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 15 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.
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.