100+ per square meter.

      Tom: I guess other considerations would be what’s the cost of one or two axis tracking, and low vs. high efficiency solar panels. I’d guess cheap rooftop is using 10-15%, satellites using 30% if they can.

      • Actually, Damien, the best metric for choosing SSP photovoltaics is to consider the Kw/Kg not raw efficiency. The cost of space transportation is a major factor in that choice of metric. There is thin-film PV, amorphous silicon, now available that is only 10% efficient but can deliver 16.8 kw/kg – that doesn’t include the rest of the satellite, just the power collection arrays. CIGS and other PV technologies may improve on that but not so far.

  10. The plans (dreams) for space-based solar include space-based manufacture with space-based materials, and bootstrapping: one plant is launched from Earth and supplies power to manufacture the rest. That largely eliminates the launch-cost objection. But the technological hurdles surely mean that we’re not talking about beginning to scale to global demand in anything short of many decades. Perhaps a separate analysis of the energy return with space-based (lunar or asteroid) materials would be informative.

  11. cheap rooftop 10 W/kg, $2/W
    space 30 W/kg (higher efficiency but expensive panels x2, 24 hours x3, transmission losses x 1/2), SpaceX GEO launch $4000/kg, $120/W

    Assume batteries and $5/W, so $7/W rooftop
    Assume 10x weight/launch reduction; $12/W. Or maybe $16+/W, since we didn’t count any cost for the space panels themselves while their cost was being dwarfed by the launch costs.
    Assuming 100x reduction: naive cost of $1.20/W hits the price floor of actually making the panels, at least $2/W.

    So you can make assumptions that make space power competitive, but you need a lot of assumptions: that SpaceX launch costs pan out and scale (and even if you believe this, you need to wait for them to actually do it), that weight/launch can be reduced by a further factor of 100, that such improvements will not also lower the cost of ground solar (plausible, if the latter is floored by batteries/storage.) If things go wrong only to the extent of getting a weight reduction of only 50x, you’ve already lost.

    This is not a good bet.

  12. If a satellite for GEOS and receiver for it are already going to be built, and the EROI for launching a solar panels is > 8 maybe it might be worth while to alter the receiver so that it also can collect microwave energy from space based solar panels.

    • This does not work. Receiving a signal for communications is a far cry from collecting all the energy. For communications, the whole Earth is bathed in a broad-ish beam signal. For solar transmission, you would not be happy with the 0.000001% efficiency resulting. EROEI would tank miserably. No piggyback options here.

  13. As a whole, humanity needs to figure out how to make carbon graphine or nanotubes tens of thousands of km long… Then, just maybe, it might be possible to create a space based race without having to deal with the rocket equation.

    Gallium Arsenide is what NASA used. It has dealt with the harsh environment (since the 70’s?) but is also rare, hence the absolute requirement of GaAs thinfilms, two axis tracking and cheap reflector optics, which should extend capacity at least a thousand fold. It is also almost twice as efficient than silicon.

    Combined, could create some very profound spin-offs. Perhaps in atomically thin carbon graphine building, electrical, and other types of materials still stranger than fiction.

    • I just wanted to edit and add that I just read “stranded resources”. Now I know that we can’t eliminate rocket equations…Thanks a lot (with bit of sarcasm) 🙂
      Perhaps we could cut “some” of the rocket (fuel) equations with a space elevator, especially, if we (I mean our descendants) could build it long enough to become a sling.
      That leads to a next thought… could an “Earth crosser” be given enough “spin” to do the same trick for many smaller pieces of itself? Possibly, there is a way, then to build a “Geo infrastructure”.

      But then again, matching velocities to GEO would seem to require almost perfect coincidences in order to not have to expend more energy or materials than its worth. And, wouldn’t the sling just roll up on itself (being too small, perhaps)?

      Thanks again for doing the math!

  14. Once again an impressive Post.

    One question that allways comes to me thinking about Energysolutions for our future is the question on how many resources would we need to build this thing.

    So thats my question. I see, pv on the ground seem to be a far better choice. But given the idea we would never the less build this thing up in space.. Do we have the resources to meet our demand ?


  15. Launch costs are a bit of a red herring. Granted, in the short-term, i.e. the next couple decades, they are an important consideration. But much of the cost of launching stuff into space is due to the fragility of the stuff being launched (people, electronic equipment). If you are launching bulk materials (e.g., sand, elemental silicon), much cheaper methods are feasible (e.g., methane cannons). I’m very glad you included the EROEI analysis, that gives a much better estimate of the limits to cost savings space-based solar power can provide.

    Gerard O’Neill included an interesting EROEI analysis for his solar power scheme. Starting around 1980, he figured that it would take until around 2005 before the program would start producing more energy than it used, and we would just now be getting a net benefit for all the energy invested.

    Once you start mining materials in outer space and manufacturing goods there, the costs should go way down, especially since the energy would not need transmitted back to Earth. That, however, is a solution for next century at best; I wholeheartedly agree that we will need to solve our Peak Oil problems by other means.

    • Right, and we should be adult enough to acknowledge that a space future is not written in stone. So there may never be an era of space mining. Notions based on the last few hundred years bear a huge bias due to the massive scale of cheap energy provided by fossil fuels. It’s anybody’s guess how we cope with the great phase change to come.

      • The EROEI analysis above only gives a relevant indication for Space Solar placed into orbit using conventional rocket technology though. Which, as is already established, is far too expensive for the purpose.

        Any solution that’s likely to meet the cost target is very unlikely to be running at anything like 100:1 launcher:payload mass; Quicklaunch and Startram for example (as two possible candidates for low-cost-to-orbit) will be FAR below this ratio. A space elevator would be lower still. Most efficient would be strapping the SPS to the back of a magic flying unicorn, but the three previous proposals are at least reasonably credible for their slightly increased energy use 😉

        IF we could get all the technologies in place to make SPS economic I think we’d find that the EROEI for the resulting solution was also EXTREMELY good. Though perhaps not for the first one (given probable fixed infrastructure costs…)

        • Many of the proposals sound like the unicorn idea to me. Easy to dream, hard to implement. If I saw the road ahead having continued energy surplus and unending growth, I might have more enthusiasm for them. But I’m not sure trends/economics are in favor of this.

        • Last time I checked, a space elevator required a material with the tensile strength about that of a carbon nanotube. The problem is that if you were to “weave” nanotubes into a “rope”, the tensile strength of the rope would be significantly lower than that of the individual nanotubes. Plus you have the additional problem of having to make a rope many thousands km long!
          So a space elevator doesn’t seem to be feasible in the near future…

    • OK, I will go along.

