Sodium Scenario
Many of Sodium's properties that make it hard to use on Earth can be a boon on the Moon. This is easier to understand with a scenario of a possible lunar use. Consider the following case:
In our Timelines we are at the early part of the Miner Period. We have enough volatiles production to generate useful by-products and a prototype shop in which to hand-build the materials and tools we need.
We are just completing a moderate-sized solar power system of 100 kilowatts on high ground. We need to use the power for an industrial operation located one kilometer away and down a slope.
We have manufactured three kilometers of power cable to do the job and now have it on large reels made from discarded decent vehicle struts. The cable is made from three materials, (Sodium, Potassium, and Iron) and is 100% lunar materials.
The cable is multi-layered and coarsely stranded. The center single strand is iron for strength. The next layer is Potassium strands, also abundant on the Moon, that provide little conduction but they bulk out the cable and help with several thermal problems. Next is a single layer of strands of Sodium that are the main electrical conductors. The final outer layer is of finer Potassium wires lying in the troughs formed by the Sodium strands. Most of the voids between strands are filled with fine glass fibers made from industrial slag. These fibers help the cable keep its shape, keep it flexible, and help heat get out. The cable has a two square centimeter cross-section which is about the diameter of a large finger. It has no outer insulation at all.
To lay the cable, we have modified a rover that was originally driven by people but is now robotic. It has a complex trenching rig at the rear that can bury cable a meter into the regolith in a very controlled manner. On its back is a mechanical a handler for one oversized cable reel. On its front is a small bulldozer blade.
The solar power station has a power conditioner that puts out 1000 volts of three-phase AC power at 2 kilohertz. The three phase configuration is optimal for industrial applications. The low RF frequency greatly reduces the amount of iron needed in transformers and motors.
The rover starts the first of three trenches at the power conditioner. In one slow and complex operation:
- It digs a trench a meter deep and 100 mm wide
- It sieves the regolith into three sizes, fines, course, and rocks.
- It packs 20 mm of fines into the bottom of the trench
- It lays the cable down in an easy serpentine pattern.
- It packs 100 mm of fines on top of the cable
- It dumps the course material back into the trench
- It tops of the trench off with the rocks.
- During these operations it keeps the trench open with two aluminum side plates.
The meter depth provides electrical insulation, radiation shielding, and thermal stability. The packed fines help remove heat from the cable and insure that sharp rocks do not damage it. The serpentine pattern allows the cable to thermally expand and contract without damage. The top rocks give a clear visual indication that something is buried here.
The rover works away from the power station until it reaches the slope. It then lowers the bulldozer blade and shifts the weight of the cable reel back. It then starts working straight down the slope rolling a mound of regolith in front to control its decent.
This process is repeated three times. At the cable ends sensors are buried to measure the current and the temperature of the cable and the surrounding regolith. The exposed ends of the cables are covered with glass or ceramic insulating tubes. The ends of the cables are placed into aluminum tubes and crimped solid. The aluminum then is bolted into the power converter connections.
In normal operation the Sodium carries almost all the power. The waste heat generated in it is transferred to the packed regolith fines. Getting rid of waste heat is the most difficult problem in power generation on the Moon. The large cable core insures that the Sodium wires will have good contact over a large area.
In the case of a failure that generates very high currents in the conductors, the Potassium wires will be the first to start to melt as Potassium has a 25 C lower melting point than Sodium. This melting will adsorb an enormous amount of heat, but only for a short time. The cable will be able to withstand a few seconds of a full short and by then the power conditioner should be able to detect the fault and open the connection. The outer layer of Potassium wires will melt on the first overload and from then on it will help improve the transfer of heat to the regolith.
The strengths of this scenario are the very large amount of lunar material used and the very small amount of Earth material needed. Its weakness is simply that we do not know enough about the thermal properties of man-disturbed regolith to know if we can successfully dump the waste heat. Thinking the problem through this way does help us understand what questions we need to ask.