Power Infrastructure

Technologies that take advantage of the strong constant sun, lunar vacuum, and low gravity for greatly enhanced performance

Four power sources make sense on the Moon, three of which are variants on solar power. There's photovoltaic solar cells directly changing sunlight into electricity. That makes the most sense at the lunar poles, where there are places with sun most of the time. There's solar thermal power, where the heat of the sun is used to drive turbines or Stirling engines. That makes sense when it can be combined with other processes for producing materials, such as for glass production. A good design at large scale can continue to produce power at a pretty steady level all through the night, that is the purpose of the STeMP units. There's space-based solar power, where photovoltaics are placed in orbits that keep them in sunlight all the time, and then that power is beamed as microwaves to the surface. This is an excellent approach once there is a bit more infrastructure, and is the largest source of power on the Moon for some time. Then there is nuclear power. Nuclear reactors are the most versatile and reliable power source and therefore always have a place.

Anywhere where there is new development far from existing infrastructure, there is a logical order in which to deploy the different power sources. Power from the reactors of the nuclear shuttles can be had immediately upon landing anywhere, day or night. A shuttle can also be fitted with a solar wing module if it is taking machinery out during the day. That powers machinery while initial setup is done. A solar panel and nuclear mix is also the best option after the shuttle leaves. The solar panels provide many times more watts per kilogram of their mass than a mobile nuclear reactor, but the reactor will power things through the night. (There are currently no power storage options that will provide consistent power for two weeks of night and have decent durability without massing more than a mobile reactor.) For best productivity in that configuration, activities that draw less power are best organized to happen during darkness. Once there are enough skyhooks set up with extensive solar power wings, part way through Phase 2, the next thing is to erect a rectenna unit that receives power beamed from those panels as microwaves. Rectennas are simple and light, that's pretty fast to do even though they are large. Finally, much later, Stirling generators can be connected to the units producing molten rock in bulk for construction and industry.

Solar Thermal Melt and Power - STeMP units

Once Lalande has a genuine town, and later for the city, generators running on stored solar heat are a significant source of power. They power Stirling engines with a gaseous working fluid, heated by a mass of molten regolith, which in turn is heated by tubes filled with molten sodium that circulate through parabolic mirrors.

Chemicals that boil off the lava are collected for use elsewhere. These are chemicals such as oxides of phosphorus, potassium, and sulfur, and minute amounts of hydrogen, carbon, and nitrogen. The plant is sized with extra capacity so that some molten rock can be drawn off and used to produce things like glass and basalt fiber materials. That lava needs to be processed further in most such cases.

Some of the components for STeMP units are simple and can be produced on-site early in colony development - mirrors, tanks, pipes, reflective heat barriers. With 3d printing of parts out of sintered metal powder and casting of molten minerals, most of the radiators, pumps, and generators can also be made.

This shows a single mirror unit, tank complex, Stirling engine, and radiator unit, for the sake of a clear explanation. Units placed in the colonies will have as many mirror, generator, and radiator units as the tank can support, and be scaled as large as simulations indicate would work well enough. That simulating remains to be done, but guesstimating from what we have now, this concept is a good option for units producing up to 10 MW of electricity, steadily day and night.

Nuclear Reactors

Unless you go back to plans from the 70s, reference to nuclear power has completely disappeared from proposals for the Moon. This is unfortunate, as the reality is, everywhere except the poles, when you start development in a new area, you need mobile nuclear power plants so you can work through the night.

Nuclear safety is quite easy on the Moon. The absence of air and water on the Moon really simplifies the use of nuclear power. There is no need to think about radioactive waste leaking over the long term because there is no water table or air to carry radioactive particles away from a waste dump. In fact, putting radiation shielding around a reactor is just a matter of piling up a sufficient thickness of the surrounding regolith, or digging a deep enough pit. There is no biosphere to protect, no weather to think about.

If it is new and has never been used, the radiation from nuclear fuel isn't dangerous at all. It's only once it has been 'burned' in a reactor that it becomes dangerous, because at that point it is full of the products of the fission that occurred during that burning. Those are super radioactive. Plain old uranium is stuff you can safely hold in your hands. The overview linked to in the sidebar makes a great comparison on slide 35 that gets this across: "Standing next to a NTP [Nuclear Thermal Propulsion] engine before launch for one year is less radiation than a diagnostic x-ray".

The wiki page for Nuclear Reactors on the Github repo summarizes several papers on the Kilopower project at NASA's Glenn Research Center. These designs aim at creating small nuclear reactors that are easy to set up and maintain because the rate of fission reactions passively corrects itself, eliminating risk of melt-down. The reactor model for the early construction phase of Cernan's Promise will be based on that design. The closed Brayton cycle reactor design shown in the sidebar will also be modeled later.

