| Solar Thermal Power Satellites by Dave Dietzler 2008 Solar Thermal vs. Silicon I don't promote silicon solar power satellites anymore...To make the millions of tons of silicon we would need to get pure enough silicon we would need thousands of tons of upported fluorine in the form of KF - postassium flouride- to make solar cells as described by Landis (1). On Earth we take white sand and react it with carbon. SiO2+ C = Si + CO2 but this is low grade silicon as the sand contains impurities and some SiC may form too. There's no white sand of rather pure SiO2 on the Moon. Then they react the silicon with HF acid or F gas to make SiF4 that is decomposed on hot fingers at 850 C leaving fairly pure silicon on the hot fingers and recovering the F. Then they go and zone refine the silicon to ultra high purity for semiconductors and solar panels. We don't have fluorine on the Moon so we'd have to upport this dangerous chemical. It would be recycled but there will always be leakage in any system. I favor the solar thermal power satellite. Gerard K. O'Neill designed a 5 GW, 80,000 tons solar thermal power satellite back in the 70s. This gives us an idea as to what these monsters would amass and how much power they'd produce. Imagine hundreds of 10 GW 160,000 tons thermal SPSs producing terrawatts of power. With shielded radiators exposed only to the 10 K temp of outer space, heat engines in space can approach near ideal Carnot efficiency. We will need not silicon but shiny foil reflectors, boilers or pipes filled with helium to focus the sunlite onto, multistage turbogenerators and shieled radiators made of various metals. The reflectors will be thin sheets of light weight magnesium that will comprise much of the SPS's mass. Magnesium has a reflectivity of 74% vs. 71% for aluminum (2,3) Magnesium production is simpler than aluminum production and does not require any upported chemicals like chlorine or LiF nor does it require the use of valuable carbon or dangerous sulfuric acid mined/made on the Moon. If only we could directly electrolyze alumina, AL2O3 or CaAl2O4 to get aluminum. Despite these difficulties, we must have some aluminum for its strength and electrical conductivity. The main metal for the frame will be aluminum, probably a magnesium-aluminum alloy because a little magnesium hardens aluminum, with titanium or steel parts at the points of greatest stress on the satellite. We will need high nickel and cobalt steel alloys for the turbine blades. Generators will have iron rotors wrapped in calcium coils and those Ca coils will be coated with aluminum vapor despoited on them in the vacuum. Silicon Disadvantages So what do we need millions of tons of silicon for? Just because solar panels have no moving parts??? Not so fast. They degrade over time and have to be annealed with heat, possibly from microwaves every year at least and radiation knocks down silicon solar panels output....Have to wonder how the solar thermal sat will stand up to micrometeors vs. the silicon panels SPS. Upports We can upport some F in the form of KF and electrolyze the salt to get F. This fluorine would be used for limited silicon PV production. We could upport some Cl in the form of CuCl3 or ZnCl2 to get some Cu and Zn up there too and recycle the stuff, but F is really corrosive and Cl is not as bad but it's corrosive too. We will have to be careful with these. Titanium is highly resistant to chorine ( see: http://en.wikipedia.org/wiki/Titanium ) so handling chlorine will be a job for titanium plumbing and storage tanks. Don't have to use steel for everything !!!! ) Let's say that we can build a lightweight silicon solar panel powersat with 1 kw/kg or 1 MW/ton or 1 GW/1000 tons A 5GW sat would amass 5000 tons and a 10 GW sat would amass 10,000 tons. Let's say that 25% to 50% of the sat's mass is silicon. We need 1250 to 2500 tons Si for the 5 GW sat and 2500 to 5000 tons for the 10 GW sat. If we build 100 of these we need 125,000 to 500,000 tons of silicon. This is for an ideal case, however. SPSs might be much more massive. Perhaps we would need 1000 tons of flourine even with recycling to do the job in a reasonable amount of time. Flourine will eat thru almost anything, even glass. Teflon resists F. Titanium is not recommed for handling flourine. From: http://www.azom.com/details.asp?ArticleID=1225 Titanium is not recommended for use in contact with fluorine gas. The possibility of formation of hydrofluoric acid even in minute quantities can lead to very high corrosion rates. Similarly, the presence of free fluorides in acid aqueous environments can lead to formation of hydrofluoric acid and, consequently, rapid attack on titanium. On the other hand, fluorides chemically bound or fully complexed by metal ions, or highly stable fluorine containing compounds (e.g., fluorocarbons), are generally noncorrosive to titanium. So, handling any but the smallest masses of F in Teflon lined pipes is out of the question. Titanium does resist chlorine par excellence. See above link to azom.com We will use chlorine in the manfuacture of silane rocket fuel and get high grade silicon as a bonus. See: Lunar Derived Propellant Rockets Must wonder if we will make silicon panels on the Moon or solar thermal powerplants once all the draw backs of making silicon on the Moon are considered. Magnesium Reflectors Magnesium is a slightly better reflector than aluminum. Lightweight magnesium reflectors might be the key to solar thermal power satellites. Producing magnesium seems to be more straight forward than aluminum production. It does not require carbon, upported chlorine, H2SO4, or an upported LiF flux as does Aluminum. Mg production simply requires MgO, CaO, FeSi and heat; all available on the Moon Magnesium is about 64% as dense as aluminum. Mg 1.74 gr/cc Al 2.7 gr/cc Thus, we would only need to launch by mass driver 1740 tons of magnesium rather than 2700 tons of aluminum, in simple numbers, to build an equivalent mass of reflectors for a solar thermal powersat. That's a saving of a thousand tons! Much less energy would be required and a lot less mass driver propellant for mass catchers that haul material from L2 to L5. If we envison an 80,000 ton solar thermal powersat like O'Neil's 5GW SPS, perhaps a fourth of it would be reflector mass. If 20,000 tons is sheet aluminum reflectors, only 12,900 tons would be magnesium reflectors or 7100 tons less. Conversly, if we use 20,000 tons of magnesium, the magnesium reflector will be equivalent to a 31,000 ton aluminum reflector, therefore 1.55 times as much volume and surface area using the same thickness of sheet metal or foil for the reflector, not considering the 3% increase in reflectivity. If we add boiler and turbine capacity to match the larger reflector the SPS generates 7.75 GW. We could upport some nanosolar panels for the initial bases. see: http://www.celsias.com/article/nanosolars-breakthrough-technology-solar-now-cheap/ http://www.nanosolar.com 1) http://www.asi.org/adb/02/13/02/silicon-production.html 2) http://www.webelements.com/magnesium/physics.html 3) http://www.webelements.com/aluminium/physics.html |
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| see: Aluminum Note |
| Brayton Cycle turbines approach 50% efficiency. Waste heat from the precooler and intercooler would go to low pressure turbogenerators with shielded space radiators using helium as a working fluid. The system would approach ideal Carnot efficiency. |
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