Solar Power Satellites: a Brief Review Gordon Woodcock ISDC May 2010

What’s the Problem SPS Is Supposed to Solve? Fossil fuels are solar energy stored in chemical form by natural processes over hundreds of millions of years. We are depleting them at rates on the order of tens of years. Present methods of fossil fuel utilization are extremely dirty and are polluting our Earth’s biosphere. It’s a race; will resource depletion or destruction of the environment win? Either way, the world economy collapses. Terrestrial renewable energy sources are not “demand” sources, that is, you cannot command them on with the throw of a switch. – Terrestrial solar is on when the Sun shines. – Terrestrial wind is on when the wind blows. – To have a demand source, you have to store the energy until you need it. – Terrestrial hydro is an energy storage method; it stores water potential energy (weather energy) until you demand it. But little developable capacity in the U.S. – Some solar concentrator thermal plants provide limited thermal storage. – Geothermal energy taps stored thermal energy of the Earth itself. – Batteries, or water electrolysis and storage of hydrogen with fuel cells can provide storage. – Research is continuing into photochemical generation of hydrogen directly from sunlight and water (plants do this by photosynthesis). To run a modern power grid exclusively (or even largely) on terrestrial renewable energy, we need enormous amounts of storage. It’s expensive! Thermonuclear fusion is a dark horse. It’s estimated to be 30 years in the future, and has been so estimated for 50 years.

Why Do We Think SPS is a Potential Solution? Continuous sunlight and fixed location over a particular location on Earth; 24/7 supply; it can be a demand source; very little storage needed. Net output x as much for given solar array area. (6 - 8x in space, 30% - 40% lost in transmission). In view of constant solar pointing, can probably reduce actual photovoltaic area by another factor of 10 to 100 by using concentrators. Power intensity on the ground: 200 – 400 W/m 2 x 80% efficiency = W/m 2 vs solar on the ground 1000 W/m 2 x 30% efficiency x 5/24 = 60 – 70 W/m 2. No cooling water or other resource depletion. Does not consume fuel. Land used for receiving sites may serve dual use (this may depend on transmission frequency; at high frequencies receiving antennas may be fairly opaque).

What are the Big Issues? Frequency selection 2.45 GHz to 100 GHz & laser (range that’s been considered). Efficiency, size, cost, RF interference. Power transmission efficiency. Expected to be frequency- dependent; needs lab work for higher frequencies. Power transmission safety. – Cell phones have been a large-scale RF irradiation experiment on the public; appears to have resolved the RF safety issue in SPS’ favor. (Even though cell phones not yet conclusively shown to be safe.) Cost of the large solar arrays needed. If they cost like current space solar arrays, costs will be hopelessly high. Cost of launch. At current launch prices, hopelessly high. Building structures on the order of miles in size in space. Energy cost to launch into space; energy payback time? – (The payback time is in the range 2 to 6 months.) Photovoltaic versus thermal cycle conversion Rainfall outages

Photovoltaic vs Thermal Cycle Conversion Comparison ItemPhotovoltaicThermal Cycle Likely Implementation Multi-bandgap Concentrator multi-bandgap Thin film Brayton (gas cycle) Rankine (2-phase cycle) Array of small Stirlings (gas cycle) EfficiencyAnticipate about 40% for multi-bandgap; 12 – 15% for thin film Brayton 25% - 40% Rankine 20% Stirlings 25% - 40% ComplexitySimplest; Concentrator systems probably deployable panels of small concentrator-cell arrangements Brayton most complex Rankine: working fluids probably liquid metal Stirlings … power per engine probably small ~ 25 – 100 kWe Construction likely very complex CostSolar arrays expensive, but made in small quantities. Terrestrial costs low enough. Thermal engines traditionally low cost but these are much different machines. Generator Output Typically DC but 3-  AC can be synthesized Typically 3-  AC

Rainfall 0.01% annual rainfall exceedance rate (Source ITU-R)

Rainfall Specific Attenuation Mm/hr Typical rain cell height 3 km, but highly variable. While 10 db or more gain margin is typical in communications systems, 3 db means half the power is lost; 10 db is 90% lost; and 20 db is 99% lost. (It’s an exponential scale.)