      So assume we build some kind of mining space craft that stays in LEO docked with a processing/manufacturing station, we rocket workers up to it. They “drive/fly” it out of orbit and rendezvous with asteroids and stuff, to mine them for Si, Al and Fe.

      We could even put the station at Earth/Moon L1 for ease in getting to both places and dock the mining craft at ISS.

      They bring the ore back to the processing station and turn that into PV cells and some kind of structural framework to attach them to. Then take them to GEO and construct the collector/transmitter device.

      Now we have to build the processing/manufacturing station and the mining craft. Get both of those to space. Power and supply them so that the human workers (I assume not robots) can survive on them.

      Perhaps you could take over the ISS and retask it when the world abandons it in 2020, but you still need to get a mining craft and the specific processing and manufacturing modules up there.

      Seems like a lot of work to begin manufacturing in space.

  16. Tom, that is a very good analysis of power satellites.

    The solution to the economic problem as the NASA study pointed out is to reduce the cost to GEO to $100-200 per kg. At $100/kg, an installed watt cost about $1.60 based on 5 kg/kW, $900/kW for the parts and $200/kW for the rectenna. That’s low enough to make synthetic fuels for a dollar a gallon on off peak power.

    The problem, as you point out, is the low payload fraction to GEO using conventional chemical rockets. You have to give up chemical rockets and go to heated hydrogen to get the the performance you need.

    But something that only recently occurred to me is that if you are building hundreds of GW of power satellites, then it’s easy to use the power from the first one for big propulsion lasers. I have run the calculation and staring with a Skylon (Reaction Engines’ design) derived vehicle which uses air and burns hydrogen till it runs out of air at 26 km and Mach 5.5 then uses a 2 GW of laser to heat hydrogen from there to orbital speed, will put 20 tons in GEO three times an hour or half a million tons per year. That’s enough to build 100 GW of new power sats per year.

    The energy payback time (a more usefuil measurement than EROEI for renewables) is about two months. Two previous versions of this scheme are on The Oil Drum. (Google henson oil drum)

    The problem is to get a seed propulsion laser to GEO. That’s expensive, the laser at $10/watt is $ 20 B and the transport to GEO using Falcon Heavy is about the same. However the value of this transport system is about $50 B a year and the gross margin is at least $25 B, so the payback time is under three years.

    If one part in ten was fed back into more transport capacity for a couple of years, the transport rate to GEO would go up to 5 million tons a year, enough for a TW of new power per year. With slightly more growth, the entire world could be off fossil fuels in less than a decade.

    If you want to see the spread sheets, ask.

    • I think skeptics will want to see the laser launch system working first, since that seems the crux of your plan. If that takes a $20B investment hump then you’ve got a problem, and convincing us probably won’t do you good anyway.

      • “and convincing us probably won’t do you good anyway.”

        That’s not the case. If the people who read this blog can find a fatal error I can quit working on it and save a lot of effort.

        If you can’t then maybe someone with money skills I don’t have can do it. Or maybe one of the big oil companies.

  17. well, that was fun! Didn’t expect you to come back to this.
    You realize that now you also have to do a post about 3He mining on the moon via a space elevator for fusion power, right? 😉
    For extra credits you can make the fusion cold or place the reactors in orbit and beam down the power via microwaves. Or both!

  18. Great post Tom, and great response above from Keith (I have spent a fair bit of time over the last months going over your posts on this topic at various other sites – a lot of work done in the field!).

    You both identify the same constraint (launch costs) but with two different attitudes. I don’t know who I agree with. Tom is certainly right today. Keith, you could be right in the future… but with the amount of time you’ve spent on this, how do you feel about the realistic 20 year roadmap to $100/kg->GEO? SpaceX is an incredible company and all, but they’re still putting all their efforts into developing relatively conventional rockets (including the awesome new work on reusable ones).

    Further, once you have ultracheap-to-orbit, you still need to build these enormous structures and have them be reliable/maintainable over many years. The usual story here is ‘cheap in scale’ – but is it really so simple? Space is a really benign environment in some ways for ultralarge structures (no atmosphere or gravity) but there are still micro-asteroids, solar storms etc. And there’s no way to build a small pilot plant that will adequately demonstrate the concept.

    On the bright side, the 100x reduction in solar cell weight is entirely feasible (and in fact already being demonstrated). Most of the mass in terrestrial solar ‘panels’ is not actually the cell, but rather the glass and panel that protect it and make it rigid. The silicon itself (if that’s what you chose to use) weighs very little. There was a group in Switzerland I found about 8 months back who are doing ultralight arrays (intended for space) and were speaking in terms of multiple kW/kg. Of course, now I can’t find them…. ahhh! Here we go, with links to the details:

    They’re talking 32kg for 50kW including all the deployment mechanisms. No mention of ensuring survival in space environment though.

    So… I’m not convinced it’s a totally dead idea that will never happen – the resource is certainly there, the result would be exactly what’s needed, and there are theoretically credible solutions for most of the problem.

    However, the gap between the theoretically credible solution and the actual implemented solution is so astronomically (haha) large that I can bring myself to bank on it as a solution in any reasonable timeframe… to say nothing of the numerous possible derailments (like finding we have to cover the whole thing with lasers to boost away micro-asteroids intent on shredding it).

    Given the potential benefits, I’d be pretty happy to have 1% of my tax going into space solar development (and I’d go work there… although only once my preferred idea fell through :-)). But banking on it as salvation? Not yet.

    • “how do you feel about the realistic 20 year roadmap to $100/kg->GEO?”

      Bad. If it takes that long it is probably not worth it. Discount economics will kill you on long term projects. On a crash basis, first launch to 2 GW of laser propulsion might be done in two years. That’s fast, but I think all the pieces are far enough developed or could be raised to that state quickly.

      “they’re still putting all their efforts into developing relatively conventional rockets” It is a consequence of the rocket equation that if you put up even one GW scale power satellite, it is worth more than a hundred times as much providing laser propulsion. So the biggest market for launch has the effect of totally changing the game, i.e., chemical rockets become totally obsolete.

      “were speaking in terms of multiple kW/kg” (PV) Have favored thermal cycles over PV in the past, but I want to solve the problem and if that will do it, I am all for it.

      “but there are still micro-asteroids, solar storms etc.”

      Micro-asteroids don’t seem to bother communication satellites over decades of time, solar storms are like getting hit with hard vacuum, there just isn’t any force to one.

