Power for Settlements
Contents
Power for Lunar Settlements
Here are some considerations for the power systems used in our lunar settlement stories.
Solar Power
An attractive source for power on the Moon is the Sun. Solar is particularly inviting in the polar regions where mountain tops are available that have solar views 75% to 99% of the time. The sun light is not diminished by an atmosphere and is never blocked by clouds. It is harsh and unrelenting.
From the surface of the Moon, the Sun appears to move slowly across the sky making a full cycle every 29 days. On rare occasions it is eclipsed by the Earth for a few minutes.
At the poles, the Sun simply circles the sky very near the horizon. At its lowest point it may be blocked by mountain tops. Other times it may be blocked by local instillations such as other power collectors.
Near the lunar equator, the Sun is visible only one half the time. The 14 day lunar nights are a major problem for power generation and are a major factor in setting a polar location for the first lunar settlement.
There are basically two types of solar power, Solar Dynamic (SD) and Photovoltaic (PV).
SD uses a heat cycle to drive a piston or a turbine which connects to a generator or dynamo. Two popular cycles for Solar Dynamic are Brayton Cycle or Stirling Cycle. Solar Dynamic systems employ a large reflector to focus sunlight to a high concentration to achieve a high temperature for the heat cycle to operate at highest possible efficiency.
PV uses semiconductors (e.g. Silicon or Gallium Arsenide) to directly convert sunlight photons into electric potential. Commonly know as “Solar cells”
Comparison of PV versus SD
PV Problems
The main problem with solar power is that PV conversion efficiency is only about 20% efficient, for rather high priced PV cells. Plus PV cells are rather heavy to ship from Earth and to soft land on Luna is very expensive.
SD Problems
SD has a much more severe pointing requirement than PV because it needs to maintain an accurate optical focus.
If a PV array drifts off a few degrees, the power level drops a few percent.
If a SD array drifts off a few degrees, the power level drops off to zero.
Space Based Solar power
Until ISRU manufacturing is online, Solar Power Satellites (SPS) represent a more economic means of supplying large scale basebar power on the Moon in the early days.
Compared to PV or SD systems, an SPS rectenna by itself is lightweight and has a conversion efficiency of over 90%.
Let us assume a PV array is 10 times heavier per watt than a rectenna.
That means we can get 5*10 = 50 times as much power per kilogram for a rectenna than for a PV array. That means a rectenna will be at least 50 times cheaper per watt of useful power than PV array. "At least" because the manufacturing cost of a rectenna will be much cheaper than for PV array.
Soft landing hardware on to the Moon is very expensive, so weight is at a premium.
The distance from L1 to Luna is about the same as the distance from Earth to a geostationary satellite.
So one could put an SPS at lunar-L1 and beam microwaves from there to supply one or more rectennas on the Moon as a very efficient way to provide energy to a power hungry moon base.
The sun angle across PV arrays on the lunar surface constantly changes, and is usually less than the 1,360 w/m^2. To maintain constant max power the PV array must have expensive and heavy steering equipment. Whereas a rectenna does not need to be steered, and always gets maximum power.
The ideal site for the lunar rectenna would be in Sinus Medii at the Lunar 0-Longitude point on the Lunar Equator, in the middle of the side which faces the Earth.
1) Because it is directly below the L-1 point, 2) it is the closest point on the lunar surface to the L-1 point, 3) Because at that point the lunar surface is at right angles to the incoming microwave beam.
All these factors permit the smallest rectenna at that location.
Without making any modifications at all to that very SPS, it could easily be maneuvered from L1 to GEO to feed a terrestrial ground based rectenna. The antenna design for L-1 to Luna will work equally well from GEO to Earth
Dumping Heat
One of the hardest things to do on the Moon is get rid of large amounts of waste heat. In any power generation system you must have both a source power and a sink for waste heat. This is basic thermodynamics and there is no way out of this requirement. Most space missions simply dump waste heat to deep space. On the surface of the Moon, this is not so simple. Half of your view is exposed to the cold of deep space, with the complication of a hot spot Sun and a warm spot Earth. The other have of your views sees the surface of the Moon itself. At the pole the average lunar surface temperature will be around 0 C and does not change drastically during the day. Here simple hear radiators can be used as they only need to avoid exposure to a slow moving Sun.
At the equator there is a completely different story. The surface of the Moon raises to about 200 C by mid lunar day. It will be very difficult to dump waste heat at this time, so it will be difficult to keep the living areas cool and do major industrial operations. Equatorial housing may need to be buried extra deep for thermal reasons.
Big Battery Power
There are at least three separate technologies promising a times-ten improvement in rechargeable batteries currently demonstrated in the laboratory. They are all based on some version of the super capacitor. The value of such a battery for electric cars on Earth alone will be in the hundreds of billions. The only problem is developing mass production techniques. The race is on.
By the time of our stories, it is reasonable to expect really good batteries will be available for use in space. These will give a lunar rover a range of at least 500 kilometers. They will also be able to power reasonable amounts of electronics and modest life support equipment through a 14 Earth day lunar night. They will not allow large industrial operations for this length of time.
Nuclear Power
During early stages of lunar exploration, some instruments can be powered by Radioisotope Power Generators (RPG). These devices provide only about 100 watts and are difficult to come by. The short lived radioisotopes needed to make them are manufactured at only a few places on Earth and only in small amounts.
In the long term it may be necessary to establish small nuclear power stations on the Moon, particularly in non-polar locations. These will mainly provide power during the long lunar night. Their daylight operation will be limited by their ability to dump waste heat. They will be expensive to build and difficult to maintain.
Story Power
In our stories, solar will be the major power source. RPGs can be used for small science stations. Big batteries may be used for electronics and life support.
Stories set some what farther in the future, may have nuclear power stations. It is reasonable to assume that if Helium 3 from the Moon is an important element of Earth side power generation, then small safe lunar nuclear power plants are a likely possibility.