Launch Cost (The Big Show-Stopper) Current launch costs are at least ten times too high, even at SpaceX. Expendable systems highly unlikely to reach acceptable cost. The only re-usable system experience we have is with the shuttle and it did not reduce payload-to-orbit cost over expendable systems Studies indicate re-usable systems can deliver acceptable cost, IF – High demand; SPS qualifies – Long life; requires better than an order of magnitude decrease in loss rate, from about 0.03 to about or better. – Low turnaround cost in terms of spares and replacement of equipment; <1% of vehicle value (manufacturing cost). – Short turnaround times, less than a week on the ground between flights. This demands automated fault detection and certification for next launch, including certification of thermal protection systems, which presently takes weeks. Simple investment analysis shows fully re-usable vehicles are not worth the investment unless demand is at least 50 – 100 launches per year. The question is how do we demonstrate low cost from re-usable systems? Two approaches: – Partially re-usable heavy lift vehicle, flyback booster. Investment analysis shows it’s justified at 3 – 5 launches per year or more, and a good investment for human space exploration. – Early smaller fully reusable passenger vehicles for space tourism to orbit.

Other Ties to Human Space Exploration Electric propulsion: Needs the same high efficiency long-life solar power generation, up to a few megawatts. Also needs reduced cost but not as low as SPS. Power beaming: May be the best way to provide planetary surface power to the Moon and Mars. – Near-term, surface solar with overnight storage can do the job for the Moon. – Dust storms are an issue at Mars since they are not entirely predictable and can reduce surface solar intensity by 60% - 80%. – High frequency links (mm-wave, laser) enable modest power links, hundreds of kW to few MW. – Both Moon and Mars have stationary locations, Earth-Moon L1 & L2 for the Moon ~ 60,000 km, and Areosynchronous orbit for Mars, ~ 20,000 km. High-frequency links may make the most sense for first-generation SPSs for Earth. These have special utility for certain situations and do not demand fully re-usable launchers at high launch rates. They will suffer rain outages but for some of these purposes that may not matter greatly.

SPS Links To Scale Earth 2.45 GHz 5000 MWe Moon 1  m Laser 400 kWe L1 GEO AMO Mars 90 GHz MWe Earth 90 GHz 70 MWe Earth 33 GHz 700 MWe Earth 5.8 GHz 2500 MWe

20 MW SPS Showing Main Structure Members The structure provides a tripod-like support between the transmitter and large reflector and a simple support of the transmitter-reflector from the subsystems box. The tripod is offset slightly from the transmitter by a ring support to keep the tripod away from the high-intensity beam emitted by the transmitter. The axis between the arrays is an axis of rotation for sun-tracking. A power slip ring in the subsystems box enables array rotation relative to the rest of the satellite. For scale, the reflector is 1 km diameter. Frequency 90 – 100 GHz. Ground spot size about 300 m.

What Should Be Done Now? Device and circuit technology for high-frequency systems: solid state amplifiers, coupling of amplifiers to transmitter antenna elements, rectifying antenna elements, phase control systems, reference phase distribution systems. Similar for lasers, plus wavelength matching lasers to receiving devices; for example, simple silicon solar cells can be 60% - 70% efficient if incoming radiation is matched to their bandgap at ~ 1 micron. Design studies and laboratory work on very high precision mesh reflector antennas for lower-power remote site power mm-wave systems. These need deployment methods, may need active figure control and must be very light per unit area. High-volume fabrication studies for very large solar arrays, to understand potential for low-cost production – Concentrator arrays with high-efficiency multi-bandgap cells; – Thin film systems; – Any other approaches for low cost. Continued thermal cycle design studies to settle that tradeoff.