      “And there’s no way to build a small pilot plant that will adequately demonstrate the concept.”

      I would argue that communication satellites have demonstrated the concept for decades. The power they deliver to the ground isn’t useful as power, but if you have ever set up a satellite antenna, it sure is measurable.

      And thanks very much for the link to the lightweight solar cells!

      • Ok – so you think there’s actually very good potential for $100/kg in 20 years… but that it’s far too slow. Could be worse – at least we get to use energy project discount rates, and of course we won’t be spending the money right now only to get returns in 20 years, rather delaying spending.

        It’s true what you say about communications sats… but on the other hand they’re not a direct equivalent in cost either! I’d hope the various concept proposals have investigated it though.

        Some of your ideas/proposals about thermal systems are really interesting too. Given the ease of creating enormous lightweight structures in space some sort of giant-mirror based concentration system could have totally different economics to an equivalent ground based system. Of course, there’s a shortage of rivers to dump waste heat into.

        Who is actively moving ahead in this field? Are Solaren or SpaceEnergy really making progress? Or is the perception that the launch tech needs to come first? Is there another market that could justify the development of low cost to orbit transport if it was only low cost in large volume?

        • “Some of your ideas/proposals about thermal systems”

          One of the problems people have not mentioned is light pressure. It’s not a problem on massive power satellites because the velocity build up over half a year is relatively small compared with orbital. But when you get to very light satellites, the reaction mass needed to counter light pressure can exceed the original mass of the satellite over ten years.

          50-60% efficient thermal power conversion cut the light pressure problem by up to 4 times.

          “a shortage of rivers to dump waste heat into”

          Definitely. Dr. Eric Dexler and I were concerned about this a long time ago in the context of keeping space colonies from cooking. Eric came up with the idea of using a pseudofluid made of a little gas and a lot of solids to get the heat out to radiators in the square km class. The paper was published in the 3rd Princeton Space Manufacturing conference.

          “Are Solaren or SpaceEnergy really making progress?”

          Last I heard, Solaren was still expecting to be funded. I have no talked to Peter Sage at SpaceEnergy for some time, but Feng Hsu (former high level NASA who works for them) has been quite active.

          “Or is the perception that the launch tech needs to come first?”

          That’s close to my perception. The low cost launch tech is like building a road into the jungle to mine a huge ore body. Nobody but he mining company is going to build the road. That’s especially true when you start using the first power sat for propulsion lasers.

          “Is there another market that could justify the development of low cost to orbit transport if it was only low cost in large volume?”

          I don’t know of one. I would be delighted if someone came up with something. I could make a case for 1000 workers at GEO, but not space tourism. The problem is that the market is essentially flat from today’s cost all the way down to about $100/kg where power sats make economic sense.

      • Ah hah! I see next big future has been posting links like crazy, including this latest one to an updated proposal for short term profitable solution by Al Globus. I’ll just go and read that for some answers to the above 🙂

        The link for others interested:

        Tom – I think it would be good to include links to this at the end of the post for those who want to do some further investigation; also to the recent IAA report on technology options etc (

        Recent and pretty comprehensive.

  19. ARPANSA’s (the Australian Radiation Protection and Nuclear Safety Agency) guidance on exposure to RF in the frequency range 6-300 GHz is a time averaged power flux density of 50 W/m2 for Occupational Exposure (occupational exposure is defined as exposure of a RF worker to RF fields when on duty, and is considered to be perfectly safe: albeit a more stringent standard is applied to exposure by the General Public).
    Full details (PDF) can be found here:

    Using ARPANSA’s guidance instead of Wikipedia’s, the impact is almost a factor of 5 (even if we round down on the assumption safety standards are relaxed, it’s still a factor or 4). Although, arguably you could just re-route aircraft, build off migratory flight paths and just fry the stray bird who flys through the beam.

    Another point to consider is that geosynchronous orbital ‘slots’ are a heavily utilised resource. Due to orbital considerations and to prevent mutual RF interference, there is a limited number of RF transmitting satellites that can be placed in geosynchronous orbit. These slots are used by communications satellites today (for much the same reasons that it is necessary to transmitting power). Which means from a economic point of view: Space-based solar faces the added hindrance of the opportunity cost of preventing the deployment of a comms sat.

  20. Borderline-irrelevant aside: a geosynchronous satellite doesn’t have an orbital period of 86 400 seconds (a mean solar day) but 86 164 seconds (23 hours, 56 minutes, 4 seconds – a sidereal day). We’re not overly interested in where the Sun is in our Earth-centred inertial frame of reference.

    • You forgot the .091 on the seconds! Yes—you are correct, but didn’t want to confuse the readers on a subtlety…

      • Fair enough, but perhaps it would have been better to just write “a day”, glossing over the different flavours, rather than write something which is not quite right.


    the Japanese Ikaros satellite, a solar sail, is producing power at about 0.8 kg/kw for the power production system. This extremely low value is accomplished by using a heliogyro thin film solar sail design, which has no structure, and thin film solar cells on part of the surface. Given reasonable improvements in thin film solar cells this figure could be 0.16 kg/kw for power production, not including other parts of the satellite.

    Towards an Early Pro table PowerSat

    Although purchasing one 12,500 kg per launch Falcon 9 costs $36.75 million, SpaceX representative Lauren Dreyer reports that packages of 1,000 launches can be purchased for $10 million apiece, which works out to $800/kg.

    Launch costs are coming down with the Spacex Heavy to $1000/kg. But volume purchases can bring that down to about $500/kg.

    Elon Musk is working to reusability to get launch costs to $10-100/kg.

    There could be cases based on the size of the space based power system where it makes more sense to use 60% efficient thermal heat engines instead of solar cells.

    Existing turbines are 10 kW/kg but can be far cheaper than solar cells.

  22. [shortened by moderator, in keeping with discussion policy]

    Photovoltaic panels in space would collect 9.6 times as much energy as the same panels on an average US site on earth. PV panels are almost never articulated in either one or two dimensions because the maintenance costs of doing so outweigh the tracking advantage, since the light scattering effect of our atmosphere makes the gain of sun tracking only about 20%. A full analysis of this comparison starts on page 17 of the Photovoltaics chapter at

    Your simple statement that “Launching a microwave dish this large should strike anyone as prohibitively difficult,” ignores the fact that large space structures such as the International Space Station can actually be and have been assembled in orbit.

