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.