# Space Based Solar Power

(at Kurzweil.net)

Space Solar Power (or Space-Based Solar Power) is undergoing a revival of interest. An excellent introduction can be found at the National Space Society’s SSP page. The presentation is biased slightly against nuclear power. But one thing which I can’t disagree with is advantage of no radioactive waste – even the best nuclear reactor fuel-cycles produce some, albeit short-lived. I believe the two, SSP and nuclear, will need to be developed side by side to provide power for all.

So what will be needed for space-based solar power to be an energy source for all? Currently photovoltaics are very expensive, especially space-rated high-efficiency cells able to take the thermal stress for extended time periods. One approach to reduce costs is go ultra-light – thin-films are actively being developed, and some space-rated designs are heading for ~4500 W/kg of PVs. Alternatively the cloudlessness of space means concentrator systems, such as the graphic above, can be used all the time – mirrors can reflect onto a high-intensity collector. Some concentrator systems tested on Earth can operate in conditions equivalent to 400-500 ‘Suns’ intensity. Very handy if, eventually, the solar system economy has developed enough to place collectors in closer orbits to the Sun – 0.05 AU would give x400 intensity of sunlight.

Of course the major cost issue, in some respects, is access to orbit. SpaceX is promising space-lift charges to LEO of $4930/kg and to GTO (s/c up to 4,680 kg) of$11,000/kg. With the Falcon-9 Heavy able to orbit 32 tons to LEO and send 19.5 tons to GTO one wonders if the charge will be lower per kg. To get from LEO to GEO, rather than just using a chemical boost to GTO, a solar-electric propulsion system could be viable. A few years ago the first version of Powersat, Inc. proposed an integrated SEP/Power-unit system to deliver its ~10 ton sub-units to GEO. There were unaddressed problems with the design, but the basic idea is a good one. An optimised design might opt for the simplest Electric propulsion system – IMO the Helicon thruster, which makes up part of Ad Astra Corp’s VASIMR. No electrode erosion issues and high-thrust for high Vex.

If we can get the cost down to ~$5000/kg then what would the cost of power be? A 1 gigawatt system needs a collector surface big enough to capture enough light to make up for system inefficiencies. The old SPS studies in the 1970s concluded that the energy transfer efficiency from the PV output to the Power-Grid on Earth could be ~63%. If we assume concentrators with ~40% efficiency, then the system efficiency is ~25.2%, meaning we have to intercept 4 GW of sunlight to get 1 GW of power to the grid. The year averaged level of sunlight is about ~1350 W/m2, so the area of the collector/mirror system is 2.94 million m2, a square about 1715 metres on its edges, or two circular collectors 1368 metres in diameter each. Obviously it’s not going to launch all at once – the Powersat concept sent thousands of sub-units up to gather together to be combined automatically. If the collectors mass 1 kg/m2 and the rest of the system masses the same equally, then the total mass is ~5,880 tons. Delivery cost at$5,000/kg is $29.4 billion. Kind of excessive, but not utterly ridiculous for such a big space-based system. Clearly the way forward is system mass reduction. My 1 kg/m2 was deliberately excessive. What if we’re looking at 0.1 kg/m2? And$2,500/kg to GEO? Then the cost is ~$1.47 billion, perhaps double that for the whole system costs, including assembly. Getting in economically viable territory. But let’s look at it from the other direction. How much could the power sell for? If we’re talking competitive with power sources on the ground then the cheapest cost for power is ~$0.04 /kW.hr. A 1 GW SPS (Solar-Power Satellite) provides 1,000,000 kW.hr/hr and might last ~30 years without major system replacements – call it 263,000 hours. Thus the wholesale energy market value is $10.52 billion at constant prices. No inflationary adjustment. End-users, like the suffering masses of my state Queensland, are paying$0.2/kWhr, thus an energy retailer would gather ~$52.6 billion in revenue over that period, non-inflation adjusted. So profits aren’t unimaginable for space-based power-companies to aim to achieve. Let’s assume space-lift is 25% total cost, thus the 1 GW SPS system has to cost ~$2.63 billion to get into orbit. That gives us a rough guide to the kind of mass-efficiency and space-lift price we want to see to make SPS a viable profit-making enterprise.

