Reflectors

REFLECTOR SYSTEMS
by Dave Dietzler 2008

Secondary reflector made of fused titanium oxide mp 2000 C.
Primary reflector magnesium

ceramic retort with fused silica window. Fused silica - SiO2, is similar to quartz

Also of interest, iron has an optical reflectivity of 65% versus 71% for aluminum and 74% for magnesium. See: http://environmentalchemistry.com/yogi/periodic/Fe.html Iron will not rust and loose reflective power on the Moon. Iron will be easier to obtain at early lunar bases than magnesium or aluminum. We can scavenge for iron fines and use serial magma electrolysis. The iron would be poured into shallow slab molds and cool by radiation into space. The solid slabs would then be rolled into plates or sheets and rolled into curved trough collectors. These would either focus sunlite onto silicon PVs to increase their output or focus onto titanium pipes with a thin outer coating or sleeve of black basalt filled with molten salt that supplies heat to steam generators and upported turbogenerators for 3 phase AC power. Alternatively, the iron reflctors could focus solar energy directly on thermoelectric of thermionic power elements. However, there is no significant amount of bismuth of tellurium on the Moon to make these power elements. We would have to look at the weight of thermoelectric and thermioniic elements upported to the Moon versus turbogenerators and the task of producing titanium on the Moon and shaping it into pipes by casting or extrusion of heated and softened titanium in addition to producing salt, which will require producing sodium or magnsium on the Moon and upporting chlorine in salt form rather than heavy insulated tanks of liquid chlorine. Salts of copper and zinc in plastic bags in rocket modules would be upported and electrolyzed on the Moon to get gasious chlorine and useful copper and zinc. The chlorine would be reacted with lunar sodium or magnesium so some limited production of Na and Mg would be rquired.

Turbines present difficulties. Blades creep and scale, steam can leak thru bearing seals, and turbines must be periodically rebalanced. The steam must be ultra pure. Waste heat radiators are also required. The complexity and mass of turbogenerator systems might easly outwiegh that of thermoelectric power elements like Peltier junctions.

It might also be possible to "sand blast" iron sheets with finely ground TiO2 obtained by hydrogen reduction of ilmenite to embed the TiO2 in the iron and make a better reflector. TiO2 is about 80% reflective.

Producing silicon for PVs on the Moon has many caveats. Regolith must be leached in HF acid or flushed with F gas to generate SiF4 that is decomposed on hot fingers of tantalum at 800-850 C. This silicon must then be zone refined and doped with boron or aluminum for p-type material and phosphorus for n-type material. The silicon must be sprayed in layers on aluminum or steel sheets and a grid electrode fashioned along with an antireflection coating. Flourine is very difficult to work with. It is also possible to use hydrogen chloride gas and impure silicon to get silane and silicon tetrachloride that can be decomposed into silicon pure enough after zone refining for solar panels. Titanium can stand up to Cl but not F. Silicon PVs degrade over time, their output is reduced by radiation and they can be damaged by micrometeorites.

A solar thermal system with thermoelectric elements and polished iron sheets or TiO2 coated iron sheets would be more robust. The sheet metal could be polished with fine regolith particles for grit. These particles have jagged edges and will make microsopic scratches. Perhaps some kind of lapidary tumbler could round off the micron sized particles sieved from regolith to make a better grit for a smoother finish.

So it comes down to the upported mass of thermoelectric generators versus the upported mass of turbogenerators as well as the added challenge of making titanium pipes sleeved in a thin layer of basalt, steam generators and waste heat radiators. Maintenance costs and reliability of each system must also be considered as well as lifetimes of each system.

These solar thermal systems must also be compared to silicon PV production on the Moon.

While silicon PV efficiencies are 10-15%, a heat engine working in space can approach nearly ideal Carnot efficiency.