    It is good that you are scratching the surface of SSP studies. I suggest you more closely examine the 24/7 deliverable power capability of SSP compared to ground level solar or even wind which are psudorandom and intermittent – not the sort of power you want for your grid. Wind power is available inversely proportional to when it is needed. Spring and fall are times of better wind availability, but power demand is at yearly minimum then. Hot summer days are mostly calm – no wind. What utilities are being forced to consider is large scale storage which is quite expensive and unnecessary for SSP, since it’s brief 70 minute outages are only at local midnight during the spring and fall equinoxes.
    When this storage is taken into account using the lowest cost option known -Compressed Air Energy storage, CAES, SSP DELIVERS about 71 times more baseload energy than the ground level solar or windmills. SSP does even better when you take into account the unreliability of our weather and also our weather forecasting.

    • I use the NREL 30-year database to tell me what a fixed panel or an articulated one will yield. And I corrected for the spectrum above and below atmosphere. I don’t see a factor of 3 error here. Your 9.6 times means 2.5 full-sun equivalent hours per day. Are you thinking Alaska?

      I never assumed single-launch for anything. A 30 meter microwave dish at 100 GHz is no joke, regardless of how many launches it takes to get the parts in place. Go bigger if you like, but the main point is that 99% of people who hear of space-based solar power are not aware of the gigantic transmitter/receiver sizes involved.

      • Space structures do not need the same support as ground based systems.

        You should look at some of the better proposals for space based solar power from Al Globus and Keith Henson.

        You will note that the Japanese Ikaros has already flown in space. The 0.8 kg per kw is already space proven.

      • Please actually read the referenced analysis, Tom, which is based on NREL data for the lower 48 and EPRI’s actual long term experimental analysis of PV installations in Texas.

        There is no hardware for 100 Ghz power transfer, besides which 100 Ghz loses too much during rain. Most notably Japan’s massive SSP project uses 5.8 Ghz Microwave. (They also have a laser initiative.)
        You really should study the available literature if you want to understand SSP.

        • Keep in mind that my ratio was not for PV as typically implemented now, but for ideal locations with tracking. The philosophy is that if you are going to the trouble of launching stuff into space, you can at least go to some trouble to maximize ground potential. In this way, I compare the best face of one to the best face of another.

          So yes, 100 GHz also struck me as high, but I was aiming for smaller antennas. Go to 16 times the wavelength and diffraction is 16 times worse, making antennas bigger. But I can see you might be forced by water in this direction.

          • Your assumptions for ground solar ignore the cost of maintaining and operating the tilting mechanisms, cleaning and washing the plates, and many other variables such as the losses due to heating in summer, which were noted in the EPRI studies. Most of these do not apply to SSP.

            Note Solarbuzz’s Cost of a (Ground) Solar Energy System –

            The cost for a 1 kilowatt peak system can range from US$8,000 to US$12,000 or €8,800 to €13,200 before sales tax and any government program assistance. Installation costs add another US$1,000-2,000 or €1,100-2,200. Assuming a 20 year life for the system, and including the cost of finance, this investment can equal a price in kilowatt hours of 30-40c/kwhr in sunny climates and 60-80c/kwhr in cloudy climates.

  23. “…I am left puzzled as to why we would want to take a harder, more expensive road to solar power.”

    Because it’s scalable and concentrateable to a degree that nothing else is. You could put up an area of mirrors and solar panels with a receiving area equivalent to a dozen earths and send that power down to ground antennas. Even with inefficiency like an energy return of 4:1, you come out ahead. Yes, this would be hugely expensive. How expensive is a planet of 9 billion with the power equivalent of the early to mid 1900s? Even with nuclear power, that’s where we’re headed.

    • Did you pick up on the part about ground receivers needing to be huge? You can throw a dozen-earth-equivalent array into space; you’ll find extreme difficulty getting the power back to the ground.

      • They certainly are huge, for 2.45 GHz and a 1 km transmitting antenna with a gausian cross section, it’s 10 by 14 km.

        However, the microwave intensity under the rectenna is much the same as what you get standing in front of a microwave oven, so the proposal is to put them over farmland since the structure blocks little light. I don’t know if the power companies should buy air rights over the farms or offer free power to the farmers who sign up.

        My biggest concern about these things is millions of birds roosting on them in the wintertime to stay warm.

        • If you phase aligned the beams from multiple stations then could you use a single ground receiver for multiple SSP’s?

          Possibly not I guess, with different origins of the beam some interference pattern would result and you’d only really get full benefit and points of reinforcement.

          On the other hand, the various rectenna/transmitter dimensions (and power densities) discussed are true for a given SPS size. They scale with separation and microwave transmitter size for a given efficiency and frequency… not for power. There would be nothing to stop you connecting 10x the space-side electrical capacity and sharing a common transmitter/receiver. The kW/m2 would increase by 10x as well, obviously, and probably become rather an issue for birdies, but otherwise only constraint is thermal limit – no issue on earth, possibly a constraint in space though.

          Another query – considering efficiency isn’t it the projected area of the Rectenna that matters? So if you’re at 45 degrees north (and considering a power sat in geostationary orbit) the 10km x 14km area mentioned above would actually need to be 1.414*10km x 14km?

          • If you have a larger antenna in space, you can shape the beam intensity to a “top hat.” That way the beam intensity is close to constant over the rectenna.

            On the size, if you have a ten km circular beam then at 45 deg the beam footprint will be an ellipse with the north south direction being 10 km x sqrt of 2.

  24. large-scale storage is required to make solar power work at a large scale.

    But, that storage could be very cheap. Daily variance can be handled by pumped storage and chemical batteries (or a myriad of other solutions, include molten salt).

    Seasonal variance can be done cheaply: Just use excess power during the summer to electrolyze seawater into hydrogen which would be stored very cheaply underground (like NG) and burnt in NG type turbines – either combined cycle for efficiency or very cheap peaker-style.

  25. A general comment to cheerleaders who anticipate radical (factor of up to 100) reduction in launch costs, given players like SpaceX, bringing the parity between space cost and ground cost in line:

    If you can do this, then also allow solar panel costs and other system component costs to make similar strides on similar timescales. Don’t hobble one while allowing the other to race ahead. The disparity between space costs and ground costs is not likely to disappear—even if we do get our wish of substantial price decreases (in both).

    • Synchrony isn’t an obvious expectation if the techs involved are different. PV prices have been crashing, but AFAIK solar thermal ones haven’t, and batteries haven’t. Going light-weight might just make ground PV more expensive for no benefit, but massively reduce the main cost in launching with today’s rockets, as that scales with mass[1]. And if one thinks there’s low-hanging fruit for lowering launch costs, which seems implausible looking at history but plausible just looking at the energetics, one isn’t committed to believing the same of PV and batteries.