If SpaceX can come through with their promise of space-access that’s “x10” cheaper – roughly a factor of 5 cheaper than their current rates – then $2,200/kg to GTO means our 1 GW SPS needs to mass <1,200 tons. Possible? Some clever SPS engineer, no doubt, will “make it so…” NB: The power-price to end-users is in$AU, which isn’t much removed from $US. Over there many states pay similar rates at$US.

According to T.A.Heppenheimer’s summary of different SPS construction plans, the idea of flying sections to GEO from LEO under their own power isn’t new…

The Boeing approach, discussed in Chapter 7, called for the powersat to be built in the shape of a single flat slab with transmitting antennas at each end. Power would be generated by silicon solar cells. The principal construction operations would be in low Earth orbit, where the construction base would build each powersat in eight sections resembling the leaves of a dining-room table. Each section (two of them would carry antennas) then would be fitted with ion-electric rocket engines and fly under its own power to geosynch. The ion engines would use electricity to eject atoms of argon at very high speeds, some 225,000 feet per second, to produce thrust.

Activities at geosynch would be strictly limited. Because each powersat section can produce much more power than it needs for the electric rockets, many of its solar arrays would be rolled up like window shades. The few crew members at geosynch would unfurl the arrays, causing the powersat sections to spread sail like a clipper ship. As each section arrived, at forty-day intervals, it would be joined to the others. A completed powersat would be activated by a ground station.

…notice the sensible power-limiting of the ion-drives, unlike the old Powersat Inc. plan which had a fully unfurled array. Where did the excess power go? That was never answered.

Nothing much to say. What about you?

## 22 thoughts on “Space Based Solar Power”

1. John Hunt says:

Harder to calculate, but what if there was sufficient industrial base on the Moon to extract and transport material to GEO (perhaps aluminum)?

2. Warren says:

Yeah, I had the same question. Solar power stations, built using material from the moon were a big part of O’Neill’s fairly popular proposals back in the ‘70’s.

3. In the long term moon-mining or even NEA mining makes sense, but the first demo versions need to be viable without the choke point of a viable moon-mining operation. Consider the logistics of setting up a viable moon-mine and you’ll see why.

4. John Hunt says:

If the only purpose for moon-mining were to provide material for the trusses and mirrors of SPSs then this might well be true. But if there were already moon-mining operations for other purposes (e.g. providing water or oxygen for fuel to LEO) then part of the infrastructure (e.g. ascenders, teleoperated equipment) for mining metals will already exist and so some of the development costs will already be paid for.

Conceivably there might be an economic dynamic whereby the market for SPSs matures before the market for lunar materials to LEO. So initially the SPSs would launch from Earth and be somewhat profitable. But, as mining operations mature on the lunar surface for multiple other reasons, eventually materials supplied from the lunar surface will become competitive with material provided from the Earth’s surface. As the lunar operations expand, the price for lunar materials would drop and so would the cost of producing a new SPS. At that point, SPSs would become increasingly profitable.

1. I think that’s a viable scenario, John, and it makes a lot of sense. We can’t make a case for SPS if it needs massive industrial facilities on the Moon, but we can make a case for mining the Moon once SPS are up and proving their worth by selling other products too.

5. Warren says:

Certainly no disagreement with your approach. And no question that It would be difficult to get to a viable moon/NEO mine, electromagnetic mass driver, or whatever unless there were substantial infrastructure already in place, shared with other projects. I’m probably asking too much here given all of the uncertainties, but your calculations are always informative. How much cheaper/more efficient would it be to construct SPS on the moon and/or in orbit given that we already have relatively self-sufficient settlement there. Can we get off of oil faster if we make the investment now?

6. Hi Warren
The Moon has lower gravity and we now know it has hydrogen in interesting quantities – which is important for making silane during the making of silicon. The amount of carbon is unknown – an issue if we’re making plastics or nanotubes. Mining kerogen from NEAs might be needed in that case, adding to the complexity. The really important ingredient is energy as pyrolytic extraction of elements from the rocks will need lots of it, if plain old industrial chemistry is too hard with such a low water supply. There’s a bunch of old NASA papers on Moon industry I haven’t read in detail which covers this stuff. But let’s look at the basics – energy costs of orbiting mass to GEO. Comparing the escape energies is straightforward – the Moon needs 2.82 MJ/kg while Earth needs 62.5 MJ/kg, thus export costs are 22 times higher from Earth, minimum.