The fuel rods for reactors weigh so little compared to the vast energy that can be tapped from them, it will be a long time before it is cheaper to fabricate fuel rods on the Moon than to import them for Earth. One ton of fuel from Earth could power a 30 MW reactor for 6 years. The soil around Lalande Crater has the Moon's highest contents of thorium, and it is reasonable to suppose that once excavation was done, deposits with much higher thorium and uranium contents would probably be found in that region. Since these deposits are also associated with a high content of many rare earth elements, it may prove economical sooner to process them if all of those elements are thus extracted. Thoria and urania, the oxides of thorium and uranium, are useful for other things besides nuclear fuel, which also improves the argument for mining them. The development that justifiees nuclear fuel production on-site in our timeline is on-site manufacture of nuclear thermal rocket engines.

Solar Photovoltaic Panels

The initial base at the north pole is powered by panels set up on talls masts. These masts can be placed in areas that already get sunlight on the surface over 80% of the time. By placing the solar panels 20 m up these masts, the proportion of the time they are in sun could be boosted to over 90% of the time. It's a matter of raising the panels above the few remaining prominences that block their view of the sun, as it skirts the horizon over the course of each lunar day. The panels then need only rotate once every 27.5 days to track the sun.

Shadows are very long always at the pole. It is like being in a place where it is always almost sunset. To minimize power loss due to some solar panels being in the shadow of others at certain times, they need to be spread far apart and placed along a curved line following the crater rim, so only two ever line up at one time. Cables between the masts terminate in a set of microwave transmitters on the mast closest to the base. Mobile machinery gets power from those tranmitters, which send a tight beam of energy to the rectennas mounted on them. The early buildings also receive power through receivers, until cables can be laid.

By the time the polar base becomes Inukshuk colony, the solar masts have been raised enough that all the panels are in the sun all the time, and the panels can be raised or lowered on their mast so the shadow of others never falls on them. It is easy enough to erect tall towers on the Moon, because the gravity is so low and there is never any wind or weather, that solar masts can be erected over a large area, and just made taller to put them always in the sun even if it takes several hundred meters before they are always above all shadows. This is important, as there are not only solar panel masts, but also solar mirrors reflecting light into greenhouses, and solar concentraters focussing sun to melt rock or perform other heat processes.

Away from the poles, where there is sun only half the time, solar panels see limited use until Cernan's Promise is largely constructed. Once the initial setup of development in an area is complete, other energy formats are superior. This changes once flywheels can be manufactured in bulk from local materials. Then, during the day, the solar panels spin up the flywheels, storing energy as rotational energy.

Solar Power Beamed from Orbit

The orbits of the timeline's three vertical skyhooks in polar orbit around the Moon put their anchor stations in the sun at all times (short of an eclipse by Earth). The equatorial skyhooks are in shadow only 15% of the time. Large wings of solar panels are attached to the anchor masses of all skyhooks early on and expanded aggressively over time. Once Cernan's Promise and Inukshuk are producing solar panels, all the skyhooks get their panels from the Moon.

The energy they collect is passed to microwave transmitters and beamed to the surface. This is one of many instances where it is advantageous that skyhook anchor masses should be as heavy as possible. The wiring from the solar cells to the transmitters can be as thick as required to carry the current, and there is no need to think about the mass of radiators, or heat sinks, or trusses. Aluminium makes good electrical wire. It takes a lot of energy to purify the metal out of lunar regolith, but that would be an early priority for that reason. The heavy things needed for these installations can come from the Moon even at an early stage of colonization - trusses, radiators, heat sinks, wiring, transformers, waveguides, and transmitters. The transmitting antennas are parabolic dishes made of wire mesh. Fabricating them at the colonies and on the skyhooks allows very large ones to be installed - no need to fit them in a rocket fairing. The larger a transmitting antenna is, the smaller a receiving antenna (rectenna) can be. Putting a bunch of large ones on the skyhook anchor allows small ones to be on the surface. (This is also helpful for the skyhook climbers, which get their power this way.)

Modern solar cells for space applications now generate around 300 W per kilogram of the system. Roughly 50% of the generated energy makes it to the electrical grid after the whole process of beaming it. The payload mass from Earth needed to supply lunar installations with power is easily accommodated in the payload budget the colonies have.

Flywheel Energy Storage

Flywheels store energy as rotation. A motor spins them up until they are rotating at thousands of times per minute. For the Moon, this would be done during the day with energy from solar panels. Energy is drawn from the flywheel by instead making the flywheel spin the motor. That causes its electromagnet to generate an electric current. This drains their energy over the lunar night when the solar panels aren't working.

To store lots of energy in flywheels for a long time - in this case, for 14 days at a time - there are a few key things needed.