      Your optimized Mojave set up was 6 hours equivalent at worst. Would it be fair to condense the problem as “cost of 4 panels, plus land, plus 16 hour storage” vs. “cost of 1 panel, plus transmission equipment, plus launch, plus receiver and land for receiver, plus GEO slot”? Marginal land is cheap so can probably be dropped as a cost. Currently launch costs dwarf everything else.

      [1] Then again, just making a satellite costs 4x what it does to launch it; space radiation and electronics don’t mix well. Are space solar panels really the same as ground ones?

    • There are market niches where space based solar makes sense. Remote bases etc…

      There are differences where space based solar does not need the supporting structure of steel and cement that a ground based solar farm does.

      The main use of space based solar or any space based power is to drive work and industry in space.

      The cost difference between LEO and GSO can be reduced for moving stuff if you have fuel depots in LEO and for space based solar they power their own engines to move up from LEO to GSO.

      Looking at space based solar to replace ground power all in one go is stupid. It is a matter of looking for applications and niches. Just as ground based solar has to find niches where the fact that it costs more than coal, natural gas, nuclear etc… does not matter.

    • Tom, my recent revelation, after working out how much laser power it took to transport half a million tons a year to GEO, is that the first power sats are worth more than a hundred time as propulsion lasers than they are for electric power.

      So if even one of them is built, it will cause a collapse in the cost of hauling parts to GEO, even lower than the $100/kg cost NASA mentioned. It’s a simple consequence of high exhaust velocity on the rocket equation.

    • Tom, as significant as the higher sunlight capture it, I think the reduction in mass is as important. I have estimated the mass of a two axis tracker that generates about a kW(avg). It’s at least 500 kg/kW. A power sat in space is going to require about 1% of the materials for one of equal output on the ground and that’s before you consider storage. Zero g and no wind really helps.

  26. Hi

    I have been waiting for others to do more analysis and I’ve read your other articles with enthusiasm, but find you make some gross generalisations (probably required for brevity).

    If we’re talking about sending power 36,000 km from geosynchronous orbit, I presume we would not balk about transporting it a few thousand kilometers across the surface of the Earth. I’d like to make some quick comments, but in general I feel you’ve made the same ‘proofs’ that the ballistic launch arguments used to prove why man could never survive escape velocity. We all know rockets changed that. My point is that analysis can seem to demonstrate anything as long as noone undertakes a critical analysis of the premise of the argument.

    I think your points at the beginning about atmospheric microwave absorption were interesting and I’ve never seen graphical presentation of those losses. I’d need to question the values, but on the surface of that it seems done and dusted. However I’d still like to mention:

    [On a yearly basis, then, getting continuous 24-hour solar illumination beats the California desert by a factor of 2.6 averaged over the year, ranging from 2.0 in the summer to 3.5 in the winter. One of my points will be that launching into space is a heck of a lot of work and expense to gain a factor of three in exposure. It seems a good bet that it’s cheaper to build three times as many panels and stick them on the ground. It’s not rocket science.]

    probably not but it could be good elevator science. The Japanese are in the process of developing it. Certainly its not a deliverable in the next 50 years.

    [but taking the 21% hit into account, we’ll just put the space gain at a factor of three and call it close enough.]

    that’s a big gain.

    [the diameter of each dish to be 330 m. These are gigantic, especially in space.]

    why? You should know that there is no ‘sag’ or other gravity effects on engineering structures.

    Thanks and keep up the good work

    • “If we’re talking about sending power 36,000 km from geosynchronous orbit, I presume we would not balk about transporting it a few thousand kilometers across the surface of the Earth.”

      The cost metric for power through wires is a penny per kWh per 1000 km. If you are going to sell power for two cent per kWh on the ground and there is a 50% loss from electricity to electricity then the power cost at the satellite has to be a penny a kWh and the loss for going 36,000 km cost a cent per kWh, same as it does to send the power 1000 km on wires.

  27. OK, I am convinced space based solar will never compete with renewable sources of baseload power for civilian use. But what about military use? The energy costs at forward operating bases for the military can be an order of magnitude more expensive than what a civilian pays for energy at home. In this situation SBSP might offer advantages over other alternatives.

    Rectennas are modular and their harvesting ability wouldn’t be completely gone if only a portion is destroyed.

    A rectenna and a base can both occupy the same geographical area.

    Rectennas don’t create large fuel depots that are targets for the enemy.

    Compared to other renewable base load energy options (wind, pv, concentrated solar all with batteries) a rectenna is is lighter and easier to deploy.

    The solar power satellite could be reaimed to a new base once you were done supplying energy to an old base.

    One solar power satellite could be designed to beam power to multiple bases simultaneously.

    I am probably missing tons of reasons why this isn’t a good idea for the military but I haven’t seen anyone point the flaws out yet.

    • Militaries already do an awful lot of things that solve military problems but are impractical for solving general societal problems. Supersonic flight being the prime example in the wild.

      Even if the military goes down the path of SBSP, Tom’s point (that it isn’t a better alternative than Terrestrial Solar for general civilian use) stands.

  28. I can just see the news stories that will come out of even having such a space solar program in place.

    “Satellites in space to generate electricity!”
    “NASA launching solar panels into space!”
    “Our energy problems finally solved!”

    Remember the “major breakthrough” in solar energy just when oil was trading at nearly $140 a barrel in 2008? Here:

    What became of it? Nothing. None of our energy problems are solved by this “revolution” in solar energy. There is one GUARANTEED way that we can solve our energy problems. And it has a success rate of 100%. It is called REDUCING energy consumption. Why do you think Tom post articles are turning off the pilot light? Because it is guaranteed to work.

    Any space related energy program will give people the hopium they need to continue driving their damn SUVs to work alone. Live in large single family houses outside of town. And put in backyard heated swimming pools.

    • Reduction puts things off, it’s not a total solution. Eventually you have to sustainably make the reduced level of energy you’re still using.

  29. “OK, I am convinced space based solar will never compete with renewable sources of baseload power for civilian use.”

    I think it will compete, even with coal.

    “But what about military use?”

    It might, but not using microwaves. For optical reason power sats just don’t scale to small sizes, at 2.45 GHz 5 GW is about as small as you want to go. Now lasers are a different matter, they can be scaled to MW sizes. But if you are going to big lasers, who needs bases?