However delivery issues even things up a bit – aerobraking is a BIG advantage. But there were designs for “harenodynamic braking” of cargos on the Moon – big sleds that allowed a vehicle to slide to a halt on the Moon. That sounds extreme but makes sense for bulk cargoes from NEA mines for example. It’s tempting to imagine just impacting cargo, but inefficient for industrial volatiles -big lumps of refractory metals and carbon are probably the only items suitable, but terminal guidance has to be VERY good.

7. John Hunt says:

> And no question that It would be difficult to get to a viable moon/NEO mine,

Well, help me understand what is so difficult about it. I am imagining that the first lunar water ice extracted would be for proof-of-concept purposes. Since the quantity is not so important, no heavy launcher would be needed. SpaceX indicates that it is willing to launch lunar X-Prize competitors at cost. Not sure which launcher they will use (Falcon 1e or Falcon 9), but I believe that an F9 would be more than sufficient to launch a lunar lander from Armadillo. It wouldn’t need to be particularly massive since it would get its return fuel from the Moon.

IF a previous prospecting mission had confirmed slabs of water ice on either lunar pole then the lander could melt this, convert it into rocket fuel and then launch a certain amount of water ice back towards Earth. Using aerobraking, a token amount of water ice could be delivered to LEO. This demonstration seems to me to be within the capabilities of a partnership between SpaceX, Armadillo & NASA ISRU if modestly funded by NASA.

Then it is a matter of scaling up the lander and ice harvesting and melting equipment. Using two F9s (an earth departure stage and a lander) the lander should be large enough to begin transporting significant amount of water fuel to LEO using a very similar approach in the demonstration. The same lander could go back and forth between the Moon and LEO for as many times as possible until the lander malfunctions.

Should the value of all water delivered by a single lander to LEO be greater than the cost of building, launching, and managing the craft, then more craft could be built and the entire operation scaled up to industrial levels. At this point we already have a lunar mining operation. Its just that the product mined is water. It requires no humans in space so we don’t have to overspend to minimize risks. If slabs of lunar ice can be found, then I see no particular reason why this operation would be beyond the capability of a company who hires experienced aerospace engineers (e.g. former NASA employees).

At this point, we have proven lunar landers/ascenders and a proven delivery method to LEO. Proceeding on to the extraction of lunar regolith (easy enough for X-Prize participants to master). I find it difficult to imagine that the extraction of metal from the lunar regolith would be that difficult given our experience of doing similar things on Earth. Delivery of metals and silicone to GEO and LEO should be within reach at that point.

Now, you’ve got the beginnings of a cis-lunar economy which is already sustainable. Next its a matter of sending up ovens and metal working equipment up to the ISS. At that point you have all that’s needed to make very large SPSs which expands the cis-lunar economy considerably.

You are already routinely landing moderately-sized landers on the Moon. So robonauts, teleoperated regolith extractors, ovens, small, metal-working equipment, greenhouses, and high-tech components could be safely delivered to the lunar surface. At this point you have a small robotic industrial complex on the lunar surface. This is leveraged to construct larger equipment and to prepare a base for humans. At that point there would be extensive experience landing and lifting off from the surface of the Moon using medium-sized landers. Since such landers get ascension fuel from the lunar surface then the amount of payload they can deliver to the Moon’s surface would probably be in the 3,000 kg range. Why couldn’t humans be delivered to the lunar surface using these medium-sized landers?

OK, so I’m no aerospace engineer so I’m undoubtedly glossing right over some significant challenges. But we’ve got the medium-lift rockets already, extensive experience docking in LEO, several different EDSs. We’re not missing or accidentally impacting the Moon much any more. We’ve done landers of several different sizes. Heat shield is figured out. Same for telecommunications. We’ve got fairly functional robonauts and experience with teleoperations. We’ve already got demonstration regolith extractors and ISRU equipment. We’ll be able to get humans to LEO a few different ways. By the time we send humans to the Moon we’ll have numerous automated landings under our belt. NASA seems to be clearing budgetary space and we’ve recently had success with NASA facilitated commercial development of space.