  • To reduce losses from drag, flywheels work in a vacuum. The Moon has plenty of that.
  • Friction losses in the bearings also need to be minimized. Suspending the flywheel's shaft magnetically eliminates friction completely. The Moon again has a big advantage here in that only a sixth of the force is needed to suspend a given mass, allowing the use of much smaller magnets for the same job.
  • If superconducting magnets are used, a high degree of stabiility can be attained and energy losses due to interacting magnetic fields are very small. The Moon has big advantages here, too. The night-time temperature on the lunar surface drops below the temperature at which several high-temperature superconductors work.
  • As long as the heat of the sun is thoroughly blocked during the day, the magnets can be kept at temperatures where they are superconducting mostly passively. Again thanks to the vacuum, simple reflective insulation can be used to ensure the heat of the sun stays out, and by putting these installations in pits, and using heavy stone frames for the flywheels, there can be enough thermal mass that heat generated by the system doesn't raise the temperature above the the desired point.
  • Stone frames are also desirable as high-speed flywheels spin so fast that they are under tremendous outward tension at their rims. Occasionally this leads to catastrophic failure in which the flywheel shatters, the fragments flying in all directions.

The energy stored in a flywheel rises with the square of its radius, the square of its rotational speed, and linearly with its mass. The most effective shape is a cylinder or wheel in which most of the mass is on the outer edge. This must be balanced with the need for the whole wheel to hold together - the material closer to the hub must bear the tension of the mass on the rim.

On Earth, where it is very difficult to maintain a vacuum, flywheels are small. The housing needed to keep the flywheel in a vacuum would become expensive of they were larger. (The market for flywheels is also in things where it wouldn't make sense to make large units.) Improved performance is attained by spinning them faster. On the Moon, where a vacuum harder than anything available on Earth is everywhere for free, making flywheels with a bigger radius could be the best way to attain high storage in a cheap system. The tension forces on the material are the same (a flywheel that is turning half as fast but has a circumference twice as big is moving at the same speed at a point on the rim), but the elecronics and control mechanisms are simpler and thus cheaper.

Professor John Vance of Texas A&M[?] designed a flexible flywheel in a gimballed support that overcame the need for high precision engineering in them. Conventional flywheels are rigid in order to accommodate the precision damping systems needed to prevent them from oscillating and vibrating. The gimbal design may also be used for rigid flywheels, but the flexible version can be made from basalt fiber or glass fiber, which can be made on the Moon, easily and in bulk. The design has several other big advantages too:

  • The tensile strength of various fiber materials, including basalt fiber and glass fiber, is much higher than steel, the material economic flywheels are usually made of. Once polymer fibers like Zylon are made on the Moon, they can be used in flywheels that spin even faster.
  • They are extremely simple to make. They are made of bundled cables coiled around each other, there is no need even for a binder in the material. Rigid steel flywheels must be forged, or better, many sheets of steel are cut, shaped, and laminated together.
  • Flywheels made of fiber materials don't fail by shattering, meaning they don't need stone enclosures to ensure flying shrapnel from such an incident can't do serious damage.
  • Because of the self-correcting feedback flexible flywheels use, engineering tolerances are much looser. This means simple permanent magnets can be used instead of superconducting ones without causing big increases in energy losses.
"Rotordynamic instability is characterized by orbital whirling of the wheel at its lowest natural frequency even while the rotor speed is much faster. It is not significantly influenced by rotor balance, and can become violent with very large and destructive amplitudes if not suppressed by proper design. In energy storage flywheels, whirl instability is typically induced by internal damping (hysteresis) in the rotating assembly. Collaborating with Dr. Richard Schneider, Dr. Vance built a flywheel in the 1970's made of woven high-strength fibers, but with no bonding agent. Instead of a steel shaft, the wheel was suspended from wound ropes that allowed the critical speed inversion to take place at very low rotor speeds. Consequently the flywheel was self-balancing, to the extent that no balance masses need to be added for smooth synchronous operation at high speeds. However, the wheel whirled at its very low natural frequency (which could easily be tracked by the human eye) as a result of the internal friction in the rotating support system. Dr. Vance solved the whirl instability problem by suspending the motor from an anisotropic gimballed support. This design will also work to stabilize flywheels of conventional design, as Dr. Vance demonstrated with a laboratory model and a computer simulation."
-- Design for Rotordynamic Stability of Vertical-Shaft Energy Storage Flywheel

Bill Gray patented and prototyped a flywheel based on this concept, called the Velkess, but was unable to bring it to market. The video in the sidebar shows a mini demo model his team made that works on the same principal. Though there are no commercial operating flywheels based on this concept, it looks very solid. The application in which this design really shines - energy storage - remains a small niche market. Only with the growth of renewable energy installations with intermittent generation, will the design get the attention needed for full analysis. Possible issues with it are ongoing off-gassing[?] from the material degrading the vacuum in its housing. (Again, not a problem on the Moon.)

Materials in a hard vacuum release a lot of gasses due to the way they respond to the absense of air pressure. This happens more in some materials than others. Sometimes the materials used in space hardware is partly determined by the need for them to not off-gas. Some materials will only off-gas a small amount, others much more. Some will stop after a period of exposure to vacuum, others will continue.

This makes deployment of a large number of flywheels on the Moon much simpler and cheaper, allowing it to be the backbone of energy storage quite early in colony development. Later in the timeline, flywheels are key to making the electric runways work. Flywheels can provide all the power the launching shuttles need in the few seconds they need it, and absorb the large amount of energy they return to the system when they land equally well.