  30. The discussion here follows a familiar template. To begin with, visionaries like Keith Henson have propose audacious megascale engineering projects incorporating many ingenious and non-obvious ideas.

    Then intelligent scientists like Tom Murphy, who specialize in other areas, apply first-order analysis to some of the more obviously outrageous aspects of these scenarios and easily show that they are wildly unreasonable.

    A chorus chimes in to scoff at the visionaries.

    But the visionaries have done their homework. They are ready, and they hit back with references to papers which have already addressed the obvious criticisms.

    The work of visionary scientists and engineers deserves careful critical scrutiny. Such scrutiny will often reveal elided points, errors, and unwarranted assumptions. But finding those takes time, first to actually read the visionaries’ work, and then to analyze and document the weaknesses. Often, one gets lost in the details, or finds that one cannot announce a definite conclusion.

    I tend to agree with Tom’s view — for the immediate future. I do not see how we could undertake such massive and high-risk projects as Keith Henson proposes, even if the costs may be estimated as comparable to those of invading Iraq or having another war with Iran. But I would not be so reflexively dismissive of these proposals on the basis of “math” that is done with insufficient attention to what has actually been proposed.

    • Thanks for the thoughtful reflection on this process. I picture you in a black and white striped shirt.

      I write my posts in a day (and change) on the weekend because it’s a “hobby” pursuit that has to coexist with my otherwise demanding job. So my approach is often first-cut analysis. Disadvantage: Tom.

      But still, I think it’s very useful to point out that solutions bandied about in the popular media are not as simple as they sound. It is absolutely worth pointing out that conventional rockets will not get us space-based solar power—that we need something more exotic first. I’m addressing the many people who don’t appreciate that it’s not as simple as rocket + panels + microwave dish.

      My analysis falls far short of definitive. But it exposes hardships that the average person would not likely intuit. That’s the service I try to provide: separating the hard from the very hard (sorry, the easy paths are no longer available, except the path of voluntary reduction).

      • [shortened by moderator]

        So there are plans proposed by Al Globus to achieve results with a few hundred million. There is already a megawatt(s) or so of space solar power for satellites in space. 200 kw or so on the space station.

        Japan is willing to spend a few billion on it. Planning 2 megawatt demo around 2020. 200 MW around 2025. Trying to get 1GW in 2030s.

        Al Globus is working out ways to get 6-30 MWe in one launch for about $20-100 million or so.

        How does this compare with nuclear fusion. Nuclear fusion has had decades of funding around the billion dollar per year level (all nations added up). No commercial power generation yet.

        I support nuclear fusion research and space based solar power research.

        Energy infrastructure is a multi-trillion per year effort.

        If you disqualify space based solar power because of it being 4 times more expensive based on a first order and wrong analysis then you should not have ground based solar because it is still over 4 times more expensive than coal and nuclear power.

        Get space based solar for expanded space based efforts and for on the ground niches and let it grow. Hopefully to the 500 MW level to potentially reduce costs of space launch per the Henson plan and other applications.

        Countries that did not fund developments of the Americas several hundred years ago missed out and were weaker than those that did.

        • “If you disqualify space based solar power because of it being 4 times more expensive ”

          It only got as low as being 4 times more expensive when 100:1 favorable assumptions were thrown in on one side. With actual tech we might be looking at 400 times more expensive.

          “Countries that did not fund developments of the Americas several hundred years ago missed out and were weaker than those that did.”

          Some countries that did fund developments of the Americas destroyed their economies and went bankrupt.

    • The so called visionaries are out there selling hopium every day. The idea that if we just implement their pet idea #5, then all of our energy problems will disappear. No need to change our way of life. After all, like Dick Cheney said, the American way of life is not negotiable. Right?

      Most Americans approach energy problems the same way that they approach weight loss. They want to continue eating big macs and french fries and bacon. If the scientists just develop a magic pill that makes me lose weight and feel great, they’ll be all set. Right? Hopium? Magic bullet solutions? Lose weight now! Make money fast!

      Any discussion of taking personal responsibility and reducing energy footprint is derided as “not thinking big”. That’s the role of visionaries. Selling magic bullet solutions to all of society’s problems.

  31. Tom has done a great service in taking the trouble to formulate arguments and write them up in a lucid fashion. My company and I are primarily involved in designing access-to-space options, with obvious bias towards hoping for and finding a silver bullet like SBSP.

    But I did some similar calculations, albeit less rigorous than Tom, and reached the same conclusions, much to my chagrin, but not too much to my surprise! Basically it would be way cheaper to build a ten times larger solar array plantation on Earth, than to build one in GEO. I even gave it the substantial benefit of lowering costs (down to $340 per pound to LEO – and higher to GEO) through fully reusable transportation as described in my interview published recently:

    And then there is maintenance as well: Can you imagine training and sending an astronaut electrician to service these large solar collector, about 5 km2 needed for a 2.5GW station (each SSP), and that too to GEO?! What would they charge per hour, and the cost of doing that often enough? And how easily they can be badly damaged by a hostile space faring adversary?

    It just does not add up.

  32. This analysis of Space Solar Power is so flawed I could wright a book based just on the flaws.

  33. This discussion series has seen substantial contributions from two people heavily invested in SBSP—namely, Darel and Keith. The former is a physicist/systems-analyst at the Space Solar Power Institute and the latter is, well, a bit of everything, judging by his Wikipedia entry. I usually strive to limit the number of comments from any one individual, but just sort-of let it play out this time (you can see more from these two at the Oil Drum re-post).

    To help myself and the readers sort out the expert opinions, I would like to know the degree to which these two are on the same page: there has not been much direct interaction/reinforcement/debate provided here. So Darel, Keith: are you largely in agreement with each other on technical details and approach, or is it more in the overall concept/drive that you share common ground? It may be helpful for the less technically informed (including myself) to sort out the degree to which the experts are converged. For instance, Keith sees laser launch as a must-have for this to fly, but I have not heard Darel say similar things. Comments?

    [note: Keith responded below—not attached to this query.]