So…why can’t we do this???

8. John Hunt says:

Oh, let me mention the economics. NASA sets aside a part of its budget (e.g. 20%) to stimulate COTS-like development and prizes where they only pay after demonstration or delivery. Lunar water ice deliveries to LEO could be sold to NASA, other governments, LEO –> GEO operations, and begin to open up LEO satellite servicing. Then, of course, SPS satellites represents a potentially tremendous amount of business. There’s moving equipment to the Moon and people from LEO to circumlunar and lunar destinations. Maybe claims on lunar real estate? Eventually metals for LEO and lunar hotels. Fuel to begin mining operations of NEOs and maybe moving a small asteroid to an L-point.

It all starts with water ice to LEO.

9. Water ice takes energy to melt and electrolyze. Hydrogen is a hard cryogen that needs significant energy for liquefaction and ongoing storage. Oxygen is easy to liquefy and store in comparison. How do we provide the power?

Metals need all sorts of extra processing to make into viable structural materials from basic ores – which need to found, mined & purified. Silicon is especially tricky to extract in pure semi-conductor grades and needs lots of energy to extract. So where does it come from? A Moon factory – if we don’t use nukes – will need an orbital array in place already for power. Significant in-space investment required before we start making arrays to sell for power… Thus a major choke point is the assumption of a Moon Mine. Eventually it might be viable, but not at the very start.

10. Just to give the previous some figures. Commercial compressed hydrogen supplying electrolyser systems use ~24 MJ/kg of water electrolysed at about 0.72 efficiency. Allow about ~1 MJ/kg more for melting the lunar ice and several MJ/kg for liquefying the products and the energy cost is ~30 MJ/kg. A ton/day production plant would need ~350 kWe supply. To launch from the Moon to GEO and then brake needs a dv of ~4 km/s, thus every ton of spaceflight launched needs 1.72 tons of propellant for the trip. Assuming a generous 50% payload fraction, that’s 3.44 tons propellant for every ton payload delivered. A 720 ton SPS, a minimal mass estimate, would then need ~2,500 tons of water electrolysed. If we’re launching an SPS a month, then we need ~82 1 ton/day production plants and 29 MWe of power available for the propellant alone.

That’s not bad. A single dedicated SPS would easily supply the lunar operations and then some more, but we have to get it up into space first.

11. BTW Is all that water propellant going to waste, one might wonder? As most of the propellant is expelled escaping the Moon a most of it should fall back to the surface. But it does pose the problem of *pollution* of the normally airless environs of the Moon with a temporary cloud of water vapour from every launch.

Of course we might contemplate a magnetic rail-gun launcher for the Moon. The question is just how much imported equipment will be needed to build a sufficiently capable rail-gun. If 1 ton is fired at 160 gees then about 4 GW of raw power is needed at peak power output to the launcher – more like 4.5 GW factoring in some inefficiency. That’s a lot of juice. To launch 720 tons of equipment in a month, to match the rockets, the 1 ton launcher needs to fire every hour. If we assume a capacitor bank or fly-wheel supplying peak power then the power supply needs only to be averaged over the hour. A 1 MWe supply will be about right. So how much will the rail-gun mass? That’s a good question. At 160 gees the launch track is ~1.95 km long.

The USN is researching rail-gun launched projectiles moving at Mach 6 or so. A bit faster and we’re into Lunar escape velocity territory. Hardened payloads could be fired from the Moon via such a rail-gun. However they’d need to only fire tiny payloads at a time – a few kilograms – thus hundreds of shots would need to be fired to make a ton.

A small(ish) rocket would be needed to circularise the payload at destination, though low-energy long flight-time orbits exist between the Moon and GEO which might allow significant mass savings. A “solid” rocket using aluminium powder gelled in LOX – a non-shock sensitive mix – could be suitable for the job.

12. moonus111 says:

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