    • I have great respect for Keith’s SSP work & interest which seems to center on the space transportation problem – a core problem being actively worked from many angles. Keith like single stage to orbit, such as lasers or Skylon; I expect a real SSP company will contract out the launch to orbit work to the low bidder. There are several good options on the horizon, so I am not committed to any particular launch path – that will change as we go forward. Launch is not today SSP’s critical path problem; transmitter design is, in my humble opinion, which is our focus – no pun intended.
      The SSP Institute’s interest is in assembling the technical / financial / business / environmental business case understanding essential to getting SSP built. That is largely an education problem, so we are a 501(c)3. I chaired the business case analysis group for the all-volunteer NSSO study five years ago. I worked for 12 years in the electric power industry doing advanced systems and strategic planning untilm1997 when I left to start the Space Solar Power Institute and Workshop. SSP is a natural extension to the existing electric power business, although there are no US companies equipped today to undertake the massive demands of SSP. The best solution to that difficulty is to bid out the public/private business from government loan guarantees just as Comsat was chartered in 1962 and created our very successful commercial satellite industry. We call it Sunsat Corp —
      The Japanese have done this. I understand the Chinese are probably assembling a similar consortium. By comparison, the US and Europeans appear to have very little funding & organization working SSP – plenty of room to grow.

  34. I have just had a thought;

    Presumably, the construction of a receiver array is less difficult than the construction of a solar panel (it is just a bunch of wires, right?) so one interesting aspect of space-based solar is that the end user country doesn’t need to have much industrial capacity, compared to one that makes and services solar panels. The guy sending up the satellite still has to have an immensely advanced industrial base, of course.

    The point is, there may be an advantage when getting energy to the developing world (much of which is, rather handily, located fairly close to the equator so the beam doesn’t get distorted much). Not having to run thousands of kilometers of wire, or support complex machines in a place with poor infrastructure, is something to consider.

    Also, correct me if I am wrong – but microwave receivers aren’t going to be massively fussed by getting a little dirty. One of the big costs of the canonical Great Sahara Desert Power Plant is all the water you need to bring in to wash sand off the solar panels!

    As I said above – none of this makes space-based solar power the big techno-fix that means we can sit back and forget about energy. But I still think it is worth including in a basket of solutions.

  35. Technical agreements . . . I don’t think Darel and I are on the same page with regard to the light pressure problem, but then I am not sure my rough analysis is correct.

    It would surprise me if Darel was in agreement with me on laser propulsion because it has only been about a month since I realized how much the cost of transport into space depends on big energy sources already *in* space.

    Even at $4000/kg it seems to be a paying proposition to lift a couple of GW of propulsion lasers (and power plant) to GEO with more or less conventional rockets then use it to power a 500,000 ton per year parts delivery pipeline. That way we could build power sats at $100/kg lift cost to GEO. Even at 5 kg/kW, that gets the transport cost per kW down to $500, about 1/3 of the cost, and permits an energy cost (based on a ten year payoff) of 2 cents per kWh.

    Done efficiently (i.e., not NASA) peak funding might be less than the ISS, or the F-22. The profit, even at selling the power sats at $1.6 B per GW, is enough to pay off the US National debt in a decade.

    I am not strongly biased in regards to the light to electric power conversion in space, Darel seems to favor lightweight PV. I slightly favor thermal over PV because it reduces the light pressure problem, as does the relatively higher mass per unit area. (And industry has a lot more capacity for turbines than PV.)

    This is all very tentative, there could be showstoppers neither of us have anticipated, there may be better ways to solve power sat engineering problems or even better ways to solve the broader problems.

    When StratoSolar (which scales to tens of TW) looked like it might come in at 1.5 cents per kWh, I spent almost two years working on it before the construction problems became intractable. If people don’t like power sats, suggest something else that will scale to tens of TW.

    I am flexible; just want to solve the energy/carbon problems before we get famines, wars and a population crash.

    • Regarding light pressure, which is 7 pounds per square mile as I recall, – it is mostly is a non-issue because it speeds up a sunsat as it revolves around one side of the earth and slows it down by an equal amount when it revolves around the other side of the earth.
      Fuel will still be needed to supplement various other techniques for doing station-keeping and battling things like diurnal vibration, thermal vibration, etc., Likely the same ion and plasma drives will do double duty boosting to GEO orbit and later doing station keeping and maneuvering.
      Keith is right, thin-film PV has better specs in my opinion. Mechanical engines are the wrong path for SSP. PV is far superior – no moving parts to break and repair, no fluids being pumped to be puncured by micrometoerites, etc., – just look at how widely used PV in space is. Mechanical machines are only used where there is no alternative, such as refrigeration for the fluids in the Hubble telescope.

      • “light pressure, . . . is mostly a non-issue”

        Doing a bit of a observation, for something dense I agree. But get a power sat down to the mass per unit area of a light sail and the light pressure will blow it away like dandelion fluff.

        The velocity, radially outward from the sun, does average out to zero over a year, but how much is the orbit distorted by the velocity build up? How much mass per unit area does it need before you can ignore the light pressure?

        The answer is probably on the web . . . somewhere.

      • “ion and plasma drives will do double duty boosting to GEO orbit ”

        I know Boeing in early studies was planning to construct power sats in LEO and self boost them to GEO. Alas! Someone at Boeing ran the numbers an even in those days, late 70s, the space junk was thick enough that the power sats would get to GEO with serious damage.

        Has someone figured out a solution I have not heard about?

  36. I want to know if GaAs and graphine could be combined to make the ultimate lightweight solar collection medium, for Earth.
    A long time ago, I read that overly optimistic book “The Millennial Project” which suggested that lasers could be used to create a sort of steam propulsion, using ice. Would such engineering be able to propel the payload to the 4,710 kmph speed necessary for GEO?

  37. Easy. Prolems attributed to space based solar can be solved quite quickly with direct wiring right from the panels overhead to the individual households. The wiring would be Helliud-type from civilization level IV. Their a bit strechy. This elegant solution bypasses all the microwave issues, etc. And hanging wires look better than humongous dishes anyway. See http://www.wikidreamia.yeah/letswakeup for more. (sic, sic and sic).

    Umm… Ok. 
    Tom, it isn’t your intimate knowledge of what Lord Kelvin and Einstein have postulated. Nor the understated eloquence that is so pleasant to read. But your patience… That, I admire. I don’t know how you do it. 

    Let’s recap in simpler words. The tower humanity has built is too tall. Its foundation shows signs of stress. It sways dangerously with slightest wind. Structural cracks are everywhere. The way to help ourselves is to scale back. Step down. Maybe dismantle a few floors. We’ll all be better off that way. Instead, everyone is talking about how to prop the building with some magic nanotube suspenders. From space. 

    • “The way to help ourselves is to scale back. Step down. Maybe dismantle a few floors. We’ll all be better off that way.”

      If you have a suggestion as to how to do this without famines from lack of low cost energy and resource wars, probably with nukes, please don’t keep it to yourself. Most “solutions” have seen are more unrealistic than power satellites by a long shot.

      Human psychology is more of a problem in finding solutions than the physics. And, being based on a million years of evolution, much harder to change.

      • Keith, don’t see my comment as disrespectful. I do hold an opinion that isn’t aligned with yours, but you are aware of humanity’s predicament and are trying to find solutions… That is to be lauded for. 

        About the solutions: I search, but see none; saying this with profound sadness, and being fully sentient about the dire implications; with a giant graveyard that Earth is thought by some to become in not so distant future. I have a feeling that best minds, like Tom, ultimately see no palatable solutions either. Mitigation by conservation tactics may only prolong the inevitable and insurmountable decline – Jevon, Chevron, “ged’er dun’ populace, female form, innate hubris, and so on will see to that. So I entirely agree with you on the evolution front – we appear unable to fight ourselves, having been tempered just so by eons. 

        As it were, a proposal to expend immense amount of energy on _anything_ in space (beyond current level of our presence, to further understanding of the universe) to support another onion layer of complexity that we as humanity are so fond of, would appear to be on a divergent path with where we should be going… I am not even going to mention that abundant energy does not refill the aquifers nor makes the soil, among other things. Scaling back – on everything –  is the only thing that addresses the whole bouquet of our problems, however imperfect and temporary band aid solution it may be. Everyone should heed Tom’s advice.

        • Actually sufficient energy can refill the aquifers or rebuild the soil. Desalinate water and pump; grind up rocks and add the dust and fertilizer to soil; make biochar and transport it to where soil’s eroded.

          Lots of high quality (low entropy) clean energy is very fungible and can do almost anything, including sucking excess CO2 out of the air and oceans and transforming it to storable form. We could blast toxic chemical waste to safe atoms and transmute radioactive waste until it wasn’t. It would be awesome if we could get it.

          Hmm, now I’m wondering if there’s any theoretical way to run a heat pump on the whole planet to cool it off. That’s a form of geoengineering I haven’t seen suggested, as opposed to reducing the sunlight we get.

          • With my logic that isn’t – no. Not in a way a gastric bypass is an acceptable answer to obesity. Consider such undertaking’s problems: another layer of complexity, unforeseen consequences , BAU delusion. 

            Earth provides, without us doing a single thing.  We mustn’t Goldbering everything we lay our sight upon – just take at a lesser rate than is being provided – for free. 
            I’ll leave my gapped line of thought at this. 

          • Damien RS has it right.

            “Consider such undertaking’s problems: another layer of complexity, unforeseen consequences , BAU delusion. ”

            Worse than the consequences of not doing anything to replace dirty fossil fuels with clean energy? SBSP isn’t any different than ground solar or wind except it could be a lot less expensive.

            “Earth provides, without us doing a single thing.”

            You are at least 8000 years out of date. The maximum human population without “doing a single thing” is of the order of 10 million. There are lots of things a disbursed population of that size can’t do. Certainly not computers, maybe not even writing.

  38. One virtue of proper pollution and sustainability taxes on fossil carbon would be that we wouldn’t feel the need to pick sides, unless we were actual investors or employees. Alternatives could compete directly in the market on a fairer basis. There’s still role for public subsidies in research, but that okay; I have no objection to monies going toward researching cheap launch technologies of whatever type. (OTOH if the buy-in is $20 billion that does call for very good math.) We uninvolved individuals shouldn’t have to pick teams; public policy should first be making sure all costs are properly accounted for.

  39. «Power from Outer Space» has a seductive charm, along with other Tom Swiftian techno-fixes. I have no doubt that some of them could well be entirely feasible. But even so, there are always limits. And assuming feasibility… feasible when?

    The distant future is a long time from this present time, and there is pressing urgency right now.

    When I was young, there were bucketfuls of talk that scientists would invent «safe cigarettes» so we could puff away and not need to quit smoking, suffer through withdrawal, etc. And besides, all those scientists would also invent a cure for cancer. (I will not deny the possibility of a safe cigarette and «a cure» for cancer some day. But not in time for many late friends and relatives.)

    Nothing new under the sun. An ancient bit of advice is appropriate, Matthew 7:13-14.
    “Enter ye in at the straight gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: Because straight is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.”

    It’s time to pull off the broad way, park the SUV, and start hiking along the narrow way.

    • Actually we now have ‘electronic cigarettes’ which give the experience of smoking a cig, and nicotine, without the smoke that bears most of the health risks to the smoker and others. So those safe cigs are kind of here. Plus nicotine patches, of course.

      Sometimes technofixes work. In fact our civilization is built on them.

  40. To me, it simply comes down to this: we haven’t gotten solar power to work on a large scale on earth yet, no way we could anticipate all of the challenges of space based generation….especially when we cannot have people there to perform maintenance and adjustments.

    So we need to expand earth solar power on a much larger scale….from there we can talk again about whether its worth to send it out to space.

  41. A proposal to only launch ultra thin and light inflatable mirrors and have them redirect sunlight to ground based solar farms at night. Efficiency is the ground based systems efficiency. Can be done at lower orbit 600 miles up with 12 or more satellites. Helps ground based solar get around the storage issue at night. Again best to do it to desert locations without clouds. Also away from places that want to do astronomy or have other impacts of turning the night into day. Just need to get astronomy into space and away from the light. Again get past a critical mass of space based power capability and infrastructure so that it is easy to make 500 MW of lasers for boosting skylon space planes to drive down costs to get to space.

    A constellation of 12 or more mirror satellites is proposed in a polar sun synchronous orbit at an altitude of approximately 1000 km above the earth. Each mirror satellite contains a multitude of 2 axis tracking mirror segments that collectively direct a sun beam down at a target solar electric field site delivering a solar intensity to said terrestrial site equivalent to the normal daylight sun intensity extending the sunlight hours at said site by about 2 hours at dawn and 2 hours at dusk each day. Each mirror satellite in the constellation has a diameter of approximately 10 km and each terrestrial solar electric field site has a similar diameter and can produce approximately 5 GW per terrestrial site. Assuming that approximately 50 terrestrial solar electric field sites are evening distributed in sunny locations near cities around the world, this system can produce more affordable solar electric power during the day and further into the morning and evening hours. The typical operating hours for a terrestrial solar electric field site can thus be extended from approximately 8 hours per day by 50% to approximately 12 hours per day. Assuming a cost of electricity of 10 cents per kWh and a projected launch cost to orbit of $1500/kg for the SpaceX Falcon Heavy launch vehicle, the cost of this mirror constellation system should be recovered in approximately 2.7 years from the additional solar electricity sales.

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